city of laurel - townnewsbloximages.chicago2.vip.townnews.com/billingsgazette.com/... ·...

CITY OF LAURELRiverside Park Bank StabilizationPreliminary Engineering Report

Prepared by:

®

March 2012

RIVERSIDE PARK BANK STABILIZATION

PRELIMINARY ENGINEERING REPORT

Prepared for:

City of Laurel, Montana

March 2012

Prepared By: Ryan Holm, EI

Reviewed By: Jeremiah Theys, PE

Approved By: Chad Hanson, PE

®

Riverside Park Bank Stabilization Preliminary Engineering Report

March 2012 i

Table of Contents

EXECUTIVE SUMMARY .............................................................................................................................. 1

INTRODUCTION ......................................................................................................................................... 3 Purpose and Need ................................................................................................................................... 3 Scope of Work .......................................................................................................................................... 3

SITE INFORMATION ................................................................................................................................... 4 Site Description & Location .................................................................................................................... 4

Water Treatment Plant Intake Structure ............................................................................................ 4 Utilities .................................................................................................................................................. 7 Site History ........................................................................................................................................... 7 Current Conditions ............................................................................................................................ 10

Hydraulic Analysis................................................................................................................................. 12 Background ....................................................................................................................................... 12 Hydrology ........................................................................................................................................... 13 Hydraulics .......................................................................................................................................... 13 Geomorphology ................................................................................................................................. 14 Shear Stress ...................................................................................................................................... 17

General Design Requirements ............................................................................................................ 18

ALTERNATIVE SCREENING PROCESS ................................................................................................... 19 Bank Stabilization Alternatives ........................................................................................................... 19

Revetment Riprap Armoring – Full Slope ........................................................................................ 19 Bioengineered Slope ......................................................................................................................... 19 Revetment Riprap Armoring – Bankfull and Below ........................................................................ 20 Sheet Pile Retaining Wall ................................................................................................................. 20 Articulated Concrete Blocks or Mats ............................................................................................... 20 No Action Alternative ........................................................................................................................ 21 Summary ........................................................................................................................................... 21

Bank Alignment Alternatives ............................................................................................................... 21 Existing Bank Alignment Alternative ................................................................................................ 21 Intermediate Bank Alignment Alternative ....................................................................................... 22 Current Bank Alignment Alternative................................................................................................. 22 Summary ........................................................................................................................................... 22

ALTERNATIVE ANALYSIS ....................................................................................................................... 23 Bank Stabilization Alternatives Description ....................................................................................... 23

Alternative 1 - Revetment Riprap Armoring – Full Slope ................................................................ 23 Alternative 2 – Revetment Riprap Armoring – Bankfull and Below ............................................... 25 Alternative 3 – Articulated Concrete Blocks .................................................................................... 25

Bank Alignment Alternatives Description .......................................................................................... 28 Alternative A – Existing Bank Alignment .......................................................................................... 28 Alternative B – Intermediate Bank Alignment ................................................................................. 28 Alternative C – Current Bank Alignment .......................................................................................... 28

Permitting & Regulatory Compliance ................................................................................................. 31 Construction Challenges ...................................................................................................................... 32 Cost Estimates ...................................................................................................................................... 32


March 2012 ii

Affected Environment .......................................................................................................................... 37 Human Environment ......................................................................................................................... 37 Public Health and Safety ............................................................................................................. 37 Socio-Economic Impacts ............................................................................................................. 39 Cultural Resources ....................................................................................................................... 40 Access to Public Lands ................................................................................................................ 41 Physical Environment ....................................................................................................................... 41 Climate & Air Quality .................................................................................................................... 41 Geology & Soils ............................................................................................................................ 42 Surface Water Resources ............................................................................................................ 42 Groundwater Resources .............................................................................................................. 44 Hazardous Facilities ..................................................................................................................... 44 Biological Environment ..................................................................................................................... 45 Vegetation .................................................................................................................................... 45 Biological Resources .................................................................................................................... 46

Basis for Selection of the Preferred Alternative ................................................................................ 47

DETAILED DESCRIPTION OF PREFERRED ALTERNATIVE ..................................................................... 53 Description ............................................................................................................................................ 53 Permitting ............................................................................................................................................. 55 Funding .................................................................................................................................................. 56 Project Schedule ................................................................................................................................... 56

List of Tables Table 1 - Hydraulic Analysis Calculated Base Flood Elevations ............................................................ 16 Table 2 - Opinion of Probable Cost - Alternative 1A .............................................................................. 33 Table 3 - Opinion of Probable Cost - Alternative 1B .............................................................................. 33 Table 4 - Opinion of Probable Cost - Alternative 1C .............................................................................. 34 Table 5 - Opinion of Probable Cost - Alternative 2A .............................................................................. 34 Table 6 - Opinion of Probable Cost - Alternative 2B .............................................................................. 35 Table 7 - Opinion of Probable Cost - Alternative 2C .............................................................................. 35 Table 8 - Opinion of Probable Cost - Alternative 3A .............................................................................. 36 Table 9 - Opinion of Probable Cost - Alternative 3B .............................................................................. 36 Table 10 - Opinion of Probable Cost - Alternative 3C ............................................................................ 37 Table 11 - Basis for Selection .................................................................................................................. 51 Table 12 - Cost Comparison & Selection Summary .............................................................................. 52 Table 13 - Opinion of Probable Cost Preferred Alternative - Alternative 2B........................................ 55

List of Figures Figure 1 – Location Map ............................................................................................................................. 5 Figure 2 – Vicinity Plan ............................................................................................................................... 6 Figure 3 – Channel Migration Exhibit ........................................................................................................ 8 Figure 4 – Channel Migration Exhibit ........................................................................................................ 9 Figure 5 – Water Surface Profile Exhibit ................................................................................................ 15 Figure 6 – Alternative 1-Revetment Riprap Armoring (Full Slope) ...................................................... 24 Figure 7 – Alternative 2-Revetment Riprap Armoring (Bankfull and Below) ...................................... 26 Figure 8 – Alternative 3-Articulated Concrete Block ............................................................................. 27 Figure 9 – Bank Alignments Alternatives Plan ...................................................................................... 30 Figure 10 – Preferred Alternative Site Plan ........................................................................................... 54


March 2012 iii

Appendices

Appendix A Site Photos

Appendix B Historic BLM GLO Maps

Appendix C Hydraulic Analysis Outputs Existing Conditions – 2007 Draft Flood Insurance Study Current Conditions – September 2011 Proposed Conditions –2012 Velocity Distribution Exhibits – 100-Year & Bankfull Flows

Appendix D Environmental Documentation

Appendix E “Evaluation of the Sediment Deposition Problems Along The Yellowstone River Near Laurel, Montana.” Prepared by U.S. Army Corps of Engineers, April 2000.


March 2012 1

EXECUTIVE SUMMARY Flooding on the Yellowstone River in April through July of 2011 caused significant erosion to the river bank adjacent to Riverside Park in Laurel, Montana. The erosion allowed the main channel flow to shift south, away from the City of Laurel Water Treatment Plant intake structure. Sediment deposition also occurred beneath the Highway 212/310 bridge and the railroad bridge, immediately upstream of the park, reducing their hydraulic capacity and increasing flow velocities. The Exxon-Mobil Silvertip crude oil pipeline was exposed by bed scour and ruptured, spilling 1,000 barrels of crude oil into the Yellowstone River. The City of Laurel began having problems drawing water into the intake in January 2012. The top of the intake, which is supposed to remain submerged in all flows, was exposed to air, and ice began to build up on the intake structure. Additionally, slush ice flows in the river, which have never affected the intake because they occur at the water surface, began clogging the intake screens. These occurrences culminated in the City having to temporarily shut off the raw water supply to the CHS Oil Refinery on January 30, 2012 in order to meet domestic demands. Since then, the water surface has been right at the top of the intake structure, often showing riffles from the bolt heads on the intake structure. Consequently, the City began making plans to reconstruct and stabilize the eroded river bank and remove sediment deposited under the bridges in an effort to return the main channel flow to directly over the intake structure. This report evaluates proposed bank stabilization treatments for the eroded river bank and examines the factors that contributed to the bank erosion and sediment deposition.

A full hydraulic analysis was performed using HEC-RAS software and compared to two Federal Emergency Management Agency’s (FEMA) Flood Insurance Studies. It was determined that the sediment deposition under the bridges caused a reduction in hydraulic capacity which aggravated an already constricted and misaligned bridge opening. This caused a backwater effect upstream of the bridges and increased flow velocities through the bridges which resulted in the severe erosion of the park land immediately downstream of the highway bridge. Current and proposed conditions were modeled to determine the effects that reconstructing the river bank and removing sediment beneath the bridges would have on base flood elevations. Additionally, the velocity distribution through the channel was analyzed to ensure the preferred alternative increased flow over the intake structure instead of allowing the flow to bypass the structure on the south as it currently does. Hydraulic analysis of the proposed improvements demonstrates improved hydraulic capacity at the bridges and reduced base flood elevations compared to current, eroded conditions. Hydraulic conditions at the intake structure will continue to worsen if action is not taken.

Several bank stabilization measures were screened for in-depth analysis including revetment riprap armoring, bioengineered stabilization, a sheet pile retaining wall, and articulated concrete blocks and mats. Additionally, three river bank alignments were developed to examine the effects of reconstructing the bank at the historic (pre-flood) location, the current (post-flood) location, and an intermediate location. Pre-screening disqualified several of the potential bank stabilization measures from further consideration; however, all three bank alignments were examined further. A No Action alternative was also disqualified for not meeting the primary goals of the project. Bank stabilization alternatives which were evaluated further included: revetment riprap armoring – full slope, revetment riprap armoring – bankfull elevation and below, and articulated concrete blocks.

The bank stabilization alternatives and the bank alignment alternatives were evaluated with a comparative analysis that considered cost, achievement of project goals, technical feasibility, environmental impacts, ease of permitting, and construction time. The preferred alternative for the Riverside Park Bank Stabilization is a revetment riprap armored slope, bankfull and below, with a vegetated overbank slope constructed along the intermediate bank alignment. This alternative was found to be the least environmentally damaging, practicable alternative.


March 2012 2

Approximately 715 linear feet of river bank will be reconstructed with 7,320 cubic yards of 48-inch nominal diameter riprap being placed from the toe to bankfull elevation. Turf reinforcement mat and upland plant species will be utilized from bankfull elevation to the top of the river bank. Approximately 5,500 cubic yards of sediment will be removed from beneath the highway and railroad bridges. These improvements will ensure reliable water supply to the intake structure, increase hydraulic capacity at the bridges, prevent river migration to the south, and protect the new river bank from future erosion.


March 2012 3

INTRODUCTION Purpose and Need Heavy rains and above average snowpack runoff caused flooding of the Yellowstone River from April 2011 through July 2011. During the flooding, the river bank adjacent to Riverside Park in Laurel, Montana experienced significant erosion. The erosion destroyed a boat ramp and exposed several underground utilities including a crude oil pipeline which ruptured and spilled crude oil into the Yellowstone River. The erosion also damaged a historic levee, significantly increasing the flood hazard to historic buildings located in the park and residences east of the park. Additionally, a large volume of sediment was deposited upstream, downstream, and beneath the Highway 212/310 and railroad bridges located immediately upstream of the park, particularly on the north side of the river. The bank erosion and sediment deposition indicated the river was migrating to the south. The City of Laurel Water Treatment Plant intake structure, which provides water to the City of Laurel and the Cenex Harvest States (CHS) Refinery, had been located in the center of the main channel flow but was now on the north edge of the main channel flows. This concerned the City of Laurel because they replaced the intake structure in 2003 at a cost of nearly $3 million due to the former intake structure being left inoperable by river channel migration. It is estimated that it will cost the City of Laurel approximately $5 million to construct a new intake when the current intake becomes inoperable, if measures are not taken to improve the reliability of operation.

Since the flood damage occurred, the City of Laurel has begun to have problems obtaining a reliable supply of water from the river. During January 2012, water over the submerged intake was at its lowest levels since installation of the new intake. This resulted in excessive ice buildup on top of the structure, slush ice developing on the intake screens, and a temporary shutdown of the raw water line from the intake to the CHS Oil Refinery because the City couldn’t meet their own domestic demands. With the recent developments concerning water availability issues at the intake structure, these problems have escalated to a severe threat to public health and safety and must be corrected as soon as possible.

The City of Laurel has secured funding from the Federal Emergency Management Agency (FEMA) to construct improvements to protect the continued operation of the intake structure. These improvements generally consist of reconstructing and stabilizing the eroded south river bank and restoring the hydraulic capacity of the upstream bridges. These measures will restore the main channel flow back to its existing (pre-flood) location and prevent sediment deposition from occurring at the intake.

Scope of Work The City of Laurel decided to take a proactive approach to prevent the new intake structure from becoming inoperable and to protect the historic buildings located in the city park. The City contracted with Great West Engineering, Inc. to prepare a preliminary engineering report to examine possible measures which would help return the main channel flow back to its existing (pre-flood) location and ensure reliable operation of the intake structure. Once an alternative has been chosen, Great West Engineering, Inc. will complete final design and construction plan preparation.

This report evaluates proposed bank stabilization treatments for the eroded river bank and examines the factors that contributed to the bank erosion and sediment deposition. A topographical survey was completed to gather current, post-flood conditions for use in a detailed hydraulic model. Existing (pre-flood), current, and proposed conditions were modeled using hydraulic analysis software to determine if proposed alternatives meet FEMA floodplain requirements. Several bank stabilization improvements were screened and feasible alternatives were further evaluated. Conceptual plans were developed for each feasible alternative. This report also recommends a preferred alternative based on the results of the analysis and input from City officials.


March 2012 4

SITE INFORMATION

Site Description & Location Riverside Park is located approximately 1.4 miles south of Laurel, Montana on the south bank of the Yellowstone River (see Figures 1 & 2). Although it is located outside of the city limits, it is a City owned and maintained park. It lies in Sections 15 & 22, Township 2 South, Range 24 East, Yellowstone County, Montana at latitude 4539’14” North and longitude 10845’29” West. It is accessed directly from U.S. Highway 212/310 which forms its western boundary. A Burlington Northern-Santa Fe railroad line parallels U.S. Highway 212/310 to the west of the highway. The topography of the park is generally flat with a manmade levee located along the river from the Highway 212/310 bridge to a point 2,500 feet downstream of the bridge. The levee consists of a standard trapezoidal cross section except west of the boat ramp where it was gently graded out on the landward side when the campground was developed. However, the top elevation of the levee was maintained all the way to the Highway 212/310 bridge abutment. Numerous cottonwood trees are found throughout the park and the area surrounding the levee is a dense upland riparian area.

In the vicinity of the project, the City of Laurel Water Treatment Plant sits on the north bank of the Yellowstone River opposite of Riverside Park. Also north of the river are the City of Laurel Wastewater Treatment Plant, a decommissioned landfill, the Cenex Harvest States (CHS) Oil Refinery, and the Billings Bench Water Association irrigation diversion structure. The CHS Oil Refinery has been in operation since the 1930s and produces 42,000 barrels per day of refined petroleum products including propane, gasoline, diesel, asphalt, and road oil. Residential and agricultural lands are the primary land use south of the river. The Yellowstone River drains approximately 8,200 square miles at Laurel, Montana and has an estimated flow rate of 56,700 cubic feet per second (cfs) in a 100-year recurrence interval flood. The project location is located approximately 2 miles upstream of the confluence of the Yellowstone River and the Clarks Fork of the Yellowstone River. Immediately upstream of the project is a 5-span steel girder bridge serving U.S. Highway 212/310 and a 3-span steel truss bridge serving the adjacent railroad.

Water Treatment Plant Intake Structure The original water treatment plant intake structure sits immediately downstream of the north center-span pier of the highway bridge; however, sediment deposition left it inoperable. A replacement intake structure, constructed in 2003, is located slightly downstream of the south center-span pier and is intended to remain submerged, even at low water. A raw water line was constructed with the new intake and is buried under the river bed. The minimum design capacity of the intake is 7 million gallons per day (MGD), with the ability to increase to 20 MGD with future improvements. During the winter months the City of Laurel draws approximately 2.7 MGD of raw water from the Yellowstone

Original intake structure and sediment deposition.

New water treatment plant intake structure, looking downstream from the highway bridge. Note low water over structure.


March 2012 5

Figure 1 – Location Map


March 2012 6

Figure 2 – Vicinity Plan


March 2012 7

Flooding in July 2011.

River, of which 1.4 MGD is supplied to the CHS Oil Refinery for process water and fire suppression.

Utilities There are a number of underground and overhead utility lines in the vicinity of the project. The Exxon-Mobil Silvertip crude oil pipeline lies 750 feet downstream of the highway bridge and was ruptured as a result of river bank erosion and bed scour at Riverside Park. A new pipeline was installed shortly after the failure, and the old, damaged pipeline is currently being removed. Another crude oil pipeline and a 12-inch natural gas transmission main operated by Williston Basin Interstate Pipeline also cross beneath the river and run through the park. An overhead power line crosses the river and terminates in an electrical drop on the south river bank. The power line provides electrical service for the buildings and RV electrical hookups in Riverside Park. A municipal water main which is suspended from the highway bridge runs underground on the south side of the river and transects the park. Additionally, there are numerous potable water hookups for RVs in the park. Buried communications lines run along Highway 212/310 on the west edge of the park.

Site History The City of Laurel was established in 1888, incorporated in 1908, and has the largest railroad yard between Seattle, Washington and St. Paul, Minnesota. Riverside Park was originally used as a German prisoner of war camp in World War II. Numerous buildings from the internment camp still stand on the property. According to the Montana State Historic Preservation Office, Riverside Park is a recorded historical site. “Site 24YL0169 is the historic Riverside Park, which is eligible for listing on the National Register of Historic Places. This site includes many buildings as well as a historic levee.” It is unclear when the levee was constructed; however, it appears on a Bureau of Land Management (BLM) General Land Office (GLO) Plat Map dated December 1906 (see Appendix B). It is speculated the levee may have been constructed during or shortly after the railroad bridge was constructed in 1890. A riprap project was proposed for the river bank and the levee in 1935, although it is unclear whether this project was completed. The park is currently used for recreation and has numerous tent and RV camp sites, picnic areas, and shower facilities. The park also provided boater access to the Yellowstone River via a concrete boat ramp.

The Yellowstone River drains a large portion of south central and southeast Montana. It has experienced flooding numerous times in the last 75 years including floods greater than the 10-year recurrence interval flow in 1943 (Q15), 1944 (Q20), 1967 (Q20), 1974 (Q30), 1975 (Q25), 1996 (Q15), 1997 (Q120), and 2011 (2 peaks – Q35 in May 2011, Q35 in July 2011). As with most rivers and streams, the Yellowstone River undergoes regular channel migration with major changes to the channel occurring during major flood events (see Figures 3 & 4). The Yellowstone River, at the project site, has generally been confined to its present location since construction of the railroad bridge (circa 1890). The original highway bridge was constructed in 1914 immediately downstream of the railroad bridge and with a slightly longer span. The active river channel has migrated within the confines of the bridges, as well as immediately upstream, which has left the river floodplain somewhat misaligned with the bridges. This has resulted in sediment deposition along the north side of the river channel, including beneath the bridges, thereby reducing hydraulic capacity and affecting downstream intakes and diversions.

The former water treatment plant intake structure is located immediately downstream of the north center-span pier of the highway bridge. During the floods of 1996 and 1997, sediment deposition on the north side of the river was so significant it prevented the intake from operating year round after the floods. Each year, City of Laurel personnel had to enter the river and construct gravel


March 2012 8

Figure 3 – Channel Migration Exhibit


March 2012 9

Figure 4 – Channel Migration Exhibit


March 2012 10

Sediment deposition beneath the railroad and highway 212/310 bridges. Looking upstream.

Sediment deposition beneath the railroad and highway 212/310 bridges. Looking downstream.

diversion berms to divert water to the intake and keep it operational. Several studies were performed to identify a solution to the problem. A brief Army Corps of Engineers (ACOE) planning study prepared in April 2000 evaluated three alternatives: bendway weirs, spur dikes, and a new intake structure. The recommended alternative was the bendway weirs because they “would be the least costly alternative and would provide a slightly better factor of safety.” Refer to Appendix E for a copy of the ACOE study.

A subsequent feasibility study, performed in March 2002, evaluated ten different alternatives (including a No Action alternative) more thoroughly and included all three alternatives from the ACOE study. This study recommended a new intake structure located downstream of the south bridge pier. During a conference call on January 11, 2002, representatives from ACOE-Omaha, ACOE-Helena, ACOE-Billings, United States Fish & Wildlife Service (USFWS), Montana Fish, Wildlife & Parks (FWP), Montana Department of Natural Resources and Conservation (DNRC), Montana Department of Environmental Quality (DEQ), Yellowstone County Disaster & Emergency Services (DES), and the City of Laurel unanimously opposed weirs and dikes, and unanimously agreed a new intake structure was the best solution.

A new intake structure was constructed in 2003 at a cost of $3 million to provide reliable, year-round availability to draw water from the Yellowstone River. It was constructed downstream of the south center-span pier and was located in the active channel.

Current Conditions Significant changes to the river occurred during the flooding in spring and summer of 2011. A large volume of sediment was deposited upstream and beneath the highway and railroad bridges. This reduced the hydraulic capacity of the bridges and created a backwater effect which caused increased velocities and scour through the southern halves of the two bridges. The increased velocities combined with the expanding flow patterns as water flowed out from the bridges caused severe erosion to the south river bank immediately downstream of the highway bridge. A large eddy formed in this area, significantly increasing the potential for future erosion. The bank was eroded for approximately 1,400 feet downstream of the bridge and up to 45 feet back from the existing (pre-flood) location. The erosion caused the existing concrete boat ramp to break up and collapse into the river. Numerous cottonwood trees were lost during the bank erosion which left an unstabilized, gravelly and cobbly river bank which is highly susceptible to further erosion. The historic levee in Riverside Park was also damaged during the flood. The erosion of the river bank caused the loss of flood protection to Riverside Park and areas south and east of the park. Future floodwaters that inundate the park and surrounding areas will be trapped behind the remaining portion of


March 2012 11

Severe bank erosion and unstabilized bank.

the levee and unable to return to the river. A historic log cabin which sat near the former top of river bank was nearly lost to erosion. City personnel used construction equipment to pull the cabin back from the eroding bank during the flooding. The cabin now sits approximately 5 feet back from the current top of bank – precariously close to the unstabilized slope. Any further erosion or a collapse of the near-vertical bank could cause the cabin to fall into the Yellowstone River.

The combination of sediment deposition along the north side of the river and erosion of the south river bank caused the active channel to migrate south away from the intake structure. It also resulted in increased deposition to a gravel bar on the north side of channel downstream of the bridges and intake structure. The migrating channel left the new water treatment plant intake structure on the edge of the active low water flow. The significant bed

scour that occurred under the south spans of each bridge and immediately downstream also lowered the water surface elevation at the intake structure during low flow. In January 2012, City of Laurel personnel witnessed the lowest water surface elevations observed since the new intake was installed in 2003. The top of the intake was exposed to air and ice began to build up on the intake structure. Additionally, slush ice flows in the river, which have never affected the intake because they occur at the water surface, began clogging the intake screens. These occurrences culminated in the City having to temporarily shut off the raw water supply to the CHS Oil Refinery on January 30, 2012 in order to meet domestic demands. Since then, the water surface has been right at the top of the intake structure, often

New water treatment plant intake structure with encroaching gravel bar, looking downstream from the highway bridge.

The highlighted area is the current main channel flow that was directly over the intake structure prior to the 2011 flooding. Note the surfacing of the intake structure bolts identified by the arrow.


March 2012 12

showing riffles from the bolt heads on the intake structure. Refer to Appendix A for photos of the site which document the flooding damage and intake operation problems.

During the flood, the Exxon-Mobil Silvertip crude oil pipeline ruptured and spilled crude oil into the Yellowstone River. Once floodwaters receded following the early July 2011 peak flow, Exxon-Mobil began planning and construction of a new crude oil pipline to replace the ruptured line. It was directional drilled 40’-70’ beneath the Yellowstone River to better protect it from bed scour and erosion. However, it comes up to standard bury depths in Riverside Park and could be susceptible to erosion damage resulting from the absence of the levee that protected the park prior to the 2011 flooding. The other existing utilities that cross the river in the vicinity of Riverside Park remain in place at shallower bury depths and are exposed in places. These other utilities are also planned for replacement prior to spring runoff in 2012.

The potential impacts of the current conditions are severe in nature if measures are not taken to mitigate the recent flood damage. First and foremost, the loss of reliable water flows to the intake structure represents an immediate threat to public health and safety. The intake structure provides water to the City of Laurel for domestic uses and fire suppression. Additionally, it supplies water the CHS Oil Refinery for industrial processes and fire suppression. A loss of water availability for even one day has the potential for catastrophic consequences. The loss of flood protection at Riverside Park has the potential to damage or destroy homes south and east of the park and the historic buildings within the park. The erosion of the river bank and the eddy currents that formed immediately downstream of the highway bridge have the potential to compromise the south bridge abutment if left unchecked. Turbidity in the river will increase as the unprotected river bank along the park is quickly eroded. The geomorphological river processes have the potential to deposit more sediment on the north side of the river, further reducing the hydraulic capacity of the bridges. If the river continues to migrate, the Billings Bench Water Association’s irrigation diversion structure may experience operational problems, and downstream properties may be impacted. The current conditions must not be allowed to continue because the potential risk to public health and safety is too great.

Hydraulic Analysis Background The Yellowstone River is a mapped floodway and floodplain through the Federal Emergency Management Administration’s (FEMA) Flood Insurance Rate Map (FIRM) program. The Yellowstone River, in Yellowstone County, had a detailed-level Flood Insurance Study (FIS) completed in November 1981 and revised in March 2000 to determine base flood elevations, water surface profiles, and floodway and floodplain limits. This study is currently the effective study; however, a draft FIS was completed in April 2007 using more accurate topographical data and updated hydrology. Since the data was gathered more recently, the 2007 FIS more closely matches the hydraulics of the river in its current condition than the effective study. Therefore, the hydraulic modeling results prepared for this report will be compared to the 2007 FIS results.

The 2007 FIS was completed by the Army Corps of Engineers using the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) software to model the hydraulic conditions. Water surface profiles were computed for the 10-, 50-, 100-, and 500-year recurrence interval flood events. Floodplain boundaries were determined for the 100- and 500-year recurrence interval flood events, and a floodway fringe was delineated to account for a future 0.5-foot rise in base flood elevation.

A topographical survey was performed in September 2011 by Great West Engineering after floodwaters had receded, and it was used to map current, post-flood conditions and for preliminary design. The vertical datum used for the topographical survey, as well as the effective FIS base flood elevations (BFE), was The National Geodetic Vertical Datum of 1929 (NGVD 29). The topographical survey was tied to elevation benchmarks “RM 34” and “RM 35” on FIRM Panel 300142 1135B. The vertical datum used for the 2007 FIS and all HEC-RAS models was The North American Vertical


March 2012 13

Datum of 1988 (NAVD 88). A conversion factor of -2.60 feet was used to convert NAVD 88 elevations to NGVD 29.

Hydrology Hydrology for the river was determined by analyzing U.S. Geological Survey stream gage data at several different locations. The Corps of Engineers estimated the 100-year recurrence interval flood to be 56,700 cubic feet per second (cfs) for the river at the project site and is considered to be the base flood. A bankfull flow corresponding to the 1.8-year recurrence interval flood event was estimated by Great West Engineering using RiverMorph software and checked using stream gage data. The calculated bankfull flow of 28,000 cfs was based on a reference cross section located approximately 500 feet downstream of the Highway 212/310 bridge. Using gage data to check the RiverMorph results, a bankfull flow of 29,000 cfs was estimated.

Hydraulics The hydraulic analysis for this project was completed using HEC-RAS (version 4.1.0) and RiverMorph programs. HEC-RAS was used to calculate water surface elevations for existing and proposed conditions to compare with the 2007 FIS water surface elevations. RiverMorph was used to calculate shear stress values for varying flow stages to be used for evaluating bank stabilization alternatives. The HEC-RAS model used for the 2007 FIS was obtained from the Montana Department of Natural Resources and Conservation and was used as the starting point for the hydraulic analysis of this project. Two scenarios were modeled in addition to the original model from the 2007 FIS. For the purposes of this report, four base flood (100-year recurrence) water surface profiles were considered and are as follows:

Effective Conditions (1981/rev. 2000) – The base flood profile from the effective FIS which was digitized from a map overlay (not modeled in HEC-RAS).

Existing Conditions (2007) – The base flood profile from the draft FIS which was modeled using the original Corps of Engineer’s HEC-RAS model. This model represents pre-flood conditions.

Current Conditions (Sept. 2011) – Topographical survey data of the channel and the floodplain in the vicinity of Riverside Park was obtained in September 2011 and added to the Draft Effective HEC-RAS model. This model represents current, post-flood conditions. It also represents proposed conditions if the current bank alignment is utilized.

Proposed Conditions (2012) – Preliminary design data was added to affected cross-sections to reflect river bank construction with a 1.5:1 slope and removal of sediment deposits beneath the Highway 212/310 and railroad bridges. Sediment will be removed to within 24 inches of the water level during construction. This model represents proposed conditions for bank stabilization and sediment removal measures.

The hydraulic analysis results are summarized and compared in Table 1 and Figure 5 (see Appendix C for full hydraulic analysis outputs). The current conditions water surface elevations clearly demonstrate the backwater effect created from aggregation upstream of and beneath the bridges when compared to the existing (pre-flood) conditions. Base flood velocities through the bridges increased from 10.4 ft/s (pre-flood) to 11.4 ft/s (post-flood), when averaged using the reported internal bridge cross-section velocities. A comparison of existing (pre-flood) and current (post-flood) cross sections immediately downstream of the bridges clearly shows the bed scour that occurred under the south spans of the two bridges. As the Yellowstone River flows through the constricted bridge openings, it accelerates and the water surface drops. As it flows out from the bridges the flow expands laterally causing increased erosion potential because the flow is directed more towards the


March 2012 14

bank than in constricted flow locations. The extensive erosion at Riverside Park was a result of the increased velocities and expanding flows.

The proposed conditions were modeled assuming reconstruction of the river bank was completed. Although proposed alternatives were evaluated further in this report, assumptions were made to facilitate preliminary design hydraulic calculations. A 1.5:1 slope constructed between the existing (pre-flood) and the current (post-flood) river bank locations was used for hydraulic analysis of proposed conditions. The sediment removal was assumed to extend to 24 inches above the water surface elevation at the time of topographical survey.

Removing sediment beneath the Highway 212/310 and railroad bridges will greatly improve their hydraulic capacity. The calculated base flood velocities through the bridges dropped to 9.9 ft/s in the proposed conditions model. Figure 5 clearly shows a reduction of the backwater effect and the elimination of a hydraulic jump immediately downstream of the bridges. The improvements restore the bridge capacity and river hydraulics to more closely match the existing (pre-flood) hydraulics documented in the 2007 FIS, as shown in Table 1.

The erosion of the south river bank in the vicinity of Riverside Park destroyed a historic levee and opened floodplain access to the river in an area that has had flood protection for at least 100 years. Hydraulic modeling estimates a 25-year flood will overtop the eroded river bank. The proposed improvements will reconstruct the damaged river bank to the historic levee elevation to match existing (pre-flood) conditions.

Geomorphology The normal bankfull width of the Yellowstone River in the vicinity of the project ranges from 350 to 600 feet wide. The bankfull width immediately upstream of the project at the highway and railroad bridges is approximately 445 feet. However, significant sediment deposition at the bridges has reduced the effective bankfull width to approximately 365 feet. The bankfull width downstream of the project at the Billings Bench Water Association diversion structure is approximately 415 feet. Although these widths fall within the range of normal bankfull width, they appear constricted based on bankfull widths upstream of the bridges and downstream of the diversion structure. Additionally, the upstream bridges and the existing levees on both north and south river banks restrict access to the floodplain. The bankfull width at the widest point between these two constrictions is 700 feet which is outside the range of normal bankfull widths and almost twice the width of the upstream and downstream constrictions. This section is clearly overwidened and is the main reason for the excessive sediment deposition in this area.

River geomorphology typically exhibits deposition on the inside of meanders and erosion on the outside of meanders. The Yellowstone River is exhibiting the opposite behavior at the project site which consists of sediment deposition on the outside and erosion on the inside. This is due to the relative overwidening of the river immediately downstream of the constriction at the bridges and the corresponding drop in velocity. Figures 3 and 4, found previously in this report, show the gravel bar on the north side of the river gradually changing in size, both growing and shrinking, in the last 60 years. The distance between the gravel bar and the water treatment plant intake structure was approximately 250 feet and remained fairly constant throughout this time. The sediment deposition that occurred during the recent flooding significantly enlarged the gravel bar and reduced the separation distance from the intake structure to 95 feet.


March 2012 15

Figure 5 – Water Surface Profile Exhibit


March 2012 16

Table 1 - Hydraulic Analysis Calculated Base Flood ElevationsEffective Conditions (1981/2000 Effective

Flood Study)W.S. Elevation (ft) W.S.

Elevation (ft)

Change in Elevation vs. 1981/2000 Effective Conditions

(ft)

W.S. Elevation

(ft)

Change in Elevation vs. Existing Conditions

(ft)

W.S. Elevation

(ft)

Change in Elevation vs. Current Conditions

(ft)

Change in Elevation vs. Existing Conditions

(ft)

142095.8 144581.34 3278.43 3275.74 ‐2.69 3275.81 0.07 3275.74 ‐0.07 0.00

141875.1 143857.33 3276.45 3274.35 ‐2.10 3274.47 0.12 3274.34 ‐0.13 ‐0.01

141622.7 143029.03 3273.68 3273.06 ‐0.62 3273.34 0.28 3273.05 ‐0.29 ‐0.01

141479.6 142559.71 3273.68 3272.37 ‐1.31 3272.77 0.40 3272.34 ‐0.43 ‐0.03

141389.2 142263.26 3272.49 3272.04 ‐0.45 3272.51 0.47 3272.01 ‐0.50 ‐0.03

141289.2 141935.15 3271.90 3271.94 0.04 3272.4 0.46 3271.9 ‐0.50 ‐0.04

141204.9 141658.58 3270.95 3271.66 0.71 3272.16 0.50 3271.63 ‐0.53 ‐0.03

141096 141301.14 3270.51 3270.73 0.22 3270.61 ‐0.12 3270.64 0.03 ‐0.09

141053 Bridge

141034.4 141099.01 3270.26 3269.48 ‐0.78 3269.41 ‐0.07 3269.89 0.48 0.41

141006 141006 3270.15 3269.52 ‐0.63 3269.46 ‐0.06 3269.75 0.29 0.23

140846.5 140482.68 3269.27 3268.79 ‐0.48 3268.6 ‐0.19 3268.55 ‐0.05 ‐0.24

140705.5 140020.15 3268.50 3267.52 ‐0.98 3267.32 ‐0.20 3267.28 ‐0.04 ‐0.24

140538.8 139473.14 3267.62 3266.35 ‐1.27 3266.35 0.00 3266.35 0.00 0.00

140367.1 138909.89 3266.74 3265.38 ‐1.36 3265.38 0.00 3265.38 0.00 0.00

140231.3 138464.46 3266.07 3264.38 ‐1.69 3264.38 0.00 3264.38 0.00 0.00

140108.3 138060.79 3265.35 3263.59 ‐1.76 3263.59 0.00 3263.59 0.00 0.00

139921.2 137447.08 3264.15 3261.01 ‐3.14 3261.01 0.00 3261.01 0.00 0.00

Note: All water surface elevations have been converted to NGVD 29 vertical datum.

Proposed Conditions (2012) Current Conditions (September 2011)River Station

(2007 Flood Study)

River Alignment Station

(This Project)

Existing Conditions (2007 Draft Flood Study)


March 2012 17

The geomorphological problems at the site have two primary components. The first component is that the constriction at the upstream bridges is causing a backwater effect and an increase in velocity through the bridges which, in turn, is causing scour in the main channel mostly south of the intake structure and erosion of the south river bank. The erosion allowed for the formation of an eddy against the south river bank which will continue to erode if left unmitigated. It also allowed the main channel flow to shift south of the intake structure. The sediment removal portion of the project will restore the hydraulic capacity of the bridges and allow for reduced velocities and reduced scour. The proposed river bank reconstruction will force the main channel flow north towards the intake structure and prevent the eddy from reoccurring, if the current bank alignment is not utilized. This is demonstrated by velocity distribution analysis using HEC-RAS. The intake structure is located between sections 141034.4 and 141006.0. Section 141034.4 is the downstream bounding cross-section of the bridge. The average channel velocities during the 100-year flood at the internal cross-section of the bridge and the two downstream cross-sections are as follows:

Section # Velocities (fps)

Existing Conditions, 2007

Current Conditions, 2011

Proposed Conditions, 2012

141053 Bridge DS 10.91 10.54 9.64

141034.4 10.56 10.18 9.32 141006.0 9.27 8.81 9.20

The average velocities in the table show that the proposed conditions reduce velocities in sections 141053 Bridge DS and 141034.4. Figure 5 illustrates the hydraulic jump immediately downstream of the highway bridge is eliminated in proposed conditions. These improvement are beneficial because they will reduce scour within and downstream of the bridges. At the cross-section downstream of the intake, section 141006.0, the low velocity of the current conditions is remedied by an increased velocity during proposed conditions. This will prevent additional deposition at the head of the gravel bar immediately downstream of the intake structure (the approximate location of section 141006.0).

More important than the average velocity of each section is the velocity distribution within each cross-section, which is more desirable in proposed conditions than in current conditions. The proposed conditions velocity distribution in section 141034.4, upstream of the intake, exhibits similar patterns as current conditions, but at generally lower velocities. Once again, this reduces scour potential upstream of the intake where the channel experienced significant scour during high flows.

The second component to the geomorphological problems is the overwidening of the river downstream of the bridges. This allows flow to spread out and slow down which causes deposition at the gravel bar island downstream of the intake. At section 141006.0, downstream of the intake, the velocity distribution shows an increase in velocity in line with the intake and especially to the left. This reduces the potential for deposition immediately downstream of the intake during both high and low flows. Refer to Appendix C for the relevant cross-sections with velocity distribution shading overlayed.

Shear Stress Shear stress is an important consideration when evaluating slope stability. RiverMorph software was used to calculate average shear stress at different stages of flow. Assuming the bankfull flow at Laurel is approximately 28,000 to 30,000 cfs, the average shear stress generated is between 2.6 and 2.9 pounds per square foot (psf). According to the Shields Entrainment Function, that


March 2012 18

magnitude of shear stress can mobilize sediment clasts up to 650 mm (25.5 inches) at typical bankfull discharge. Rosgen’s shear stress data (which were collected in-situ from gravel-based streams in the western US) suggest a slightly smaller mean diameter for particles that can be mobilized at that shear level.

The highest shear stress developed under usual circumstance occurs at a water-surface elevation of about 3,268 feet (NAVD 88) on the reference cross section. At that stage the shear stress exceeds 3 psf at an estimated discharge of 38,750 cfs. Beyond that, the model assumes that a certain proportion of floodplain becomes available to flow, which reduces the overall stream power by allowing lateral movement of the water, and the average shear stress falls slightly.

The model, however, predicts only average shear across the channel. In the case of Riverside Park, the railroad and Highway 212/310 bridges and their approaches block the floodplain. Any over-bank flow is directed back into the main channel in such a manner that the bridges essentially act as a constriction to flow. Water will mound upstream of the bridge, resulting in increased velocity and, consequently, increased shear. Locally, shear stresses can be considerably higher than the average shear value predicted by models. Depending upon the geometry of the channel, shear stress at the eroded river bank could easily be twice the average shear, or 6 psf, and could mobilize material 36 inches or greater in diameter.

General Design Requirements The project is intended to protect the operation of new water treatment plant intake, reconstruct the eroded river bank adjacent to Riverside Park, and restore hydraulic capacity to the Highway 212/310 and railroad bridges through sediment removal. The proposed improvements must promote river training in order to maintain reliable operation of the new water treatment plant intake structure. The improvements should also promote channel stability and hydraulic efficiency, resist erosive forces, and be cost effective. The bank stabilization improvements must be able to resist the estimated shear stress and retain the bank fill material. Alternatives using vegetation must account for the time to establish the vegetation. The upstream end of the bank stabilization improvements must tie into the riprap placed around the highway bridge abutment to provide a smooth and erosion-resistant transition. The downstream end of the project must terminate in manner that prevents flanking and undermining such as a cutoff wall or terminal rock refusal.

Recently, the Montana Fish, Wildlife and Parks committed money to reconstruct the demolished boat ramp as part of the project. The boat ramp improvements should include a cut or break in the bank stabilization treatment which would allow for a one lane boat ramp at 8%-10% longitudinal slope. Most importantly, the bank stabilization treatment must terminate on each side of the boat ramp in a way that prevents flanking during high water.

The sediment removal beneath to the bridges is considered necessary to the project due to the hydraulic conditions it creates downstream of the bridge. The amount of sediment removed will depend on the water surface elevation during construction. Sediment will be removed to an elevation 24 inches above water level.

The proposed improvements must also restore flood protection to the park and residential areas to the south and east. The river bank will be constructed to match the historic levee elevation and will be graded gently away from the river to blend with the existing park topography. The proposed improvements must also tie into undamaged portion of the historic levee.


March 2012 19

ALTERNATIVE SCREENING PROCESS The alternative screening process was used to briefly examine the viability and suitability of numerous bank stabilization techniques and alignments. In order to simplify the alternative screening process, two aspects of the project were considered. Bank stabilization alternatives included the measures, systems, or treatments applied to the reconstructed river bank to minimize or prevent future erosion. Bank alignment alternatives were the different alignments or locations the river bank could be reconstructed. Although there are many different bank stabilization techniques and an infinite number of possible alignments, site characteristics, and past experience narrowed the screening process to include the following alternatives.

Bank Stabilization Alternatives

Revetment Riprap Armoring – Full Slope This alternative utilizes rock riprap placed from the river bed to approximately the base flood elevation (100-year recurrence) at a 1.5:1 (H:V) slope. A toe keyway 1 to 2 times the riprap thickness is constructed along the toe of the slope to prevent undermining and slope failure. Rock refusals are used at the beginning and end of the riprap to prevent flanking, i.e. water eroding behind the riprap. Conventionally, geotextile is placed under riprap to prevent substrate soils from mixing with the riprap. However, empirical evidence suggests the geotextile contributes to slope failure by increasing the severity of the slip plane under the riprap. Instead, wetland plants such as willows are planted in the riprap and once they establish, they help key the riprap.

This alternative provides effective erosion resistance as long as flows do not flank the riprap. However, a full height riprap bank increases the likelihood floodwaters will find a way behind the riprap causing it to fail. Additionally, the volume of rock required to riprap the entire river bank is quite costly. Many consider a fully riprapped slope to be aesthetically displeasing. Full height riprap slopes do not provide a natural river bank and can lead to increased velocities offshore. This can promote downcutting erosion into the river bed. Downcutting is a form of scour that cuts down through the river bed at the toe of the riprap slope. Without protection, in the form of an armored toe keyway, downcutting can result in the failure of the riprap slope when the bottommost rocks fall into the scour hole created by downcutting. Riprap slopes also have the potential to direct river currents at other, unprotected river banks downstream of the riprap which can lead to accelerated erosion. Consequently, a full-height riprap slope can be difficult to meet guidelines set forth in the Clean Water Act.

Bioengineered Slope A bioengineered slope involves wetland and upland vegetation planted with turf reinforcement mat to stabilize the slope. Additionally, logs vanes and root wads are placed near the toe to protect it from undercutting through direct protection and deflection of the current. Log vanes placed angling upstream help redirect river currents toward the center of the channel and away from the river bank. Rootwads help slow shoreline velocities and promote sediment deposition along the shore. They also provide aquatic species habitat.

This alternative is lower cost than many of the hard armor treatments; however, on a river as large as the Yellowstone River, the erosion protection provided by a bioengineered slope is significantly reduced compared with hard armor. Rootwads have been found to be notoriously difficult to anchor in large water bodies because of their natural buoyancy. The Yellowstone River would most likely sweep any rootwads placed at the toe of the river bank downstream during the first year’s annual peak flow. Likewise, log vanes are also difficult to anchor. The location of the project on the inside bend of the meander also renders log vanes mostly ineffective to deflect the river current. They are most effective around an outside bend. Vegetated slopes are generally only practical at or above the


March 2012 20

bankfull elevation (approximately Q1.8) because it is rare for permanent vegetation to establish below this elevation in the active channel.

Revetment Riprap Armoring – Bankfull and Below This alternative utilizes a combination of revetment riprap placed from the bankfull elevation down to the river bed. A riprap toe keyway, rock refusals, and willow plantings within the riprap are also used in this alternative. Above bankfull, wetland and upland vegetation planted with turf reinforcement are used to stabilize the upper slope and overbank area.

This alternative provides many of the benefits of both previous alternatives without some of the drawbacks. Combining bioengineering with hard armoring provides a high level of erosion protection while maintaining a somewhat natural river bank configuration. The cost is lower than a fully riprapped slope due to a smaller volume of rock. The upper portion of the slope remains more useable for recreation purposes which is important given the project is adjacent to Riverside Park. Additionally, this alternative meets the guidelines in the Clean Water Act. As with the other riprap alternative, this alternative has the potential to direct river currents to other, unprotected river banks which can lead to accelerated erosion downstream. Given this project is located on an inside bend, this possibility is greatly reduced. The alternative can also promote downcutting resulting from increased velocities offshore of the riprap slope which may eventually cause a slope failure.

Sheet Pile Retaining Wall A sheet pile retaining wall uses heavy-gauge, corrugated steel sheet piles to create a vertical wall or bulkhead to retain bank material and prevent erosion. Sheet piles are driven in a similar manner as conventional round or H-piles. They interlock by matching up the corrugations on adjacent sheets. Sheet pile retaining walls can be constructed up to bankfull elevation and provide a sloped overbank area, or they can be constructed up to base flood elevation when space is limited. Typical installations require an embedment of 2 to 3 times the exposed height.

This alternative provides complete erosion resistance to material behind the wall as long as flood waters do not overtop the wall. Downcutting erosion generally occurs at the exposed toe of the retaining wall due to increasing velocities along the wall and no downcutting protection. Consequently, potential downcutting depths must be taken in to account when determining embedment depth and will generally increase the required embedment. Sheet pile is also very expensive and is generally only cost effective when there isn’t space available to lay back the river bank to more natural slopes. Sheet pile is often considered aesthetically displeasing, and a retaining wall acts as an impediment or a hazard to both recreational users on land and in the water. Generally, permitting agencies will not permit a sheet pile retaining wall unless extraordinary circumstances exist. For this project, there is a very real possibility that the gravelly, cobbley river alluvium will make it impossible to drive sheet pile to the required embedment depths.

Articulated Concrete Blocks or Mats This alternative utilizes manufactured concrete blocks which interlock or are cabled together to provide erosion protection. These units typically provide more erosion protection than riprap because their interlocking ability or cabling significantly resists displacement. The void areas can be filled with soil and stone, and they allow for vegetation planting to provide extra stability and visual concealment. Blocks and mats can be installed over the entire slope or used in conjunction with a vegetative stabilized upper slope treatment. Cabled articulated mats require significant anchoring to prevent the river from peeling the mat off the river bank; especially when used in rivers as large as the Yellowstone River.

Manufactured products such as blocks or mats are generally more costly than riprap; although, they also provide an increased level of erosion protection. They also allow vegetation to grow out through their voids which can help anchor soil placed in the voids and the subgrade they are installed on. It is possible for soil to be eroded from the voids if shear stresses are high or vegetation doesn’t fully establish. Articulated concrete blocks or mats often give a “manmade” look to a river channel which


March 2012 21

can be undesirable on natural rivers and streams. Articulated concrete blocks often create significant increases in surface roughness which creates a less hydraulically efficient channel. This can cause increases in water surface elevations. Permitting can also be difficult for installing manufactured products in the Yellowstone River.

No Action Alternative This alternative proposes to take no action to repair the eroded bank and prevent the Yellowstone River channel from migrating to the south. It would allow erosion to continue along Riverside Park and may threaten the continued operation of the water treatment plant intake and irrigation diversion structures, or the highway bridge abutment if the river migrates further south.

This alternative is the least expensive alternative in the short term; however, if the river continues to migrate to the south it will eventually be the most expensive alternative if the intake or diversion structures had to be reconstructed again. Additionally, the loss of water for domestic, industrial, irrigation, and fire suppression uses would be disastrous for area residents, businesses, and farmers. It may result in the CHS Oil Refinery raw water line having to be taken offline for an extended period of time which would cause an emergency shutdown of the refinery. It is unlikely the refinery would resume production if it is shut down. This alternative also allows the Yellowstone River to continue eroding parkland, and it potentially threatens the historic structures in Riverside Park and the buried crude oil and natural gas pipelines. The No Action alternative is not practicable due to the potential threat to public health and safety it perpetuates.

Summary The previously discussed alternatives represent a wide spectrum of impact to the Yellowstone River ranging from no impact to severe impact. All of these alternatives have been used in other projects around the United States with varying degrees of success. Revetment riprap armoring, articulated concrete blocks, and sheet pile retaining walls are all effective measures to protect against erosion on a river as large as the Yellowstone River. Riprap and concrete blocks combined with vegetated upslope treatments simulate natural river bank conditions most closely.

However, each alternative has drawbacks associated with it. A bioengineered slope does not provide the required level of erosion protection. A sheet pile retaining wall may not be technically feasible given the cobbley nature of the site soils. It may not be possible to drive the sheet piles to required embedment depth. Sheet pile retaining walls and articulated concrete blocks are very expensive due the fact they are manufactured products. To a lesser degree, a full riprap slope is also expensive compared to a partially riprapped slope due to the larger volume of rock. Nearly all alternatives will be difficult to obtain permit approval for, but full riprapped slopes, sheet pile retaining walls, and full articulated concrete block slopes will be the most difficult. The No Action alternative is not a practicable alternative because of the urgent threat to public health and safety.

After evaluating the benefits and drawbacks, the alternatives that were considered further include: Revetment Riprap Armoring – Full Slope, Revetment Riprap Armoring – Bankfull and Below, and Articulated Concrete Blocks.

Bank Alignment Alternatives

Existing Bank Alignment Alternative This alternative reconstructs the river bank so the proposed bankfull alignment matches the historic, existing (pre-flood) bankfull alignment. Aerial images pre-dating the flood were used to determine the alignment of the existing river bank. The existing bank alignment would restore the bank to the historic location which it has occupied for at least 100 years prior to the recent flooding. It would require the most fill material and would restore the most park land. This alternative would also cause the largest impact to the river. It would be most effective in stopping river migration to the south and diverting flows towards the intake structure. Consequently, it would provide the greatest protection to the continued operation of the intake structure. This alternative would provide the


March 2012 22

most consistent bankfull width for the river which has geomorphological benefits. However, it would also require the most digging within the channel, the most fill in the floodway, and ultimately the greatest impact to the current conditions of the river.

Intermediate Bank Alignment Alternative This alternative reconstructs the river bank between the existing (pre-flood) and current (post-flood) locations. The alignment restores the river bank’s gradual, curving alignment similar existing conditions, whereas currently, the river bank has an irregular alignment resulting from differing amounts of erosion and river bank loss. Hydraulic modeling indicates this alternative would return the river hydraulics to closely match existing conditions. This alternative would increase velocities at the intake structure and downstream preventing deposition which could render the intake inoperable. It prevents the ability of the water to eddy immediately downstream of the bridge which provides additional erosion protection to the highway bridge abutment. This alternative balances the impacts to the Yellowstone River with the urgent need to improve the river hydraulics at the intake structure. It also requires a moderate amount of fill which will come from the sediment removal portion of the project.

Current Bank Alignment Alternative This alternative utilizes the current, eroded bankfull elevation alignment and regrades the bank as necessary for the selected stabilization treatment. This alternative requires the least amount of earthwork, but it will be the most difficult to construct because of the irregularities of the alignment. In addition, the irregularities reduce the hydraulic efficiency of the channel. This alternative also results in a loss of park land beyond what was already lost because the slope would be laid back. Consequently, sediment removed from beneath the bridges will have to be disposed of offsite instead of being used for fill material onsite which increases the project costs. This alternative provides no improvement to the hydraulic conditions which have caused recent disruptions of service for the water treatment plant intake structure. It allows velocities to remain high at the upstream bridges which experienced significant bed scour during recent flooding. It also allows the main channel flow to remain south of the intake structure and overwidened resulting in sediment deposition in the immediate vicinity of the intake structure. It is likely the sediment island downstream of the intake would continue to grow and eventually completely bury in the intake structure, which is what happened to the former intake structure. This alternative minimizes fill placement in the current river channel. Due to the fact no hydraulic improvements are realized with this alternative and it does not improve the operating conditions of the water treatment plant intake structure, this alternative is not practicable.

Summary The bank alignment alternatives vary from most to least effective at improving the hydraulic conditions to keep the water treatment plant intake structure operational. Impacts the environment are evaluated further in the Affected Environment section. All of the bank alignment alternatives are evaluated further in the alternative analysis.


March 2012 23

ALTERNATIVE ANALYSIS This section examines each viable alternative identified in the prescreening summary more closely. Schematic plans and cost estimates were developed to assist in comparing the alternatives. Selection criteria of cost effectiveness, technical feasibility, environmental impacts, ease of permitting, and construction time were used to evaluate and select a preferred alternative. Since the bank stabilization and bank alignment alternatives may be combined interchangeably, each combination of bank stabilization treatment and alignment will be compared for cost purposes.

Bank Stabilization Alternatives Description

Alternative 1 - Revetment Riprap Armoring – Full Slope This alternative utilizes approximately 9,800 cubic yards of rock riprap placed from the river bed to approximately the base flood elevation (100-year recurrence) at a 1.5:1 (H:V) slope (see Figure 6). Based on shear stress calculations, the minimum rock must be 36 inches in diameter. Therefore, a nominal diameter of 48 inches is used with rock diameters ranging from 36 to 72 inches. As a rule of thumb, riprap slope thicknesses should be approximately 2 times the median diameter (D50, approx. nominal diameter) or 1 times the maximum diameter (D100), whichever is greater. For this project, a thickness of 8 feet is used for the riprap revetment.

The most common failure of riprap slopes results from toe scour which allows the riprap slope to slide on the natural slip plane created by the subgrade on which the riprap sits. The roughness of the riprap slope causes a decrease in velocity in flow against the riprap but an increase in velocity in flows out near the toe of the river bank. This causes downcutting which undermines the foundation rock of the riprap slope and allows rock above it shift as well. A toe keyway, 1 to 2 times the riprap thickness along the toe of the slope, is used to prevent undermining and slope failure. For this project, a keyway 8 feet thick and 8 feet wide is used to support the riprap slope. The minimum width is used due to the extreme thickness of the riprap section.

Another common failure mechanism is called flanking, where flood waters erode and get behind the riprap slope at the upstream or downstream ends. This particularly affects riprap slopes that confine the floodplain to the limits of the floodway. Energy and shear stress build as the flood stage rises to the base flood elevation in the floodway. Rock refusals are thickened edges of riprap used to prevent flanking and are used in this alternative.

Conventionally, geotextile is placed under riprap to prevent substrate soils from mixing with the riprap. However, empirical evidence suggests the geotextile contributes to slope failure by increasing the severity of the slip plane under the riprap. Instead, wetland plants such as willows will be planted in the riprap, and once they establish, they help key the riprap. Bundles of willow stakes are placed in the riprap at a 45 degree angle and at an elevation such that they will establish in the capillary fringe just above the base flow water surface elevation. Some topsoil or loamy fill material may be needed to be placed in the riprap to help the wetland plants establish roots due to the thickness of the riprap.

Maintenance of this alternative includes periodic inspection of the slope and adjustment or replacement of displaced or lost rock. The projected service life for this alternative is 100 years if maintained properly.


March 2012 24

Figure 6 – Alternative 1-Revetment Riprap Armoring (Full Slope)


March 2012 25

Alternative 2 – Revetment Riprap Armoring – Bankfull and Below This alternative utilizes approximately 7,300 cubic yards of rock riprap placed from the river bed to approximately the bankfull elevation (1.8-year recurrence) at a 1.5:1 (H:V) slope (see Figure 7). This is the minimum amount of armoring necessary to protect the bank with riprap. The sizing criteria for the rock are the same as Alternative 1; a nominal diameter of 48 inches and a thickness of 8 feet. A toe keyway, rock refusals, and willows planted in the riprap would likewise be used in this alternative.

The bankfull elevation is the river stage above which permanent vegetation such as grass and brush start growing. Below this elevation is considered the active channel and usually has minimal vegetation and mostly exposed gravels and cobbles. The bank stabilization treatment above the bankfull elevation involves placing turf reinforcement mat and planting vegetation. Turf reinforcement mat consists of a straw and coconut fiber matrix sandwiched between synthetic netting, and is generally anchored in place using landscaping staples. The turf reinforcement mat is permanent and provides primary stabilization until vegetation establishes and then continues to assist in stabilizing the river bank.

On small streams, wetland species are usually chosen to stabilize the stream banks. However, the Yellowstone River has such a large difference between the bankfull and the low flow water surface elevations that it is unlikely wetland species would survive above the bankfull elevation. During times of low flow, the capillary fringe would be too low to support the wetland species. Consequently, upland species such as dogwood, choke cherry, and range grasses would be planted.

Maintenance of this alternative includes periodic inspection of the slope, adjustment or replacement of displaced or lost rock, and repair or replacement of displaced or torn turf reinforcement mats. Initially, stabilization plantings would need to be cared for to ensure they establish within the first growing season after planting. The projected service life for this alternative is 100 years if maintained properly.

Alternative 3 – Articulated Concrete Blocks This alternative utilizes approximately 2,560 square yards of manufactured concrete blocks placed from the river bed to approximately the bankfull elevation (1.8-year recurrence) at a 1.5:1 (H:V) slope (see Figure 8). The blocks are designed to interlock with each other to provide additional erosion protection beyond that provided solely by their weight. Many block designs also incorporate void space to facilitate placing topsoil and planting wetland species within them. Willows would be planted in the blocks as in Alternatives 1 and 2. Proprietary names such as A-Jacks, Toskanes, and Armorloc are examples of some concrete block products.

Above bankfull elevation, turf reinforcement and upland stabilization plantings would be used as in Alternative 2.

Maintenance of this alternative includes periodic inspection of the slope, adjustment or replacement of displaced or lost blocks, and repair or replacement of displaced or torn turf reinforcement mats. Initially, stabilization plantings would need to be cared for to ensure they establish within the first growing season after planting. The projected service life for this alternative is 100 years if maintained properly.


March 2012 26

Figure 7 – Alternative 2-Revetment Riprap Armoring (Bankfull and Below)


March 2012 27

Figure 8 – Alternative 3-Articulated Concrete Block


March 2012 28

Bank Alignment Alternatives Description See Figure 9 for an illustration of the different bank alignment alternatives.

Alternative A – Existing Bank Alignment This alternative reconstructs the river bank so the proposed bankfull alignment matches the historic, existing (pre-flood) bankfull alignment. Approximately 715 linear feet of river bank is reconstructed with this alternative, and the reconstructed bank extends out into the area of eroded river bank almost 50 feet horizontally. Aerial images pre-dating the flood were used to determine the alignment of the existing river bank which was present in this location for at least 100 years.

Approximately 5,700 cubic yards of fill material is necessary for this alternative and is obtained by removing sediment from beneath the highway and railroad bridges. The fill is placed and compacted in lifts and armored upon completion. Construction equipment is able to place fill from the existing river bank and build the reconstructed bank out into the river while remaining on dry ground throughout construction.

This alternative matches the existing conditions which remained mostly unchanged for more than 100 years. It would be most effective in stopping river migration to the south and diverting flows towards the intake structure. This alternative would provide the most consistent bankfull width for the river which has geomorphological benefits. However, it would also require the most digging within the channel, the most fill in the floodway, and ultimately the greatest impact to the current conditions of the river.

Alternative B – Intermediate Bank Alignment This alternative reconstructs the river bank between the historic (pre-flood) and current (post-flood) locations. The alignment restores the river bank’s gradual, curving alignment similar historic conditions; whereas currently, the river bank has an irregular alignment resulting from differing amounts of erosion and river bank loss. Approximately 715 linear feet of river bank is reconstructed with this alternative, and the reconstructed bank extends 35 feet horizontally out into the existing eroded area.

Approximately 1,400 cubic yards of fill material is necessary for this alternative and is obtained by removing sediment from beneath the highway and railroad bridges. The fill is placed and compacted in lifts and armored upon completion. Construction equipment is able to place fill from the existing river bank and build the reconstructed bank out into the river while remaining on dry ground throughout construction.

Hydraulic modeling indicates this alternative would return the river hydraulics to closely match existing conditions. This alternative would increase velocities at the intake structure and downstream preventing deposition which could render the intake inoperable. It would also reduce velocities within the bridges where significant scour has already taken place. It prevents the ability of the water to eddy immediately downstream of the bridge which provides additional erosion protection to the highway bridge abutment. A more consistent bankfull width compared to the current conditions would be another benefit of Alternative B. This alternative reduces the impact to the Yellowstone River compared to Alternative A by reducing the amount of river bed disturbance and encroachment into the channel.

Alternative C – Current Bank Alignment This alternative utilizes the current, existing bankfull elevation alignment and regrades the bank from its existing near-vertical slope for the selected stabilization treatment. Approximately 730 linear feet of river bank is reconstructed with this alternative. Due to laying back the slope, approximately 5,200 cubic yards of excess material must be disposed of offsite in addition to the sediment removed from beneath the bridges. This removes a large volume of natural river substrate from the river system.


March 2012 29

This alternative provides the least disturbance to the current, damaged conditions Yellowstone River in the vicinity of the project by minimizing the amount of river bed disturbed. However, this alternative provides no improvement to the hydraulic conditions which have caused recent disruptions of service for the water treatment plant intake structure. It allows velocities to remain high at the upstream bridges which experienced significant bed scour during recent flooding. It also allows the main channel flow to remain south of the intake structure resulting in sediment deposition in the immediate vicinity. This alternative would allow the sediment island on the north side of the channel to continue to expand, eventually burying the intake structure in sediment. This alternative minimizes fill placement in the current river channel. Construction difficulty along this alignment would also be most difficult due to the irregularities of the existing river bank.

The potential threat to public health and safety resulting from this bank alignment is severe. Additional impacts are discussed below in the Affected Environment section. Due to the fact no hydraulic improvements are realized with this alternative and it does not improve the operating conditions of the water treatment plant intake structure, this alternative is not practicable.


March 2012 30

Figure 9 – Bank Alignments Alternatives Plan


March 2012 31

Permitting & Regulatory Compliance Construction in or around streams and rivers requires federal, state, and local permits to be obtained prior to construction. The project will be designed and constructed in accordance with state and federal stream permitting guidelines. The project will require permits from the U.S. Army Corps of Engineers (USACOE), the Montana Department of Fish, Wildlife, and Parks (MFWP), the Montana Department of Environmental Quality (MDEQ), the Montana Department of Natural Resources and Conservation (MDNRC), and Yellowstone County.

A 404 Permit from the ACOE is required for placement of fill within waters of the United States. This project is required to comply with the National Environmental Policy Act (NEPA) which requires the project to utilize the least environmentally damaging practicable alternative (LEDPA). Preliminary discussions with the ACOE have indicated this project will not qualify under the nationwide or regional general permit, and therefore, will require an individual permit. Individual permits are reviewed using 404(b)(1) process which includes obtaining public comment. The 404 permit appears to be the most difficult permit to obtain for a project of this type.

The ACOE has indicated that compensatory mitigation will be required to offset the potential adverse environmental impacts created by this project. Compensatory mitigation includes restoration, enhancement, establishment, and/or preservation of aquatic and riparian resources. Riverside Park experienced anthropogenically accelerated stream bank erosion caused by increased velocities and expanding flows resulting from the railroad and Highway 212 bridges which are misaligned with the Yellowstone River and hydraulically undersized. According to Appendix D(h) of the ACOE Montana Stream mitigation Procedure, “Compensatory mitigation might not be required for projects addressing anthropogenically accelerated stream bank erosion beyond the applicant’s control.” The original scope of this project was to simply return the river bank to the location it has occupied for 100 years and protect it in that location. However, it became apparent early on that permitting would very difficult, so the project was modified to minimize the scope of bank reconstruction to only that which was necessary to provide reliable flows to the intake structure. Additionally, the project will provide modest geomorphological benefits in an impaired reach of the Yellowstone River. Therefore, it is our opinion that compensatory mitigation is not warranted.

However, in an effort to practice good stewardship and to expedite the 404 permitting process the City of Laurel has agreed to several compromises. One of which is that the City of Laurel will be purchasing 1,235 stream mitigation credits from EcoAsset Management Inc., an approved mitigation bank, to offset any perceived adverse impacts created by the project.

A SPA 124 Permit is administered by the MFWP for projects affecting the bed or banks of any stream in Montana and is used to protect and preserve fish and wildlife resources. A 318 Permit is obtained from MDEQ for projects temporarily violating state surface water quality standards for turbidity. Its purpose is to protect water quality and minimize sedimentation and turbidity. A 401 Certification is also obtained from MDEQ and is for projects that dredge and fill within state waters. A Stormwater Discharge Permit is also required by MDEQ for stormwater discharges during construction for projects disturbing more than 1 acre. All of these state agencies have been contacted regarding the project and have indicated for their support for the project and that their respective permits will be issued once the 404 permit is obtained.

The Yellowstone River is considered a Navigable River, and therefore, a State Lands Permit is required for work which takes place below the low water mark. This project will be disturbing the river bed regardless of which alternatives are selected, so this permit will be required. The project also requires a Floodplain Development Permit from Yellowstone County because the Yellowstone River lies within a FEMA mapped floodplain. The Floodplain Development Permit has already been issued.


March 2012 32

Construction Challenges All alternatives present challenges during construction stemming from need to excavate and place rock or blocks in the channel. Construction will be completed at low water to minimize work in the river; however, the toe and lower portions of the slope will be constructed below the water line. Construction equipment will be able to operate from the bank to minimize the potential for contamination from petroleum products.

Due to the size and weight of the riprap and concrete blocks, handling them will be difficult and may require specialized equipment. Many styles of concrete blocks incorporate lift eyes into the construction to assist in moving and placing. Some styles of block come in pieces which are lighter weight and easier to move but require a minimum amount of assembly. Concrete blocks also require precise placement to achieve interlock between the blocks. This can add construction time to the project.

The sediment removal in the vicinity of the railroad and highway bridges will require construction equipment to enter the floodway. Excavation will be completed during low flows and will remain at least 24 inches above the current water level. Local bridges along the proposed access route to the floodway may not have the load rating to carry fully loaded dump trucks leaving the site. An alternate access route may have to be investigated or lighter loads/more trips may be required.

Cost Estimates Since the bank stabilization and bank alignment alternatives may be combined interchangeably, a cost estimate has been prepared for each combination: 1A, 1B, 1C, 2A, 2B, 2C, 3A, 3B, and 3C (see Tables 2 through 10). The estimates include costs associated with engineering design, project construction, and construction engineering. Cost estimates are based on past bid tabs on similar projects and quotes received from material suppliers. Engineering design costs include permitting, preliminary and final design, and construction document preparation. Construction engineering costs include construction inspection and staking. A 10 percent contingency has been added to account for unforeseen expenses. A 10 percent mobilization item accounts for bonding, insurance, transportation of labor and equipment, and other costs that are typically not included in other bid items.

A present worth analysis was not completed because the service life and anticipated maintenance costs are comparable for each alternative.


March 2012 33

Table 2 - Opinion of Probable Cost - Alternative 1A

ITEM NO. DESCRIPTION QUANTITY UNIT UNIT PRICE TOTAL PRICE

1 Mobilization 1 LS $125,490.00 $125,490

2 Clearing & Grubbing 1 LS $2,000.00 $2,000

3 Unclassified Excavation & Embankment 8275 CY $12.00 $99,300

4 54" Nom. Dia. Riprap (48" nom. 36" min., 72" max.) 8960 CY $120.00 $1,075,200

5 Imported Topsoil (6" depth) 495 CY $35.00 $17,325

6 Upland Stabilization Plantings 4690 SY $10.00 $46,900

7 Wetland Stabilization Plantings 1 LS $6,500.00 $6,500

8 Hydroseeding 4690 SY $1.10 $5,159

9 Turf Reinforcement Mat (NA Green SC250) 310 SY $8.00 $2,480

$1,380,354

10% $138,035

11% $151,839

8% $110,428

$1,780,657

ENGINEERING DESIGN

CONSTRUCTION ENGINEERING

TOTAL

REVETMENT RIPRAP ARMORING - FULL SLOPE

EXISTING BANK ALIGNMENT

SUBTOTAL

CONTINGENCY

Table 3 - Opinion of Probable Cost - Alternative 1B


1 Mobilization 1 LS $122,380.00 $122,380



4 Excess Channel Material Disposal 5790 CY $10.00 $57,900







$1,346,146

10% $134,615

11% $148,076

8% $107,692

$1,736,528

ENGINEERING DESIGN


TOTAL

REVETMENT RIPRAP ARMORING - BANKFULL AND BELOW

INTERMEDIATE BANK ALIGNMENT

SUBTOTAL

CONTINGENCY


March 2012 34

Table 4 - Opinion of Probable Cost - Alternative 1C


1 Mobilization 1 LS $123,870.00 $123,870










$1,362,517

10% $136,252

11% $149,877

8% $109,001

$1,757,647

ENGINEERING DESIGN


TOTAL

REVETMENT RIPRAP ARMORING - FULL SLOPE

CURRENT BANK ALIGNMENT

SUBTOTAL

CONTINGENCY



1 Mobilization 1 LS $110,550.00 $110,550



4 54" Nom. Dia. Riprap (48" nom. 36" min., 72" max.) 7460 CY $120.00 $895,200






$1,216,016

10% $121,602

11% $133,762

8% $97,281

$1,568,660

ENGINEERING DESIGN


TOTAL



SUBTOTAL

CONTINGENCY


March 2012 35



1 Mobilization 1 LS $102,160.00 $102,160










$1,123,722

10% $112,372

11% $123,609

8% $89,898

$1,449,601CONSTRUCTION ENGINEERING

TOTAL



SUBTOTAL

CONTINGENCY

ENGINEERING DESIGN



1 Mobilization 1 LS $103,830.00 $103,830










$1,142,069

10% $114,207

11% $125,628

8% $91,366

$1,473,269CONSTRUCTION ENGINEERING

TOTAL



SUBTOTAL

CONTINGENCY

ENGINEERING DESIGN


March 2012 36



1 Mobilization 1 LS $161,030.00 $161,030



4 Articulated Concrete Blocks 2800 SY $500.00 $1,400,000






$1,771,296

10% $177,130

11% $194,843

8% $141,704

$2,284,971

ENGINEERING DESIGN


TOTAL

ARTICULATED CONCRETE BLOCKS


SUBTOTAL

CONTINGENCY



1 Mobilization 1 LS $151,820.00 $151,820










$1,669,982

10% $166,998

11% $183,698

8% $133,599

$2,154,277

ENGINEERING DESIGN


TOTAL



SUBTOTAL

CONTINGENCY


March 2012 37



1 Mobilization 1 LS $152,150.00 $152,150










$1,673,589

10% $167,359

11% $184,095

8% $133,887

$2,158,930

ENGINEERING DESIGN


TOTAL



SUBTOTAL

CONTINGENCY

Affected Environment The National Environmental Policy Act (NEPA) requires the Federal Government to coordinate Federal plans, functions, and resources to achieve various environmental, economic, and social goals through the use of a systematic, interdisciplinary analysis of Federal actions that have an impact on the human environment. This is accomplished through the use of a deliberative, written environmental review. For this type of project, an Environmental Assessment (EA) is initiated to determine the potential significance of impacts to the human environment. If the EA determines the proposed action will have significant impacts, then either an Environmental Impact Statement (EIS) must be prepared or the effects of the proposed action must be mitigated below the level of significance and documented in a mitigated EA.

An EA must document the purpose and need for the proposed action, the affected environment, an analysis of alternatives including a No-Action alternative, and an analysis of the impacts to the human environment of the different alternatives, including an evaluation of appropriate mitigation measures. The alternative analysis must consider direct, secondary, and cumulative impacts to the human environment. Alternatives that are not practicable and do not meet the project purpose are eliminated. The Least Environmentally Damaging Practicable Alternative (LEDPA) is the remaining alternative with the fewest adverse impacts to the environment.

Human Environment

Public Health and Safety The City of Laurel water treatment plant intake structure, constructed in 2003, is located slightly downstream of the south center-span pier and is intended to remain submerged, even at low water. The intake provides water to the City of Laurel water system for domestic and fire suppression uses. Additional water is supplied to the CHS Oil Refinery for process water and fire suppression.


March 2012 38

In the wake of the 2011 flooding, the combination of sediment deposition along the north side of the river and erosion of the south river bank caused the active channel to migrate south away from the intake structure. It also resulted in increased deposition to a gravel bar on the north side of channel downstream of the bridges and intake structure. The channel migration left the new water treatment plant intake structure on the edge of the active low water flow. The significant bed scour that occurred under the south spans of each bridge and immediately downstream also lowered the water surface elevation at the intake structure during low flow. In January 2012, City of Laurel personnel witnessed the lowest water surface elevations observed since the new intake was installed in 2003. The top of the intake was exposed to air, and ice began to build up on the intake structure. Additionally, slush ice flows in the river, which have never affected the intake because they occur at the water surface, began clogging the intake screens. These occurrences culminated in the City having to temporarily shut off the raw water supply to the CHS Oil Refinery on January 30, 2012 in order to meet domestic demands. Since then, the water surface has been right at the top of the intake structure, often showing riffles from the bolt heads on the intake structure.

City of Laurel Public Works Director, Kurt Markegard, documented the recent intake problems in a letter to Great West Engineering dated February 16, 2012, and is quoted as follows:

“The water supplied to the CHS Oil Refinery was disrupted on January 30, 2012, due to low river flows and a buildup of ice over the top of the intake structure.”

“The river flow over the water intake on January 30th was the lowest that any City employee has seen it since the intake was installed in the river in 2003.”

“I feel that it is imperative that the river channel migration to the south be stopped and actually restored at the bridge structure in order to prevent the City of Laurel from running out of water for domestic, industrial, and fire suppression efforts.”

The letter, which is enclosed in Appendix D, also included photos of the current condition intake structure which show the low water levels at the intake structure and the main channel flow bypassing the intake to the south. The photos also show the severe erosion that occurred at the Riverside Park river bank in the background. Photos of the site may be found in Appendix A.

The City has already had to shut down water to the CHS Oil Refinery once. It is a very real possibility that the City may not be able draw the water necessary to meet their demands – even with the refinery offline – during low flows this spring, late summer, and fall. If this occurs, it will constitute a public health and safety emergency and the city will be forced to enter the river channel and take whatever measures are necessary to restore flow to the water intake.

The City of Laurel was recently contacted by an adjacent property owner that lives immediately west of the north railroad bridge abutment. The property owner is concerned the sediment deposited during the 2011 flooding will cause his property to flood during upcoming spring flows. Flood waters during 2011 crept to the edge of his lawn and he was forced to vacate his home in the interest of personal safety. Correspondence regarding this property owner is included in Appendix D.

Direct Impacts: Bank Alignment Alternatives A and B will shift the main channel flow back to the existing (pre-flood) condition, which was directly over the water intake. This will restore the reliable water flow to the intake structure.

Alternatives C and No Action will not restore the existing flow patterns and will not improve the reliability and availability of water to the intake structure. This is a serious threat to public health and safety.

Bank Stabilization Alternatives 1 – 3 protect the river bank from erosion thereby protecting public facilities in the park. Alternative 1 provides a slightly greater level of protection.


March 2012 39

Secondary Impacts: The reliable availability of water provided by Alternatives A and B will ensure the City of Laurel can provide domestic water to its residents. The intake will also have capacity to support fire fighting operations in the event of a fire in Laurel. The CHS Oil Refinery will also have raw water available for industrial processes and fire suppression. These alternatives will reduce the backwater effect upstream of the bridges and lower water surface elevations which will provide a measure of safety and flood protection to the aforementioned property owner.

Alternatives C and No Action will not improve the intake water supply problems. A loss of service could endanger the lives of residents of Laurel due to the lack of potable water or insufficient fire flows. According to CHS Oil Refinery personnel, the CHS Oil Refinery uses raw water for steam production and cooling water, two critical utilities for the entire refinery. If the raw water line is shut down, as it was on January 30, 2012, the refinery must begin an emergency shutdown immediately. The recent disruption was short enough in duration that the refinery was able to resume operations without completely shutting down.

The risk of a refinery accident or emergency also increases exponentially when raw water is shut off. The refinery employs 320 people and contracts work to an additional 100-500 independent contractors. Additionally, 100 employees in the Pipelines and Terminals group support refinery operations. An accident at the refinery caused by a loss of raw water availability could threaten the safety of all of these employees.

Alternatives C and No Action will not improve the hydraulic conditions immediately upstream of the bridges which will not reduce the flood hazard to the aforementioned property owner.

Cumulative Impacts: Due to the unique nature of the water treatment plant intake structure and the localized hydraulics around it, cumulative impacts to the associated public health and safety impacts are not applicable.

Socio-Economic Impacts Socio-economic factors include a wide range of indicators, some of which are community demographics, demand for public services, employment and income levels, economic development, government spending levels, community values, and aesthetic quality. Many of these factors can be impacted all types of projects.

Direct Impacts: The immediate impacts of Alternatives A and B include the restoration of reliable water availability to the intake, which will ensure the availability of potable water and fire flows in the City of Laurel. Alternatives C and No Action will not restore the existing flow patterns and will not improve the reliability and availability of water to the intake structure. This is a serious threat to the economic stability of Laurel.

Secondary Impacts: The potential socio-economic impacts of this project are severe if no action is taken. During the floods of 1996 and 1997, sediment deposition on the north side of the river was so significant it prevented the former City of Laurel water treatment plant intake structure from operating year-round after the floods. This represented a major threat to public health and safety for the people served by the Laurel water system. As a result, a new intake structure was constructed in 2003 at a cost of $3 million to provide reliable, year-round operation. It is estimated that it will cost the City of Laurel approximately $5 million to construct a new intake when the current intake becomes inoperable, if measures are not taken to improve the reliability of operation.

The reliable availability of water provided by Alternatives A and B will ensure the City of Laurel can provide domestic water to its residents and businesses, and raw water to the refinery. Alternatives C and No Action will not improve the intake water supply problems. A loss of service could cost residents and business owners a considerable amount of money depending on the nature of their respective water demands. According to CHS Oil Refinery


March 2012 40

personnel, the CHS Oil Refinery uses raw water for steam production and cooling water, two critical utilities for the entire refinery. If the raw water line is shut down, as it was on January 30, 2012, the refinery must begin an emergency shutdown immediately. The recent disruption was short enough in duration that the refinery was able to resume operations without completely shutting down.

The refinery is the largest employer in Laurel and employs 320 people and contracts work to an additional 100-500 independent contractors. Additionally, 100 employees in the Pipelines and Terminals group support refinery operations. If they have to shut the refinery down, their operation license will expire and they will have to meet new startup conditions. It may be impossible for the refinery to meet the new startup conditions. If the refinery were to shut down, most employees would permanently lose their jobs. This would cause an economic catastrophe in Laurel.

Alternatives C and No Action also have the potential to affect the ability for the Billings Bench Water Association irrigation diversion structure to function properly. The irrigation canal fed by the diversion structure flows for 62 miles and serves countless acres of farmland. A loss of irrigation flows in the canal could cause a crisis for farmers that rely on the canal for irrigation water.

Cumulative Impacts: This reach of the Yellowstone River is surrounded by critical infrastructure, and significant measures have been taken to limit the ability of the river to meander. The existing historic levee, including the section damaged by the recent flooding, is illustrated on BLM General Land Office maps dated 1906, making the levee at least 100 years old. Although this impairs the river’s ability to function naturally, it is the reality on this reach of the river. Critical infrastructure adjacent to this reach of the Yellowstone River include a refinery, a water treatment plant, a wastewater treatment plant, a decommissioned landfill, a major irrigation diversion structure, buried crude oil (2) and natural gas (1) pipelines, a highway bridge, and a railroad bridge. This is a situation in which the human environment is best served by maintaining the restricted nature of the river.

Cultural Resources As previously noted, historic buildings remain standing on the Riverside Park property. According to the Montana State Historic Preservation Office, Riverside Park is a recorded historical site. “Site 24YL0169 is the historic Riverside Park, which is eligible for listing on the National Register of Historic Places. This site includes many buildings as well as a historic levee.” These buildings, which date from World War II, are located more than 100 feet outside of the proposed construction limits and will not be disturbed during construction. A historic log cabin within the park sits near Highway 212/310 and was almost lost to flood waters. The cabin was moved off its concrete slab foundation (not original) under emergency circumstances during flooding in 2011. Once river bank reconstruction is completed, the foundation may be repaired or replaced and the cabin moved back to its previous location.

Direct Impacts: All alternatives, except the No Action alternative, would reconstruct the river bank to match the historic levee elevation. This would restore flood protection to Riverside Park, the buildings contained therein, and the areas surrounding it. The No Action alternative would perpetuate the current conditions which protect the park from floods smaller than the 25-year event.

Secondary Impacts: The No Action alternative would allow the park and historic buildings to be inundated by the 25-year flood or larger events. This could damage or destroy the historic buildings. It also would direct flood waters behind the intact portions of the historic levee and impact surrounding homes. All improvement alternatives would protect against flooding up to the 100-year event.


March 2012 41

Cumulative Impacts: The cumulative impacts of projects similar to this provide protection to cultural resources in the floodplain. This is a beneficial impact.

Access to Public Lands The flooding and erosion that occurred at Riverside Park caused the destruction of a concrete boat ramp and left the bank of the Yellowstone River at a near-vertical slope. Park land and vegetation was also lost to the river. Consequently, public access has been severely limited. The river bank is no longer safe for fishing access by foot and the loss of the boat ramp has limited boating and recreation opportunities on this section of river as the nearest public boat ramp is 5 miles upstream.

Direct Impacts: The project will restore the walking accessibility to the river bank by returning the bank to its pre-flood slope. Additionally, the boat ramp will be reconstructed. Land and vegetation will be restored for the benefit of users of the park. Public access to a variety of public lands will be restored and enhanced as a result of this project. Alternative 1, a full slope of riprap, and Alternative 3, articulated concrete blocks, will provide a slight obstacle to park users to accessing the water. They will, however, not prohibit access to the river. The No Action alternative would provide no improvement to the current conditions and would limit access to public lands.

Secondary Impacts: Public access to a variety of public lands will be restored and enhanced as a result of this project. The No Action alternative would provide no improvement to the current conditions and would limit access to public lands.

Cumulative Impacts: The cumulative impacts of projects similar to this provide protection to public facilities in the floodplain and enhance access to those facilities and surrounding public lands. This is a beneficial impact.

Physical Environment

Climate & Air Quality The project is located approximately 1.4 miles south of Laurel, Montana where U.S. Highway 212/310 crosses the Yellowstone River. The project is situated at an elevation of 3,270 feet in the Yellowstone River Valley. Average temperatures range from 26.5 degrees F in December to 72.8 degrees F in July and annual precipitation is 13.6 inches. See Appendix D for climate data from the National Oceanic and Atmospheric Administration (NOAA).

An oil refinery owned and operated by Cenex Harvest States (CHS) is located approximately 1,000 feet northwest of the project. This facility has the single largest effect on local air quality and is subject to the conditions of the Montana Department of Environmental Quality (DEQ) Air Quality Operating Permit it operates under.

Direct Impacts: No alternatives would affect the climate in the project vicinity. All alternatives, except the No Action alternative, have an equal potential to generate temporary dust during construction. This would have a minor impact on the air quality in the vicinity of the project and would be considered a nuisance at most. Additionally, due to close proximity to the Yellowstone River, soils are anticipated to be moist, thereby inhibiting dust being released into the atmosphere. All alternatives involving construction equipment (i.e. all except no-action) would produce exhaust fumes from the equipment. The increased exhaust fume levels would be negligible given the close proximity of Highway 212/310 and the railroad, both being larger sources of exhaust fumes.

Secondary Impacts: No alternatives would affect the climate in the project vicinity. Alternatives that do not improve the hydraulic conditions around the water treatment plant intake structure have the potential to cause future impacts to air quality. If the intake structure is unable to draw enough water to meet demands, fire suppression water flows may not be present to fight a fire in the City of Laurel or at the CHS Oil Refinery which could


March 2012 42

lead to smoke and toxic fumes entering the atmosphere. Alternatives correcting the hydraulic problems at the intake structure would not have any known secondary impacts to air quality.

Cumulative Impacts: No cumulative impacts have been identified for any of the alternatives.

Geology & Soils “The project area lies in an unglaciated portion of the Missouri Plateau of the northern part of the Great Plains Province. Sedimentary rocks, consisting primarily of sandstone underlie the site. River gravel, with depths from two to more than ten feet, overlay the bedrock in the river channel, and on the terraces above the river channel. The Yellowstone River has cut into the bedrock more deeply on the south side of the river than on the north.” (Feasibility Study for Mitigating Laurel’s Water Supply Problem, HKM Engineering, 2002)

The soils outside of the river channel are predominantly composed of sandy to clayey loam to a depth of more than 5 feet, according to the NRCS Soil Survey of Yellowstone County. Haverson-Hysham loams, 0 to 1 percent slopes (Map Unit Hh) are present in the floodplain on the south side of the river. Hysham-Laurel silty clay loams, 0 to 2 percent (Map Unit Hy) are present in the floodplain on the north side of the river. Alluvial land, mixed (Map Unit Al) is a mix of gravelly sand and loam and is shown on the north river bank. Riverbed substrates vary in size from silts to cobbles.

Direct Impacts: All alternatives, except the No Action alternative, would disturb the south river bank from the highway bridge to downstream of the former boat ramp. Temporary sedimentation in the river would occur during construction but would be minor. Bank Stabilization Alternatives 1 – 3 would cause about the same amount of disturbance. Bank Alignment Alternatives A and B would each disturb 715 linear feet of bank, and Alternative A would disturb the most river bed in the eroded bank area. Bank Alignment Alternative C would disturb 730 linear feet of bank. Alternative C would also result in a large volume of river sediment removed beneath the bridges being disposed of offsite and completely removed from the river system. Alternatives A and B would utilize this sediment in bank reconstruction, thereby keeping the sediment in the river system. The No Action alternative would have no direct impact to the geology or soils.

Secondary Impacts: Armoring river banks often causes an increase in scour near the toe of the armored banks. However, hydraulic modeling indicates Alternatives A and B would cause a reduction in velocities through the bridge where excessive scour has been a problem. They would also increase velocities downstream of the intake structure where abnormal sediment deposition is occurring. This would restore a more normal sediment transport regime to the river. Alternatives C and No Action would maintain the current conditions causing scour in the main channel beneath the bridges and excessive deposition downstream of the intake structure. Alternatives 1 – 3 would have no effect on secondary impacts.

Cumulative Impacts: Adding bank armoring in many locations on a river can often have a detrimental cumulative impact to the geomorphology of the river and downstream river banks. However, the river in the vicinity of the project has been constrained with riprap and levees for more than 100 years. The project intends to restore the existing conditions of the river so no cumulative impacts are anticipated.

Surface Water Resources The Yellowstone River, in Yellowstone County, had a detailed-level Flood Insurance Study (FIS) completed in November 1981 and revised in March 2000 to determine base flood elevations, water surface profiles, and floodway and floodplain limits. This study is currently the effective study; however, a draft FIS was completed in April 2007 using more accurate topographical data and updated hydrology. Due to the fact the data was gathered more recently, the 2007 FIS more closely matches the hydraulics of the river in its current condition than the effective study. The Corps of


March 2012 43

Engineers estimated the 100-year recurrence interval flood to be 56,700 cubic feet per second (cfs) for the river at the project site and is considered to be the base flood. According to the study and the effective Flood Insurance Rate Map (FIRM) for the project area, the project lies within a designated 100-year floodplain. A historic levee was constructed more than 100 years ago and is situated along the south river bank at Riverside Park. A portion of the levee was destroyed during the recent flooding.

River geomorphology typically exhibits deposition on the inside of meanders and erosion on the outside of meanders. However, the Yellowstone River is exhibiting the opposite behavior at the project site which consists of sediment deposition on the outside and erosion on the inside. This is due to the relative overwidening of the river immediately downstream of the constriction at the bridges and the corresponding drop in velocity.

The water treatment plant intake structure, constructed in 2003, is located slightly downstream of the south center-span pier and is supposed to remain submerged, even at low water. The minimum design capacity of the intake is 7 million gallons per day (MGD), with the ability to increase to 20 MGD with future improvements. During the winter months the City of Laurel draws approximately 2.7 MGD of raw water from the Yellowstone River of which 1.4 MGD are supplied to the CHS Oil Refinery for process water and fire suppression.

Direct Impacts: All alternatives, except the No Action alternative, would cause minor sedimentation in the river during construction resulting in a short-term degradation to water quality. The sediment removal portion of the project would immediately restore hydraulic capacity to the railroad and highway bridges which was lost due to excessive sediment deposition. The No Action alternative would maintain the restricted hydraulic capacity of the bridges. No direct impacts to the floodplain are anticipated due to construction taking place during low flows.

Secondary Impacts: Although allowing the river access to the floodplain is generally desirable, the floodplain at Riverside Park does not return to the river. Allowing limited access to the floodplain only at the project location would substantially increase the flood hazard to the historic buildings in Riverside Park and residences east of the park because flood waters would be trapped behind the remaining sections of the levee. Additionally, the floodplain is completely blocked on both sides of the river at the upstream bridges and downstream at the levees (which exist on both sides of the river). For this project, reconstructing the bank to the historic levee elevation provides the greatest benefit to the human environment as required by NEPA.

The Yellowstone River naturally moves within the meander zone on unimpaired reaches. This reach of the Yellowstone River is surrounded by critical infrastructure, and significant measures have been taken to limit the ability of the river to meander. Although bank armoring and levees impair the river’s ability to function naturally, it is the reality on this reach of the river and has been for more than 100 years. Critical infrastructure adjacent to this reach of the Yellowstone River include a refinery, a water treatment plant, a wastewater treatment plant, a decommissioned landfill, a major irrigation diversion structure, buried crude oil (2) and natural gas (1) pipelines, a highway bridge, and a railroad bridge. This is a situation in which the human environment is best served by maintaining the restricted nature of the river. Alternatives 1 – 3 will equally stabilize the bank and prevent river migration. Alternative 2 would provide the most natural river bank replacement with rock and upland riparian vegetation.

No long term impacts to water quality are anticipated as a result of any of the construction alternatives. It is very unlikely downstream properties will be impacted by the placement of fill or bank stabilization. The existing river bank and levee have been relatively stable for 100 years prior to last year’s flooding. For Alternatives A and B, once a more natural


March 2012 44

geomorphological regime is established which deposits sediment on the inside bend where the riprap is proposed, minimal energy will be deflected to downstream river banks.

Alternatives A and B will improve the ability of the Yellowstone River to convey high flows in the vicinity of the bridges and Riverside Park, as demonstrated by the HEC-RAS modeling results. The backwater effect upstream of the bridges will be reduced and the hydraulic jump immediately downstream of the bridges will be eliminated.

Alternatives C and No Action will perpetuate the backwater effect, hydraulic jump, and geomorphological problems. It is likely the island on the north side of the channel will continue to grow and move closer to the water intake, eventually completely covering it. This has the highest potential to occur with the No Action alternative. It is unclear what impacts to downstream properties may occur as a result of Alternatives C and No Action. They have the potential to allow the river geometry to change which could impact the ability of the Billings Bench Water Association irrigation diversion structure to divert water, especially during low flows.

The No Action alternative would allow floodwaters to inundate, damage, and possibly destroy the historic buildings located in Riverside Park and the residences south and east of the park. This alternative would also allow the current river bank, which is highly susceptible to erosion, to erode at an accelerated rate. This would cause an increase in turbidity and a degradation of water quality and could impact the buried pipelines in the vicinity of the project, even causing a rupture. A future crude oil spill would cause a major degradation of water quality to the Yellowstone River and the surrounding ecosystem.

Cumulative Impacts: Adding bank armoring in many locations on a river can often have a detrimental cumulative impact to the geomorphology of the river and downstream river banks. However, the river in the vicinity of the project has been constrained with riprap and levees for more than 100 years. The project intends to restore the existing conditions of the river so no cumulative impacts are anticipated.

Groundwater Resources Groundwater resources are not used by the water treatment plant intake or any aspects of this project.

Impacts: No direct, secondary, or cumulative impacts to groundwater resources are anticipated as a result of this project.

Hazardous Facilities The Montana Department of Environmental Quality (DEQ) maintains records of environmental impairments such as petroleum spills, leaking underground tanks, and abandoned mines. Searching the DEQ databases can provide important information for the site and the surrounding area; however, they do not cover unreported environmental hazards. A records search was performed which returned the following results in the vicinity of the project site:

Abandoned Mine Sites: 0 sites Underground Storage Tank (UST) Facilities: 0 sites Underground Tank Leaks: 0 sites Petroleum Tank Release Compensation Sites: 0 sites Remediation Response Sites: 0 sites

The DEQ also administers several environmental cleanup programs including the Montana State Superfund (CECRA) and the Voluntary Cleanup Program (VCRA). A records search was performed and no CECRA or VCRA sites are in the vicinity of the project site.

The United States Environmental Protection Agency (EPA) also maintains records of major environmental cleanup projects. A records search of the EPA’s databases produced no records of


March 2012 45

CERCLA or National Priority List sites in the vicinity of the project. The Cenex Harvest States (CHS) Oil Refinery is listed as a Resource Conservation and Recovery Act (RCRA) site. The CHS Oil Refinery has been in operation since the 1930s and produces 42,000 barrels per day of refined petroleum products including propane, gasoline, diesel, asphalt, and road oil.

There are a number of underground pipelines in the vicinity of the project. The Exxon-Mobil Silvertip crude oil pipeline lies 750 feet downstream of the highway bridge. The Silvertip line ruptured in July 2011 as a result of river bank erosion and bed scour at Riverside Park and spilled approximately 1,000 barrels of crude oil into the river. A new pipeline was installed shortly after the failure, and the old, damaged pipeline is currently being removed. Another crude oil pipeline and a 12-inch natural gas transmission main operated by Williston Basin Interstate Pipeline also cross beneath the river and run through the park.

Direct Impacts: All alternatives, except the No Action alternative, have the potential to impact the buried pipelines during construction. The potential impacts will be mitigated through coordination with pipeline owners to ensure no facilities are damaged during construction.

Secondary Impacts: Bank Alignment Alternatives A and B will ensure adequate flow to the water treatment plant intake, which also provides 1.4 million gallons per day of raw water to the CHS Oil Refinery. Alternatives C and No Action will not ensure adequate flows to the intake structure. This has the potential to disrupt raw water used for industrial processes and fire suppression at the refinery which could cause a hazardous materials emergency. These alternatives also maintain higher average velocities in the river which contribute to additional bed scour. This could cause damage to the existing pipelines running under the river and could result in a future crude oil spill in the river.

The No Action alternative does not prevent river bank erosion in the vicinity of the new Silvertip pipeline. Although the pipeline is buried deep below the river bed it does come up to standard bury depths in Riverside Park. River bank erosion and floodplain inundation could cause damage to the pipeline. A future oil spill would be a severe hazardous material emergency.

Cumulative Impacts: No cumulative impacts are anticipated as a result of this project.

Biological Environment

Vegetation Based on information from the U.S. Fish & Wildlife Service’s National Wetland Index, there appears to be riverine and palustrine wetlands in the vicinity of the project. An emergent class of wetlands which consist of erect, rooted, herbaceous hydrophytes (plants which grow in water or saturated soil) are located on the bar across the Yellowstone River from and downstream of Riverside Park near the Billings Bench Water Association diversion structure, and upstream of the railroad bridge. Permanently flooded riverine wetlands are located in the river channel.

An upland riparian buffer is present immediately downstream of the project along the intact portion of the historic levee. This riparian buffer is well-established and appears to be self-sustaining without any maintenance or human input. The riparian buffer is assumed to be a Great Plains Cottonwood/Western Snowberry Community Type (Hansen and others, 1995) because the buffer appears to be native riparian vegetation for the area.

The flooding of 2011 caused the south river bank to erode for approximately 1,400 feet downstream of the bridge and up to 45 feet back from the existing (pre-flood) location. Numerous cottonwood trees were lost during the bank erosion which left an unstabilized, gravelly and cobbly river bank which is highly susceptible to further erosion.


March 2012 46

Direct Impacts: The proposed improvement alternatives may result in temporary sedimentation due to constructing the bank stabilization measures down to the river bed. Sediment removal activities in the channel will be conducted above water so no additional sedimentation should result. Efforts will be made to adhere to best management practices to minimize any disturbance to wetlands. Riverine wetlands would be disturbed by all Bank Alignment Alternatives with Alternative A causing the most disturbance and Alternative C causing the least disturbance. The disturbance of riverine wetlands will be mitigated by planting wetland species throughout the bank stabilization portion of the project. This will also provide additional slope stability and erosion protection. Bank Stabilization Alternatives 2 and 3 will utilize upland riparian vegetation planted in the reconstructed bank above bankfull elevation to stabilize the fill. Alternative 1, a full riprap slope, will utilize very little upland riparian vegetation.

The No Action alternative will avoid direct impacts to existing wetlands and riparian areas.

Secondary Impacts: The long term impacts to riverine wetlands disturbed by construction in the riverbed will be mitigated by planting wetland species in and amongst the riprap or block revetments. It is possible that the existing palustrine wetlands located on the sediment island across the main channel from Riverside Park will be washed away if the geomorphological conditions improve as a result of Alternatives A and B. However, the loss of these wetlands will be mitigated because normal geomorphological patterns will cause deposition along the south river bank creating an opportunity for new palustrine wetlands to establish. The No Action alternative will not disturb any existing wetlands, nor will it realize any geomorphological improvements which could facilitate the creation of new wetlands.

Cumulative Impacts: Adding bank armoring in many locations on a river can often have a detrimental cumulative impact to the geomorphology of the river and downstream river banks. However, the river in the vicinity of the project has been constrained with riprap and levees for more than 100 years. This project also intends to promote normal geomorphology which can provide the opportunity for new wetlands to develop. Additionally, a significant amount wetland and upland riparian species are proposed which will enhance the vegetation in the area.

Biological Resources There are several different information sources to determine the biological resources present in the region. The Montana Natural Heritage Program (MNHP) maintains lists of plant and animal species of concern. Typically these lists are organized by county so the presence of a plant or animal on the list generally means that species was found in the county at least once, not that it is likely to be found at the project site. A records search of the MNHP databases was performed and found 8 animal species of concern and no plant species of concern. The species include Black-tailed Prairie Dog, Baird’s Sparrow, Great Blue Heron, Yellow-billed Cuckoo, Pinyon Jay, Bald Eagle, Spiny Softshell Turtle, and Yellowstone Cutthroat Trout.

Breeding and non-breeding bald eagle activity occurs in the general project area along the Yellowstone River. Coordination with local Montana Fish, Wildlife and Parks (FWP) wildlife staff will be necessary to determine whether active eagle nesting occurs in this area prior to construction commencement. To the maximum extent practicable, construction activities should be scheduled so as not to disrupt nesting birds during the breeding season (approximately April-August). If work is proposed to take place during the breeding season or at any other time which may result in take of migratory birds, their eggs, or active nests, all practicable measures will be taken to avoid and minimize take, such as maintaining adequate buffers, to protect the birds until the young have fledged. Active nests may not be removed.


March 2012 47

The Yellowstone River at Laurel is a transitional water body that is primarily dominated by cold and cool water aquatic species. The following list includes the species of fish documented by Montana Fish, Wildlife and Parks to inhabit this reach of river.

Black Bullhead Brook Stickleback Brook Trout Brown Trout Burbot Channel Catfish Common Carp Emerald Shiner Fathead Minnow Flathead Chub Goldeye Green Sunfish Largemouth Bass Longnose Dace Longnose Sucker Mottled Sculpin Mountain Sucker Mountain Whitefish Rainbow Trout River Carpsucker Sand Shiner Sauger Shorthead Redhorse Smallmouth Bass Stonecat Walleye Western Silvery Minnow White Sucker

Direct Impacts: The proposed improvement alternatives may result in temporary sedimentation due to constructing the bank stabilization measures down to the river bed. Sediment removal activities in the channel will be conducted above water so no additional sedimentation should result. Efforts will be made to adhere to best management practices to minimize any disturbance to aquatic species. The No Action alternative will not directly impact aquatic species.

If an active nest(s) is(are) present, seasonal restrictions and construction/development distance buffers specified in the 2010 Montana Bald Eagle Management Guidelines: An Addendum to Montana Bald Eagle Management Plan will be considered. The No Action alternative will not directly impact bald eagles.

Potential impacts to all species of concern include loss of habitat due to construction. Given the limited size of the project limits, the limited length of construction time, and the fact it will be constructed in low water, effects to the species of concern should be minimal.

Secondary Impacts: Potential long term impacts to biological resources include loss of habitat and introduction of pollutants. Revetments often degrade river bank habitat; however, wetland species will be planted in the bank stabilization treatment to mitigate the loss of bank habitat. Improvements to river geomorphology in Bank Alignment Alternatives A and B will also provide improvement to aquatic species habitat in the river channel. The No Action alternative will minimize disturbances to habitat; however, it will not provide the benefits that will result from the improvement alternatives.

Cumulative Impacts: This reach of the Yellowstone River is impaired with regard to natural function and aquatic species habitat. Some degradation of aquatic habitat occurred during the recent flooding. Alternatives A and B will make modest improvements to aquatic habitat compared to the current conditions (No Action alternative) and Alternative C due to geomorphological improvements. All improvement alternatives will contribute to a temporary disruption of habitat along the river banks.

Basis for Selection of the Preferred Alternative Table 11 presents a ranking of each alternative based on the environmental impacts evaluated in the Affected Environment section of this report. It also identifies the Least Environmentally Damaging Practicable Alternative (LEDPA) for each category of alternatives as required by NEPA. Table 12 presents a ranking of each alternative based on a comparative evaluation. Refer to Table 13 for a summary of the comparative evaluation rankings and a cost comparison.

The primary purpose of this project is to protect critical infrastructure in the vicinity of Riverside Park near Laurel, Montana which is necessary for public health and safety. If left uncorrected, the channel migration that occurred during the flood will prevent the ability of the City of Laurel’s Water Treatment Plant intake structure to draw a sufficient amount of water to meet their demands. All of


March 2012 48

the bank stabilization alternatives achieve these goals. The Existing Bank Alignment and the Intermediate Bank Alignment alternatives achieve these goals. The Current Bank Alignment, Alternative C, does not achieve these goals because it provides no improvement to the hydraulic conditions in the vicinity of the intake structure. The No Action Alternative also does not achieve these goals. Therefore, Alternative C and the No Action Alternative are not practicable alternatives.

The environmental impacts of each alternative were fully evaluated in the Affected Environment section of this report. A qualitative score was developed for each alternative and represents the severity of the impacts to each type of affected environment. The practicability of each alternative, which was evaluated in the Alternatives Analysis Description section of this report, was accounted for with a “Yes” or “No”. The environmental scores of each alternative were summed and the alternative in each category (stabilization or alignment) with the highest score and a “Yes” practicability rating were identified as the Least Environmentally Damaging Practicable Alternative (LEDPA). See Table 11 for the environmental ranking of each alternative and identification of LEDPA. For the environmental rating in Table 12, Comparative Evaluation Ranking, the LEDPA in each alternatives category was assigned a value of +1. The alternatives not identified as LEDPA were assigned a value of -1.

The ability of the City of Laurel to obtain the necessary permits to construct the project is another consideration. Based on recent discussions with the ACOE and the requirements of NEPA, only the least environmentally damaging practicable alternative may be permitted. Since LEDPA is already accounted for in the Environmental Impacts score, no additional scoring was performed with regard to the ease of permitting.

Other considerations also have an important bearing on the preferred alternative analysis. The technical feasibility varies for each of the bank stabilization treatments. In general, riprap is not difficult to place, requiring only minor adjustment to achieving a keyed installation. Concrete blocks require more precise placement and must follow specific patterns to achieve interlock. Of the different bank alignments considered, the Existing Bank Alignment and the Intermediate Bank Alignment are easier to construct than the Current Bank Alignment because of the irregularities in the shape of the existing bank. Technical Feasibility is not applicable to the No Action Alternative; therefore, it is assigned a value of 0.

The City of Laurel is very motivated to complete this project as soon as possible to ensure the reliable availability of river flows to the Water Treatment Plant intake structure, especially considering the recent problems they have had drawing water during low flows. Therefore, time is of the essence, and that means the duration of construction is very important. Due to the complexities of installation of the articulated concrete blocks and the additional volume of rock to be placed with the full riprap slope, the riprap armoring – bankfull and below is the alternative with the shortest construction time. Likewise, due to the irregularities of the Current Bank Alignment and the additional volume of fill required for the Existing Bank Alignment, the Intermediate Bank Alignment should provide for the quickest construction time. Since the No Action Alternative maintains the current conditions, it is the quickest alternative to implement.

Table 13 compares the project costs for each alternative combination and incorporates the evaluation rankings to determine a preferred alternative. Since the No Action Alternative does not achieve the project goals, is not practicable, and has no cost, it was omitted from the cost comparison. The cost estimates reveal that Revetment Riprap Armoring – Bankfull and Below is the least expensive bank stabilization treatment. Although the cost difference between the different bank alignments is nearly negligible, the Intermediate Bank Alignment is the least expensive. Therefore, Alternative 2B is the least expensive alternative with a total cost of $1,449,601. This represents of savings of $23,000 to $835,000 compared to the other alternatives.

Based on the results of the environmental evaluation which identified LEDPA, the comparative evaluation which made up the basis for selection, and the cost comparison, the preferred alternative


March 2012 49

for the Riverside Park Bank Stabilization Project is a revetment riprap armored slope, bankfull and below, with a vegetated overbank slope constructed along the intermediate bank alignment.


March 2012 50

Table 11 – Environmental Impact Ranking

ALT

ER

NA

TIV

E 1

R

evet

men

t Rip

rap

Arm

orin

g - F

ull S

lope

ALT

ER

NA

TIV

E 2

R

evet

men

t Rip

rap

Arm

orin

g - B

ankf

ull a

nd

Bel

ow

ALT

ER

NA

TIV

E 3

A

rticu

late

d C

oncr

ete

Blo

cks

ALT

ER

NA

TIV

E A

E

xist

ing

Ban

k A

lignm

ent

ALT

ER

NA

TIV

E B

In

term

edia

te B

ank

Alig

nmen

t

ALT

ER

NA

TIV

E C

C

urre

nt B

ank

Alig

nmen

t

NO

AC

TIO

N

ALTE

RN

ATI

VE

PUBLIC HEALTH & SAFETY +1 0 0 +1 +1 -1 -1

SOCIO-ECONOMIC 0 0 0 +1 +1 -1 -1

CULTURAL RESOURCES 0 0 0 0 0 0 -1

ACCESS TO PUBLIC LANDS -1 0 0 0 0 0 -1

CLIMATE & AIR QUALITY 0 0 0 0 0 -1 -1

GEOLOGY & SOILS 0 0 0 0 +1 0 -1

SURFACE WATER RESOURCES -1 +1 0 0 +1 -1 -2

GROUNDWATER RESOURCES 0 0 0 0 0 0 0

HAZARDOUS FACILITIES 0 0 0 0 +1 0 -1

VEGETATION -1 0 0 0 0 0 +1

BIOLOGICAL RESOURCES -1 0 0 0 0 -1 0

-3 +1 0 +2 +5 -5 -8

Y Y Y Y Y N N

N Y N N Y

PH

YS

ICA

L E

NV

IRO

NM

EN

TENVIRONMENTAL IMPACT RANKING

HU

MA

N

ENV

IRO

NM

ENT

TOTAL

LEAST ENVIRONMENTALLY DAMAGING PRACTICABLE ALTERNATIVE (LEDPA)

IS ALTERNATIVE PRACTICABLE?

BANK STABILIZATION ALTERNATIVES

AFFECTED ENVIRONMENT

BANK ALIGNMENT ALTERNATIVES

BIO

LOG

ICA

L E

NV

IRO

NM

EN

T


March 2012 51

Table 12 – Comparative Evaluation Ranking

ACHIEVES PROJECT GOALS

ENVIRONMENTAL IMPACTS

TECHNICAL FEASIBILITY

CONSTRUCTION TIME TOTAL

ALTERNATIVE 1Revetment Riprap Armoring - Full Slope

ALTERNATIVE 2Revetment Riprap Armoring - Bankfull and Below

ALTERNATIVE 3Articulated Concrete Blocks

ALTERNATIVE AExisting Bank Alignment

ALTERNATIVE BIntermediate Bank Alignment

ALTERNATIVE CCurrent Bank Alignment

NO ACTION ALTERNATIVE -1 -1 0 +1 -1

COMPARATIVE EVALUATION RANKING

+1

+1

0

+1

+1

0

-1

BA

NK

ALI

GN

ME

NT

ALT

ER

NA

TIV

ES

0

-1

+1

-1

0

-1

+1

-1

BA

NK

STA

BIL

IZA

TIO

N

ALT

ER

NA

TIV

ES

+1 0-1

0

+1

0

0

+3

-1

0

+3

0

-2

0

+1


March 2012 52

Table 13 - Cost Comparison & Selection Summary

CONSTRUCTION COST

TOTAL COST (incl. contingency

& engineering)COST RANKING

TOTAL BANK STABILIZATION

RANKING

TOTAL BANK ALIGNMENT

RANKING

TOTAL ALTERNATIVE

RANKINGALTERNATIVE 1ARevetment Riprap Armoring - Full Slope

Existing Bank Alignment

ALTERNATIVE 1BRevetment Riprap Armoring - Full Slope

Intermediate Bank Alignment

ALTERNATIVE 1CRevetment Riprap Armoring - Full Slope

Current Bank Alignment

ALTERNATIVE 2ARevetment Riprap Armoring - Bankfull and Below


ALTERNATIVE 2BRevetment Riprap Armoring - Bankfull and Below


ALTERNATIVE 2CRevetment Riprap Armoring - Bankfull and Below


ALTERNATIVE 3AArticulated Concrete Blocks


ALTERNATIVE 3BArticulated Concrete Blocks


ALTERNATIVE 3CArticulated Concrete Blocks


$1,346,146 0

$1,380,354 0

0

$1,216,016

$1,123,722

$1,362,517 0

$1,568,660

+3$1,142,069

$1,771,296

$1,669,982

$1,673,589

$1,780,657 0

$1,736,528 0

$1,757,647

$2,154,277 -1 -1

+1 +3

$1,449,601 +2 +3

$1,473,269 +1

0

+3

-2

0

+3

-2 +2

-3

+1

$2,158,930 -1 -1

0$2,284,971 -2 -1

-4

COST COMPARISON & SELECTION SUMMARY

+3

-2

0

+3

-2

+4

+8


March 2012 53

DETAILED DESCRIPTION OF PREFERRED ALTERNATIVE

Description The City of Laurel is proposing to reconstruct and stabilize a section of Yellowstone River bank which was severely eroded in recent flooding in order to protect the continued operation of the City’s water treatment plant intake structure. The preferred alternative involves the reconstructing the river bank along an alignment between the historic (pre-flood) and current (post-flood) river bank locations (see Figure 10). Revetment riprap armoring will be placed from the river bed up to the bankfull elevation. Upland vegetation and turf reinforcement mat will be placed from the bankfull elevation up to the top of bank. These stabilization measures will protect the river bank from further erosion and will help prevent the Yellowstone River from migrating south. Sediment will also be removed from beneath the Highway 212/310 and railroad bridges to restore their hydraulic capacity. Refer to Table 13 on the following page for a detailed cost estimate for the project.

Approximately 7,320 cubic yards of rock riprap will be placed from the river bed to approximately the bankfull elevation (1.8-year recurrence) at a 1.5:1 (H:V) slope. Based on shear stress calculations, the minimum rock must be 36 inches in diameter. Therefore, a nominal diameter of 48 inches will be used with rock diameters ranging from 36 to 72 inches. The riprap section will have a thickness of 8 feet and the toe keyway will be 8 feet thick and 8 feet wide to support the riprap slope. Rock refusals are thickened edges of riprap used to prevent flanking and will be utilized at the upstream and downstream ends of the riprap. Conventionally, geotextile is placed under riprap to prevent substrate soils from mixing with the riprap. However, empirical evidence suggests the geotextile contributes to slope failure by increasing the severity of the slip plane under the riprap. Instead, wetland plants such as willows will be placed in the riprap at a 45 degree angle and at an elevation such that they will establish in the capillary fringe just above the normal water surface elevation. Some topsoil or loamy fill material may be needed to be placed in the riprap to help the wetland plants establish roots due to the thickness of the riprap.

The bank stabilization treatment above the bankfull elevation will include placing turf reinforcement mat and planting vegetation. Turf reinforcement mat consists of a straw and coconut fiber matrix sandwiched between synthetic netting and is generally anchored in place using landscaping staples. The turf reinforcement mat is permanent and provides primary stabilization until vegetation establishes and then continues to assist in stabilizing the river bank. Upland species such as dogwood, cottonwood, and range grasses will be planted above bankfull elevation to provide additional stabilization.

The river bank will be reconstructed between the historic (pre-flood) and current (post-flood) river bank locations. The alignment will restore the river bank’s gradual, curving alignment similar historic conditions, whereas currently, the river bank has an irregular alignment resulting from differing amounts of erosion and river bank loss. Approximately 715 linear feet of river bank will be reconstructed with this alternative. Earthwork required for construction of the river bank will include 4,265 cubic yards of excavation and 5,625 cubic yards of embankment construction. Approximately 1,360 cubic yards of fill material will be obtained by removing sediment from beneath the highway and railroad bridges. The fill material will be placed and compacted in lifts and armored upon completion. Construction equipment will be able to place fill while operating from the existing river bank. The equipment will build the reconstructed bank out into the river while remaining on dry ground throughout construction.

Additional sediment will be removed from beneath the highway and railroad bridges to restore their hydraulic capacity. Sediment will be removed to an elevation 24 inches above water level so the volume of sediment removed will depend on the water surface elevation during construction.


March 2012 54

Figure 10 – Preferred Alternative Site Plan


March 2012 55

An estimated 5,500 cubic yards of sediment will be removed and used as fill material for river bank construction or be disposed of offsite. Construction equipment access to the sediment removal area will be via Nutting Road located west of the railroad bridge and adjacent to the north river bank.

Maintenance of the reconstructed and stabilized river bank includes periodic inspection of the slope, adjustment or replacement of displaced or lost rock, and repair or replacement of displaced or torn turf reinforcement mats. Initially, stabilization plantings will need to be cared for to ensure they establish within the first growing season after planting. The projected service life for these improvements is 100 years if maintained properly.

Table 14 - Opinion of Probable Cost Preferred Alternative - Alternative 2B

Revetment Riprap Armoring - Bankfull and Below



1 Mobilization 1 LS $102,160.00 $102,160










SUBTOTAL $1,123,722

CONTINGENCY 10% $112,372

ENGINEERING DESIGN 11% $123,609

CONSTRUCTION ENGINEERING 8% $89,898

TOTAL $1,449,601

Permitting Construction in or around streams and rivers requires federal, state, and local permits to be obtained prior to construction. The project will be designed and constructed in accordance with state and federal stream permitting guidelines. The project will require permits from the U.S. Army Corps of Engineers (USACOE), the Montana Department of Fish, Wildlife, and Parks (MFWP), the Montana Department of Environmental Quality (MDEQ), the Montana Department of Natural Resources and Conservation (MDNRC), and Yellowstone County.

A 404 Permit from the ACOE is required for placement of fill within waters of the United States. This project is required to comply with the National Environmental Policy Act (NEPA) which requires the project to utilize the least environmentally damaging practicable alternative (LEDPA). Preliminary discussions with the ACOE have indicated this project will not qualify under the nationwide or regional general permit, and therefore, will require an individual permit. Individual permits are reviewed using 404(b)(1) process which includes obtaining public comment. The 404 permit appears to be the most difficult permit to obtain for a project of this type.


March 2012 56

The ACOE has indicated that compensatory mitigation will be required to offset the potential adverse environmental impacts created by this project. Compensatory mitigation includes restoration, enhancement, establishment, and/or preservation of aquatic and riparian resources. Riverside Park experienced anthropogenically accelerated stream bank erosion caused by increased velocities and expanding flows resulting from the railroad and Highway 212 bridges which are misaligned with the Yellowstone River and hydraulically undersized. According to Appendix D(h) of the ACOE Montana Stream mitigation Procedure, “Compensatory mitigation might not be required for projects addressing anthropogenically accelerated stream bank erosion beyond the applicant’s control.” The original scope of this project was to simply return the river bank to the location it has occupied for 100 years and protect it in that location. However, it became apparent early on that permitting would very difficult so the project was modified to minimize the scope of bank reconstruction to only that which was necessary to provide reliable flows to the intake structure. Additionally, the project will provide modest geomorphological benefits in an impaired reach of the Yellowstone River. Therefore, it is our opinion that compensatory mitigation is not warranted.

However, in an effort to practice good stewardship and to expedite the 404 permitting process the City of Laurel has agreed to several compromises. One of which is that the City of Laurel will be purchasing 1,235 stream mitigation credits from EcoAsset Management Inc., an approved mitigation bank, to offset any perceived adverse impacts created by the project.

A SPA 124 Permit is administered by the MFWP for projects affecting the bed or banks of any stream in Montana and is used to protect and preserve fish and wildlife resources. A 318 Permit is obtained from MDEQ for projects temporarily violating state surface water quality standards for turbidity. Its purpose is to protect water quality and minimize sedimentation and turbidity. A 401 Certification is also obtained from MDEQ and is for projects that dredge and fill within state waters. A Stormwater Discharge Permit is also required by MDEQ for stormwater discharges during construction for projects disturbing more than 1 acre. All of these state agencies have been contacted regarding the project and have indicated for their support for the project and that their respective permits will be issued once the 404 permit is obtained.

The Yellowstone River is considered a Navigable River, and therefore, a State Lands Permit is required for work which takes place below the low water mark. This project will be disturbing the river bed regardless of which alternatives are selected, so this permit will be required. The project also requires a Floodplain Development Permit from Yellowstone County because the Yellowstone River lies within a FEMA mapped floodplain. The Floodplain Development Permit has already been issued.

Funding The project will be funded with the Federal Emergency Management Agency’s (FEMA) Public Assistance grant program. This programs pays for providing emergency services during a disaster, clean up of disaster debris, repair or replacement of damaged infrastructure, and hazard mitigation improvements. Public Assistance funds are available to communities affected by major disasters. The program is structured to provide 75% of the funds for repair or replacement and the community and/or the state pay the remaining 25%. The Riverside Park Bank Stabilization project has received approval of funding through the Public Assistance program. The State of Montana has committed to paying the remainder of the costs for the project.

Upon completion of final design, the project will be let for competitive bids. The project will most likely be awarded to a single contractor contingent on the winning bid falling within the funding limits.

Project Schedule The City of Laurel would like to see this project constructed in Spring of 2012. The project must be constructed during low water to minimize turbidity in the Yellowstone River. If construction cannot


March 2012 57

be completed prior to spring runoff, it will be delayed until Fall of 2012. In order to meet this aggressive timetable, permits will need to be obtained by March of 2012. Permit applications have already been submitted to the respective agencies and are currently being reviewed.

Once permits are issued, letting the contract is anticipated to take 30 days. At the earliest, construction could begin at the beginning of May 2012. However, historically this is the beginning of spring runoff and increased flows which will require construction to be postponed until fall of 2012.

APPENDIX A

SITE PHOTOS

APPENDIX B

HISTORIC BLM GLO MAPS

APPENDIX C

HYDRAULIC ANALYSIS OUTPUTS

EXISTING CONDITIONS

2007 DRAFT FLOOD INSURANCE STUDY

CURRENT CONDITIONS

SEPTEMBER 2011

PROPOSED CONDITIONS

2012

VELOCITY DISTRIBUTION EXHIBITS

BANKFULL FLOW

VELOCITY DISTRIBUTION EXHIBITS

Q100 FLOW

APPENDIX D

ENVIRONMENTAL DOCUMENTATION

APPENDIX E

“EVALUATION OF THE SEDIMENT DEPOSITION PROBLEMS ALONG THE YELLOWSTONE RIVER NEAR LAUREL, MONTANA”

Prepared by U.S. Army Corps of Engineers

April 2000

city of laurel - townnewsbloximages.chicago2.vip.townnews.com/billingsgazette.com/... ·...

Documents