4.9 geomorphology 4.9.1 regulatory settinggeomorphology 4.9-2 sacramento area flood control agency...

4.9 GEOMORPHOLOGY

This section describes geomorphic conditions and processes with the potential to be affected by implementation of the proposed project. The section analyzes temporary and short-term impacts from construction activities, potential impacts during the interim period as vegetation plantings are established, and foreseeable long-term impacts after project implementation. Potential geomorphic impacts analyzed include bank erosion, channel incision, sediment transport, and sedimentation.

4.9.1 REGULATORY SETTING

FEDERAL

Sacramento River Bank Protection Project

Originally authorized by Section 203 of the Flood Control Act of 1960, the Sacramento River Bank Protection Project (SRBPP) is a long-term flood risk management project designed to enhance public safety and help protect property along the Sacramento River and its tributaries. The goal of the SRBPP is to evaluate the levees bordering the river and reduce stream bank erosion along them to minimize the threat of a flood along the Sacramento River. While the original authorization approved the rehabilitation of an estimated 430,000 linear feet of levee, the 1974 Water Resources Development Act added an estimated 405,000 linear feet to the authorization and a 2007 bill authorized another 80,000 linear feet, for a total of approximately 915,000 linear feet of levee protection. The U.S. Army Corps of Engineers (USACE) is currently developing a Post-Authorization Change Report, including an environmental impact statement, to address the effects of the latest authorization. USACE, Sacramento District is responsible for implementation of the SRBPP in conjunction with its non-Federal partner, the California Central Valley Flood Protection Board (CVFPB). The SRBPP provides guidelines for levee evaluation and bank protection for the proposed project.

U.S. Army Corps of Engineers Levee Design Criteria

Levees included in the project study area are Federally authorized and fall within the jurisdiction of USACE. The levee evaluation for the project area conforms to the engineering criteria established by USACE for the assessment and repair of levees. The primary USACE guidance is contained in Engineering Manual (EM) 111--1913 Design and Construction of Levees (USACE 2000). In addition, Engineering Technical Letters (ETL) 1110-2-569, Design Guidance for Levee Underseepage (USACE 2005) and 1110-2-555, Design Guidance on Levees (USACE 1997), contain guidance that is applicable to the proposed levee improvements.

STATE

Urban Levee Design Criteria

California Government Code Sections 65865.5, 65962, and 66474.5 requires that levees and floodwalls in the Sacramento-San Joaquin Valley provide protection against a flood that has a 1-in-200 chance of occurring in any given year. The Urban Levee Design Criteria (ULDC) prepared by the California Department of Water Resources (DWR) (2012) provides engineering criteria and guidance for civil engineers in meeting the government code requirements, and offers this same guidance to civil engineers working on levees and floodwalls anywhere in California. The ULDC also provides engineering criteria and guidance for DWR’s urban levee evaluations and

North Sacramento Streams, Sacramento River East Levee, Lower American River, and Related Flood Improvements Project DEIR AECOM Sacramento Area Flood Control Agency 4.9-1 Geomorphology

participation in urban levee projects. These guidelines apply to the levee accreditation components of the proposed project.

REGIONAL AND LOCAL

Regional Flood Management Plan

The Regional Flood Management Plan (RFMP) for the Lower Sacramento/Delta North Region (FloodProtect 2014), also known as FloodProtect, is the regional follow-on to the DWR 2012 Central Valley Flood Protection Plan (CVFPP). The overarching goal of FloodProtect is to develop long-term, sustainable integrated flood management in the region that involves all regional stakeholders. This includes recommending feasible structural and nonstructural improvements to achieve 200-year level of protection in urban and urbanizing areas including Sacramento, while providing environmental enhancement. These guidelines apply to the levee accreditation, vegetation management, and Conservation Strategy components of the proposed project.

Sutter County General Plan

No Sutter County General Plan policies related to geomorphic conditions apply to the proposed project.

Sacramento County General Plan

The following policies from the Sacramento County General Plan of 2005-2030 Conservation Element regarding geomorphic conditions apply to the proposed project (Sacramento County 2011:2, 9-59).

► Policy CO-25: Support the preservation, restoration, and creation of riparian corridors, wetlands and buffer zones. (Applies to project-related planting components of the Conservation Strategy and Natomas East Main Drainage Canal [NEMDC]/Steelhead Creek Corridor Management Plan [CMP] pertaining to geomorphology.)

► Policy CO-26: Protect areas susceptible to erosion, natural water bodies, and natural drainage systems. (Applies to project-related components of the levee improvements pertaining to geomorphology.)

► Policy CO-96: Reduce dependence on traditional levee protection methods where those methods conflict with habitat preservation efforts and where alternate methods exist which are compatible with preservation efforts and offer an acceptable level of bank stabilization. (Applies to project-related components of the levee improvements and Conservation Strategy pertaining to geomorphology.)

► Policy CO-97: Work with appropriate regulatory agencies to reduce bank and levee erosion by minimizing erosive wake activity generated by recreational and commercial boating. (Applies to project-related components of the levee improvements pertaining to geomorphology.)

► Policy CO-98: Coordinate with federal, state and local agencies overseeing levee and bank stabilization to investigate and, whenever possible, utilize biotechnical or nonstructural alternatives to other conventional stabilization methods. (Applies to project-related components of the levee improvements and Conservation Strategy pertaining to geomorphology.)

AECOM North Sacramento Streams, Sacramento River East Levee, Lower American River, and Related Flood Improvements Project DEIR Geomorphology 4.9-2 Sacramento Area Flood Control Agency

► Policy CO-100: Encourage construction of structures for flood control and stormwater quality purposes using currently approved scientific methods to prevent erosion and stabilize the banks. (Applies to project-related components of the levee improvements and Conservation Strategy pertaining to geomorphology.)

► Policy CO-101: Stabilize the banks of rivers and streams in a manner that increases flood protection and increases riparian habitat functions. (Applies to project-related components of the levee improvements and Conservation Strategy pertaining to geomorphology.)

► Policy CO-106: Realigned or modified channels should retain topographic diversity including maintaining meandering characteristics, varied berm width, naturalized side slope, and varied channel bottom elevation. (Applies to project-related components of the levee improvements, Conservation Strategy, and NEMDC/Steelhead Creek CMP pertaining to geomorphology.)

► Policy CO-107: Maintain and protect natural function of channels in developed, newly developing, and rural areas. (Applies to project-related components of the Conservation Strategy and NEMDC/Steelhead Creek CMP pertaining to geomorphology.)

► Policy CO-122: River and stream maintenance should allow natural vegetation in and along the channel to assist in removal of nutrients, pollutants, and sediment and to increase bank stabilization, while minimizing impacts on conveyance. (Applies to project-related components of the Conservation Strategy and NEMDC/Steelhead Creek CMP pertaining to geomorphology.)

City of Sacramento 2030 General Plan

The following policies from the City of Sacramento 2030 General Plan Environmental Resources Element regarding geomorphic conditions apply to the proposed project (City of Sacramento 2009:2, 303-327).

► Policy ER 1.1.7: Construction Site Impacts. The City shall minimize disturbances of natural water bodies and natural drainage systems caused by development, implement measures to protect areas from erosion and sediment loss, and continue to require construction contractors to comply with the City’s erosion and sediment control ordinance and stormwater management and discharge control ordinance. (Applies to all project-related components pertaining to geomorphology.)

4.9.2 ENVIRONMENTAL SETTING

GENERAL GEOMORPHIC CHARACTERISTICS OF THE PROJECT STUDY AREA

The project study area is located in the Sacramento Valley portion of the Great Valley geomorphic province of California. The Great Valley is bordered by the Coast Ranges to the west, the Sierra Nevada Mountains to the east, and the Klamath Mountains to the north. This geomorphic province stretches approximately 400 miles north-to-south and approximately 50 miles east-to-west, and is characterized by a relatively flat alluvial plain.

Two major river systems drain the Great Valley: the San Joaquin River to the south and the Sacramento River to the north. The two rivers meet south of the City of Sacramento at the Sacramento-San Joaquin Delta (Delta), where flows eventually discharge to the Pacific Ocean via the San Francisco Bay. The Sacramento Valley has been a sediment deposition zone throughout most of the last 180 million years. Sediments eroded from the


adjacent mountain ranges have been transported by tributary watercourses to settle on the channel bottom and adjacent floodplain. Sediment deposits in the Great Valley vary between 3–6 miles in thickness.

Over the past 11,700 years, the Sacramento River has functioned as a dynamic fluvial system, frequently overtopping its banks and flooding while meandering across the valley floor (Helley and Harwood 1985). Much of the dynamics of the Sacramento River were driven by contributions of flow and sediment from its tributaries, particularly in the Sierra Nevada Mountain range. The largest tributary in the immediate project study area is the American River, which joins the Sacramento River just north of downtown Sacramento.

Geomorphic processes and conditions of the Lower Sacramento River and its tributaries have been increasingly modified since the discovery of gold in the region in 1849. These modifications include flood-control levee construction, in-channel dredging, alteration to natural flow and sediment regimes due to dam construction and water diversions, residual effects of hydraulic mining in the Sierra Nevada Mountains, and land reclamation. Planform geometry of the Sacramento River in the project study area has been fixed by levees and bank protection measures since the late 1800s. Hydraulic mining in the Sierra Nevada during second half of the 19th century led to large volumes of sediment delivered by tributaries such as the American River, which were subsequently deposited in the Sacramento River (James 1989, 1991). This exacerbated flooding due to the decreased cross-sectional area, and coincided with several levee failures. This in turn resulted in additional levee construction (James 1999).

To the north of the American River, several smaller tributaries drain the Sierra Nevada Mountains and Foothills, predominantly towards the southwest direction. Historically, flood flows from these streams would enter and flood what is now the Natomas flood basin, forming a series of marshes and lakes, and eventually reaching the Sacramento River. Urbanization in the Sacramento area has led to the development of the Natomas Basin. To allow development within the Basin, the NEMDC, also referred to as Steelhead Creek, was constructed to divert flows away from the Basin. NEMDC/Steelhead Creek intersects these streams, which include Dry Creek, Robla Creek, Arcade Creek, and Magpie Creek, at a perpendicular angle, delivering flows directly to the American River. NEMDC/Steelhead Creek and several of its tributaries are located in the North Sacramento Streams Levee Improvements area.

Substrate composition beneath the existing levees is highly relevant to the levee being susceptible to underseepage, therefore an understanding of the geomorphology of the project study area, along with geological and geotechnical investigations, is used to predict potentially problematic levees (Llopis et al. 2007). The historical dynamics of the Sacramento River and its tributaries left behind a mosaic of heterogeneous and permeable coarse-grained abandoned channels, also known as paleochannels. Locations where the modern levee systems overlay either historically or recently deposited permeable material present the highest risk of underseepage (Fugro William Lettis & Associates 2010, 2011; Kleinfelder 2013). In addition, legacy deposits of hydraulic mining material present additional hazards to levee foundations. Thus, understanding the geomorphic processes behind the distribution of paleochannels and the historical land use in the Basin are important to determining areas where analysis related to levee improvements for the proposed project should be initiated. The geomorphic conditions specific to the North Sacramento Streams and Sacramento River East Levee Improvements areas, where levee improvements are proposed, are discussed in more detail below.


NORTH SACRAMENTO STREAMS LEVEE IMPROVEMENTS

Geomorphic Setting

The following summary is adapted primarily from the geotechnical report by Kleinfelder (2013) and the geological and geomorphic assessment conducted by Fugro William Lettis & Associates (2011). The North Sacramento Streams Levee Improvements area includes leveed reaches of NEMDC/Steelhead Creek and its tributary Arcade Creek. Flows in Arcade Creek originate from the northern Sierra Nevada Foothills and drain west-southwest, where they are intercepted by NEMDC/Steelhead Creek and diverted south to the American River, approximately 1.5 miles east of the confluence with the Sacramento River.

NEMDC/Steelhead and Arcade Creeks are entrenched in semi-consolidated alluvial sediments of the Riverbank and Modesto Formations, dating back to the late Pleistocene (125,000 years ago). These two formations are composed of alluvial fans and terraces derived from erosional episodes in the Sierra Nevada and deposition on the Sacramento Valley floor. They represent a period of substantial deposition and fan building followed by local erosion and backfilling, which formed medium dense to very dense deposits. More recent (less than 11,700 years old) and loosely consolidated Holocene deposits are eroded into the Riverbank and Modesto Formations and generally follow the southwest-trending drainage pattern of streams in the North Sacramento Streams Levee Improvements area, eventually forming alluvial fans at the mouths of the creeks. Layers of sediment from the flooding of the American River Basin also have accumulated throughout the North Sacramento Streams Levee Improvements area.

Levee Through-Seepage, Underseepage, and Levee Erosion

The levees constructed along the banks of creeks in the North Sacramento Streams Levee Improvements area differ from other project levees built along large river systems such as the Sacramento and Feather Rivers, which generally overlay recently deposited alluvium, including hydraulic mining deposits (James 1999). The levees in the North Sacramento Streams Levee Improvements area are generally built over relatively old, less permeable deposits (Fugro William Lettis & Associates 2011). Fugro William Lettis & Associates (2011) performed detailed surficial geologic mapping and geomorphic assessments of streams in the North Sacramento Streams Levee Improvements area, which include NEMDC/Steelhead Creek, Dry Creek, Robla Creek, Arcade Creek, and Magpie Creek. The purpose was to develop a conceptual geomorphic model to extrapolate stratigraphy between subsurface explorations and create a spatially continuous geologic map. The mapping provides information on the type and distribution of surface and shallow subsurface deposits likely to underlie project levees, and thus partially control seepage processes. Based on the results of this mapping, they identified the reaches of levee to be most susceptible to underseepage due to shallow subsurface stratigraphy.

In general, Fugro William Lettis & Associates (2011) found that most of the levees in the North Sacramento Streams Levee Improvements area are underlain by the relatively low-permeability Riverbank and Modesto Formations, which may inhibit underseepage. They noted that these formations, along with coarse-grained alluvium, may contain buried coarse-grained paleochannels that may provide localized pathways for underseepage. Reaches identified with potential susceptibility to underseepage consist of the eastern NEMDC/Steelhead Creek Levee, which traverses the mouth of Arcade Creek, as well as both the north and south levees in the lower reach of Arcade Creek.


To identify specific work sites, URS Corporation (2014b) synthesized existing information on geology and geomorphology from several previous reports, particularly those by Kleinfelder (2013) and Fugro William Lettis & Associates (2011). Combining the geologic mapping and geomorphic assessments with geotechnical analysis and modeling, URS Corporation (2014b) developed a Problem Identification Report (PIR). The PIR identified levee reaches where improvements are required to meet criteria (Table 4.9-1). These improvements consist of approximately 3.5 miles on the north and south banks of Arcade Creek and approximately 0.5 mile of levee along the east bank of NEMDC/Steelhead Creek (about 4 miles total), which were evaluated in the Alternatives Analysis Report prepared by URS Corporation (2014a).

Table 4.9-1. North Sacramento Streams Levee Reaches Requiring Improvement

Reach Approximate Levee Stationing

Reach Length (feet)

Geotechnical Conditions Requiring Improvement

NEMDC East Levee NAS-NE-2 3034+00 to 3051+00 1,700 Underseepage

NEMDC East Levee NAS-NE-3 3130+00 to 3135+00 500 Underseepage Stability

Arcade Creek South Levee NAS-ARS-1 4000+00 to 4013+00 1,300 Underseepage Stability

Arcade Creek South Levee NAS-ARS-3 4031+18 to 4068+45 3,727 Underseepage Through-seepage

Stability

Arcade Creek South Levee NAS-ARS-4 4068+45 to 4110+00 4,155 Underseepage Through-seepage

Stability

Arcade Creek North Levee NAS-ARN-1 5023+00 to 5033+00 1,000 Underseepage

Arcade Creek North Levee NAS-ARN-3 5038+00 to 5075+00 3,700 Underseepage Stability

Arcade Creek North Levee NAS-ARN-4 5075+00 to 5112+43 3,743 Underseepage Through-seepage

Note: NEMDC = Natomas East Main Drainage Canal Source: URS Corporation 2014a, data compiled by cbec Eco Engineering in 2014

Levee Erosion

MBK Engineers (2014) conducted an analysis to determine whether the levees in the North Sacramento Streams Levee Improvements area were prone to substantial erosion during the base flood. They combined several types of assessments, including historical, hydraulic, geometric, geotechnical, and field assessments to determine the adequacy of existing embankment protection. The results of the analyses suggested that none of the levees in the North Sacramento Streams Levee Improvements area are susceptible to substantial erosion, primarily due to low modeled velocities and short flow durations.


SACRAMENTO RIVER EAST LEVEE IMPROVEMENTS

Geomorphic Setting

This summary is primarily adapted from the geotechnical report prepared by Kleinfelder (2013) and a surficial geological mapping and geomorphic assessment conducted by Fugro William Lettis & Associates (2010), in conjunction with several other related documents.

The levee improvements associated with the Sacramento River East Levee Improvements area span approximately 8.1 miles from south of R Street to north of the East Bay Municipal Utility District (EBMUD) Freeport facility in the Pocket area. This stretch of levee begins just downstream of the confluence of Sacramento and American Rivers and downtown Sacramento. The Sacramento River East Levee Improvements area has been mapped by a number of geologists on a regional scale, including published maps by Jennings et al. (1977), Wagner et al. (1981), and Helley and Harwood (1985). Most recently, the Sacramento River East Levee Improvements area was mapped by Fugro William Lettis & Associates (2010) for DWR’s Urban Levee Evaluation project. Fugro William Lettis & Associates mapped surficial geologic units at a scale of 1:20,000, the most detailed in the Sacramento River East Levee Improvements area. They also developed a conceptual geomorphic model to describe the complex fluvial processes that determine the surface and subsurface distribution of different sized alluvial materials.

The present-day Sacramento River system has been shaped by thousands of years of complex river processes. Historically, the Sacramento River meandered freely, eroding the banks along the outside of beds and depositing sediment along the inside of bends (Shlemon 1967). During frequently occurring flood events, coarse sediment was deposited along near the river bank, and fine sediments were carried across the floodplain as floodwaters disperse. Where floodwaters concentrated, overflow channels formed, and abandoned meanders formed oxbow lakes. This historical dynamics of the river and subsequent confinement by levees left behind a mosaic of abandoned activated channels, also known as paleochannels.

Today, these abandoned channels are characterized as infilled depressions and historical lake bodies, underlain by coarse-grained material. In summary, Fugro William Lettis & Associates (2010) concluded that much of the levee in the Sacramento River East Levee Improvements area is underlain by geologically young, unconsolidated, silty and sandy fluvial deposits, which have a relatively high potential for shallow subsurface seepage given certain hydraulic conditions.

With the cessation of hydraulic mining in the early 1900s, combined with the construction of dams on the tributaries to the Sacramento River through the mid-20th century, the Sacramento River system experienced a drastic reduction in sediment supply. As a result, this was followed by a period of bed incision and channel widening (cbec Eco Engineering 2011). New levees were constructed and additional levees were fortified to reduce lateral erosion, but this also exacerbated channel incision. Channel incision has since reduced or reached equilibrium; however, bank erosion continues to occur in the leveed reaches (Northwest Hydraulic Consultants [nhc] 2010).

The present day geomorphology of the Sacramento River in the Sacramento River East Levee Improvements area is characteristic of Delta waterways, which are bordered by levees that are low-gradient with a deep, sandy bed with low-velocity flatwater. Flows are controlled by a network of dams and the river is subject to diurnal tidal fluctuation. Local changes in depositional bars and in-channel sedimentation features have been observed, but are minimal in affecting channel morphology (cbec Eco Engineering 2011).


Levee Through-Seepage, Underseepage, and Levee Erosion

Seepage beneath and through segments of the levee systems around Sacramento has been identified as a substantial risk to the stability and reliability of the system. Through-seepage is seepage through a levee embankment that can occur during periods of high river stages. Underseepage problems occur in locations where levees are constructed on low-permeability foundation soil (silt and clay) underlain by higher-permeability layers (sand and gravel). Excessive underseepage makes the affected levee segment susceptible to failure during periods of high river stage. Under these conditions, seepage travels horizontally under the levee and then is forced vertically upward through the low-permeability foundation layer, often referred to as the “blanket.” Failure of the blanket can occur either by uplift—a condition in which the blanket does not have enough weight to resist the confined pressure acting upon the bottom of the blanket—or by piping (internal erosion) caused by water flowing under high vertical gradients through the erodible blanket and carrying fine soil particles out of the foundation materials.

The location of permeable layers are directly related to geomorphic history of the Sacramento River East Levee Improvements area, more specifically the natural levees, meander scrolls, and paleochannels of historical channel configurations, are described above. The Sacramento River East Levee Improvements area has been mapped by a number of geologists on a regional scale, including published maps by Jennings et al. (1977), Wagner et al. (1981), and Helley and Harwood (1985). Most recently, the Sacramento River East Levee Improvements area was mapped by Fugro William Lettis & Associates (2010) for DWR’s Urban Levee Evaluation project. Fugro William Lettis & Associates mapped surficial geologic units at a scale of 1:20,000 and developed the most detailed geologic mapping available for the Sacramento River East Levee Improvements area.

Fugro William Lettis & Associates found a complex relationship of fluvial deposits at the surface and beneath the eastern floodplain of the Sacramento River. In general, they found that most of the levee in the Sacramento River East Levee Improvements area is underlain by unconsolidated, permeable material, and thus susceptible to shallow subsurface seepage. In addition, they also mapped reaches that were determined to be more prone than others to underseepage because they are underlain coarser materials of paleochannels and Holocene meander scrolls. GEI Consultants, Inc. and HDR, Inc. (HDR) (2014a) performed a levee embankment and foundation stability evaluation on the levee within the Sacramento River East Levee Improvements area, incorporating the findings of Kleinfelder (2013) and Fugro William Lettis & Associates (2010), and determined approximately 5.1 miles of the 8.1 miles evaluated had subsurface conditions that required measures to address seepage concerns. Table 4.9-2 provides a summary of reaches within the Sacramento River East Levee Improvements area with potential seepage issues.

Levee Erosion

As discussed above, levees along the Sacramento River continue to confine the flow into a relatively narrow channel, and erosion is still present. In addition, the system that had an artificially high sediment load (primarily due to the residual effects of hydraulic mining) during the time of levee construction is now a sediment-starved system. A sediment-starved river often experiences increased erosive energy (Kondolf 1997). Although bank erosion is a natural process, it could have detrimental effects on levees by undercutting the foundation, which can increase underseepage and decrease levee stability, and ultimately lead to levee failure. Additionally, it is suspected that most of the levees were constructed with unconsolidated hydraulic mining deposits dredged from the river (cbec Eco Engineering 2011), potentially making these reaches more susceptible to erosion.


Table 4.9-2. Preliminary Geotechnical Evaluation of Sacramento River East Levee Reaches for Federal Emergency Management Agency (100-year flood) Accreditation

Approximate Levee Stationing Meets Criteria

Does Not Meet Criteria Potential Deficiencies1

1000+00 to 1040+00 -

1040+00 to 1080+00 -

1080+00 to 1125+75 Through-seepage, underseepage exit gradients above 0.5 and stability FOS <1.4

1125+75 to 1150+00 Underseepage exit gradients above 0.5 and stability FOS <1.4

1150+00 to 1185+00 -

1185+00 to 1206+00 Underseepage exit gradients above 0.5

1206+00 to 1244+00 -


1261+75 to 1285+75 -




1418+00 to 1424+50 -


1490+00 to 1540+00 Underseepage exit gradients landward of the levee toe above 0.5


1570+00 to 1600+00 -


1640+00 to 1710+29 -

1710+29 to 1730+00 -

1730+00 to 1770+00 -

Notes: FOS = Factor-of-Safety 1 Potential deficiencies identified based upon minimum criteria per USACE EM-1110-2-1913 Source: GEI Consultants, Inc. and HDR 2014a, data compiled by cbec Eco Engineering in 2014

nhc (2010) conducted an evaluation of the historical channel migration of this portion of the Sacramento River and concluded that lateral channel movement in the Sacramento River East Levee Improvements area has been limited due to the active measures led by USACE in implementing and maintaining revetments along the bank line using rock material as the primary use for erosion protection (as part of SRBPP). However, shallow wake erosion and deep toe scour erosion remain a concern.

The existing levees along the Sacramento River are in close proximity to the river banks. Since 2005, DWR, SAFCA, and USACE have implemented a number of levee repair and enhancement projects. cbec Eco Engineering (2011) completed a field inventory of bank protection measures from the Sacramento Deep Water Ship Canal (approximately River Mile 58) downstream to approximately River Mile 52, or the Pocket area.


Downstream of Chicory Bend (River Mile 54) also in the Pocket area, the levee is largely protected by rock revetments, however upstream of this point, approximately half of the levee is protected.

MBK Engineers (2014) evaluated erosion potential by considering historical performance, hydraulic analysis of the 100-year flood event, geometric criteria, geotechnical soil conditions, and field observations. In summary, one erosion site was recommended for immediate bank protection measures prior to Federal Emergency Management Agency (FEMA) accreditation, eight erosion sites were recommended for corrective action within a 3-year time window of accreditation, and 11 sites were identified as requiring regular monitoring before and after a high-water event, but not requiring immediate or short-term corrective action (Table 4.9-3). The nature of erosion at the sites requiring repair ranges from shallow, wake-generated erosion to deeper toe scour erosion (GEI Consultants, Inc. and HDR 2014a). The nine sites, which comprise approximately 3,000 linear feet and require immediate or short-term corrective action, are included in the proposed project.

Table 4.9-3. Recommended Action for Each Erosion Site along Sacramento River East Levee

Site (USGS RM) Length (feet) Recommended Action 1 46.76 162 Monitor

2 47.6 100 Maintenance


4 48.7 500 Monitor



7 49.95 200 Monitor


9 50.4 89 Monitor

10 51.63 50 Monitor

11 51.85 100 Monitor

12 52.65 117 Monitor


14 54.8 500 FEMA Action



17 56.62 262 Monitor

18 56.73 100 Monitor

19 58.5 386 Monitor

20 59.6 2000 Monitor

Notes: FEMA = Federal Emergency Management Agency; USGS RM = U.S. Geological Survey River Mile Text in bold indicates FEMA action Source: MBK Engineers 2014, data compiled by cbec Eco Engineering in 2014


AMERICAN RIVER AND BEACH LAKE LEVEES HIGH-HAZARD LEVEE ENCROACHMENT AND VEGETATION REMOVAL

American River

This portion of the American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area is located along the leveed reaches of the river from the confluence with the Sacramento River upstream to approximately 1 mile east of Watt Avenue. Like the Sacramento River and many of its tributaries that originate in the Sierra Nevada Mountains, the geomorphic conditions are highly modified. These modifications include flood-control levee construction, in-channel dredging, alteration to natural flow and sediment regimes due to dam construction and water diversions, residual effects of hydraulic mining in the Sierra Nevada Mountains, and land reclamation.

Prior to the discovery of gold in the Sierra Nevada, the Lower American River was a low-gradient meandering system with a well-developed and connected floodplain. The American River was also affected by hydraulic mining in the 1800s, which caused several feet of channel bed aggradation as well as floodplain deposition (Gilbert 1917). As the American River began to recover from hydraulic mining sediment deposits in the early 1900s, several dams were constructed between 1930 and 1960 on the American River and its tributaries for flood control and hydropower. The dams trapped sediment and converted the system to a sediment-starved river, exacerbating channel incision. Along with confinement by flood control levees, this resulted in thalweg (location of strongest flow in the deepest part of the river channel) incision of approximately 6.5 feet between 1924 and 1970 (James 1997). Channel incision and hydromodification have resulted in less frequent inundation of the adjacent floodplains, and reduced riparian habitat.

Beach Lake

This portion of the American River and Beach Lake Levee High-Hazard Levee Encroachment and Vegetation Removal area is located along approximately 2.5 miles of the north levee of the Morrison Creek floodplain (i.e., the Beach Lake Levee) from the Interstate 5 (I-5) crossing upstream to the Union Pacific Railroad (UPRR) crossing and confluence with Union House (Beacon) Creek. The Morrison Creek floodplain includes Beach Lake, a perennial wetland and wildlife refuge.

Before interventions beginning in the mid-1800s, the floodplain was once a vast seasonal wetland that, during winter and spring floods, stretched from Morrison Creek 10 miles to the south along the Sacramento River to the Mokelumne River floodplain. Beach and Stone Lakes were large perennial wetlands within this complex. Levee construction along the Sacramento River and Delta islands in the mid-1800s decreased flooding frequency and converted floodplain to farmland. Construction of the UPRR and I-5 bisected the floodplains and constricted flood flows. Morrison Creek and its tributaries, which include Laguna, Florin, and Elk Grove Creeks, were relocated and channelized throughout the mid-20th century. Levees were constructed along Morrison Creek and its floodplain to protect the rapidly urbanizing south Sacramento area, and were raised in 1999 to provide 500-year protection.

NATOMAS EAST MAIN DRAINAGE CANAL/STEELHEAD CREEK CORRIDOR MANAGEMENT PLAN

The environmental setting related to the NEMDC/Steelhead Creek CMP activities related to geomorphology would be the same as described above for the North Sacramento Streams Levee Improvements.


4.9.3 ENVIRONMENTAL IMPACTS AND MITIGATION MEASURES

METHODOLOGY

This analysis of the geomorphic impacts of the proposed project focuses on the effects of both the construction and the long-term operations of the levee improvements. Temporary and short-term impacts on geomorphology could occur from ground-disturbing activities and other construction-related activities along the levee reaches where improvements would occur over the course of the anticipated construction seasons (1 year for North Sacramento Streams and 2 years for Sacramento River East Levee). Plantings may require several more seasons to establish and stabilize disturbed areas. The analysis of environmental impacts focuses on how the presence of improved levees and other proposed project elements may affect geomorphic conditions and processes.

SIGNIFICANCE CRITERIA

The thresholds for determining the significance of impacts for this analysis are based on the environmental checklist in Appendix G of the State CEQA Guidelines. The proposed project would have a significant impact on geomorphology if implementation of the proposed project would:

► substantially alter the existing drainage pattern of the site or area, including through the alteration of the course of a stream or river, in a manner that would result in substantial erosion or siltation on- or off-site.

In addition to the thresholds listed above, the proposed project would have a significant impact on geomorphology if implementation of the proposed project would increase any of the following:

► through- or underseepage (which would decrease levee stability), ► bank erosion or bed incision (which may increase seepage or decrease levee stability), or ► substantial scour/sediment deposition (which may alter geomorphic conditions of the river channel).

IMPACT ANALYSIS

IMPACT GM-1

Potential Temporary and Short-term Erosion during Construction. Ground-disturbing activities associated with levee reconstruction, encroachment removal, vegetation management, the Conservation Strategy, and the NEMDC/Steelhead Creek CMP could expose slopes that are susceptible to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this temporary and short-term impact would be potentially significant throughout the project study area.

North Sacramento Streams Levee Improvements

Levee improvements in the North Sacramento Streams Improvements area consist of approximately 3.5 miles on the north and south banks of Arcade Creek and approximately 0.5 mile of levee along the east bank of NEMDC/Steelhead Creek (about 4 miles total), which were evaluated in the Alternatives Analysis Report prepared by URS Corporation (2014a). Construction-related activities associated with levee improvements would entail ground-disturbing activities that could expose slopes that are susceptible to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow


patterns in the river. Therefore, these proposed project elements would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.

An analysis performed to assess existing levee encroachments for compliance with FEMA, USACE, CVFPB, and ULDC criteria, identified 209 encroachments in the North Sacramento Streams Improvements area, including 139 encroachments on Arcade Creek and 70 encroachments on NEMDC/Steelhead Creek (Mead & Hunt 2013: Exhibit 4). Approximately 14 encroachments have been identified on Arcade Creek as “unacceptable” per USACE rating guidelines including permitted and unpermitted structures (Mead & Hunt 2013: Exhibit 4). These high-risk encroachments consist predominantly of dense and low-height landscape vegetation, and as retaining walls. No encroachments are rated “unacceptable” on NEMDC/Steelhead Creek. Their removal would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.

Levee accreditation would involve the removal of high-hazard levee vegetation including trees, woody shrubs, and non-walkable herbaceous groundcover. Approximately 14 high-hazard trees would be removed along Arcade Creek in the North Sacramento Streams Improvements area (AECOM 2014). Their removal would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.

Mitigation Measure: Implement Mitigation Measure GEO-1 (Acquire Appropriate Regulatory Permits and Prepare and Implement a Storm Water Pollution Prevention Plan and Associated Best Management Practices).

Timing: Submittal of the State Construction General Permit NOI and SWPPP before the start of earthmoving activities, and implementation of BMPs throughout project construction.

Responsibility: Sacramento Area Flood Control Agency.

Significance after Mitigation: Implementation of Mitigation Measure GEO-1 would reduce potentially significant temporary, short-term construction-related erosion impacts to a less-than-significant level by requiring preparation and implementation of a SWPPP with appropriate BMPs such as source control and revegetation to reduce erosion and maintain surface water quality conditions in adjacent receiving waters.

Implementation of the Conservation Strategy in the North Sacramento Streams Improvements area would compensate for adverse effects associated with levee improvements, encroachment removal, and vegetation management. The Conservation Strategy would entail riparian plantings in various locations in the project study area. Riparian plantings, including riparian trees, are proposed to occur on the floodplain of Arcade Creek. Riparian plantings are also proposed on approximately 7 acres of the Novak parcel (located northeast of the intersection of Garden Highway and Powerline Road) and on approximately 6 acres of the Riego North site (located on the east side of the Sacramento River near the intersection of Garden Highway and Riego Road).


Riparian plantings are also proposed at either Site A (which is also Borrow Site 3) or Site B on the north side of Robla Creek. These plantings would take place on the landside of the levees.

Vegetation planted in flood-prone areas would be monitored during and after the flood seasons following construction, as a standard adaptive management practice in floodplain restoration. Flood damage to plantings would be repaired and associated post-flood conditions (such as sedimentation and deposition of flood debris) would be assessed for their implications on original Conservation Strategy design objectives and continued operations and maintenance.

Plantings associated with the Conservation Strategy would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition downstream in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measures GEO-1 and GM-1, described below, have been identified to address this impact.




Mitigation Measure GM-1: Implement Conservation Strategy Measures to Reduce Erosion and Sediment Transport.

SAFCA shall implement the following measures as part of the Conservation Strategy to reduce erosion and sediment transport:

• CM-1: Limit Ground Disturbance to Construction Areas and Avoid and Limit Disturbance to River and Creek Banks and Habitats when Feasible. Ground disturbance shall be limited to construction areas, including necessary access routes and staging areas. The number of access routes, size of staging areas, and total area of the project activity shall be limited to the minimum necessary. When possible, existing access routes and points shall be used. All roads, staging areas, and other facilities shall be placed to avoid and limit disturbance to river and creek banks and habitat when feasible.

• CM-2: Clearly Mark Project Construction Limits. To minimize ground and vegetation disturbance during project construction, project limits shall be clearly marked, including the boundaries of designated equipment staging areas; ingress and egress corridors; stockpile areas for spoils disposal, soil, and materials; and equipment exclusion zones.


• CM-4: Avoid Disturbing or Exceeding the Minimum Vegetation Removal Necessary. Disturbance or removal of vegetation by machinery shall not exceed the minimum necessary to complete project construction and operations.

• CM-5: Replant or Reseed with Native Species and Monitor and Maintain Growth to Ensure Success for Areas Requiring Vegetation Removal. When vegetation removal is required, the disturbed areas shall be replanted or reseeded with native species and monitored and maintained to ensure the revegetation effort is successful. If erosion control fabrics are used in revegetated areas, they shall be slit in appropriate locations as necessary to allow for plant root growth.

• CM-6: Limit Rock Riprap for Erosion Protection. The amount of rock riprap and other materials used for bank protection shall be limited to the minimum needed for erosion protection and establishment of planting benches.

• CM-11: Install, Monitor, and Maintain Erosion Control Measures that Minimize Soil or Sediment from Entering Waterways or Wetlands. Erosion control measures that minimize soil or sediment from entering waterways and wetlands shall be installed, monitored for effectiveness, and maintained throughout construction operations.

• CM-13: Avoid Use of Materials in Locations Where it Can Erode from Normal or Expected High Flows. No material shall be placed in a manner or location where it can be eroded by normal or expected high flows. Jute netting or another non-monofilament erosion control fabric shall be used to cover soil that is placed over or mixed into riprap or other revetment materials.

• CM-14: Implement Precautionary Measures to Minimize Turbidity/Siltation during Construction. Precautions to minimize turbidity/siltation shall be implemented during construction. This may require placing barriers (e.g., silt curtains) to prevent silt and/or other deleterious materials from entering downstream reaches.

• CM-15: Inspect Sediment and Turbidity Control Barriers Daily during Construction for Proper Function and Replace Immediately if Not Functioning Effectively. Performance of sediment and turbidity control barriers shall be inspected at least once each day during construction to they are functioning properly. Should a control barrier not function effectively, it shall be immediately repaired or replaced. Additional controls shall be installed as necessary.

• CM-16: Remove Sediment from Sediments Controls and Dispose of Properly. Sediment shall be removed from sediment controls once the sediment has reached 1/3 of the exposed height of the control. Sediment collected in these devices shall be disposed of away from the collection site at designated upland disposal sites.

• CM-17: Treat Water with Silt or Mud from Construction Activities to Prevent it from Entering Live Waterways. Water containing mud or silt from construction activities shall be treated by filtration, or retention in a settling pond, adequate to prevent muddy water from entering live waterways.


• CM-18: Treat All Disturbed Soils with Appropriate Erosion Control. All disturbed soils shall undergo appropriate erosion control treatment (e.g., sterile straw mulching, seeding, planting) prior to the end of the construction season, or prior to October 15, whichever comes first.

Timing: During and after project construction.


Significance after Mitigation: Implementation of Mitigation Measures GEO-1 and GM-1 would reduce potentially significant temporary, short-term construction-related erosion impacts to a less-than-significant level by requiring preparation and implementation of a SWPPP with appropriate BMPs to reduce erosion, and by requiring implementation of other measures such as revegetation to reduce sediment transport and maintain surface water quality conditions in adjacent receiving waters.

Sacramento River East Levee Improvements

In total, approximately 5.1 miles of the Sacramento River East Levee located south of R Street and north of the EBMUD Freeport facility, out of the approximately 8.1 miles of levee evaluated, require remediation to address seepage or stability concerns in the Sacramento River East Levee Improvements area. In addition, one erosion site was recommended for immediate corrective action with bank protection prior to FEMA accreditation, eight erosion sites were recommended for corrective action within a 3-year time window of accreditation, and 11 sites were identified as requiring regular monitoring before and after a high-water event but not requiring immediate or short-term corrective action.

Construction of levee improvements would occur between April and October when rainfall is the least likely and streamflows are lowest. However, construction-related activities associated with levee improvements would entail ground-disturbing activities that could expose slopes that are susceptible to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, these proposed project elements would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.

An analysis performed to assess existing levee encroachments for compliance with FEMA, USACE, CVFPB, and ULDC criteria, identified 612 encroachments in the Sacramento River East Levee Improvements area (Mead & Hunt 2013: Exhibit 4). Approximately 91 encroachments have been identified as “unacceptable” and need to be removed. Encroachment removal would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.

Levee accreditation would involve the removal of high-hazard levee vegetation including trees, woody shrubs and non-walkable herbaceous ground cover. Approximately 615 high-hazard trees would be removed in the Sacramento River East Levee Improvements area (AECOM 2014) and approximately 25 percent of trees considered for removal are on the waterside slope of the levee, and approximately 75 percent of the trees are on the landside slope or near the levee toe within the levee easement area. Vegetation removal would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease


levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.





Implementation of the actions associated with the Conservation Strategy in the Sacramento River East Levee Improvements area would compensate for adverse effects associated with levee improvements, encroachment removal, and vegetation management. The Conservation Strategy would also entail riparian plantings in several locations along the Sacramento River East Levee, as well as on approximately 75 acres located north of the Stone Lakes National Wildlife Refuge.

Approximately 3,000 linear feet of bank protection is proposed at nine sites on the Sacramento River East Levee. Conceptual designs for these sites blend engineered bank stabilization with on-site habitat features and biotechnical elements that optimize replacement habitat, minimize construction impacts, and ensure long-term bank and levee slope stability. Riparian trees and shrubs would be planted on recontoured middle and upper bank slopes in natural soil. Large root wads and other instream woody material would be anchored in the rock slope and on low benches just below and above the summer/fall water surface elevation. Existing lower and middle bank riparian vegetation and large woody material would be retained on-site as much as is feasible without sacrificing geotechnical bank stabilization by placing rock under and within the exposed root mass of mature trees.

Numerous patches of nonnative grassland gaps occur in the woodland canopy on high benches between the waterside levee slope and the upper banks of the Sacramento River, amounting to approximately 10 acres of area potentially suitable for planting. The project proposes removal of nonnative grassland and planting activities, as part of the Conservation Strategy. Proposed plantings in the Stone Lakes National Wildlife Refuge would be adjacent to, but not located in, Morrison Creek.



Planting and related components of the Conservation Strategy would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition downstream in lower energy portions of the channel, which could affect flow patterns in the Sacramento River. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measures GEO-1 and GM-1, described below, have been identified to address this impact.




Mitigation Measure: Implement Mitigation Measure GM-1 (Implement Conservation Strategy Measures to Reduce Erosion and Sediment Transport).




American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal

An analysis performed to assess existing levee encroachments for compliance with FEMA, USACE, CVFPB, and ULDC criteria, identified 833 encroachments in the American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area, consisting of an estimated 816 from the American River Levee and an estimated 17 from the Beach Lake Levee (Mead & Hunt 2013: Exhibit 4). Approximately 79 encroachments from the American River Levee and 1 from the Beach Lake Levee have been identified as “unacceptable” per USACE rating guidelines including permitted and unpermitted access gates, bridges, irrigation, landscaping, retaining walls, stairways, and structures (Mead & Hunt 2013: Exhibit 4), and should be removed. Encroachment removal would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.

Levee accreditation would involve the removal of high-hazard levee vegetation including trees, woody shrubs and non-walkable herbaceous ground cover. Approximately 242 high-hazard trees would be removed in the American


River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area, consisting of an estimated 150 from the American River Levee and an estimated 90 from the Beach Lake Levee (AECOM 2014). Vegetation removal would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measure GEO-1, described below, has been identified to address this impact.





Implementation of the activities associated with the Conservation Strategy in the American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area would compensate for adverse effects associated with encroachment removal and vegetation management. Riparian plantings are proposed on approximately 7 acres of the approximately 270-acre Woodlake site (located on the north side of the American River between State Route 160 and the Union Pacific Railroad tracks). Removal of nonnative vegetation and planting activities would occur on this site. In addition, proposed riparian plantings would occur on approximately 20 acres in the Upper Beach Lake Wildlife Area (Bufferlands) and on approximately 7 acres of the Camp Pollock site.


Planting and other associated activities with the Conservation Strategy would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition downstream in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measures GEO-1 and GM-1, described below, have been identified to address this impact.









Natomas East Main Drainage Canal/Steelhead Creek Corridor Management Plan

In NEMDC/Steelhead Creek, beavers and their constructed dams have caused unwanted (i.e., large scale) channel sedimentation, resulting in multi-braided and shallow channel conditions. Dense stands of vegetation affect river-flow patterns, and destabilize stream banks which can lead to erosion. The NEMDC/Steelhead Creek CMP would involve restoring the existing NEMDC/Steelhead Creek channel to create a deeper, single-thread, low-flow active channel.

The NEMDC/Steelhead Creek CMP would implement vegetation management practices in NEMDC/Steelhead Creek that emphasize protection of large shade trees and removal of nonnative invasive species. The long-term objective is to promote growth and retention of large shade trees along the low-flow channel that typically grow much taller than the maximum height (and depth) of major flood stages. Vegetation management in NEMDC/Steelhead Creek would selectively target removal and thinning of wide, dense stands of sandbar willow and would prioritize the removal, control, and long-term management of nonnative invasive shrubs. On Lower Dry Creek, SAFCA would develop concept plans to stabilize the channel by minimizing erosion to over-steepened banks, further reducing the source of sediment entering NEMDC/Steelhead Creek.

As described in Chapter 3, “Project Description,” efforts to minimize fish entrapment sites on NEMDC/Steelhead Creek would be coordinated with SAFCA’s proposed Site 18A Culvert Replacement Project. In addition, the proposed approximately 13-acre 2K Borrow Site would be graded and contoured to support the establishment of seasonal wetland habitats as described in Conservation Strategy component of the project description, contained in Chapter 3.


Implementation of the NEMDC/Steelhead Creek CMP components would require ground-disturbing activities, which could temporarily increase susceptibility to erosion. Excessive erosion could decrease levee stability and cause sediment deposition downstream in lower energy portions of the channel, which could affect flow patterns in the river. Therefore, this proposed project element would have a potentially significant impact. Mitigation Measures GEO-1 and GM-1, described below, have been identified to address this impact.








IMPACT GM-2

Potential Long-Term Increase in Channel Bed Incision and Bank Erosion Attributable to Project Design. When confined between levees, stream energy has the potential to erode the banks and cause bed degradation or incision. However, the proposed project would not raise levee heights or water surface levels, and thus would not increase or intensify these geomorphic processes. Bank protection would be installed at summer water surface elevations, minimizing the potential to deflect erosion energy downstream at high flows. Therefore, this impact would be less than significant for the North Sacramento Streams and Sacramento River East Levee Improvements. There would be no impact for the American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area because there would be no project-related activities that would affect channel bed incision or bank erosion.


In general, the various proposed seepage control strategies would not increase or intensify existing geomorphic processes because the proposed levee improvements are predominantly subsurface geotechnical improvements and would not significantly alter the levee surfaces on the waterside of the levee. Construction of the Arcade Creek Levee toe slurry cutoff walls would require construction of a bench along the toe. After the bench is completed, the top and waterside slope would be covered with riprap to control erosion over the completed cutoff


wall. Above the bench, all disturbed construction areas would be revegetated. Additionally, because the proposed project would not involve an increase in the height of the Arcade Creek Levee, and thus would not increase or intensify these geomorphic processes. Therefore, this proposed project element would have a less-than-significant impact.

Mitigation Measure: No mitigation is required.

Activities associated with encroachment removal, vegetation management, and the Conservation Strategy would have minimal long-term effect channel bed incision or bank erosion because disturbances would be temporary and stabilized with vegetation. Furthermore, removal of dense flow-impeding vegetation from the levee channel and low-flow channel enhancement designs, as part of the Conservation Strategy, are expected to mitigate for channel modifications and vegetation management by providing a setting that favors native species and reduces the need for maintenance. After vegetation establishes, it would provide resistance to erosive forces. Encroachment removal and vegetation management would not raise water surface levels appreciably, and thus would not increase or intensify the geomorphic processes of channel bed incision or bank erosion. Therefore, these proposed project elements would have a less-than-significant impact.



In general, the various proposed seepage control strategies would not increase or intensify existing geomorphic processes because the proposed levee improvements are predominantly subsurface geotechnical improvements and would not significantly alter the levee surfaces on the waterside of the levee. They would not increase the height of the Sacramento River East Levee or water surface levels appreciably. Therefore, this proposed project element would have would have a less-than-significant impact.


The proposed project includes placement of embankment protection material such as riprap or other selective rock material in the erosion zone and landscaping fill to reestablish the slope above the erosion zone, to protect the levee bank from erosive forces due to wave action and/or high flow velocities. The nature of erosion at the sites requiring repair ranges from shallow, wake-generated erosion to deeper toe scour erosion. The proposed project includes work at nine erosion sites along the Sacramento River East Levee.

For shallow wake erosion, reconstruction of the riverbank would be performed by incorporating a rock berm at the toe of the slope in combination with soil fill to reestablish the river bank slope. The rock placed at the toe would be constructed at the summer water surface elevation to buffer the effects of wave wash and to provide geotechnical stability for the upper bank. As described in the Conservation Strategy, the soil fill slope would be revegetated with a variety of native woody and herbaceous plants for future cover, terrestrial/aquatic habitat, and to minimize the potential for erosion.

The deep scour erosion repair concept is based on revetment toe protection guidelines developed by USACE. Bioengineered features would also be incorporated in the repair at elevations above the summer water surface elevation, incorporating woody material, soil fill, and bank revegetation to minimize potential for erosion.


Armoring the river banks has the potential to deflect erosive energy downstream to unarmored reaches. However, because the bank protection measures would be installed at or near summer water surface elevations, the potential to deflect energy at high flows—when erosive forces are greatest—is minimal. The Conservation Strategy includes additional riparian plantings of up to 5,000 linear feet where there would be no rock slope protection to provide mitigation for bank protection impacts. Therefore, these proposed project elements would have a less-than-significant impact.

Mitigation Measures: No mitigation is required.

Activities associated with encroachment removal, vegetation management, and Conservation Strategy in the Sacramento River East Levee Improvements area could potentially increase channel bed incision or bank erosion. The removal of encroachments and “high-risk” vegetation would decrease channel roughness and could increase flow conveyance and erosive forces and alter sediment transport patterns. However, these effects are anticipated to be localized and minor. Once established, the plantings proposed in the Conservation Strategy would counteract for some of the changes in channel roughness and sediment transport. The proposed project elements would not raise levee heights or water surface levels appreciably—and where they do would be localized and minor—thus they would not increase or intensify these geomorphic processes. Therefore, these proposed project elements would have a less-than-significant impact.



Activities associated with encroachment removal, vegetation management, and the Conservation Strategy in the American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area could potentially increase channel bed incision or bank erosion. The removal of encroachments and high-hazard trees would decrease channel roughness and could increase flow conveyance and erosive forces and alter sediment transport patterns. However, these conditions are anticipated to be localized and minor. Once established, the plantings proposed in the Conservation Strategy would counteract for some of the changes in channel roughness and sediment transport. The project elements would not raise levee heights or water surface levels appreciably—and where they do would be localized and minor—thus they would not increase or intensify these geomorphic processes. Therefore, these proposed project elements would have a less-than-significant impact.



In NEMDC/Steelhead Creek, beavers and their constructed dams are causing unwanted sedimentation within the channel resulting in multi-braided and shallow channel conditions. Dense stands of vegetation found in the system affect river flow patterns, and destabilize stream banks which can lead to erosion. The NEMDC/Steelhead Creek CMP would involve restoring the existing NEMDC/Steelhead Creek channel and the lower approximately 1.5 miles of Dry, Robla, and Arcade Creek channel to create a deeper, single-thread, low-flow active channel. A lower channel elevation is a desired effect that would increase sediment transport capacity, create a seasonally inundated floodplain, and enhance fish passage. Removal of dense flow-impeding vegetation from the levee channel and low-flow channel enhancement designs are expected to mitigate for channel modifications and vegetation management by providing a setting that favors native species and reduces the need for maintenance.


The NEMDC/Steelhead Creek CMP would implement vegetation management practices in NEMDC/Steelhead Creek that emphasize protection of large shade trees and removal of dense willow thickets and nonnative invasive species. The long-term objective is to promote growth and retention of large shade trees along the low-flow channel that typically grow much taller than the maximum height (and depth) of major flood stages. Vegetation management in NEMDC/Steelhead Creek would selectively target removal and thinning of wide, dense stands of sandbar willow and would prioritize removal, control and long-term management of nonnative invasive shrubs.

The removal of beaver dams and dense vegetation, and confinement of low flows to a single-thread channel stabilized with planted trees would decrease roughness and increase flow velocities and the sediment transport capacity of the streams during medium to high flows. This could change the sediment transport environment from a zone of deposition to a zone of transport and increase erosive forces, and thus result in an increase in sediment deposition to unknown downstream areas, which could affect flow patterns. Increased conveyance and decreased sedimentation are both desired effects of the proposed project for both flood control and environmental purposes. The potential increase in sedimentation downstream would likely be temporary as the channel equilibrates to the new configuration. The proposed project would not change depositional processes beyond what naturally occurs as a channel adjusts to flows and sediment transport. Therefore, this proposed project element would have a less-than-significant impact.


IMPACT GM-3

Long-Term Potential for Levee Seepage. Through- and underseepage have the potential to weaken and potentially cause levee failures. The proposed project would decrease through- and underseepage, thus reducing the risk of levee failures. Therefore, the proposed result would result in a beneficial effect in the North Sacramento Streams and Sacramento River East Levee Improvement areas. There would be no impact in the American River and Beach Lake Levees High-Hazard Levee Encroachment and Vegetation Removal area because no levee improvements are proposed.


Encroachment removal, vegetation management, and the Conservation Strategy would not affect levee seepage. Therefore, these proposed project elements would have no impact.


Through‐ and underseepage has the potential to weaken levee foundations. Levee failures could contribute significant volumes of sediment and material to the stream channels, which could alter flow patterns. The proposed levee improvements in the North Sacramento Streams Levee Improvements area (e.g., cutoff walls, waterside toe slurry cutoff wall with landside slope replacement, footprint and impact zone for cutoff walls, relief wells, clay blanket on waterside slope, landside berms, or toe drains) would decrease through- and underseepage, thus reducing the risk of levee failures. Therefore, this proposed project element would have a beneficial effect.






Through‐ and underseepage has the potential to weaken levee foundations. Levee failures could contribute significant volumes of sediment and material to the stream channels, which could alter flow patterns. The proposed levee improvements in the Sacramento River East Levee Improvements area (e.g., cutoff walls, waterside toe slurry cutoff wall with landside slope replacement, footprint and impact zone for cutoff walls, relief wells, clay blanket on waterside slope, landside berms, toe drains, or bank protection measures) would decrease through- and underseepage, thus reducing the risk of levee failures. Therefore, this proposed project element would have a beneficial effect.






Activities associated with the NEMDC/Steelhead Creek CMP would not affect levee seepage. Therefore, this proposed project element would have no impact.


RESIDUAL SIGNIFICANT IMPACTS

Impacts from potential long-term increase in channel bed incision and bank erosion attributable to project design (Impact GM-2) and long-term potential for levee seepage (Impact GM-3) would be less than significant. Therefore, no residual significant impacts would occur.

Impacts from the potential temporary and short-term erosion during construction (Impact GM-1) would be potentially significant. However, implementation of Mitigation Measures GEO-1 and GM-1 would reduce these impacts to a less-than-significant level. Therefore, no residual significant impacts would occur.


4.9 geomorphology 4.9.1 regulatory settinggeomorphology 4.9-2 sacramento area flood control agency...

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