& sedimentation update · elevation of sedimentation of the reservoir from the 2012 survey to...

20

Twitchell Reservoir

Results of 2018 Aerial Survey & Sedimentation Update

Prepared by The Santa Maria Valley Water Conservation District &

MNS Engineers Inc.

Survey Date: November 2018 Report Date: FINAL August 5, 2019

Twitchell Reservoir – Results of 2018 Aerial Survey

THIS PAGE INTENTIONALLY LEFT BLANK


of 57

Table of Contents Executive Summary ........................................................................................................................ 5 Introduction ..................................................................................................................................... 9 Limits of Area Mapped ................................................................................................................. 10 Topographic Survey Description .................................................................................................. 10 Comparison to Previous Topographic Surveys............................................................................. 11 Deliverables .................................................................................................................................. 11 Property Boundaries...................................................................................................................... 12 General Discussion on Sedimentation .......................................................................................... 14 Historical Streamgage Flow Data ................................................................................................. 17 Volume Study & Discussion: ....................................................................................................... 19 Change in Sediment Levels Since 2012........................................................................................ 19 Chart of Historical Elevation vs. Capacity ................................................................................... 24 Affect on Flood Control and Water Conservation Storage ........................................................... 28 Sedimentation: A Pictorial History at the Outlet Structure .......................................................... 33 Downstream Sediment .................................................................................................................. 37 Observations and Conclusions ...................................................................................................... 39 Appendices .................................................................................................................................... 40

Enclosures: Maps Included in this report Binder are two full-size maps: Hard Copy of Twitchell Reservoir Topographic Map Hard Copy of the Sedimentation Levels Comparison 2012 vs. 2018 Map (Heat Map) Data Disk Included in this report Binder is one data CD containing: 1958 Original Drawings 1960 Aerials 1999 Survey Files 2007 Survey Files 2012 Survey Files: 2018 Survey Files

Aerial Drawings AutoCAD Drawings Capacity Spreadsheets and Calculators Misc. Reference PDF Files

Past PowerPoint Presentations


of 57



of 57

Green: Sediment Deposition Red: Scour

Upper Reservoir Basin

Lower Reservoir Basin

Executive Summary This Report was prepared by the Santa Maria Valley Water Conservation District and MNS Engineers Inc. (MNS) to update the volume capacity tables for Twitchell Reservoir and provide current data regarding sedimentation influx to the reservoir. A new aerial survey was performed in November 2018 as the basis for this report. Survey mapping was performed from the lowest point of the basin, at elevation 526 feet, up to the 692 foot elevation. The lowest point in the reservoir is now 20’ above the lowest measured point in the 2012 survey. Flight photography was taken in September, 2018, and processed in November 2018. The specifications were for 2’ contour intervals. Aerial targets were set at 20 locations including 10 check shots around the project area. The field survey was performed September 4 through 11, 2018. The horizontal basis for the project is on the NAD83 (Epoch 2010.00) datum.

Figure 1: Sediment Change 2012-2018 (Full size map included in the report binder)


of 57

The reservoir had an original total design water capacity of 240,000 ac-ft at the spillway crest, achieved at elevation 651.5 ft, with a design 100-yr sediment capacity of 40,000 Acre-feet. Historic inflow of sediment is as follows:

Year Water Capacity at el. 651.5 ft Sediment (Acre-feet) Below el. 651,5 ft (Below Spillway)

1953 (Design) 240,000+/- 0 1981 224,399 15,714 1997 203,499 36,614

Projected 100-yr sediment accumulation (1959-2059) 40,000 2000 198,339 41,714 2007 197,756 42,357 2012 194,971 45,142 2018 194,277 45,836

Included in the Appendix of this report is a full size exhibit showing graphically the change in elevation of sedimentation of the reservoir from the 2012 survey to the 2018 survey. Changes in sediment elevation are color coded to graphically show scour and deposition areas by depth. As described in the discussion above on capacity, the overall capacity of the reservoir has continued to be reduced due to sedimentation. The total sediment now in the reservoir below the spillway (elev. 651.5) is 45,836 acre-feet, which is a reduction of storage in the conservation allocation of 202 acre-feet since 2012. Summarized Results: 1. Overall, 121.08 acre-feet of new sediment was washed into the lower basin of the reservoir

(Defined as the El 566 contour line. 2. 81 Acre-feet of sediment that the 2017 storms brought to the immediate area of the intake

structure occlude over 50% of the inlet capacity of the outlet works. 3. 84.71 acre-feet of sediment increase was washed below the 530 El contour line. 4. 81.13 acre-feet of sediment increase was washed below the 527 El contour line. 5. The top of the intake structure is at El 526. This means that the 81.13 acre-feet of sediment

increase below the 527 El contour line is directly impacting the Intake Structure. 6. Sediment levels at the Intake Structure itself have increased 43.4 feet, inundating the lower

portal opening, and inundation ¾ of the upper opening and trash racks. Additional Observations: 1. Sediment has flowed into and blocked the outlet tunnel between the Inlet Structure and the

release gates, located in the center of the dam. Gate operation has verified that this occlusion is approaching 100% blockage.

2. Sediment has filled the stilling basin and Keyhole structures. Estimated sediment in the Stilling Basin is 120 CY and in the Keyhole is 3800 CY. See attached drawing of these two features.

Statement of Severity of This Problem: This inflow of sediment is the largest single event impacting operations of the Dam since it’s construction. This is a Category III Dam and works in conjunction with the Santa Maria Levee as the first line of defense against flooding the Santa Maria Valley and its communities. The


of 57

District must expedite work to remove sediment impeding operations and assure the outlet works remain functional so that the dam can perform its flood control function.


of 57



of 57

Introduction Originally, Twitchell Dam and Reservoir started as part of “The Santa Maria Project”, one of three "seacoast projects" along the Central Pacific Coast aimed at capturing and storing floodwaters that would otherwise "waste to the sea.” The others are the Cachuma and Ventura River Projects. Constructed by the United States Army Corps of Engineers (USACE) and the Bureau of Reclamation (BOR) in 1958 on behalf of the Santa Barbara County Water Agency (SBCWA), Twitchell dam has been operated by the Santa Maria Valley Water Conservation District (SMVWCD) since completion. The Santa Maria Project was authorized in 1954 and is a water conservation and flood control project providing full and supplemental irrigation water to approximately 35,000 acres of cropland. The BOR constructed Twitchell Dam and Reservoir, formerly called Vaquero Dam and Reservoir, and the U.S. Army Corps of Engineers constructed a system of river levees.

The Cuyama River, with its principal tributaries Alamo Creek and Huasna River, is the main source of water for the project. The drainage basin, comprising approximately 1,135 square miles above Twitchell Dam, lies along the southern boundary of San Luis Obispo County and the northern edge of Santa Barbara County. All water used within the area is obtained by pumping from the ground-water reservoir.

Twitchell Reservoir is located in both Santa Barbara and San Luis Obispo Counties, and is located approximately 6 miles northeast from the City of Santa Maria. Twitchell Dam is an earth-fill structure, has a structural height of 241 feet, of which 216 feet are above streambed, a crest length of 1,804 feet, and contains approximately 5,833,000 cubic yards of material. Twitchell Dam and Reservoir are designed for the protection of the Santa Maria Valley from flood and drought. The dam catches excess rain runoff from the Cuyama watershed and stores it in the reservoir protecting the valley from flood. Water is slowly discharged into the Santa Maria River, which serves as the main recharge source for the local aquifer. The aquifer provides water for the residents and agricultural industry of the Santa Maria Valley. The Santa Maria Water Conservation District is providing this updated topographic mapping of the Twitchell Reservoir basin and report for the principal purpose of updating data tables showing reservoir capacity at any given level and other tables and/or graphs that show the relationship of capacity, level, area, etc. and comparisons to previous studies.

TWITCHELL RESERVOIR


of 57

This Survey and study were performed to the same degree of accuracy as the 2007 and 2012 surveys, which set a standard for a higher degree of accuracy than earlier surveys. The data derived was compared with the last survey to determine current capacities to be used in a revised capacity table. This data is critical to determine water reclamation and flood control capacities as well as to monitor siltation in the reservoir. Previous topographic and/or topographic/bathymetric combined surveys have been performed in 1958, 1981, 1991, 2000, 2007 and 2012. The District began, in 2000, a policy goal of performing a topographical survey and preparation of a Sediment Report every five years, or following a severe storm event. Limits of Area Mapped Mapping was done for the reservoir from the lowest point in the reservoir of 526 feet to the crest of the dam at 692 feet. The lowest measured point in the reservoir is 20’ higher than that measured in 2012. Additionally, the downstream Cuyama River Valley was photographed from the dam for a distance of approximately 5,000 feet. The downstream portion was not processed to develop topography for that area. The downstream portion was mapped in 2007 to establish a baseline for downstream sedimentation. Topographic Survey Description MNS Engineers Inc. performed an aerial topographic survey of the reservoir based on a flight photography taken in September, 2018, and processed in November 2018. The specifications were for 2’ contour intervals. Aerial targets were set at 20 locations including 10 check shots around the project area. The following detailed information related to the survey is provided to assist in documenting the methods of this survey and in providing a basis for consistency in future surveys. The field survey was performed September 4 through 11, 2018. The aerial control panels for the flight were set using Trimble R8 & R10 receivers set at 10 second logging intervals. The Trimble receivers are state-of-the-art GPS units utilizing the Global Navigation Satellite System (GNSS). To assure accuracy with the GPS units, the occupation time was set at 15 minutes with double occupations for redundancy checks. Baseline processing was performed in Trimble Business Center software version 4.10. The network adjustment of the vectors was performed at a 95% confidence level for all stations using the least squares adjustment software StarNet version 9.0. All points adjusted had a relative ellipse error not exceeding 0.2’. The horizontal basis for the project is on the NAD83 (Epoch 2010.00) datum. Three CORS stations: ORES, P513 and P516 were first incorporated into a free weighted adjustment including NGS HPGN Station, PID FV2078. The measured versus record coordinate values in US survey feet are as follows:


of 57

Points marked with an * are differences in ellipsoid heights from the held 1929 datum on the project benchmark PID FV0943 (Q 304).

A fully constrained adjustment was performed fixing the record horizontal values of CORS stations ORES, P516 and HPGN Station FV2078.

The vertical datum for the project is NGVD 29 and based upon benchmark PID FV0943 with an elevation of 727.21 feet.

Comparison to Previous Topographic Surveys This 2018 survey effort, in following protocols established for the 2007 and 2012 surveys, matches the highest level of accuracy for a topographic map of the reservoir to-date. The last pre-2007 survey, performed in 2000, produced a 2-foot contour map, but was flown at an elevation appropriate for 4-foot resolution accuracy. Applying national standards for accuracy, or ½ of a contour, the 2007 survey is accurate to +/- 1 foot. The 2000 survey is accurate to +/- 2 feet. A close examination of the differences between the contour lines between these maps shows that the 2000 contours are less angular and represent the actual shape of the ground contours. The 2007, 2012, and 2018 maps, as expected with their more detailed contours, has resulted in more accurate capacity tables with a slightly greater volume than would be calculated using the previous resolution. This added accuracy is estimated to result in a capacity of up to 1 or 1-1/2% greater than previously mapped. Deliverables Included with this report are the following Items:

1. Report containing updated area-capacity tables for the reservoir, quantification and discussion of current and past influx of sediment into the reservoir, and documentation regarding the method of survey.

CONTROL dN dE dZ

FV2078 0.0260 -0.0263 -2.7552*

ORES 0.0068 0.0077 -3.2970*

P513 0.0874 -0.0693 -2.8782*

P516 0.0251 0.0158 -2.8291*


of 57

2. Full Size Prints: Aerial Topo Map (without ortho-photo), Aerial Topo Map (with ortho-photo), Aerial Map with Sedimentation and Scour Exhibit

3. Data DVD: Containing electronic submittal of maps in AutoCAD and PDF formats, report in both word and PDF format, copies of relevant historical documentation regarding past surveys and sedimentation influx into the reservoir.

Property Boundaries This survey effort did not attempt to compile or retrace District boundaries or other private property boundaries surrounding or including the reservoir. The following “Twitchell Reservoir and Associated Parcels Exhibit" is for reference only and was created from Assessor’s Parcel Maps.


of 57

Twitchell Reservoir and Associated Parcels Exhibit

H.R. Perrett

H.R. Perrett

H.R. Perrett

H.R. Perrett


of 57

General Discussion on Sedimentation To provide continuity in the documentation of the sedimentation history of the reservoir the following information is provided.

The reservoir had an original total design water capacity of 240,000 ac-ft at the spillway crest, achieved at elevation 651.5 ft. This capacity has steadily been reduced due to siltation. In spite of the on-going siltation issues, the Bureau of Reclamation has determined and documented in their 2016 Comprehensive Facility Review (CFR) that the Dam was in good condition and is well maintained. 1953 Design – Per the Army Corps of Engineers Reservoir Regulation Manual for Twitchell Reservoir, “The amount of sediment expected to be deposited in Twitchell Reservoir was determined by the U. S. Bureau of Reclamation on the basis of an analysis of runoff samples. A relationship between sediment load and discharge was developed, and using a flow-duration curve, a long term sediment yield was determined. It was estimated that 400 acre-feet of sediment per year would be deposited. Under the assumption that this rate of deposition would continue during the 100-year period following completion of the dam, 40,000 acre-feet of storage was allocated to sedimentation. It was assumed that this sediment would fill the reservoir to elevation 504 at the end of a 100-year period.” 1953 Design vs. 1981 Survey –15,714 acre-feet of sediment was observed to have accumulated in the reservoir, and the pattern of sediment quickly building around the Outlet Structure is also observed. The lower portal of the Outlet Structure was closed in an effort to reduce sediment passing downstream, and choking the Stilling Basin and other elements of the outlet works. See 1979 photo on page 15. Major storms contributing to the sedimentation occurred in 1962, 1967, 1969, 1978, and 1983 (see historical peak rainfall data, page 11). 1981 vs. 1997 Survey – 20,900 additional acre-feet of sediment was observed to have accumulated in the reservoir, again significantly raising the elevation of the reservoir bottom at the Outlet Structure. The lower portal of the Outlet Structure was reopened and the operation strategy of seasonal flushing was adopted to keep the Outlet Structure open. Major storms


of 57

contributing to the sedimentation occurred in 1991, 1992, 1993, and 1995 (see historical peak rainfall data, page 11). 1997 vs. 2000 Survey – In a June 22, 2000 letter to the Directors of the Santa Maria Valley Water Conservation District from the County of Santa Barbara, it was noted that there were two major differences between the 1997 elevation-capacity table and the 2000 table. The differences were: 1) The 2000 survey shows slightly greater capacity (about 400 acre-feet) than the 1997

survey at elevations below 540 feet (see Table 3). Both bathymetric and aerial photography data were used to generate the 1997 survey because at that time there was water in the reservoir. The bathymetric survey was necessary to determine the topology beneath the water surface and data was collected from a barge at an elevation of about 540 feet. The 2000 survey, however, was performed when the reservoir was completely empty of water. It was based entirely on aerial photography and therefore the data was more accurate. Another factor that could have increased capacity below 504 feet is the mobilization of sediment near the outlet works during the 1998 flood control releases.

2) The 2000 survey shows a loss of over 5,100 acre-feet capacity since 1952 throughout the

entire reservoir (below 651.5 feet elevation), 4,217 of which is in the conservation pool (below 623 feet elevation) (see Survey Results summary table). The reason for this significant loss of capacity is the significant storm season of 1998 and the record inflow of sediment and debris that occurred throughout the entire reservoir system.

1998 Flood: On February 23, 1998, severe flooding throughout the Cuyama River Basin resulted in damage to agricultural land and roads; a record flood peak of 26,200 cubic feet per second (ft3/s) occurred at 2330 hours at the U.S. Geological Survey stream-gaging station on the Cuyama River below Buckhorn Canyon, near Santa Maria (11136800). In response, the U.S. Geological Survey, in cooperation with the Santa Barbara County Flood Control District and Water Agency, undertook a study to assess the magnitude and frequency (exceedence probability) of the flood, the distribution of tributary flood peaks in the basin, and factors that contributed to flooding. Continuous flow data has been collected at the Cuyama River below Buckhorn Canyon since October 1959; the previous record peak, 17,800 ft3/s, occurred on February 25, 1969 (U.S. Geological Survey, 1969). Stream-gaging station 11136800, Cuyama River below Buckhorn Canyon, is located on the downstream side of the bridge over the Cuyama River on State Highway 166, 1.5 miles downstream from Buckhorn Canyon and 13 miles northeast of Santa Maria. The drainage area of the Cuyama River Basin upstream from this gaging station is 886 square miles. The river valley trends northwest to southeast and drains the east side of the Sierra Madre Range, the north side of the Pine Mountains at the southern end of the basin near the headwaters of the Cuyama River, and the western face of Mt. Pinos. The Cuyama River flows into Twitchell Reservoir downstream from gaging station 11136800, and then joins with the Sisquoc River to form the Santa Maria River, which flows to the west to the Pacific Ocean near Guadalupe. The exceedence probability of the February 23,1998 flood peak at gage 11136800, calculated using 41 years of stream flow records, was 0.04, or equivalent to a 25-year flood. This exceedence


of 57

probability compares well with the 24-hour precipitation frequency for most of the basin (Miller, 1973), which indicates that the storm of February 23 was a 25-year storm. As an emergency project following the 1998 storms, which followed erosion-enhancing fires in the Cuyama River drainage basin, a contractor removed sediment both upstream and downstream of the dam to maintain the outlet works clear. 2000 vs. 2007 Survey –In 2002 the SMVWCD contracted with Madonna Construction to remove sediment around the Outlet Structure on the upstream side of the dam, and the debris racks were cleaned, repaired and painted. Sediment fills the reservoir eliminating any water storage capacity to the 490 elevation. This is close to approaching the 100-year sediment level of elevation 504 predicted in 1953. Major storms contributing to the sedimentation occurred in 2001, and 2005. 2007 vs. 2012 Survey –A detailed discussion of current sedimentation and reservoir capacity follows. In 2010-11 the SMVWCD contracted with Wood Brothers Construction to remove sediment in the Stilling Basin, Keyhole area and downstream channel for approximately 4,000 feet below the dam. Sediment now fills the reservoir eliminating any capacity to the 508 elevation. This is more than the 100-year sediment level of elevation 504 predicted in 1953. Significant fire events in the upstream watershed continue to contribute to the sedimentation in the basin. Major storms contributing to the sedimentation occurred in the winter of 2010-11, at which time a natural disaster declaration was made in Santa Barbara County. 2012 vs. 2018 Survey – Between 2012 and the 2017 severe storm events, only minor dredging work was needed in the Keyhole to control cattails. In 2013 structural repairs in the Outlet Works Tunnel was performed while some water was retained in the reservoir, and with the tunnel free of any sediment. The reservoir was empty and dry beginning in 2014 through most of 2015. Severe storms in 2017 centered on the Huasna and on the Cuyama watersheds brought record flows into the reservoir along with record sediment into the lower basin, impacting the outlet works. Sediment now fills the reservoir eliminating any capacity to the 524.4 elevation. This is significantly more than the 100-year sediment level of elevation 504 predicted in 1953. Significant fire events in the upstream watershed continue to contribute to the sedimentation in the basin. The Thomas and Alamo fire each took a toll on the watersheds feeding the reservoir. Major storms contributing to the sedimentation occurred in 2017, at which time a natural disaster declaration was made in San Luis Obispo and Santa Barbara Countys.


of 57

Historical Stream Gage Flow Data Cuyama River Flows: In order to understand the history of the rainfall and associated stream flows associated with the Twichell Reservoir and to see the correlation to sediment inflow, the following historical flows for USGS stream gage 11136800 (Cuyama River below Buckhorn Cyn near Santa Maria CA) is provided (Ref. USGS Website).

A discharge hydrograph from the Cuyama River at Buckhorn Cyn. from the 2012 Survey and the current 2018 is shown below:


of 57

It can be seen that the 2017 storms, during the peak event, discharged water into the reservoir through the Cuyama River at the rate of approx. 15,500 CFS. Huasna River Flows: the following historical flows for USGS stream gage 11137900 (Huasna River near Arroyo Grande, CA) is provided (Ref. USGS Website).

A discharge hydrograph from the Huasna River at Grande from the 2012 Survey and the current 2018 is shown below:

It can be seen that the 2017 storms, during the peak event, discharged water through the Huasna River into the reservoir at the rate of approx. 6,200 CFS. Alamo River Flows: Although stream flow data on Alamo Creek/River Watershed is not kept, emergence evacuation orders were issued along the creek in 2017.


of 57

Volume Study & Discussion: This report includes Area Tables that calculate and report the area of the reservoir in square feet, acre-feet and cumulative acre-feet in 0.1’ increments of elevation from the lowest point on the reservoir to the 692 contour. The table values were calculated using the areas derived from the 2’ contour mapping with infilling by interpolation. The report also includes Elevation/Capacity tables which report the cumulative capacity of the reservoir in 0.1’ increments, a table which shows comparisons of volume to previous reports, and copies of the mapping plotted at an appropriate scale. Change in Sediment Levels Since 2012 As previously mentioned, the reservoir had an original total design water capacity of 240,000 ac-ft at the spillway crest, achieved at elevation 651.5 ft.

Year Water Capacity at el. 651.5 ft Sediment (Acre-feet) Below el. 651,5 ft (Below Spillway)

1953 (Design) 240,000+/- 0 1981 224,399 15,714 1997 203,499 36,614

Projected 100-yr sediment accumulation (1959-2059) 40,000 2000 198,339 41,714 2007 197,756 42,357 2012 194,971 45,142 2018 194,277 45,836

Included in the Appendix of this report is a full size exhibit showing graphically the change in elevation of sedimentation of the reservoir from the 2012 survey to the 2018 survey. Changes in sediment elevation are color coded to graphically show scour and deposition areas by depth. As described in the discussion above on capacity, the overall capacity of the reservoir has continued to be reduced due to sedimentation. The total sediment now in the reservoir below the spillway (elev. 651.5) is 45,836 acre-feet, which is a reduction of storage in the conservation allocation of 202 acre-feet since 2012. Overall Basin Observations: By studying of the graphical sedimentation exhibit “Sedimentation levels 2012-2018” included in the report appendix, the following trends can be observed:

1. There has been continuing migration of sediment from the upper “arms” of the reservoir toward the lower reservoir basin.


of 57

2. Channels continue to erode in the existing sediment in the confluence area of the upper “arms” of the reservoir and the re-suspended sediment is moving downstream downstream into the lower basin.

3. There is an unusual large influx of sediment into the area immediately around the Outlet Works of the Dam.

4. The sudden influx of sediment around and into the Outlet Works has appeared to fill the upper ½ of the Outlet Tunnel (Between the Intake Structure and the Gates).

Lower Basin Observations in Context of FEMA-4308-DR-CA 2017 Storms: Background:

The 2017 Storms followed two decades of significant fires in the watershed for Twitchell reservoir, which has severely impacted the sensitivity of the watershed to debris flows and sedimentation at Twitchell Dam. The 2017 storms were significant storm events in the watershed, and severe sedimentation impacts on the outlet works were suspected. The water releases were unusually full of sediment, impacting downstream water-flow control structures: The Stilling Basin (in which the critical-to-subcritical flow/hydraulic jump occurs, and the


of 57

“Keyhole” which also slows the flow as water enters the downstream river channel). Unusually high volumes of sediment were passed through the Dam.

As soon as possible, the reservoir was drained through controlled releases so that an aerial topographic survey of the basin could be completed. The purpose of the survey was to determine

impacts to the Outlet Works (Upstream Intake Structure) by sediment brought down in the heavy flows of the 2017 storms. This effort is also in support of our claim for sediment removal funding from FEMA-4308-DR-CA 2017 Storms.

This summary of findings is the result of Post-Storm Basin Capacity Survey (2018) comparing to the most recent Pre-storm Basin Capacity Survey (2012).

The information summarized herein is the result of a comparison of the Topographic surveys pre-flood (2012) and post flood (2018). Surveys are normally performed every 5+/- years or more frequently, if a significant storm occurs. The results are as follows:

October 2018

General Appearance 2007 to 2015 (Last time reservoir was empty was 2015). Generally corresponds with 2012 Survey


of 57

Summarized Results: 1. Overall, 121.08 acre-feet of new sediment was washed into the lower basin of the

reservoir (Defined as the El 566 contour line. See map Below

2. The photographs above show visually the impact from the measured 81 Acre-feet of sediment that the storms brought to the immediate area of the intake structure.


of 57

Intake Structure

Lower Portal

Sediment Level El 527

Sediment Level El 525

Twitchell Reservoir Lower Basin

3. 84.71 acre-feet of sediment increase was washed below the 530 El contour line. 4. 81.13 acre-feet of sediment increase was washed below the 527 El contour line. 5. The top of the intake structure is at El 526. This means that the 81.13 acre-feet of

sediment increase below the 527 El contour line is directly impacting the Intake Structure.

6. Sediment levels at the Intake Structure itself have increased 43.4 feet, inundating the lower portal opening, and inundation ¾ of the upper opening and trash racks.

Additional observations:

1. Sediment has flowed into and blocked the outlet tunnel between the Inlet Structure and the release gates, located in the center of the dam.

2. Sediment has filled the stilling basin and Keyhole structures. Estimated sediment in the Stilling Basin is 120 CY and in the Keyhole is 3800 CY. See attached drawing of these two features.

Statement of Severity of this problem:

This inflow of sediment is the largest single event impacting operations of the Dam since it’s construction. This is a Category III Dam and is the first line of defense against flooding the Santa Maria Valley and its communities. The District must expedite work to remove sediment impeding operations and assure the outlet works remain functional so that the dam can perform its flood control function.


of 57

Reservoir Elevation (Feet)

Cumulative Sediment Increase at El 530 Contour:

84.71 AF Cumulative Sediment

Increase at El 566 Contour: 121.08 AF

Cumulative Volume (Acre-Feet)

Contour Elevation (Feet)


of 57

Tunnel flow into Stilling Basin. Note sediment

Tunnel flow into Stilling Basin. Note sediment

Keyhole. Note full of sediment

Ele va tio n

2012 Cumulative

Water Volume (AF)

2018 Cumulative

Water Volume (AF)

Difference (Water Volume Change at given Elevation -AF)

566 19167.65 19046.57 -121.08565 18330.28 18216.79 -113.49564 17527.29 17417.53 -109.76563 16743.56 16635.11 -108.45562 15969.31 15862.91 -106.40561 15204.48 15100.17 -104.31560 14449.85 14347.92 -101.93559 13705.69 13608.86 -96.83558 12974.20 12886.26 -87.94557 12257.95 12179.91 -78.04556 11557.85 11489.08 -68.77555 10872.81 10815.11 -57.70554 10202.56 10158.00 -44.56553 9549.38 9517.73 -31.65552 8914.53 8894.71 -19.82551 8297.48 8287.29 -10.19550 7697.58 7693.93 -3.66549 7113.08 7114.65 1.57548 6542.94 6550.09 7.15547 5989.28 6000.68 11.41546 5454.52 5466.80 12.28545 4937.79 4949.95 12.16544 4439.82 4453.30 13.48543 3963.36 3980.64 17.28542 3510.56 3533.69 23.13541 3084.19 3111.88 27.69540 2685.94 2713.05 27.11539 2316.85 2337.35 20.50538 1974.80 1985.70 10.90537 1654.84 1656.32 1.48536 1356.72 1346.98 -9.74535 1083.21 1057.33 -25.88534 839.03 793.37 -45.67533 630.47 568.57 -61.90532 458.01 385.82 -72.19531 321.79 242.44 -79.35530 223.46 138.74 -84.71529 156.81 68.53 -88.28528 114.65 27.14 -87.52527 89.56 8.43 -81.13526 72.58 1.70 -70.89525 59.88 -59.88524 49.84 -49.84523 41.66 -41.66522 34.86 -34.86521 29.06 -29.06520 24.03 -24.03519 19.64 -19.64518 15.89 -15.89517 12.81 -12.81516 10.28 -10.28515 8.13 -8.13514 6.28 -6.28513 4.69 -4.69512 3.36 -3.36511 2.31 -2.31510 1.49 -1.49509 0.88 -0.88508 0.45 -0.45507 0.17 -0.17506 0.04 -0.04


of 39

Chart of Historical Elevation vs. Capacity The following chart shows a graphical representation of the Change in Capacity over time due to siltation based on the results of the 1981, 1987, 2000, 2007, 2012 and 2018 topographic surveys


of 57

.



of 39

Affect on Flood Control and Water Conservation Storage The Army Corps of Engineers indicated in their 1960 “Reservoir Regulation Manual for Flood - Control Storage at Twitchell Dam and Reservoir” that “It is anticipated that the Regional Director of the Bureau of Reclamation will, at reasonable intervals, make necessary field surveys and office studies to prepare estimates of the volume and location of sediment deposits in the reservoir. If the results of such studies show that the 40,000 acre-feet of storage allocated to sedimentation has been depleted, and the net storage initially allocated to flood control (89,000 acre-feet) or conservation (110,000 acre-feet), respectively, is reduced by an amount exceeding, 10 percent of the allocation for either purpose, the operating plan described herein with respect to storage allocations shall be reviewed with the view of reestablishing an equitable distribution between the primary reservoir uses.” Field surveys have been conducted to-date not by the BOR, but by the SBCWA. Although there has been reference to the 621.8’ water surface elevation as the “official” boundary of the conservation storage pool, this has not been formally adopted by the BOR. The Following table compares the changes in capacity at elevations 623 (Original 40,000 acre-feet of storage conservation elevation) and 651.5 (Spillway Elevation).

Capacity at Capacity at Difference in Elevation 623 Capacity as Elevation 651.5 Capacity as Accumulation Capacity at Elev Accumulation Boundary of Water Percent Crest of Percent of Silt 623 From of Silt Conservation Storage of Design Spillway of Design at Elev. 623 Previous Survey at Elev. 651.5

Year ac-ft % ac-ft % ac-ft ac-ft ac-ft 1958 Design 150,000 100.0% 240,113 100.0% -0- -0- -0- 1981 Survey 135,615 90.4% 224,399 93.5% 14,385 14,385 15,714 1997 Survey 116,422 77.6% 203,499 84.8% 33,578 19,193 36,614 2000 Survey 112,205 74.8% 198,339 82.6% 37,795 4,217 41,774 2007 Survey 110,482 73.7% 197,756 82.4% 39,518 1,723 42,357 2012 Survey 108,128 72.1% 194,971 81.2% 41,872 2,354 45,142 2018 Survey 107,926 71.9% 194,277 80.9% 42,074 202 48,836

We can see from this table that the total sediment below the spillway (elev. 651.5) is 45,142 acre-feet, which is a reduction of storage in the conservation allocation from the design conditions of 27.9%. Other elevation capacities for purposes of comparison are listed below:

Other Elevations of Interest Capacity at El 560 Capacity at El 590 Capacity at El 600 Capacity at El 621.8

Year ac-ft ac-ft ac-ft ac-ft

2018 Survey 14,348 44,932 60,296 104,998

2012 Survey 14,450 45,111 60,511 105,188

2007 Survey 15,691 46,643 62,394 107,535

2000 Survey 18,399 49,601 64,986 109,314

1997 Survey 18,881 52,040 68,002 113,433

1981 Survey 29,803 68,013 85,463 132,625

1958 Design 41,117 81,261 99,250 147,950


of 57

The following table looks at the various pool capacities (sediment, conservation and flood control) at elevation 628.8: Historical Pool Capacities at Elevation 621.8

Year Surveyed Volume

at El 621.8

Sediment Reserve Capacity at El

621.8 Approx. Sediment Volume at El 621.8

Conservation Capacity at El 621.8

Flood Control Capacity below spillway

1958 Design 147,950 40000 -0- 107,950 >89,000 1981 Survey 132,625 25615 14,385 93,565 >89,000 1997 Survey 113,433 6422 33,578 73,433 >89,000 2000 Survey 109,314 2205 37,795 69,314 >89,000 2007 Survey 107,535 482 39,518 67,535 >89,000 2012 Survey 105,188 0 41,872 63,316 >89,000 2018 Survey 104,998 0 42,952 62,046 >89,000

We can observe that the reserved flood control capacity remains reserved below the spillway, but the conservation pool shrinks with accumulation of sediment.


of 57



of 39

The chart shows the square root of the area for ease in charting. The chart is valuable to see the change in surface areas at the lower levels of the reservoir as sediment fills the reservoir.


of 57



of 39

Sedimentation: A Pictorial History at the Outlet Structure The following Photos are provided as a pictorial frame of reference for the sedimentation at the outlet structure.

1958

1979


of 57

1996

2000


of 57

2007

2010


of 57

2018

No Pictures from 2011 or 2012 are available because of water in the reservoir.

Photo Illustrating Depth of Sediment Compared to height of Intake Structure


of 57

2018


of 57

Downstream Sediment This survey Did not include processing of a topographic map downstream of the dam. The 2012 survey was the last survey that mapped the area below the dam and represents the third survey for the downstream area of the dam. The first was performed in 1999, and the second was performed in 2007. It was not in the scope of this study to graphically depict or quantify deposits of sediment downstream. However, the area downstream of the dam was covered in the aerial photography and a topo can be processed when or if it is determined desirable to generate the data. The downstream stilling basin, keyhole (plunge-pool) and channel has been maintained to approximately 4000 feet downstream to remove sediment and keep water flowing and keeping the water in its original channel. This is performed in areas consistently maintained by the District as a part of their on-going downstream sediment management program.


of 57

Observations and Conclusions The current survey (December 2018) was performed at a similar resolution to the previous 2012 survey of the Twitchell Reservoir. The overall capacity of the reservoir has continued to be reduced due to sedimentation. The total sediment now in the reservoir below the spillway (elev. 651.5) is 48,836 acre-feet, which is a reduction of storage in the original (1958) conservation allocation of 28.1%. The 2012 survey and analysis showed a total of 45,142 acre-feet of sediment in the reservoir. The 2018 survey shows an increase of 3694 acre-feet of sediment since 2012, or a 8.2% increase. At the water conservation storage elevation of 623 (water conservation storage elevation boundary), the capacity has changed from 108,128 acre-feet in 2012 to a 2018 capacity of 107,926 acre-feet. There has been continuing migration of sediment from the upper “arms” of the reservoir toward the lower reservoir basin. Channels continue to erode in the existing sediment in the confluence area of the upper “arms” of the reservoir and the re-suspended sediment is moving downstream downstream into the lower basin. There is an unusual large influx of sediment into the area immediately around the Outlet Works of the Dam. The sudden influx of sediment around and into the Outlet Works has appeared to fill the upper ½ of the Outlet Tunnel (Between the Intake Structure and the Gates).


of 39

Appendices Appendix A: Aerial Target Map Appendix B: 1958 Elevation vs. Capacity Table Appendix C: 1995 Bathymetric & 1997 Aerial Survey Elevation vs. Capacity Table Appendix D: 2000 Aerial Survey Elevation vs. Capacity Table Appendix E: 2007 Area & Capacity Tables, 1-foot Increments Appendix F: 2012 Area & Capacity Tables, 1-foot Increments Appendix G: 2018 Area & Capacity Tables, 1-foot Increments Maps Included in this report Binder are two full-size maps: Hard Copy of Twitchell Reservoir Topographic Map Hard Copy of the Sedimentation Levels Comparison Dec. 2012 to Nov. 2018 Map Data Disk Included in this report Binder is one data CD containing: 1958 Original Drawings 1960 Aerials 1999 Survey Files 2007 Survey Files 2012 Survey Files 2018 Survey Files

Aerial Drawings AutoCAD Drawings Capacity Spreadsheets and Calculators


of 57

Appendix A: Aerial Target Map


of 57

Appendix B: 1958 Survey Elevation vs. Capacity Table


of 57


of 57

Appendix C: 1995 Bathymetric & 1997 Aerial Survey Elevation vs. Capacity Table


of 57


of 57

Appendix D: 2000 Aerial Survey Elevation vs. Capacity Table


of 57


of 57

Appendix E: 2007 Area & Capacity Tables, 1-foot Increments


of 57


of 57

Appendix F: 2012 Aerial Survey Elevation vs. Capacity Table


of 57


of 57

Appendix G: 2018 Aerial Survey Elevation vs. Capacity Table


of 57

& sedimentation update · elevation of sedimentation of the reservoir from the 2012 survey to...

Documents