Updated Hydraulic and Sediment Transport Modeling for Cache Creek Settling Basin, Woodland California

K. J. Carr, A. Ercan, and M. L. Kavvas, M.ASCE

UCD J. Amorocho Hydraulics Laboratory, Department of Civil & Environmental Engineering, University of California, Davis, Davis, CA 95616; PH (530) 754-4399e-mail: kjcarr@ucdavis.edu

Abstract Cache Creek Settling Basin, located two miles east of City of Woodland, in California was constructed to preserve the floodway capacity of Yolo Bypass by entrapping sediment. Cache Creek is the main source of mercury entering the San Francisco Bay-Delta. Entrapping sediment will help to entrap mercury, so the settling basin also helps preserve aquatic life of the San Francisco Bay-Delta. 2-dimensional hydraulic and sediment transport models are under development to be coupled under historical flow conditions and preliminary results are presented.

Introduction

Cache Creek flows from Clear Lake in Lake County, California to the Yolo Bypass outside of Woodland, California and is fed by a watershed that covers 1150 square miles. A system of levees provides flood protection to agricultural land and the City of Woodland along the lower reach of Cache Creek and guides flow into the CCSB. The CCSB is located about 2 miles east of the City of Woodland on the boundary of the city proper and the surrounding agricultural lands. Bound by levees on all sides, the basin encompasses roughly 3600 acres that are utilized for agriculture when the basin is dry.

As shown in Figure 1, Cache Creek enters the training levees in the north-west corner of the basin and is guided south roughly two-thirds of the 2.8 mile length of the basin before opening up into the south-west region of the basin proper. A 1740-foot wide roller compacted concrete weir is built into the east levee, approximately 2 miles from the west levee. A low-flow channel roughly parallels the south levee, leading from the training channel to the low-flow outlet in the south-east corner of the basin. The low-flow outlet is comprised of two 5-foot wide by 4-foot tall flap-gated box culverts controlled by a dual sluice gate system. The low-flow outlet is believed to pass no more than 580 cubic feet per second (cfs). The training channels and outlet weir are designed to pass flood flows of up to 30,000 cfs, representing approximately 10 to 20-year flood protection.

The Finite Element Surface Water Modeling System, Two-dimensional Depth-averaged Flow and Sediment Transport Model (FST2DH) developed by David C. Froehlich, Ph.D is used for hydraulic and sediment transport modeling of the CCSB.

The Surface Water Modeling System (SMS) is used for pre-processing and post-processing of model data.

Figure 1. Aerial View of CCSB

Background

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Originally built in 1938, the CCSB was designed to control sediment deposition and increase sediment storage capacity, protecting loading to the Yolo Bypass. The basin has been modified many times since 1938 to redistribute sediment settling patterns and increase sediment storage capacity. In the US Army Corps of Engineers (USACE) 1961 Operations Manual of the Cache Creek Levees and Settling Basin, it is stated that “encroachment of debris deposits in the Settling Basin cannot be predicted in advance” and thus sediment retention was to be increased as needed by removal of interior portions of the training levee that feed into the basin proper. With regard to controlling sediment loading to the Yolo Bypass the operations manual states, “It will be necessary to meet the problem as it arises in the future.” This management strategy has not changed significantly in the last 50 years, as little has been done to analyze sediment retention efficiency.

In 1992, the basin was radically altered. Surrounding levees were raised 12 feet and a new outlet weir, 5 feet higher than the previous weir, was built representing current conditions. At the time of alteration it was believed that an additional 50 years of sediment storage was being provided, with an average trap-efficiency of 50 percent, for an average retention of 340 acre-feet of sediment per year. This postulated trap efficiency is based on an action plan outlined in the USACE 2007 Cache Creek Settling Basin Operations and Maintenance Manual, in which the outlet weir is to be raised an additional 6-feet at year 25 (2017) of the project, or when the trap-efficiency is 30%. Also beginning in year 25 of the project, 400 foot sections of the interior training levee will be removed every five years starting with a section 1100 feet upstream from the current terminus of the training channel. Each subsequent 400 foot section will be removed 1100 feet upstream from the section removed previously. Hydraulic and sediment transport modeling is to be completed for the current and all proposed configurations.

A number of abandoned, un-reclaimed and partially reclaimed mercury mines in the Cache Creek basin have permanently altered the landscape of the Cache Creek watershed, increasing sediment erosion and sediment loading in runoff to Cache Creek. Additionally, Cache Creek has been subject to aggregate mining which has increased channel incision and sediment loading in the creek. Sixteen miles of Cache Creek, between the towns of Capay and Yolo, have been extensively mined for aggregates (Thayer, 2009). Historically, Cache Creek has been mined in an excessive manner, in which channel wide mining has been applied. Kondolf et al. described the mining as “a visually impressive example of channel-wide mining, with the entire active channel excavated over a width of 460 m (1500 ft), creating a broad, flat surface that was likened to an airport by local residents”. This type of channel wide mining, which is now illegal, eliminates natural features such as bars and permanently alters the sediment transport mechanisms of the creek. Further evidence of the effect of aggregate mining has been found downstream of Capay Bridge, where aerial photographs have documented the extension of an in-bed gravel pit upstream. The continued migration of the gravel pit contributed to the near failure of the Capay Bridge in the 50-year flood of 1995 (Kondolf et al.).

Suitability of FST2DH for Modeling the CCSB

FST2DH simultaneously simulates the movement of water and non-cohesive sediment in rivers, estuaries and coastal waters. The Galerkin finite element method is applied to solve either steady-state or time-dependent systems of equations for two-dimensional depth averaged surface-water flow and for the transport of non-cohesive sediment by surface waters. The depth averaged surface water flow equations are forms of Navier-Stokes equations, found by integrating three-dimensional mass and momentum transport equations with respect to the vertical coordinate assuming vertical velocities are negligible. A momentum flux correction coefficient can be applied to compensate for any vertical velocity variations by assuming a logarithmic vertical velocity profile. Changes in bed level by two-dimensional depth averaged flow are modeled with the sediment continuity equation.

The two-dimensional model is applicable to the CCSB as vertical velocities and accelerations are believed to be small in the basin compared with obvious 2-D horizontal velocities and accelerations. FST2DH is particularly well-suited to the flow events of interest in the CCSB such as early rainy season flows expected to be carrying larger loads of suspended sediment and flood flows that will radically alter the amount of wetted area in the basin.

The suitability of FST2DH to basin conditions and the models accuracy have been evaluated through the modeling of a number of physical experiments previously conducted at the University of California, Davis J. Amorocho Hydraulics Laboratory (JAHL). Sturgeon passage experiments completed in 2005 provided detailed data to be used in the comparison of water surface elevation and velocity (Carr, 2006). The experiments provided complicated water surface and velocity profiles owing to the pattern of contraction and expansion used in the sturgeon passage design. The baffles shown in Figure 2 were repeated 5 times, approximately every 12 feet in an 80 foot flume. Steady state modeling of the experiments using FST2DH showed the model to be accurate with little disagreement between measured and predicted velocities and water surface elevation after the first run of the model. After the first model simulation the average percent error in water surface elevation was 12 percent, and the average percent error in velocity was 10 percent. After adjustments to the roughness value used in modeling predictions of both water service elevation and velocity improved, yielding average percent errors below 5 percent. Comparison of modeled and measured centerline water surface elevations is shown in Figure 3,

comparison of modeled and measured centerline velocities is shown in Figure 4.

Figure 2. Baffle system used in sturgeon passage experiments.

Centerline Water Depth

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Figure 3. Comparison of FST2DH modeled and experimental centerline water surface elevations.

Depth Averaged Centerline Velocity

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Figure 4. Comparison of FST2DH modeled and experimental centerline velocities.

Testing of FST2DH sediment transport capabilities was accomplished in the comparison of modeled and measured results of a roughness study completed at the

JAHL (Chen et al., 2009). The experiment ran clear water over a bare soil bed in a 6 foot tall, 4 foot wide flume with 65 foot long experimental section to calculate roughness. Pre-experiment and post-experiment bed heights are known. Modeling was accomplished by inputting known volumetric discharge, pre-experiment flume bed coordinates, weir height at the outlet, and experimentally determined roughness values. Sediment grain size distribution of the bed is unknown. Water surface height and bed elevation changes predicted by the model for three different experimental set-ups were compared to measured results. Comparison of modeled and measured results for a velocity of 1 ft/sec and depth of 5.2 feet (experiment S1) are shown in Figures 5 and 6. Predicted water surface elevation values were extremely accurate with no adjustment necessary for tuning parameters such as bed shear and roughness.

FST2DH was able to fairly accurately model the sediment transport during the experiments, capturing the magnitude and location of erosion in the flume. Isolated discrepancies between the real and model sediment surface is attributed to movement of hard-packed, clumped soil which FST2DH can not easily predict, but was visually observed during experimentation.

Comparison of experimental and physical results has shown the model to be fairly accurate. The model’s predictive ability will be enhanced through calibration using results of physical experiments utilizing sediment from the CCSB in which the grain size distribution of the bed and input sediment volumes are well known.

S1 Water Surface Elevation

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Figure 5. Comparison of FST2DH modeled and experimental centerline water surface elevations.

S1 Elevation Change - Centerline

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Figure 6. Measured and modeled elevation change from experiment S1.

Modeling of the CCSB

To begin effectively modeling the CCSB, the training channel was isolated and modeled with real flow data from USGS site 11452600, Cache Creek at Road 102, taken on February 23, 2009 (USGS, 2009). Daily flow was 2240 cfs with an upstream boundary water surface elevation of 38.74 feet. Velocity results for the simulation are shown in Figure 7. Modeling was also accomplished at a documented low flow condition (500 cfs, 37 ft) and for a theoretical maximum flow condition (30,000 cfs, 41.5 ft). Realistic results were available after extensive editing of the grid mesh. Due to a lack of data, comparison of modeled velocities and water surface elevations in the training channel with actual values is not possible at this time.

Modeling of the training channel alone helped to reveal where mesh refinement is necessary and where the grid can be relaxed, altering processing time. Training channel modeling of the water surface elevation, as shown in Figure 8, has revealed the necessity to model an additional 5000 feet upstream of the present model inlet in order to eliminate boundary affects and assure confidence in the modeled results in the training channel. An accurate modeling of the channel is crucial for accurate modeling of the settling basin as a whole.

Figure 7. Training channel velocities with 2240 cfs flow.

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Figure 8. Centerline water surface elevation of 2240 cfs simulation.

Models of the basin as a whole have been initiated for steady state and dynamic flow conditions. The elevation contour for the basin is shown in Figure 9. Evaluations of maximum flows of 30,000 cfs, with a downstream (weir) boundary condition of 41.5 feet, and median flows of 14,200 cfs, with a downstream boundary condition of 40.54

feet have begun. Preliminary model runs revealed the need for extensive augmentation to the mesh grid, and better definition of the low flow channel which travels east to west along the southern levees of the basin. Upon completion of the intensive mesh edit, weir and low flow flap gate behavior will be analyzed and dynamic sediment transport modeling initiated.

Figure 9. Elevation contour of CCSB.Reference:Carr, K. (2006). Swimming Behavior Response of Adult White Sturgeon, Acipenser

Transmontanus, to Velocity, Turbulence Intensity and Turbulent Kinetic Energy in a Vertical Slot Passageway (Master’s Thesis). University of California, Davis.

Chen, Z.Q., Kavvas, M.L., Bandeh, H., and Tan, E., Carlon, J., Griggs, T., Lorenzato, S. (2009). Study of the roughness characteristics of native plant species in California floodplain wetlands. Report to Department of Water Resources, State of California. June, 2009.

Kondolf, G. M. (1997) “Hungry Water: Effects of Dams and Gravel Mining on River Channels.” Environmental Management 21(4), 533-551.

Thayer, Robert. “Cache Creek Nature Preserve.” Putah and Cache: A Thinking Mammals Guide to the Watershed. University of California, Davis. June 8, 2009. <http://bioregion.ucdavis.edu/book/13_Lower_Cache_Creek/13_03_circ_ccnp.html>

US Army Corps of Engineers (1961). Supplement to Standard Operations and Maintenance Manual, Sacramento River Flood Control Project: Unit no. 126 Cache Creek Levees and Settling Basin, Yolo Bypass to High Ground. US Army Corps of Engineers Sacramento District. November, 1961.

US Army Corps of Engineers (2007). Cache Creek Basin, California: Cache Creek Settling Basin Enlargement: Operations and Maintenance Manual. US Army Corps of Engineers Sacramento District. March,2007.

USGS (2009). “USGS Real-Time Water Data for USGS 11452600” USGS Cache Creek Inflow to Settling Basin Nr Woodland CA. February 23 , 2009. < http://waterdata.usgs.gov/nwis/measurements/?site_no=11452600>