Sessom Creek Sand Bar Removal HCP Task 5.4.6

Transcription

1 Sessom Creek Sand Bar Removal HCP Task Prepared by: Dr. Thomas Hardy Texas State University Dr. Nolan Raphelt Texas Water Development Board January 6, 2013 DRAFT 1

2 Introduction The confluence of Sessom Creek and the San Marcos River maintains a sediment bar that has grown substantially over the past decade. The bar has widened, deepened, and constricted the San Marcos river channel downstream of the confluence and covered a small stand of Texas wild rice (TWR). The sediment bar has become vegetated with both littoral and terrestrial plants, and recreationists currently use the river channel adjacent to the bar as it provides a shallow swimming area. To minimize and mitigate the impacts of incidental take from recreation, Texas State University and the City of San Marcos agreed to conducted a study of the sediment dynamics between Sessom Creek and the San Marcos River. The study specifically aimed to evaluate options for sediment bar removal that minimizes impacts to listed species while providing the best possible hydrodynamic configuration of the channel at the Sessom Creek confluence and downstream sections of the San Marcos River within Sewell Park thus protecting and/or improving TWR conditions within these sections of the San Marcos River. Methods Hydrodynamic Model The U.S. Army Corps of Engineer s Adaptive Hydraulics (AdH) Model (AdH version 4.31; Berger et al., 2013) using the 2 dimensional Navier Stokes shallow water equations was utilized for existing conditions and all alternatives simulations. One of the major benefits of ADH is its use of adaptive numerical meshes that can be employed to improve model accuracy without sacrificing efficiency (see Figure 1). Figure 1. Example of adaptive mesh properties to simulate sediment flow. DRAFT 2

3 AdH allows sediment transport to be coupled to bed and hydrodynamic changes to evaluate the flow dependant dynamics of sediment delivery from Sessom Creek to the confluence area of the San Marcos River and downstream stretches within Sewell Park under existing and alternative channel configurations. Computational Mesh, Channel Roughness and Hydraulic Calibration Data Three dimensional channel topographies for the upper San Marcos River were measured with standard survey instruments using a systematic irregular sampling strategy as part of the ARRA funded San Marcos Observing System (SMOS) (Figure 2). Figure 2. Field measurements of channel topography in the San Marcos River Calibration discharges (2.57 and 6.36 m 3 /s) and corresponding longitudinal profiles of the water surface elevation were measured throughout the study reach. GPS was used to map homogeneous polygons of aquatic vegetation and river channel substrate (Figure 3). The topography data was utilized to generate the starting triangular irregular network and the corresponding vegetation and substrate data integrated over the spatial domain to assign initial model roughness (e.g., Figure 4). Calibration of the model followed standard engineering practice and model parameters were adjusted until observed and predicted water surface profiles were less than 0.05 meters over the model domain under existing conditions. DRAFT 3

4 Figure 3. Aquatic vegetation and substrate polygons for the upper San Marcos River. Figure 4. Example of the triangular irregular network with integrated aquatic vegetation and substrate data for use in the AdH model. DRAFT 4

5 Modeled Flows and Suspended Sediment Inputs Flow partitioning between the western and eastern spillways of Spring Lake were assumed to be 90 and 10 percent respectively based on limited concurrent flow measurements at these sites. Modeled flows for the boundary conditions in the western and eastern spillways and Sessom Creek were derived from the Sessom Creek flow measurements and corresponding 15 minute USGS gage data for the San Marcos River at San Marcos ( ) with the assumed flow partitioning from Spring Lake. Non volatile suspended solids and discharge data within Sessom Creek for storm events (Dec through Nov , unpublished data by Dr. Weston Nowlin and Dr. Benjamin Schwartz at Texas State University) were utilized in the modeling. For modeling purposes, sediment input from Sessom Creek was assumed to be composed of silt (80%), very fine sand (15%), and fine sand (5%). It was assumed that Spring Lake acts as a complete sediment trap during the evaluated storm events. A representative sequence of storm flows and corresponding sediment loading as shown in Figure 5. Figure 5. Flows and sediment loadings from Sessom Creek utilized in modeling scenarios. Existing Conditions DRAFT 5

6 Figure 6 shows the existing conditions of the channel topography and orientation of the outflow of Sessom Creek. It also shows the severe northern bank erosion area that threatens to undermine the back side of the channel armoring in the main channel of the San Marcos River below the western spillway. This bank erosion is due to the orientation of the outflow from Sessom Creek that angles into the northern bank and will continue without some form of bank stabilization or reorientation of the Sessom Creek outflow direction. Figure 6. Channel configuration under existing conditions at the Sessom Creek confluence. Simulations under existing conditions clearly show that the current channel configuration will continue to propagate the confluence sediment bar. This is in part due to the difficulty in overcoming the bulk momentum of the main stem San Marcos River by the Sessom Creek flows entering at the point where flow separation is occurring at the downstream toe of the existing concrete bank protection area on San Marcos River right. It is also evident that the entrance angle of Sessom Creek will continue to cause the northern bank to erode and ultimately cause undercutting of the large cypress adjacent to the northern bank and erode the backside of the concrete wall causing structural failure. Partial Sediment Bar Removal DRAFT 6

7 Figure 7 shows the modified channel to allow the outflow from Sessom Creek to enter the San Marcos River at a more favorable angle to promote downstream sediment entrainment. Figure 7. Alternative channel configuration at the Sessom Creek confluence with the San Marcos River. Figure 8 shows the velocity vectors during the peak runoff event simulated for the study. The simulation suggests that this orientation would provide for a better integration of the hydraulics of Sessom Creek flows into the main stem San Marcos; however, the northern shoreline on the left side of the Sessom Creek confluence will still exhibit significant erosion potential and endangerment of the retaining wall. Figure 9 shows the expected fine sand flow dynamics during this event and as can be seen the highest concentration remains adjacent to the Sessom Creek confluence and will likely result in sand bar building over time. Figure 9 shows the velocity field at nominal low flows at the confluence and clearly shows the flow separation and reverse velocity field that will result in continued sediment accumulation over time in this section of the channel. DRAFT 7

8 CITY OF SAN MARCOS ATTACHMENT 2 JANUARY 6, 2013 Figure 8. Simulated velocity field under peak flow conditions in the San Marcos River and Sessom Creek. Figure 9. Fine sand plum at the Sessom Creek confluence during peak runoff event. DRAFT 8

9 Figure 10. Simulated velocity field under nominal low flow condtions in the San Marcos River and Sessom Creek. Alternative Sessom Creek Orientation Figure 11 provides a plan view in which the Sessom Creek outflow has been modified into a closed culvert and the outflow located within the straight section of the San Marcos River. The alternative was examined as it would eliminate the erosion problem along the northern shoreline at the confluence and allow for the outflow of sediment to be entrained in the higher velocity fields of the San Marcos River without a flow separation vortex to avoid sediment accumulation as shown in Figure 12. Although subtle, this orientation pushes the velocity field and entrained sediment along the river left half of the channel and maintains a higher transport rate. This orientation also moves the flow field to river left and provides some protection for the existing Texas wild rice stands that are located mid channel to river right below the highway bridge. DRAFT 9

10 Figure 11. Reorientation of Sessom Creek into a closed culvert. Figure 12. Fine sand plum at the Sessom Creek confluence during peak runoff event. DRAFT 10

11 Discussion Hydraulic and sediment transport modeling at the Sessom Creek confluence suggest that under the existing or channel modification scenarios that the northern bank of the San Marcos River will continue to erode and endanger the integrity of the existing retaining wall on right below the Saltgrass restaurant. Changes the physical orientation (shape) of the existing sediment bar at the confluence will only provide a short term solution as the upstream bank orientation will continue to result in flow separation and a reverse flow vortex that will promote sediment accumulation over time. The alternative configuration of changing the outflow structure at Sessom Creek would appear to provide the necessary protection to the northern bank and allow the entrainment of outflows from Sessom Creek into the flow of the San Marcos River. This configuration also eliminates the reverse flow vortex and reduces the potential for future sediment bar formation at the confluence of Sessom Creek and the San Marcos River. Recommendation The Facilities Department at Texas State University have reviewed the simulations and are interested in collaborating with the City of San Marcos and the HCP on an integrated project to address not only the sediment bar at Sessom Creek but also address the bank erosion problem along the northern shoreline at the confluence. Literature Cited Berger, R.C., J.N. Tate, G.L. Brown, and G. Savant. Adaptive Hydraulics: A two dimensional modeling system developed by the Coastal and Hydraulics Laboratory Engineering Research and Development Center. USERS MANUAL: Guidelines for Solving Two Dimensional Shallow Water Problems with the Adaptive Hydraulics Modeling System. 99 pp. DRAFT 11