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West Bay Diversion Evaluation 1-Dimensional Modeling CWPPRA Technical Committee

West Bay Diversion Evaluation 1-Dimensional Modeling CWPPRA Technical Committee and Peer Review Team Presentation 2 December 2009 Freddie Pinkard ERDC - CHL. West Bay Diversion Evaluation 1-Dimensional Modeling. Purpose Model Capabilities, Limitations, & Requirements Modeling Approach

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West Bay Diversion Evaluation 1-Dimensional Modeling CWPPRA Technical Committee

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  1. West Bay Diversion Evaluation 1-Dimensional Modeling CWPPRA Technical Committee and Peer Review Team Presentation 2 December 2009 Freddie Pinkard ERDC - CHL

  2. West Bay Diversion Evaluation 1-Dimensional Modeling Purpose Model Capabilities, Limitations, & Requirements Modeling Approach MVK Regional HEC-6T Base Model ERDC HEC-6T Model Results and Conclusions Sensitivity Analyses

  3. West Bay Diversion Evaluation 1-Dimensional Modeling Purpose ●The 1-dimensional modeling provides the means of assessing the long term impacts (50 year simulations) of the diversion as well as providing upstream sediment boundary conditions for the multi dimensional models. ●The results of the 1-dimensional modeling will be integrated with the results of the geomorphic assessment and the multi dimensional models in order to provide a complete evaluation of the impacts of the West Bay Diversion.

  4. 1-Dimensional Modeling Model Capabilities, Limitations, and Requirements HEC-6T Sedimentation In Steam Networks ● is a 1-dimensional movable boundary open channel flow numerical model designed to simulate and predict changes in river bed profiles resulting from scour and/or deposition over moderate time periods ● provides computed total sediment discharge passing each cross section and the volume of deposits or scour accumulated at each cross section from the beginning of the simulation

  5. 1-Dimensional Modeling Model Capabilities, Limitations, and Requirements HEC-6T Advantages ●Allows for long term simulations ●Ability to simulate dredging activities ●Allows for the diversion of both water and sediment and calculates that impact on downstream sediment transport HEC-6T Disadvantage ●Uses average hydraulic and sediment parameters to simulate 3-dimensional processes in 1 dimension

  6. 1-Dimensional Modeling Model Capabilities, Limitations, and Requirements Input Requirements ●Channel Geometry ●Upstream Boundary Conditions ● Flow ● Sediment Loads ●Bed Material Gradations ●Distributary / Diversion Outflow and Sediment Concentrations ●Water Temperature ●Downstream Boundary Conditions Water Surface Elevations ●User Defined Sediment Transport Function

  7. 1-Dimensional Modeling Model Capabilities, Limitations, and Requirements Two PhasedCalibration ●Water Surface Profiles ●Sediment Observed Deposition and/or Erosion Measured Loads Dredging Records

  8. 1-Dimensional Modeling - Modeling Approach ●Utilize MVK developed regional HEC-6T model ●Add cross sections through study reach to better define channel geometry ●Compare ERDC collected diverted flow, sediment concentration, and bed material gradation data to model values ●As model input parameters are modified, check model calibration ●Determine subsidence and sea level rise rates

  9. 1-Dimensional Modeling Modeling Approach ●Develop typical discharge hydrograph ●Develop downstream water surface elevations to match the discharge hydrograph ●Develop composite dredge template ●Run hydrograph for 4 alternative scenarios West Bay Diversion Closed + No PAA Dredging West Bay Diversion Closed + PAA Dredging West Bay Diversion Open + No PAA Dredging West Bay Diversion Open + PAA Dredging

  10. 1-Dimensional Modeling Modeling Approach ●Compare sediment deposition locations and quantities and dredging locations and quantities through the PAA for the 4 50-year simulation scenarios to determine the impact that the West Bay Diversion has on the PAA

  11. 1-Dimensional Modeling MVK Regional HEC-6T Model ●Model Limits - Vicksburg, MS (RM 427.28) to Pilots’ Station in Southwest Pass (RM -18) ● Channel Geometry - 1992 Comprehensive Hydrographic Survey ●Cross Section Distribution

  12. 1-Dimensional Modeling MVK Regional HEC-6T Model ●Bed Material Gradations ● Nordin and Queen samples collected in 1989 ● 2 year frequency discharge run for 30 days to rework bed material gradations. ●Water Temperature – monthly averages from measured values at Tarbert Landing ● Downstream Water Surface Elevations – monthly average stages for the NOAA gage at Grand Isle East Point

  13. 1-Dimensional Modeling MVK Regional HEC-6T Model ●Diversion / Distributary Locations

  14. 1-Dimensional Modeling MVK Regional HEC-6T Model ● Diversion Flow Distributions – percentage of Mississippi River flow upstream of diversion estimated from measured data or calculated ●Sediment Concentration Diversion Ratios – ratio of sediment concentration in the diverted flow compared to sediment concentration in the Mississippi River derived by grain size for a range of flows from measured data or estimated using the Rouse equation ●Sediment Transport Function – Toffaleti / Meyer-Peter Muller

  15. 1-Dimensional Modeling MVK Regional HEC-6T Model ●Impact of Salinity – approximated by varying the silt and clay shear threshold deposition coefficients ●Water Surface Profile Calibration – gage stations

  16. 1-Dimensional Modeling MVK Regional HEC-6T Model ●Sediment Calibration ● observed deposition downstream of the Old River Complex ● observed erosion at Smithland Crossing ● measured sediment transport at Tarbert Landing and at Belle Chasse ● reported dredge volumes in Southwest Pass and Above Head of Passes

  17. 1-Dimensional Modeling ERDC HEC-6T Model ●Model Limits - Vicksburg, MS (RM 427.28) to Pilots’ Station in Southwest Pass (RM -18) ●Cross Section Distribution

  18. 1-Dimensional Modeling ERDC HEC-6T Model ● Analyze ERDC Collected Data / Compare With MVK Model Input / Revise Input As Required ● Flow Distribution At Diversions / Distributaries ● Sediment Concentration Ratios Between Diversions / Distributaries And The River ● Bed Material Gradations From RM 19.6 to RM -18

  19. 1-Dimensional Modeling - ERDC HEC-6T Model Flow Distribution – Baptiste Collette Bayou, Grand Pass, West Bay Diversion, and Cubits Gap

  20. 1-Dimensional Modeling - ERDC HEC-6T Model Flow Distribution – West Bay Diversion

  21. 1-Dimensional Modeling ERDC HEC-6T Model ● Analyze ERDC Collected Data / Compare With MVK Model Input / Revise Input As Required ● Flow Distribution At Diversions / Distributaries ● Sediment Concentration Ratios Between Diversions / Distributaries And The River ● Bed Material Gradations From RM 19.6 to RM -18

  22. 1-Dimensional Modeling - ERDC HEC-6T Model West Bay Diversion Sediment Concentration Diversion Ratios Derived From ADH 2-D Model

  23. 1-Dimensional Modeling ERDC HEC-6T Model West Bay Diversion Sediment Concentration Diversion Ratios

  24. 1-Dimensional Modeling ERDC HEC-6T Model ● Analyze ERDC Collected Data / Compare With MVK Model Input / Revise Input As Required ● Flow Distribution At Diversions / Distributaries ● Sediment Concentration Ratios Between Diversions / Distributaries And The River ● Bed Material Gradations From RM 19.6 to RM -18

  25. Bed Sample Locations Grand Pass West Bay Diversion

  26. Looking Downstream BSS-18 BSS-17 River Mile 5.5 ERDC Bed Sample Locations

  27. 1-Dimensional Modeling ERDC HEC-6T Model Bed Material Gradations At Site BSS-17

  28. 1-Dimensional Modeling ERDC HEC-6T Model Bed Material Gradations At Site BSS-18

  29. West Bay Diversion Cubits Gap Head Of Passes Bed Sample Locations

  30. BSS - 23 BSS - 26 Looking Downstream River Mile 2.5 ERDC Bed Sample Locations

  31. 1-Dimensional Modeling ERDC HEC-6T Model Bed Material Gradations At Site BSS-23

  32. 1-Dimensional Modeling ERDC HEC-6T Model Bed Material Gradations At Site BSS-26

  33. 1-Dimensional Modeling ERDC HEC-6T Model Subsidence Rates Derived From NOAA Technical Report NOS/NGS 50

  34. 1-Dimensional Modeling - ERDC HEC-6T Model BSS - 23 Typical Hydrograph At Vicksburg 1984 - 2009

  35. 1-Dimensional Modeling - ERDC HEC-6T Model ● Develop Downstream Boundary Condition ● Daily 8:00 AM Stage At Grand Isle, East Point Gage ● Rectify Datum Issues ●Sediment Transport Function – Toffaleti ● Extended the moveable bed limits and utilized a realistic dredging template (750 feet wide at -50 feet, MLG minus 1 foot overdepth / advance maintenance dredging)

  36. 1-Dimensional Modeling - ERDC HEC-6T Model ● Composite Dredge Template – extended dredging width to add an end area equivalent to match the PAA dredging. Width varied from 940 to 970 feet depending upon PAA dredging elevation (-41, -44, or -48 feet) at -50 feet, MLG ● Model under estimates reported dredging quantities in Southwest Pass but responds well in other reaches. Computational analysis instead of computational model.

  37. 1-Dimensional Modeling – ERDC HEC-6T Model Navigation Dredge Template vs. Composite Dredge Template Utilized Option Of Filling Dredge Cut First, Then Deposits Equally Across The Cross Section River Mile 2.46 – West Bay Diversion Open

  38. 1-Dimensional Modeling – Results Total Sediment Load for the Ms River Between RM 80 and RM -20 Relative to Total Sediment Load at RM 12.5

  39. 1-Dimensional Modeling – Results Total Sediment Load for the Ms River Between RM15 and RM -5 Relative to Total Sediment Load at RM 12.5 Baptiste Collette Bayou Grand Pass Cubits Gap West Bay Diversion Head Of Passes

  40. 1-Dimensional Modeling – Results River Mile 2.46 – West Bay Diversion Closed

  41. 1-Dimensional Modeling – ERDC HEC-6T Model River Mile 2.46 – West Bay Diversion Open

  42. 1-Dimensional Modeling – Results River Mile 2.46 West Bay Diversion Open / No PAA Dredging

  43. 1-Dimensional Modeling – Results River Mile 2.46 West Bay Diversion Open / With PAA Dredging

  44. 1-Dimensional Modeling – Results Average Dredging Per Cycle Upstream of Head of Passes During 10 Year Periods BSS - 23

  45. 1-Dimensional Modeling – Results Percentage of Dredging Due to Opening West Bay Diversion BSS - 23

  46. Sensitivity of Model Modifications ● Dredging Template Dimensions and Moveable Bed Width ● Sediment Concentration Diversion Ratios ● Subsidence Rates ● Daily Downstream Stages ● Increased Cross Sections to Better Define Channel Geometry

  47. Sensitivity Analyses (Recommended For 12 Month Effort) ●Future Flows (Typical Hydrograph) ●Future Development of West Bay Diversion ●Different Transport Functions ●Impact of Different Diversions / Distributaries

  48. 1-Dimensional Modeling – Conclusions ●West Bay Diversion can not be analyzed in isolation. ● Approximately 40% - 50% of the total sediment load at RM 12.5 is diverted and/or deposits upstream of Head of Passes. ● The 1-D model performs well in reproduction of deposition and dredging locations but under estimates the best available estimates of dredging quantities in Southwest Pass. ● From Cubits Gap downstream to Head of Passes, the channel functions as an efficient sediment trap. BSS - 23

  49. 1-Dimensional Modeling – Conclusions BSS - 23 ● Although the West Bay Diversion diverts only 7% of the total flow, the computed impact on dredging is disproportionably large. The 1-D model indicates that the West Bay Diversion accounts for 20% – 25% of the dredging required above Head of Passes including the PAA. ● Due to the influence of salinity and tides, a 3-D or laterally averaged hydrodynamic / salinity / sedimentation model may be required to resolve the sedimentation processes in Southwest Pass.

  50. West Bay Diversion Evaluation 1-Dimensional Modeling Questions / Comments

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