Palo Alto Flood Basin Hydrology Corrected Final Report Prepared for July 2016 Schaaf & Wheeler Consulting Civil Engineers Palo Alto Flood Basin Hydrology SCVW.20.14 Table of Contents Introduction ............................................................................................................................................... 1 Purpose ..........................................................................................................................................................................................................1 Background ..................................................................................................................................................................................................1 Previous Studies .........................................................................................................................................................................................3 Summary of Work .....................................................................................................................................................................................3 Information Gathering and Site Visit ..........................................................................................................................................3 Updated Flood Basin Hydrology and Hydraulics ..................................................................................................................4 Flood Basin Modeling .............................................................................................................................. 5 Palo Alto Flood Basin ...............................................................................................................................................................................6 Mayfield Slough ..........................................................................................................................................................................................6 Tide Gate ........................................................................................................................................................................................................8 Inflow to the PAFB ....................................................................................................................................................................................9 Tributary Watershed .........................................................................................................................................................................9 Precipitation ....................................................................................................................................................................................... 14 Runoff Curve Numbers .................................................................................................................................................................. 16 Calibration of Antecedent Moisture Condition and Base Flow .................................................................................... 17 Runoff Calculations ......................................................................................................................................................................... 2 4 Tide Boundary Conditions................................................................................................................... 27 Astronomic Tides .................................................................................................................................................................................... 27 Establishing a Coincident Boundary Condition ......................................................................................................................... 28 Observations from the Storm of Record ................................................................................................................................ 28 Assessing the Conditional Probability of Coincident High Tide .................................................................................. 29 Palo Alto Flood Basin Performance .................................................................................................. 33 Existing Condition Simulations ........................................................................................................................................................ 33 Starting WSEL for Simulations ................................................................................................................................................... 33 Tidal Timing Shift ............................................................................................................................................................................. 33 Levee Containment Elevations ................................................................................................................................................... 33 100-year Water Surface Elevations in PAFB .............................................................................................................................. 34 Summary of Tidal Shift Impact .................................................................................................................................................. 34 Statistical Analysis of Random Tide Shift .............................................................................................................................. 35 Future Planning Scenarios .................................................................................................................. 38 Sea Level Rise ........................................................................................................................................................................................... 38 Sea Level Rise Impact with Existing Flood Basin Configuration ................................................................................. 39 Tide Gate Structure and Flood Basin Modifications ................................................................................................................ 40 Gravity Remediation Alternatives ............................................................................................................................................ 43 Pumping ............................................................................................................................................................................................... 45 Conclusions ............................................................................................................................................................................................... 46 Appendices (digital under separate cover) Schaaf & Wheeler i Corrected Final Report July 2016 Palo Alto Flood Basin Hydrology SCVW.20.14 Tables Table 1. 1% Rainfall Coefficients and Depths for TDS Equation with 17.5" MAP ........................................................ 14 Table 2. Hyetographs ("Gages") Used in HEC-HMS ................................................................................................................... 16 Table 3. Matadero Creek Diversion Adjustment February 2-3, 1998 ............................................................................... 19 Table 4. Matadero Creek Diversion Adjustment February 13, 2000 ................................................................................. 19 Table 5. Flood-Frequency Statistics for Matadero Creek ........................................................................................................ 19 Table 6. Flood-Frequency Statistics for San Francisquito Creek ......................................................................................... 20 Table 7. Statistics for Flood-Frequency Correlation ................................................................................................................. 21 Table 8. Watershed Parameters for San Francisquito Creek at Stanford ........................................................................ 22 Table 9. Calibration of Storage Coefficient R ................................................................................................................................ 23 Table 10. Calibration of AMC for Watershed Modeling .............................................................................................................. 23 Table 11. Inflow Volume to PAFB ......................................................................................................................................................... 24 Table 12. Summary of Watershed Discharges ................................................................................................................................ 25 Table 13. Mean Tide Cycle at Dumbarton Bridge .......................................................................................................................... 27 Table 14. San Francisco Bay Boundary Conditions ...................................................................................................................... 30 Table 15. Comparison of Coincident 100-year Design Tide to Previous Design Tides ................................................ 32 Table 16. Summary of Tide Shift Impact on PAFB Stage ............................................................................................................ 35 Table