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Trends in Suspended Sediment Input to the San Francisco Bay from Local Tributaries

This presentation discusses the spatial and temporal trends in suspended sediment records in the San Francisco Bay, focusing on dominant geomorphic processes, spatial and temporal variability, and the implications of decreasing sediment yields.

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Trends in Suspended Sediment Input to the San Francisco Bay from Local Tributaries

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  1. Trends in Suspended Sediment Inputto the San Francisco Bayfrom Local Tributaries • Presented by • Setenay Bozkurt • s.bozkurt@pwa-ltd.com • Philip Williams & Associates, Ltd. • San Francisco, CA • December 2005

  2. PROJECT OBJECTIVE • To detect spatial and temporal trends in suspended sediment records • No emphasis on the absolute amounts of sediment entering the Bay • McKee et al, 2003 for sediment delivery to the Bay

  3. DISCUSSION TOPICS • CONCEPTUAL MODEL • GEOMORPHIC PROCESSES • SPATIAL VARIABILITY • TEMPORAL VARIABILITY • SUSPENDED SEDIMENT RECORDS • SPATIAL TRENDS • TEMPORAL TRENDS • CONCLUSIONS

  4. GEOMORPHIC PROCESSES

  5. GEOMORPHIC PROCESSES Dominant sediment sources: Landslides, gullies, bed/bank erosion, rills and sheetwash

  6. DISCUSSION TOPICS • CONCEPTUAL MODEL • GEOMORPHIC PROCESSES • SPATIAL VARIABILITY • TEMPORAL VARIABILITY • SUSPENDED SEDIMENT RECORDS • SPATIAL TRENDS • TEMPORAL TRENDS • CONCLUSIONS

  7. CONCEPTUAL MODEL FOR SPATIAL VARIABILITY • Drainage area • Topography • Climate • Hydrology • Geology/Soils • Land use/ Land Cover • Processes

  8. DISCUSSION TOPICS • CONCEPTUAL MODEL • GEOMORPHIC PROCESSES • SPATIAL VARIABILITY • TEMPORAL VARIABILITY • SUSPENDED SEDIMENT RECORDS • SPATIAL TRENDS • TEMPORAL TRENDS • CONCLUSIONS

  9. Native American Life in California

  10. Arrival of the Spanish… and their cows

  11. All of this occurred before the invention of the modern “low impact” cow

  12. California in the 20th Century

  13. California in the 20th Century: Urban and suburban growth

  14. Urbanization tends to increase stormwater runoff: • peak flows • volume • frequency Post-Development Hydrograph Modification Impacts Pre-Development Runoff Time

  15. Erosion impacts: • small, frequent flows move most sediment • hydromodification increases frequency of small flows • increased sediment transport = erosion Hydrograph Modification Impacts

  16. Incision in Bay Area streamsdue to hydromod

  17. Sediment Yield: Agricultural sources

  18. CONCEPTUAL MODEL FOR TEMPORAL VARIABILITY Modified from Wright and Schoellhamer, 2004

  19. DISCUSSION TOPICS • CONCEPTUAL MODEL • GEOMORPHIC PROCESSES • SPATIAL VARIABILITY • TEMPORAL VARIABILITY • SUSPENDED SEDIMENT RECORDS • SPATIAL TRENDS • TEMPORAL TRENDS • CONCLUSIONS

  20. SPATIAL TRENDS IN SUSPENDED SEDIMENT RECORDS • 19 watersheds • Look for correlation of suspended sediment yield with: • drainage area • annual runoff • relief • % sand in soils • % area with landslides • mean annual precipitation • rainfall threshold statistics • % area with steep slopes • % sandy soils • % area of rangeland and barren land • % area of rangeland with steep slopes • % impervious area

  21. SPATIAL CORRELATIONS • Significant correlations between sediment yield and • Drainage area: r2 = 0.79 • Mean annual runoff: r2 = 0.78 • Relief: r2 = 0.68

  22. SPATIAL CORRELATIONS • No multiple regression possible • due to small data set • Several parameters weakly correlated: • % sand in soils • % landslide area • % impervious area

  23. DISCUSSION TOPICS • CONCEPTUAL MODEL • GEOMORPHIC PROCESSES • SPATIAL VARIABILITY • TEMPORAL VARIABILITY • SUSPENDED SEDIMENT RECORDS • SPATIAL TRENDS • TEMPORAL TRENDS • CONCLUSIONS

  24. TEMPORAL TRENDS IN SUSPENDED SEDIMENT RECORDS • Do sediment loads change over time? • Compare sediment concentrations for a given discharge range over long term

  25. TEMPORAL TRENDS IN SUSPENDED SEDIMENT RECORDS

  26. PREDICTING SUSPENDED SEDIMENT INPUT TO THE BAY • Sediment rating curves: Qs vs. Q • Sediment loads correlated well for all stations • r2 > 0.70 • Scatter due to: • Inherent random changes in any system at any given time • Nonlinear relation between discharge and concentration • Hysteresis and sediment lag of floods • Better correlation and predictability if low discharges are excluded

  27. SEDIMENT RATING CURVES

  28. DISCUSSION TOPICS • CONCEPTUAL MODEL • GEOMORPHIC PROCESSES • SPATIAL VARIABILITY • TEMPORAL VARIABILITY • SUSPENDED SEDIMENT RECORDS • SPATIAL TRENDS • TEMPORAL TRENDS • CONCLUSIONS

  29. SUMMARY and CONCLUSIONS • Suspended sediment records are spatially and temporally limited • Significant functional relationship between discharge and sediment load for all stations • rating curves can be used to estimate sediment yields and to extrapolate data for prediction

  30. SUMMARY and CONCLUSIONS • Decreasing sediment yields in • Alameda Creek at Niles • Colma Creek at South San Francisco • Cull Creek above the Reservoir • Drainage area, mean annual runoff, and relief • are the best variables to predict sediment yield from local watersheds

  31. SUMMARY and CONCLUSIONS • Significance of decreasing suspended sediment trends • implications of decreasing sediment yields for large scale restoration projects around the Bay • Need for more quantitative measurements on: • dominant geomorphic process e.g. gullies, landslides, channel erosion • Need for more suspended sediment measurements

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