Trends in Suspended Sediment InputTrends in Suspended Sediment Inputto the San Francisco Bayto the San Francisco Bay

from Local Tributariesfrom Local Tributaries

Presented by

Setenay Bozkurt

s.bozkurt@pwa-ltd.com

Philip Williams & Associates, Ltd.

San Francisco, CA

December 2005

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

DISCUSSION TOPICS

• CONCEPTUAL MODEL– GEOMORPHIC PROCESSES– SPATIAL VARIABILITY– TEMPORAL VARIABILITY

• SUSPENDED SEDIMENT RECORDS– SPATIAL TRENDS– TEMPORAL TRENDS

• CONCLUSIONS

GEOMORPHIC PROCESSES

GEOMORPHIC PROCESSES

Dominant sediment sources:

Landslides, gullies, bed/bank erosion, rills and sheetwash

DISCUSSION TOPICS

• CONCEPTUAL MODEL– GEOMORPHIC PROCESSES– SPATIAL VARIABILITY– TEMPORAL VARIABILITY

• SUSPENDED SEDIMENT RECORDS– SPATIAL TRENDS– TEMPORAL TRENDS

• CONCLUSIONS

CONCEPTUAL MODEL FOR SPATIAL VARIABILITY

Drainage area

Topography

Climate

Hydrology

Geology/Soils

Land use/ Land Cover

Processes

DISCUSSION TOPICS

• CONCEPTUAL MODEL– GEOMORPHIC PROCESSES– SPATIAL VARIABILITY– TEMPORAL VARIABILITY

• SUSPENDED SEDIMENT RECORDS– SPATIAL TRENDS– TEMPORAL TRENDS

• CONCLUSIONS

Native American Life in California

Arrival of the Spanish… and their cows

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

California in the 20th Century

California in the 20th Century: Urban and suburban growth

Hydrograph Modification Impacts

Run

off

Time

Pre-Development

Urbanization tends to increase stormwater runoff:

peak flows

volume

frequency

Post-Development

Hydrograph Modification Impacts

Erosion impacts:

small, frequent flows move most sediment

hydromodification increases frequency of small flows

increased sediment transport = erosion

Incision in Bay Area

streamsdue to

hydromod

Sediment Yield: Agricultural sources

CONCEPTUAL MODEL FOR TEMPORAL VARIABILITY

Disturbance Expected effect on sediment yieldGrazing Increase

Urban/suburban development Initial increase, then decrease

Channelization Increase

Bed and bank protection Decrease

Dams and reservoirs Decrease

Channel dredging Decrease

Other factors (agricultural development and logging)

Increase

Modified from Wright and Schoellhamer, 2004

DISCUSSION TOPICS

• CONCEPTUAL MODEL– GEOMORPHIC PROCESSES– SPATIAL VARIABILITY– TEMPORAL VARIABILITY

• SUSPENDED SEDIMENT RECORDS– SPATIAL TRENDS– TEMPORAL TRENDS

• CONCLUSIONS

SPATIAL TRENDS IN SUSPENDED SEDIMENT RECORDS

19 watershedsLook 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

SPATIAL CORRELATIONS

Significant correlations between sediment yield and

– Drainage area: r2 = 0.79– Mean annual runoff: r2 = 0.78– Relief: r2 = 0.68

SPATIAL CORRELATIONS

No multiple regression possible

due to small data set

Several parameters weakly correlated:

% sand in soils

% landslide area

% impervious area

DISCUSSION TOPICS

• CONCEPTUAL MODEL– GEOMORPHIC PROCESSES– SPATIAL VARIABILITY– TEMPORAL VARIABILITY

• SUSPENDED SEDIMENT RECORDS– SPATIAL TRENDS– TEMPORAL TRENDS

• CONCLUSIONS

TEMPORAL TRENDS IN SUSPENDED SEDIMENT RECORDS

Do sediment loads change over time?

Compare sediment concentrations for a given discharge range over long term

TEMPORAL TRENDS IN SUSPENDED SEDIMENT RECORDS

Alameda Creek

Colma Creek

Cull Creek

San Lorenzo Creek

2 - 10% Qw Range10 - 20% Qw Range

-1 -0.5 0 0.5 1

No Trend

Negative Small / ModerateTrend Positive Small / ModerateTrend

Kendall Tau Value

PREDICTING SUSPENDED SEDIMENT INPUT TO THE BAY

Sediment rating curves: Qs vs. QSediment loads correlated well for all stations

r2 > 0.70Scatter 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

SEDIMENT RATING CURVES

DISCUSSION TOPICS

• CONCEPTUAL MODEL– GEOMORPHIC PROCESSES– SPATIAL VARIABILITY– TEMPORAL VARIABILITY

• SUSPENDED SEDIMENT RECORDS– SPATIAL TRENDS– TEMPORAL TRENDS

• CONCLUSIONS

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

SUMMARY and CONCLUSIONS

Decreasing sediment yields inAlameda Creek at NilesColma Creek at South San FranciscoCull Creek above the Reservoir

Drainage area, mean annual runoff, and reliefare the best variables to predict sediment yield from local watersheds

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