Post on 27-Mar-2015
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