historical sedimentation in the san francisco estuary bruce jaffe 1, theresa fregoso 1, amy...
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Historical Sedimentation in the San Francisco Estuary
Bruce Jaffe1, Theresa Fregoso1,
Amy Foxgrover1, Shawn Higgins2
1 United States Geological Survey2 Kamman Hydrology and Engineering
San Pablo Bay
Outline
• The Estuary in the past 150 years
• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments
• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change
Outline
• The Estuary in the past 150 years
• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments
• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change
Jaffe et al., 1998; Capiella et al., 1999; Foxgrover et al., 2004; Jaffe and Foxgrover, 2006a; Jaffe and Foxgrover, 2006b; Jaffe et al., 2007; Fregoso et al., 2008; Jaffe, 2009
Outline
• The Estuary in the past 150 years
• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments
• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change
The Data
Image obtained from NOAA photo library (www.photolib.noaa.gov)
Sample Data- South San Francisco Bay
Generation of Bathymetric Models
• Digitize soundings, contours, and marsh extent
• Georeference data• Error checking • Generate 25 or 50m bathymetric grid using TopoGrid
San Francisco Estuary
in the 1980s
• Area: ~1200 km2 • Average depth: 7 m• Median depth: 3 - 4 m• Deepest point: ~120 m
(Golden Gate)
The morphology of the estuary has changed
1850s 1890s 1920s
1950s 1980s
Generation of Sedimentation Grids
• Correct to a common vertical datum
• Adjust for subsidence (as needed)
• Difference the grids
Difference Grids
Net sediment gain of ~250 million cubic meters from 1850s - 1980s
Suisun Bay
Loss
52 Mcm
San Pablo Bay
Gain
339 Mcm
Central Bay
Gain
71 Mcm
( loss of 48 Mcm from borrow areas)
South Bay
Loss
90 Mcm
( loss of 38 Mcm from borrow areas)
The patterns of sediment gain and loss are complex
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1950s
1950s to 1980s
San Pablo Bay
Suisun Bay
South Bay
Central Bay
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
1850s to 1890s- gain of ~290 Mcm
Hydraulic mining
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1950s to 1980s
San Pablo BaySuisun Bay
1890s to 1950s
South Bay
Central Bay
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
After hydraulic mining
1890s to 1920s- loss of ~100 Mcm
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1950s to 1980s
San Pablo Bay
1890s to 1950s
Suisun Bay
South Bay
Central Bay
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
1920s to 1950s- gain of ~100 Mcm
SF Bay Area population increase
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1890s to 1920s
1920s to 1950s
1950s to 1980s
1850s to 1890s
1950s to 1980s
San Pablo Bay
Suisun Bay
1890s to 1950s
South Bay
Central Bay
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
300
Mcm
0
200
100
-100
1950s to 1980s- loss of ~200 Mcm
Water projects
From the 1850s-1980s
Circular bays near the center of the Estuary bays (San Pablo and Central) had net deposition
Elongate bays at ends of estuary (Suisun and
South) had net erosion
From 1950s-1980s
All bays were erosional
-100
0
100
200
300
400
500
1850 1900 1950 2000
Suisun BaySan Pablo BayCentral BaySouth Bay
Cum
ulat
ive
Sed
imen
tatio
n (1
0 6
x m
3)
Year
Erosion
DepostionNet
Net
Question:
Will the bays continue to erode?
-100
0
100
200
300
400
500
1850 1900 1950 2000
Suisun BaySan Pablo BayCentral BaySouth Bay
Cum
ulat
ive
Sed
imen
tatio
n (1
0 6
x m
3)
Year
Erosion
Depostion
?
?
??
-100
0
100
200
300
400
500
1850 1900 1950 2000
Suisun BaySan Pablo BayCentral BaySouth Bay
Cum
ulat
ive
Sed
imen
tatio
n (1
0 6
x m
3)
Year
Erosion
Depostion
?
?
?
Partial answer:
Not necessarily….
South San Francisco Bay had net deposition from 1983 to 2005
No data for other bays
Outline
• The Estuary in the past 150 years
• Sediment connectivity of sub-embayments
• Sediment delivery control on Sediment delivery control on tidal flat changetidal flat change
Sediment Connectivity
Sub-embayment#1
Sub-embayment#2
Sediment Exchange
Determining the degree of sediment connectivity
• Modeling studies
• Tracers (natural and anthropogenic)
• Patterns and quantities of erosion and deposition
Determining the degree of sediment connectivity
• Patterns and quantities of erosion and deposition– Similar behavior (erosion and deposition occur at sub-embayments at the same time)
– Opposite behavior of subembayments with similar magnitude (sediment eroded from one sub-embayment transported and deposited in another embayment)
– Quantity of deposition greater than local tributary supply (sediment coming from somewhere else)
Net
Net
South SF Bay example of similar behavior (inferred sediment
connectivity)
Outline
• The Estuary in the past 150 years
• Sediment connectivity of sub-Sediment connectivity of sub-embaymentsembayments
• Sediment delivery control on tidal flat change
Tidal flats of the estuary have changed
1850s 1890s 1920s
1950s 1980s
San Pablo Bay
Central Bay
Suisun Bay
South Bay
1850s1890s1920s1950s1980s
0
20
40
60
80
100
km2
0
20
40
60
80
100
km2
0
20
40
60
80
100
km2
0
20
40
60
80
100
km2Tidal flat area decrease >50%
Intertidal mudflat area responded to changes
in sediment supply to San Pablo Bay
Mudflats widened where sediment was abundant
Jaffe et al., 2007
Summary- Historical Sedimentation in the San Francisco Estuary
• The estuary in the past 150 years– The patterns of gain and loss of sediment are complex
– There were periods with net gain (e.g., hydraulic mining in mid-1800s) and periods with net loss (e.g., water projects in mid- to late 1900s)
• Sediment connectivity of sub-embayments– Patterns and quantities of erosion and deposition point towards a connected system
• Sediment supply controls on tidal flats– In San Pablo Bay, increased tidal flat area during the hydraulic mining period corresponded to increased sediment supply. Decrease in tidal flat area during the late 1900s corresponded with a decrease in sediment supply.
Sediment gains and losses- 1950s to 1980s
Sediment removal from 1950s to 1980s
Areas outlined in green are where sediment was removed
> 50 Mcm of sediment removed from the system
~60% of sediment loss from Central Bay was from human activities
1850s1890s1920s1950s1980s
Central Bay
South Baykm2
km2
Marsh area decrease >90%