Historical Sedimentation in the San Francisco Estuary

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21 Φεβ 2014 (πριν από 3 χρόνια και 4 μήνες)

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Historical Sedimentation in the

San Francisco Estuary

Bruce Jaffe
1
, Theresa Fregoso
1
,

Amy Foxgrover
1
, Shawn Higgins
2


1
United States Geological Survey

2
Kamman Hydrology and Engineering




San Pablo Bay

Outline



The Estuary in the past 150 years



Sediment connectivity of sub
-
embayments



Sediment delivery control on tidal flat change

Outline



The Estuary in the past 150 years



Sediment connectivity of sub
-
embayments



Sediment delivery control on tidal 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
-
embayments



Sediment delivery control on tidal 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 km
2



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

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

Hydraulic mining

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

After hydraulic mining

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

SF Bay Area
population increase

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

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 Bay
San Pablo Bay
Central Bay
South Bay
Cumulative Sedimentation (10
6
x m
3)
Year
Erosion
Depostion
Net

Net

Question:


Will the bays continue to
erode?



-100
0
100
200
300
400
500
1850
1900
1950
2000
Suisun Bay
San Pablo Bay
Central Bay
South Bay
Cumulative Sedimentation (10
6
x m
3)
Year
Erosion
Depostion
?

?

?

?

-100
0
100
200
300
400
500
1850
1900
1950
2000
Suisun Bay
San Pablo Bay
Central Bay
South Bay
Cumulative Sedimentation (10
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 tidal 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
-
embayments



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

1850s

1890s

1920s

1950s

1980s

0

20

40

60

80

100

km
2

0

20

40

60

80

100

km
2

0

20

40

60

80

100

km
2

0

20

40

60

80

100

km
2

Tidal 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


1850s

1890s

1920s

1950s

1980s

Central Bay

South Bay

km
2

km
2

Marsh area decrease >90%