spatial and temporal trends in tidal flat shape in san francisco bay shape in san francisco bay josh...

Spatial and Temporal Trends in Tidal Flat Shape in San Francisco Bay

Josh Bearman, Carl Friedrichs, Bruce Jaffe, Amy Foxgrover

Aerial Photo of flats near Dumbarton Bridge, South San Francisco BayCourtesy http://asapdata.arc.nasa.gov

1) On tidal flats, sediment (especially mud) moves away from high concentration

areas and towards areas of weaker energy.

2) Tides and/or abundant sediment supply favor a convex upward profile;

waves and/or sediment loss favor a concave upward profile.

3) South San Francisco Bay provides a case study supporting these trends,

both in space and in time.

Main Points










Main Points

Visit Josh at TheHotSeats.net

• First ponds leveed in 1854

• Currently 26,000 acres of salt ponds in South Bay

• October, 2000• 61% of ponds sold to large

conglomerate of GOs, NGOs, private foundations.

South Bay Salt Pond Project

High energy wavesand/or tides

Higher sediment concentration

Lower sediment concentration

Tidal advection

Tidal advectionHigh energy waves

and/or tidesLow energy waves

and/or tides

Low energy wavesand/or tides

What moves sediment across flats? Ans: Tides plus concentration gradients; (i) Due to energy gradients:

1

Higher sediment concentration

Lower sediment concentration

Tidal advection

Tidal advectionSediment source from

river or local runoffLow energy waves

and/or tides

What moves sediment across flats? Ans: Tides plus concentration gradients; (ii) Due to sediment supply:

“High concentrationboundary condition”

Net settling of sediment

“High concentrationboundary condition”

Net settling of sediment

2

Maximum tide and wave orbital velocity distribution across a linearly sloping flat:

x x = xf(t)

Z(x)

z = R/2

z = - R/2x = 0

h(x,t)

h(t) = (R/2) sin wtx = L

z = 0

0 0.2 0.4 0.6 0.8 1

1.4

1.2

1.0

0.8

0.6

0.4

0.20 0.2 0.4 0.6 0.8 1

3.0

2.5

2.0

1.5

1.0

0.5

x/L x/L

UT9

0/U

T90(L

/2)

UW

90/U

W90

(L/2

)

Spatial variation in tidal current magnitude

Landward Tide-Induced Sediment

Transport

Seaward Wave-InducedSediment Transport

Spatial variation in wave orbital velocity

3










Main Points

South San Francisco Bay Tidal Flats:

1

23

4

5

6

7

89

10

11

12

0 4 km

700 tidal flat profiles in 12 regions, separated by headlands and creek mouths.

Semi-diurnal tidal range up to 2.5 m

San Mateo Bridge

Dumbarton Bridge

South San Francisco Bay

MHW to MLLW

MLLW to - 0.5 m

6

San Mateo Bridge

Dumbarton Bridge

South San Francisco Bay MHW to MLLW

MLLW to - 0.5 m

Dominant mode of profile shape variability determined through eigenfunction analysis:

Ampl

itude

(met

ers)

Across-shore structure of first eigenfunction

Normalized seaward distance across flat

First eigenfunction (deviation from mean profile)90% of variability explained

Mean + positive eigenfunction score = convex-upMean + negative eigenfunction score = concave-up

Mean concave-up profile (scores < 0)Hei

ght a

bove

MLL

W (m

)

Mean profile shapes

Normalized seaward distance across flat

Mean tidal flat profile

Mean convex-up profile (scores > 0)

12 3

45

6 7

8

10

11

12 Profile regions

4 km

9

7

12 3

45

6 7

8

10

11

12 Profile regions

4 km

10-point running average of profile first

eigenfunction score

Convex

Concave

Convex

Concave

8

4

0

-4

Eige

nfun

ction

sco

re

Tidal flat profiles

4

2

0

-2

Regionally-averaged score of first

eigenfunction

9

Significant spatial variation is seen in convex (+) vs. concave (-) eigenfunction scores:

12

3 4

5 6

7

8

9 10

1112

8

-- Tide range & deposition are positively correlated to eigenvalue score (favoring convexity).-- Fetch & grain size are negatively correlated to eigenvalue score (favoring concavity).

12 3

45

6 7

8910

11

12 Profile regions

4 km

Profile region1 3 5 7 9 11

4

2

0

-2

3

2

1

0

Aver

age

fetc

h le

ngth

(km

) Convex

Concave

Eige

nfun

ction

sco

re

r = - .82

Fetch Length

1 3 5 7 9 11

4

2

0

-2

40

30

20

10

0

Profile region

Mea

n gr

ain

size

(mm

) Convex

Concave

Eige

nfun

ction

sco

re

r = - .61

Grain Size

1

.8

.6

.4

.2

0

-.2

-.41 3 5 7 9 11

4

2

0

-2

Profile region

Net

22-

year

dep

ositi

on (m

) Convex

Concave

Eige

nfun

ction

sco

re

Depositionr = + .92

Profile region1 3 5 7 9 11

4

2

0

-2

2.5

2.4

2.3

2.2

2.1

Mea

n tid

al ra

nge

(m)

Convex

Concave

Eige

nfun

ction

sco

re

Tide Ranger = + .87

9

12 3

45

6 7

8910

11

12 Profile regions

1 3 5 7 9 11

4

2

0

-2

r = + .94r2 = .89

Profile region

Observed ScoreModeled Score

Eige

nfun

ction

scor

e

Modeled Score = C1 + C2 x (Deposition)+ C3 x (Tide Range) – C4 x (Fetch)

Convex

Concave

Increased tide

range

Increased

deposition

Increased

fetch

Increased

grain size

Convex-upwards

Concave-upwards

Seaward distance across flat

Flat

ele

vatio

n

San Mateo Bridge

Dumbarton Bridge


MHW to MLLW

MLLW to - 0.5 m

Tide + Deposition – Fetch Explains 89% of Variance in Convexity/Concavity

10










Main Points

(Jaffe et al. 2006)11

12 3

45

6 7

8910

11

12 Regions

4 km

Eige

nfun

ction

scor

eEi

genf

uncti

onsc

ore


eigenfunction score


eigenfunction

12

12 3

45

6 7

8910

11

12 Regions

4 km

Eige

nfun

ction

scor

eEi

genf

uncti

onsc

ore


eigenfunction score


eigenfunction

Inner regions (5-11) tend to be more convex

12

San Mateo Bridge

Dumbarton Bridge


MHW to MLLW

MLLW to - 0.5 m

Sed load at delta

San Jose

Variation of External Forcings in Time:

(Ganju et al. 2008)

13

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9

Region 12Region 11Region 10

1900 1950 2000 1900 1950 2000 1900 1950 2000Year Year Year

- Trend of Scores in Time (+ = more convex, - = more concave)

14

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9


1900 1950 2000 1900 1950 2000 1900 1950 2000Year Year Year

- Trend of Scores in Time (+ = more convex, - = more concave)- Outer regions are getting more concave in time (i.e., eroding)- Inner regions are not (i.e., more stable)

Inner regions

Outerregions

Outerregions

14

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

6

4

2

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9


1900 1950 2000 1900 1950 2000 1900 1950 2000S

edim

ent

Dis

ch. (

MT

)S

edim

ent

Dis

ch. (

MT

)S

edim

ent

Dis

ch. (

MT

)S

edim

ent

Dis

ch. (

MT

)Year Year Year

Inner regions

Outerregions

Outerregions

* * *

* *

*

*

*SIGNIFICANT

- Trend of Scores in Time (+ = more convex, - = more concave)CENTRAL VALLEY SEDIMENT DISCHARGE- Outer regions become more concave as sediment discharge decreases

15

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9


1900 1950 2000 1900 1950 2000 1900 1950 2000Year Year Year

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

1

0

-1

PD

O In

dex

PD

O In

dex

PD

O In

dex

PD

O In

dex

Inner regions

Outerregions

Outerregions

- Trend of Scores in Time (+ = more convex, - = more concave)PACIFIC DECADAL OSCILLATION- No significant relationship to changes in shape

16

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9


1900 1950 2000 1900 1950 2000 1900 1950 2000Year Year Year

.3

0

-.3

.2

0

-.2

.6

.3

0

0

-.4

.3

0

-.3

0

-.2

-.4

.4

0

.6

.3

0

1

.5

0

.6

.3

0

.2

0

-.2

0

-.3

B

edch

ang

e (m

)

Bed

chan

ge

(m)

B

edch

ang

e (m

)

Bed

chan

ge

(m)

Inner regions

Outerregions

Outerregions

*SIGNIFICANT

- Trend of Scores in Time (+ = more convex, - = more concave)Relationship to preceding deposition or erosion- Inner and outer regions more concave after erosion, more convex after deposition

* *

* *

**

17

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9


1900 1950 2000 1900 1950 2000 1900 1950 2000Year Year Year

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

20

15

10

San

Jo

seR

ain

fall

(in

) S

an J

ose

Rai

nfa

ll (i

n)

San

Jo

seR

ain

fall

(in

) S

an J

ose

Rai

nfa

ll (i

n)

Inner regions

Outerregions

Outerregions

*SIGNIFICANT

- Trend of Scores in Time (+ = more convex, - = more concave)SAN JOSE RAINFALL- Inner regions more convex when San Jose rainfall increases

*

* *

18

SanJose

12 3

45

6 7

8910

11

12 Regions

4 km

Sco

reS

core

Sco

reS

core

-1

-2

0

-2

4

2

0

2

1

-1

0

-1

1

0

-1

4

2

0

0

-1

0

-2

0

-2

2

1

0

1

-1

Region 1

Region 4

Region 7 Region 8

Region 5

Region 2 Region 3

Region 6

Region 9


1900 1950 2000 1900 1950 2000 1900 1950 2000Year Year Year

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

1.8

1.7

T

idal

Ran

ge

(m)

T

idal

Ran

ge

(m)

T

idal

Ran

ge

(m)

T

idal

Ran

ge

(m)

Inner regions

Outerregions

Outerregions

- Trend of Scores in Time (+ = more convex, - = more concave)CHANGES IN TIDAL RANGE THROUGH TIME- No significant relationships to temporal changes in tidal range

19

12

3

4

5

67

89

10

11

12

Significance (slope/std err)

Region Mult Reg Rsq CV Seds SJ Rainfall Dep/Eros

r1 0.82 4.21 ––– –––

r2 0.73 3.19 ––– –––

r3 0.71 3.07 ––– –––

r4 0.55 2.10 ––– –––

r5 0.95 8.18 ––– 3.43

r6 0.53 ––– ––– 1.51

r7 0.35 ––– 1.39 –––

r8 0.47 ––– 1.29 1.12

r9 0.66 2.03 ––– 2.4

r10 0.94 3.41 ––– 7.77

r11 0.46 1.05 ––– 1.37

r12 0.51 1.39 ––– –––

Temporal Analysis: Multiple RegressionLess Central Valley sediment discharge: Outer regions more concave.

More San Jose Rains: Inner regions more convex.

Recent deposition (or erosion): Middle regions more convex (or concave)

San Jose

20










Main Points

spatial and temporal trends in tidal flat shape in san francisco bay shape in san francisco bay josh...

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