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

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

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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





Main Points

South San Francisco Bay Tidal Flats:

1

23

4

5

67

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 MLLWMLLW 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)Heig

ht 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

scor

e

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

scor

e

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

scor

e

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

scor

e

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

scor

e 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



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



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

Scor

eSc

ore

Scor

eSc

ore

-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

Scor

eSc

ore

Scor

eSc

ore

-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

Scor

eSc

ore

Scor

eSc

ore

-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 2000Se

dim

ent

Dis

ch. (

MT)

Sedi

men

tD

isch

. (M

T)Se

dim

ent

Dis

ch. (

MT)

Sedi

men

tD

isch

. (M

T)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

Scor

eSc

ore

Scor

eSc

ore

-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

PDO

Inde

xPD

O In

dex

PDO

Inde

xPD

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

Scor

eSc

ore

Scor

eSc

ore

-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

ange

(m)

B

edch

ange

(m)

B

edch

ange

(m)

B

edch

ange

(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

Scor

eSc

ore

Scor

eSc

ore

-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

Jos

eR

ainf

all (

in)

San

Jos

eR

ainf

all (

in)

San

Jos

eR

ainf

all (

in)

San

Jos

eR

ainf

all (

in)

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

Scor

eSc

ore

Scor

eSc

ore

-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

)

Tid

alR

ange

(m)

T

idal

Ran

ge (m

)

Tid

alR

ange

(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

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Main Points

Documents

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