1

WATERSHED SPECIFIC AND REGIONAL SCALE SUSPENDED

SEDIMENT LOAD ESTIMATES FOR BAY AREA SMALL TRIBUTARIES

Mikołaj Lewicki and Lester McKeeSources Pathways and Loading

WorkgroupDecember 8th 2008

Item #3a

2

Project Impetus• Information on suspended sediment loads is of great importance in a

watershed/estuary management

• sediment impacts– water quality– recreation– aquatic habitat– commercial shipping operations

• 40% of the total suspended sediment load entering the Bay may be supplied by the small tributaries (8,180 km2 - an area less than five percent of the Bay’s total watershed area upstream from the Golden Gate)

• Our objective was to develop a new watershed-specific and regional estimates of current annual average suspended sediment loads

Item #3a

3

METHODS

• Local Settings– Mediterranean climate 95% of the precipitation during the winter – Variations in the annual precipitation 300-1500mm/yr– 1,600 km2 behind dams

• Our study is based on flow and sediment discharge of – 29 gaged watersheds in the Bay Area watershed (W.Y. 1957-

2007– 177 station years– Sub watershed sizes2.0-1,639 km2

– Monitored drainage area 4,237 km2 (52% of the area of interest)

Item #3a

4

METHODS• We were focused on identifying statistically important relationships

between watershed characteristics, or different physical processes and the annual suspended sediment loads specific for a watershed

• The tested watershed characteristics included: – drainage area

– watershed slope

– different flow parameters such as daily discharge, or peak discharge

– geology/soils (represented by the erodibility factor)

– land use

• Regression analyses were performed between the individual spatial parameters and measured suspended sediment loads to test the ability of individual parameters to explain the variation in a dependent (suspended sediment load) variable.

Item #3a

5

METHODSStudy area has– Disproportional distribution of sediment monitoring stations (most in non urban

sub watersheds)– Extensive urban growth in the lower watersheds,

Urban and non-urban watersheds loads must be computed differently. – Flow based method– Land use based method

Flow based method– In non-urban watersheds were identified local statistical relationships between

sediment yields and watersheds characteristics.

Land use based method– A land use specific sediment production was used to estimate sediment loads

per unit area.

Item #3a

6

Flow based method• ~ 90% of the sediment load occurs during just a few days per year (annual peak discharge).

• USGS sediment rating curves are flow dependent

• We found that for the non-urban watersheds, annual unit peak discharge correlated with annual unit sediment load provides the most accurate sediment yield predictions

Q (s/A) =f(QA)

• Rantz (1971) identified three distinct hydro-geomorphic provinces in the Bay Area (defined by climate and geology). We stratified discharge and suspended sediment loads data into Rantz (1971) provinces and found a significant improvement in correlation between the unit annual peak discharge and unit suspended load yield.

East Bay

North Bay Peninsula

Item #3a

7

• In order to apply this method across all the watersheds peak flow was estimated in ungaged areas using regressions from Rantz (1971)

Q (2/ 5/ 10/ 25/ 50) =f(A, P)

For example:

• Where Q represents 2, 5, 10, 25, 50 years peak discharge (cfs), A represents drainage area (miles2), and P represents a watershed average precipitation (inches).

• Regional flood frequency distribution was applied to watersheds without flow record and the resulting discharge estimates were combined with the locally derived suspended sediment regression equations to estimate contemporary average suspended sediment loads for each watershed.

• For watersheds with sediment monitoring stations, watershed-specific load rating curves were derived

• If a monitored watershed had four or less years of record, its rating curve was derived by merging its record with a neighboring monitored watershed of a similar size.

Flow based methodItem #3a

8

• In the lowland urbanized watersheds there is only 14 years of the total 177 station years (just two watersheds were urbanizing during the period when data was collected)

• Due to lack of sufficient data, the same method as applied for non-urban areas could generate order of magnitude errors (likely an over prediction of sediment loads).

• Therefore, a land use based estimation method was applied based on data extracted from published literature on land use specific sediment loads per unit area. Typical ranges of expected erosion rates scaled by the watersheds sizes were applied.

Sediment production rates estimated for selected land use type classes (metric t/km2/year) (regional erosion rate).

• DR=0.417762*A-0.134958-0.127097 – where DR is the delivery ratio (decimal fraction) that decreases as watershed size

increases and A is the watershed area (miles2).

Land use based methodItem #3a

9

DATA ANALYSIS • We used all gaging station suspended sediment and flow data to determine sediment loads in

relation to watershed size, location, and discharge.

• For watersheds with reservoirs, the area above the reservoirs was not clipped out of the overall watershed area.

• Physical variables tested:

– Flow, Drainage area, Topography, Slope, Precipitation, Land use, Geology

• Construction of a multiple regression model failed due to insufficient available input data.

Item #3a

10

Sediment Load variability (-)

1

10

100

1,000

10,000

100,000

1,000,000

0 1 10 100 1,000 10,000

Peak dischrge variability (-)

Sed

imen

t lo

ad v

aria

bil

ity

(-)

RESULTS• Measured annual suspended sediment loads in Bay Area watersheds vary by orders of

magnitude between years

• Up to one order of magnitude of this variability is related to the variability of sediment supply and two or more orders of magnitude are likely associated with inter-annual peak discharge variations

Item #3a

Peak Discharge Variability (-)

Sed

imen

t Lo

ad V

aria

bilit

y (-

)

Large Watersheds

have less sediment

input variability

11

San Francisco Bay Annual Suspended Sediment Loads

0.01

0.1

1

10

100

1000

10000

100000

0.001 0.01 0.1 1 10

Unit discharge (cms)

Uni

t Lo

ad (m

etri

c to

ns)

East Bay

North Bay

Peninsula

Power (East Bay)

Power (North Bay)

Power (Peninsula)

RESULTS• Given the influence of a watershed size on annual average flow and suspended sediment loads

among watersheds, the best way to compare one watershed directly to another is to normalize annual average loads by the area of the watershed

• Estimated unit export in Bay Area watersheds varied from 31- 1130 t/km2 Greatest unit exports tended to occur in smaller watersheds. For example, Wildcat Creek 327 t/km2 whereas Alameda Creek watershed an average annual unit export of 64 t/km2

• The largest unit sediment exports were associated with two watersheds that were urbanizing when the USGS was making measurements (e.g., Colma Creek watershed export = 1,136 t/km2; Zone 6 Line B watershed export =13,493 t/km2).

• For urban watersheds near the Bay margin, annual average unit suspended sediment export was estimated to varied from 44 to 788 t/km2

Item #3a

Uni

t Loa

d (t

)

Unit Discharge (cms)

12

Regional Loads Item #3a

Bay Segment Load (t/year)

Rivers 25,357

Suisun Bay 223,863

Carquinez Strait 28,993

San Pablo Bay 420,589

Central Bay 259,159

South Bay 293,732

Lower South Bay 239,560

Total 1,491,254

13

DISCUSSION• Our results vs. previous estimations

• Sources of discrepancy– 25% of the increase can be associated with the previous choice of area

(6,650 km2) for some of the previous research.

– New watershed boundaries (smaller watersheds have higher yields)

– A new statistical method

Item #3a

14

Implications

• Long term average input into the Bay is ~40% higher than previously estimated

• Pollutants that absorb to fine sediments as: mercury, PCB, and a variety of other trace metals and trace organic contaminants

• Sediment dredging

• Aquatic habitat impact

• Wetland restoration

Item #3a

15

Questions?

16

Drainage Area, peak discharge, geology, slope and land use correlated with sediment load

Item #3a