1 WATERSHED SPECIFIC AND REGIONAL SCALE SUSPENDED SEDIMENT LOAD ESTIMATES FOR BAY AREA SMALL TRIBUTARIES Mikołaj Lewicki and Lester McKee Sources Pathways and Loading Workgroup December 8 th 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 km 2 - an area less than five percent of the Bays 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 mm/yr –1,600 km 2 behind dams Our study is based on flow and sediment discharge of –29 gaged watersheds in the Bay Area watershed (W.Y –177 station years –Sub watershed sizes2.0-1,639 km 2 –Monitored drainage area 4,237 km 2 (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 METHODS Study 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(Q A ) 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 (miles 2 ), 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 method Item # 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/km 2 /year) (regional erosion rate). DR= *A –where DR is the delivery ratio (decimal fraction) that decreases as watershed size increases and A is the watershed area (miles 2 ). Land use based method Item # 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 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 (-) Sediment Load Variability (-) Large Watersheds have less sediment input variability

11 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 t/km 2 Greatest unit exports tended to occur in smaller watersheds. For example, Wildcat Creek 327 t/km 2 whereas Alameda Creek watershed an average annual unit export of 64 t/km 2 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/km 2 ; Zone 6 Line B watershed export =13,493 t/km 2 ). For urban watersheds near the Bay margin, annual average unit suspended sediment export was estimated to varied from 44 to 788 t/km 2 Item # 3a Unit Load (t) Unit Discharge (cms)

12 Regional Loads Item # 3a

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 km 2) 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