Anna Kunkel

Maidment--GIS

Term Paper--Fall 2002

Suspended Sediments in South San Francisco Bay

 

Objective

 

            The objective of this project was to determine how different factors affect the suspended sediment concentration in the south San Francisco Bay.  The factors examined were location, season, and precipitation.

 

What are Suspended Sediments?

 

Suspended sediments are defined as solid particles transported in a fluid media or found in deposit after transportation by flowing water, wind, glacier and gravitational action.  Their concentration in a water body is affected by many factors.  In rivers, the concentration depends on the water’s flow rate, turbidity, soil erosion, urban runoff, and wastewater and septic system effluent, while in lakes, decaying plants and animals, bottom-feeding fish, and wind/wave action play a larger role.  For the San Francisco Bay, river discharge and wind/wave action are the largest contributors of suspended sediments.  When rivers and streams discharge into the bay, the lack of turbidity and large concentration of particles causes them to settle within certain lengths of the river mouth.  These distances can become large during the rainy season.  Wind/wave action resuspends sediments in shallow water, which are then transported to deeper water by tidal currents, especially during spring tides.

 

Why are they important?

 

Suspended sediments play a key role in shaping the characteristics of a body of water, such as south San Francisco Bay.  Besides transporting adsorbed toxic substances, they block light from reaching submerged vegetation, which in turn slows down photosynthesis.  This effect causes less dissolved oxygen to be released into the water by the plants and in extreme cases, results in death of the plants.  More oxygen is then required to decompose these plants.  Since suspended solids absorb heat from sunlight, they increase the water temperature, decreasing the dissolved oxygen level even more.  When the dissolved oxygen level in a body of water becomes too low, fish kills occur.  Suspended sediments also trigger changes in morphology of an area.  As sedimentation and resuspension occur, particles are moved to different areas, altering the depths of waterbodies.  The sediment plume of the Mississippi River is an example of this phenomenon.

             

Examination of area

 

            San Francisco Bay covers 11,776 km2 and encompasses the hydrologic unit coverages (HUCs) 18050001 through 18050006.  These HUCs are further divided into 29 basins, which contain a vast web of rivers and streams that

feed into the bay.  The basemap of the area, Figure 1,  illustrates the hydrologic network.  The bay itself can also be                     

divided into four different regions, starting from the north:  Suisun Bay, San Pablo Bay, Central Bay, and South Bay.                                     Figure 1:  Mississippi River Sediment Plume

This paper’s focus is on the water quality of the South Bay.                                                                                                                                                                

 

 

                               

 

                    Figure 2:  San Francisco Bay and South Bay with USGS Testing Sites                                              Figure 3: Submersible Instrument Package

 

       

        The US Geological Survey has 36 testing sites in the San Francisco Bay, 21 of those being in the South Bay.  These sites, located along a deep central channel,  are monitored by a monthly cruise, starting at Site 1 in the San Pablo Bay and ending at Site 36 in the South Bay.  Water quality data is gathered for all sites on one day each month, and measurements are taken at various water depths at each site.  A submersible instrument package (Figure 3) is used to measure the level of salinity, dissolved oxygen, temperature, and suspended sediments.  The suspended sediment concentration is calculated from linear regression between the discrete measurements of suspended sediments and the OBS voltage.  Discrete measurements are made by weighing the mass of solids collected onto filters after drying, while the OBS voltage is the output of the optical backscatter sensor.

        For this paper, the concentration of suspended sediments was examined from January 2001 to December 2001.  The testing information was downloaded from the website and entered into an excel spreadsheet for analysis. Since the suspended sediment concentration was measured for various depths for a particular site,  the first step was to determine a relationship between the suspended sediment concentration and the depth. Four sites, spread out over the tested area, were examined (Figure 4).  For three of these sites, the variation with depth was minimal.  Therefore, the measured concentration was assumed to be independent of depth, and the mean concentration per testing day was used to represent each site.  Figure 5 shows the concentration measured at each testing site for each monthly cruise.  Large variations in suspended sediment amounts are observed in the southern testing sites (Sites 30-36).  The suspended sediment levels also seemed to fluctuate with time. 

 

 

 

 

Figure 4:  Suspended Sediment Concentration at Various Depths for 4 Testing Sites          

                                                                                                                                                  

                                                                                                                                                       Figure 5:  Suspended Sediment Concentration at the Testing Sites for the  .                                                                                                                                                                      2001 Monthly Cruises

 

Location

The suspended sediment concentrations gathered by USGS were found to vary more for the southern testing sites (Sites 29-36) than for the northern testing sites throughout the year.  One explanation for this inconsistency is attributed  to the location of the sites, as shown in Figure 6.  The southern testing sites are closer to the shore, while the northern testing sites are farther away from the shore.  The proximity of the southern sites to land allows them to be affected by land runoff and river discharge.  Rivers are able to carry large amounts of suspended sediments due to their comparatively large velocities.  As they empty into a lake, large sediments settle out near the river mouth, while fine sediments are transported farther into the lake.  If a testing site is located within the sphere of influence of a river, the results would reflect the river’s suspended sediment concentration.  As shown on the map, Corte Madera Creek, Stevens Creek, and Coyote Creek are all situated near Sites 34, 35, and 36 and could influence the results from these sites.

Another factor related to site location is the effect of wind/wave action in resuspending the particles.  When the wind blows, an internal wave in the lake, called a sieche, is formed.  If this internal wave forms in a shallow region of the lake, it will be dissipated by friction with the lake bottom, thereby, resuspending particles from the lake bottom.  In deeper regions of the lake, the internal waves do not come in contact with the bottom, minimizing the resuspension of particles.  Therefore, the concentrations found by testing sites in shallower regions are affected by this phenomenon. Sites 34-36 were found to be the shallowest with depths between 8 and 9 meters.  Upon calculation of  the average depth for the northern and southern testing sites, the northern sites were found to be considerably deeper (average depth = 14.6 m) in relation to the southern sites (average depth = 11.7 m).  The shallowness of the southern sites permits them to be affected more by wind/wave action, which factors into the large differences in suspended sediment concentration.

 

 

              Southern Testing Sites                             Northern Testing Sites

 

                                                                           

                                                                                                                Figure 6:  Southern vs. Northern Testing Sites

 

 

 

Seasonal                                                                                                                                              Figure 7:  Suspended Sediment Concentration at the Sites for Different Seasons

As shown in Figure 5, the suspended sediment concentration varies with time of year, as well as the site location.  In order to examine a larger overall trend, the months were grouped into seasons in the following manner: Winter (December to Febuary), Spring (March to May), Summer (June to August), and Fall (September to November).  Figure 7 shows the suspended sediment concentration of the sites for the different seasons.  Winter and fall were shown to have higher amounts of suspended sediments than spring and summer.  This result is expected since winter and fall are considered the rainy season for the area. 

 

 

Precipitation

Since the seasonal variation of suspended sediment concentrations was explained due to rainfall, the relationship between the concentrations and the precipitation was investigated.   Intuitively, the two factors should be related.  As the precipitation increases, the river discharge, which includes the river volume and velocity, increases, bringing with it more suspended sediments.  These suspended sediments have a large horizontal velocity and will travel farther into the bay before settling, thereby effecting the results of nearby testing sites.  To determine the actual relationship, precipitation data for 2001 was found for the Bay Area.  The California Department of Water Resources has a network of CDEC stations around the state that collect climatic and river information, including wind speed and direction, precipitation, and river flow.  Precipitation data was collected from 11 sites in the San Francisco Bay Hydrologic Area, as seen in Figure 8.  The sites were taken from around the bay area to determine the precipitation over the entire area.  Figure 9 illustrates the monthly accumulated precipitation for the CDEC sites over the year.  From this graph, the rainy season occurs from November to March, which is what was predicted from the seasonal information.  Since the suspended sediment concentration changes in the same manner as the precipitation, a proposed relationship between the two seems valid.  Therefore, the level of suspended sediment increases as the rainfall increases. 

 

 

Figure 8: CDEC Testing Sites for the Bay Area                                                                Figure 9:  Accumulated Precipitation at the CDEC sites over 2001

 

 

 

 

Figure 10: Accumulated Precipitation over the Bay Area

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Spatial Variance of Precipitation

    The spatial variance of rainfall over the bay area was also examined for the months of February and November.  The kriging function of ArcGIS was used to create a raster from the precipitation data of the CDEC sites.  The resulting maps (Figure 10) show that the amount of rainfall is the largest in the north and decreases moving south.  No further conclusions could be made due to the lack of sites testing the direct influence of rain into the south San Francisco Bay.

 

Conclusion

    This project was an investigation of how three factors, location, time and precipitation, effect the concentration of suspended sediments in the south San Francisco Bay.  The southern testing sites experienced large variations in suspended sediment levels due to their proximity to the shore and river outlets.  The shallowness of these sites also made them more susceptible to wind/wave action compared to the northern sites.  Time of year was also determined to be a factor due to seasonal variations of precipitation.  During the rainy season (fall and winter), the suspended sediment concentrations increased dramatically due to increased flux of suspended sediments into the system.  

 

References

 

http://water.usgs.gov/lookup/getspatial?huc250k, USGS Retrieval for Spatial Data Set HUC250k.

 

http://water.usgs.gov/lookup/getspatial?erf1, USGS Retrieval for Spatial Data Set ERF1.

 

http://sfbay.wr.usgs.gov/access/wqdata, USGS Water Quality for San Francisco Bay.

 

http://www.gis.ca.gov, The California Spatial Information Library.

 

http://bcn.boulder.co.us/basin/data/FECAL/info/TSS.html, BASIN: General Information on Solids from the City of Boulder.

 

http://visibleearth.nasa.gov/Land_Surface/Erosion_Sedimentation/, Visible Earth Images.

 

http://cdec.water.ca.gov/, California Data Exchange Center, California Department of Water Resources.