170 



the suspended aggregates, Cq is the concentration of suspended solids in 

 the flooding waters, and t is time. Cq = Cq during a rising tide, and 

 Co = during a falling tide. 



Laboratory tests on San Francisco Bay muds showed that the median 

 settling velocity (by weight) in cm/s of the suspended aggregates is 

 described by 



Ws = kcV3 (10.5) 



where k was found to be 110 when the concentration is in g/cm^ . Equations 



10.4 and 10.5 are combined to solve for concentration C through a finite 

 difference scheme, and from that the rate of growth of marsh elevation, y^,, 

 given the density of the marsh soil. The value of the ambient 

 concentration in the channel, Cq, was obtained by calibrating the 

 calculated y^ change against measurement of the same at specific sites 

 within the bay. 



As would be expected, the rate of rise of marsh elevation is strongly 

 dependent on Cq , as demonstrated by computations for two values of Cq in 

 Fig. 10.7. The computations begin with an initial marsh level (0.15 m) in 

 1930 when a levee was removed to allow tidal waters to flood the marsh 

 area. The figure shows a more rapid rate of rise during the early period, 

 when the marsh was most frequently flooded, and a slowing rate as a steady 

 rate of rise was approached. 



10.5 RESEARCH NEEDS 



Present day capability in predicting the evolution of the morphology 

 of the estuarine mouth and adjacent shorelines due to sea level rise is 

 limited. So is our ability to quantitatively evaluate wetland response to 

 sea level rise. Predictive capabilities for sedimentation within the 

 estuary are better, although in areas where high suspension concentrations 

 occur, the physics is poorly understood. Furthermore, the precise nature 

 of chemical and biological variability is not clearly known; hence we are 

 not in a position to establish the quantitative significance of physical 

 versus chemical versus biological control in estuarine sedimentation 

 processes. 



