b. Concentration of Suspended Sediment. Sediment is assumed to enter the 

 harbor at a uniform concentration, c, through the water column. This 

 vertical uniformity may result from mixing in the channel even where outside 

 waters are stratified. Mixing will usually increase as the ratio of the tidal 

 prism to channel cross section increases. 



In many cases the concentration of entering sediment will vary with time 

 in the tidal cycle. Since this variation will affect the time and distance 

 the sediment has to settle to be deposited, time dependence must be con- 

 sidered. When a linear change is assumed, and the concentration at the 

 beginning of the floodtide cycle, c-, the concentration averaged over the 

 floodtide cycle, c, and the period T, of the complete tidal cycle are 

 known, a simple relationship is used to account for concentration change with 

 time during a flooding cycle sequence 



('"I 1 ) 



4| 

 k s = x " ' (2) 



where k is the correction factor used for suspended-sediment concentration 

 which varies linearly with time. The values of c^ and c are obtained from 

 analyses of water samples collected over one or more rising water sequences 

 near the harbor site. 



c. Settling Characteristics of Suspended Solids . The rate at which 

 suspended particles settle is a critical factor when determining whether they 

 will be deposited or removed while still in suspension as the outside water 

 surface drops and fluid leaves the harbor. Because a mass of sediment par- 

 ticles in nature displays a wide range of settling velocities, a settling 

 velocity distribution must be considered. For each particle settling 

 velocity, v , there is a corresponding particle size, given as sphere 

 diameter, d , in millimeters in which the sphere has a density, p = 2.7 

 grams per cubic centimeter. Caution is recommended when using a grain-size 

 distribution instead of a settling velocity distribution. Grain diameter is 

 often determined in the laboratory as the effective diameter based on Stokes' 

 Law using distilled water. However, particle aggregation in the fluid in 

 which the sediment was collected (often saline) may increase the settling 

 velocity in nature. Therefore, settling velocity distribution should be 

 measured in the ambient fluid at the normal fluid temperature expected in 

 the harbor. 



d. Harbor Geometry . The plan area of the basin at sill elevation, A. , 

 the slope of the basin sidewall, a, and the elevation of the sill, z , are 

 required parameters (Fig. 1). These geometric characteristics of the harbor 

 are used to determine the volume of water and the mass of suspended sediment 

 which will enter the harbor during each floodtide cycle. The parameters are 

 also needed to determine the vertical distance through which a suspended- 

 sediment particle must fall to be deposited and the area over which the 

 sediment will be deposited. 



2. Solution . 



Three steps required to determine the shoaling rate are: (a) predict the 

 total mass of sediment that will enter the harbor; (b) predict the part of 

 that mass which will be deposited; and (c) predict the shoaling rate (increase 

 in bottom elevation) which will result. 



