The total mass deposited in the basin, Mj, is 



M, = M P 



sb 



(7) 



c. Step 3 — Shoaling Rate in Harbor . It may be desirable to predict the 

 rate of bottom elevation rise caused by sediment deposition. The sediment 

 mass (step 2) must then be converted to sediment volume, and specifically to 

 the volume of the water-sediment mixture which forms the deposit in the 

 harbor. That part of a unit volume of deposited material, V j, with a 

 density of pj, which is composed of sediment of density p ? , is 



's> 



'sd 



Pd - Py 



Po - P, 



(8) 



where < V gd < 1.0 or 



is the density of water. The mass of sediment per 



unit volume of deposited material, 



,T V ■ P £ 



sd 



and the shoaling rate, S r , in the basin (not sidewalls) is 



S_ = 



A b "V 



(9) 



(10) 



The assumption is that sediment which would be deposited on the basin side- 

 walls moves downslope aud is deposited on the basin bottom (A. = area of basin 

 at sill elevation). Thus, during deposition, the basin sidewall slopes are 

 assumed to remain constant and in their original position. 



IV. EXAMPLE PROBLEM 



Dillingham Harbor is an enclosed half-tide harbor located off Nushagak 

 Bay, about 550 kilometers southwest of Anchorage, Alaska (Fig. 6). Sedimenta- 

 tion has been a major maintenance problem requiring almost continuous dredging 

 during the summer months. In addition to the high cost, the dredging inter- 

 feres to some extent with navigation by fishing boats and commercial barges. 

 Flow into and out of the basin is caused by tidal fluctuations in Nushagak 

 Bay. Water entering during floodtide carries large quantities of suspended 

 materials, some of which settle below sill elevation before ebb tidal waters 

 can remove them as the tide lowers. 



GIVEN ; Data obtained in the vicinity of Dillingham Harbor in 1969, 1973, and 

 1974 (Everts, 1976b), are: 



(a) Tidal Characteristics . 



= 6 meters 



Mean tidal range: 



Tidal period: T = 12.4 hours 



(b) Suspended-Sediment Characteristics. 



= 0.8 kilo- 



Suspended-sediment concentration at t = 0: 

 gram per cubic meter (800 milligrams per liter) 



Suspended-sediment concentration at t = T/2: c(T/2) =0.2 

 kilogram per cubic meter (200 milligrams per liter) 



13 



