Dillingham is located 500 kilometers southwest of Anchorage, Alaska, in the 

 upper reaches of Nushagak Bay (Fig. 6), The local tidal range is 6 meters and 

 suspended-sediment concentrations vary from 50 to 1,500 milligrams per liter of 

 fine-grained material (1 to 100 micrometers) . Local and transient fishing boats 

 and commercial barges up to 15 meters long use the harbor. The harbor basin 

 area is 21,500 square meters at the project elevation of +0.6 meter (MLLW) . The 

 sill elevation is +1.8 meters (MLLW), and sidewall slopes are 1:5. Moorage is 

 provided for 140 boats. Sedimentation has averaged almost 2 meters per year 

 since the harbor was constructed in 1961. Nearly continuous dredging during the 

 ice-free season is now required to keep the harbor in use. 



Nyshagak Boy 



Figure b. Location map of Dillingham Harbor, Alaska, showing the 



three elements which comprise a half-tide harbor: basin 

 sill, and navigation channel. ' 



Flow into and out of the basin is almost entirely tide-dominatea w?ith neg- 

 ligible inflow from Scandinavian Creek. The channel is parabolic in shape 

 (Fig. 7), with a top width near the basin of 45 meters. At low tide, the chan- 

 nel is nearly dry (Fig. 7); near the time of high tide, the channel is bank- 

 filled near the entrance to the basin (Fig. 6). In this region an average 1.3 

 X 10 cubic meters ebbtide prism passes through the channel cross section, 

 which has a bank-filled area of about 160 square meters. Figure 4 shows the 

 cross-sectional area to ebbtide discharge relationship for Dillingham Harbor. 



4. Channel Geometry . 



a. Cross-Sectional Area . The cross-sectional area/discharge relationship 

 of the channel at Dillingham Harbor (Fig. 4) closely approximates that of nat- 

 ural channels in similar material in a region where the tidal hydrograph is 

 similar in shape, if not amplitude. In the range of 300 to 1,500 milligrams 



