and the depth is 



-H 



in which w is the channel width. 



c. Channel Bottom Elevation . Channel bottom elevations can be approxi- 

 mated using discharge-dependent width/depth ratios (w/d) . The w/d ratio is 

 < 10 (average = 6) for discharges of > 80 cubic meters per ebb cycle, and > 10 

 (average = 25) for discharges < 80 cubic meters per ebb cycle. Bottom eleva- 

 tion may be approximated using these values only for the region above MLLW. 

 The depth to which a channel will be maintained by tidal flow, unless the river 

 discharge and tidal flow is larger than about 10^ cubic meters per ebb cycle, 

 is at or just slightly below MLLW. Ship Creek and the Dillingham Harbor Channel 

 bottom depths, at the MLLW shoreline of Knik Arm and Nushagak Bay, respectively, 

 are about -1 meter (MLLW) . The depth of Eagle River at its estuary mouth is 

 unknown . 



IV. DISCUSSION AND CONCLUSIONS 



1. A cross-sectional area/ebb-tidal prism relationship exists between nat- 

 ural tidal-flat channels and stream channels, and artificial navigation chan- 

 nels which provide access to enclosed harbor basins. This relationship may be 

 used to estimate the stable cross-sectional area, shape, and depth of a short 

 navigation channel before harbor construction. 



2. A controlling factor is the ebb-tidal prism. For a navigational chan- 

 nel, this prism is dependent on the proposed harbor basin volume between the 

 low water and the high water mark. Thus, the size of the harbor controls the' 

 stable dimensions of the navigation channel. 



3. An important assumption in the procedure outlined is that negligible 

 bedload sediment is carried into the channel. In the example area, the average 

 suspended -sediment concentration varies from 300 to 1,500 milligrams per liter. 

 Scour versus deposition in the channels within this range is such that the re- 

 lationship did not vary by an amount greater than the possible errors involved 

 in estimating the cross-sectional area of the ebb-tidal prism. 



4. In areas where sand-sized sediment dominates, the relationship shown in 

 Figure 4 will vary, probably because of sand transported and deposited by long- 

 shore currents. The sediment-size difference is also probably a factor, as is 

 the difference in channel depths relative to MLLW. 



5. A critical factor in predicting the cross-sectional area of a navigation 

 channel, when using natural channels as analogs, is a similarity of composition 

 of the sediment comprising the boundaries of the channels, i.e., channel erodi- 

 bility. The comparison between channels cannot be made unless this similarity 

 exists. 



6. The relationship shown in Figure 4 may be used for navigation channels 

 which cross intertidal areas in Alaska where the sediments comprising those 

 areas are predominantly highly compacted silts and muds which contain less than 

 2 percent clay materials. 



