A METHOD TO PREDICT THE STABLE GEOMETRY OF A CHANNEL 

 CONNECTING AN ENCLOSED HARBOR AND NAVIGABLE WATERS 



hy 

 Craig H. Everts 



I . INTRODUCTION 



A desirable criterion for the design of an enclosed harbor is that the 

 channel connecting the harbor with navigable waters be self -maintaining. This 

 condition may prevail where sediment movement is negligible, or in the case of 

 moving sediment, where bottom shear stress caused by tidal or river discharge 

 is sufficient to prevent deposition and thus maintain acceptable channel dimen- 

 sions. 



A method is presented to predict the stable configuration of a navigation 

 channel connecting open tidal waters with an enclosed harbor. The stable cross- 

 seqtional area, cross-sectional shape, and bottom elevation of the channel are 

 considered. A relationship between these variables and the water discharge 

 through the channel is determined using the geometric characteristics of nearby 

 natural channels and the hydraulic regimes that sustain the channels. Using 

 appropriate field data, the method may be applied to the design of a navigation 

 channel in any region where natural tidewater drainage or river channels exist. 



An example is given using data obtained from a navigation channel at the 

 harbor of Dillingham, Alaska, and from natural drainage channels on a tidal 

 flat and in rivers near Anchorage, Alaska. The resulting relationship (tidal 

 prism/channel cross section) may be used when sediments are like those on north- 

 ern tidal flats, i.e., highly compacted glacial silt' and mud-sized material gen- 

 erally lacking in clay materials and organic constituents. 



II. METHOD 



1. Background . 



Theory does not exist to cover all circumstances of channel design because 

 of the complex nature of the problem, which involves varying tidal ranges, 

 varying quantities of sediment in transport, and varying compositions of channel 

 bank and bed material from one location to another. However, two widely used 

 procedures for estimating channel cross-section dimensions are the "tidal prism 

 relationship," which is applied to sandy tidal inlets, and the "regime theory," 

 which is mostly applied to river channels. 



A tidal prism is defined as the total amount of water that flows into a 

 harbor or estuary or out again with movement of the tide, excluding any fresh- 

 water flow (Allen, 1972). O'Brien (1931) first developed a power function form 

 of the relationship 



A = CP" (1) 



where 



A = cross-sectional area of an inlet gorge below mean sea level (MSL) 



C = constant 



