A METHOD TO FORECAST SEDIMENTATION RATES RESULTING FROM 

 THE SETTLEMENT OF SUSPENDED SOLIDS WITHIN SEMIENCLOSED HARBORS 



by 



Craig H. Everts 



I. INTRODUCTION 



Some harbors experience sedimentation largely as a result of the fallout 

 of suspended material. This report presents a method to forecast the 

 sedimentation rate caused when waters laden with suspended solids enter a 

 semienclosed harbor during a rising tide, or rising river stage, and settle 

 out before they can be removed in suspension as the water level subsequently 

 declines. The method can be used in planning harbor maintenance expenses 

 before construction in an area where significant quantities of sediment are 

 transported in suspension. Geometric characteristics of the proposed harbor 

 are considered; consequently, the method may also be used to evaluate design 

 tradeoffs (i.e., sedimentation rate versus harbor size, project depth, and 

 channel characteristics). 



II. HARBOR CHARACTERISTICS 



The general case of an enclosed harbor basin connected by a channel to 

 navigable waters is considered (Fig. 1). The basin and channel may be of any 

 size and shape as long as the rise and fall of the water surface inside the 

 basin is nearly in phase with and of the same amplitude as that outside the 

 basin. Sidewalls may be sloping or vertical. A sill in the navigation 

 channel may be at any elevation, including the elevation of the channel 

 bottom; i.e., no sill, the most common case for enclosed harbors. A sill is 

 used in high tidal range areas to reduce excavation costs when constructing 

 the basin. It provides flotation for vessels in the basin at low tide stages, 

 but restricts navigation in and out of the basin to times of higher tidal 

 elevations. Harbors in Alaska with such sills are called "half-tide" harbors. 













V/////A 





y//. 



WA 









♦ Slope 





Estuary 



No Sill Condition 



Figure 1. Definition sketch of an enclosed harbor. Three major components of a half-tide 

 harbor are illustrated: basin, sill, and navigation channel. The half-tide 

 harbor is the most general case of an enclosed harbor; in most situations a sill 

 will be absent. When the sill is absent, and where the basin bottom elevation is 

 above MLLW, the basin will be dry at or near the time of low tide. a g = MHHW 

 tide elevation; Au = plan area of basin; n = water surface elevation; z g = sill 

 elevation; a = sldewall slope. 



