A SIMPLE COMPUTER MODEL FOR EVALUATING COASTAL INLET HYDRAULICS 



by 

 William N. Seelig 



I. INTRODUCTION 



This report describes a method for estimating coastal inlet veloc- 

 ities, discharge, and bay levels using the simple numerical model of 

 Seelig, Harris, and Herchenroder (in preparation, 1977]^. The model 

 can be used for sea level fluctuations caused by astronomical tides, 

 storm surges, seiches, or tsunamis. A digital computer program is used 

 because of the large number of computations. A run on a CDC 6600 com- 

 puter generally costs less than $5 for a tidal cycle. 



II. PREDICTING INLET HYDRAULICS 

 1. Systems Modeled with Computer Program . 



An inlet-bay system co.isists of a "sea" (e.g., ocean or lake] con- 

 nected to a "bay" by one or more inlets (Fig. 1). The computer model 

 will predict bay levels, inlet velocities, and discharge as a function 

 of time given the geometry of the system and the water level fluctua- 

 tions in the sea. It is assumed that the sea is much larger than the 

 inlet and bay and that the bay is large compared to the inlet. 



Tlie model is designed for systems where the bay water level rises 

 and falls uniformly throughout the bay. This occurs when the wavelength 

 in the bay is much longer than the longest axis of the bay: 



Tf •'gd^ >> Hay ' f^l^ 



where ^ ^ ^ 



T^ = forcing wave period 



g = acceleration of gravity 



^haxj = '^'^pth of bay 



Hau ^ length of bay 



2. Procedures for Use of Computer Program . 



Step 1 . Evaluate the inlet geometry by using maps, charts, hydro- 

 graphic surveys, and dredging records to determine the depth of water 

 throughout the inlet. The side slope of the inlet at mean water level 



^SEELIG, W.N., HARRIS, D.L., and HERCHENRODER, B.E., "A Spatially 

 Integrated Numerical Model of Inlet Hydraulics," GITI Report 14, U.S. 

 Army Corps of Engineers, Coastal Engineering Research Center, Fort 

 Belvoir, Va. , and U.S. Army Engineer Waterways Experiment Station, 

 Vicksburg, Miss, (in preparation, 1977). 



