ESTIMATION OF FLOW THROUGH OFFSHORE BREAKWATER 

 GAPS GENERATED BY WAVE OVERTOPPING 



by 

 William N. Seelig and Todd L. Walton, Jr. 



I. INTRODUCTION 



Offshore breakwaters are often constructed to protect harbors or eroding 

 coasts from wave action. Wave overtopping of segmented breakwaters generates 

 a seaward flow through breakwater gaps. Low discharges and low water velocities 

 are usually desirable in gaps between breakwaters. High velocities may be a 

 hazard to navigation or swimmers, and large net exit flows through the gaps 

 may transport sediment out of the breakwater area. This report illustrates a 

 technique for predicting the net discharges and mean velocity through breakwater 

 gaps caused by overtopping. 



II. HYDRAULICS OF DETACHED BREAKWATERS 



Since the cost of a breakwater increases with the height of the structure, 

 it may be necessary to build the structure to allow some wave overtopping (Fig. 

 1) . A moderate overtopping rate, which helps maintain water quality by encour- 

 aging circulation, may also be desirable. If the breakwater is impermeable the 

 volumetric rate of overtopping per unit length by breakwater, q Q (assuming 

 no return flow over the breakwater), may be predicted (Weggel, 1976; U.S. Army, 

 Corps of Engineers, Coastal Engineering Research Center, 1977) by 



„-(*w 3 ) 1/2 <HS) 



0.1085 



(1) 



PROTECTED 

 REGION 



hj) = P - ponding level for on 

 enclosed system 



SWL 



7777777777777, 



Figure 1. Definition sketch. 



