Note that in this first approximation of breakwater gap flow that the waves 

 are assumed to approach approximately normal to the breakwaters and shoreline, 

 so the longshore current can be neglected. Other effects such as diffraction, 

 refraction, reflection, and wave-current interactions have not been considered. 



If incident wave conditions do not vary rapidly with time, a condition will 

 be reached where water flowing into the zone protected by the breakwaters will 

 equal the exit flow through breakwater gaps or inlets. The equation describing 

 this condition is 



% \! - F7 m = V^i^b (C d Bd s (N - 1) + 2C de A ce ) (6) 



where N is the number of breakwaters, A ce is the cross-sectional flow area 

 between the breakwaters and shoreline at the end of the system, and B is the 

 gap width between breakwaters (Fig. 4). Solving equation (6) and putting into 

 dimensionless form, the dimensionless velocity, 



V =■ ■' = 



c dV^^ 



becomes a function of a single coefficient, K, where 



K = 



V2g~^[c d Bd s (N - 1) + 2C dc A ce ] (7) 



q *N 



Figure 5, which gives the relation between dimensionless velocity and K, shows 

 that any combination of factors causing K to increase will produce a smaller 

 dimensionless velocity. For example, keeping all other factors constant, if the 

 gap spacing B is increased, K will increase and V" will be reduced. 



K and the resulting velocity may be easily solved using equation (7) and 

 Figure 5 . The computer program BWFL0W2 can also be used to solve the velocity 

 and flow through breakwater gaps due to overtopping. This program is recom- 

 mended if a large number of calculations are needed. The program is also sug- 

 gested any time irregular wave conditions are assumed, because irregular wave 

 overtopping rates are a complex function of the overtopping rate given by equa- 

 tion (1). The cost of running BWFL0W2 is a few cents per condition of interest. 



It is recommended that V" be kept below 0.5 foot (0.15 meter) per second 

 for extreme design conditions. Velocities much higher than this value could 

 transport significant amounts of sediment out of the breakwater system and may 

 cause scour around the breakwaters. Recall that V is an average velocity 

 through the gap and that local velocities in breakwater vicinity may be con- 

 siderably higher. 



14 



