Endpassing considerations are similar to the overpassing process except 

 that the controlling factor is the seaward growth of the fillet. The updrift 

 beach will build seaward until the breaker zone has shifted seaward enough to 

 allow the longshore drift to pass around the end of the groin. 



Tide and storm effects continuously change the water level at a groin, 

 which in turn changes the apparent groin height and length. The result is 

 variable bypassing. For example, as water level rises, overpassing can 

 increase; as water level falls, the breaker line moves seaward and endpassing 

 can increase. 



The combination of all the factors discussed makes prediction of the 

 percentage of longshore transport bypassing difficult. Only gross percentage 

 estimates are possible based on engineering experience and judgment. As an 

 example, the estimates of the percentage of longshore transport stopped by a 

 groin on the Atlantic coast, where a normal breaker depth of 1.8 meters (6 

 feet) is assumes, are as follows: 



(1) For high groins extending to a 3-meter or more water depth, use 100 

 percent of the total longshore transport. 



(2) For high groins extending to a 1.2- to 3-meter (4- to 10-foot) depth 

 below MLW (or mean lower low water, MLLW) , or for low groins extending to 

 a depth more than 3 meters, use 75 percent of the total longshore 

 transport . 



(3) For high groins extending from MLW to 1.2 meters below MLW (or MLLW), 

 or for low groins extending to a depth less than 3 meters below MLW, use 

 50 percent of the total annual rate of longshore transport. 



Similar percentages can be estimated proportionally by assuming that the 

 normal breaker zone for the gulf coast and less exposed shores of the Great 

 Lakes ranges from 0.9- to 1.2-meter (3- to 4-foot) depths; more exposed shores 

 of the Great Lakes approach the 1.8-meter depth. The Pacific coast ranges 

 from 2.1- to 3-meter (7- to 10-foot) depths depending on exposure. 



Rule 6: The longshore drift that is aolleeted in the updrift fillet is 

 prevented from reaching the downdrift area, where the sand 

 balance is upset . 



This simple rule has surfaced many times with the addition of groins 

 downdrift of a groin system as a followup to a progressive erosion problem. 

 This problem can be reduced by using beach nourishment concurrent with the 

 groin construction, which more rapidly reestablishes the natural longshore 

 transport past the groins. (Due to the reorientation of the shoreline, the 

 initial longshore transport rate is seldom fully reestablished.) 



3 . Functiona l Desi gn. 



a. Groin Height . For functional design purposes, a groin may be con- 

 sidered in "three sections (see Fig. 5-12): horizontal shore section (HSS), 

 intermediate sloped section (ISS), and outer section (OS). 



5-39 



