currents or a combination of tidal and river flow. This must be carefully 

 considered, particulary in design of the inner toe of the structure. As 

 the jetties' primary purpose is to prevent the passage of littoral material 

 through the littoral drift zone, the uprush area must be impermeable or 

 sand tight. Other design considerations are much the same as for a break- 

 water. 



(c) Groins . The structural design and the selection of stone 

 sizes and gradation for a groin are much the same as for a jetty. The 

 primary difference is that whereas in a jetty, no sand should pass through 

 or over the structure, most groins are part of a beach stabilization 

 program and it is usually desirable to permit some littoral sand to pass 

 around, over, or through the groin. This is not to imply that the permea- 

 bility of armor stone can be designed to allow a selected amount of sand to 

 bypass, since current design procedures are not capable of designing 

 successful functioning permeable groins. Many attempts have been made to 

 design groins with a varying degree of permeability. For rubble-stone 

 groins, it is usually adequate to design the elevation of the impermeable 

 core through the nearshore and foreshore area to the desired beach profile. 

 The voids in the armor rock will generally be adequate to pass the surplus 

 sand through to the downdrift beach. Groins usually terminate just seaward 

 of the breaker zone in from 1.8 to 3.6 meters (6 to 12 feet) of water 

 (MLW) , and the seaward end is designed against the largest breaking wave 

 possible at that depth, taking tidal elevations into account. The breaker 

 zone is an area of constant turbulence and care must be taken to properly 

 place, as well design, the bedding layer or the structure will fail. 

 Considerable success has been experienced in recent years in replacing this 

 bedding layer of stone (or combining it) with filter cloth. 



(2) Seawall or Revetment . A stone rubble seawall or revetment is 

 used to protect the shore, or a shore structure, against erosion by wave 

 action or currents (Fig. 7). It may be a trapezoidal gravity seawall- type 

 structure, backfilled by shore material, or it may be a form of sloped 

 revetment against a shore bank of earth, wood, steel, or concrete. 



(a) Current Protection . A revetment designed to protect 

 against currents, tidal or river, is designed much the same as a river 

 revetment except that, in the case of tidal currents, the flow may be 

 reversible. When river and tidal currents combine, tidal elevations must 

 be considered to determine the stage of maximum or critical velocities. 

 Also in bays or large river mouths, consideration must be given to local 

 wind waves, residual swell, or seiching from the open sea. In the case of 

 river mouth entrances, the revetment may simply represent a transition 

 section from the steady flow river revetment to the wave exposed jetty. In 

 other cases, where a jettied entrance connects the open sea to a bay or 

 wide mouth and the channel is of such width as to create currents, the 

 shoreline facing the channel must be revetted. In the same manner as for a 

 breakwater, there must be a layer of armor rock, an underlayer, and a 

 filter layer. Special consideration should be given to ensure stability of 

 the toe of the structure because of the unidirectional flow of most 

 currents . 



(b) Wave Protection . The design of a rock rubble face of a 

 seawall or revetment against wave forces is similar to that of the seaward 



48 



