Nearshore slopes are usually gentle seaward of the bar. Slopes are 

 steeper inshore of the bar and may be as steep as 1 on 5 at the waterline with 

 coarse sand. Analyses of profiles at eroding beaches indicate that it may be 

 assumed that the slope seaward of a depth of 8 meters (26 feet) will remain 

 nearly unchanged, that the point of slope break E will remain at about the 

 same elevation, and that the profile shoreward of the point of break in slope 

 will remain nearly unchanged. Thus, the ultimate depth at the wall may be 

 estimated as follows: 



(a) In Figure 5-2, let B represent a water depth of 8 meters, 

 E the point of slope break at the depth of about 2 meters (6.5 feet), 

 and C the present position of the berm crest. If it is desired to build a 

 structure with an economic life estimated at 50 years at point A and it is 

 found that n is the annual average loss of beach width at the berm, then in 

 50 years without the structure this berm will retreat a distance 50n to 

 point D. 



(b) From D to the elevation of point E, draw a profile D-F 

 parallel to C-E, and connect points B and F. This dashline,, D-F-B, will 

 represent the approximate profile of beach after 50 years without the struc- 

 ture. The receded beach elevation at the structure's location will be 

 approximated by point A' . Similar calculations may be made for anticipated 

 short-term beach losses caused by storms. Storm erosion generally results in 

 a greater loss of beach material above the mean low water (MLW) level, because 

 the superelevation of the water level (storm surge) allows storm waves to act 

 on the upper part of the beach. 



Other factors considered in planning and design are the depth of wall 

 penetration to prevent undermining, tiebacks or end walls to prevent flanking, 

 stability against saturated soil pressures, and the possibility of soil slump- 

 ing under the wall . 



III. PROTECTIVE BEACHES 



1 . Functions . 



Beaches can effectively dissipate wave energy and are classified as shore 

 protection structures of adjacent uplands when maintained at proper dimen- 

 sions. Existing beaches are part of the natural coastal system and their wave 

 dissipation usually occurs without creating adverse environmental effects. 

 Since most beach erosion problems occur when there is a deficiency in the 

 natural supply of sand, the placement of borrow material on the shore should 

 be considered as one shore stabilization measure. It is advisable to investi- 

 gate the feasibility of mechanically or hydraulically placing sand directly on 

 an eroding shore, termed heaoh restoration^ to restore or form, and subse- 

 quently maintain, an adequate protective beach, and to consider other remedial 

 measures as auxiliary to this solution. Also, it is important to remember 

 that the replenishment of sand eroded from the beach does not in itself solve 

 an ongoing erosion problem and that periodic replenishment will be required at 

 a rate equal to natural losses caused by the erosion. Replenishment along an 

 eroding beach segment can be achieved by stockpiling suitable beach material 

 at its updrift end and allowing longshore processes to redistribute the 

 material along the remaining beach. The establishment and periodic replenish- 

 ment of such a stockpile is termed artificial heach nourishment. Artificial 



5-6 



