Consider the beach shown in Figure 5-2 where the solid line represents 

 an average existing profile. It is desired to place a seawall at point A 

 as shown. From prior records, either the loss of beach width per year or 

 the annual volume loss of material over an area which includes the profile, 

 is known. In the latter case, the annual votyme loss may he converted to 

 an annual loss of beaoh width by the general rule: loss of 1 cubic yard 

 of beach material is equivalent to loss of 1 square foot of beach area on 

 the benn. This inale is applicable primarily at the ocean front. In 

 shallow, protected bays, the ratio of volume to area is usually much less. 



+ 20 I— 



+ 10 — 



o - 10 — 



m - 20 — 



-30 — 



-40 



MLW 



Figure 5-2. Effects of Erosion. 



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 30 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 30 feet, E 

 the point of slope break at the depth of about 5 feet, and C the present 

 position of the berm crest. If it is desired to build a structure at A 

 whose economic life is estimated at 50 years, and it is found that n is 

 the annual loss of beach width at the berm, then in 50 years without the 

 wall 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 dashed line, D F B, 

 will represent the approximate profile of beach after 50 years without the 

 presence of the wall„ The receded beach elevation at the wall's location 

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



5-6 



