5o32 LIMITATIONS 



Whether to provide a protective beach, with or without groins, de- 

 pends on the availability of suitable sand for the purpose. Artificial 

 nourishment is usually quite costly on a unit length basis when applied 

 to short segments of shore, because the widened beach protuding seaward 

 of its adjacent shores erodes rapidly. This results in high nourishment 

 costs, but is not necessarily a limitation if artificial nourishment over 

 a short length of beach prevents the enlargement of that problem area to 

 dovmdrift shores. However, difficulties could be encountered in financing 

 a shore protection method which provides protection beyond the immediate 

 problem area. 



5.33 PLANNING CRITERIA 



Planning of a protective beach by artificial nourishment requires: 



(a) Determination of the predominant direction of longshore trans- 

 port and deficiency of material supply to the problem area; 



(b) Determination of the composite average characteristics of the 

 existing beach material or native sand in the zone between the 30-foot 

 depth and the dune or cliff line (the zone of active littoral movement); 



(c) Evaluation and selection of borrow material for initial beach 

 fill and periodic nourishment, including the determination of any extra 

 amount of borrow material required for placement based on the comparison 

 of the native beach sand and borrow material; 



(d) Determination of beach berm elevation and width; 



(e) Determination of wave-adjusted foreshore slopes; 



(f) Determination of feeder-beach (stockpile) location. 



5.331 Direction of Longshore Transport and Deficiency of Supply . The 

 methods of determining the predominant direction of longshore transport 

 are outlined in Section 4.5. The deficiency of material supply is the 

 rate of loss of beach material; it is the rate at which the material 

 supply must be increased to balance the transport by littoral forces to 

 prevent net loss. If no natural supply is available, as on shores down- 

 drift from a major barrier to longshore transport, the net rate of long- 

 shore transport will approximate the deficiency in supply. Comparison^ 

 of surveys of impoundment or eroding areas over a long period of time is 

 the best method of estimating the rate of nourishment required to maintain 

 stability of the shore. Since surveys suitable for volume measurement are 

 rarely available, approximations computed from changes in the shore posi- 

 tion, as determined from aerial photographs or other suitable records, are 

 often necessary. For such aomputations ^ the relationship in which 1 square 

 foot of change in beach surface area equals 1 cubic yard of beach material 

 appears to provide acceptable values on exposed seacoasts. For less ex- 

 posed shores, this ratio would probably result in volume estimates in 

 excess of the actual value. 



5-8 ( 



