the areas between the initial postgroin shoreline and the original shoreline 

 position. As sand is added to the compartment, the shoreline will build out 

 to the stabilized groin-adjusted shoreline. The estimation of the initial 

 groin-adjusted shoreline is important since it represents a maximum erosion 

 condition and is, therefore, essential in the design of the shoreward limit of 

 the groin. 



(3) Downd rif t Shore Alinement . The major factor in an adjustment of 

 the downdrift beach is the reduction in the longshore transport while the 

 updrift side of the groin and the updrift groin compartments of a groin system 

 are naturally filling. The time period for a natural filling to take place 

 can be estimated by assuming that the percentage of longshore drift not reach- 

 ing the downdrift area is being trapped updrift. This sand will fill the 

 updrift groin and groin compartments until the adjusted updrift beach aline- 

 ment (Fig. 5-14) and the adjusted intermediate beach alinement (Fig. 5-15) are 

 attained. The sediment trapped in these updrift fillets is prevented from 

 reaching the downdrift area, which results in downdrift erosion. If artifi- 

 cial fill is used to form the updrift fillets, the longshore transport will 

 bypass the groin and reach the downdrift area sooner than if the natural 

 longshore drift were depended on to form the fillets. Therefore, artificial 

 filling is usually preferred. The following steps can be used to determine 

 the position of the downdrift shoreline: 



(a) Estimate the time required for the updrift side of the groin to 

 fill (see Sec. VI, 3, g of this chapter). 



(b) Draw an adjusted shoreline, r-s, which represents the berra crest 

 line shown in Figure 5-16 such that the area r-s-o accounts for the 

 deficit volume of longshore drift determined from the time period for the 

 updrift groin or groins to fill. Use the natural beach profile of the 

 study area to find the volume corresponding to the area r-s-o (see Sec. VI, 

 3,g of this chapter). 



BEACH 



Groin -Adjusted 

 Shoreline 



Originol 

 Shoreline 



Direction of Net 

 Longshore Transport 



OCEAN 



Figure 5-16. Downdrift beach alinement. 



5-43 



