1^4?. With the construction of the groin a b c, the tendency is for 

 the updrift beach to accrete as littoral drift is intercepted, and for 

 the dox^rndrift beach to erode as the norraal supply of material is with- 

 held by the groin. Now at the seaward end of the groin, the sand 

 elevation on either side of the groin will remain the same so that any 

 advance on the updrift side must be accompanied by a steepening of the beach 

 as between b and c. Conversely, any recession on the downdrift side of 

 the groin must be accompanied by a flattening of the beach as between d 

 and c, ~ 



lii.6. The changes in beach slopes are directly related to two 

 principles governing the movement of beach materials; (ij the beach slope 

 is a function of grain size, e,g» for the same wave conditions, the coarser 

 the beach material the steeper the slope on which it will stand; (2) the 

 beach slope is a function of the wave characteristics, e.g. for the same 

 beach material, high steep waves cause offshore movement of material and 

 flatter slopes, whereas low long iJaves v.aa^e onshore movement of material 

 and steeper slopes. 



Iii7. Referring to the updrift side of the groin. Figure 60 with the 

 interception of littoral drift the shore line begins to advance from a 

 to b resulting in an unstable slope* WTien the slope becomes unstable, 

 the'waves attempt to recreate a stable condition by moving material off- 

 shore to c_ whence it can move on downcoast or farther offshore. In this 

 offshore movement, the coarser fractions of the sand are left behind 

 resulting in a material having a larger median diameter and a steeper 

 stable slope. This action tends to continue until all of the material 

 in littoral drift is passing around or over the groin or until the updrift 

 beach has reoriented itself to a position normal to the wave orthogonals 

 and the downdrift movement of material shoreward of c_ comes to a halt 

 for all practical purposes, 



1U8, With the reorientation of the shore line, the waves cease to 

 diverge as they approach shore which results in slightly higher wave action, 

 This higher wave action tends to move additional material offshore thus 

 slowing the advance of the shore line or accelerating the gradation of 

 the beach material. This combined action continues until a balance has 

 been reached betX'jeen grain size and beach slope. 



II49. On the downdrift side of the groin, the shore line starts to 

 recede from a towards d as the normal supply of material is interrupted. 

 As the elevation remains constant at c_, this results in a flatter slope 

 and the beach is again unstable but in an opposite direction than on the 

 updrift side. With the flatter slopes, the waves are competent to move 

 material into the eroding area from seaward of £ h. This action somewhat 

 nuHliiies the tendency of the shore line to recede. Recession of the 

 doT^mdrift shore line will continue until sufficient material passes around 

 or over the groin to stabilize the shore line. 



l50. Figure 6I shows the general position of the shore line to be 

 expected for a field of 2 or more groins. The positions shown assumes 



78 



