PART IV: METHODS OF INVESTIGATION 



23. In order to quantify the amount of beach erosion as measured from 

 historic maps, a technique developed by the US Army Coastal Engineering 

 Research Center (1973) was used. This technique recognizes that changes in 

 area of beach can be used to estimate the volume of sediment eroded, if the 

 average berm height and the offshore depth limit of effective wave base are 

 considered. This same relationship was used to calculate the amount of ma- 

 terial deposited in the back-barrier area by estimating the depth limit of 

 sand penetration from historic hydrographic charts (Appendix A). Where marsh 

 was present the marsh elevation was used as the depth limit. This relation- 

 ship can be expressed as: 



AV = AA(a - b) 



where 



AV = volume of sediment gained or lost 



AA = change in area of beach or back barrier 



a = average berm height 



b = offshore (or bay depth) beyond which natural changes of sand bottom 

 are insignificant 



24. For Isles Dernieres, the values of a were variable along the is- 

 lands* length, ranging from 0.36 to 0.59 m. The value of b for the offshore 

 was determined to be 3.65 m by the US Army Corps of Engineers (1973) and was 

 variable on the bayside. Using the values chosen for each time interval, a 

 constant can be calculated using the above relationship for each period. 



Example: Where a = 0.61 m and b = 3.65 m 



3 3 



4? = constant = ^> [ +0 .61 m - (-3.65 m) ] = 4.26 ^ 

 AA lm 3 m 2 



25. Therefore, for each square meter of beach change on Isles 

 Dernieres, using these constants, 4.26 nr of sediment is lost or gained. 

 Using this method, the value for square feet of beach surface gained or lost 

 can be converted into cubic metres of sediment gained or lost. 



26. The Ott Compensating Planimeter was used to calculate the area of 

 sediment eroded and deposited during the intervals for which historic data 



16 



