coast of Florida (Walton and Adams, 1976), but quantities elsewhere vary 

 widely according to local conditions. 



6. Beach Stability . 



Although a beach may be temporarily eroded by storm waves and later partly 

 or wholly restored by swells, and erosion and accretion patterns may occur 

 seasonally, the long-range condition of the beach — whether eroding, stable, or 

 accreting — depends on the rates of supply and loss of littoral material. The 

 shore accretes or progrades when the rate of supply exceeds the rate of loss. 

 The shore is considered stable (even though subject to storm and seasonal 

 changes) when the long-term rates of supply and loss are equal. Thus, conser- 

 vation of sand is an important aspect of shore protection. 



V. CAUSES OF SHORELINE EROSION 



Before embarking upon any method of coastal protection, it is important to 

 identify and understand both the short- and long-term causes of coastal ero- 

 sion. Failure to do this may result in the design and placement of shore 

 protection measures which actually accelerate the process that the protection 

 measure was intended to alleviate. Although the most serious incidents of 

 coastal erosion occur during storms, there are many other causes, both natural 

 and man-induced, which need to be examined. 



Natural causes of erosion are those which occur as a result of the response 

 of the beach to the effects of nature. Man-induced erosion occurs when human 

 endeavors impact on the natural system. Much of the man-induced erosion is 

 caused by a lack, of understanding and can be successfully alleviated by good 

 coastal zone management. However, in some cases coastal erosion can be due to 

 construction projects that are of economic importance to man. When the need 

 for such projects is compelling, the coastal engineer must understand the 

 effects that the work will have on the natural system and then strive to 

 greatly reduce or eliminate these effects through designs which work in 

 harmony with nature. 



Natural and man-induced causes of erosion, as discussed below, are given in 

 Table 1-1. 



1. Natural Causes . 



a. Sea Level Rise . A long-term rise in sea level relative to the land 

 exists in many areas of the world. This rise results in a slow, long-term 

 recession of the shoreline, partly due to direct flooding and partly as a 

 result of profile adjustment to the higher water level. 



b. Variability in Sediment Supply to the Littoral Zone . Changes in the 

 world's weather pattern that cause droughts can result in a reduction in the 

 occurrence of floods on rivers supplying sediment to the coastal zone. 



c. Storm Waves . Steep waves from a coastal storm cause sand to be trans- 

 ported offshore with temporary storage in a bar or shoal. Later partial 

 recovery of the beach may be made through natural transport of this material 

 onshore by longer period, flatter waves. But, in most cases, some material is 

 permanently lost into the greater offshore depths. 



1-15 



