PART VII: VERTICAL SEAWALL TESTS 



Background 



207. Vertical seawalls placed on eroding beaches are designed to protect the land shoreward of the 

 seawall location. Recently, seawalls have been cited as being either the cause of erosion to the beach 

 fronting the seawall or contributing to increased rates of erosion to the beach profile (e.g., Pilkey and 

 Wright 1988). This criticism and the projected rise in sea level will undoubtedly require engineers and 

 coastal planners to make tough decisions on whether protection of upland investments warrants placement 

 of structures such as seawalls. Because these decisions will be difficult, they ought to be based, to the 

 maximum extent possible, on scientific facts and knowledge of the impacts of seawalls on fronting and 

 adjacent beaches. 



208. Opinions concerning the impact of seaweills on beaches are widely varied, mainly because 

 insufficient scientific evidence is available to substantiate claims of the parties debating the issue. To date, 

 the most comprehensive review and analysis of existing data and studies pertaining to seawall effects is 

 that of Kraus (1988), which critically reviewed approximately 100 scientific papers related to seawall effects 

 on the beach. The reader is referred to Kraus for his conclusions regarding seawall effects and additional 

 details. (Note that Kraus (1988) appears in a volume dedicated to examining the effects of seawalls on 

 beaches and the different viewpoints on the topic.) 



209. One particular point addressed by Kraus (1988) was whether the volume of sand scoured locally 

 on the profile in front of a seawall is greater or less than the volume eroded on adjacent beaches without 

 seawalls. Expressed in another way, "Is the volume of sand being denied to the profile by the seawall 

 similar to the volume of additional erosion observed in front of the seawall?" Kraus cites several field 

 studies that indicate the volume of sediment withheld is approximately the same as the additional eroded 

 volume over the profile of the seawalled beach. In one of the cited studies, Birkemeier (1980) used aerial 

 photography to conclude that the eroded volumes of seawalled profiles and adjacent natural profiles were 

 nearly the same. In another study, Kriebel (1987) reported that posthurricane field measurements on the 

 Florida west coast indicated that the ". . . volume of sand lost due to scour at the seawall was 

 approximately equal to the volume eroded on the adjacent beach without a seawall". 



210. Dean (1986) presented logical arguments founded on the principle of sediment conservation in 

 discussing the potential effects of coastal armoring on fronting and adjacent beaches. One of Dean's 

 proposed "approximate principles" for the 2-D case was that the local volumetric scour in front of a coastal 

 structure should be equal to or less than the volume that would have eroded if the structure had not been 

 in place. 



84 



