problem. Much work remains to te done toward gaining a better under- 

 standing of this feature of the shore. 



BffROVEMENT tVORKS 



Shore Structures. Shore structures are generally designed for one 

 of two purposes: to protect the shore against erosion or to improve or 

 shelter a navigation channel or harbor. Each design properly requires 

 an answer to three basic questions: 



a. What must be the dimensions of the structure to cause 

 it to function as required? 



b. What will be the forces imposed on the structure? 



c. What materials can most reasonably meet the required 

 strength and endurance characteristics? 



The status of research for various types of shore structures relating 

 to shore protection are given briefly in the following paragraphs. 



Beach Groins . Research is actively under way at the Beach Erosion 

 Board on the proper height, length, spacing and profile of groins. 

 Little work is being done to measure the forces exerted on groins, 

 required strength being estimated largely on the basis of experience. 

 A report on the use of steel in shore structures including groins(26) 

 was issued in 1952 and one dealing with the use of timber^26a) was 

 issued in 1955. The Portland Cement Association has a booklet on 

 concrete ^27) ^n^j ^^g Asphalt Institute may possibly issue one in the 

 near future. Recent studies at the Waterways Experiment Station for 

 the Bureau of Yards and Docks on the stability of rock mounds have 

 added greatly to our knowledge of the stability of rock structures in 

 wave action(28)^ 



Seawalls . The influence of the shape of seawalls on wave run-up 

 and quantity of overtopping water has been the subject of extensive 

 tests for the Beach Erosion Board at the Waterways Experiment Station 

 over the past three years. Vertical-face, stepped-face, curved-face, and 

 sloping-face breakwaters have been tested over a wide range of wave 

 conditions. Some of the results have been published(29) ^nd a compre- 

 hensive report is expected to be published in a year or so by the 

 Beach Erosion Board. Work still needs to be done on erosion at the toe 

 of seawalls. At least two exploratory studies^ -^^'^^^ have been made 

 recently of the wave forces developed against seawalls; however much 

 additional work is needed before a quantitative understanding is gained. 

 Studies of the suitability of materials for use in seawalls are 

 essentially the same as those mentioned for "beach groins" in the pre- 

 ceding paragraph. 



