is to provide or maintain a beach. 

 Groins can be designed in various con- 

 figurations to do any of the following: 



o build or widen a beach by trap- 

 ping littoral drift; 



o stabilize a beach by reducing the 

 rate of sand loss; 



o prevent accretion in a downdrift 

 area by acting as a littoral bar- 

 rier. 



The above functions all assume 

 existence of either a sandy beach and/ 

 or a littoral supply of sand. Groins 

 can affect areas both updrift and dot n- 

 drift. The functions of building, or 

 stabilizing a beach may have the effect 

 of starving an adjoining area. 



Site Characteristics and Environmental 

 Conditions 



Groins are constructed on many 

 types of shorelines, but most commonly 

 on shallow, sandy, or shingle beaches. 

 Since they can be used to prevent ero- 

 sion, build or widen beaches, or prevent 

 downdrift accretion, their siting on the 

 shoreline is dictated by their intended 

 function (Figures 15, 16, and 17). For 

 a groin or groin system to function, 

 there must be a supply of sand provid- 

 ed by littoral transport. Other than 

 this common characteristic, no generali- 

 zations can be made concerning environ- 

 mental conditions or groin sites. 



Placement Constraints 



Engineering . A groin must be de- 

 signed for a specific site. There is no 

 best design , optimum choice of construc- 

 tion, nor ideal length or spacing between 

 groins that can be applied generally to 

 all situations. The substrate of the site 

 must be studied to determine structural 

 limitations, material availability, and 

 maintenance requirements (U.S. Army 

 Corps of Engineers 1973b). Other char- 

 acteristics of a shoreline must also be 

 known before a single groin or a groin 

 field is constructed. These are angle 

 of wave approach, volume of littoral 

 drift, wave strength, current, and 

 shoaling patterns (Horikawa and Sonu 

 1968). 



If the objective of the groin or 



groin field is to trap sand and to mini- 

 mize sand movement downcoast, groins 

 should be built to a height that will 

 prevent normal high water from carrying 

 sand over them. When continued movement 

 of sand is desired, the height of groins 

 should be near to or below normal high 

 tide level (Balsillie and Eerg 1973). 

 Length of groins is also dependent on 

 the degree of littoral drift obstruction 

 desired and on existing and desired 

 beach slope (U.S. Army Corps of Engi- 

 neers 1973b). Length is measured from 

 the groin's landward end at the berm to 

 its seaward end. The seaward end usually 

 extends to the point where incoming 

 swells exert the greatest force on the 

 sand bottom (Coen-Cagli 1932). 



The spacing of groins in a groin 

 field is subject to a number of factors. 

 As a general rule, groins should be sep- 

 arated by a distance tv/o to four times 

 their length (Savage 1959, U.S. Army 

 Corps of Engineers 1973b). However, 

 spacing must assure that minimum beach 

 width is maintained. A more detailed 

 discussion of the factors involved in 

 groin spacing is found in the Shore Pro- 

 tection Manual (U.S. Army Corps of Engi- 

 neers 1973b). 



Though the majority of groins are 

 straight, some are built with a length- 

 wise curve or are L-, Z-, or T-shaped 

 (Balsillie and Berg 1973). Their crests 

 can be level or can slope downwards to- 

 ward the seaward end (U.S. Army Corps of 

 Engineers 1973b). In a groin field, suc- 

 cessive downdrift groins can be made 

 progressively shorter or lower, with the 

 latter variation being preferable (Coen- 

 Cagli 1932). 



Whatever the design of groins, 

 starvation of downdrift beaches should 

 be prevented. If a newly constructed 

 groin will capture nearly all littoral 

 drift, artificial nourishment is desir- 

 able to assure a supply of sand to down- 

 coast beaches (Sanko 1975). Another 

 method of filling behind a groin in- 

 volves placing a weir or series of weirs 

 along its length. This allows a portion 

 of the littoral drift to continue down- 

 drift (U.S. Army Corps of Engineers 

 1973b). This structure is effective only 

 if there is no movement of the stone 

 material . 



34 



