i.ith present laethods does not ordinarily warrant its use. For p11 prrctic?-! 

 purposes^ short groins can.be constructed normal to the beach, c.s this 

 xjill approxiiiiate very closely the resultant direction of wave approach. 

 Long groins though usually built normal to the shore may v/arrant an 

 effort to determine the resultant wave direction, 



173, Order of Groin Construction . - This applies only to sites 

 vjhere a ^-J^oi^'i field is under consideration. Here two conditions arise: 

 (l) where the ^roin litld will be filled and it is desired to stabilize 

 the new beach in its advanced position; and (2) cohere littoral drift is 

 depended upon to make the fill and it is desired to stabilise the existing 

 beach or build additional beach with a minimum of detriment-cl effect on 

 downdrift areas. 



I7J4. In the first instance the only interruption to littoral drift 

 will be between the time the groin field is constructed and the time the 

 artificial fill is made. In the interests of economy, the fill inust be 

 placed at one time, especially if it is being accomplished by hydraulic 

 dredge. Accordingly to reduce the time interval between groin construction 

 and deposition of fill, all groins should be constructed concurrently or 

 as rapidly as practicable if constructed consecutively. Deposition of 

 fill should comrience as soon as the stage of groin construction will 

 permit. 



17!;^, In the second instance nn groin can fill until all of the 

 preceding updrift groins have been filled. The time required for the 

 entire field to filj i^nu material to resume its unrestricted ino'^oiiiunt 

 doxmdrift may be excessive so that severe dmuage r-!=^y I'esult^ /.ccci-dingly 

 only the groin or group of groins at the downdrift end should be con- 

 structed initially. The second groin, or group, should not be started 

 until the first has filled- and m^tterial passing around or over the ^roins 

 has again stabiliiied the downdrift beach, Alt>iout.:h this method may in- 

 crease mobilisation costs, it will not only aid in holding damage to a 

 rainijniim, but will provide a practicable guide to spacing of . roins to 

 vfrif3'- the previously coniputed spacingo 



CFFi:.lG>iE 3.^.1:;AI'..<ATEIio 



17u. Jse. - An off short breakwater differs from other breal i.'aters 

 in that it is generally parallel to and is not f:oiiiiected with shore. 

 This i.ype of structure seldom has been constructed rolely for shore pro- 

 tection because of its comparatively high first cor-t and the difficulty 

 of minor maintenance, if required. However, undei' ^.ome specific con- 

 diticns., this cost may be justified. 



17Y. Effect On The Slxore Lin e. - T>i6 effectj; of an offshore brtnk- 

 wfiber on the shore line pre partially illustrated by Fii.cure C£, An off- 

 shore br-akv/nter hp.s an effect on v,hi-. i">!iore regjjtien similrr tci thrt of 

 nny other structure, such as a groin, i hi';h modifies the rate of littoral 

 drift. It is probably the most effective means of completely intercepting 



90 



