260. Line E.5: YDG(I) Enter the lengths of the diffracting groins 

 and jetties YDG(I) as measured from the x-axis in the order of cell number 

 in which they occur (NDG values from small to large cell numbers correspond- 

 ing to the locations given at Line E.4). 



261. Line E.6: DDG(I) Enter the depths at the tips of the dif- 

 fracting groins and jetties DDG(I) in the order of cell number in which they 

 occur (NDG values from small to large cell numbers corresponding to the 

 locations given at Line E.4). 



F. Groins/ietties 



262. Line F . 1 . This section requests general information pertaining to 

 both nondiffracting and diffracting groins and jetties (and shore -connected 

 breakwaters). If there are no groins or jetties on the grid (values of 

 entered at both Lines D.l and E.l), then Lines F.2-F.5 may be skipped. If 

 there are groins of any type, responses to Lines F.2-F.5 must be given. (If 

 there are no groins or jetties on the grid, Lines F.2-F.5 will not be read by 

 GENESIS, and values remaining there may be arbitrary.) 



263. Line F.2: SL0PE2 Groins impound sand on the side of pre- 

 dominant direction of drift, implying that the beach slope near a groin is 

 milder than the equilibrium slope. An estimate of this slope SL0PE2 should 

 be made by reference to measurements at the site or to other data. GENESIS 

 uses this value in calculation of sand bypassing around the seaward tips of 

 groins and jetties. 



264. Line F.3: PERM(I) . Permeabilities PERM(I) (called "P" in 

 the main text) of the groins and jetties must be assigned. Permeabilities 

 should be given in order of increasing cell location of the structures as they 

 appear on the grid, irrespective of whether the structure is nondiffracting or 

 diffracting. 



265. The permeability coefficient empirically accounts for transmission 

 of sand through and over a groin. (Bypassing of sand around the seaward end 

 of groins is automatically calculated by GENESIS.) A permeability value of 

 1.0 implies a completely transparent groin, whereas a value of 0.0 implies 

 a high, impermeable groin that does not allow sand to pass through or over it. 

 (Note: A completely transparent groin is not necessarily equivalent to a 

 natural beach (no groin): a representative beach slope (Line F.2) must be 



113 



