northern end of the grid. This type of boundary condition allows sediment to 
move freely alongshore across the boundary from either side. The full grid 
consists of 537 cells and spans a coastal reach of 26.9 km (16.6 miles). This 
is the longest known extent to be simulated with a shoreline change model. 
Representation of 
structures in the model 
97. As discussed in Part II, a massive seawall and numerous groins 
have been constructed on the north New Jersey coast in an attempt to reduce 
erosion and control the position of the shoreline. To accurately simulate 
shoreline change, the influence of these structures on the longshore sediment 
transport rate and shoreline position must be represented in the model. 
98. Seawall. A seawall functions to prevent landward retreat of the 
shoreline. Although only portions of the project coastline are backed by an 
actual seawall, roads and buildings located immediately landward of the pres- 
ent shoreline serve as an effective seawall (transport restrictor) since 
erosion would not be permitted beyond such facilities. Topographic sheets 
from the 1985 shoreline survey indicate the position of an effective seawall 
for the full length of the modeled area. A seawall introduces a constraint on 
the longshore sediment transport rate, in addition to constraining the 
possible position of the shoreline. Implementation of the seawall constraint 
in the model is complex; the report by Hanson and Kraus (1986) should be 
consulted for further details. 
99. The seawall constraint is imposed at the same level of approxima- 
tion as the assumptions used to derive the one-line model. Wave reflection, 
scouring, and flanking are not simulated. This description is believed to be 
reasonable, provided the beach slope in front of the seawall does not appreci- 
ably deviate from that of the neighboring beach. This restriction is equiva- 
lent to assumption a of the one-line theory. 
100. Groins. The positions and lengths of groins were obtained 
directly from 1985 aerial photographs and corresponding topographic maps. 
Based on careful inspection of the photographs, 91 groins were placed on the 
full grid with, for example, 19 groins lying in the 8-mile-long stretch from 
Sea Bright to Monmouth. Six groins lie to the north, between the project's 
north terminus and Sandy Hook Gateway Recreation Park. Only groins judged to 
be efficient at trapping sand were entered in the model; very short groins and 
nonfunctioning groin remnants were not included. Classification of a groin as 
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