80 



Sea level rise will generally result in a loss of efficacy of existing 

 groin projects. Increased water level will allow more overtopping by 

 waves , and eventually flanking could occur at the landward end of the 

 structure as the shoreline retreats. Groins with long useful lives may 

 require lengthening and raising, while those with shorter life spans should 

 be replaced with redesigned structures. 



Figure 6.10 shows the groin field at Long Branch, New Jersey, where 

 the longshore drift is from south to north. This project has succeeded in 

 trapping sand and building a beach, but apparently at the expense of the 

 North Long Branch shoreline. 



Jetties - are shore -perpendicular structures, usually of rubble mound 

 construction, placed at tidal inlets in order to stabilize their position' 

 and maintain a navigable channel. Shoreline response to construction of 

 jetties is similar to that of groins, but on a larger scale as jetties are 

 usually very long. A large fillet is formed on the updrift side of the 

 inlet, with the downdrift shoreline often subject to severe erosion. 

 Jetties serve to increase the velocities in a tidal inlet, which deepens 

 the cross-section and pushes the ebb shoals offshore, entrapping even 

 larger amounts of sediment. Without mechanical bypassing of sand from the 

 updrift to downdrift side of an inlet, the downdrift beach will erode until 

 the updrift fillet and ebb shoals are large enough to shunt sand across the 

 inlet. 



Jetties may exacerbate shoreline retreat as sea level rises in the 

 following manner. A rise in mean water level increases the depth of a bay 

 and increases the hydraulic efficiency of an inlet, thereby increasing the 

 tidal prism. An increase in tidal prism increases the velocities through 

 the inlet which in turn pushes the ebb shoal offshore into deeper water. 

 The shoal then demands more sediment from the neighboring shorelines until 

 the regional longshore transport rate is restored (if possible). As noted 

 previously, long-term sea level rise will also promote creation of 

 additional inlets, which if stabilized will each demand sediment to 

 maintain ebb shoals . 



As with groins, sea level rise will tend to reduce the efficacy of 

 jetties due to overtopping and possible breaching at the shoreward end. 



