S tructure Functions 



Jetties located at coastal entrances 

 to bays or rivers usually have multiple 

 purposes, including: 



stabilize the inlet location; 



direct and confine flow 



vent or reduce channel 



from littoral drift material; 



protect vessels using the entrance 



from wave or current action. 



o 







or pre- 

 shoaling 



When taken together, these func- 

 tions have aspects of groins and break- 

 waters, as well as jetties. Jetties locat- 

 ed inside of estuaries or along rivers 

 may have the single function of direct- 

 ing and confining flow to reduce chan- 

 nel shoaling. Sometimes, these struc- 

 tures concurrently function as groins 

 because they may stabilize or otherwise 

 change the movement of material along 

 an estuarine or riverine beach. 



Site Characteristics and Environment 

 Conditions 



Jetties are usually placed on one 

 or both sides of an inlet, extending 

 from above high water on the shoreline 

 but beyond low water (Ortolano and Hill 

 1972). They sometimes extend out to 

 the depth of the associated navigation 

 channel (U.S. Army Corps of Engineers 

 1971b) and usually extend beyond the 

 surf zone (Gifford 1977). Most jetties 

 are found at river or bay openings into 

 the ocean or the Great Lakes. Natural 

 and man-made inlets, when unaltered, 

 usually interrupt the longshore move- 

 ment of sand. This causes bar formation 

 in the inlet mouth (U.S. Army Corps of 

 Engineers 1971b). Jetties are also lo- 

 cated at natural and man-made inlets 

 through barrier beaches (U.S. Army 

 Corps of Engineers 1973b). 



Placement Constraints 



Engineering . A number of factors 

 must be considered when choosing a de- 

 sign and a site for a jetty. The Shore 

 Protection Manual (U.S. Army Corps of 

 Engineers 1973b) recommends careful 

 study of the following: 



(1) The tidal prism and cross- 

 section of the gorge in the 

 natural state; 



(2) Historical changes in inlet 

 position and dimensions..; 



(3) Range and time relationship 

 (lag) of tide inside and out- 

 side the inlet; 



(4) Influence of storm surge or 

 wind setup on the inlet; 



(5) Influence of the inlet on 

 tidal prism of the estuary and 

 effects of freshwater inflow 

 on the estuary; 



(6) Influence of other inlets on 

 the estuary; and 



(7) Tidal and wind-induced cur- 

 rents in the inlet. 



Hydraulic Factors 

 Improved Inlet. 



of Proposed 



(1) Dimensions of the inlet...; 



(2) Effects of inlet improvements 

 on currents in the inlet, and 

 on the tidal prism, salinity 

 in the estuary, and on other 

 inlets into the estuary; 



(3) Effects of waves passing 

 through the inlet; and 



(4) Interaction of the Hydraulic 

 Factors (item b) on Navigation 

 and Control Structure Factors 

 (item c and d). 



Navigation Factors 

 Improved Inlet. 



of the Proposed 



Hydraulic 

 Inlet. 



Factors of the Existing 



(1) Effects of wind, waves, tides 

 and currents on navigation 

 channels; 



(2) Alignment of channel with re- 

 direction and natural channel 

 of unimproved inlet; 



(3) Effects of channel on tide, 

 tidal prism and storm surge of 

 the estuary; 



(4) Determination of channel di- 

 mensions based on design ves- 

 sel data and number of traffic 

 lanes; and 



(5) Other navigation factors such 

 as: 



(a) Relocation 

 channel to 

 site; 



(b) Provision 

 pans ion of 

 sions; and 



of navigation 

 alternate 



for future ex- 

 channel dimen- 



28 



