sloped sides and protect the base of the 

 structure from scour and undermining 

 (U.S. Army Corps of Engineers 1973b)' 



Sheet piles are not satisfactory in 

 high wave energy environments, but 

 can be used where the wave climate is 

 less severe. Steel, used as sheet piles, 

 should be coated to prevent corrosion. 

 Timber must be treated with preserva- 

 tives to prevent attack by marine inver- 

 tebrates, such as borers and gribbles. 

 Concrete is immune to pests, but an im- 

 proper mix can deteriorate rapidly in 

 seawater (U.S. Army Corps of Engi- 

 neers 1973b). 



Expected Life Span 



Rubble mound jetties can last up 

 to 50 yr if properly designed and main- 

 tained (U.S. Army Engineer District, 

 Portland 1975b). Maintenance includes 

 both replacing displaced armor compo- 

 nents after particularly severe storms 

 and major repairs every 15 to 20 yr to 

 replace broken, worn, or lost compo- 

 nents. Sheet pile jetties, whether steel, 

 timber or concrete have shorter life- 

 spans due to abrasion by sand and wa- 

 ter-borne debris, corrosion by salt wa- 

 ter, and attack by borers or gribbles 

 (U.S. Army Corps of Engineers 1973b). 

 They can last anywhere from 10 to 35 

 yr depending on the conditions of their 

 environment. 



Summary of Physical and Biological 

 Impacts 



C onstruction effects . As with any 

 major construction activity in the coast- 

 al zone, placement of jetties causes some 

 temporary disturbance, such as turbid- 

 ity caused by res us pension of bottom 

 sediments. Toxic substances present in 

 sediments can be released (Carstea et 

 al. 1975a). Noise, and air and water 

 pollution will accompany construction 

 activities (U.S. Army Engineer District, 

 St. Paul 1976a). During construction, 

 nearshore currents can be disrupted. 

 Erosion and accretion can occur locally 

 in patterns quite different from those 

 previously existing or those which de- 

 velop after completion (Anderson 1975). 

 Suspended sediments may reduce pri- 

 mary productivity and smother benthic 

 organisms (Cronin et al. 1971). The 



area covered by the jetty will be lost 

 as a bottom habitat (Virginia Institute 

 of Marine Science 1976), but a new type 

 of habitat will be created. 



Chronic effects. The presence of 

 jetties at a river or bay mouth alters 

 both river outflow and tidal currents 

 (U.S. Army Engineer District, Portland 

 1975b). These alterations are often felt 

 well into the estuary and may have wide- 

 spread effects. Altered rates of nutri- 

 ent and sediment accumulation can occur 

 in salt marshes. Salinity and tempera- 

 ture changes can occur. The tidal prism 

 can be altered since overall circulation 

 patterns within an estuary are affected 

 by the change in water flow through a 

 stabilized channel. The flushing charac- 

 teristics of the estuary can be changed 

 and wave height often increased in its 

 lower regions (Carstea et al. 1975a). 



Outside the estuary, the most sig- 

 nificant effect of jetties is the alter- 

 ation of littoral transport. Littoral 

 transport is obstructed by the jetties; 

 sand is impounded updrift and eroded 

 downdrift. If a single jetty is install- 

 ed, the opposite side of the inlet can 

 erode severely. Also, a shoal can form 

 at the tip of a single jetty updrift of 

 an inlet and eventually fill in the in- 

 let. The influence of a jetty extends 

 well beyond its immediate vicinity. 

 Downdrift beaches retreat due to sand 

 starvation unless measures are taken to 

 bypass the sand that accumulates updrift 

 of the jetties (Ketchum 1972). Changes 

 in foredune height have been reported 

 downdrift of jetties (Demory 1977). 



The channel formed by jetties often 

 migrates from the original location to 

 an area adjacent to one of the jetties, 

 scouring the bottom and causing 

 turbidity. As the channel nears the 

 jetty, the scouring action can erode the 

 base of the jetty and necessitate re- 

 pairs or strengthening of the structure 

 base. Sandbars tend to migrate seaward 

 in the presence of jetties (Kieslich and 

 Mason 1975). Dredging is usually requir- 

 ed to maintain a channel of sufficient 

 depth since the tidal currents are inad- 

 equate to keep the channel scoured (U.S. 

 Army Corps of Engineers 1973b). 



The placement of jetties destroys 



31 



