(c) Effects of harbor facili- 

 ties and layout on chan- 

 nel alignment. 



d. Control Structure Factors. 



(1) Determination of jetty length 

 and spacing by considering 

 the navigation, hydraulic, 

 and sedimentation factors; 



(2) Determination of the design 

 wave for structural stability 

 and wave runup and overtop- 

 ping considering structural 

 damage and maintenance; and 



(3) Effects of crest elevation and 

 structure permeability on 

 waves in channel. 



e. Sedimentation Factors. 



(1) Effects of both net and gross 

 longshore transport on method 

 of sand bypassing, size of 

 impoundment area, and chan- 

 nel maintenance; and 



(2) Legal aspects of impoundment 

 area abd sabd bypassing pro- 

 cess. 



f. Maintenance Factor. Dredging will 

 be required, especially if the cross- 

 section area between the jetties is 

 too large to be maintained by the 

 currents associated with the tidal 

 prism. " 



To be effective in preventing the 

 shoaling of a navigation channel, jetties 

 must be impermeable. However, imper- 

 meability causes downdrift sand starva- 

 tion so methods have been developed to 

 bypass the sand which accretes behind 

 the updrift jetty. These include bypass 

 pumping, placement of the weirs in an 

 otherwise solid jetty, sand transfer 

 plants, and dredging. Dredging is also 

 used to remove bars which form in the 

 channels lacking adequate currents to 

 maintain depth by scour (U.S. Army 

 Corps of Engineers 1973b). 



Socioeconomic . The size, type, and 

 construction materials of the jetties de- 

 pends, in part, on available funds for 

 construction and maintenance (U.S. 

 Army Corps of Engineers 1973b). Nei- 

 ther the construction nor maintenance 

 should severely hamper commercial or 

 recreational use of the area around the 

 jetty site (Persaud and Wilkins 1976). 

 It is also important to consider the final 

 appearance of a jetty, either alone or 

 as part of the overall shoreline scene 



(Snow 1973). Jetties provide a spot for 

 fishing, and safe passage to and from a 

 harbor for small craft. In addition, 

 during the construction period there may 

 be a beneficial economic effect in the 

 area (U.S. Army Enqineer District, Port- 

 land 1976e). 



Bi ological . When planning jetty 

 construction, the effects of the struc- 

 ture on area wildlife propagation and 

 movement should be considered (Coastal 

 Plains Center for Marine Development 

 Service 1973). Migratory runs of fish 

 may be affected by changes in an inlet. 

 Construction activities should be care- 

 fully planned to avoid fish migration or 

 spawning runs (Persaud and Wilkins 

 1976). Dredging to bypass or remove 

 accumulated sand should also be sched- 

 uled for times of relatively low produc- 

 tivity (Thompson 1973). Care should be 

 taken in the choice of downdrift sand 

 release sites to avoid movement of sand 

 onto productive fish and shellfish areas 

 or rich plant communities (Cronin et al. 

 1969). If the accumulated sand is not 

 returned to the littoral drift, it 

 should be disposed of carefully. 



Construction Materials 



Jetties along the United States 

 coastlines are usually built of rubble 

 or quarried stone (U.S. Army Corps of 

 Engineers 1973b). Other materials oc- 

 casionally used, particularly in the 

 Great Lakes, are steel sheet pile cells, 

 cassions, and timber, steel, or concrete 

 cribs. Prefabricated concrete components 

 (Figure 14) are sometimes used on the 

 outer layer. 



Caps on rubble mound jetties are 

 often concrete embedded with large 

 stone. Thick bedding layers of gravel 

 or stone often extend out from the fac- 

 ing layer affording the toe of the 

 structure protection from scour. Medi- 

 um-sized stone is usually placed between 

 the large stone or shaped concrete fac- 

 ing. Most cribs, cassions, and sheet 

 pile cells are filled with dredged mate- 

 rial, gravel, or small-sized quarry 

 stone and capped with concrete or large- 

 sized cover stones lying on a bedding 

 layer. A stone mattress and riprap are 

 usually placed at the base of the verti- 

 cal walls. These toe structures have 



29 



