increased volume of sand shoaled into the artificial channel is, in part, 

 beach material from adjacent shorelines, although to what extent shoaling 

 occurs is unknown and a subject for further research. The increase in channel 

 shoaling may be a nonlinear response to increasing channel depths. Figures 4- 

 87 and 4-88, for example, are plots of the cumulative dredging history of the 

 ebb-tidal delta portion of the channel which has been maintained to a designed 

 depth. The slope of the mass-dredging-versus-time curve is the average 

 shoaling rate in the channel during that period. When the design or natural 

 depth has been increased, the increase in shoaling has been significant. 

 Figure 4-87 shows that increasing the depth of Pensacola Inlet, Florida, from 

 9.75 meters (32 feet) to 11.25 meters (37 feet) has more than doubled the 

 shoaling rate in the channel. Similar effects are seen in Figure 4-88 for 

 East Pass, Florida. 



Channel dredging can have significant effects on adjacent shorelines, 

 although such effects are difficult to predict or assess. Many of the deeper 

 navigation channels in tidal inlets are dredged by hopper dredges which, due 

 to draft limitations, must dump the channel material offshore in depths of 

 water where the material, typically a large part beach sand, is removed from 

 the littoral system. Although the limiting water depth for material dumped 

 offshore of a beach to return to the beach is generally unknown, a few 

 monitored offshore dumping tests suggest that material dumped in water depths 

 greater than 5.5 meters (18 feet) will not return to the nearshore littoral 

 system. The paragraphs that follow describe three trials in which offshore 

 dumping of sand-sized material failed to provide beach material to the 

 nearshore beach system. 



Offshore dumping of sand with the intent to nourish the beach was first 

 attempted at Santa Barbara, California, in the fall of 1935. The Santa 

 Barbara harbor was dredged by hopper dredges; 154,000 cubic meters (202,000 

 cubic yards) of material was moved. Material was dumped in about 6.7 meters 

 (22 feet) of water approximately 1 mile east of the Santa Barbara harbor 

 breakwater and about 305 meters (1,000 feet) offshore. The sand formed a 

 mound about 670 meters (2,200 feet) long and 1.5 meters (5 feet) high. It was 

 expected that the waves would move the sand onshore and eastward. Surveys 

 made in 1946 showed that the mound at that time was at no point more than 0.3 

 meters below its 1937 depth and did not move appreciably (U.S. Army Corps of 

 Engineers, 1950a). 



Offshore dumping of sand in 5.5 to 6.0 meters (18 to 20 feet) of water 

 (MLW) was employed at Atlantic City, New Jersey, during the period April 1935 

 to September 1943. It was concluded (U.S. Army Corps of Engineers, 1950b) 

 that the material which amounted to 2,717,000 cubic meters (3,554,000 square 

 yards) of sand did not significantly provide nourishment for the beach. 



Offshore dumping of sand by hopper dredge was carried out at Long Branch, 

 New Jersey, in April 1948 and was monitored throughout 1948 and 1949 by the 

 Beach Erosion Board and the U.S. Army Engineer District, New York, at Long 

 Branch, New Jersey (Hall and Herron, 1950). The purpose of the monitoring 

 test was to determine the feasibility of restoring an eroding shore by 

 employing natural forces to move material, dumped in relatively deep water, 

 shoreward toward the beach. The material was dredged from New York Harbor 

 entrance channels (grain size dcQ = 0.34 millimeter) and was placed in a 

 ridge about 2.1 meters (7 feet) high, 1100 meters (3,700 feet) long, and 230 



4-180 



