PART III 



COASTAL WATERS 



The coastal zone is one of the most productive and critical areas for 

 fish and wildlife. In Part III, impacts are divided into dredging site impacts 

 (effects of the removal operation) and disposal operation impacts (effects of 

 dredged material disposal). We believe some of the most severe long-term 

 impacts from dredging are caused by the physical changes in estuaries due to 

 "new-work" dredging. Maintenance dredging and dredged material disposal pose 

 less severe threats, except when the sediments contain high levels of contam- 

 inants. 



ASSESSMENT OF IMPACTS AT THE DREDGING SITE 

 Water Column Impacts 



Potential water column impacts at the dredging site include increased 

 turbidity, increased oxygen demand, and releases of contaminants and nutri- 

 ents - especially free sulfides, hydrogen sulfide, and ammonia. The release 

 of these constituents varies widely with different types of equipment, and 

 even within the same equipment type, depending on effectiveness of operation, 

 state of maintenance, and deployment. Turbidity associated with the dredging 

 operation is not usually as great as turbidity associated with the disposal 

 operation. Employment of known good dredging procedures by dredge operators 

 will greatly reduce dredge-induced turbidity (Huston and Huston 1976). The 

 greatest concern about the dredging operation is turbidity caused by hopper 

 dredge overflow and clam shell dredging. 



A more chronic type of turbidity is associated with the sediments from 

 the excavated channel. These sediments become available for resuspension by 

 wave action or currents, until they are finally transported by natural forces 

 from the area or become biologically fixed (Taylor and Saloman 1967). The new 

 channel becomes a trap which retains sediments that are frequently resuspended 

 by boat traffic or maintenance dredging. Thus, the net result of new channel 

 construction may be a general increase in turbidity (Taylor and Saloman 1967). 

 In contrast, maintenance dredging, although it may produce a temporary in- 

 crease in turbidity, may decrease long-term turbidity by deepening the channel 

 and thus decreasing the resuspension of sediments by boat traffic. 



Bottom Impacts 



The use of a section of water bottom for a navigational channel often 

 precludes its utilization for normal aquatic production. During the initial 

 channel construction and at each maintenance dredging, 75% or more of the ben- 

 thic organisms are removed from the site (U.S. Army Engineer District, San 

 Francisco 1975). Recolonization of a new channel is often rapid and original 

 biomass is sometimes reached in 2 weeks to 4 months (Chesapeake Biological 

 Laboratory 1970, Slotta et al. 1973, Taylor undated, and U.S. Army Engineer 

 District, San Francisco 1974). However, recolonization is usually by opportun- 

 istic species which are less valuable in the food chain. Original species 

 diversity is seldom achieved (U.S. Army Engineer District, San Francisco 1975; 

 Taylor undated). Although original biomass may often occur, Taylor and Saloman 



17 



