10 



movement and overflow (Collins, 1995). This may partly explain the plume traces on the 

 November 2 ebb survey which are small and patchy; the bucket cycle was aperiodic and 

 intermittent throughout the surveying period. 



The acoustic survey data also reveal that at the source of dredge operations the 

 horizontal distribution of dredged sediments at the surface is extensive but tends to decrease 

 with depth, i.e., conical in shape. This counters one of the more basic assumptions in most 

 models of dredged sediment plumes, that of a homogenous vertical line source (e.g., . 

 Christodoulou 1974, Schubel 1978, Wechsler and Cogley 1977, Collins 1995). Some of 

 these models also assume a widening, or increasing horizontal distribution away from the 

 source. The acoustic data exhibit progressively patchy and asymmetric distributions away 

 from the dredging source. Additionally, in nearly all cases the traces showed that the 

 shoreward, or shallow water, boundary of the plume in the areas immediately downstream 

 are more well defined than the deep water margin within the navigational channel. This 

 characteristic appears to be the result of gravitational flows driven by the density contrast 

 between the sediment laden plume and ambient waters. This contrast decreases at the deep 

 water plume edge; the combined effects of particulate settling, advection, and mixing cause 

 the boundary to become more diffuse and difficult to define. Such spatial variability in the 

 distribution of the plume sediment would seem to indicate that, unless models are able to 

 reproduce these features, it can be expected that modeling results are overestimating the mass 

 of sediment downstream and in areas receiving settling material. 



The DAISY data also indicate several instances when suspended material resulting 

 from proximate dredging activities intruded out of the channel and into the shoal areas, as 

 evidenced by suspended material concentrations much larger than background. At the 

 beginning of the deployment period these events were quite well defined and short lived. 

 Resulting maxima seldom exceeded 1(X) mg«l'. These events are similar to those previously 

 observed during monitoring of dredge disposal operations within Long Island Sound (Bohlen 

 et al. 1992) and are representative of the passage of relatively small portions of the 

 suspended material plume. A majority of the spikes are occurring at times of maximum 

 current, suggesting the material is advected from afar. If the site was a sink for the dredged 

 sediment, spikes would occur at slack when material settled, and in the early stages of the 

 tide, as the freshly deposited sediment would be easily resuspended. 



During the latter part of the deployment, the data show several major perturbations in 

 the suspended material concentrations persisting over nearly two days during which maxima 

 approached 8(X) mg-1 '^ Since for a significant portion of this time the dredge was operating 

 well to the south of the DAISY site, the dredging operations cannot be identified as the 

 principle source of these perturbations. Most of these larger events are clearly correlated 

 with stream flow, wind stresses, and discharges from municipal outfalls and CSOs. 

 However, the causes of the remaining events are not so obvious. There appears to be no 



Dredged Sediment Dispersion in New Haven Harbor 



