23 



4.0 DISCUSSION 



The absence of any large areas of gain or loss at CSDS between May and August 

 1992 and the transformation from sand over mud to all sand at seven of the stations surveyed 

 by REMOTS® support the active bed transport processes deduced from the May 1992 survey 

 at CSDS (Wiley 1994). Although there is a predominant east- west transport direction at the 

 site, the dispersion rate for fine-grained materials was suspected to be markedly retarded by 

 sand armor that migrates over the more cohesive, less erosive fine-grained dredged material. 



Comparing the May to August bathymetric surveys showed a clustering of positive 

 depth differences in the southern portion of the survey area. This coincides with the known 

 location of a sand wave field. Migration of sand waves (part of the east- west transport 

 supported by previous studies [Wiley 1994]) could have caused the accumulations noted 

 between May and August 1992. The north-south alignment of the areas of accumulation are 

 also parallel to the wave crests. 



At the North Cove buoy location, up to 50 cm of dredged material had accumulated 

 between December 1991 and May 1992 (Wiley 1994). The two areas of 20 cm loss in 

 August were to the east and west of this area of accumulation. Based on the bathymetric 

 measurements, there was no change in the water depth at the center of the disposal mound 

 between May and August, again supporting the relative stability of the fine-grained deposit. 



In May, the REMOTS® photographs on the mound showed fine-grained material at 

 the center surrounded by sand over mud on the flanks. By August fine-grained dredged 

 material was visible only at the two stations closest to the center of the mound. Sand over 

 mud was found at two stations on the flanks and at two stations almost 300 m to the 

 southwest. Where there was dredged material or sand over mud in May but sand in August, 

 it is likely that the fine-grained material remains buried under an accumulation of sand. 

 When the photographs showed sand over mud in May, the sand may have been acting as a 

 barrier to the erosion of the mud. How thick the layer of sand could be over the mud is 

 unknown. 



The absence of any detectable accumulations where the sand had moved over the mud 

 may be a result of compaction of the mud under the sand. The change at F9 from sand in 

 May to sand over mud in August 1992 may have been due to a loss of sand, allowing the 

 REMOTS® camera to penetrate through to the dredged material. The new stations surveyed 

 in August detected mud under the sand at stations D6 and D7. This extended the previous 

 estimation of the location of the fine-grained dredged material boundary to the south. 



Because sand deposits limit REMOTS® camera penetration, the absolute boundary of 

 the fine-grained dredged material deposit remained unknown after the sediment profile 



Monitoring Cruise at the Cornfield Shoals Disposal Site, August 1992 



