49_ 



The results of the December 1991 survey display the MQR mound with a diameter of 

 approximately 425 m and a minimum depth of 18.0 m (Figure 3-20). The addition of 

 65,000 m^ of UDM and CDM at the CDA 93 buoy has created a new apex at 

 4r08.627' N, 72° 53.864' W, approximately 100 m northeast of the 1991 mound apex. 

 Depth difference calculations between the July 1994 and December 1991 datasets show a 

 1.5 m increase in mound height south-southwest of the CDA 93 buoy position (Figure 3-21). 

 A total volume of 15,300 m^ of additional sediment was found within the detectable 

 footprint of the dredged material deposit. The remaining 49,700 m^ of material spread 

 down the flanks of the MQR mound, in layers too thin to be detected by standard 

 hydrographic techniques. The majority of the detectable dredged material accumulation was 

 concentrated over the northeast quadrant of the MQR mound with a smaller deposit visible 

 along the southern flank (Figure 3-22). 



3.2.2 Surface Sediment Characterization 



Acoustic sediment surface classification is based on the premise that bulk sediment 

 properties (i.e., bulk density, porosity, and grain size) affect the interaction between an 

 acoustic signal and the sediment column. Penetration of sound in sediment is both a 

 function of the system frequency and the impedance contrast between the water column and 

 the sediments. 



Acoustic impedance (vr), the product of the density and the velocity of sound in a 

 layer of sediment, is also affected by differences in porosity, surface roughness, and grain 

 size, among other factors (LeBlanc et al. 1992). Sound penetrates deeper in softer sediment 

 since the impedance of high-water content silts and clays is more like that of the water 

 colunm, resulting in an increase in the amount of acoustic signal lost in the sediment and a 

 decrease in the strength of the returning signal. A weaker signal return translates as a 

 "softer" surface sediment type. In contrast, a stronger signal return translates into a 

 "harder" sediment type. 



Using these principles, SAIC developed the Sediment Acoustic Characterization 

 System (SACS) to remotely characterize surface sediments and distinguish between dredged 

 material deposits and ambient bottom. This system was utilized over the southern half of 

 the July 1994 bathymetric survey area, and a plot of the acoustic signal remms was 

 generated (Figure 3-23). From SACS data, most of the surface sediments at CLIS can be 

 interpreted as "softer" less dense material (fine sand, silt, and clay). The plot also shows 

 remms of 96.0 dB to 104.0 dB, which suggests that patches of dense "harder" material exist 

 in the vicinity of the MQR, NHAV 83, STNH-S, and NHAV 74 mounds. The majority of 

 these increases in surface sediment density can be attributed to the consolidation and de- 

 watering of dredged material in these historic mounds. However, REMOTS® sediment 



Monitoring Cruise at the Central Long Island Sound Disposal Site, July 1994 



