15 



3.0 RESULTS 



Results from the monitoring surveys are summarized in this section. First, the 

 surface topography and texture of the cell was measured using side-scan sonar (Section 

 3.1) and bathymetry (Section 3.2). The video data were useful in corroborating the 

 surface sediment topography and recolonization, and are incorporated throughout the 

 presentation of results. Next, the information gathered that documented layer thicknesses 

 within the cell, including core (Section 3.3) and subbottom (Section 3.4) results, is 

 presented. 



3.1 Side-scan Sonar Results 



The side-scan sonar data over the cell area showed distinct acoustic regions of the 

 cell (Figure 3-1). The southern end of the cell consisted of a uniform low reflectance area 

 interpreted as a flat surface. Strong backscatter from the central and northern portions of 

 the cell indicated a harder bottom, and a rough, uneven topography. Throughout the 

 central portion of the cell, clamshell bucket markings were noted clearly, with dark 

 shadows indicating strong changes in topography that we termed artificial "sand waves" 

 for further discussion. Two of the three dredge cuts (NAE 1997), aligned east- west, 

 showed individual ridges caused by clamshell bucket passes. The third most northern cut 

 was partially obscured because of the sand that was disposed in the northern part of the cell 

 after dredging (Section 1.2). Spud marks from the dredge were noted along the northern 

 edge. 



Although the eastern edge of the pit was in shadow, there was a bright spot in the 

 lower southeast comer indicating a topographic feature, consistent with bathymetric results 

 (Section 3.2). This hard acoustic return (dark crescent) suggested that it was present above 

 the ambient sediments to rise above the edge shadow, and hard enough for a strong 

 acoustic return. Further interpretation of this feature is discussed below. 



The final distinctive feature of the side-scan sonar results was the scalloped edges of 

 the cell. The shape of the cell walls still appeared to reflect much of the original clamshell 

 markings from the excavation. Although sequential bathymetric surveys indicated some 

 changes in the cell wall orientation, the sidescan results indicated that the BBC was firm 

 enough to retain much of the original dredged topography. 



3.2 Bathymetric Results 



Weeks Marine provided two bathymetric datasets for evaluation of the survey data, 

 including the survey conducted after the excavation of the cell prior to any disposal 

 (predisposal survey; 29 June 1997), and the final postcap survey conducted at the end of 

 the project (25 July 1997). The surveys were conducted using different survey parameters: 

 the predisposal survey was conducted over a large area with east- west lanes, at 50 ft lane 



MONITORING RESULTS FROM THE FIRST BHNIP CONFINED AQUATIC DISPOSAL CELL 



