119 



5.0 DISCUSSION 



5.1 Assessment of the Footprint of the Capped Disposal Mound 



The demonstrated ability to form a discrete deposit of dredged material on the 

 complex topography of the seafloor at the PDS was a key element to the success of the 

 Royal River Project. Prior to the project, the DAMOS Capping Model was used to predict 

 the size of the capped dredged material mound. The Capping Model is a tool to 

 approximate the size of a mound formed from point dumping of material in a specified 

 water depth. This model has proven useful in managing the deposition of dredged material 

 at other disposal sites with subtle relief. This model does not include a correction for 

 bottom topography. Therefore, the measured footprint of both the pseudo-UDM and CDM 

 deposits on the floor of PDS was evaluated relative to the shape of the mound as predicted 

 by the DAMOS Capping Model. 



Based on the amount of dredged material disposed at the PDA buoy during the 

 Royal River Capping Experiment (39,500 m^ pseudo-UDM and 22,200 m^ CDM), the 

 DAMOS Capping Model predicted the formation of a conical pseudo-UDM deposit 

 approximately 1.2 m high, with flanks extending up to 250 m from the central point of 

 disposal, and a 20 cm thick cap. Although the capping volume was less than what is 

 generally required in an acmal capping project (>50 cm), the model was still considered 

 useful despite the small volumes used. These results were assessed in light of the 

 measured deposit of pseudo-UDM (Section 5.1.1) and cap (Section 5.1.2) material. 



5.1.1 Pseudo-Unacceptably Contaminated Dredged Material (Pseudo-UDM) 



Typically in a capping project, sequential bathymetric surveys are used to determine 

 the overall shape and height of a disposal mound > 20 cm thick (the resolution of the 

 bathymetric method), and sediment-profile photographs are then used to map the apron of 

 material of < 20 cm. In the Royal River Project, bathymetry detected accumulation of 

 dredged material in close proximity to the PDA buoy position following disposal of the 

 pseudo-UDM. The bathymetric footprint consisted of two lobes, with an overall width of 

 approximately 300 m (Figure 4-12). The footprint of the mound was concentrated within 

 the namrally occurring basin feamre detected in the southern quadrant of PDS. 



Due to the complex bottom topography at PDS, however, there was a degree of 

 uncertainty associated with the bathymetric results due to survey artifacts from replicate 

 surveys over strong topographic feamres. The result is that the thickness of dredged 

 material may have been overestimated in areas, especially along the eastern lobe which was 

 located over a pre-existing topographic high (Figure 4-13). Because of the uncertainty of 

 the bathymetric footprint, and the overall low height of the pseudo-UDM deposit, the 



The Portland Disposal Site Capping Demonstration Project, 1995-1997 



