16 



3.0 DEEP WATER DISPOSAL AND 

 DEEP WATER CAPPING 



The results from mathematical 

 modeling predictions and from monitoring 

 actual capping operations in up to 60 m 

 water depth form the basis for evaluating 

 the feasibility of capping dredged material 

 in deeper water. Studies have shown that 

 the crucial steps in successful capping are 

 the initial formation of a distinct dredged 

 material mound and the subsequent 

 accurate placement of cap material. 

 Disposal operations conducted at depths 

 greater than 60 m (MBDS, Port Gardner, 

 WA, and Elliott Bay, WA) have formed 

 distinct mounds that were mapped by 

 postdisposal monitoring. Once the areal 

 extent of a mound is mapped, cap material 

 can then be placed over the mound 

 accurately. 



The transfer of capping technology to 

 areas where the water depth is greater than 

 60 m requires an understanding of how 

 dredged material acts as it travels through 

 the water column. Any change in the 

 behavior of the descending dredged 

 material as water depth increases will 

 indicate the need for a change in the 

 design of the capping operation. 

 Fortunately, empirical and theoretical 

 information on the fate of dredged material 

 disposed in deep water is available. 



Dredged materials go through three 

 phases of descent independent of the water 

 depth at the disposal site: convective 

 descent, dynamic collapse, and passive 

 dispersion (SAIC 1987; Figure 3-1). 

 During convective descent, the material is 

 transported to the bottom under the 



influence of gravity. Sediments dredged 

 with a clamshell dredge retain most of 

 their consolidated nature during descent. 

 At dynamic collapse, which occurs when 

 the dredged material reaches the bottom 

 (or a level of neutral buoyancy), the 

 vertical momentum is transferred to 

 horizontal spreading. The loss of 

 momentum from the disposal operation 

 initiates the passive dispersion phase where 

 ambient currents and turbulence determine 

 the transport and spread of material. 



As the water depth increases, the time 

 the material spends in the convective 

 descent phase in the water column 

 increases. A model of dredged material 

 disposal at the New London Disposal Site, 

 20 m depth, calculates the material 

 remaining in convective descent for 12 

 seconds. If the water depth is increased to 

 100 m, convective descent time increases 

 to 102 seconds. Even with dredged 

 material reaching bottom during the 

 convective descent phase for the deeper 

 water disposal sites, the time that the 

 material spends in the water column during 

 descent can affect disposal design and 

 operation. At the 90 m depths found at 

 MBDS, the material will take 90 seconds 

 to reach bottom based on a descent 

 velocity of 1 m/sec (Bokuniewicz et al. 

 1978). An increase in descent time can 

 increase water entrainment. Due to the 

 entrainment of water and the residual 

 dispersal of sediment washing out of the 

 disposal vessel, some dredged material 

 will remain in suspension in the water 

 column. Estimates of the amount of 

 dredged material remaining in suspension 

 range from 3 to 5% (dry mass basis based 

 on in situ observation or modeling; 



Deep Water Capping 



