Participants

• Eric Adams, Ph.D., MIT

• Gordon Ruggaber, Ph. D., MIT

Objectives

• To improve experimental data in support of mathematical models that predict sediment plume trajectories, growth rates, velocities, and composition.

Approach

To date, few laboratory or field studies have been performed to quantify the trajectories, growth rates (due to entrainment of ambient water), velocities, and internal particle compositions of sediment plumes and clouds - requirements for the successful placement of both dredged and capping materials. Cloud momentum at the point of bottom impact determines the degree of lateral spreading which occurs during sediment placement and the amount of mixing that takes place during subsequent placement of cap materials. Together, these factors determine the final configuration of dredged and capping materials on the ocean floor as well as the amount of fine materials (i.e., silt and clay particles) lost to the water column.

Flow visualization experiments were conducted using a glass-walled tank, special sediment release and capture (i.e., "trap") mechanisms, and various cohesive and non-cohesive particles. Particle sizes were scaled to real-world dimensions through the cloud number (N_c), defined as the ratio of particle settling velocity to the characteristic cloud velocity. An "inverse" integral model was developed in which the conservation equations were solved for α and κ using measured velocity and radius data. Based on the "inverse" model results, particle cloud experiments were simulated with an integral model using constant and time-varying α and κ.

Status Report

The non-cohesive sediments evolved rapidly into "thermals" with asymptotic deceleration and large growth rates (α = 0.2 -0.3). The particles eventually organized into "circulating thermals," with linear growth rates obeying buoyant vortex ring theory. In this phase, large particles (N_c > 10^-4) produced laminar-like vortex rings with small α (0.1 - 0.2). Compared to the cohesive sediments, which exhibited a wide range of growth ratesm changes in water content and initial momentum of the non-cohesive particles produced 10 - 20 % variations in α.

Material not incorporated into the cloud upon release formed a narrow "stem" behind the cloud, which contained as much as 30 % of the original mass depending on the release conditions. Much of the "stem" material either re-entrained into the cloud later in descent or reached the bottom shortly after it. Material not incorporated into the "stem," which may be advected by ambient currents, was found to be only a small fraction (<1 %) of the original mass.

Inverse integral model results suggest that C_D and κ are close to zero within the "thermal" phase. In the "circulating thermal" phase, the reduction in α caused by large particles (N_c 10^-4) increased κ to a value similiar to that of a solid sphere. Integral model results confirm the suitability of using constant coefficients for modeling particle clouds with N_c less than 10^-4. When N_c is greater than 10^-4, time-varying α and κ are required to properly simulate cloud behavior in the "circulating thermal" phase.

Participant Information

• Eric Adams is a Senior Research Engineer with the MIT Department of Civil and Environmental Engineering with interests in environmental fluid mechanics and water quality modeling.

• Gordon Ruggaber completed his Ph.D in Environmental Engineering in MIT's Department of Civil and Environmental Engineering in June 2000.