Field study of the mechanics of the placement of dredged material at open-water disposal. Volume 1: Main text and appendices A-I.

A field study has been made of the mechanics of the placement of dredged material at five locations, an estuarine site on the Atlantic and one on the pacific coast, two sites in the Great Lakes, and one in the open ocean. The objective was to observe all of the processes by which dredged material is emplaced on the bottom at a disposal site. Instrument arrays were designed to define the transit of dredged material in time and space from the moment of its release until its final deposition. Close attention was given to accurate timing of events and to the placement of instruments close to the discharging vessel. Methods used included optical transmittance, acoustic pulse echo and water flow measurements with instrument arrays, and water sampling by continuous pumping. Additional observations were made to characterize the mechanical properties of the dredged material, its quantity, and the rate at which it is released into the receiving water. Placement proceeds by a three-step process at all localities: descent through the water column, impact with the bottom, and spread of a bottom surge generated by the impact. The field observations demonstrate the dependence of the placement process on water depth, the currents at the disposal site, and the properties of the dredged material. Accurate, controlled placement is possible under a wide range of disposal site conditions. Control of the placement can be attained through selection of dredged m ethod, design uf the containing vessel, and choice of disposal site characteristics. (A)