CSD Spillage Model for sand and rock

Spillage is a problem for many dredging projects that make use of a Cutter Suction Dredge (CSD). In addition to higher energy consumption for delivering the targeted depth, spillage can lead to a variety of environmental issues. Equally important is the operator's uncertainty as to how much spillage can be expected when drafting a tender. For this reason, this paper presents a tool to predict spillage rates when cutting sand or rock named the Sand-Rock Cutting Spillage Model. The tool focusses on centrifugal advection and rapid redeposition of sediment, the most significant spillage mechanisms. Centrifugal advection is defined as spillage due to the centrifugal forces acting on disintegrated soil inside the cutter as described in the Sand Cutting Spillage Model of Werkhoven et al. (2018). Rapid redeposition is induced by low mixture velocities that cause suspended particles to settle back onto the bank before reaching the suction mouth. This paper derives an improved mathematical foundation for the Sand Cutting Spillage Model and describes five model mechanisms that more accurately capture a number of observed dynamics. First, to more accurately predict cutter head flows, the internal and external cutter head densities are included. Second, the addition of axial flow in the cutter is investigated. Third, a formulation for the effects of rapid redeposition is incorporated. Fourth, an enhanced representation of the geometry of the cutter allows for a more accurate spillage prediction for the cutting of sand. The tool results compare well with measurements of Miltenburg (1983). For the cutting of rock, Den Burger (2003) observed that the downscaled rock material in his experiment easily settled at the cutter bottom. To this end, the fifth contribution of the Sand-Rock Cutting Spillage Model is the introduction of a concentration difference for rapid redeposition flow in comparison to other flow terms. The five mechanisms underlying the Sand-Rock Cutting Spillage Model bring the model in good agreement with experimental data for rock spillage rates by Den Burger (2003).