CSD spillage is defined as “any soil that is dislodged above the lowest cutter tip trajectory of a single swing, but is not sucked into the suction pipe”. In addition to higher energy consumption and material wear for delivering the targeted depth, spillage can lead to a variety of environmental issues. As of yet, no analytical model exists in literature that can estimate spillage rates for a given set of cutting parameters. This thesis presents the Sand-Rock Cutting Spillage Model (SRCSM), an engineering model that is particle size-agnostic and makes use of cutting parameters that are all available to the dredge operator. Prediction accuracy of 5 percentage point is achieved with a two-disc potential flow model complemented with empirical closing relationships. A triad of forces governs flow in the cutter head for typical cutting conditions: a centrifugal, suction and gravitational force are considered. For the centrifugal pump effect, and centrifugal pump effect only, the flow inside the cutter is considered steady, non-gravitational, inviscid and non-axial. This allows for the derivation of a pressure-discharge affinity law from the Navier-Stokes. The axial pump effect is governed by the mixture velocity at the suction mouth. It is hypothesized that the pressure difference over the discs drives an inflow at the disc closest to the nose. Centrifugal advection and rapid redeposition spillage are considered the two most significant spillage types out of the six classified. Centrifugal advection can be determined by identifying the onset of radial outflow at the disc near the cutter ring. The magnitude of rapid redeposition flow and its concentration depend on mixing effects that are proportional to the ratio particle settling velocity and the mixture velocity squared. The model is calibrated with three coefficients. User input parameters are the cutter geometry, cut-type factor fd,type (-1 for under-cut), bank slope angle ξ, cutter inclination angle λ, bank height h, step size lstep, rotational velocity ω, settling velocity vts, swing velocity vs, mixture velocity vm and material densities. Spillage=f(Dring,Dnose,Dpipe,b,fd,type,ξ,γ,h,lstep,ω,vts,vs,vm,ρq,ρb,ρw ) For calibration, an inverse flow number θ-1 is used that is proportional to the ratio of centrifugal flow over mixture flow. Spillage rates from SRCSM are in high agreement with reference data for sand (Miltenburg, 1983) and rock (Den Burger, 2003) in an under-cut swing. A sensitivity analysis suggests that most cutter head dynamics are adequately incorporated. The model is less reliable for (non-typical) inverse flow numbers of  θ-1= 6 [-] and higher due to a mixture velocity that drops below zero. In addition, the model is calibrated for a relatively high cutter inclination angle of 45 [deg] and bank angle of 45 [deg]. Caution should be observed with the results. It is also suggested that mixing effects related to the swing velocity are incorporated more explicitly in the model. For typical sand cutting conditions, the highest spillage reduction (-4.6%) is achieved by a 1 [%] smaller step size. For rock, the highest spillage reduction (-0.63%) is achieved for a 1 [%] decrease in swing velocity. Spillage appears to follow the theorem of Ellington (1934): it don’t mean a thing if it ain’t got that swing.

Great Lakes Dredge & Dock (GLDD) is one of the biggest dredging companies in the USA which provides engineering services in dredging and land reclamation. GLDD came up with an idea to develop a working model that can estimate spillage when using a cutter suction dredger (CSD) for cutting soft material like sand. The main reason for this is because not every material that is cut from the seabed contributes to production. A part of the material is lost since it doesn’t enter the suction mouth and contributes to spillage. Spillage can be defined in a number of ways. GLDD defines spillage as “the difference in elevation between the maximum depth at which the dredge cuts (cutting depth) and the depth obtained after dredging (after dredging depth)”. Theoretically, spillage is defined as “the material that is displaced from the seabed which does not enter the suction pipe and thus does not contribute to production”. The aim of this thesis is to identify and describe the types of spillage that occur while dredging soft material like sand and to use this information to create a numerical model that estimates spillage. Four types of spillage are found, namely; soil that disintegrates and is directed away from the cutter head (Type-1), soil that enters the cutter head but not the suction mouth (Type-2), soil that does not enter the cutter head when cutting large bank heights (Type-3) and breaching of the bank after the cutter head has passed (Type-4). The numerical model focuses only on Type-2 spillage. The foundation of the model is based on the theory that the cutter head of a CSD behaves like a combination of an axial and centrifugal pump with an inflow present near the nose and an outflow present near the cutter ring. For simplicity, the cutter head is divided into two slices with the top slice (2) having an inflow and the bottom slice (1) having an outflow. It is assumed that the outflow from the bottom slice circulates back to the top slice and re-enters the cutter head. Equations for pressure and flow are derived from the pump affinity laws for each slice. A constant, β, is a parameter that influences the flow and an initial value is assumed which is later calibrated. Using the concept of the continuity equation, an implicit equation for the height of the bottom slice (w1) is found. In the model, spillage percentage is defined as the ratio of the circulating flow to the total outgoing flow. Real life data from The Texas (GLDD) is used in the model. The constant β is calibrated using spillage values obtained from hydrographic surveys. Sensitivity analyses are performed on the various operational parameters of the dredge as well as on β. To study the influence of β on the relationship between the operational parameters and spillage, a number of β-values are used to plot data. For the validation of the model, the spillage value obtained from the model is compared with the spillage values obtained from the hydrographic surveys. As the spillage is in close proximation with values observed in the industry, the model is said to be working as intended. In order to further validate the model, the results obtained from the model are compared with the results obtained from experiments performed by Marco den Burger and the results are almost similar. Thus, it can be concluded that although the model is simple, it is able to provide realistic values of spillage.