1. A method comprising: evaluating data representing observed sediment erosion and deposition behavior in a hydrocarbon reservoir; adjusting a parameter value in a set of equations so that the output of the set of equations accurately matches the observed sediment erosion and deposition behavior for sediment sizes throughout a range of about 10 microns to about 10 centimeters; defining a grain size distribution of an initial condition of a sediment bed in the hydrocarbon reservoir; applying the set of equations with the adjusted parameter value to the initial condition of the sediment bed, wherein outputs of the set of equations express how a fluid flow affects the erosion and deposition of sediments at the initial condition of the sediment bed; adjusting the grain size distribution of the initial condition of the sediment bed based on the outputs of the set of equations, to create a subsequent sediment bed condition having a grain size distribution associated therewith; re-applying the set of equations with the adjusted parameter value to the subsequent sediment bed condition a pre-determined number of times; re-adjusting the grain size distribution of the subsequent sediment bed condition after each re-application of the set of equations with the adjusted parameter value; creating a model of the hydrocarbon reservoir using the subsequent sediment bed condition; and outputting the model of the hydrocarbon reservoir.