Cohesive Sediment Algorithms in ROMS and Sediment Test Cases

Chris Sherwood (1), Larry Sanford (2), Alan Blumberg (3), Courtney Harris (4), John Warner (1), and Rich Signell (1)

(1) U. S. Geological Survey Woods Hole Science Center
(2) University of Maryland, Center for Environmental Science
(3) Stevens Institute of Technology, Dept. of Civil, Environmental, and Ocean Engineering
(4) Virginia Institute of Marine Science


We have added cohesive-sediment algorithms to the ROMS sediment-transport model that allow ROMS to simulate erosion, deposition, and biodiffusive mixing of sand, muds, or mixed sediments. The new code implements algorithms developed by Sanford and Blumberg (Modeling resuspension and deposition with a dynamically varying mixed sediment bed 18th Biennial Conference of the Estuarine Research Federation, 2005). The model assumes that cohesive sediments have a critical stress for erosion that varies (typically increasing) with depth in sediments, determined by various physical and biological properties or processes. This shape of the profile is most readily determined from field or laboratory measurements using instruments like the Gust chamber or Sea Carousel. Erosion and deposition alter this profile, but it is reestablished over a characteristic time scale. The critical stress profile limits the amount of sediment that can be eroded during events with time scales much shorter than the profile restoration time scale. This behavior contrasts with that of non-cohesive material (e.g., sand or silt with less than about 5-10% mud), for which the critical shear stress depends on the grain size and density, rather than depth in the bed. Mixed sediments exhibit cohesive behavior when the mud content exceeds a threshold (15-30%), and a gradual mix of erosion properties is implemented between the limiting thresholds. Biodiffusion is implemented and can redistribute cohesive and non-cohesive sediments within the bed. A few conceptual test cases demonstrate the use and implications of these algorithms in ROMS.