Most of the sediment transport equations were developed with sand and/or gravel data. Therefore, most silt and all clay particles are outside of the range of applicability of the sediment transport functions implemented in HEC-RAS. In most systems, these particles are wash load, material only found in the bed in trace amounts, because transport capacity always exceeds supply. Some modelers will just ignore fines as throughput load, arguing that if fines never interact with the bed in the model reach, the model is insensitive to them and they add unnecessary complexity and parameters to the model. However, sometimes fines must be modeled explicitly. In reservoirs and other backwater or low energy zones, silt and clay can deposit and clay lined channels, both natural and engineered, can erode, causing local and downstream problems.
Fine sediment transport is further complicated by electrostatic and electrochemical forces. These particles are not just outside of the empirical range of the equations, but they often erode and deposit by fundamentally different processes. These forces cause fine particles, particularly clay, and "stick" to the bed surface, so that fine erosion and deposition are often not primarily functions of sediment size. These processes make fine deposition and erosion fundamentally different than the cohesionless sand and gravel transport.
HEC-RAS considers the smallest five grain classes 'fine sediment.' HEC-RAS applies the cohesive method selected to these grain classes. In the default grain classes, these five grain classes are the clay and silt classes, and are all finer than 0.0625 mm. If the user edits these, the cohesive methods will still apply to the first five grain classes, regardless of their size. However, if more than 20% of the active layer is cohesive, then the model considers the sediment 'matrix supported,' assuming cohesive sediment is abundant enough to fill the voids and regulate the erosion rate of all particles.
HEC-RAS includes three cohesive methods: applying the standard transport equations, or two different implementations of the Krone and Partheniades approach.