One of the most important hydraulic parameters for sediment modeling is the hydraulic warm-up period. Unless the prototype starts dry and pushes a wetting front through the domain, it is essential to use a hydraulic warm up period for the a 2D sediment model. A hydraulic model can often pass through a wetting phase at the beginning of a simulation without affecting the final results (this is not best practice but can produce acceptable results in some cases). If the sediment model runs without a warmup period, it will scour as the wetting front initialized the model and bias the result.
HEC-RAS will only run hydraulics during the first half of the warmup period, and then will add sediment warm up during the second half. Choose a warmup period that wets the mesh fully during the first half. HEC-RAS does not populate a warmup period by default. Define this parameter by slecting *OptionsComputational Options and Tolerances…* and selecting the 2D Flow Options tab.
Define the Initial Contidions Time (hrs) and the Ramp Up Fraction (the portion of the Initial Conditions Time the model will take to gradually get up to the full warm up flows – often 0.1). Run the hydraulic model (without sediment) to make sure this time is sufficient to reach an initial steady-state flow condition.

Figure 2 1. HEC-RAS Unsteady Computation Options and Tolerances editor.