Bed Mixing Time Steps (SPI)

The mixing and armoring algorithms update several times per computational interval. HEC-RAS subdivides the computational increments into mixing time steps or Bed Exchange Iterations. Set the Bed Exchange Iterations (SPI factor in HEC-6) to control the mixing updates per computation increment. For example, if Bed Exchange Increments variable is 10 (default) HEC-RAS will update the mass and gradation of the bed layers every 10% of the computation increment, updating bed layer gradations and exchange increments ten time between backwater and bed change computations. Sorting and armoring iterations are important to track supply limitation in order to keep the model from over predicting erosion. However, they also affect run times. The recommended range for this parameter is 1 to 50. Start with a high Bed Exchange Iteration Per Time Step value and drop it as much as possible without changing the results. (see Sediment Computational Options and Tolerances)

Minimum Bed Change Before Updating Hydraulics

Similarly, hydraulic parameters are not automatically computed after each computational increment unless one of the cross sections has undergone appreciable change. This conserves computational resources since the frequency with which hydraulic computations are performed drives run times. This parameter is, by default, set equal to the Minimum Bed Change Before Updating Cross Section parameter so that every time the cross sections change, the hydraulics are recomputed but can be edited separately.

Minimum Bed Change Before Updating Cross Sections

HEC-RAS does not update the cross sections every computational increment. Instead it tracks erosion or deposition at a cross section until a minimum bed change is achieved. When this threshold is exceeded at one of the cross sections the bathymetry is re-computed. The default is 0.02 feet which will generally be exceeded in streams with relatively active beds. This tolerance can be increased to lower run times or decreased to make the model more sensitive to bed change.

Modeling Note: Min Bed Change and Stability

The hydraulic and bed change thresholds are legacy features. They do not often improve run times dramatically and can introduce instabilities, particularly in simplified channels. Test the sensitivity of these parameters. If they do not improve run times, consider setting them to 0.

Volume Change Method

HEC-RAS computes deposition or erosion in mass. Then it coverts mass to a uniform vertical bed change, applied to all wetted nodes in the movable bed limits. This computation requires iteration to converge on a precise solution, which is computationally expensive. Instead, HEC-RAS converts the computed bed change into mass change (by computing the area change between the original and updated cross sections), compares it to the computed mass change, and carries remainders over into the next time step. HEC-RAS selects this option by default. It is recommended unless results are being compared with HEC-6 which does not use this option.

Modeling Note: Volume Check and BSTEM

HEC-RAS will not apply the volume check for cross sections selected for USDA-ARS Bank Stability and Toe Erosion (BSTEM) modeling (even if it is checked), because volume change in that module is incompatible with this feature.