Reach Average Depth Primer
RAS2FIM is based on determining a 'reach averaged' flooded depth along a one-dimensional river for a constant steady state flow rate. One dimensional hydraulic models are composed of a minimum of two cross sections where the distance between cross sections (reach lengths) are specified. RAS2FIM utilizes the computational engine in the HEC-RAS software developed by the U.S. Army Corp of Engineers. The HEC-RAS engine calculates water surface elevations for 1-D models from one cross section to the next by solving the energy balance equation with an iterative procedure called the standard step method.
For RAS2FIM a few considerations are made when computing water surface profiles in HEC-RAS:
- All flow is considered to be sub-critical.
- Reach boundary conditions are set at the downstream most cross section by assuming a normal depth with an energy slope of 0.5%.
- Flow is constant down the reach being analyzed.
