Both HEC-RAS and HEC-2 utilize the Standard Step method for balancing the energy equation to compute a water surface for a cross section. A key element in the solution of the energy equation is the calculation of conveyance. The conveyance is used to determine friction losses between cross sections, the flow distribution at a cross section, and the velocity weighing coefficient alpha. The approach used in HEC-2 is to calculate conveyance between every coordinate point in the cross section overbanks (see figure below). The conveyance is then summed to get the total left overbank and right overbank values. HEC-2 does not subdivide the main channel for conveyance calculations. This method of computing overbank conveyance can lead to different amounts of total conveyance when additional points are added to the cross section, without actually changing the geometry. The HEC-RAS program supports this method for calculating conveyance, but the default method is to make conveyance calculations only at n-value break points (see figure below).
