This option can be used in both steady flow and unsteady flow water surface profile calculations. The friction loss between adjacent cross sections is computed as the product of the representative rate of friction loss (friction slope) and the weighted-average reach length. The program allows the user to select from the following previously defined friction loss equations:
Any of the above friction loss equations will produce satisfactory estimates provided that reach lengths are not too long. The advantage sought in alternative friction loss formulations is to be able to maximize reach lengths without sacrificing profile accuracy.
(.Friction Loss Evaluation v6.0:2), the average conveyance equation, is the friction loss formulation that has been set as the default method within HEC-RAS. This equation is viewed as giving the best overall results for a range of profile types (M1, M2, etc). Research (Reed and Wolfkill, 1976) indicates that (.Friction Loss Evaluation v6.0:3) is the most suitable for M1 profiles. (Suitability as indicated by Reed and Wolfkill is the most accurate determination of a known profile with the least number of cross sections.) (.Friction Loss Evaluation v6.0:4) is the standard friction loss formulation used in the FHWA/USGS step-backwater program WSPRO (Sherman, 1990). (.Friction Loss Evaluation v6.0:5) has been shown by Reed and Wolfkill to be the most suitable for M2 profiles.
Another feature of this capability is to select the most appropriate of the preceding four equations on a cross section by cross section basis depending on flow conditions (e.g., M1, S1, etc.) within the reach. At present, however, the criteria for this automated method (shown in Table 4-1), does not select the best equation for friction loss analysis in reaches with significant lateral expansion, such as the reach below a contracted bridge opening.
The selection of friction loss equations is accomplished from the Options menu on the Steady Flow Analysis window.
Table 4-1 Criteria Utilized to Select Friction Equation
Profile Type | Is friction slope at current cross section greater than friction slope at preceding cross section? | Equation Used |
| Subcritical (M1, S1) | Yes | |
| Subcritical (M2) | No | |
| Supercritical (S2) | Yes | |
Supercritical (M3, S3) | No | |