Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.
In HEC-RAS conveyance for a given cross section is divided into the main channel, left overbank and right overbank. The distribution of conveyance is controlled by geometric features of the cross section itself, such as the terrain, bank station locations, ineffective flow triggers, and Manning’s n values. The latter three are typically subjective in how they are defined at any given location. It is important for the modeler to define these parameters to accurately define the hydraulics in the cross section, but also to maintain numerical stability for unsteady flow modeling.
Copyright © RASModel.com. 2009. All rights reserved.
In HEC-RAS conveyance for a given cross section is divided into the main channel, left overbank and right overbank. The distribution of conveyance is controlled by geometric features of the cross section itself, such as the terrain, bank station locations, ineffective flow triggers, and Manning’s n values. The latter three are typically subjective in how they are defined at any given location. It is important for the modeler to define these parameters to accurately define the hydraulics in the cross section, but also to maintain numerical stability for unsteady flow modeling.
A convenient way to check for appropriately defined conveyance distribution is a quick scan of Standard Table 2, in the Profile Output Table. Standard Table 2 displays the flow in the left overbank, main channel and right overbank (Q Left, Q Channel, Q Right). The modeler should view this table and look for cross sections that show a sudden change in distribution. Any sudden change is a good indication of poorly placed bank stations or ineffective flow triggers.
The attached table illustrates how Standard Table 2 can be used to locate areas of sudden changes in flow or conveyance distribution.
