Layout for a RAFTS storage routing model
A catchment is divided into sub-catchments that depend on junctions of streams and internal ridge lines. There is considerable latitude in setting out RAFTS models. Most users will locate nodes at the center of sub-catchments, even though they perform routing for the whole sub-catchment down to its outlet.
Rainfall is converted to rainfall excess (rainfall - losses) at the sub-catchment nodes and catchment routing occurs in these nodes as well. If desired, translation of flows or routing based on kinematic wave hydraulic principles can be carried out in the stream routing reaches between nodes.
To apply these procedures in DRAINS, you need to select a RAFTS storage routing model in the Hydrological Model dialog box. In the Main Window you should set out a system with nodes at each sub-catchment outlet, at the junction of streams and at the outlet. Draw appropriate overflow routes between the nodes, and attach sub-catchments to the nodes where these are located. The overflow routes in this type of model use the same property sheet as those occurring in a pipe system, but are meant to represent the main channels running through a catchment. They can do this as either a simple link with a translation time delay or lag, or as a channel section in which kinematic wave routing takes place.
Provide information for sub-catchments by opening their pop-up (right mouse button) menu, and selecting edit data to open the following property sheet:
The catchment name, area, percentage impervious, sub-catchment slope and Manning's n need to be entered. The sub-catchment slope is the slope of the catchment along the main drainage line plus any side lines extending to the top of the sub-catchment. The Manning's n represents the average roughness of the total catchment surface, including pervious and impervious fractions, rather than that of a stream channel. Manning's roughness values for impervious and pervious areas are typically 0.015 and 0.040, respectively. A value of 0.025 is typical for mixed catchments. A sub-catchment along a heavily overgrown creek may have a higher value, from 0.05 to 0.1. Urbanised surfaces will have lower values, say 0.015 or 0.020.
If the RAFTS hydrological model required has different losses, a different routing parameter, BX, or storage exponent, n, to the default model specified in the Storage Routing Hydrological Model property sheet, it can be selected from the Hydrological Model buttons. Different RAFTS models can be set up in the Hydrological Model Data Base and applied to different sub-catchments throughout a system.
Within a RAFTS-type model, it is only necessary to specify a name for nodes. However, if this model is intended to interface with a Horton (ILSAX) model, DRAINS IL-CL (Time-Area-Routing) or open channel links (prismatic, irregular or multi-channel), a surface level must be provided at the connecting node.
As noted above, the links used are the same as those used as overflow routes in pipe systems, and share the same property sheets. The simplest option is to translate hydrographs along links without changing their shape, which requires that, along with the reach name, a time be specified on the first page of the overflow route. This can be zero.
The other option is to perform routing through the stream link using a kinematic wave procedure that is a simplification of the full hydrodynamic calculations applied to pipes and channels in the unsteady flow hydraulic calculations. This requires the reach length:
In both cases a cross-section must be chosen from the Overflow Route data base on the second page:
The safe flow criteria developed for overflow routes can also be applied here, though they are not considered in design.
The RAFTS model can be linked to drainage system components including pipes, open channels and sub-catchments employing the Horton (ILSAX) hydrological model. It cannot be intermixed with other types of storage routing model.
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