Routing:Lake and Channel Routing
A strength of the GSSHA model is the capability to couple surface runoff with channel hydraulics. A new feature added in GSSHA 2.0 is the ability to simulate simple lake storage and routing. Channel flow is simulated as a 1-D finite volume system of links and nodes. Lakes act as storage mechanisms that are able to grow and shrink, changing the water surface elevation in response to inflows and outflows. Both the channel networks and lakes are defined in WMS using feature arcs to create the channel links or outline the lake area. WMS automatically maps the vector streams to the grid cells in order to prepare the input necessary for GSSHA to simulate the interaction between overland and channel flow. Each segment of the channel network must be assigned cross-sectional parameters, and hydraulic structures, lakes, and detention basins may be placed at any point in the network.
WMS has several tools for creating, assigning attributes, and numbering the lakes, links, and nodes. How these tools can be used to easily create the necessary data for channel routing and lake storage is the focus of this chapter. The next section discusses the “nuts and bolts” of the input parameters required by GSSHA. This is followed by a detailed discussion on how to build the proper channel files using WMS.
Besides the identification of grid cells containing the stream channels and the definition of their cross-sectional parameters, several physical constants and simulation parameters for the channel routing and lake storage algorithms must be defined. All of these parameters are defined in the GSSHA Job Control dialog of WMS as discussed in Chapter 3.
Channel routing is defined by either selecting the Diffusive Wave or MESH routing method. If no channel routing is defined, only surface runoff calculations are performed. If the Diffusive Wave method is defined, then the following parameters must also be set:
- Node length - the distance between computational stream channel nodes. This value must be uniform throughout the channel network. The actual node lengths are set by redistributing the verticies along all of the stream arcs and may vary slightly from arc to arc. The minimum permissible node length will vary depending on the velocity of the water in the channels, but a good rule of thumb would be to make them no shorter than 1/2 the cell size.
- Routing time-step - the computational time-step in seconds. At present this value must be equal to the time-step used for overland flow routing.