Model Construction:Overland flow routing options
GSSHA offers several optional processes that can be selected along with the basic overland flow routing described in Chapter 3. These optional processes are selected in the GSSHA Job Control Parameters dialog box under Surface Flow. Toggle on the desired options and then supply the needed inputs as described below. The optional processes are:
- Interception.
- Initial Depth.
- Area Retention.
- Retention Depth.
Interception
Interception is defined in GSSHA using two distributed parameters: an interception coefficient (b) and a storage capacity (a). The interception rate (i) is expressed as:
i(t) = r(t) while I <= a
i(t) = b * r(t) while I > a
where:
| r(t) | denotes rainfall intensity at time t |
| a | is the storage capacity, mm |
| b | is the interception coefficient, unitless fraction between 0.0 and 1.0 |
| I | is the cumulative interception depth |
The interception storage capacity, a, must be specified in units of meters, and the interception coefficient, b, must be a unitless fraction between 0.0 and 1.0. For a table of storage capacity and interception coefficient values, see Gray (1970), section 4.6, or Bras (1990), p 233.
These two parameters may vary in space, depending on vegetation and land use distributions. The storage capacity parameter, as well as interception coefficient parameter, is usually created from an index map of the vegetation cover. Typically, interception is not simulated in GSSHA; the effects of interception are included in the surface retention and infiltration parameters.
Initial depth
This value represents the beginning overland depth value in the grid cells. This feature is rarely used, since the overland flow plane is usually dry prior to the first storm event. Initial depths are specified in units of meters.
Area reduction
This dimensionless fraction is used to reduce the area over which there is infiltration opportunity after the end of rainfall because of flow concentration in micro-topography.
Retention
This feature allows the simulation of storage as a result of micro-topography. If the depth of water in a grid cell is less than the retention depth, no overland flow is routed. This feature has the effect of increasing the amount of infiltration. The retention depth is specified with the Mapping Table tied to index maps of land use or vegetation. Retention depth is specified in units of millimeters.
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