WMS:GSSHA Lakes and Wetlands

Defining Wetlands and Lakes

Wetlands can be created as feature polygons in a GSSHA coverage. Lake attributes can also be defined at feature polygons, but it is recommended to define lakes as point attributes in a GSSHA coverage. Lakes can be defined at a point by defining a detention basin hydraulic structure at a point attribute on a stream in a GSSHA coverage.

After creating arcs representing your lake boundary, follow these steps to finish setting up your lake:

1. Create an embankment arc downstream from your lake that represents the dam elevation for the lake. Make sure the type of arc is set to be an embankment arc and that the elevations of the arc are set in the embankment arc editor. When you create your embankment arc, the grid edges defined by your embankment arc must be downstream from the outlet point/detention basin hydraulic structure. If the edges are not downstream from the outlet (you can display the edges to make sure they are outside), you will get an error after you define your lake water surface elevations.
2. Select the arcs representing your lake boundary and build polygons using these selected arcs.
3. Go to the polygon/point hydraulic structure attributes and set your minimum water surface elevation, initial water surface elevation, and maximum water surface elevation in the GSSHA Polygon Attributes dialog.
4. Define any outlet structures, such as culverts, weirs, rating curves, scheduled discharges or rule curves, for the feature node immediately downstream from the lake.

(If defining a lake using a polygon) After outlining the area with feature arcs and converting those feature arcs to a polygon, double-click the polygon to bring up the feature attributes dialog. The polygon type then needs to be assigned to either lake or wetland.

Lakes in GSSHA

The important hydrologic feature of a lake is its ability to store water and route water flows. To accomplish this, an initial water surface elevation and a storage capacity curve must be generated for a lake. The storage capacity curve can be created either manually, through the XY Series Editor accessible by clicking on the XY series button, or by using the Detention Basin Calculator. In the Detention Basin Calculator the storage capacity curve can be defined as a set of Volume-Elevation curves or Area-Elevation curves. The Detention Basin Calculator can also approximate the storage capacity curve by assuming the lake has a pyramidal shape (with constant side slope). In the feature attributes dialog, the storage capacity curve can be generated by assuming the lake has a polygonal footprint with vertical sides and a flat bottom. One important note here is that WMS can compute a storage capacity curve from the bathymetry data for a lake or reservoir. To learn more about this option review the TIN-related tutorials in the WMS 8.1 tutorials.

For infiltration from the lake to the groundwater the same infiltration options as are set for the rest of the simulation will be applied. The water surface elevation of the lake will be used when calculating the hydraulic gradient for the infiltration routines.

Static Lakes

One option of note is the static lake option. Lakes can be specified as static features on the overland and channel. This can be computationally expedient because the calculations required for lakes taking and releasing overland cells and channel nodes is removed. The lake is assumed to occupy the maximum lake extent specified. To make a lake static, turn on the Static lake option in the GSSHA Hydraulic Structures dialog. The Min/Max/Init WSE must still be specified. All cells below the Max WSE become lake cells and any water that enters the cell through any mechanism will be added to the lake. If the lake area is actually highly variable, then the accuracy of this approximation may suffer.

Various GSSHA detention basin options

Lake Rating Curves

A lake rating curve option was also added. A stage/volume/area curve can be specified for a lake by toggling the Define rating curve option and defining the curve. The rating curve stage values must go up to at least the Max WSE.

Rating curves can be used with or without the static lake option turned on.

If the lake rating curve is specified, the lake stage/area/volume calculations will be performed using the rating curve. The lake will still interact with the stream and the overland based on the stage of the lake unless the Static lake option is toggled on.

Wetlands

Wetland areas store small amounts of water and act as important aquifer recharge areas. The areal parameters are assumed to be homogenous over the polygon. Currently, the wetlands areas will act as small lakes with slightly different infiltration parameters. A different hydraulic conductivity value can be specified for the sediments responsible for the wetland condition if a high water table is not the principle cause of the wetlands. Currently the wetlands and lakes do not have any special sediment trapping or contaminant removal parameters. They are simple treated as modifying the stream and overland flow processes. Lakes and wetlands may be in-stream or out-of-stream features.

Lakes and Wetlands in GSSHA

Lakes and wetlands must be used in conjunction with a rating curve (or other curve or hydraulic structure, if applicable) at the outlet to describe how they release the stored water. The rating curve is set up as a hydraulic structure at the outlet node of the lake or wetland.

The wetland areas are treated as simple flow storage areas with special sediment conditions. The wetland sediments are treated as a lens that must be saturated before it will release water into the groundwater domain. Thus five parameters must be specified for the wetlands area. These are the maximum storage depth, the retention depth (sediment lens thickness), the base elevation, the hydraulic conductivity of the sediment lens, and the hydraulic conductivity of the soil just below the sediment lens (retention depth).

Related Topics:

• GSSHA Feature Polygons