WMS:Delineate Floodplain

There are several options that must be defined in the Floodplain Delineation dialog shown in order to delineate a flood plain in WMS. Each of these options are explained and described in detail below.

Select a TIN

Most of the time you will be working with a single TIN and therefore this option will be dimmed out. However, it is possible to have more than one TIN in WMS and in such cases you will need to specify the TIN on which the flood plain delineation will be performed.

Select the Stage Scatter and Data Set

Water surface elevation data used to derive the floodplain are imported and processed in WMS as scatter data sets. In addition to the scatter set (the xy locations of water surface elevation points), a particular data set must also be specified. It is possible to have a single scatter set (locations of water elevations) with multiple data sets representing the water levels themselves. This will often occur when multiple scenarios of a hydraulic model are run. See the information on preparing stage files for more information about creating scatter data sets for floodplain delineation.

Select a Flood Barrier Coverage

A flood barrier coverage allows you to incorporate flood barriers representing natural or artificial barriers that are not represented explicitly by elevations in a TIN. The floodplain delineation process considers these barriers during water level interpolation. The resulting flood depth become closer to reality rather than a mere interpolation. For example, an embankment or a road is not always represented in a TIN.

In order to delineate the floodplain properly, these barriers must be considered in a hydraulic model as well as in floodplain delineation process. This option not only incorporates such existing barriers in the process, it also provides the flexibility to professionals so that they can consider proposed structures and evaluate the “what if” scenarios.

Maximum Search Radius

Water surface elevations are determined for each TIN vertex by using interpolation from a set of "nearest" water surface elevations in the stage scattered data set. The maximum search radius defines the limiting distance that will be used when collecting the nearest stage scatter points. If the Use Flow Paths option is turned on then the "radius" distance is the flow distance path, whereas if the option is turned off it is the straight line (as the crow flies) distance.

Using Flow Paths

The flow path option in the floodplain delineation process ensures that the sources of water, i.e. the water levels, and the areas flooded (TIN vertices for which water surface elevation is interpolated) are hydraulically connected. This is an important option because if not applied, the process may interpolate water levels while ignoring obstructions between the water levels and the point of interpolation.

Quadrants

The quadrant option ensures the water levels are selected for interpolation from different directions instead of being biased by a particular direction. When using this option water elevations used for interpolation are selected equally from the four primary quadrants surround the point of interpolation. If no water levels, or an insufficient number, are found in a quadrant the process proceeds using that many fewer water levels for interpolation.

Number of Stages

A number of stages used for interpolation must be defined. If the quadrant option is turned on then you will need to specify the number of closest stage points to find within each quadrant. Without the quadrant option then the total number of nearest stages are specified. It is not required that the number specified be found, interpolation will proceed as long as one possible water elevation scatter point is found to meet the specified criteria. For example if 2 stages from each quadrant is specified, and one quadrant has zero possible choices and another quadrant only 1, then 5 points will be used in interpolation.

Resulting Data Sets

The floodplain delineation tool generates two different types of data sets that can be used for contouring and further analysis with the TIN. First of all a data set of water surface elevations at each TIN vertex contained in the flood plain is calculated and stored. Secondly TIN elevations are subtracted from the water surface elevations to create a flood depth data set. Both, or either of the data sets can be specified for calculation. The data set(s) are TIN data sets and are managed by selecting the active data set for the TIN.

Related Topics

• Overview of Flood Plain Delineation
• Differences From Earlier Versions
• Preparing Stage Data
• Stochastic Modeling