WMS:NSS Computing Peak Discharges

National Flood Frequency Regression Method dialog

Setting up the analysis consists of the following steps, all done within the National Streamline Statistics Method dialog. The dialog can be used to do the follow:

1. Delineate the watershed and compute the basin area and other geometric parameters. Enter the value manually if not having delineated from a DEM, drainage coverage, or TIN.
2. Select the State the watershed is in and assign the Maximum Flood Region.
3. Select the region equation that the watershed is in and while highlighted from the Available Equations window choose the Select button so that it will be listed in the Selected Equations window. If the watershed overlaps more than one region then select the other regions as well.
4. Variable values derived from a DEM or Drainage coverage will automatically be placed in the appropriate edit fields. Other variables not computed will need to be defined.
5. Select the Compute Results button to estimate the peak flows for the 2, 5, 10, 25, 50, 100 and 500-year recurrence intervals.

A hydrograph can be estimated by selecting one of the peak flow rows from the Results table and choosing the Compute Hydrograph button.

National Streamline Statistics Method Dialog Overview

The dialog is divided into four sections: Basin Information, Regional regression equations, Variable Values, and Results.

Basin Information

• Basin Name

Displays the basin ID by default. A different name can be manually entered if desired.

• Total Basin Area

  The total basin area includes the area of all regions overlapped by the watershed. If a terrain model is used to compute areas, this value will be defaulted to the area computed for the selected basin.

• State

  The NSS regression equations are separated by state. The State drop down menu is used to specify the state that the watershed is located in. Once the state is specified, the available regions will appear in the regional equations text window. Besides the 50 US states, an equation for Puerto Rico and a custom defined equation can be selected from the available choices.

• Max Flood Region
• Units

Units can be displayed in either "English" or "Metric".

Regional Regression Equations

Each state is divided up into hydrologically similar regions with separate regression equations. Once the state is selected, the available regions appear in the text window below the name.

• Available Equations
• Selected Equations
• Select→
• ←Remove
• Compute Overlapping Areas

Variable Value

• Restore Computed Geometric Values

Results

• Weighting Options
• Compute Results

Estimate the peak flows for the 2, 5, 10, 25, 50, 100 and 500-year recurrence intervals.

• Max Flood Envelope
• Compute Hydrograph
• Export

Related Topics

• Overview of NSS
• Hydrographs
• GIS Overlay