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

For comparison and evaluation, the NSS program compares each extrapolated 500-year flood peak discharge with the maximum flood-envelope curves given by Crippen and Bue (1977) and Crippen (1982). The map below shows the different flood regions, and should be used to determine the appropriate region for the watershed. A default value has been determined for each region, but this value may be overwritten.

• Units

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

Map showing maximum flood regions in the United States

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 this section of the dialog.

• Available Equations

Lists all available regional regression equations for the selected State.

• Selected Equations

Lists all regional regression equations that have been selected.

• Select→

Moves selected equations from the Available Equations list to the Selected Equations list.

• ←Remove

Deletes a selected equation from the Selected Equations list.

• Compute Overlapping Areas

Opens a dialog that shows overlapping regions in the Selected Equations section. See NSS Overlapping Regions for more information.

Variable Values

As regions are selected in the Regional regression equations section, the relevant parameters for the regional equations are added to the Variable values spreadsheet. Parameters that can be computed by WMS are defaulted to their computed values at the time the regression equation is selected for use. If desired, override defaulted values and later return to the computed values by clicking on the Restore Computed Geometric Values button.

Parameters not computed by WMS when computing basin data can sometimes be computed using the generic GIS Overlay calculator. This option (under the Calculators menu) can be used for example to determine the percentage of lake or forest cover from the drainage coverage and a land use layer.

The NSS program includes in its database the range of parameter values for the watersheds used in developing the regression equations. The applicable range, along with the expected units are displayed on the same row as the parameter edit field.

The following values are typically displayed in this field.

Variable Name Abbreviation Value

Units Minimum Maximum

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

A hydrograph can be estimated using the dimensionless hydrograph derived by the NSS program once peak flows have been computed. When computing a hydrograph a lag time must be defined. The lag time can be computed using either of the available methods in WMS, or by entering it manually.

• Export

Allows saving the results as a Comma-Separated Values File (*.csv).

Overlapping Regions

The Regional regression equations section of the National Flood Frequency Regression Method dialog.

Example of the Compute Overlapping Areas dialog.

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.

Selecting Overlapping Regions

Select the region(s) the study watershed overlaps by clicking on the region name in the Regional regression equations available text window and then Select → button. The region named will be moved to the Selected Equations text window, and any relevant parameters will be added to the values spreadsheet. If a mistake is made the regional equation can be moved back again using the ← Remove button after selecting the Selected Equations window.

The USGS Report has a description and applicable maps for each state. These maps can be used to identify which regions are overlapped by the study watershed. It is also possible to download one of these maps, register the image, and create an NSS coverage. With an NSS coverage, and providing consistent coordinate systems are used, the overlapping regions can be computed automatically. The NSS tutorial demonstrates how to do this for a state.

The website for the USGS report and individual state reports/maps can be found at:

http://water.usgs.gov/software/NSS/

Related Topics

• Overview of NSS
• GIS Overlay