In Cooperation with the Vermont Agency of Transportation

Introduction

Estimates of the magnitude and frequency of streamflow are needed to safely and economically design bridges, culverts, and other structures in or near streams. These estimates also are used for managing floodplains, identifying flood-hazard areas, and establishing flood-insurance rates, but may be required at ungaged sites where no observed flood data are available for streamflow-frequency analysis. The U.S. Geological Survey (USGS), in cooperation with the Vermont Agency of Transportation (VTrans), developed regression equations for estimating peak-flow frequency for recurrence intervals of 2, 5, 10, 25, 50, 100, and 500 years and estimates of the 25, 50, and 75 percent flow duration (daily discharges exceeded 25, 50, and 75 percent of the time) for ungaged, unregulated streams in Vermont. Generalized-least-squares regression (Tasker, 1987) was used to relate basin characteristics to flow-frequency characteristics for the peak-flow equations. Ordinary-least-squares regression was used to develop the flow-duration equations.

The Vermont Flow-Frequency (VTFF) Tool, a geographic information system (GIS) application, was created to automate the determination of basin-characteristics required for the flow-frequency equations. The tool then applies the equations to estimate peak-flow frequency and flow duration for a user selected point on a stream. The tool is built upon the National Hydrography Dataset (NHD) and NHD Watershed Tool. In addition, a link between the VTFF Tool and the National Flood Frequency (NFF) program provides the ability to plot frequency curves and synthetic hydrographs, and adjust flow-frequency curves for urbanization. The application customized for Vermont also calculates standard error of prediction, prediction intervals, and expected probability adjustments.

ArcView window showing delineated watershed.

To Download Reports
For a report on how the regression equations were developed Water Resources Investigation Report, 02-4238 (PDF 3.5 MB)

For a report on how to use the Vermont Flow-Frequency Tool Open-File Report 02-494, available only online (PDF 1.0 MB)

Vermont Flow-Frequency Tool

GIS Application and Supporting Programs

• Click here to download the Vermont Flow-Frequency (VTFF) Tool*.
• The VTFF tool also requires the NHD toolkit available from the National Hydrography Dataset Web site http://nhd.usgs.gov
• An optional program, the National Flood Frequency (NFF) program, provides the ability to plot frequency curves and synthetic hydrographs, and adjust flow-frequency curves for urbanization. Click here to obtain NFF

*Requires ArcView 3.2 and the Spatial Analyst for ArcView 3.2 Extension

Datasets Required

National Hydrography Dataset
The Vermont Flow-Frequency (VTFF) Tool requires the 1:24,000-scale National Hydrography Dataset (NHD) and the NHD toolkit, available at http://nhd.usgs.gov.

NHD Watershed Tool Support Datasets
The NHD Watershed Tool Support (WATSUP) datasets will be available from the Vermont Center for Geographic Information (VCGI). Digital topographic maps, can also be obtained from VCGI, and used as a location map for site selection.

The NHD WATSUP datasets can also be used with the NHD Watershed Tool for those interested in only delineating watersheds for any point along an NHD stream. The NHD Watershed Tool is an ArcView 3.2 extension, upon which the VTFF Tool was built. The NHD Watershed Tool is available at the NHD Web site.

NOTE: It is not recommended to load both the NHD Watershed Tool and the VTFF Tool in the same ArcView project.

Areas at or above 1,200 feet
The Vermont Flow-Frequency Tool uses a GRID dataset that identifies areas at or above 1,200 feet as one of the five variables to estimate peak flow characteristics and flow duration. The data, named grid_1200ft, is available by major subbasins, which corresponds to the NHD workspace directory names. Each grid_1200ft will be organized in separate 8-digit subbasin flow frequency folders. For example, to download grid_1200ft for the Lamoille River Basin (Subbasin 02010005), click on the 02010005 link below. Once the zipped file is downloaded, use a file compression utility, such as WinZip to uncompress 02010005_ff.zip. The uncompressed folder 02010005_ff should reside in the same directory where the 02010005_24shp and 02010005_ws directories are located. For more information on the directory structure for the VTFF tool refer to OFR-02-494. To download Grid_1200ft by major subbasin click on the basin names below.

Other subbasins will be made available once the corresponding NHD has been developed.

Optional Datasets

PRISM Climatic data
The Vermont Flow-Frequency (VTFF) Tool uses a mean annual precipitation dataset. The mean annual precipitation (1961-1990) used for this application was developed by the PRISM climate mapping system at Oregon State University. PRISM is a proprietary dataset and available from Climate Source <http://www.climatesource.com>.

The application does not need the PRISM precipitation dataset to estimate peak flow characteristics. The PRISM dataset is required to estimate flow duration with the VTFF Tool.

Once obtained from Climate Source, the PRISM mean annual precipitation data will need to be projected into the Vermont State Plane Coordinate System, units in meters, with a horizontal datum NAD83. The PRISM dataset used to build the regression analysis was also resampled to a 30-meter resolution using a bilinear resampling algorithm. The PRISM precipitation data used in the VTFF Tool consists of the data being clipped to the corresponding NHD workspace watershed extent, and named precip_grd in the NHD workspace_ff folder (For example, 01080101_ff).

For more information on the directory structure for the VTFF Tool refer to the online report OFR-02-494.

National Elevation Dataset
The National Elevation Dataset (NED) is used to obtain additional basin characteristics for published regression equations based on earlier studies. The VTFF Tool will not automate the computation of these earlier equations using these additional characteristics. For more information about NED, visit the NED website.

NED data projected into the Vermont State Plane Coordinate System can be obtained from VCGI. The NED data should be clipped to the corresponding NHD workspace watershed extent, and named dem_grid in the NHD workspace_ff folder (For example, 01080101_ff). For more information on the directory structure for the VTFF Tool, refer to the online report OFR-02-494.

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

For technical questions contact: Scott Olson (603) 226.7815, solson@usgs.gov