In Cooperation with the
Vermont Agency of Transportation
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 (Medina and 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
ArcView window showing delineated watershed.
To Download Reports
For a report on how the regression equations were developed: Water Resources Investigations 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
and Supporting Programs
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
The Vermont Flow-Frequency (VTFF) Tool requires the 1:24,000-scale
National Hydrography Dataset (NHD) and the NHD toolkit, available
Tool Support Datasets
The NHD Watershed Tool Support (WATSUP) datasets will be available
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
NHD stream. The NHD Watershed Tool is an ArcView 3.2 extension,
upon which the VTFF Tool was built. The NHD Watershed Tool is
at the NHD Web site.
NOTE: It is not recommended
to load both the NHD Watershed Tool and the VTFF Tool in the same
Areas at or above
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
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
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
Other subbasins will be made available once the corresponding
NHD has been developed.
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
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
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 at http://ned.usgs.gov/.
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
Medina, K.D., and Tasker, G.D., 1987, Analysis of surface-water data network in Kansas for effectiveness in providing regional streamflow information; with a section on theory and application of generalized least squares: U.S. Geological Survey Water Supply Paper 2303.
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, firstname.lastname@example.org