PROGRAMS AND PLANS - Technical Announcement of Availability of Updated Flood-Frequency Information


In Reply Refer To:                                         November 29, 1976
EGS-Mail Stop 415




SURFACE WATER BRANCH TECHNICAL MEMORANDUM NO. 77.04

Subject:  PROGRAMS AND PLANS - Technical Announcement of
          Availability of Updated Flood-Frequency Information


Enclosed for your information is a copy of a technical
announcement describing the availability of flood-frequency data
at WRD district offices.  This announcement has been distributed
to water-oriented technical and professional journals and to
Federal agencies.

This project provides an opportunity to publicly meet a part of
our agency's primary obligation - the evaluation of the nation's
water resources and hazards.  Along with this opportunity there
are, however, some constraints and responsibilities.

One of the constraints concerns our ability to respond to data
requests.  In recognition of our limited manpower,we must keep
this project from becoming a burden.  Our obligation requires only
that we have flood-frequency analyses available for inspection at
the district offices.  Some districts may find both less effort
and a better public response by providing easily reproducible
summaries of the results.  The method of responding to information
requests is a district decision which should reflect both our
obligation and our need to minimize manpower requirements.

Our responsibilities include providing explanations of our motives
and our analytical methods.  We may anticipate questions about our
computation routines and why our new estimates differ from either
previous estimates or established regulatory values.  We should
note in our explanations that we have followed computational
guidelines in WRC Bulletin 17, but that Bulletin 17 provides
numerous analytical alternatives which do not guarantee unique
frequency estimates.  Our estimates are based upon decisions by a
Geological Survey analyst about the representativeness of sample
data, outliers, historic information, and skew coefficients.
Alternate decisions by another analyst may provide different
estimates of comparable reliability.  Through WATSTORE, we are
prepared to use some different analytical assumptions as
requested, but at the requestor's expense.

Of particular importance is a clear response to questions about
differences in our estimates and established regulatory values.
Note that we have no regulatory authority.  Our function is to
provide reliable estimates of future flood frequency which we have
approached through analysis of available data and the basic
assumption that past experience is the best indicator of the
future.  Note also that as more data are accumulated over the
years, our estimates will change.  Establishing regulatory (or
design) values is a function of other Federal, State, and local
agencies, and the basis of their analyses may involve different
analytical assumptions and different objectives.

Requests for frequency information for gaging stations in other
districts should be referred to the district which has data-
collection responsibility for the site.  This will minimize our
providing different answers computed by different WRD analysts.

Call the Surface Water Branch for further information about either
the technical or policy aspects of this project.




                                       Harry H. Barnes, Jr.
                                       Chief, Surface Water Branch

Enclosure


WRD DISTRIBUTION:  A, S, FO-L, PO



TECHNICAL ANNOUNCEMENT

USGS Water Resources Division
December 1, 1976


Further Information:
Arthur G. Scott (703-860-6837)

FLOOD-FREQUENCY CURVES TO BE AVAILABLE FOR MORE THAN 9,000 SITES

Flood-frequency curves for all gaged sites in the United States
that have at least ten years of recorded natural (unregulated)
streamflow discharges will be prepared and made available for
inspection after about January 1, 1977, by district offices of the
U.S. Geological Survey's Water Resources Division

At least 9,000 sites on streams and rivers in all 50 States will
be made available through the program.

By establishing how frequently a particular flood flow is likely
to be equaled or exceeded at least once on the long-term average,
flood-frequency curves are used by public and private agencies for
design purposes and to determine, for example, insurance rates and
flood zone regulations.

These curves will be defined in accordance with the new Federal
guidelines for determining flood-flow frequency as detailed in
Bulletin 17 of the United States Water Resources Council and
represent the U.S. Geological Survey's interpretation of the
Council's guidelines.

The analyses are based on the use of weighted skew values.  Skew
values are computed from station records and regionally from local
studies where available, or from the Water Resources Council
national map of regional skew values and then weighted on the
basis of the number of years of flow record.  The analyses also
include consideration of an, where appropriate, adjustment for
high and low outliers, historic peak data, and conditional
probability adjustment for zero flows or truncated flow record.
In addition to the flood-frequency curve, data are available on
confidence limits and expected probability.  The analyses do not
include a comparison of the computed frequency curve with others
in a hydrologically similar region.

If alternate analyses are required, users may request these
through the National Water Data Storage and Retrieval System
(WATSTORE) utilizing the U.S. Geological Survey's peak-flow data
file.  These analyses must be made at the user's expense.
Analyses may include the use of different skew values, alternate
treatment of historic information, and alternate treatment of high
and low outliers.

These flood-frequency data, for each state, are available for
inspection only at the U.S. Geological Survey Water Resources
Division district office in one of the following cities (listed
under United States Government Department of the Interior - in the
telephone book):

ALABAMA, Tuscaloosa                 MISSISSIPPI, Jackson
ALASKA, Anchorage                   MISSOURI, Rolla
ARIZONA, Tucson                     MONTANA, Helena
ARKANSAS, Little Rock               NEBRASKA, Lincoln
CALIFORNIA, Menlo Park              NEVADA, Carson City
COLORADO, Lakewood(Denver)          NEW JERSEY, Trenton
CONNECTICUT, Hartford               NEW MEXICO, Albuquerque
FLORIDA, Tallahassee                NEW YORK, Albany
GEORGIA, Doraville (Atlanta)        NORTH CAROLINA, Raleigh
HAWAII, Honolulu (includes          NORTH DAKOTA, Bismarck
  American Samoa and Guam)          OHIO, Columbus
IDAHO, Boise                        OKLAHOMA, Oklahoma City
ILLINOIS, Champaign                 OREGON, Portland
INDIANA, Indianapolis               PENNSYLVANIA, Harrisburg
IOWA, Iowa City                     PUERTO RICO, San Juan
(includes
KANSAS, Lawrence                      the Virgin Islands)
KENTUCKY, Louisville                SOUTH CAROLINA, Columbia
LOUISIANA, Baton Rouge              SOUTH DAKOTA, Huron
MARYLAND, Towson (includes          TENNESSEE, Nashville
  Delaware and District of          TEXAS, Austin
  Columbia)                         UTAH, Salt Lake City
MASSACHUSETTS, Boston (includes     VIRGINIA, Richmond
  Maine, New Hampshire, Rhode       WASHINGTON, Tacoma
  Island, and Vermont)              WEST VIRGINIA, Charleston
MICHIGAN, Okemos                    WISCONSIN, Madison
MINNESOTA, St. Paul                 WYOMING, Cheyenne