Guidelines for Identifying and Evaluating Peak Discharge Errors
In Reply Refer To: July 2, 1992
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 92.10
SUBJECT: Guidelines for Identifying and Evaluating Peak
Discharge Errors
Recent examinations of U.S. Geological Survey (USGS) files,
especially peak-flow files, by District personnel have identified
significant errors. Some of these errors have resulted from
entering incorrect station numbers, entering information under the
wrong station number, failing to correct for digital recorder
errors, and data entry (keypunch) errors. In addition, questions
have been raised about the accuracy of peak-flow determinations in
steep streams with highly movable beds. There is concern that
some peaks that have been computed as water flows were actually
debris flows. Also, questions are being asked about the accuracy
of assigned roughness values in channels with coarse bed material.
Some investigators suggest that actual Mannings "n" values are
higher than those typically assigned to such channels. This
memorandum has been assembled to call attention to the importance
of our peak-flow data base and to describe procedures to ensure
the integrity of that data base.
The USGS data bases are used for a wide variety of purposes and by
many public and private entities. It is critical that data we
provide to the nation be as accurate as current technology allows.
It is imperative that keypunch or other forms of mechanically
based errors be identified in an expeditious manner and correct
values entered into the data base. It is also important that
peak-discharge events that may actually have been debris flows be
critically examined.
The burden of maintaining the accuracy of our data files lies with
the Districts. The California District has been involved in
searching for anomalies in their peak-flow files and correcting
erroneous values. We encourage all Districts to review and
revise, if necessary, their data bases using similar procedures.
The following suggested procedures are based, in large part, on
the California District's experience.
1. Transfer data from the WATSTORE peak-flow file to a local
relational data base. Use of a relational data base will simplify
the retrieval and display of the data. All data elements in the
peak-flow file should be transferred, including county and
hydrologic-unit codes, latitude, longitude, dates, discharges,
gage heights, historical information, and qualification codes, for
both annual and secondary peaks above the threshold.
2. Retain a copy of all data as retrieved from WATSTORE for
future use in quality assurance of the review and updating
processes (step 13).
3. Proofread the data against the published record. This step is
important because there is concern that an unknown fraction of the
data have been corrupted by clerical errors such as entering data
under the wrong station number. Most of this work can be done by
clerical staff, concurrently with technical review of the data by
the hydrologic staff. The clerical work can be eased by using the
data base to print out old data by station number and date for
comparison with the mid-60's flood compilations (USGS Water-Supply
Papers 1671-1689) and to print out the newer data by year and
station for comparison with the annual data reports. Check all
data elements, including station numbers, dates, gage heights, and
qualification codes. Keep a permanent copy of the data listing
showing a check mark for each item checked.
4. If any discrepancies are found, keep a permanent record of the
correction and the basis for the correction. Prepare necessary
input and images for updating the WATSTORE peak-flow file, but do
not change either the local data base or the WATSTORE file until
the review is complete.
5. Concurrently, with the clerical review, conduct a technical
review of the data with emphasis on identifying and critically
examining apparent outliers. Use data-base graphical facilities
to plot discharge versus drainage area by State, county, or
hydrologic unit code. Draw smooth envelope curves that enclose
about 95 percent of the data, and identify apparent outliers (low
as well as high).
6. Where appropriate, compare recorded peaks with values computed
by the Rational Formula or by appropriate regional flood-frequency
equations, or with data from the daily-values and measurement
files. Identify apparent outliers.
7. When apparent outliers are found, remember that just because a
peak is anomalous does not mean that it is incorrect; this is just
a screening criterion. When an apparent outlier is found:
a. Compare the peak with the published values and with original
data; look for transposed numbers and values repeated from
previous years, and ensure that the station ID is correct.
b. Determine the basis of the peak.
(1) If it was an estimate, check that it was coded as such.
(2) If it was an indirect measurement, review the measurement;
give special scrutiny to measurements rated poor or based on one
or two cross sections.
(3) If the peak was from a rating extension, look for other
evidence to verify the extension and the peak.
(4) Use data-base graphical facilities to plot discharge against
gage height to check for datum changes and possible misapplication
of shifts.
(5) Make a permanent record of the review, including the
conclusion reached, the basis for the conclusion, and any
necessary corrections or revisions.
(6) Prepare any necessary input cards for updating the WATSTORE
peak-flow file, but do not change either the local data base or
the WATSTORE file.
8. Determine if the site is susceptible to debris flows and,
therefore, if the peak could have been caused by a debris flow
rather than by a clear-water flood. Review photographs taken
during indirect discharge measurements for evidence of debris
flows.
9. Determine if the site is susceptible to scour and fill.
Review photographs taken during indirect discharge measurements
for evidence of channel scour.
10. Review the location and drainage area for the site in
question. This has been found to be especially important for
sites smaller than 5 square miles and for sites at which drainage
areas were determined on 15-minute maps. This has required some
field verification.
11. Review historic-peak (and "highest-since") information.
Review the definition of items "historic" and "systematic" on
pages C-44 and 45 of the WATSTORE manual, vol. 4, Ch. I, and
ensure that the historic-peak qualification code (see pages A-18
and A-19 of WATSTORE manual, vol. 4, Ch. I) is used properly.
Determine the specific technical basis and exact source of all
historical information and make a permanent record of it; if the
specific basis and exact source cannot be determined, acknowledge
that fact in the record and provide whatever information is
available. If any specific information is available for a
historic peak (e.g., gage height or "highest since" year), ensure
that it is stored in the peak-flow file, with a historic-peak
qualification code, even if the discharge cannot be determined.
Ensure that values subject to high uncertainty (greater than 25
percent) are coded as estimates. Discharges subject to very high
uncertainty (greater than 50 percent) either should not be stored
in the file or should be stored as lower-limit estimates with a
"greater-than" qualification code; both of these cases are treated
the same by the J407 flood-frequency program.
12. If any revisions are necessary, make a permanent record of
the revision and the basis for it. Prepare input cards for
updating the WATSTORE peak-flow file. After all reviews have been
completed and all corrections and revisions have been determined,
run a job to update the WATSTORE data base. Do not update the
local data base as the data as originally retrieved will serve as
documentation for revisions to WATSTORE.
13. Retrieve the data from the (updated) WATSTORE peak-flow file
and compare it with the data previously retrieved (Step 2). The
PRIMOS CMPF command, or the UNIX DIF command, can be used to print
only the differences between the old and new data sets; review the
differences to ensure that all updates were made correctly and
that no inadvertent changes were made. Keep a permanent record of
this review for quality assurance.
14. Keep a permanent machine-readable copy of the updated
WATSTORE peak-flow data. This file can be used as the basis for
future comparisons with the WATSTORE peak-flow file, for quality
assurance of the annual peak-flow records-production process.
The procedures noted above are aimed at examining extreme high
(and low) peaks. These procedures should not be taken to imply
that other values, such as those within the envelope curves of
plots of discharge versus drainage area (step 5), are error free.
Such errors could be found using procedures outlined above, but it
becomes more difficult to decide which peaks to choose for such an
analysis.
As noted, simple and obvious errors should be corrected without
delay. Such problems require little hydrologic judgment to
ascertain that corrections are needed. However, anomalies that
result from possible mischaracterization of debris flows as water
flows, or from possible errors in assigned roughness values
require careful scrutiny and sound judgment before data values are
revised. Experts usually carefully reviewed high discharges and
most indirect measurements at the time the information was
obtained. Our understanding of some processes has, however,
advanced since the time of the collected information. The
reviewer should be careful to make technical revisions only where
there are technical reasons to do so. Revisions, especially those
based on roughness characteristics, should not be based simply on
"second guessing" by different personnel.
Anomalous indirect measurements or anomalous peaks derived from
ratings based on indirect measurements should be critically
examined by hydrologists with debris flow knowledge and with
knowledge of movable bed dynamics. It is not possible to provide
specific guidelines and criteria on which to base judgments as to
the character of any specific flow or conclusions that any
indirect measurement is erroneous because of improperly assigned
"n" values. Some general guidelines regarding recommended
indirect measurement technique include obtaining a minimum of
three cross sections, limiting conveyance ratios to between 0.7
and 1.4, avoiding sections that result in flow moving in and out
of critical flow, and other general criteria as given in
"Measurement of Peak Discharge by the Slope-Area Method,"
Techniques of Water-Resources Investigations Book 3, Chapter A2,
(Dalrymple and Benson, 1968) and in the "Users Manual for WSPRO -
A Computer Model for Water Surface Profile Computations," Report
No. FHWA-IP-89-027, 187p (Shearman, 1989).
Violation of any one of the guidelines and criteria given in the
references mentioned above does not necessarily invalidate any
indirect measurement. The decision to accept or reject a
measurement requires the hydrologic judgment of qualified persons
in each District. Measurements that are questionable and
difficult to assess should be reviewed by specialists outside the
District, such as the Regional Surface-Water Specialists or others
known to be authorities in surface-water hydraulics or debris
flows.
It is recognized that a thorough review of the entire flood-peak
data file requires substantial investments of time, money, and
efforts of clerical, computer, and hydrologic personnel. However,
the flood data that we collect and disseminate serves important
national safety-related purposes. There are growing national
concerns for safety and for quality-assurance of basic data and
scientific analyses of safety-related issues. The effort of
reviewing our flood data base and making necessary corrections and
revisions, therefore, will be well worthwhile to the USGS and to
our cooperators. Such a review might be included in plans for
statewide or regional flood-frequency studies; reviews for
subsequent studies would be much simplified by preservation of
machine-readable files of the updated data base, as suggested in
step 14. It would be desirable to have the review completed
before the transfer of the peak-flow file from WATSTORE to NWIS-
II.
Charles W. Boning
Chief, Office of Surface Water
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