In Reply Refer To:
Mail Stop 415
June 9, 2014 revised August 13, 2014
REVISED (Changes are underlined or
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 2014.06
SUBJECT: Requirements for the collection, validation, and input of peak-stage verification data at U.S. Geological Survey streamgages
Policy Summary: Independent verification of the annual peak stage is required at all U.S. Geological Survey (USGS) discharge and stage-only streamgages; verification of other peaks is strongly encouraged. After determining that an independently measured peak stage is a valid representation of peak stage at the gaging station, the independent peak stage must be compared with the corrected (data corrections applied) peak stage value from the data recorder. If the two peaks agree within their determined accuracy, the recorded peak stage value is to be used for the annual peak stage. If the independent peak stage value is higher than the recorded peak stage value beyond the determined accuracy of each measurement, then the independent peak stage value is to be input into the representative gage-height time-series, such as the data corrected time-series, and used for the annual peak stage.
Background: The annual peak stage is one of the most important and utilized pieces of information collected at all USGS streamgages. These data are used in the design of hydraulic infrastructure, such as bridges, culverts, and dams, and for mapping flood risk. In most cases, the annual peak streamflow is determined from the recorded annual peak stage. The importance of having an independent means to verify recorded peak stage values is alluded to in Rantz (1982), Benson and Dalrymple (1967), Sauer and Turnipseed (2010), and in a number of Office of Surface Water Technical Memoranda; 81.04, 91.09, 93.07, and 06.05. Beyond verifying recorded peak stage values, it is important that the time series of recorded stage values contains all valid measurements of the peak stage, including those obtained from independent sources such as crest-stage gages (CSG), maximum-stage float-tape clips, or surveyed high-water marks (HWM). Having an auxiliary device to measure the peak stage, such as a CSG, provides a peak stage backup in case the primary recorder is subject to localized hydraulics, such as drawdown, malfunctions, or suffers flood-related damage. Currently, most electronic data recorders are set to measure stage in fifteen minute intervals. An accurate maximum river stage during a flash flood event may not be captured by a recorder set at fifteen minute intervals.
Policy: Independent verification of the annual peak stage is required at all USGS streamgages; verification of all other peaks is strongly encouraged. Traditional methods for verification utilize properly located and maintained CSGs, and maximum-stage float-tape clips. For stations not amenable to these traditional methods, such as sites that are heavily impacted by frost heave and ice flows, diligent collection and survey of high-water marks is an acceptable alternative. In these instances it is required to document that a search for high-water marks was made during all site visits, and this protocol must be explicitly spelled out in the station description and the water science center surface-water quality-assurance plan. Further, other well maintained sensors that are independent of the electronic data logger that record stage in a continuous fashion, such as a strip chart, or another electronic sensor with a measurement frequency that is short enough to capture the peak stage, ideally no greater than one minute and not to exceed five minutes, also are acceptable alternatives. Such electronic sensors must be capable of meeting a stage accuracy goal of + or - 0.03 ft as outlined in Office of Surface Water Technical Memorandum 89.08. For this memo “recorded peak stage values” refers to peak stage values from the electronic data logger and “independent peak stage values” refers to peak stage values obtained from CSGs, maximum-stage float-tape clips, surveyed HWMs, or other acceptable continuous auxiliary sources.
It is imperative to determine the validity of independent peak stage values in relation to any given event at the time it is collected or soon thereafter. This is to be documented in the “gage height record” element of the station analysis. Assuming that the equipment used for verification is properly installed, maintained, and located in the gage reach or gage pool, valid independent peak stage values should always either be equal to or higher than valid recorded peak stage values. Independent peak stage values that are lower than valid recorded peak stage values are suspect, and suggest a problem with the installation, location, or condition of either the verification or recording gage equipment that must be rectified. If an independent peak stage is thought to be affected by water pile up, draw down, debris, or other phenomena such that it does not represent the peak water surface for the event at the gage it must be discredited.
The validated independent peak stage value must be compared with the recorded peak-stage value to determine the annual peak stage at a gaging station. The recorded peak-stage value must have gage height and datum corrections applied before the comparison is made. If the two peaks agree within their determined accuracy, the recorded peak stage value is considered verified and is to be used for the annual peak stage. Differences between peak stage values that are 0.05 ft or less (at discharge and stage-only sites) are negligible unless the difference in computed discharge (at discharge sites) is greater than 8 percent. If the independent peak stage value is higher than the recorded peak stage value, outside of the determined accuracy of each measurement, and is outside of the range provided above, then the independent peak stage value is to be input into the data-corrected gage-height time-series and used for the annual peak stage.
For independent peak stage values that are input into the data-corrected gage-height record, a time must be assigned that is different from an existing value. Generally, the time for the independent peak stage value should be input about halfway in time from the highest recorded peak stage and either the value before or after it.
A plot of recorded peak stage value versus independent peak stage values should be maintained for all stations to help assess the validity of both sensors, as this relation should generally remain stationary. These plots should contain all pairs of peak stage values obtained, not just those associated with the annual peak.
While independent verification of the annual peak stage is required verification of secondary peaks is also strongly recommended. This is particularly important when working up streamflow records for time periods shorter than one year, the current standard operating procedure of the USGS. Following the recommended procedures (below) should account for all pertinent peaks.
Considerations for independent peak stage recording devices: Peak stage recording devices include CSGs, maximum-stage float-tape clips, surveyed HWMs, or other acceptable continuous auxiliary sources. Maximum-stage float-tape clips are installed on a float tape located inside of a stilling well. These devices are capable of recording the maximum stage that occurred within the stilling well since the last time they were reset. Due to this capability, an independent peak gage height can be obtained for all site visits. CSGs are installed in locations where contact with the river stage occurs infrequently (2 to 3 times per year). Due to this characteristic, independent peak stage values cannot be obtained for all site visits. Further, during periods of well below average streamflow, such as a drought, annual peak verification may prove difficult. Under these circumstances documentation of CSG inspections is critical. However, even under drought conditions, considerations must be made to obtain some form of independent peak stage verification that may include looking for and surveying HWMs. CSGs and other peak verification devices should be located such that about 2 to 3 peaks can be recorded during an average year. If the device fails to meet this target, its location must be adjusted. There are situations where CSGs cannot be located within the same low-water gage pool as the primary stage sensor. While this is less than ideal, this can be done if the relationship between the independent peak stage and recorded peak stage values is tracked and found consistent in time. It is important to note that this consistent relationship can, and will likely, vary by stage.
Considerations for setting gage height recording intervals less than 15 minutes: Since the transition to electronic data recorders that began in the 1990s gage height recording intervals generally have been set to 15 minutes. For most streams gaged by the USGS, this recording interval appears to fit well with the rate that stage changes. However, there are streams gaged by the USGS with drainage characteristics that lead to very rapid stage changes. These rapid stage changes, at times, are not tracked adequately with a gage height recording interval of 15 minutes. Evidence for this includes well located and maintained peak verification devices, such as maximum float tape clips, crest-stage gages, or high-water marks that consistently record peak-stage values that are substantially greater than the electronic data recorder peak stage. Other evidence includes discharge measurements made during significant stage changes where additional gage height readings, beyond the set interval of 15 minutes, would have proved beneficial. In the past, recording intervals were partially determined by limitations associated with digital storage, both in the recorders themselves and in our databases. Such limitations no longer exist and therefore, gage height recording intervals should be decreased so that recorded peaks fall within the criteria outlined above (within 0.05 ft of independent stage readings or 8 percent of computed discharge).
Peak Flow File considerations: This memo also serves as a reminder that entries in the peak flow files of the USGS should be consistent with the unit value record within the guidelines specified above and as recommended in OSW Technical Memorandum 06.05.
Routine Steps for peak verification:
Questions concerning the policies outlined in this memo may be addressed to Jim Kolva (firstname.lastname@example.org).
Acting Deputy Chief, Office of Surface Water
Benson, M.A., and Dalrymple, T., 1967, General field and office procedures for indirect discharge measurements: U.S. Geological Survey Techniques of Water Resources Investigations, book 3, chap. A1, 30 p. (available on line at http://pubs.usgs.gov/twri/twri3-a1/)
Rantz, S.E., and others, 1982, Measurement and computation of streamflow: U.S. Geological Survey Water-Supply Paper 2175, 631 p. (available on line at http://pubs.usgs.gov/wsp/wsp2175/)
Sauer, V.B., and Turnipseed, D.P., 2010, Stage measurement at gaging stations: U.S. Geological Survey Techniques and Methods bood3, chap. A7, 45 p. (available on line at http://pubs.usgs.gov/tm/tm3-a7/pdf/tm3-a7.pdf)
USGS, 1981, “EQUIPMENT & SUPPLIES--Use of Bubble Gage Manometers in high sediment concentrations”, Technical Memorandum 81.04, USGS, Surface Water Branch, (available online at /admin/memo/SW/sw81.04.html)
USGS, 1989, “Policy statement on stage accuracy”, Technical Memorandum 89.08, USGS, Office of Surface Water, (available online at /admin/memo/SW/sw89.08.html)
USGS, 1991, “Accuracy of U.S. Geological Survey manometer and pressure-sensor bubble gages”, Technical Memorandum 91.09, USGS, Office of Surface Water, (available online at /admin/memo/SW/sw91.09.html)
USGS, 1993, “Policy statement on stage accuracy”, Technical Memorandum 93.07, USGS, Office of Surface Water, (available online at /admin/memo/SW/sw93.07.html)
USGS, 2006, “Approval and revision of time-series data in ADAPS 4.6”, Technical Memorandum 06.05, USGS, Office of Surface Water, (available online at /admin/memo/SW/sw06.05.html)
Distribution: GS-W All