Stage at each gage site is measured to about 0.01 ft accuracy and is recorded at 15 minute (or shorter) intervals using either a float gage or pressure transducer. These stage records are converted to flow records using stage-discharge relationships (see below).

Crest-stage recorders (floating cork gages) indicate high water marks and provide a backup source of high water in case recording gages are compromised in flooding conditions.

Stilling wells are installed at each site, either in the channel or via piping to the channel, to provide quiescent locations for sensing of stage.

1. General Approach to Discharge

   Measurement and Flow Ratings

   In a measurement of discharge (volumetric flow per unit time), a number of velocity measurements are taken across a transect located near the gage using a wading rod and propeller current meter (usually a mini (pygmy) velocity meter). Velocities (ft/sec) for each sub section are multiplied by their respective sub section areas (sq. ft.) and these discharges (cubic feet per second) are then summed to get a total discharge measurement for the section. Generally, an attempt is made to include enough velocity measurements across the transect such that no more than 10 % of the total discharge is contained in each sub section, although this may not be achievable for higher flows. Each total discharge measurement, along with the concurrent stage reading constitutes a point on a stage-discharge rating curve. Over time, discharge measurements are obtained for a range of stages so that a complete, accurate and current stage-discharge rating curve (graph) can be constructed.

   USGS field crews visit all sites about every six weeks to download data, check and calibrate equipment, and to do a low flow discharge measurements to check the stage-discharge rating for shifts, and to observe the channel section for potential changes in the high flow rating. A single USGS hydrologist is assigned to the BES stations to provide continuity and to maximize familiarity with the stations. These USGS visits are augmented by weekly visits by BES field crews who do a number of checks under the guidance of a Forest Service Hydrologist (see below, II. Stream Gage calibration & Flow Rating QAQC: Weekly Checks).

   High flow velocities cannot be measured in-stream, and are metered by USGS crews from bridges near the gaging station. Discharge measurements during runoff events are repeatedly conducted until the high flow portion of the rating is defined. Extremely high flow discharges that cannot be measured are estimated by a slope area curve method using cross section geometry, channel roughness and surveyed elevations of high points of the hydraulic grade line indicated by debris left by the receding limb of the hydrograph.

   Data are recorded at 15 minute intervals at the larger watersheds and at 5 minute intervals at small watersheds. Records are downloaded electronically and processed at the USGS office where adjustments are made for calibration problems, backwater corrections (due to debris and ice buildup), missing records, shifts in the rating, etc. Small changes in the low flow portion of the stage-discharge rating are handled using "v-shift" adjustments to the stage record. Larger changes warrant the use of a newly defined curve, based on recent discharge measurements.

   Flows are reviewed by USGS hydrologists using standard USGS data processing software. A Forest Service hydrologist also reviews the final flow records before processing for exploratory analyses, plots, and load computations.

   Two flow record files are produced. The first, referred to as "unit value" files contain the stage data recorded in the field at 15 minute (or less) intervals. The second, the "average daily flow" files contain an average flow for each day, and are produced using USGS software which accesses the unit value file stage records, stage corrections, and stage-discharge ratings and shift information.

2. Open Channel Sites

   These include Gwynns Falls main channel stations at Carroll Park, Villa Nova, and Gwynnbrook. In-stream discharge measurements are used to construct ratings. These sites are maintained with the goal of maintaining a +/- 10 % accuracy (standard for all USGS sites). Hydraulic controls at these sites are provided by downstream channel morphology and roughness, although at Gwynnbrook high flows may be controlled to some extent by the capacity of the Gwynnbrook Avenue bridge cells, some of which are blocked by sediment deposition. However, ratings shifts are frequent due to scour and deposition in the channel, especially after large runoff events.

3. Sites with Primary Devices

   The Pond Branch small watershed has a compound triple notch sharp edged weir, which needs frequent maintenance in terms of removal of debris from the weir and digging out of sediment from behind the weir. This weir was installed almost 30 years ago and experienced some problems with leakage around both sides. Packing the streambank with hydraulic concrete has alleviated these early problems with leakage around the sides of the weir. A combination of in-stream discharge measurements and volumetric measurements (using a large graduated cylinder and a stopwatch) are used to check the rating.

   The Glyndon small watershed has a wooden structure which functions to some degree as a modified broad crested weir (with a very large angle "notch"), but was designed to primarily to stabilize the channel and stage-discharge relations. Problems have been experienced with sediment deposition and leakage of flow through the banks and under the weir. Plans are under way to install a metal weir plate that will improve accuracy and reliability. Instream discharge measurements are done here.

   The Rognel Heights small watershed uses a stage and velocity sensor in a round 3.5 ft diameter storm drain pipe to record flow. Since this is a small urban watershed, there is an extremely rapid response time to rainfall events and it is very difficult to safely obtain in-stream discharge measurements the usual way. There is normally no dry weather flow but when there is (high groundwater sometimes seeps into the pipe through leaky seams), low flow checks on the rating are done by taking volumetric flow measurements on the weekly BES visits and at the six-week USGS visits. Stage checks on high water at the gage are done by measuring the elevation of chalk marks inside the pipe which have been scoured by runoff events and and by crest stage recorders in the channel downstream. These are converted to flow using Manning's open channel uniform flow equation.

   The McDonogh small watershed has a metal compound weir with a single V-notch in the center. Sediment must be removed from behind the weir periodically. Volumetric and in-stream discharge measurements are used to check the rating.

   The Dead Run subwatershed site has an old, somewhat damaged, modified broad crested concrete weir just a few meters downstream of the Kernan Drive bridge. This weir provides a control, but is buried upstream and the site has characteristics of an open channel site in terms of maintenance of the rating. In-stream discharge measurements are done here.

   The Baismans Run site flow is controlled by a concrete broad crested weir with a V-notch located on the upstream side of the Ivy Hill Rd. bridge. Sediment deposition under the bridge may and upstream of the weir may affect the rating. In-stream discharge measurements are used to check the rating.

4. Flow Data Publication

   Flow data are published on the USGS web (with links on the BES web site) and in the following publications annually:

   WDR-MD-DE-99-1 By Robert W. James, Richard W. Saffer, Anthony J. Tallman Water Resources Data, Maryland and Delaware, Water Year1999, Volume 1. Surface-Water Data 407 p.

5. USGS Methodologies and Protocols

   The methodology and QAQC protocols that USGS uses to collect flow data at stream gaging stations can be found at:

   http://oregon.usgs.gov/pubs_dir/twri-list.html

   These publications, the Techniques of Water-Resources Investigations of the U.S. Geological Survey (TWRI) is "a series of manuals that describe procedures for planning and conducting specialized work in water-resources investigations. The material is grouped under major subject headings called books and is further divided into sections and chapters. For example, Section A of Book 3 ("Applications of Hydraulics") pertains to surface water. The chapter, the unit of publication, is limited to a narrow field of subject matter."

   TWRIs may be obtained from:

   USGS Branch of Information Services
   Box 25286, Federal Center
   Denver, Colorado 80225
   ph: (888) ASK-USGS (275-8747)
   fax: (303) 202-4693
   e-mail: infoservices@usgs.gov
   

   The following TWRIs are relevant to QAQC and methods employed in gaging and processing flow data for BES stream stations.

   Book 3. Application of Hydraulics

   Section A. Surface-Water Techniques

   3-A1. General field and office procedures for indirect discharge measurements, by M. A. Benson and Tate Dalrymple: USGS--TWRI Book 3, Chapter A1, 30 pages. 1967

   3-A2. Measurement of peak discharge by the slope-area method, by Tate Dalrymple and M.A. Benson: USGS--TWRI Book 3, Chapter A2, 12 pages. 1967

   3-A3. Measurement of peak discharge at culverts by indirect methods, by G. L. Bodhaine: USGS--TWRI Book 3, Chapter A3, 60 pages. 1968

   3-A4. Measurement of peak discharge at width contractions by indirect methods, by H.F. Matthai: USGS-TWRI Book 3, Chapter A4, 44 pages. 1967

   3-A5. Measurement of peak discharge at dams by indirect methods, by Harry Hulsing: USGS--TWRI Book 3. Chapter A5, 29 pages. 1967

   3-A6. General procedure for gaging streams, by R.W. Carter and Jacob Davidian: USGS--TWRI Book 3, Chapter A6, 13 pages. 1968

   3-A7. Stage measurement at gaging stations, by T.J. Buchanan and W.P. Somers: USGS--TWRI Book 3, Chapter A7, 28 pages. 1968

   3-A8. Discharge measurements at gaging stations, by T.J. Buchanan and W.P. Somers: USGS--TWRI Book 3, Chapter A8, 65 pages. 1969

   3-A9. Measurement of time of travel in streams by dye tracing, by F.A. Kilpatrick and J.F. Wilson, Jr.: USGS--TWRI Book 3, Chapter A9, 27 pages. 1989

   3-Al0. Discharge ratings at gaging stations, by E.J. Kennedy: USGS--TWRI Book 3, Chapter A10, 59 pages. 1984

   3-A11. Measurement of discharge by the moving-boat method, by G.F. Smoot and C.E. Novak: USGS--TWRI Book 3, Chapter A11, 22 pages. 1969

   3-A12. Fluorometric procedures for dye tracing, Revised, by J. F. Wilson, Jr., E.D. Cobb, and F.A. Kilpatrick: USGS--TWRI Book 3, Chapter A12, 34 pages. 1986

   3-A13. Computation of continuous records of streamflow, by E.J. Kennedy: USGS--TWRI Book 3, Chapter A13, 53 pages. 1983

   3-A14. Use of flumes in measuring discharge, by F.A. Kilpatrick and V.R. Schneider: USGS--TWRI Book 3, Chapter A14, 46 pages. 1983

   3-A15. Computation of water-surface profiles in open channels, by Jacob Davidian: USGS--TWRI Book 3, Chapter A15, 48 pages. 1984

   3-A16. Measurement of discharge using tracers, by F.A. Kilpatrick and E.D. Cobb: USGS--TWRI Book 3, Chapter A16, 52 pages. 1985

   3-A17. Acoustic velocity meter systems, by Antonius Laenen: USGS--TWRI Book 3, Chapter A17, 38 pages. 1985

   3-A18. Determination of stream reaeration coefficients by use of tracers, by F.A. Kilpatrick, R.E. Rathbun, Nobuhiro Yotsukura, G.W. Parker, and L.L. DeLong: USGS--TWRI Book 3, Chapter A18, 52 pages. 1989

   3-A19. Levels at streamflow gaging stations, by E.J. Kennedy: USGS--TWRI Book 3, Chapter A19, 31 pages. 1990.

   3-A20. Simulation of soluble waste transport and buildup in surface waters using tracers, by F. A. Kilpatrick: USGS--TWRI Book 3, Chapter A20, 38 pages. 1993

   3-A21. Stream-gaging cableways, by C. Russell Wagner: USGS--TWRI Book 3, Chapter A21, 56 pages. 1995

   Book 4. Hydrologic Analysis and Interpretation

   Section A. Statistical Analysis

   4-A1. Some statistical tools in hydrology, by H.C. Riggs: USGS--TWRI Book 4, Chapter A1, 39 pages. 1968

   4-A2. Frequency curves, by H.C. Riggs: USGS--TWRI Book 4, Chapter A2, 15 pages. 1968

   Section B. Surface Water

   4-B1. Low-flow investigations, by H.C. Riggs: USGS--TWRI Book 4, Chapter B1, 18 pages. 1972

   4-B2. Storage analyses for water supply, by H.C. Riggs and C.H. Hardison: USGS--TWRI Book 4, Chapter B2, 20 pages. 1973

   4-B3. Regional analyses of streamflow characteristics, by H.C. Riggs: USGS--TWRI Book 4, Chapter B3, 15 pages. 1973

   Section D. Interrelated Phases of the Hydrologic Cycle

   4-D1. Computation of rate and volume of stream depletion by wells, by C. T. Jenkins: USGS--TWRI Book 4, Chapter D1, 17 pages. 1970

   Book 8. Instrumentation

   Section A. Instruments for Measurement of Water Level

   8-A1. Methods of measuring water levels in deep wells, by M.S. Garber and F.C. Koopman: USGS--TWRI Book 8, Chapter A1, 23 pages. 1968

   8-A2. Installation and service manual for U.S. Geological Survey manometers, by J.D. Craig: USGS--TWRI Book 8, Chapter A2, 57 pages. 1983

   Section B. Instruments for Measurement of Discharge

   8-B2. Calibration and maintenance of vertical-axis type current meters, by G.F. Smoot and C.E. Novak: USGS--TWRI Book 8, Chapter B2, 15 pages. 1968

   Book 9. Handbooks for Water-Resources Investigations

   Section A. National Field Manual for the Collection of Water-Quality Data

   9-A1. Preparations for water sampling, by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. Iwatsubo: USGS--TWRI Book 9, Chapter A1, [variously paged]. 1998

   9-A2. Selection of equipment for water sampling, edited by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. Iwatsubo--TWRI Book 9, Chapter A2, [variously paged]. 1998

   9-A3. Cleaning of equipment for water sampling, edited by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. Iwatsubo--TWRI Book 9, Chapter A3, [variously paged]. 1998

   9-A4. Collection of water samples, edited by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. Iwatsubo--TWRI Book 9, Chapter A4, 152 p. 1999

   9-A5. Processing of water samples, edited by F.D. Wilde, D.B. Radtke, Jacob Gibs, and R.T. Iwatsubo--TWRI Book 9, Chapter A5, 128 pages. 1999

   9-A6. Field measurements, edited by F.D. Wilde and D.B. Radtke: USGS--TWRI Book 9, Chapter A6, [variously paged]. 1998

   9-A7. Biological indicators, edited by D.N. Myers and F.D. Wilde: USGS--TWRI Book 9, Chapter A7, [variously paged]. 1997

   9-A8. Bottom-material samples, by D.B. Radtke: USGS--TWRI Book 9, Chapter A8, [variously paged]. 1997

   9-A9. Safety in field activities, by S.L. Lane and R.G. Fay: USGS--TWRI Book 9, Chapter A9, [variously paged]. 1997

Problems are reported to USGS weekly (by emailing the field database), unless serious changes in the rating or any large changes in gage calibration are seen, in which case a phone call is placed to appropriate USGS personnel on return to the office.

1. Inside and Outside Gage Check

   The outside gage (usually a staff gage) is read to within 0.01 ft and compared to the inside gage (recorder) reading (if accessible). If the outside gage is no longer hydraulically connected to where stage is being measured (leaves, enlargement of sediment bar, ice), debris is cleared or a channel is dug and an additional reading is taken after the water surface level reaches equilibrium (usually within 10 minutes). Readings are taken before and after any alterations so that any errors in the flow record can be annotated and pro-rated correctly.

2. Low Flow Rating Check

   Any objects or surfaces in the channel or pipe that are "felt" by flowing water cause changes its elevation (i.e., depth or stage) and therefore in the rating. The dominant object or surface is the feature that determines the elevation of the water at the point where stage is recorded. It is referred to as the hydraulic control, or just the control. The low flow control is usually a riffle or primary measuring device device such as a weir (see illustration).

   The control is inspected for any changes at every visit. Blockages (leaves, ice, woody debris) which change the water surface elevation are removed. The outside gage elevation is recorded before and after debris removal (after equilibrium in stage is reached (as described above).

   Special notation of a zero flow condition, especially if flow is not truly zero but is by-passing the gage instrumentation in one way or another (e.g., passing around or under the weir.)

   The condition of primary devices (weirs, flumes, pipe sections) are also evaluated. This includes observations of damage to weir edges, proper aeration under the nappe (the pouring water at a notch), any shifting that might affect the level, and evidence of leakage at the sides of the weir (through hidden flow channels in the streambank). The buildup of sediment behind the weir is monitored and recorded (the minimum water depth should be about twice the maximum head expected (stage above zero flow.) Also, inspections are made of stage or flow sensors and intake pipes for any shift in position (especially elevation) or blockage (by trash) that would affect the functioning or reliability of any stage measuring sensors.

3. High Flow Rating Check

   Generally, different objects will control the water surface elevation of high flows than those which control at low flows, and some imagination in the field is required to see what these might be at a given site. It is important to note changes because high flow discharge measurements are done infrequently, usually at the suspicion of a change in the control. It is better to note such suspicions earlier rather than later to produce accurate estimates of high flows.

   Observations are made of any sudden, large scale changes (e.g., from a large runoff event, ice debris damage, increased or decreased sediment loads) to the stream cross section, gradient, and floodplain that may affect the water surface elevation at the gage in high water, including:

   1. significant scouring or deposition of sediment in the stream bed of the main channel,
   2. deposition of, or change in the status of, large woody debris (logs, snags) or other urban debris (shopping carts, concrete) in the measured reach or downstream, and
   3. any large scale changes in channel bars, stream banks or flood plains that would increase or decrease resistance to flowing water at high stages.

4. Digital Photos

   Digital photos are taken quarterly from the same location which show a close up of the station and sensor/intake, the high and low flow controls and an upstream and downstream view from the stage measuring point.

5. Selected Station Specific Notes

   Carroll Park
   Watch for trash (plastic bags especially) that might wrap around the intake, possibly hampering accurate high flow stage readings or causing lag times in the recording of rising stages.

   Villa Nova
   Dig out channel to staff gage; watch for large debris which may hang up in or cause damage to the intake.

   Gwynnbrook
   Watch for changes in the extensive sediment bars downstream of the dry culverts in the Gwynnbrook Ave. Bridge.

   Dead Run
   Watch for blockage in the low flow concrete weir, especially in the damaged sections; look for leaf debris which impedes low flow both on the weir and downstream of the weir.

   Baismans Run
   Look for ratings changes at the new, continuously gaged location and at the downsteam staff gage position; look for sediment accumulation at the new control and record chalk mark elevations as indicators of the last storm.

   Glyndon
   Watch for leakage through flow channels in the streambank and streambed; perform a volumetric discharge measurement at very low flows (stopwatch, ca. A 5 second minimum collection time).

   Pond Branch
   Watch for resumption of leakage through the streambank or damage to patches made previously with hydraulic cement; watch for sediment build-up upstream of the weir to the extent it approaches to within two tenths of a foot of the elevation of the invert of the notches. Perform a volumetric discharge measurement at very low flows

Storm event and seasonal hydrographs are examined for any suspicious shapes or responses (e.g., a straight-line record during periods when flows should be declining or rising.) Flows are also compared to historical values as an additional check.

USGS is notified immediately of any concerns or questions regarding flow record accuracy.