Presentation on theme: "Measurement of Stream Discharge by Wading Water Resources Investigations Report 00-4036 By K. M. Nolan and R. R. Shields The U. S. Geological Survey measures."— Presentation transcript:
1Measurement of Stream Discharge by Wading Water Resources Investigations Report By K. M. Nolan and R. R. ShieldsThe U. S. Geological Survey measures stream discharge at over 7000 sites nationwide. Each year thousands of discharge measurements are made. The vast majority of those measurements are performed by wading the stream using the velocity-area method. For example, this is how over 75% of the 53,000 discharge measurements made by the USGS in 1990 were done.The training presentation that follows describes how a current meter measurement should be made when wading the stream and using the velocity-area method. The next slide, which will appear automatically, describes what to do if you need help viewing or navigating through the class.
2How class worksInstructions for viewing and navigating through this training class can be found in the “Intro.html” file, which is on the CD-ROM containing this presentation.Instructions for viewing and navigating through this training class are contained in the “Intro. html” file, which is on the CD-ROM containing the presentation. This file discusses narrations, videos and how you can navigate through the class. You can use the link on this slide to view the “Intro.html” file. Following this link will open up a web browser, which should be closed before continuing with the class.
3Class CreditListed as Training Class SW1271 with the National Training Center of U.S. Geological SurveySupervisor may ask to see a copy of the completed testThis class is listed as training class SW1271 with the National Training Center of the USGS. Supervisors can submit paper work to provide training credit to employees. You may need to show your supervisors results of taking the test, which can be found at the end of the class.
4Links to reference materials Occasionally text on slides will be linked to additional reference materials. Some are official memorandums that explain procedures and USGS policies. These require either a web browser or Adobe PDF reader to view. Others links will take you to the online materials, such as the training class SW4230, Surface-Water Field Methods. To view such links the computer you are using must be connected to the Internet.Most of the concepts contained in this training class on wading measurements are discussed in detail in the web-based class SW4230. Feel free to use it to gain additional background on the methods and procedures discussed in the following presentation. The link on the bottom of this page can be used to test your computers internet connection.You should close any web browser you open before proceeding to the next slide.USGS memosLinked Text
5Agenda Horizontal angle correction How this class works Computing subsection width and depthRecording velocityRecording dischargeFinalizing measurementMean Gage HeightAccuracyThe controlPoint of zero flowPlotting on ratingTestReferencesAcknowledgementsHow this class worksThe velocity-area conceptMaking the measurementMeasuring widthMeasuring depthWading rod useMeasuring velocitySite selectionSafetyMeasurement notesBlackfoot River as an exampleStarting the measurementThe front sheetInside the note sheetJob Hazard AnalysisThe class agenda is shown here. You can jump to any part of the class by clicking on an agenda topic. It is recommended that you view the entire presentation at least once before jumping around in the class. You can return to this slide at any time by clicking on the agenda button.
6THE VELOCITY-AREA METHOD Discharge = (Area of water in cross section) x (Water velocity)Cross section areaxWater VelocityThe most practical method of measuring stream discharge is through the velocity-area method. Discharge is determined as the product of the area of the water and the water velocity
7Channel cross section is divided into numerous sub sections Measuring the average velocity of an entire cross section is impractical, so the USGS uses what’s called the mid-section velocity area method. Using this method the channel cross section is divided into a number of sub-sections.Most natural channels must be broken down into 20 or 30 sub-sections to adequately characterize their irregular geometry. The discharge of each sub-section is determined by measuring it’s area and water velocity.Discharge of each sub-section = Area x Average Water Velocity
8Area of each sub-section determined by directly measuring width and depth Area = Width x DepthDepthThe area of each subsection is determined by directly measuring width and depth
9Water velocity in each sub-section is estimated using a current meter to measure water velocity at selected locationsAverage water velocity in each sub-section is estimated using current meters to measure water velocity at selected locations..For shallow sections average velocity is estimated by measuring the velocity at 0.6 of the depth below water surface. When depths are large, the average velocity is best estimated by averaging velocities measured at 0.2 and 0.8 of the depth below the water surface.
10Stream discharge is sum of discharges in all sub-sections Total Discharge = ((Area1 x Velocity 1) + (Area2 x Velocity2) + ….. (Arean x Velocityn))123nThe sum of the incremental discharges is the total discharge of the stream!
11Making the Measurement Verticals should be spaced so no subsection has more than 10% of the total dischargeIdeal measurement has no more than 5% of total discharge in any subsectionShould have between 20 and 30 subsectionsSpacing between verticals should be closer in those parts of cross section with greater depths and velocities.This schematic diagram conceptualizes the velocity-area method in detail. You can see that values of depth and velocity at each observation point (or vertical) are applied to a sub-area whose width extends half way to the preceding and following observation points. We will show examples of how width is determined in subsequent slides.For now it is important to recognize that enough subsections must be established to adequately represent the uneven nature of most channels. In general, subsections should be spaced so no subsection contains more than 10% of the total discharge. Ideal measurements have no more than 5% of the total discharge in a subsection. This means that most measurements should have between 20 and 30 subsections. Subsections should be placed closer together in those parts of the cross section with greater depths and velocities.
12Measuring Cross-Section Area Width Channel width is generally measured directly using a tape or tag line.
13Measuring Cross-Section Area Width (cont.) Taglines usually have marksOne mark every 2 ftSome taglines have:Two marks every 10 ftThree marks every 100 ft.Distances between marks are estimated or measuredMarks or beads on taglines indicate distance. Taglines generally contain a single mark or bead every 2 feet. Some taglines have two marks or beads every 10 feet, and three marks or beads every 100 feet. Distance between marks is usually estimated. The video sequence on this slide shows several activities associate with stringing a tagline.
14Measuring Cross-Section Area Depth Wading rod is marked every 0.1 footDuring wading measurements depths are usually measured using marks on a wading rod
15Wading Rod Use Top Setting Mechanism The top-setting wading rod is used most commonly in the U.S.G.S.. The top-setting mechanism found on this rod allows the hydrographer to set the meter at the proper depth without reaching into the water.The rod is placed in the stream so the base plate rests on the streambed. Water depth is read on the graduated main hexagonal rod. The video shows depth being set on a top-setting wading rod. Additional detail on how to set a top-setting rod is contained on the next slide.
16Wading Rod Use (cont.) 2.08 ft. 1.04 ft. 0.52 ft. Rod set so meter placed at 0.6 (2.6 ft)108642234SettingRodDepthScale560.2 depth72.08 ft.0.6Depth0.8Depth82.6 ft.TotalDepthWading rods contain single marks every tenth of a foot, double marks every half foot, and triple marks every foot. The round “setting” rod is graduated in feet. When the “setting rod” is moved to read the depth using the top-setting scale the meter is automatically placed at 0.6 the depth. The example on this slide shows the rod set for a total depth of 2.6 feet.The meter can be placed at 0.8 the depth from the water surface by dividing the depth of water by 2 and using this new value to “set” the rod. When the depth of water is multiplied by 2 and this new value is used to set the rod, the meter will be set at 0.2 the depth from the water surface.1.04 ft.0.52 ft.
17Wading Rod Use (cont.)Must estimate depth when velocity causes “pile-up” on rod.Visually extend water surface to rod.The velocity head associated with the flow of water will cause water to pile up on the wading rod. When this occurs the hydrographer must estimate the depth by visually extending the water surface to the rod. The velocity head should not influence the depth measurement. The video shows water piling up on a rod.
18Wading Rod Use Stand beside and downstream of rod You should stand in a position that least affects the velocity of water passing the current meter. It is generally best to face the bank so water flows against your leg. The wading rod should be held at the tag line. You should stand about 3 inches downstream and at least 18 inches from the wading rod. This will minimize your affect on flow past the current meter. The video demonstrates the proper positioning.
19Velocity Determination USGS generally uses Price current metersAA for large depthsPygmy for shallow depthsStandard AA MeterWhen depths are large the USGS generally uses the standard AA vertical axis current meter to measure velocity. When depths are shallow (that is, generally less than 1.5 feet) a pygmy meter is used. You should see Office of Surface Water Memorandum 85.07, which was later revised by memo , for a description of proper meter use.See OSW memos and 85.14Pygmy Meter
20Meter UseThe table on this slide shows conditions recommended for using the standard AA meter and the pygmy meter. Some departure from conditions shown here is permissible. For example, if depths in most subsections are greater than 1.5 feet, you should not substitute a pygmy meter for the few sections that are less than 1.5 feet deep and vice versa. An AA meter set closer than 0.5 feet to the stream bottom will under register velocity. Any deviation from recommendations shown in this table should be noted and the measurement downgraded.
21Spin Tests A spin test should be performed on all meters: Before each field tripWhen performance is suspect.Before and after repairsA log of all spin tests must be maintainedSee OSW memo for policy on spin testsA spin test should be performed on all meters between field trips, when performance is suspect, and before and after repairs.The video sequence on this slide shows the start and finish of a spin test. You must maintain a log of all spin tests. Office of Surface Water memo presents current policy on spin tests.
22Meter PerformanceMeter performance should also be checked before and after each measurementSpin meter and note if meter spins freely and comes to gradual stopMeter performance should be checked before and after each measurement. This can be done by noting if the meter spins freely and comes to a gradual stop.
23Meter CareMeter care and maintenance is discussed in OSW memorandum 99.06Meters must be maintained during and after use in the field. Office of Surface Water memorandum discusses meter care in detail. This memo describes when meters should be cleaned, how meter care and performance should be tracked, the parts that need to be inspected and cleaned and the type of of oil that should be used on meters.
24Headset and StopwatchVelocity is determined by placing meter in stream and counting number of revolutions in a measured amount of timeWater velocity is determined by counting meter revolutions for a given amount of time.Revolutions have traditionally been counted using a stop watch and headset. The headset is used to listen for clicks made by a wire contact as it completes an electric circuit. The circuit is completed either on every meter revolution or on every 5th meter revolution depending upon which meter post the headset is connected to. Time is usually kept using a stop watch. The stopwatch is started simultaneously with the first “click”, which is counted as “zero”, not “one”.Stop Watch and Headset
25Digitizer, Aquacalc and DMX 1 Newer units are now available that compute velocity and/or dischargeDMXComputational units such as current-meter digitizers, Aquacalcs and DMX units are also available for recording meter revolutions. These units work best when used with current meters fitted with either magnetic or optic heads. Unlike meter heads with wire contacts (which are often called “cat wiskers”), these heads do not relay on electrical contacts and, therefore, produce cleaner signals. Wire contacts have been known to produce spurious readings when used with computational units.Current-meter digitizers keep track of time and revolutions and produce direct readings of velocity. Depths and stationing can be keyed into Aquacalcs and DMX units. Both units can, therefore, be used to compute discharge. We will not discuss the details of how to use either the Aquacalc or the DMX. It should be pointed out, however, that users of these units bear some responsibility for archiving measurements if they are available only in electronic forms.Aquacalc1Use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government
26Marking MeterMeter revolutions can sometimes be counted manually by marking one meter cup.Meter revolutions can also be counted manually by marking one of the meter cups. Marking one of the meter cups can provide a backup means of measuring velocity if electronic systems fail.The video shows a meter with a marked cup.
27Rating TableVelocity can then determined using a current-meter rating tableIf you know the number of revolutions meter cups make in a given amount of time, velocity can be determined using a rating table (as shown here) or using a rating equation (as shown on the next slide) for the meter being used..Partial listing of standard rating #2
28Rating Table Equations Equations for standard Rating Tables:For AA meterV = RFor pygmy meterV = RR = Revolutions per secondStandard rating equations for the AA and pygmy meter are shown here. These are standard ratings #2. The use and development of these equations are described in Office of Surface Water Memo PDF files containing rating tables for both meters can be viewed using the links provided on this page. Be ware that the PDF file for the Pygmy meter rating table comes up vertically in the PDF file. You must scroll down to see the table.See:OSW Memo 99.05Standard rating table #2 for AA meterStandard rating table #2 for Pygmy meter
29Velocity from Digitizer, Aquacalc, and DMX Current meter digitizers Aquacalcs and DMX units have equations for rating table built in.These devices provide direct computation of velocityCurrent-meter digitizers, Aquacalcs and DMX units have rating equations built into them. These devices, therefore, compute velocity directly. The video shows Ron Shields reading velocity information from a current-meter digitizer.
30Measure velocity for at least 40 seconds Velocity should be measured for at least 40 secondsEvens out short-term velocity fluctuationsVelocity varies across the stream, with depth, and with time. Because velocity varies with time we measure it for at least 40 seconds. This evens out naturally occurring short-term fluctuations and thus provides a better estimate of average velocity.The video shows velocity fluctuations that occurred while making a measurement with a pygmy meter.
31Average VelocityThe goal is to represent the average velocity in the verticalMeasured at 0.6 the depth when depths are shallowMeasured at 0.2 and 0.8 the depth when depths are large. These two velocities are averaged to represent average velocity in the verticalBecause velocity varies with depth we must sample velocity in the vertical in such a way as to best estimate average velocity in the vertical. Research has found that velocity typically varies as shown in this graph. Velocity is typically fastest at the surface and decreases near the bottom due to friction with the streambed.When depths are shallow and a typical velocity profile exists research has shown that average velocity can be estimated by measuring it at 0.6 the depth. When depths are larger, the average of velocities measured at 0.2 and 0.8 the depth should be used to represent average velocity in the vertical.Typical velocity profile
32Velocity Measurement Methods Guide to velocity-measurement methodsThis table shows the recommended depths for using the “0.6 depth method” and the “0.2 and 0.8 depth method”.
33Non Standard Conditions Use of 0.6 and 0.2/0.8 methods assume velocity profile is logarithmic.Velocity should decrease closer to bottom due to frictionIf velocity at 0.8 depth is greater than velocity at 0.2 depth or if velocity at 0.2 depth is twice the velocity at 0.8 depth then the velocity profile is considered abnormal and the three-point method must be used (see next slide).Use of the 0.6 and 0.2 and 0.8 methods assumes a standard logarithmic velocity profile. If the 0.8 depth velocity is greater than the 0.2 velocity, or if the 0.2 velocity is twice the 0.8 velocity the profile is considered to be abnormal and the three-point method must be used.The video describes how you can save time by not moving the meter up and down the rod so much when using the 2-point method.
34Three-Point MethodThree-point method computed by averaging velocity measured at 0.2 and 0.8 the depth and averaging that result with velocity measured at 0.6 the depth.Average velocity is computed using the 3-point method by averaging velocity measured at 0.2 and 0.8 the depth and averaging that result with velocity measured at 0.6 the depth.
35Site SelectionReach should be straight and uniform for a long enough distance to provide uniform flow through the measuring sectionStreambed should be stable and free of large rocks, weeds, and protruding obstructions.It is difficult to make a high quality discharge measurement if the reach chosen for the measurement is poor. The reach should be straight and uniform for a long enough distance to support uniform flow through the cross section being measured. In addition the streambed should be stable and free of large rocks, weeds, and protruding obstructions.It is often necessary to, “improve” a cross section before making a measurements. For example, large rocks can be moved to make the cross section more uniform. It might be necessary to build temporary dikes to eliminate slack water or shallow depths. The cross section should NOT be modified after the measurement is started.Upstream view of excellent measuring section, Little Blackfoot River
36Site Selection (cont.) Riffle Straight and uniform for a distance long enough to support uniform flowMeasurement SectionControlFlowRiffleWThe diagram on this slide summarizes attributes of an ideal measurement section, as described on the previous slide. Ideally, the measuring section should be 5 channel widths below the most upstream riffle and approximately 2 channel widths above the control.5W2W
37Safety - First “Safety First, Every Job, Every Time” (Department of Interior Policy)Throw BagStandard PFDThere is inherent danger in making wading discharge measurements. No discharge measurement is worth putting yourself or fellow employees in danger. Safety must be the first consideration in deciding if, when, where, and how to make a measurement. This includes deciding what safety equipment needs to be available when measurements are made.Inflatable PFD
38Safety – On Line http://wwwhif.er.usgs.gov/uo/safety.html All USGS safety policies related to wading discharge measurements are described by documents available on the Water Resources Division’s Internal Home Page for Safety. You can view this home page by clicking on the URL shown here. You must be connected to the internet and using a USGS computer to view this internal home page. If you are not linked to the internet or are using a non-USGS computer, memos that pertain directly to wading measurements and operation of gaging stations can be viewed by following links on the next two slides.
39Safety – Memo 99.32 Water Resources Division Memorandum 99.32 The main reference for safety issues related to wading measurements is Water Resources Division Memorandum This memo describes general policy related to making wading measurements. It outlines use of personal floatation devices, the need to protect against hypothermia, prohibits making measurements when rescue would be dangerous or difficult, and requires that a job hazard analysis be developed for all sites where discharge measurements are made.Water Resources Division Memorandum 99.32
40Safety – miscellaneous memos Click on page image to view memoAlthough WRD memorandum is the main reference for current safety policy related to discharge measurements, several other WRD Memorandums contain valuable safety information. Links to those memos and a brief description of each one are contained on this slide.
41Safety – Responsibilities WRD Memo “Both supervisors and employees will be held accountable if safety policies are not followed.”It is important to remember that both supervisors and employees are responsible for safety. Failure to adhere to current policy can result in sever penalties up to, and including, separation from government service.Employees injured as a result of deliberately disregarding safety policy may be denied workman’s compensation.Winter measurementSnake River near Moran, WY
42Safety - PFDs See WRD Memo 99.32 Personal Flotation Devices (PFDs) must be worn unless the job hazard analysis states otherwise.WRD Memo mandates that Personnel Floatation Devices, or PFDs, be worn during all measurements unless the job hazard analysis for the site states otherwise. Approved PFDs are listed in memo Both standard models as well as suspender-type floatation devices can be used if they meet WRD specifications.Suspender-type PFDStandard PFD
43Safety- HypothermiaProtection against hypothermia is required when conditions warrant.WRD Safety Home Page contains information on protection against hypothermiaUse of cold water protective Personal Flotation Devices increases protection against hypothermiaMemo requires protection against hypothermia. Information on WRD’s Safety Home Page describes environmental conditions that lead to hypothermia and summarizes things that can be done to prevent it. Use of cold water protective floatation devices, such as the float coat shown here, increases protection against hypothermia.Float Coat
44Safety - Rescue Look downstream and think rescue When preparing to make a measurement always consider possible hazards and prepare for the worst. One of the most important things to consider is conditions downstream and think about how, or if, a rescue could be conducted.This downstream view of a measurement site on the Clark Fork shows a series of riffles, which are not associated with significant velocity increases, rapids or drop-offs. Also, there are few overhanging trees or branches that could snag or trap a swimmer underwater. A self-rescue would be likely below this measuring site, should the hydrographer fall into the stream. A measurement should not be attempted if rescue would be dangerous or difficult to execute.Downstream view of measurement section, Clark Fork, near Gold Cr., MT
45Safety - RescueThrow bags contain a line that can be thrown to colleagues who need help.See Mississippi report on use of multi-person field tripsThrow bags contain a line that can be thrown to colleagues who need help. They are excellent pieces of safety equipment when measurements are made by more than one person. A report by USGS, WRD’s Mississippi District provides guidance on the use of 2-person field trips, which may be of interest.
46Wading Safely When wading, proceed carefully Use wading rod to probe bottom ahead of youThe video on this slide shows Ron Shields using a wading rod to probe the stream bottom ahead of him as he wades the Clark Fork in Montana. You should look for loose or slippery rocks, deep holes and shifting sand bottoms as you proceed across the stream.One rule-of-thumb, often used when wading a stream is that you should not attempt to wade when the product of depth times velocity is greater than 10. This rule-of-thumb must be used with caution because it does not take into account the many factors associated with the site conditions and the limitations of the individuals involved.
47Wading Safely (cont.)Wade carefully and think about hazards downstreamThe video on this slide shows Ron Shields wading a small steep stream in Montana. How would you perform a self-rescue if you were to slip and fall wading this stream?How would you execute a self-rescue at this site?Have a plan before you begin
48Safety – Conditions Around You Snake River near Moran, WYWhen wading a stream you must be aware of hazards from upstream as well as downstream. You must look for floating ice and debris. You must be aware of dam releases. You should get release schedules for dams and advise dam operators that you will working downstream below the dam.Although it will be more of hazard on rivers that are too deep to wade, boat traffic can occasionally pose problems when wading streams.Be aware of potentially dangerous conditions upstream and downstreamIceDebrisDam ReleasesBoats
49Safety – Measuring Width Use laser distance meter, rather than a tag line, to measure channel width and distances when boat traffic is possibleOne way to cut down on hazards associated with boat traffic is to use laser distance meters to measure distance, instead of tag lines. Tag lines can injure boaters and ruin their equipment.Laser distance meter
50Safety - TractionUse of felt soles or metal cleats on the bottom of your waders can significantly improve traction.Be careful, felt bottoms can become slick if algae is present.Felt soles or metal cleats placed on the soles of waders can significantly improve traction on slippery stream bottoms. Be careful, however, algae can sometimes clog felt bottoms and make them slippery.
51Safety - WadersBelts can prevent waders from rapidly filling with water, if you fall below the water level.BeltIt will be much more difficult to recover from falling into a stream if your waders fill with water. Belts can prevent water from rapidly filling waders and give you time to recover without the weight and drag you would have to overcome if your waders are filled with water. In this photograph, Ron Shields is wearing a fanny pack with belt that serves this purpose.
52Safety--In-Pool Training Some WRD offices have scheduled in-pool training to familiarize employees with the use of personal floatation devices and with how it feels to be submersed while wearing waders and hip boots. This video shows parts of such training done in the WRD’s Utah District Office. All such training activities should be done with trained life guards present.
53Measurement NotesMeasurements are documented in measurement note sheets. Following the measurement, these sheets become legal documents that may be reviewed by other USGS personnel as well as by the general public. It is, therefore, important that these note sheets be filled out carefully and completely.Original information should never be erased from measurement note sheets. Original data include gage readings, depths, revolutions and time. Examples of information on a measurement note sheet that is derived from original data include the total discharge on the front sheet and mean gage height.
54Measurement Notes -Example Gage HouseThe process of filling out measurement note sheets will be examined by following a measurement made on the North Fork Blackfoot River on September 13, 1999The process of filling out measurement note sheets will be examined by following a measurement made on the North Fork of the Blackfoot River near Ovando, Montana on September 13, 1999.Upstream view, North Fork, Blackfoot River
55N. Fk. Blackfoot River Downstream View Upstream View These photographs show several views of the N. Fork Blackfoot site
56Gage HouseThis video shows the gage house at the North Fork Site
57Starting the Measurement N. Fork Blackfoot River Discussion of measurementOverview of gage siteThe video sequence on this slide discusses the measurement and shows channel conditions at and below the North Fork Blackfoot gage house.
58Selecting Measuring Section The reasons for selecting the measuring section on the Blackfoot and the quality of that section are discussed on this video sequence.
59Measuring Section Downstream view showing control riffle This un-narrated video shows the channel downstream of the Blackfoot site including the riffle that controls the stage-discharge relation at this stage.Downstream view showing control riffle
60Stringing TaglineThis video shows the tag line being installed on the left bank of the North Fork.
61Meter and Rod SetupA Price-AA meter and current meter digitizer were used for the measurement. The video shows the rod and meter setup.
62Meter and Rod Setup (cont.) Here is another video that shows the rod and velocity meter being prepared for the measurement.
63Determining Subsection Spacing Remember, try to have no more than 5% of flow in any one subsectionThis video discusses how subsection spacing was selected for the measurement. Remember, we try to have no more than 5% of the total discharge in any one subsection.
64Assessing Subsection Spacing Can keep track of sectioning by using rating to estimate what discharge will be.You should have some idea what the discharge will be before you start the measurement.. The estimated value for the North Fork measurement is discussed in the video.
65Assessing Subsection Spacing (cont.) Try to monitor flow in each section as you proceed.It is most important to monitor flow in the deepest/fastest subsections to judge compliance to the 5% rule.As discussed on this video, knowing approximately what the discharge will be and computing the discharge as you go will allow you to assess how well you are doing in terms of limiting flow in each subsection to no more than 5% of the total discharge. It might be more important for you to watch your footing and to watch for floating debris than it will be to compute the discharge for each subsection as you go. If this is the case, you should at least try to compute the discharge for the deepest and fastest section or two, this way can be sure that you are not dramatically violating the 5% rule.
66Starting the Measurement – The Second Subsection This video shows the second subsection being established for the Blackfoot measurement. This is the first subsection where velocity and depth can be measured.
67Job Hazard AnalysisA portion of the job hazard analysis for the North Fork measurement is shown here. You can click on the reference to the analysis to see the complete form.Job Hazard Analysis for North Fork Blackfoot River Site
68Front Sheet Summarizes: 1. Measurement results 2. Gage operation 3. Channel conditions4. Evidence of high-water5. Water Samples taken during visitThe “Front Sheet” of a measurement note sheet summarizes measurement results, gage operation, channel conditions, evidence of high-water marks and the type of water samples taken during the visit.Because the front sheet summarizes gage operation and the condition of the channel, it provides information needed to adequately compute a daily discharge record. For example notes on high-water marks provide checks against peak stages recorded between visits to the gage site. Notes on the condition of the control allow the hydrographer to assess how the measurement plots relative to the station’s stage-discharge relation. Information on gage operation allows the hydrographer to correct gage height recorded between visits to the gage.
69Summary Information - Blackfoot Measurement Information collected during the measurement made on the North Fork Blackfoot River is shown here in red.
70Summary Information - Blackfoot Measurement (cont.) The portion of the note sheet show here summarizes condition of the gage and channel at the time of the measurement made on the North Fork Blackfoot. Gage height at this site is sensed by a Design Analysis H-355 gas pressure system. Data are recorded by a graphic recorder as well as by a H-350 data logger. Data are telemetered by a Sutron 8004 Data Collection Platform.Recorders are referenced to a set of staff gages located adjacent to the gage house (This is the outside gage on the note sheet). The video describes equipment in the gage house.Gage equipment 11Use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government
71Inside the note sheetDischarge measurement data are written inside the measurement note sheet. The width and depth of each subsection and the velocity recorded at one or more points in the vertical of that subsection are recorded individually. These data are summed to determine total discharge.
72Recording Width and Depth Distance from initial point - usually measured from left bankDepth - usually measured directly using a wading rod.Width is computed from data entered in the “Distance from initial point” column. The “initial point” is usually on the left bank of the stream. Depth is measured directly using the wading rod.The video shows the tagline being set up on the right bank.
73Recording Angle Correction Angle coefficient- correction applied if flow is not perpendicular to the cross sectionNeeded because we must determine flow normal to the stream cross section.Angle of the current can be assessed by observing floating particles in the waterCorrection can be determined using measurement note sheet as guide (see next slide)We must apply an angle coefficient if flow is not perpendicular to the cross section. Without such a correction we would over estimate the amount of water flowing downstream.The angle of the current can be assessed by observing floating particles in the water. The next slide shows how that angle can be used to determine the appropriate angle coefficient.
74Determining a horizontal angle correction Point of originRead angle correction hereTagline or tapeNoteSheetThis diagram show how the correction factors listed around edges of the inside of the note sheet can be used to determine the horizontal angle correction. This is the correction needed to reduce the discharge to the component normal to the measurement cross section.The procedure is demonstrated in the video.
75Computing width and depth of individual subsections Example of computing sub-section widthAssume the following measurements appliedStarting point - 0 feetNext point feetNext point feetNext point feetMeter locations135The values of depth and velocity at each observation point apply to a subsection whose width extends half way to the preceding and following observation points. We can follow the process of determining width and depth using example points shown here.
76Recording Width Meter locations (ft.) 1 3 5 0.5 1.5 Widths 2 ? 1350.51.5Widths2As pointed out in this slide, width of each subsection is the difference between the preceding and following points divided by two.The term “mid-section” is sometimes confusing because it infers that the observation point is in the middle of the subsection. This is only true if widths are equal on both sides of the subsection in question. Notice that velocity and depth of the subarea at 1 foot are measured off center.?Width at point 0 = (1 - 0)/2 = 0.5Width at point 1 - (3 - 0)/2 = 1.5Width at point 3 = (5 - 1)/2 = 2.0
77Recording Width and Depth Meter locations1350.5Depth, in this example, is zero, therefore no flow1.5Because there is no preceding point at the starting point, which is the edge of water, the width is half the distance to the following point. In the example shown here, we cannot measure depth in the first subsection. We, therefore, assume discharge, is zero. For this reason it is good to keep the first subsection as small as possible.When the edge of water is a vertical bank, depth and velocity will probably not be zero. In this situation it is necessary to estimate velocity at the end vertical. This is usually done by assuming velocity is some percentage of velocity in the adjacent vertical. The link on this page will take you to a table from Rantz, which presents data that can be used to estimate velocity when the edge of water is a vertical bank.Widths2Note: See table from Rantz (1982) for help estimating velocity when edge of water is vertical.?
78Recording Width and Depth (cont.) Data from the North Fork Blackfoot RiverData entered for the measurement on the North Fork are shown here. REW stands for “right edge of water” which was measured at All observations shown here were made at 0.6 depth.
79Recording Velocity Velocity measured for at least 40 sec. Rating table for meter is used to determine velocity “at point”Any needed horizontal angle correction is applied to velocityVelocity data are entered on the measurement note sheet next to data on width and depth. Velocity is measured for at least 40 seconds. The number of revolutions and the observation time are entered and the velocity determined from the rating table for the meter being used. If a horizontal angle coefficient is listed in the far left column of the measurement note sheet, that coefficient is applied to velocity. The resulting value is entered in the “Adjusted for horizontal angle” column.
80Recording Velocity (cont.) Velocity measured at a single point is entered in the “At Point” columnIf velocity is measured at more than one point in the vertical the mean must be computed.Velocity measured at a single point is entered in the “At Point” column. If velocity is measured at more than one point in the vertical the mean must be computed. Occasionally, abnormal velocity profiles will be encountered. For example, sometimes the velocity measured at 0.8 the depth will be faster than velocity measured at 0.2 the depth. When this occurs velocity should also be measured at 0.6 the depth. The average of the 0.2 and 0.8 readings should be averaged with the .6 reading to get the mean velocity in the vertical.
81Checking Meter Condition Condition of meter should be checked periodically to make sure debris is not affecting performanceIt is important to periodically check the condition of the meter during a discharge measurement. You need to make sure that meter performance has not been affected by debris built up on the meter.The video shows a pygmy meter being checked during a measurement.
82Velocity from Rating Table See OSW memo for the most recent ratings for AA and pygmy meters.Velocity is determined from the rating table appropriate for the meter being used. Devices such as current-meter digitizers, Aquacalcs and DMX units have rating equations built into them. These devises, therefore, provide a direct readout of revolutions, time and velocity.Partial listing of standard rating #2
83Recording Discharge Data from North Fork Measurement Discharge for individual subsections is computed by multiplying area of the subsection by the average velocity of the subsection. Area is determined by multiplying width times depth. This slide shows data from the North Fork measurement.Velocity was measured at 0.6 depth for all sections shown here. The velocity at 0.6 depth is therefore used as the average velocity for all subsections shown in this measurement note sheet. If there were horizontal angle corrections, those coefficients would have been multiplied times the velocity to obtain the adjusted velocity.
84Finalizing Measurement Include finish timeSum width, area and dischargeIntermediatetimeFinishtimeYou begin finalizing the measurement by summing data on the measurement note sheet. Partial discharges are usually computed to one significant figure more than will be used to report the total discharge. The finish time, total width, total of all widths (which should agree with the total width), total area and total discharge are entered.This slide presents data from the North Fork measurement.Total areaTotal widthTotal dischargeTotal of all widths
85Finalizing Measurement (cont.) Record left edge of water, finish time, etc.A check of the discharge computation later revealed a discharge of 205 cubic feet per secondIllustrates value of checking measurementsThis video shows the North Fork Measurement being completed. Note that the finished time was noted and that the final discharge was computed as soon as the measurement was completed. A check of the discharge computation later revealed a discharge of 205 cubic feet per second.
86Finalizing Measurement (cont.) Fill in Width, Area, Velocity, Gage height, and Discharge on Front SheetAverage velocity is (Discharge / Area)Summary data from inside the note sheet are transferred to the front sheet.Total width, area, average velocity, gage height and total discharge should be entered on the front sheet. Average velocity is computed by dividing total discharge by total area. This is also where you describe how your meter was working, the type of meter used and how far off the rating this measurement plotted.
87Finalizing Measurement (cont.) Fill in Width, Area, Velocity, Gage height, and Discharge o Front SheetAverage velocity is (Discharge / Area)Information from the North Fork measurement is shown here in red.
88Finalizing Measurement (cont.) Read gageRecord times and gage heights recorded during measurement on front sheet.All gages should be read after the measurement is completed and those readings should be recorded on the front of the measurement note sheet. A record of gage heights during the measurement will allow you to compute the mean gage height for the measurement. The starting and ending times of the measurement are usually recorded. Interpolated gage heights associated with the beginning and end of measurements are shown inside parentheses.Information from the North Fork Measurement is shown here in red. You will note from the last slide that a gage height of 2.79 feet was applied to the measurement. This gage height, which was the gage height being recorded at the time, was within the error associated with reading the outside staff gage. Had inside and outside gage readings differed significantly, the measurement would have been referenced to the outside gage and appropriate corrections would have been applied to recorded gage heights. This is because the outside gage is the reference gage at this site, and has been set to water surface elevation by levels. It is the gage used to develop the stage-discharge relation.
89Determining Mean Gage Height Compute mean gage height for measurement, if neededMean gage height should be computed if change in stage is greater than 0.15 feet or if change in stage has not been uniform during measurementWeighting can be done using either partial discharge or time as the weighting factorSee Discharge Measurements, Part 2 in training class SW4230 for a description of how weighting should be done.Mean gage height should be computed by weighing the measurement either by discharge or time if gage height changes by more than 0.15 feet during the measurement, or if changes in stage were not uniform during the measurement. A record of intermediate gage heights will be needed to compute weighted gage heights. Mean gage height for smaller changes in stage can be computed by averaging the starting and ending gage heights.Information in the on-line training class SW4230 describes gage-height weighting procedures in detail. You can use the link on this page to go to the second section on Discharge measurements in that training class. Weighting procedures are described in the subsection entitled “Computation of mean gage height of discharge measurements”.
90Measurements During Rapidly Changing Stage Weighted mean gage height is not truly applicable when stage is changing rapidlyYou can reduce measurement time using the following procedures in the order listed:Use only the 0.6-depth methodReduce velocity-observation time to about secondsReduce the number of subsections to 15 – 18Using any of the shortcuts listed above will reduce measurement accuracySee further discussion in SW4230The weighted mean gage height is not truly applicable when stage is changing rapidly. You can reduce measurement time, and therefore the amount of stage change that occurs during a measurement, by using only the 0.6-depth method even though the 2-point or 3-point methods are called for. If you need to further reduce the amount of time needed for the measurement you can reduce velocity-observation time to between seconds. This is sometimes referred to as using “half-counts”. If you need to shorten the measurement time even more you can try reducing the number of subsections to between 15 and You must be aware that using any of these shortcuts will reduce measurement accuracy. Text in Surface-Water training class SW4230 discusses these shortcuts further.
91Finalizing Measurement (cont.) Downstream view through Blackfoot measuring reachThe portion of the front sheet shown here describes how the measurement was made, how good you think it was, how the gage was operating, and the condition of the channel at the time of the measurement.
92Assessing Measurement Assessing accuracy of measurementSemi-quantitative based upon:1. Cross section uniformity2. Velocity uniformity3. Stream bed conditions4. Etc.Blackfoot measuring sectionIt is up to you to assess the accuracy of the measurement. This, semi-quantitative assessment is based upon how uniform the cross section is, how uniformly velocity is distributed horizontally and vertically, how stable the stream bed is, and other factors. A complete description of factors affecting the accuracy of discharge measurements can be found in Open-File Report by Sauer and Meyer.See “Determination of Error in Individual Discharge Measurements” by Sauer and Meyer announced in OSW memo 93.14
93Assessing Measurement (cont.) Measurements are rated as being either Excellent, Good, Fair or Poor. The percent errors associated with each of these ratings is shown here. It should be possible to make an excellent measurement in a stable uniform channel. On the other hand it will probably only be possible to make a poor measurement in some channels.Remember, the rating of the measurement is not necessarily a reflection of your measuring technique. Nor is it based upon how the measurement plots relative to the rating. The measurement rating assessment is based upon measuring conditions and, therefore, should be made in the field immediately after completing the measurement.
94Finalizing Measurement -- The Control Very important – Observations of control forms basis for developing shifts to ratingsThe control that is in effect during the measurement must be identifiedWater SurfaceRiffleSection ControlYour observations of what feature is controlling how deep the water is in the gage pool at the time of your measurement is very important. When these “controls” change the stage-discharge relation for the station will also change.During low flow the control may be a riffle downstream or, less commonly, the configuration of the entire channel below the gage. As flow increases the entire channel will most probably become more important in controlling the stage-discharge relation. The diagram on this slide illustrates these two types of controlling conditions.ChannelControlChannelControlStream Bottom
95Finalizing Measurement -- The Control (cont.) Photos on this slide illustrate section control below a gage pool and how that control can drown out as stage increases. The photo on the right shows the rock riffle below the gage becoming less effective in controlling the stage-discharge relation. At the stage shown in the photo on the right the overall shape of the channel below the gage is starting to control the stage-discharge relation rather than just the riffle.Section Control drowning out-- Channel becoming controlSection Control
96Finalizing Measurement – Describing channel conditions This slide shows information describing channel and measurement conditions during the North Fork measurement. Note that riffles 250 feet below the gage were identified as the control and that no algae or debris was noticed on those riffles. It is not sufficient to note whether or not the control was clear. The physical feature, or features, that represent the control must be identified.The video shows the control.
97Finalizing measurement - Point of Zero Flow Point of zero flow (also called Gage Height of zero flow)Useful in developing stage-discharge relation (rating)First approximation for scale offsetCan be used as another point on the ratingRepresented by the lowest (deepest) point on section controlWhenever possible the point of zero flow should be measured before or after a measurement. The PZF is very useful in developing stage-discharge relations. This is because it can be used as the first approximation of the scale offset as well as another point that can be used to develop the stage-discharge relation.
98Point of Zero Flow (cont.) Gage PoolControl SectionThese two diagrams shows how the PZF should be measured. The PZF is determined by measuring the deepest point on the control and subtracting that measurement from the gage height. The pile up on the rod caused by the velocity head should be included in your reading.Include Velocity HeadFlowDeepest Point on ControlGage PoolControl Section Perpendicular to Flow
99Point of Zero Flow (cont.) North Fork Blackfoot RiverThe video sequence on this slide show the PZF being measured on the N. Fork.
100Point of Zero Flow (cont.) Another exampleThis video shows another example of a PZF being measured.
101High-water marks on North Fork staff gage Look for high-water marksImportant aid in working streamflow recordAllow you to verify peak stages recorded between visits to siteHigh-water marks on North Fork staff gageBe sure to look for marks left by high water that might have occurred between visits to the site. This information will prove valuable when compiling the streamflow record because it will allow you to verify stages recorded between visits to the gage.
102Measurement and the Stage-Discharge Relation Compute how far measurement plots from rating and plot on ratingBefore leaving the gage site you should determine how the measured discharge compares to the discharge from the stage-discharge relation for the gage height of the measurement. The North Fork measurement plotted minus 16 percent from the rated discharge. This was likely due to fill deposited in the channel after spring runoff.Normally, if a measurement plotted 16% from the rating a check measurement would be required to insure that the measurement was correct. In this case, however, previous measurements already verified that the control had filled and that you should expect measurements to plot off the rating. If a check measurement is made, it should be made at a different cross section using different equipment. This is a good place to note that you would never be able to assess the need to make a check measurement if you did not compute the measurement before leaving the field site.The video shows the North Fork measurement being compared to the rating.
103TestClick on image on the left to start Excel file containing the test.You will have to allow Excel the use of macros for the test to function properlyYou should open the Excel file as “read only”.You can install the Excel Viewer, if you do not have Excel on your computerClicking here will start installation
104Wading Discharge Measurements Using the Mid-Section Method By K Wading Discharge Measurements Using the Mid-Section Method By K. Michael Nolan and Ronald R. Shields Listed as Training Class SW1271 with National Training Center of U.S. Geological SurveyThis concludes the training presentation. You can either quite the presentation, view a list of selected references that you may want to rely on for additional information on stream discharge measurements, or view our acknowledgement of people who contributed to this report.QuitView selected referencesAcknowledgements
105Selected References Quit Buchanan, T.J., and Somers, W.P., 1969, Discharge measurements at gaging stations: USGS Techniques of Water-Resources Investigations, Book 3, Chapter A8, 65 p. (In revision)International Organization for Standardization, 1983, Measurement of liquid flow in open channels, Handbook 16, 518 p.Nolan, K.M. and others, Surface-water field techniques training class, USGS WRIR , (http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods)Rantz, S.E., 1982, Measurement and Computation of Streamflow:Volumes I and II, USGS Water Supply Paper 2175, 631 p.V.B.Sauer and R.W. Meyer, Determination of Error in Individual Discharge Measurements”, USGS Open-file reportQuit
106AcknowledgementsThe authors would like to thank Glen Hess, Vern Sauer, and Lucky Sultz for their review of preliminary versions of this report. We appreciated the help of Ken Thompson during field work at the North Fork Blackfoot River. Funding for production and distribution of this report was provided by the Safety Committee of the Water Resources Division.Quit