Presentation on theme: "Introduction to Acoustic Measuring Equipment"— Presentation transcript:
1Introduction to Acoustic Measuring Equipment U.S. Geological SurveyTEchnical training in Support of Native American Relations (TESNAR)Introduction to Acoustic Measuring EquipmentKlamath Falls and Chiloquin, ORSeptember, 19 – 23, 2011Mark Uhrich, USGS, Portland, ORMarc Stewart, USGS, Central Point, ORGlen Hess, USGS, Portland, OR
2Brief History of ADCPSOcean going boats used “speed logs” to measure speed of the boat.“The first commercial ADCP, produced in the mid-1970’s,was an adaptation of a commercial speed log” (Rowe and Young, 1979).1980s Doppler technology continue to involveEarly 1990s ADCP become more widespread in the USGS and other agencies.2012 Acoustic based instruments become the most common instrument type used in the USGS (Flowtrackers and ADCPs)
3Acoustic Instrument What Why and When Much of the material in the presentation is borrowed from USGS Hydroacoustic Classes
4Acoustic Instruments TRDI Rio Grande ADCP TRDI StreamPro SonTek RiverSurveyorHere are some of the commonly used ADCP’s.SonTek Flow Tracker
5FlowTracker (acoustic point measurements) Ceramic transducers send and receive pulses of soundCenter transducer transmits the sound, while the transducers on the arms are receiversLocation of velocity measurement is called the sample volumeSample volume is located about 4 inches from the transmitting transducerMeasures velocity based on the Doppler shift
6What Is an ADCP? LET’S LOOK AT THE NAME “a” “d” “c” “p” AcousticDopplerCurrentProfilerSoundWaves and theShifts are used to measureWater VelocityProfilesWhat is an ADCP?First its acoustic, meaning that it uses sound to make its measurements.It uses sound to measure velocity using the Doppler shift principle. Whether you realize it or not you are familiar with this principal. It is what changes the pitch of the horn on a passing car or train. A sound approaches you the sound waves or compressed and you hear a higher pitch. When the sound is moving away the sound waves or stretched an you hear a sound with a lower frequency.Using this technology through water the ADCP measures the velocity of the water.The ADCP measures profiles of water velocity, rather than sampling the water column at discrete points.
7Sound WavesWater wave crests and troughs are points of high and low water elevations.+CrestTrough-Sound wave “crests” and “troughs” consist of bands of high and low air or water pressure.Now we will discuss the basic concepts of how hydroacoustic instruments work.You are probably familiar with water waves. Water waves have crests and troughs which are high and low water elevations.[CLICK]Sound waves are similar but their crests and troughs are areas of high and low pressure in whatever medium the sound is traveling through, which in the cause of an adcp is the water in the stream[CLICK]The frequency of the sound may be audible to the human ear such as the sound of a trumpet.Or it may be a frequency that we cannot hear such as the sound transmitted from a transducer.TrumpetADCP Transducer
8How does an ADCP instrument work? Uses Doppler shift to measure water velocityThe Doppler effect is the change in a sound's observed pitch (frequency) caused by the relative velocities of the sound source and receiver.Hydroacoustic instruments can use the change in frequency, called Doppler shift, to determine speed.I am sure you have heard the pitch of a siren on a police car, ambulance, fire truck or horn on a train sound higher in pitch as it approached you and lower in pitch as it drove past and away from you.This change in sound caused by the vehicle coming towards you and then passing you and driving away from you is the Doppler shift.As a sound source travels towards you, the sound waves are compressed and the sound you hear has a higher pitch or frequency than the sound actually being transmitted.Conversely, as a sound source travels away from you the sound waves are stretched and the sound you hear has a lower pitch are frequency than the sound actually being transmitted. By measuring the change in sound frequency and knowing the frequency of the sound being transmitted the speed of the sound source can be computed.Note: either the source of the sound or the sink (listener) can move or both.GOOD link to more info on the doppler shift:
9The Basic Doppler Equation fD = fS * V/CfD = Doppler Shifted FrequencyfS = Source Frequency (frequency of ADCP)V = Velocity of scatterers in waterC = Speed of Sound (dependent on water char.)The equation for computing the velocity of the sound source from the Doppler shifted sound frequency is shown here.We know source frequency and Speed of sound (based on our instrument and the salinity and temperature of the water)The instrument can measure the doppler shifted frequencyTherefore, we can compute Velocity
10Scatter Velocity Assumption V = fD / 2fS * C V = water velocity = scatterer velocity Important We assume that, on average, scatterer velocity equals water velocity Violation of this assumption will lead to errors in water velocity computation.As we have already mentioned, hydroacoustics instruments do not measure the velocity of the water because pure water is acoustically transparent, but measures the velocity and direction of small particles and organisms, called scatterers, in the water column. We assume water velocity = scatterer velocity. If this assumption does not hold we introduce errors into our water velocity computations.Note: The 2 in the equation is result of two Doppler shifts, one as the sound goes out and another as it returns
11When the scatter velocity may not be equal to the water velocity Water-velocity measurement is biased toward the fish velocityWaterFish:WaterStationary object:RockWater-velocity measurement is biased toward zeroExamples where the scatterer velocity is not equal to water velocity. (Click) In case 1 the fish is the scatterer– the velocity of the fish is measured, and the fish may be moving in a completely different speed and direction than the water. (Click) A large stationary object such as a rock will bias velocities toward the speed of the object which is zero.
12Importance of Speed of sound (C) V = (fD /2fS *)CImportantSpeed of sound (C) must be computed accurately by the instrument.A temperature error of 4o C or salinity error of 12 ppt will result in a 1% velocity errorThe instrument must have an accurate temperature sensor and must be configured for the correct salinityRule of thumb: Specific conductance generally below 5000 uS/cm should not significantly affect CPolicy: All acoustic instruments must have independent temperature check (within 2 degrees C)Speed of sound must be computed accurately to get the velocity measurement right. In the equation for speed of sound underwater, temperature is the largest factor affecting C. Therefore temperature must be accurately measured by the instrument at the transducer head to accurately compute C. Salinity is another Importantfactor.(Click)A temperature error of 2 degrees celsius or a salinity error of 5 ppt would result in approximately a 1 percent error in measured velocity.
13ADCP Speed AnalogyPicture at time T1Picture at time T2STo measure velocity, ADCPs listen to the returns at two separate timesWe are going to measure the speed of an automobile on a road using a strobe light and a high speed camera. Consider road at night with cars moving at a steady rate of speed. The posts have been installed within the camera’s field-of-view and are a known distance apart. A strobe light is actuated and while the road is illuminated the camera takes two high-speed photographs. When the investigator examines the photograph negatives he finds that by lining up or synchronizing the images of the cars on the two photographic negatives he can determine the distance traveled by the cars by measuring the apparent shift in position of the reference posts. He can then calculate auto speeds by dividing the distance between the posts by the lag-time between the two photos.If the strobe flashes become acoustic pulses, the cars become reflective particles in the water column, and the pictures become the received reflected signals, this scenario becomes roughly analogous to the workings of a narrow-band ADCP system.The drawback with such a system is that the strobe pulse dissipates very fast and the two photos must be taken within a very short interval (while the same cars are still illuminated by the single strobe). This means that time lags are very short, and the distance traveled by the cars (reflectors) is very short, and therefore, the car speeds cannot be measured very precisely. Because of the above-mentioned limitations, velocity measurements made using the narrow band technology are noisy.A more accurate measurement could be made if the time lag could be longer, however, just like the strobe, the acoustic energy dies out very quickly. The time lag cannot be longer than the pulse length and in fact must be much shorter. But for a more accurate measurement we need to make the time lag longer.V=S/(T2-T1)
14PhasePhase is the fraction of a wave cycle elapsed relative to a point – or when thought of as the wheel on the left, how much it has rotatedUsually ADCPs use PHASE CHANGE to measure the speed, instead of measuring the change in frequency of the wave (how far the cars have travelled)The above animation shows a rotating wheel. On the wheel there is a blue blobwhich goes round and round. When viewed 'flat on' we can see that the blob ismoving around in a circle at a steady rate. However, if we look at the wheelfrom the side we get a very different picture. From the side the blob seems tobe oscillating up and down. If we plot a graph of the blob's position (viewedfrom the side) against time we find that it traces out a sinewave shape whichoscillates through one cycle each time the wheel completes a rotation. Here,the sine-wave behavior we see when looking from the side 'hides' the underlyingbehavior which is a continuous rotation.GIF from
15Phase Change – Like measuring how much the wheels on the cars have rotated Picture 1Picture 2Because we don’t know the direction the cars are traveling, we must account for both positive and negative values (we measure a half rotation either direction)Set the time between pictures (lag) to optimize the tire rotation for the expected speed– Longer time (lag) = more precise measurementToo long of time between picture (long lag) may cause the distance car travels to exceeds a half rotation and result in a measurement error called ambiguity errorShort time (lag) limits precision (increased random noise), but decreases possibility of an ambiguity errorUsually the ADCPs use PHASE CHANGE to measure the speed, instead of simply measuring the change in frequency (or how far the cars have travelled.Phase Change is like measuringBecause we don’t know the direction the cars are traveling, we must account for both positive and negative values (we measure a half rotation either direction)Set the time between pictures (lag) to optimize the tire rotation for the expected speed– Longer time (lag) = more precise measurementToo long of time between picture (long lag) may cause the distance car travels to exceeds a half rotation and result in a measurement error called ambiguity errorShort time (lag) limits precision (increased random noise), but decreases possibility of an ambiguity errorhow much the wheels on the cars have rotated
16What Does This Mean? Lag = Time between pulses in a ping Long lag = accurate measurementsLong lag = low ambiguity velocityExceed ambiguity velocity = ambiguity errorAmbiguity error = inaccurate measurementsLag needs to be optimized based on maximum speedSonTek usually has short lags (no chance for ambiguity errors but noisy – pictures close together and wheel hasn’t turned much in lower velocites)TRDI – usually has longer lags that need to be adjusted for conditions (less noise, but chance for ambiguity errors if not adjusted correctly)So what does all of this mean? What do you need to understand from this information.First long lags lead to accurate measurements. However, in Doppler just like in life nothing is free.Long lags result in a low ambiguity velocity, therefore, limiting the maximum velocity that can be measured.If the ambiguity velocity is exceed, there is an ambiguity error, which will be a significant error in your data and cause inaccurate velocities and maybe discharges.Therefore, we need to optimize the lag (ambiguity velocity) based on the maximum speed of the water and boat.
17Identifying Ambiguity Errors Ambiguity errors are usually very obvious in single ping data.[CLICK]In the stick-ship plot ambiguity errors result in very high velocities often in the wrong direction.A contour plot of the error velocity can also be used to find ambiguity errors. Remember that the error velocity should be randomly distributed, however, it is clear from this plot that there are vertical strips. These are caused by ambiguity errors.
18CorrelationHow well the two pictures can be aligned If there is too long of a time between pictures, cars may be in different locations relative to each other, or the to pictures could contain totally different cars and the distance S may not be determinedSBack to our highway analogy, the correlation corresponds to how well the cars from the two pictures can be aligned.
20TransducersProduce sound waves (pulses) and then listen to returning sound wavesceramic element protected with a urethane coatingADCPs use the same transducer to both transmits and receive the pulses.Hydroacoustic instruments use transducers to transmit and receive sound waves. A transducer consists of a ceramic element that is caused to deform or vibrate by application of an electrical current. The ceramic element is usually protected by urethane. The transducers used in ADCPs are monostatic, meaning they both transmit and receive sound waves.
21Measures Velocity Parallel to Beam Hydroacoustic instruments and only measure velocity of the scatterers parallel to the beam. This is also called a radial velocity.If the scatterers in this diagram are moving in the direction of the large arrow, only the component of the vector parallel to the acoustic beam would be measured.What velocity would we measure if we pointed a transducer straight down in the water column? Only the vertical velocity, because the horizontal velocity would be perpendicular to the beam. This is why the ADCP has multiple transducers that are tilted in different directions.
22Need Multiple Transducer Since each Transducer only measures the velocity component parallel to the beam, multiple transducers are needed4 beams can be resolved into: x, y, z and error velocitiesADCP’s can only measure the velocity components parallel to the beams present in the ADCP. However, if we assume that the velocity is the same in each beam then we can use trigonometric relations to resolve the beam velocities into something more useful to use, velocity in the horizontal (x and y) and vertical (z) directions. Therefore, beam velocities from three beams are necessary. Sontek typically only produces three beam systems and thus can measure the x, y, and z components of the velocity field. RDI however, produces four beam systems. The fourth beam is not required to compute the horizontal and vertical velocities but it does allow the computation of what has been called the error velocity.M9 only uses 4 beams at a time to compute a velocityRiverRay forms 4 beams from the single phased array transducer
23(High error or D velocity) Velocity ErrorsThe difference between beam pair vertical velocities is reported in software as Error or D VelocityThe redundancy within the 4 transducers (only 3 would be needed for 3D vectors) is Since the doppler shift is directional and each beam is only measuring a small component of velocity parallel to the beams, when the radial velocities are resolved into horizontal and vertical components, the assumption is made that all beams are measuring a homogeneous volume of water (seeing the same velocity magnitude and direction). Remember the instrument is looking at the individual beams not the entire volume of water contained inside the boundary of the four beams.What are the potential sources of error?A vortex or eddy in one beamRandom errorsSome instruments have an additional fourth beam. The fourth beam is not required to compute the horizontal and vertical velocities but it does allow the computation of what has been called the error velocity, which is the difference in vertical velocity for the beam pairs.The error velocity gives a an indication of flow homogeneity and is an indicator of the validity of the assumptions used to compute the horizontal and vertical velocity components.?Homogeneous(Low error velocity)Non-homogeneous(High error or D velocity)
24Error Velocity Contour Error velocities should be randomly distributedareas of high area error velocities may occur when water is not flowing at similar magnitudes and direction in all beams (example: turbulence)Error velocities may also be the result of an instrument measuring one beam velocity wrongBehind Bridge Pier
25Types of PulsesHow the pulses are transmitted into the water and sampled can vary and be optimized for the conditionsThis configuration is commonly called “water mode”Some of the newer ADCPs automatically adjust the configurations for the environment on the flyUntil recently the majority of ADCPs currently in use must be set up prior to data collection
26Depth Cells (Bins) Time A B C Distance From ADCP Blank Bin 1 Bin 2 TransmittingBlankingGate 1Gate 2Gate 3Gate 4TimestartendABCechoechoechoechoBlankBin 1cell 1Bin 2cell 2This graph explains how observations are compartmented into bins.Bins cover sections of the probed depth. They represent information about the average velocity for specific areas within the cross-section.In this graph, we see how information for a single transducer is organized to create the Bins.Vertically, the distance from the ADCP increases towards the top.Horizontally, time increases towards the right.In A, the transducers start to transmit signals in the water.The signal travels diagonally in this graph. It gets further away from the ADCP and time goes by. Already, signals are echoed back but the transducers are not listening.In B, the ADCP stops transmitting. A period of rest is then required to let vibrations in the transducers calm down. Only then will they be used to listen. This represents the blanking distance.In C, the ADCP is ready to start listening to echos. All the information obtained after C is then correlated and averaged until Gate 1 is reached. The information for the area covered forms the Bin 1 data.Then, information obtained after Gate 1 is also correlated and averaged until Gate 2. This represent the Bin 2 data.The process continues for the following bins.Bin 3cell 3Distance From ADCPBin 4cell 4
27ADCP’s Profile (Ensemble) Depth CellADCP’s can be deployed from a boat and measure velocity profiles. It is similar to having a whole string of velocity meters deployed. The cup meters measures the velocity in discrete layers. The ADCP performs a similar processes small parts of the entire echo individually thus producing multiple velocity measurements. The layers are frequently referred to as a depth cell or bin.
28ADCP Measured Water Velocity The water velocity measured by an ADCP deployed on a moving boat is the velocity of the water passing by the ADCP, which we will refer to as the relative water velocity (it is relative to the instrument). This relative water velocity includes the velocity of both the boat and water. Therefore, as the boat moves faster, the relative water velocity measured by the ADCP increases. Since we are interested in the actual water velocity referenced to a fixed location, the velocity of the boat must be measured and removed from the relative water velocity measured by the ADCP.The faster the boat travels,the faster the velocity of the water relative to the ADCP.
29Boat Speed (Bottom Tracking) ADCP’s can also measure the speed of the instrument or boat by measuring the Doppler shift of a pulse off of the bottomThis is called bottom tracking and assumes that the streambed is stationarySediment transport on or near the streambed can affect the Doppler shift of the bottom-tracking pulses, which can result in the measured boat velocity being biased in the opposite direction of the sediment movement. This is referred to as a Moving Bed conditionADCP’s can also measure the speed of the instrument or boat by measuring the doppler shift of a pulse off of the bottomThis is called bottom tracking and assumes that the streambed is stationarySediment transport on or near the streambed can affect the Doppler shift of the bottom-tracking pulses, which can result in the measured boat velocity being biased in the opposite direction of the sediment movement. This is referred to as a Moving Bed condition
30Depths Bottom Track pulses are also used to measure depth Typically 4 beam depths are averagedSonTek also can use a vertical beam dedicated to depth
31Acoustic Profiler Discharge Measurement ADCP Discharge MeasurementDischarge is the total volume of water flowing through a given cross-section of water per unit of time. With an ADCP and WinRiver the total volume discharge is computed for each ADCP ensemble (vertical profile) from a moving boat. There is no need to stop and make individual measurements.ADCP Measures Flow through some arbitrary surfaceAn important feature of this system is that discharge can be measured over an irregular path. Instead of measuring flow through a PLANE like we do with Price AA measurements, with an ADCP we are measuring flow through a SURFACE (you may think of it as an irregular shaped weir). If you project the ship track onto the bed, imagine a surface rising vertically above that projected ship track. The SOFTWARE (WinRiver) is computing flow through THAT surface, the hardware measures the velocity, depth, and ship track.A single pass across the river is called a transect, a discharge measurement is usually comprised of multiple transects averaged together
32Computation of Discharge Measured Q = ∑(V x A)V = Velocity perpendicular to boat path for the ensembleA = Depth Cell Size x WidthWidth = boat speed x time since last valid ensembleAssumption made: the measured boat and water velocities are representative of the boat and water velocities since the last valid data. The longer it has been since the last valid data, the greater the error may be in this assumptionThe above is equal to the cross product of the boat and water speed x depth cell size and the time since last ensemble, which is how software computes Q in a depth cellIn this picture, we see rectangles which are the representation of measured Depth Cells.The different colors represent the different velocity of the flow passing through these surface areas.For each of these surface elements, the discharge can then be easily calculated.It is the product of the average flow velocity perpendicular to the surface and the surface area.Q = Velocity x Area.The total Q for the measured area is then the summation of every partial discharge over every bin.But as we will see, there are unmeasured sections, and to obtain the total discharge over an entire cross section, the data that is not available will have to be modeled in order to complete the picture.
33Unmeasured Top and Bottom Unfortunately the ADCP is unable to measure the entire water column.At the top, the ADCP must be immersed in the water and there is a blanking distance below the transducer where data cannot be collected. The blanking distance is a result of the transducers being both the sound source and sink (speaker and microphone). Think of a Chinese gong. When it is stuck it continues to vibrate and send out sound for a period of time. The operation of the transducers are similar. When power is sent to the transducers they vibrate to send the sound into the water column. These transducers cannot be used to listen to the backscattered acoustic energy until the ringing (vibration) has died down to a level that will not contaminate the received acoustic energy. The distance that the sound moves during this period is the blanking distance.The ADCP also cannot measure all the way to the streambed. When acoustic transducers produce sound, most of the energy is transmitted in the main beam. Unfortunately there are also side lobes that contain less energy that propagate from the transducer as well. These side lobes are not a problem in most of the water column because they are such low energy. However, when the side lobe strikes the streambed, the streambed is a good reflector of this acoustic energy and much of the energy is reflect back to the transducer. Because of the slant of the beams the acoustic energy in the main beam is reflecting off of scatters in the water column near the bed at the same time that the side lobe is reflecting from the streambed. The energy in the main beam reflected from these scatters in the water column is relative low compared to the energy sent out from the transducer and the energy in the side lobe returned from the streambed is sufficient to contaminate the energy from the main beam near the bed. Therefore, there is an area near the bottom that cannot be measured due to side lobe interference. This distance is computed as (1-cos(system angle))*100. So for a 20 degree system it is 6% of the range from the transducer.
34Measured and Unmeasured Areas DepthValid Ensembles (Profiles)Top (Estimated) LayerThe middle, light blue is where the ADCP measures velocity and directly computes dischargeThe top layer (gray) is where the velocities can not be measured, due to the ADCP draft and blanking distanceThe bottom layer is estimated do to side lobe interferenceThe edges are too shallow to measure and also need to be estimated.Middle (Measured) LayerBottom (Estimated) LayerEdges (Estimated)
35How To Estimate Top and Bottom Q? Q in the top and bottom unmeasured areas is estimated for each ensemble, based on the measured dataThe typical method is to use a power fit of the measured data, but other options are available when this is not validVelocity cross product (X-value), ft2/s2f1f3f4f5fnf2MeasuredEstimatedPower fitFree surfaceDistance from the bed (Z), feetWhat assumption could we make to estimate the top and bottom discharges? What assumption is made in cup meter measurements?We could assume a logarithmic velocity profile. [CLICK] [CLICK] An alternative is to use the 1/6th power law is essentially the same as the logarithmic profile but is computationally easier to fit to the data.This provides a much better estimate of the discharge in the bottom unmeasured area.The ADCP software defaults to a 1/6th power, but depending on the shape of the measured data other powers can be entered by the user.There are other methods that are software specific and will be discussed later.This
36Power Curve Limitations Unidirectional FlowBi-directional FlowDistance from the bed (Z), feetDistance from the bed (Z), feetUsing the power curve in unidirectional flow conditions is very common, however, using the power fit in bidirection flow (salinity, temperature, and wind-induced) condition can produce erroneous results. [CLICK] The power curve does not fit the data at all and even predicts a bottom discharge with the wrong sign.(-)(+)(-)(+)Velocity cross product (f-value), ft2/s2
37Examples of profiles affected by Wind Range from BottomRange from BottomWater VelocityWater VelocityDepending on direction, wind can either cause the profile to bend either way at the water surfaceThe magnitude of this may cause the standard power fit to be a poor choice for top extrapolation, in this case the software has options to only use data near the surface for estimating the top Q
38Estimating Shore Discharges MEASURE the edge distance.Average multiple ensembles to get an accurate depth and velocityMeasured by ADCPdm=last measured depthdmVm = last measured velocityVmL = distance from last ensemble to edge of waterLMeasured by UserThe averaged measured velocity is multiplied by the averaged measured depth, the measured length, and finally by a coefficient to account for the shape of the edge (.35 for triangle and .91 for square)It is recommended that when the depths near the shore become too shallow for two good bins in the profile that the transect be stopped and the discharge in the edge be estimated from the available information. So what information is available? [CLICK] the ADCP has the depth of the last ensemble [CLICK] and the velocities in the last ensemble [CLICK]. To ensure accurate depths and velocities it is recommended that the boat be held in position and 10 ensembles be collected, [CLICK] which will be automatically averaged in WinRiver. We also have access to a value that [CLICK] can be measured by the user, [CLICK] the distance from the end of the transect to the edge of water. [CLICK] it is important that this distance be measured. Visual estimates are typically too short and will lead to inaccurate discharges.[CLICK] Now what else to we know? First, just like at the bottom [CLICK] we know the velocity has to go to zero at the bank. Second, [CLICK] if we assume either a triangular or rectangular edge we can compute the missing area using the depth from the ADCP and the distance we measured to shore. Third, we know that Q=AV.
39Online Training Class SW1271 Ideal ReachThe diagram summarizes attributes of an ideal measurement section, as previously described. Ideally, the measuring section should be 5 channel widths below the most upstream riffle and approximately 2 channel widths above the controlFrom: Water Resources Investigations Report By K. M. Nolan and ShieldsOnline Training Class SW1271
40Computation of Total Discharge Top QLeft QMiddle QRight QWe have explained how the “Middle Q” or measured discharge is computed and how the unmeasured discharge information is modeled and estimated. Throughout our exploration of the process, we have seen that even though the analysis of data requires advanced physics, the application of the principles for the discharge computation is relatively simple.The ultimate goal of this presentation was to understand how Acoustic Doppler Current Profilers (ADCPs) acquire and process data to measure discharge. The last remaining step is the computation of the total discharge and this is straight forward:Once the middle section data is obtained and once information about the cross section edge shape, distances to shore and instrument draft is provided, the software can then compute the sum of all discharge sections for the cross section. This value is a simple summation of the Left, Right, Top, Bottom and Middle Qs.Based on this review of ADCP principles, it should now be easier to understand the implications of ADCP implementation procedures described in other lessons and ultimately get the maximum precision from this instrument.Bottom QTotal Q = Left Q + Right Q + Top Q + Bottom Q + Middle Q
41Online Training Class SW1271 Where ….Ideal ReachReach - Straight and uniform for a distance that provides for uniform flowStreambed - stable free of large rocks, weeks or other obstructionsA poor cross section = poor measurement regardless of the accuracy of your point velocitiesThe diagram summarizes attributes of an ideal measurement section, as previously described. Ideally, the measuring section should be 5 channel widths below the most upstream riffle and approximately 2 channel widths above the controlFrom: Water Resources Investigations Report By K. M. Nolan and ShieldsOnline Training Class SW1271
42Site Selection Still Critical Reach - Straight and uniform for a distance that provides for uniform flowStreambed - stable free of large rocks, weeks or other obstructionsA poor cross section = poor measurement regardless of the accuracy of your point velocities