Discharge measurements with ADCP

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SW1321 Streamflow Measurements Using ADCPs Training Course


Additional Equipment needed:

  • Laser Rangefinder to measure shore distances,
  • Field Book for note taking,
  • Handheld radios for communications.


Data you will need to know before making a measurement:

  • Transducer depth below water surface
  • Maximum water depth and velocity
  • Magnetic declination (see NOAA)

Notes you should take in the field:

  • General cross section shape (record in field notes, needed for estimating flow along edges)
  • Bank shapes
  • Flow description (steady/unsteady, turbulence, eddies, slack water, reverse flows, etc).
  • Start and stop points (distance to shore)
  • configuration

Required steps for QAQC:

Other notes

  • RTK-GPS is recommended, but check the GGA string to make sure it is working (5 for Novatel RTK, 9 indicates WAAS is enabled)
  • ADCP baud rate should be 115K, GPS baud rate is set at 39K. Initially (upon receiving the purchase of a new instrument), the Rio Grande baud rate needs to be programmed using BBTalk or equivalent.
  • Measure (not estimate) your edge distances w/ laser rangefinder or tape measure. Describe the edge (triangle or square?) in order to get the best estimate of flow in the edge.
  • Measure your ADCP transducer depth below water surface with ruler or tape measure and keep an eye on it during the measurement to see if it changes.
  • Beam 3 forward or offset 45 deg. If offset put a note in your field book and need to enter this into Winriver II. Note: The new streampros (w/ compass) need to be in the proper orientation at all times. The arrow indicates the forward direction and it is 45 degrees offset. The power and communication unit needs to be oriented forward as well (as it is in a trimaran set up), if not you can apply a 180 degree correction. Make a note of all orientations.
  • Gage height. Record the gage height before and after the measurement to make sure there is no big changes in stage/flow during measurement. If there is no staff gage at the location of discharge measurement, install a temporary rebar in the river and do a tape down measurement before/after
  • Water temperature. Field check the water temperature with a portable probe at the location of your measurement to make sure the adcp temperature is correct.
  • Use WinRiver II to QA/QC discharge measurement in the field!
  • For RDI ADCPs The water modes / cell size configuration are a trade off between range, random noise and resolution. Small cells will be noisier but have good resolution. Large cells will give you less noisy data, but lower resolution.

Follow the following excerpt from USGS QAQC procedures by Oberg et. al.: USGS SIR 2005-5183 "Quality Assurance Plan for Discharge Measurements using Acoustic Doppler Current Profilers"

"It is important to select appropriate measurement cross sections for streamflow measurements. Many ADCP measurement problems can be solved by moving to a better measurement section. The guidelines provided in USGS Water- Supply Paper 2175 (Rantz and others, 1982, p. 139) should be followed when using an ADCP, except for those guidelines that relate to depth and velocity requirements for current-meter measurements. Rantz states: “The first step in making a conventional current-meter measurement of discharge is to select a measurement crosssection of desirable qualities. If the stream cannot be waded, and high-water measurements are made from a bridge or cableway, the hydrographer has no choice with regard to selection of a measurement cross-section. If the stream can be waded, the hydrographer looks for a cross-section of channel with the following qualities:

1. Cross-section lies within a straight reach, and streamlines are parallel to each other.

2. Velocities are greater than 0.5 ft/s (0.15 m/s) and depths are greater than 0.5 ft (0.15 m).

3. Streambed is relatively uniform and free of numerous boulders and heavy aquatic growth.

4. Flow is relatively uniform and free of eddies, slack water, and excessive turbulence.

5. Measurement section is relatively close to the gaging station control to avoid the effect of tributary inflow between the measurement section and control and to avoid the effect of storage between the measurement section and control during periods of rapidly changing stage.


It will often be impossible to meet all of the above criteria, and when that is the case, the hydrographer must exercise judgment in selecting the best of the sites available for making the discharge measurement. If the stream cannot be waded and the measurement must be made from a boat, the measurement section selected should have the attributes listed above, except for those listed in item 2 concerning depth and velocity.”

General guidelines for selection of an ADCP measurement section are listed below.

• Desirable measurement sections are roughly parabolic, trapezoidal, or rectangular. Asymmetric channel geometries (for example, deep on one side and shallow on the other) should be avoided if possible (Simpson, 2002), as should cross sections with abrupt changes in channel-bottom slope. The streambed cross section should be as uniform as possible and free from debris and vegetation or plant growth.

• Measurement sections with velocities less than 0.30 ft/s should be avoided if it is possible to do so, and an alternative measurement location is available. Although measurements can be made in low velocities, boat speeds must be kept extremely slow (if possible, less than or equal to the average water velocity) requiring special techniques for boat control (Simpson, 2002).

• Depth at the measurement site should allow for the measurement of velocity in two or more depth cells at the start and stop points near the left and right edges of water.

• Sites with very turbulent flow, for example, evidenced by standing waves, large eddies, and non-uniform flow lines, should be avoided. This condition is often indicative of non-homogenous flow, a condition that violates one of the assumptions required for accurate ADCP velocity and discharge measurements.

• Measurement sections having local magnetic fields that are relatively large as compared to the Earth’s magnetic field should be avoided. Large steel structures, such as overhead truss bridges, are a common source for these large local magnetic fields and may result in ADCP compass errors.

• When using DGPS, avoid locations where multi-path interference is possible (signals from the satellites bounce off structures and objects such as trees along the bank) or where reception of signals from GPS satellites is blocked."

Other notes... from Bob

  • See some cataraft pictures here:

http://werc.engr.uaf.edu/~bob/discharge/2011_Itkillik_ADCP_Setups/

  • Decent NRS Strap layout for securing the cataraft frame to the pontoons:

http://werc.engr.uaf.edu/~bob/discharge/field_book_2011_may/Busey_Umiat_2011-pg4_5.jpg Further Notes from Bob written in June 2011. Specifically about the Upper Itkillik at break up but probably useful more generally.

  • We found it good to have a third cone at the downstream don't go below here point. If the water is big and fast and you need to do an open loop that finishes below, More cones along the bank would be helpful to guide to.
  • Turns out not all the time is it good to go with the current. Standing waves are to be avoided if at all possible (more of these on the left bank of the Itkillik at break up).
  • Still need to figure a good way to catch the edges in moving water where no one can get out. Typically we think of that in the bad distance to shore number but spending a couple minutes at the edge trying to get into proper position is another piece of error.
  • In turbulent water like the day after peak I think but can't say for sure that less of a diagonal might be better. I don't know though. Waves are tricky and especially with the engine variability.
  • With the wave you lose ensembles first to the off kilter adcp and then again when the engine changes speed to compensate for the new water. So, a single bad ensembles quickly grows to several.
  • So, do the diagonal to minimize time in the standing wave. (Much easier said than done). In the bow, I almost never noticed the standing waves except in the bad ensembles.
  • In the bigger water we found it worked well to have the discharge book on shore and have the computer person call out just the left bank and right bank numbers to person 3. Once the river slowed, the book moved to the boat for a couple days but it was convenient to work the radios on the Itkillik.
  • Get a bluetooth keyboard/touchpad combo and put it in an aloksak, hopefully one that can have a string tied onto it.

http://werc.engr.uaf.edu/~bob/discharge/2011_Itkillik_ADCP_Setups/large-6.html

  • For achilles and plan b, it'd be nice to be able to tilt the computer screen at an angle.

Idle Brainstorming about the difficulty of seeing the laptop screen out on the water:

  • Open Question: would a glass cleaner be suitable for keeping the laptop drybag clear?
  • how about a pelican case with connections epoxied in place rather than the dry bag... Not sure about this one but I think the rover set up could definitely be improved.
  • If we could figure out how to do it safely without the danger of losing a computer to the drink, the actionpacker was the easiest way to run everything the scrren although... The shape isn't the greatest, the slender base compare to the top makes it want to slide off the knees.
  • Swivel chair is key for the cataraft.
  • For engine on cataraft bow pipe is raised fifteen to thirty degrees from horizontal and the bar goes two to four inches past the black rugged tube surface
  • for rowing in this configuration, port oar blade is in stern, starboard oar is in the bow. In an emergency, the driver will hand the starboard oar to the computer person and the computer person will swing around to face downstream and grab the port oar.
  • for rowing in general, we found shifting the rowing frame back four to six inches (just a couple inches onto the black for the bow portion of the frame). Also, it was fine rowing in the motor position.
  • with row-only, the driver and computer person trade places. I found myself as the computer person not getting fully onto the frame, just the upper body to work the computer. Legs and all were stretched back on the end of the starboard pontoon.
  • with a motor set up, the third person will push the party out. With the rowing configuration, the computer person can hold the cataraft in position for the edge bins and then hop on for the transect proper (this is where the value comes in for not getting all the way onto the frame). At the transect conclusion, the computer person hops out, holds for ten bins and stops the transect.
  • It took a bit of figuring out (didn't do this method on the first 5/31 transect) but this procedure really limited edge estimate error I think among other things.
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