Discharge measurements with ADCP

Follow the following USGS QAQC procedures (from USGS 2005): USGS SIR 2005-5183 QAQC Report

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 gagingstation 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 nearby bridges or buildings) or where reception of signals from GPS satellites is blocked.

Additional Equipment needed:

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