Chapter III - Open-Channel Flow Measurement

Abstract

Large dimensions and depth make flow measurement in open channels complex. The dimensionless Froude number classifies flow into subcritical, critical (wave celerity), and supercritical flow. Various methods, such as hydraulic radius slope, area velocity method, volumetric tank, tracer method, and open channel flow measurement by hydraulic structure are discussed in this chapter. The selection of a primary flow device and installation of these are also covered. Fluid mechanics, including Bernoulli's energy equation, Manning, Chezy–Kutter's, and Darcy–Weisbach friction equation are discussed at length to form the basis of selection and to establish the discharge (including Shen) equation. Flume and weir are used as primary flow element. There are various kinds of flumes, including rectangular, trapezoidal, U, Venturi, Parshall, Palmer–Bowlus, cutthroat, and H flumes. Different weirs, including rectangular, Cipoletti, broad-crested, overshoot gate and Sutro weirs are discussed. For each of these, discharge equations have been established for both modular and submerged flow conditions. From the discharge equation it is established that discharge is related to the upstream level/head for open channel flow. However, for submerged flow, downstream head measurement is necessary. The head can be detected by level measurement or by measuring hydrostatic pressure in an open channel/still well. Mechanical level, sensing staff gage, ultrasonic level and velocity sensing, online pressure-sensing, including the bubbler method and pressure transmitters are commonly used instruments. All these are discussed in this chapter.