Open channel flow of water has been used in aquaculture production for many years. Distribution canals, raceways and drainage ditches are some examples. Since the beginning of civilization man has been interested in flow in open channels.
Attempts to record the levels on the Nile River date back to 3500 B.C. In 52 A.D. Sextus Julius Frontinus as Water Commissioner of Rome attempted to determine the quantity of water delivered to each user by measuring the cross-sectional area of each discharge spout.
The technology available today is much more accurate but, for the most part, is an adaptation of these earlier concepts. The flow of water in open channels can be an effective and efficient way to move water for aquaculture.
Often, the large flow rates needed in aquaculture require large, costly piping systems. Open channels offer an alternative for movement of large flows over long distance if the land slope is appropriate and water losses to evaporation and percolation are acceptable.
Open channels should be designed and maintained to handle the needed flow. This fact sheet will discuss the design of open channels and measurement of flow in open channels.
Methods of determining flow in open channels:
By definition, an open channel flow is flow in any channel in which the liquid flows with a free surface. There are several ways to determine flow in open channels. Some are discussed below.
Time gravimetric:
With this method the entire stream flow is collected in some type of container for a measured length of time. The flow rate is calculated by dividing the volume of water collected by the time to collect it. This is the “bucket and stop watch” technique. Most practical considerations limit the use of this method to low flow rates.
In aquaculture uses, if the container is large enough, i.e., a pond of known volume, the technique may have practical application.
Dilution:
With this procedure the flow rate is measured by determining how much the flowing stream dilutes an added tracer solution, usually a radioactive material or fluorescent dye. The dilution technique uses theoretical formulas to indicate stream flow and does not alter the flow. A disadvantage of this method is that it requires costly equipment that is not very rugged in field use.
Velocity area:
In this method, the flow rate is calculated measuring the crosssectional area of the channel and multiplying that area by the mean flow velocity across the area.
Hydraulic structure:
In this method some type of hydraulic structure is placed in the stream. The purpose of the hydraulic structure is to produce a relationship between the liquid level (head) and the flow rate of the stream. These structures are normally either weirs or flumes as shown in Figure 1.
Restrictions caused by weirs or flumes will alter the flow of the stream.
Slope-hydraulic radius:
In this method the slope of the water surface, the cross-sectional area and the wetted perimeter over a length of uniform section of the channel are used to measure flow using a resistance equation such as Manning’s Formula.
The wetted perimeter is the length of wetted surface of the cross-sectional area and is an indicator of the efficiency of the channel shape and therefore the resistance to flow. The Manning Formula requires a knowledge of the crosssection of the channel, the slope of the channel, the water depth and a roughness factor dependent on the channel surface.
Authors:
J. David Bankston, Jr. and Fred Eugene Baker
