Chapter 4 - SELECTION OF WATER MEASURING DEVICES

4. Selection Guidelines

Selection of a water measurement method can be a difficult, time-consuming process if one were to formally evaluate all the factors discussed above for each measuring device. Of course, this difficulty is one reason that standardization of measurement devices within a district is so popular. However useful devices are sometimes overlooked when similar devices are automatically selected. The purpose of this chapter is to provide some preliminary guidance on selection so that the number of choices can be narrowed down before a more thorough analysis of the tradeoffs between alternatives is performed.

(a) Short List of Devices Based on Application

Site conditions for a water measurement device quickly narrow the list of possible choices, because most devices are only suitable under a limited number of channel or conduit conditions. Table 4-1 provides a list of the most commonly used measurement methods for each of several applications.

Table 4-2 provides an abbreviated table of selection criteria and general compliance for categories of water measurement devices. The symbols (+), (0), and (-) are used to indicate relative compliance for each selection criteria. The (+) symbol indicates positive features that might make the device attractive from the standpoint of the associated selection criteria. A (-) symbol indicates negative aspects that might limit the usefulness of this method based on that criteria. A (0) indicates no strong positive or negative aspects in general. A (v) means that the suitability varies widely for this class of devices. The letters (na) mean that the device is not applicable for the stated conditions. A single negative value for a device does not mean that the device is not useful and appropriate, but other devices would be preferred for those selection criteria.

Table 4-1

Application-based selection of water measurement devices

1. Open channel conveyance system
1. Natural channels
1. Rivers
1. Periodic current metering of a control section to establish
2. stage-discharge relation
3. Broad-crested weirs
4. Long-throated flumes
5. Short-crested weirs
6. Acoustic velocity meters (AVM - transit time)
7. Acoustic Doppler velocity profiles
8. Float-velocity/area method
9. Slope-area method
2. Intermediate-sized and small streams
1. Current metering/control section
2. Broad-crested weirs
3. Long-throated flumes
4. Short-crested weirs
5. Short-throated flumes
6. Acoustic velocity meters (AVM - transit time)
7. Float-velocity/area method
2. Regulated channels
1. Spillways
1. Gated
1. Sluice gates
2. Radial gates
2. Ungated
1. Broad-crested weirs (including special crest shapes, Ogee crest, etc.)
2. Short-crested weirs
2. Large canals
1. Control structures
1. Check gates
2. Sluice gates
3. Radial gates
4. Overshot gates
2. Other
1. Long-throated flumes
2. Broad-crested weirs
3. Short-throated flumes
4. Acoustic velocity meters
3. Small canals (including open channel conduit flow)
1. Long-throated flumes
1. Broad-crested weirs
2. Short-throated flumes
1. Sharp-crested weirs
3. Rated flow control structures (check gates, radial gates, sluice gates, overshot gates)
4. Acoustic velocity meters
5. Other
1. Float-velocity area methods
4. Farm turnouts
1. Pipe turnouts (short inverted siphons, submerged culverts, etc.)
1. Metergates
2. Current meters
3. Weirs
4. Long-throated flumes
5. Short-throated flumes
2. Other
1. Constant head orifice
2. Rated sluice gates
3. Movable weirs
2. Closed conduit conveyance systems
1. Large pipes
1. Venturi meters
2. Rated control gates (orifice)
3. Acoustic velocity meters (transit time)
2. Small and intermediate-sized pipelines
1. Venturi meters
2. Orifices (in-line, end-cap, shunt meters, etc.)
3. Propeller and turbine meters
4. Magnetic meters
5. Acoustic meters (transit-time and doppler)
6. Pitotmeters
7. Elbow meters
8. Trajectory methods (e.g., California pipe method)
9. Other commercially available meters

The process of narrowing down options might start with table 4-1 to examine the main methods to consider. Table 4-2 can then be used to get an idea of the general positive and negative features of various methods. In narrowing down the options, different applications will place different weight on the selection criteria, so no universally correct selection exists. Finally, a preliminary design for several candidate methods selected should be performed so that details on cost, hydraulics, operations, etc., can be more thoroughly examinedCfollowed by final selection, design, and construction.

(b) Example

We want to measure the flow entering a small farm turnout ditch that serves an agricultural field. The ditch is trape-zoidal, concrete-lined and has a rectangular metal sluice gate that is opened by hand to divert flow into the ditch from a canal lateral. No power is available at the site. The ditch carries a flow of about 10 cubic feet per second (ft³/s). Field survey measurements taken during an irrigation indicate about a 0.75-ft drop in the water surface from the gate to the downstream channel. The irrigation flow transports fine sediment and numerous tumble-weeds. Water is diverted to the field on a 2-week rotation for a period of about 24 hours. The measurement device will be used to establish a known flow rate through the headgate for crop yield management and water use accounting. Typically, the water surface in the lateral remains fairly constant during an irrigation; therefore, a single measurement per irrigation will meet current needs. However, in the future, more frequent measurements may be desired. The irrigator would like to install a device that costs less than $500.

Table 4-1 identifies a number of devices that are typically used for farm turnouts. Our site requires we select a device or method that can be used in an open channel. Therefore, common measurement devices given for this application are: current meters, weirs, flumes, and rated sluice gates (headgates). Next, the advantages or disadvantages for each of these devices should be considered with respect to the measurement goals and the site conditions. Table 4-2 is used to assist in comparing the attributes of devices. Typically, only a few selection constraints are high priorities. The selection priorities for the example are likely: meeting available head, cost, accuracy, and debris passage goals. Head loss is the highest priority because it is a physical constraint of the site that must be met to provide good measurement. Current meters provide the least head loss followed by long-throated flumes (including broad-crested weirs), short-throated flumes, and sharp-crested weirs. Sluice gates rate low in terms of head loss; however, for this application, the gate is part of the site and will not provide additional head loss. Based on our highest priority, current metering, a long-throated flume or rating the headgate are good choices. Next, consider the cost of devices including: initial cost, data collection time, and maintenance. Rating the headgate and a long-throated flume are considered to be a lower cost than current metering largely because of the time involved in data collection. Accuracy of measurement and debris passage favor a long-throated flume.

This example selection process identifies a long-throated flume as potentially the best device followed by rating of the headgate. These two methods of measurement are recommended for additional detailed design and evaluation prior to the final selection.