What is the cheapest flow measurement device to install, that is durable, accurate, and easy to maintain? The California Department of Water Resources (CDWR) Red Bluff office wrote a memo in October of 2010, detailing findings on flow measurement devices over decades of watermaster service. Part of the memo is summarized here – not direct quotes:
Economics of Flow Measurement Devices
CDWR watermasters helped water right holders to install and operate a variety of structures over eight decades. Physical factors such as flow range, ditch width and slope, soil type, whether or not a site is protected from livestock, and access to the site, play a large role in picking the right device. Cost is also important, including design, installation, replacement, operation, maintenance and repair.
The following chart shows the relative costs that the watermaster service and water right holders have experienced for various devices. The chart shows that pre-built weirs are the least expensive to install and repair. Flumes are the least expensive to operate, but are the most expensive to design and construct or install.
Some considerations for the proper use of weirs are:
- Always seal leaky flashboards with sediment, horse manure if available, and if necessary, sheet plastic. Use newer flashboards, replaced every year or two, and trim boards on site to ensure a level top board, and reduce gaps prior to use.
- 1.5 inch (2″ nominal lumber e.g. 2″ x 8″) flashboards are not measurably less accurate than a thin steel plate metal with sharp edges, provided that the head on the weir is great enough to cause separation of the water from the upstream edge of the flashboard. The air bubble on top of the board is easy to see when the weir is working right. The minimum required head is around 0.25 feet. As an example, CDWR watermasters compared the use of flashboards to
sharp-crested weirs on November 20, 2008, as they have done several times in the past. Differences were recorded and weirs with flashboards had flows that were both slightly less and slightly more than the flows calculated for sharp-crested weirs. Any difference is well below the margin of error when taking into account all possible errors. In summary, 1.5-inch flashboards provide results indistinguishable from sharp-crested weirs for the use of measuring diverted flows.
- CDWR designed the Briggs-manufactured Twin-Track Weirs© with two flashboard slots. The upstream flashboard slot holds the nominal 2-inch wide weir flashboards. The downstream slot provides the air gap for the nappe, as specified in the USBR Water Measurement Manual, 3rd Edition, 2001, in Figure 7-8 on Page 7-13.
- CDWR measures weir heads with weir sticks, as specified in the USBR Water
Measurement Manual, 3rd Edition, 2001, on Page 13-4. CDWR has checked the use of weir sticks numerous times and found the difference between the head measured with a weir stick and that at a staff gage nearly always to be 0.00 feet (the same to the hundredth of a foot).
- Even when diverters are careful, the amount delivered may occasionally be somewhat more than the water right as streamflows vary due to diurnal fluctuation, changes in upstream diversions, and from increased flows due to storm events. However, more than half the time when flows are not at the legal amount, they are less than the water right amount for the following reasons:
- Floating debris sometimes accumulates on the upstream side of a diversion headgate, reducing flow.
- Water right holders who grow hay shut off their diversions to cut, dry, bale and haul hay for several days, reducing their total volume of diversion.
- Irrigation season damage to ditches requires diversions to be reduced or stopped during repairs.
- Weed growth or sediment in the ditch reduces the hydraulic capacity of ditches.
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