Flow Measurement with a Venturi Flume Under Submerged or Unsubmerged Conditions
* Can accurate water level measurement technologies be developed that will enable practical field use of submerged venturi flumes for measuring flow at limited-head-availability sites using either physical or electronic level sensing techniques?
Need and Benefit
Obtaining an accurate discharge measurement in open channels at low-head-availability sites where critical flow measurement devices cannot function properly has historically represented a costly and time consuming task. As canal operators seek to upgrade their water management capabilities, the ability to measure flow throughout the system--frequently including low-head-availability sites--is a key requirement. Until recently, stream gaging has been the predominant means by which discharge is measured at many sites that are not suitable for a critical-flow measurement structure.
A growing list of products featuring acoustic doppler profiling (ADP) technology are now becoming available that offer the ability to measure flow with varying degrees of accuracy. Unlike critical flow measurement structures, ADP flow meters create no head loss. They are a viable option for measuring flow at low head availability sites. ADP products are available over a wide price range. The high-end ADP systems with more features are economically practical only for key primary measurement sites at some of the bigger irrigation districts. Reclamation personnel who have field experience with ADP equipment in the lower cost range report that a significant calibration effort is required for these units--often accomplished by stream gaging or other means of measurement of modest accuracy. ADP equipment can be characterized as an emerging technology that one Reclamation water specialist has termed "a complex electronic device." The observed consistency and reliability in performance of the ADP equipment in the field to date has been mixed.
Like the ADP equipment, a venturi flume can be used to measure open channel discharge under either submerged or unsubmerged conditions, provided flow depths can be determined with a sufficient degree of precision. With a venturi flume, there is a physical relationship between discharge and the flow depths being measured in the upstream and the throat sections of the venturi flume. Knowing the two flow depths and the flume geometry, an analytic solution for discharge exists. Flow rate is obtained by solving a combination of the energy and continuity equations.
In 2004 study at Reclamation's Water Resources Research Laboratory, accurate discharge measurements with a submerged venturi flume were obtained from water levels measured with a point gage (level resolution = 0.001 foot) using a methodology that took up to 30 minutes per discharge reading. This study confirmed the viability of measuring discharge using a submerged venturi flume; however, the measurement techniques employed would be impractical for most field applications.
The focus of the current proposal would be to identify robust, practical and cost effective means of measuring both upstream and throat water levels--with the required degree of precision--to measure flows accurately with a submerged venturi flume under field conditions. The goal is to identify/develop both electronic and manual level measurement methodologies suitable for this application. With development of this level measurement technology, a submerged venturi flume could offer cost savings and improved performance reliability over other available methods for measuring open-channel discharge at low-head-availability sites.
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