When that doesn’t work, we look for ways to fix the hydraulics. (equalization, pump cycles, gate/valve position, et al). Simple changes in operation are often enough to minimize frequency, or magnitude, of submergence.
In this case, even as actual flow approaches zero, a typical flow meter is sensing peak flow unaware of the downstream condition. Here, we find the flume completely submerged. Even at the lower flow conditions shown here, the tailwater is already a gathering concern.
In this photo, we see clear evidence of substantial submergence on the sidewall. In all Parshall Flumes above 10 feet, transition submergence is 80%. Parshall Flumes with a throat section (W) from 1 inch to 8 feet, this ratio varies from 55% to 78%.
#Is flume free free
The point at which the flow changes from free flow to submerged flow is called transition submergence and is expressed as a percentage, which is the ratio Hb/Ha. Submergence in Parshall flumes is identified with a ratio of two level measurements expressed as: xx % = Hb/Ha. While most of the flumes we visit operate under free flow conditions, those cases of submergence we have found were almost always unidentified much less compensated for, or remedied. Identifying Submergence in Parshall Flumes Too, this error tends to be cumulative over time often adding up to huge differences over a week/month/year. A submerged flume problem can quickly drive daily totals out of balance. When a plant’s total influent and effluent delta is much more than can be explained in the treatment process, we begin to suspect submergence. On several inspections, we’ve found a flume operating in submerged conditions some continually …others just during prolonged periods of peak flow. We often hear of plants where the total flow is “out of balance” with more total influent than total effluent or the other way around. If the flume is in submergence, most flow meters will detect an increase in level, but not the decrease in velocity causing it to over-report. When flow cannot exit the flume quickly enough, water pushes back upstream potentially to the free flow level measurement location known as Ha. Submergence occurs “when downstream conditions are such that the flow conditions upstream are affected”… ¹ We use their findings to identify submergence, determine the impact on flow accuracy, and to help with a remedy.ĭischarge through a Parshall flume is considered “ free flow when the tailwater (or downstream) conditions do not affect the flow conditions upstream and the discharge may be evaluated by measuring only one depth of flow, the upstream depth” (Ha). The bulk of the research on submerged flow in Parshall flumes comes from Water Research Laboratory at Utah State University. Luckily, the Parshall flume is the exception. While any primary device could be affected by submerged flow conditions, there isn’t much data to be found on the topic. As often as we use Parshall flumes for open channel flow measurement, it is important to understand “free versus submerged flow” conditions as it greatly impacts the accuracy of the installation…or the lack thereof.
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