The top surface of the piers under the crest of flume should be level, such that when the flume is resting on them, it will be at the correct elevation. If no blocks are available, pour piers (perpendicular to the flow) under the area where the flume is to be located. Block the underside of the flume such that the crest is level and at the correct elevation. Large flumes may also require blocks or other support on the underside of the flume to support the interior weights during installation. This usually requires at least 18" on each side. Consideration should be given to the following suggestions concerning flume installation.Ĭhannel or vault should be designed with adequate clearance at the sides to allow grout placement and worker access for chaining during installation. They will not prevent a flume from floating during installation. The anchor clips on the flume exterior are designed to secure the flume against the grout once it is cured.
This usually results in less than optimum dimensional accuracy at the metering station. The buoyant forces of grout can cause a flume to float during installation. Large flumes have large flat bottoms that require particular attention during installation. The positioning procedure described above can also be used to secure larger flumes in the correct position. The finished surface should be sloped toward the flume so that any water will drain back into the flume. The first pour of grout should just cover the bottom of the flume and allowed to set before additional pours are made. After the all-thread rods are firmly anchored in place, the flume can then be locked in place and leveled by using a nut and washer on both sides of the anchor clips. Either grout the rods in place below the flume or drill holes and epoxy them in place. Very small Parshall flumes can be set in place as follows:Īll-thread rods can be embedded in the concrete below the flume, with the rods aligning with the anchor clips on the flume's exterior. The flume's internal dimensions must not be distorted due to grout pressure.The flume must not float out of position due to grout pressure.The flume must be installed with the flume's crest.The flume must be installed at the elevation specified by the engineer.
When setting the flume in concrete, pay attention to the following points: Submerged flow conditions are usually avoided to allow use of the standard discharge tables and depth measurements at Ha only. Observe elevations during design & installation so that modular flow, also known as "Free Flow" conditions are always present. The standard flow tables must be corrected when the Submergence Ratio, Hb/Ha, expressed as a per cent, exceeds the following values: Submerged flow conditions occur when the resistance to flow in the downstream channel becomes sufficient to reduce the velocity, increase the flow depth, and cause a backwater effect at the Parshall flume.
A large fall or steep slope immediately downstream of the metering station can eliminate the possibility of submerged flow conditions. Long, narrow, flat or undersized channels can result in a backwater effect at the flume and should be avoided. Downstream channel: The downstream channel should not permit submerged flow conditions to occur.Crest of the flume: The crest of the flume (the floor of the converging section where depth measurements are made) must be level both longitudinally & transversely.A 1:4 sloping ramp upstream should be provided for flumes that must be installed above the channel floor. A slope of almost flat, to 3% maximum, for very small flumes, and 2% maximum for larger flumes is the ideal slope value. The upstream channel slope should not allow excessive velocity at the flume. Channel turns, tees, elevation drops or other obstructions should be avoided. Upstream conditions: Upstream conditions should promote laminar flow conditions at the flume inlet.What are the three areas you should be concerned with for successful installation of a Parshall flume? Flumes are restrictions in the channel and consideration should be given to the effect of the resulting backwater on upstream drains and channel walls or banks. In general, the smallest flume of adequate capacity is selected. Sizing of flumes is based on anticipated normal and maximum flows. What size Parshall Flume should I purchase?
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