Parshall Flume Installation
Sizing of flumes is based on anticipated normal and maximum flows. In general, the smallest flume of adequate capacity is selected. 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.
What are the three areas you should be concerned with for successful installation of a Parshall flume?
- Upstream conditions: Upstream conditions should promote laminar flow conditions at the flume inlet. Channel turns, tees, elevation drops or other obstructions should be avoided. The upstream channel slope should not allow excessive velocity at the flume. A slope of almost flat, to 3% maximum, for very small flumes, and 2% maximum for larger flumes is the ideal slope value. A 1:4 sloping ramp upstream should be provided for flumes that must be installed above the channel floor.
- 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.
- Downstream channel: The downstream channel should not permit submerged flow conditions to occur. Long, narrow, flat or undersized channels can result in a backwater effect at the flume and should be avoided. A large fall or steep slope immediately downstream of the metering station can eliminate the possibility of submerged flow conditions.
What is submerged flow?
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. The standard flow tables must be corrected when the Submergence Ratio, Hb/Ha, expressed as a per cent, exceeds the following values:
50% for 1", 2" & 3" flumes
60% for 6" & 9" flumes
70% for 1 Ft. through 8 Ft. flumes
Observe elevations during design & installation so that modular flow, also known as "Free Flow" conditions are always present. Submerged flow conditions are usually avoided to allow use of the standard discharge tables and depth measurements at Ha only.
Small Parshall Flume Installation
When setting the flume in concrete, pay attention to the following points:
- The flume must be installed at the elevation specified by the engineer.
- The flume must be installed with the flume's crest
- The flume must not float out of position due to grout pressure.
- The flume's internal dimensions must not be distorted due to grout pressure.
Very small Parshall flumes can be set in place as follows:
All-thread rods can be embedded in the concrete below the flume, with the rods aligning with the anchor clips on the flume's exterior. Either grout the rods in place below the flume or drill holes and epoxy them in place. 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. The flume can then be grouted in place. The first pour of grout should just cover the bottom of the flume and allowed to set before additional pours are made. The finished surface should be sloped toward the flume so that any water will drain back into the flume.
Large Parshall Flume Installation
The positioning procedure described above can also be used to secure larger flumes in the correct position. Large flumes have large flat bottoms that require particular attention during installation. The buoyant forces of grout can cause a flume to float during installation. This usually results in less than optimum dimensional accuracy at the metering station.
The anchor clips on the flume exterior are designed to secure the flume against the grout once it is cured. They will not prevent a flume from floating during installation. Consideration should be given to the following suggestions concerning flume installation.
Channel or vault should be designed with adequate clearance at the sides to allow grout placement and worker access for chaining during installation. This usually requires at least 18" on each side.
Large flumes may also require blocks or other support on the underside of the flume to support the interior weights during installation. Block the underside of the flume such that the crest is level and 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. 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. Piers should be located so the floor of the flume rests on piers, not the ribs.
Set flume in place & check that flume crest is level in both directions. Shim where required.
Wire anchor clips on flume's underside to vault floor or place rebar bent into a "U" shape through the anchor clip. Check that flume's crest is still level. Run two chains from side to side on underside of flume for "chaining" during grouting. Chains should extend above to of flume, with enough additional length to allow a 2 man, side to side, "tug of war" for agitation. Vibrator sticks can be used in lieu of chaining.
Large flumes must also be weighted and lined on the interior to prevent floating or distortion due to the grout's hydrostatic pressure. The sidewalls of flumes that are 2 ft. or taller must be lined and braced to prevent distortion. Line interior floor with plywood and adequately weight inside floor of flume to prevent floating during grouting. Pallets with 55 gallon drums of water or sandbags are a typical weight source. Weight must be adequate to resist buoyant forces of the grout. If there is any doubt as to the weight required, calculate the interior volume of the flume, from the bottom of the flume to the top of the first pour of grout. The weight required must be equal to the weight of the grout that is displaced by this volume. Grout is 2.5 times heavier than water. Water is 62 lbs./cu.ft., grout can be estimated at 155 lbs./cu.ft.
Only grout one section between piers at a time. The use of a grout hose may be required. Flow grout in from one side of flume only. To keep upward force to a minimum, do not let grout get a depth of more than 2" up the sidewall of the flume before agitating and moving to the next section.
Use vibrator sticks or agitate with chains against underside of flume to be sure all air pockets are removed & grout is in place along entire underside of section being grouted. Remove chains & let grout cure before proceeding to next section.
Good concrete and grout techniques should be observed. Non-shrink grout is not required. The use of plasticizers and too much water in grout can result in the excess water coming out of the grout & pooling between the exterior of the flume floor and the grout. It may result in an unwanted void after curing.
Repeat procedure for each area between piers, allowing the grout to cure before proceeding to next section. Wire anchor clips on flume exterior walls, rebar, or place bent rebar through anchor clips. Line interior sidewalls of flume with plywood & 2x4's to prevent distortion. Grout sidewalls in 6" lifts, letting each lift cure before proceeding. The finished surface should be sloped toward the flume so that any water will drain back into the flume.
Source: Global Water Instrumentation Inc.