|H FLUME INSTALLATION GUIDELINES
What size H Flume should I purchase?
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.
What are the three areas you should be concerned with for successful installation of a H flume?
Upstream conditions: Upstream conditions should promote laminar flow conditions at the flume inlet. Channel
turns, splits, elevation drops or other obstructions within five times the height of the flume should be avoided. The width
of the channel should equal the width of the flume mouth. The upstream channel slope should not allow excessive
velocity at the flume. Excessive velocity will cause turbulence at the measuring point and reduce accuracy. A slope of
almost flat, to 2% maximum, for very small flumes, and less slope for larger flumes is the ideal gradient.
Crest of the flume: The crest of the flume (floor of the flume) must be level both longitudinally & transversely. The
V shaped structure should be pointed downstream.
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.
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 H flume. The standard flow tables
must be corrected if a submerged flow condition exists.
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
H flumes were developed by the US Forest Service as small portable flumes to measure water flow When setting the flume in
concrete, pay attention to the following points:
- The flume must be installed with the floor of the throat level.
- 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.
Small H 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.
The positioning procedure described above can also be used to secure larger flumes in the correct position. Large flumes
have large bottoms that require particular attention during installation. The buoyant forces of grout can cause a flume
to float or distort during installation. This usually results in less than optimum dimensional accuracy at the monitoring
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
The channel should be designed with adequate clearance at the sides to allow grout placement. This usually requires at least
18 inches 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 so that the throat 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 throat of the flume should be level, so 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 throat 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 throat is still level. The use of vibrator sticks should be minimized as excessive use may distort the flume's
sidewalls or bottom.
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. Install
temporary wooden cross braces and plywood lining inside the flume to prevent distortion due to the grout pressure. Sandbags
are a typical weight source. Additional weight can be positioned by placing a board across the top of the flume and using a 5
gallon bucket filled with sand or other heavy items. 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
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 inches up the sidewall of the
flume before agitating and moving to the 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. A large flume
should be grouted in several lifts, with the first one only reaching slightly above the invert of the flume. Grout sidewalls in
6 inch lifts, letting each lift cure before proceeding. Before proceeding with the next pour, be sure the grout is hardened to the
point where the pressure of the next lift cannot be transmitted through the previous pour and possibly float the flume. Continue
with additional pours, letting each set, until the surface is even with the top of the flume. The finished surface should be
sloped toward the flume so that any water will drain back into the flume.