5. Site Characteristics Related to Locating, Selecting, and Setting Flumes

Proper location of the flume is important from the standpoint of accuracy and ease of operation. For convenience, the flume should be located near the diversion point and near the regulating gates used to control the discharge. Flumes should be readily accessible by vehicle for both installation and maintenance purposes. All structures for measuring or regulating the rate of flow should be located in a channel reach where an accurate head can be measured. The survey of a channel to find a suitable location for a structure should also provide information on a number of relevant factors that influence the performance of a future structure.

(a) Approach Conditions

Flumes should not be installed too close to turbulent flow, surging or unbalanced flow, or a poorly distributed velocity pattern. Poor flow conditions in the area just upstream from the measuring device can cause large discharge indication errors. In general, the approaching flow should be tranquil. Tranquil flow is defined as fully developed flow in long straight channels with mild slopes, free of curves, projections, and waves.

Studies of approach requirements for closed conduits have led to the acceptance of 10 diameters of straight pipe as sufficient for pipe meters claiming to be accurate to within 0.5 to 1 percent. By the usual hydraulic analogy, open channel flow would require 40 times the hydraulic radius of straight, unobstructed approach channel. The hydraulic radius is the area of flow section divided by the wetted perimeter, which becomes d/4 for full pipes; hence, the suggested 40 times hydraulic radius approach distance. These requirements can probably be relaxed because open channel measuring flumes claim accuracy to a wider margin of 2 to 5 percent. However, for a rectangular channel that is twice as wide as it is deep, 40 times the hydraulic radius is numerically equal to 10 top widths.

Bos et al. (1991) gives approach length requirements stated in various terms of flow depths, head readings, and widths as follows:

Approach velocities less than 1 foot per second (ft/s) encourage aquatic pests, insects, and sediment deposition, so the approach velocity should exceed 1 ft/s if at all practical. To prevent wave interference of head measurement, the Froude number of the approaching channel flow should be less than 0.5 for the full range of anticipated discharges and should not be exceeded over a distance of at least 30 times the measurement head before the structure.

It is recommended that a check be made of the approach velocity condition by current meter measurements, especially when using baffles. In any case, approach condition should be verified visually. Visual inspection should be made for obvious boils and backflows and unstable surface conditions.

(b) Channel Flow Characteristics and Operational Needs

For accurate measurements, sufficient head loss must be created to obtain a unique discharge versus head relationship. This relationship assures that submergence limits have not been exceeded and modular flow exists. To prevent submergence altogether or to assure that excessive submergence does not occur, the designer needs to know whether the downstream water surface elevation relationships are consistent and do not change with season or whether they are influenced by operation of gates, reservoir operation, or other laterals. The channel water levels greatly influence the sill height necessary to keep the downstream water surface below the submergence limit, thus obtaining modular flow for the needed discharge range.

The amount of downstream flow resistance and, hence, the water surface elevation, is likely to vary with sediment deposits, debris, canal checking operations, vegetative growth, and aging. For a new design, careful assessment of friction, including the effects of relative roughness, is required. A thorough appraisal of needed canal operations is required to determine the frequency of measuring different discharges, including the normal design flow and the maximum design flow.

To select or design an appropriate flume for installation in an existing channel, full advantage should be taken of making field measurements at different discharges to obtain thorough knowledge of channel performance at the site. After tentatively selecting the flume location, information should be obtained on the maximum and minimum flows to be measured, the corresponding flow depths, the maximum velocity, and the dimensions of the channel at the site. These measurements should include channel widths, side slopes, depths, and the height of the upstream banks with special attention to their ability to contain the increased depth caused by the flume installation.

(c) Erosion And Scour

Ideally, the selected channel reach should have a stable bottom elevation. In some channel reaches, sedimentation occurs in dry seasons or periods. These sediments may be eroded again during the wet season. Sedimentation may change approach velocity or may even bury the structure, and the erosion may undercut the foundation of the structure.

Based on the channel water levels and the required sill height, in combination with the discharge versus head relationship of the structure, ponding at the upstream structure should be assessed. Excessive ponding commonly causes sedimentation difficulties because of the subsequent reduction in the approach flow velocities. To avoid upstream sedimentation, the sill height plus the measuring head should be about the same as for the approach channel over as much of the discharge measuring range as practical. This arrange-ment will help reduce sedimentation upstream from the structure.

The required drop may exceed the capacity of soil lined channels to resist scour, and foundations may scour. Thus, rock armoring may be needed to prevent undermining. For more details on sediment handling, see Bos (1989) or Bos et al. (1991).