Flumes & Weirs

H flume installed

*Peter van der Merwe (Pr Tech Eng) experience of the 1930s saw the establishment and mandate of the Soil Conservation Service in 1935 to conserve the nation’s soil and water resources. With this mandate, the researchers of the Soil Conservation Services began the investigation to develop a flume suitable for measuring agricultural flows. The result was the H flume, so called because it was the eighth in a series of flumes investigated.

The flume was accepted, as it combined the flow sensitivity of a v-notch weir with the flat floor and self-cleaning properties of a flume. The H series of flumes are more than flumes but modified weirs with a v-shaped throat and no diverging or discharge section. The design allows for a wider range of low flow sensitivity as well as high flow rate measurement.

The flume is ideal for edge-of-field, low average flow run-off monitoring and substantially higher rainfall flows. The flat floor of the H flume means that it passes sediments and smaller debris with ease. While originally developed for agricultural run-off monitoring, the versatility of this flume can be used in several different applications: • edge-of-field monitoring • earthen channels and furrows • monitoring landfill leachates • watershed monitoring • dam seepage • industrial discharge • sewage treatment works (screened/ treated flows). H flumes work well in mine applications, as their range of flows is large and their flat floors readily pass large amounts of sediment without clogging. Even with the integral approach section, sediments do not affect H flumes as they do many flumes and all weirs. At low flows, when sediments tend to drop out in the approach section, water channels through any accumulated sediments and, as the flow rises, these sediments ultimately push through the flume.

is an independent consultant who advises on hydraulic matters relating to open-channel flow monitoring. He has eight years of experience in the design, manufacture and supply of a wide range of flumes and weirs, and has consulted and supplied products to over 170 clients, both local DEMYSTIFYING THE H FLUME

The H (hybrid) flume primary device for open-channel flow measurement and monitoring offers great versatility across many applications. By Peter van der Merwe*

The American Dust Bowl

andinternational. Unlike the Parshall (ASTM & ISO), rectangular long-throat (ISO) and Palmer-Bowlus (ASTM) flumes, the H (hybrid) flumes have not been defined in a standard, but by several research publications, the most common being:

The dimensions for the H flumes are reviewed in two primary publications: • Brakensiek, D, Osborn, H, Rawls, W,

Field Manual for Research in

Agricultural Hydrology: Agriculture

Handbook No. 224, February, 1979 • Gwinn, W, Parsons, D, ‘Discharge

Equations for HS, H, and HL Flumes’,

Journal of the Hydraulics Division , Vol. 102, No. HY1, January, 1976. Metric conversions of the H flumes and their discharge tables, combined with the development of a standard best-fit equation, were proposed by: • Bos, M, Discharge Measurement

Structures, 3rd Edition , International

Institute for Land Reclamation and

Improvement, Publication 20, 1989. The approach sections for HS/H/HL flumes are based upon the research of:

H flume for a mine project

H flume construction

• Gwinn, W, ‘Chute Entrances for HS, H, and HL Flumes’, Journal of Hydraulic

Engineering , Vol. 110, No. 5, May, 1984.

Flow equations and tables Operators in the field find it easier to use calibrated flow tables instead of flow equations.

H flume flow tables can be calculated to the top of the flume. Unlike other flumes, there is no designed freeboard with the standard dimensions. In practice, a freeboard is recommended in the event that, when maximum flows occur, the flume could overflow with loss of flow accuracy. The unique shape of the H flume – more weir than flume – has meant that the discharge equation for the flume is three separate flow regimes: low, transitional and main.

The flow equations, however, can be difficult to calculate with typical applications; the need to transition from one equation to another, as the level in the flume rises and falls, becomes needlessly cumbersome. To simplify flow calculations, MG Bos developed a single, standard, best-fit equation for the data: Log Q = A + B log H + C [log H] 2

Where: Q = discharge in m 3 /s H = upstream head in metres A, B & C are the constants

Bos’ equation and table require only three dimensionless constants for each flume size and the level at the point of