CHAPTER 8 - FLUMES

6. Workmanship

Flumes require accurate workmanship for satisfactory performance. Short flumes will provide reasonably accurate flow measurements if the standard dimensions are attained during construction. For accurate flow measurement, the flow surfaces must be correctly set or placed at the proper elevation, the crest must be properly leveled, and the walls must be properly plumbed. Although long-throated flumes can be computer recalibrated using as-built dimensions to correct for moderate form slipping or errors of construction, correcting for throat-section slope in the direction of flow is not always satisfactory. In any case, adequate care during construction is preferable. The modified broad-crested weir flume has only one critical flow surface, and it is level.

Flumes should be set on a solid, watertight foundation to prevent leakage around and beneath the flume and prevent settlement or heaving. Collars or antiseep walls should be attached to either or both the upstream and downstream flanges of the flume and should extend well out into the channel banks and bottom to prevent bypass flow and foundation settlement caused by erosion. A stable foundation without significant settling or leakage must be secured at reasonable costs.

The flumes can be built of wood, concrete, galvanized sheet metal, or other materials. Large flumes are usually constructed on the site, but smaller flumes may be purchased as complete flumes and placed in one piece. Others are provided in bolt-together pieces which are assembled onsite. Some of these flumes are made of lightweight materials, which are then made rigid and immobile by careful earth backfill or by placing concrete outside of the walls and beneath the bottom.

When making a number of relatively small concrete flumes of the same size, use of portable and knockdown reusable forms is economical and practical. These forms require high quality design and workmanship. Good construction practice should be used in placing footings, setting the forms, and pouring and tamping wall concrete to provide smooth surface finishes. Accuracy of the short flumes depends on correct flume dimensions, proper setting, and proper use. As flume size decreases, the influence of a small dimensional error becomes more prominent, and the importance of this care increases.