Text A Open Cannels

Two types of conduits are used to convey water, the open channel and the pressure conduit. The open channel may take the form of a canal, flume, tunnel or partly filled pipe. Open channels are characterized by a free water surface.

The problems of canal location are similar in many respects to highway location, but the solution may be more difficult since the slope of canal bottom must be downgrade and changes in slope should be avoided, because excessive slope may result in erosion of the channel bottom. The velocity at which scour will begin depends on the bed material. The soil in which the canal is constructed is often permeable. Thus, it may be economical to line a canal in order to reduce water seepage. Various types of lining are used to reduce seepage losses from canals. Clay, asphalt, cement mortar and reinforced concrete have been utilized successfully. For large and important canals a concrete lining is usually most satisfactory because of its technical characteristics.

Numerous structures are necessary for the proper operation of canals. The usual diversion structure is an overflow dam. Canal intakes are ordinarily located a short distance upstream from the diversion structure and serve to regulate the flow in the canal. Fish screens are often provided at the intakes as well. The distance from the intake to the canal should be long enough to permit smooth flow. If the change in elevation for a canal is large, it may be necessary to use chutes or drops to avoid excessive slope in the canal. A chute ordinarily consists of an intake structure, a long section, an outlet designed to dissipate excessive kinetic energy.

Because of the high velocity of the flow in the chute, the entire structure is usually concrete-lined. A drop is similar to a chute but the change in elevation is effected in a shorter distance. In some cases a vertical drop is used. If a canal meets an obstruction such as a highway embankment or hill, a culvert or tunnel may be used to carry water through the obstruction. If the canal must cross a highway cut, stream or gully, a flume or a pipe may be used to convey the water. In some cases an inverted siphon may be used.

Regulation of flow in the canal and the distribution of the water are facilitated by various structures. A check is a short, concrete-lined structure, placed in the canal and provided with piers so that flashboard or gates can be used for flow regulation. The main purpose of the check is to raise the upstream water level to permit water diversion. Turnouts are usually pipes passing through the canal embankment for diversion of flow from the main canal to a smaller distribution canal. Wasteways are canals or pipes used to return excess water to the stream. They are necessarily provided with gates and are sometimes used for sluicing sediment from the canal. Excess water is discharged into a river or a stream. Excess storm water may be prevented from entering a canal by use drainage culverts provided for under the canal or an overchute (flume passing over the canal). In either case, it is essential that the culvert or overchute have adequate capacity to pass the floodwaters that are expected.

Settling basins are sometimes constructed just below to the point of water diversion so that sediment can be collected and returned back to the river. A settling basin consists of a large, shallow basin through which the water passes with a low velocity (1 ft/sec or less). As much as 90 per cent of the suspended sediment can be collected in a well designed settling basin. Sand traps are sections or depressed canal bed located upstream from a check structure and provided with wasteways or sluiceways so that accumulated sediment can be sluiced from the canal.

A flume is a channel of wood, concrete or metal which is usually supported aboveground. Flumes are used to convey water over terrene where construction of canals is difficult or expensive. They are often employed to carry a canal over a depression. Wooden flumes are usually of rectangular cross section, but triangular and semicircular sections are also used. Various types of wood have been used in timber flumes, but redwood and cypress are superior to all others. A good redwood flume may have a life of 50 yr., while pine and fir flumes may be used for about 20 yr. Semicircular steel flumes are widely used. The flume channel is made of thin steel sheets rolled to semicircular form. All metal which comes in contact with the flowing water should be galvanized or covered with a suitable protective paint. The useful life of a well-maintained steel flume will range from 15 to 50 yr.

The most permanent type of flume is a properly constructed concrete flume. However, care must be taken to avoid cracking. Suitable contraction joints must be provided at each pier. Small concrete flumes are often constructed of precast sections supported on steel, timber or concrete piers. Large flumes are usually cast in place with concrete piers. Flumes and canals are sometimes covered to minimize evaporation, pollution and freezing.

Occasionally it is cheaper to convey water by tunnel than by flume or canal. Tunnels are usually made use of circular or horseshoe section. If the tunnel material is unstable, a concrete or steel liner may be used to prevent collapse. Tunnels driven through rock need not be lined but their interior surfaces are usually lined with gunite to improve hydraulic characteristics. Tunnels may be full or partly full of water. When tunnels are full of water, they act as pressure conduits. When tunnels are partially full of water they act as an open channel.