Open Channel Flow

Abstract

Introductory paragraphs: Open channel flow occurs when a free water surface in a channel is at atmospheric pressure. Common examples of open channel flow are rivers, streams, drainage ditches, and irrigation canals. Open channel flow may also occur in pipes if the pipe is not flowing full and the water surface is at atmospheric pressure. Open channel design is common in many applications of soil and water conservation, including drainage and irrigation ditches, grassed waterways, reservoir spillways, and large culverts. In all these applications, the designer must consider channel shape, slope, hydraulic roughness or resistance to flow, and in many cases, channel resistance to erosion. Channels may be earth or concrete lined, vegetated, lined with impervious material such as rubber or fabric, or lined with erosion resistant material such as large rock or high strength geotextile materials. Channels may be left in a natural condition, shaped to achieve a desired capacity, or designed to minimize bed erosion. In some cases, channels may be confined by vertical sides made from materials that are resistant to erosion or sloughing. A properly designed, earth-lined open channel should provide (1) velocity of flow such that neither serious scouring nor sedimentation will result, (2) sufficient capacity to carry the design flow, (3) hydraulic grade at the proper depth for good water management, (4) sideslopes that are stable, and (5) minimum initial and maintenance costs. Additional requirements must be met for carrying irrigation water, such as low seepage loss. Details on open channel flow and design can be found in Chow (1959), French (1985), and Henderson (1966).