tailieunhanh - Open channel hydraulics for engineers. Chapter 5 spillways

Spillways are familiar hydraulic structures built across a stream to control the water level. This chapter emphasizes the classification of weirs and spillways as well as the application of hydraulic formulas for designing their shape and dimensions. | OPEN CHANNEL HYDRAULICS FOR ENGINEERS ----------------------------------------------------------------------------------------------------------------------------------- Chapter SPILLWAYS _ . Introduction . General formula . Sharp-crested weir . The overflow spillway . Broad-crested weir _ Summary Spillways are familiar hydraulic structures built across a stream to control the water level. This chapter emphasizes the classification of weirs and spillways as well as the application of hydraulic formulas for designing their shape and dimensions. Key words Spillway; weir; crest; design head . INTRODUCTION Spillways are used at both large and small dams for letting flood flows pass, thereby preventing overtopping and failure of the dam. A spillway as sketched in Fig. is the most common type. Three zones can be distinguished: the crest, the face and the toe – each with its separate problems. crest face toe Fig. . General view of a spillway A weir is a notch of regular form through which water flows. The term is also applied to the structure containing such a notch. Thus a weir may be a depression in the side of a tank, a reservoir, or a channel, or it may be an overflow dam or other similar structure. Classified in accordance with the shape of the notch, there are rectangular weirs, triangular or V-notch weirs, trapezoidal weirs, and parabolic weirs. Weirs are usually designed to control water levels in rice fields or wetlands. They are commonly used as a means of flow measurement. Of primary importance for hydraulic structures considered in this chapter is the magnitude of backwater level they cause upstream of the structure for the given discharge; that is, the head-discharge relationship for the structure. Both gradually varied

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