Dams and Weirs.pdf

This guide is for all parties who have an interest in the construction, operation and maintenance, refurbishment or demolition of river weirs. Its aim is to make them aware of the wide range of issues that are relevant in planning and implementing such works, and by doing so, to ensure that mistakes are avoided and potential benefits are maximised.

A theoretical approach is proposed to compute the hysteresis of a controlled hydraulic jump by a broad-crested sill in a rectangular channel, when the inflow is generated by a gate and a standard spillway with an abrupt slope. The attention is focused on the calculation of both the maximum and minimum inflow Froude numbers compatible with the formation of the hydraulic jump in the energy dissipator stilling basin. When the inflow is generated by a gate, the theoretical development shows that the inflow Froude number is linked to the relative sill height by a cubic equation which can be solved using trigonometric functions. When the inflow is generated by a standard spillway with an abrupt slope, the inflow Froude number and relative sill are well-defined by an involutive function.

NOTE: Some of the graphs contained in this section are copied from the Los Angeles Hydraulics Manual and we wish to give them credit for their efforts. Also, applicable graphs are for 8 " curb heights which may not meet Rio Rancho standards. A weir is a barrier in an open channel, over which water flows. A weir with a sharp upstream corner or edge such that the water springs clear of the crest is a "sharp crested weir". All other weirs are classified as "weirs not sharp crested". Weirs are to be evaluated using the following equation: Q = CLH 3/2 where: Q = Discharge in cfs C = Discharge coefficient from Handbook of Hydraulics, King and Brater, 5th Edition (or comparable) L = Effective length of crest in feet H = Depth of flow above elevation of crest in feet (approach velocity shall be disregarded in most applications) Applications Weirs are generally used as measuring and hydraulic control devices. Emergency spillways in which critical depth occurs and overflow-type roadway crossings of channels are the most common applications of weirs. Channel drop structures and certain storm drain inlets may also be analyzed as weirs. Special care must be exercised when selecting weir coefficients in the following cases: a. Submerged weirs b. Broad crested weirs c. Weirs with obstructions (i.e., guardrails, piers, etc.)

This Technical Brief is concerned with the typical small dam (up to about three metres high) which is built across a stream to form a reservoir. It provides guidance on planning, design and construction, but professional help should always be sought before building any dam whose failure could endanger lives, property or the environment. Care must also be taken to avoid the health hazards of reservoirs, including schistosomiasis and polluted water; and the rights of existing users of the water and land must be protected. A reservoir is useful where the available flow in the stream is sometimes less than the flow required for water supply or irrigation, and water can be stored from a time when there is surplus, for example, from a wet season to a dry season. In addition to the simple earth dam, alternatives to consider are using the sub-surface (groundwater) dam (see The Worth of Water, pages 97-100) or using wells. These may be preferable for environmental and water-quality reasons. Simple earth dams can be built where there is an impervious foundation, such as unfissured rock, or a clay subsoil. The channel upstream should preferably have a gentle slope, to give a large reservoir for a given height of dam. An ideal dam site is where the valley narrows, to reduce the width of the dam. Figure 1. Section through a small earth dam and reservoir

Open-File Report

Table 1. Basic data for each dam and reservoir. File name: DAMCARDS. Column 1. Country, limited to the first 8 letters except for the following abbreviations, gt. brit, s. africa, usa and ussr; all letters are lower case. Column 2. Dam name, limited to the first 10 letters or variant thereof, if there are two similarly named dams within the country; See cross-reference if in doubt. All letters are lower case. Column 3. Year of completion of the dam; only the last two numbers are given so that 1915 becomes 15. "99" indicates completion date unknown. Column 4. Type(s) of dam: 1, earth and/or rock fill; 2, arch; 3, buttress; 4, gravity; 5, multiple arch; 6, other. Column 5. Maximum height of the dam in meters. Column 6. Maximum depth of the water in meters. Column 7. Method of determining the water depth: b, reported by the U.S, Bureau of Reclamation; c, reported by the U.S. Army Corps of Engineers; e, estimated; m, maximum water depth attained, which is a lower value than the planned maximum depth; r, reported from one of the references cited in Table 5. Column 8. Maximum capacity of the reservoir in 100,000,000 cubic meters. Column 9. Code expressing our measure of confidence in whether or not impoundment has triggered seismicity: 0, no data; 1, tested and definitely not; 2, probably not; 3, data are inadequate to render a judgement; 4, case is questionable but might be induced; 5, probably induced; 6, accepted as induced. Column 10. Maximum magnitude of the earthquake in question, given in Richter magnitude. Table 2. Geologic information. File name: GEOLOGY. An asterisk means no data. Columns 1 and 2. As in table 1. Column 3. Faults in the vicinity. y, yes; n, no; u, unknown. Column 4.

Circular, 1996

took all of the photographs in this report. Mike Yard and Dave Wegner. of the Bureau of Reclamation. and Brad Dimock. of the Grand Canyon River Guides. critically reviewed early drafts ofthe manuscript. George Weiss ofthe U.S. Geological Survey helped our thought processes both on the river and off. Designed and illustrated by Jeanne Sandra DiLeo. Our thanks to you all. IV

Dam Failures

A Dam is an abstraction or barrier built across a river or stream. At back Of the barrier, water gets collecting to form a pool of water that is termed as the upstream side of the dam. And the other one is the downstream part of the dam.