Water for Every Farm - Yeomans Keyline Plan by Ken B. Yeomans & P.A.(dec.) Yeomans
Author:Ken B. Yeomans & P.A.(dec.) Yeomans [Yeomans, Ken B.]
Language: eng
Format: mobi
Publisher: Keyline Designs
Published: 2008-06-02T16:00:00+00:00
b) Spillway
The spillway of a dam, like all other features of good farm irrigation dam construction, has to be correct for the dam to be a good one. Usually the construction of the spillway will produce a surplus of earth which is then used in the building of that end of the wall.
The batter between the end of the wall of the dam and the bottom level of the spillway should be about 1 in 3 or flatter so that in the grassing of the dam site, wall and excavation area, convenient travel with cultivating equipment is possible. A section of the spillway therefore, will show a batter slope of 1 in 3 falling from the end of the wall of the dam, to a dead level spillway bottom of a given width, with a similar batter rising from the spillway bottom to the rising land on the high side of the spillway and away from the wall of the dam.
Shrinkage allowance, freeboard height and spillway size therefore provide that, on the completion of the dam and its subsequent settlement, there will be 0.9 metres (three feet) of wall everywhere above top water level at the point where water commences to flow out of the dam and through the spillway. The design of the spillway is such that the type of flood likely to occur any time in 50 years would be by-passed, with the spillway carrying little more than 0.3 metres (one foot) of water across its full width and when this happens, there would still be a further freeboard of 0.6 metres (two feet) to compensate as a safety measure for bigger floods.
Larger spillways are necessary for farm dams built in the lower parts of large primary valleys and for creek dams, since they usually have considerably more catchment area than a dam that is higher in a valley.
To secure the necessary width of spillway with a dead flat floor, considerable material may have to be excavated into the rising land near the wall of the dam. In these circumstances an appreciable amount of earth may have to be moved, that is, earth greatly in excess of the needs of the spillway bank. The construction of the wall of such a dam then, is designed so that the earth of the spillway is used in the construction of that part of the wall adjacent to the spillway. It is advisable to construct the spillway before the earth in the centre of the wall approaches its finished height and when there is still plenty of wall area unfilled and available for the use of the material excavated from the spillway.
On occasions where the spillway of a dam involves very considerable earth moving the construction of the spillway may be completed by placing the excavated material into the wall site immediately the site preparation is completed.
In spillway construction, earth has to be excavated down to a specific level and supervision should ensure that the earth is not excavated too deeply
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