Your Dam Your Responsibility (PDF~1.2MB)
Your Dam Your Responsibility (PDF~1.2MB) Your Dam Your Responsibility (PDF~1.2MB)
Appendix C The width of the discharge channel must be sized to suit the gradient. This is required to make sure the water flow will not be too fast or restricted. Table C-1 below provides the discharge channel width required for the slope of your channel. Table C-1 for Up to 25-Hectare Catchment Discharge Channel Slope (% vertical to horizontal, or fall in m. per 100m) 3% 4% 5% 6% 7% 8% 9% 10% 15% Discharge Channel Width in metres 4 4 5 6 7 7 7 10 13 For dams that have a catchment greater than 25-hectares and less than 50-hectares, the following size may be used: Figure C-3 Where a long entry channel is required this channel width must not be less than the width the spillway crest and the level must not be higher than the crest section. C The width of the discharge channel must be sized to suit the gradient. This is required to make sure the water flow will not be too fast or restricted. Table C-2 below provides the discharge channel width required for the slope of your channel 77
78 C Appendix C Table C-2 for over 25 up to 50-Hectare Catchment Discharge Channel Slope (%) 3% 4% 5% 6% 7% 8% 9% 10% 15% Discharge Channel Width in metres 6 8 9 11 13 14 15 17 22 In some cases there may be some potential for trimming of the spillway size if a suitably qualified engineer is engaged to consider all the factors relating to your specific dam site. Outlet Pipe General Outlets should be constructed in undisturbed ground in the foundation. The outlet pipe may be a welded steel (preferred), polyethylene or ductile iron pipe below the embankment to discharge the stored water for supply or environmental flow purposes. The type of pipe selected should be able to take the embankment and water pressures. Taking into account a minimum service life of around 50 years, some wall thickness allowance for corrosion should be made for steel and ductile iron pipes. PVC pipe is not considered suitable due to its potential for cracking. Many failures occur along the outlet pipe due to lack of compaction around the pipe particularly under the lower third of the laid pipe. The outlet pipe should be concrete encased because it is very difficult to get suitable soil compaction around a pipe. The trench for the outlet pipe should have vertical sides up to the top of the pipe then battered 1H to 1 V side slopes to allow good compaction of backfill. (Note that this requires careful attention to detail: seek advice). Installation The pipe should be laid in the trench supported at intervals off the floor of the trench (75mm high concrete blocks) to ensure that concrete can flow under the pipe to fully encapsulate it. Prior to pouring the concrete the pipe should be filled with water to test for leaks. When the outlet pipe is being encased, care should be taken that it is not only supported from the floor of the trench but that it is also held down at intervals to stop it floating in the fresh concrete. Filling the pipe with water will help in this regard. Size of Pipe The minimum size should be 200mm to protect against blockage. This size of pipe is also useful should you need to lower the level of the water if an embankment problem develops. The diameter of the outlet pipe should also be large enough to provide adequate supply and environmental flows. Pipe manufacturers have strength and flow chart information readily available.
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Appendix C<br />
The width of the discharge channel must be sized to suit the gradient. This is required to<br />
make sure the water flow will not be too fast or restricted. Table C-1 below provides the<br />
discharge channel width required for the slope of your channel.<br />
Table C-1 for Up to 25-Hectare Catchment<br />
Discharge Channel Slope (% vertical to horizontal, or fall in m. per 100m)<br />
3% 4% 5% 6% 7% 8% 9% 10% 15%<br />
Discharge Channel Width in metres<br />
4 4 5 6 7 7 7 10 13<br />
For dams that have a catchment greater than 25-hectares and less than 50-hectares, the<br />
following size may be used:<br />
Figure C-3<br />
Where a long entry channel is required this channel width must not be less than the<br />
width the spillway crest and the level must not be higher than the crest section.<br />
C<br />
The width of the discharge channel must be sized to suit the gradient. This is required to<br />
make sure the water flow will not be too fast or restricted. Table C-2 below provides the<br />
discharge channel width required for the slope of your channel<br />
77
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