Your Dam Your Responsibility (PDF~1.2MB)

Appendix C
will be needed. In stable soils well-established grass cover 100–200mm long will provide
good protection without obstructing flow. In more erodible soils additional protection is
likely to be needed such as graded rock or beaching. The spillway cut should be flat to
make sure the flow is evenly distributed across the width of the channel. Do not allow
traffic ( vehicles or cattle) across or along the spillway channel, as any rutting will create a
starting point for erosion.
Spillways must be located such that water discharging from them is directed away from
the embankment or the embankment interface with the hillside. If this is not done, the
discharge water may cause erosion of the embankment and ultimately failure of the dam.
Ideally, a spillway should have a control section across its width, constructed of concrete
with its surface at the overflow level, but extending at the sides about 300mm above
the overflow level. Installation needs some care if this is to prevent erosion lowering the
overflow level, which is its main purpose.
Figure C-1
C
Freeboard
Freeboard is also an important protection and is defined as the vertical distance between
the full storage (spillway crest) level and the embankment crest. The amount of freeboard
required depends on the storage size, expected wave heights and the desired spillway
capacity. For situations covered here the freeboard should not be less than 1.0m.
Size of Spillway
The size of the spillway is determined from the likely rainfall intensity for the area,
catchment characteristics and the nature of the spillway being used. When sizing a
spillway you need to consider all factors that will affect the speed of rainfall run-off into
the dam. For example timbered areas will not normally be as fast as grass areas however
rocky catchments generally have the fastest run-off rate.
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