| Detention Dams and Drop Structures |
These are small on farm floodwater control dams. Detention dams are designed to moderate peak floodwater flow rates during high intensity rainfall events (flash floods) by collecting the water in a contained area and releasing it at a controlled flow rate through a small diameter outlet pipe. These have been used to control rill and gully erosion in the Bay of Plenty since the mid 1970s. Detention dams are constructed in dry watercourses that only carry water during heavy rainfall events. A minimum storage volume of 60 m3/ha (total 2,600 m3) of catchment and a spillway height of 2 m or less (to minimise the cost) is the criteria for a dam site. The dams are designed to cater for a 1 in 10 year rainfall event. Emergency spillways are incorporated into the design to allow for larger storm events. The catchment sites for such dams is 40 hectares.
 | | Impounded stormwater in a detention dam |
When constructing a detention dam consider:
- the bottom of selected dam sites are excavated
- embankments are constructed with 800 mm compacted freeboard and a slope of 3:1
- pipe intake is protected from animal treading by a small headwall
- an intake grill is installed where there is a risk of leaf litter or other debris clogging the pipes
- topsoil is respread and grass/legume seed mix sown
- the cost is $2,500 - $6,000 depending on the site, size and extent of work involved
Drop structures are designed to carry floodwaters over abrupt changes in channel gradients with plunge pool formations. The turbulence caused by floodwaters can cause plunge pools to undermine the surrounding banks and collapse the upstream channel floor. This process is known as headward erosion in permanent streams and gully head erosion in normally dry gullies.
Box Flumes
Flumes are designed to carry water flow away from the face of a gully head escarpment and drop it into a plunge pool far enough downstream to prevent undercutting of the escarpment. Box flumes are usually constructed of timber using tongue and groove linings or resin bonded plywood. They are built as a chute with floor, sides and cross bracings. Wing walls at the intake are extended into the sides and an apron from the floor of the flume is embedded into the floor of the channel.
Box flumes have high design and construction standards and are expensive. These work best in small permanently flowing streams where there is a headward erosion problem.
Pipe Flumes
Rigid pipe flumes have the same function as box flumes but are not suitable for permanently flowing streams. These require large pipe sizes, adequate pipe support and the formation of an embankment (bund) with a spillway at the intake. Flexible pipe flumes duct water directly to the lower channel level avoiding the formation of plunge pool. These are commonly used for the outflow of detention dams.  | | Flexible pipe flume |
Reinforced Embankments
These are used in normally dry gully floors to control scouring by flood water where there is a drop in gully floor levels of 1 m or less. They comprise a sand bag wall or geotextile fabric apron and level sill which distributes flow evenly across the embankment. The maximum flow rate for reinforced embankments is 2 m/sec. Flows more than this will require some other form of flow control.
Environment Bay of Plenty design and build detention dams and drop structures in accordances with specific technical standards and the provision of the Regional Land Plan.
Fact sheets on land management are available from Environment Bay of Plenty
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