Sediment basins are ponds with open water that capture coarse sediment and litter carried by stormwater. They intercept stormwater before it reaches the waterway, and slow it down to allow the coarse sediment to fall to the bottom.
How sediment basins work
Sediment basins work in the following way:
Water enters the sediment basin, slowing down to less than 0.5m/s – during this time the water level can temporarily rise by up to 350mm (called the extended detention depth).
Gravity pulls the coarse and medium sediment to the bottom.
The cleaner water stays at the top of the pond and flows through the outlet structure.
When to use sediment basins
Sediment basins are used in catchments with high sediment loads, such as construction sites, to protect downstream treatments from sediment. Use a sediment basin:
during development construction
upstream of a wetland or raingarden
at the drainage outlet of industrial open air industry sites
Sediment basins are almost always used upstream of a constructed wetland or large raingarden, in a combination called a treatment train. This enables them protect wetlands and waterways from large sediment deposits, while the wetland or raingarden downstream further treats stormwater nutrients and toxins.
Designing a sediment basin
A well-designed sediment basin should be just large enough allow time to capture most of the target coarse sediment. It should only capture a small amount of finer particles and contaminants, the majority of which should be treated by a constructed wetland or raingarden downstream.
Key design considerations for sediment basins are:
target sediment size – typically 125 µm or larger
design flow rate
volume of storage for sediment
The captured sediment should have low enough levels of contamination for cost-effective disposal.
A sediment basin needs to be cleaned out when the sediment builds up to 0.5 metres below the water level, which usually takes about two to five years. To clean out a sediment basin:
The sediment basin is drained of water to allow access
An excavator reaches from the side or a bob-cat enters the basin to clean out the sediment
Wet sediment is placed in a nearby de-watering area to dry, reducing the weight and cost of disposal
For an example of a sediment basin clean out, view our case study:
Tips and advice
When designing a sediment basin, check that it:
has a permanent pool water depth of at least 0.5m above the sediment accumulation zone
is ‘right-sized’ and not oversized
has allowed for maintenance access
can be dewatered without draining any downstream wetland, and has a site for de-watering the sediment
There are a number of hold points during a project where checks need to be made:
For detailed information on how to design sediment basins, see the design guidance and examples provided in Chapter 4 'Sedimentation Basins' in the WSUD Engineering Procedures available for purchase from CSIRO Publishing.