Attenuation refers to the temporary storage and then discharge of stormwater to avoid flooding. Rainwater is collected from properties where it feeds into a system that has been designed to collect excess rainwater for controlled release into stormwater drains. Excess water can be held over longer periods so the possibility of flooding in vulnerable downstream areas is therefore reduced or avoided.
Attenuation looks to bring about a reduction in the force of stormwater by carefully controlling the flow. The slow rate of discharge avoids downstream flooding and the erosion of waterways due to heavy flows.
Components needed for your complete attenuation system include silt trap or catchpit chamber as a prefilter, soakaway crates for stormwater storage, impermeable membrane with protection fleece (this can either come as individual components so that you can wrap the tank yourself or in two pieces as a pre-fabricated shoebox kit) and finally a flow control unit such as an orifice chamber.
Utilising Attenuation as a SuDS technique
The Geocellular system is used to control, store and manage rainwater surface runoff. Its flexible nature means that attenuation tanks using geocellular modules are lightweight and easy to install. The system can be installed in both traffic and non-trafficked areas and is capable of providing high storage volume capacity and managing high flow events.
The Four Stages of Attenuation
- Attenuation Stage 1:
Stormwater flows into a silt trap (or catchpit) to catch debris, improve water quality & prevent blockages further in the system.
- Attenuation Stage 2:
The filtered stormwater enters the pre-built geocellular crate system at a size that will hold the required level of stormwater.
- Attenuation Stage 3:
Stormwater is held in the tank which is wrapped in an impermeable membrane liner - this can be purchased by the roll or as a pre-built shoebox
- Attenuation Stage 4:
A flow control chamber then manages the discharge rate of stormwater into the main surface water drainage network.
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