Attenuation
What is an attenuation tank, and how does it actually work?
A buried box that does nothing most of the time — until it rains. Here's what an attenuation tank is, what it's made of, and why the voids matter.
An attenuation tank is an underground store for stormwater. Most of the year it sits empty. During a storm it fills, holding back the surge of runoff a development creates, then lets it out slowly through a flow control so the downstream network is never overwhelmed. Attenuate simply means to reduce — here, to reduce the peak flow.
What it's made of
Modern tanks are built from geocellular crates — modular plastic units that lock together into a block of the required volume. The clever part is the void ratio: 95–97% of the crate is empty space available to hold water, so you get a lot of storage in a compact footprint.
The crate block is wrapped: a protective geotextile fleece against damage, then an impermeable membrane so the tank holds water rather than letting it seep away. Every membrane joint is heat-welded into a continuous barrier, and a test weld is recorded before backfill.
How the water moves through it
Runoff enters through an inlet, fills the void space, and leaves through an outlet fitted with a flow control set to the discharge rate your planning condition allows. When the tank is full and the storm keeps coming, that's what the design volume is for — it buys time until the flow can drain away.
Why the build quality matters
A tank only works if it holds its designed volume and its joints stay sealed underground for decades. That's why we weld rather than tape, test the welding on every job, and hand over documentation — an attenuation tank is infrastructure you never see again once the ground goes back over it, so it has to be right the first time.
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