Attenuation: crates, pipe or tank?

You need to hold a volume of stormwater underground and let it out slowly. That is the whole brief.

There are three common ways to do it, and arguments about which is best usually miss what actually decides it.

The three options, honestly

Geocellular crates are modular plastic units assembled into a block and wrapped. Large-diameter pipe is exactly what it sounds like, run in lengths with manholes. An in-situ tank is a concrete structure built to fit.

All three store water. They differ in how they handle depth, loading, awkward shapes and, crucially, who can get inside them afterwards.

Notice how many rows are about the hole and the access rather than the product.

The excavation is the job

This is the part that surprises people who have only priced the system.

Whatever you put in the ground, you first have to make a hole big enough, keep it stable, and deal with whatever comes out of it. Those costs scale with depth and ground conditions, not with which catalogue you ordered from.

We covered why that is not a linear relationship in why drainage depth changes your whole programme. The same logic lands on attenuation with extra force, because an attenuation hole is wide as well as deep.

Maintenance access decides more than people expect

Here is a question worth asking before anything is ordered.

In fifteen years, when this silts up, who opens it and how?

Large-diameter pipe answers that easily. You put manholes on it and someone runs a camera through. An in-situ tank has access designed in. A crate system depends entirely on the inspection channels specified at the start.

None of these is wrong. But if the adopting body or the site owner is going to want a survey, pick the system that makes a survey possible.

You cannot retro-fit maintenance access. It is designed in, or it is not there.

What we want to see before pricing

The drainage strategy and the discharge rate. Levels. A site investigation. Anything known about water.

With those, the choice usually makes itself. Without them, any price is a guess, and a guess priced low is worse for you than a fair price because it moves later.

Attenuation rarely arrives on its own, either. It comes with the rest of the drainage package, and often with the road that sits over it, which is where the adoption and inspection regime starts to matter.

A note on what gets you signed off

Whatever system goes in, the thing that gets it accepted is evidence.

Testing records, as-built drawings, a CCTV survey where the specification asks for one. Collect those while the work is happening. Nobody has ever enjoyed producing them a year later.

Where the system sits under an adoptable road, the S278 and S38 inspection regime applies to the drainage as well as the surface, so the records matter twice.

If you have a drainage strategy and need the civils priced properly, send it over. We would rather look at the real levels than estimate from a description.