Attenuation tanks in Worcestershire

Development in a river county

The Severn runs through the middle of the county, joined by the Teme below Worcester and the Avon at Tewkesbury, with the Stour coming in at Stourport. Much of the low ground beside them floods, and towns along their banks sit downstream of every new estate built upstream.

So drainage strategies here are built around holding water back. The planning condition sets a discharge rate, the engineer works out the storage, and the choice is usually between a buried crate tank, an open basin or both.

Tanks and basins

We lined a 3,500m² attenuation basin for a housing development in Worcester in 1mm HDPE, welded on site and tested. On the same kind of scheme, a crate tank does the job where the land is needed for houses, roads or parking, because it stores water below a surface that can still be used.

Many Worcestershire estates use both: a basin where there is open space and a crate tank where there isn't. We install both, which keeps one welding standard and one set of records across the site.

• Crate tanks under roads, parking and open space
• Lined attenuation basins in HDPE and other membranes
• Heat-welded seams and a recorded test weld on each
• QA and handover documentation for each element

Clay vales and the M5 corridor

Much of the county sits on red mudstone clays, which hold water and make soakaways unreliable. That pushes most sites towards attenuation. The M5 corridor through Bromsgrove, Droitwich and Worcester brings employment sites and larger housing schemes. Further out, in the Vale of Evesham and the rural west, the jobs are smaller: farm buildings, village housing and business units.

In the Vale of Evesham, horticulture and packing sheds bring their own drainage needs, with large glasshouse and shed roofs on flat land. On flat sites like these, the levels decide a lot: the tank often has to be shallow and wide, with the outlet set to reach a ditch or brook without pumping. We build to whatever footprint and depth the engineer's design sets.

How we work

We come down from our Tamworth base on the M42 and M5. Digging is not ours: the excavation is already formed by your groundworker when we get there. After a formation check, the fleece and membrane are laid, crates assembled at 95–97% void, inlet, outlet and flow-control connections made, and a test weld recorded, all before the groundworker backfills.