Flood, Drainage and SuDS – Page 6

Geocellular storage: silt, inspection and adoption

Silt is what kills a geocellular attenuation tank, and adoption is the question to settle before the layout is fixed. Here is the evidence a reviewer expects.

Interception losses and the first 5mm of rainfall in SuDS

Interception losses deal with small frequent rainfall rather than the design storm. Attenuation tanks provide none of it, and the evidence is a separate calculation.

Adoption of SuDS: management company or section 104

Undertaker adoption, highway adoption, local authority adoption and private management each take different components on different terms. The route chosen shapes the drainage design, so settle it at concept stage.

Cumulative downstream effects of a surface water discharge

Rate control is not volume control. Here is how in-combination surface water effects arise, how to assemble the committed development list and how to frame an assessment that a lead local flood authority can review.

The sequential test for a change of use application

National guidance exempts most change of use applications from the sequential test. What matters instead is whether the vulnerability classification of the use rises, which decides whether the exception test applies.

Retrofitting SuDS on a constrained urban site

Brownfield drainage retrofit starts from the evidenced existing discharge rather than a greenfield rate. Disconnection before storage, options where there is no open ground, and how to write up a solution that falls short of greenfield.

Pumped surface water drainage: when it is accepted

Lead local flood authorities accept pumped surface water systems only where gravity discharge cannot be achieved, and they want the levels that prove it. What the design has to include and how to justify it in the drainage strategy.

Rain gardens and tree pits in adopted highway

Street SuDS gets refused on maintenance and utilities, not performance. Answer those two in the authority's own format and with a detail they have adopted before.

Pond and basin safety: slopes, benches and freeboard

Design the safety into a basin at concept stage and the objection never lands. Slopes, benches, visibility, freeboard and the exceedance route explained.

Treatment train design for a car park outfall

Parking and service yards carry silt, metals and hydrocarbons that roof water does not. Here is how to build and evidence a treatment train that a drainage officer will accept.

The simple index approach to surface water quality

Water quality is assessed separately from attenuation, and a sealed tank provides no treatment at all. How hazard and mitigation indices are compared, and where a detailed assessment takes over.

Flood risk standing advice and where it stops

Standing advice lets a council decide smaller applications without consulting the Environment Agency. It does not remove the need for a flood risk assessment, and four categories fall outside it.

Safe access and egress: dry routes and alternatives

A dry route is the benchmark for safe access and egress. Where one cannot be provided, the argument moves to hazard rating, refuge and evacuation, and the evidence has to be drawn rather than asserted.

Finished floor levels and freeboard: justifying the numbers

A finished floor level is only as good as the design flood level it sits above. Where that level comes from, why the freeboard is what it is, and how to draw it.

Riparian ownership duties on a development site

If a watercourse runs along or through your plot you are probably a riparian owner, with duties you did not agree to and rights that decide whether your drainage strategy can be built.

Infiltration testing to BRE Digest 365 and common errors

A single bucket test in a topsoil pit is not an infiltration rate. What the method requires, how the rate is derived, and the mistakes that get results thrown out.

Finished floor levels and freeboard: setting them defensibly

A finished floor level is only as defensible as the flood level it sits above. Get the design event, the allowance and the datum right, and the freeboard argument becomes straightforward.

Highway drainage connections and the adoption agreement

Highway drains are not sewers, and the right to connect to a public sewer does not extend to them. Who adopts what, and why the ownership plan comes first.

Drainage on a site over abandoned mine workings

Over shallow mine workings, soakaways can wash fines into voids, drive mine gas and mobilise contaminants. What replaces infiltration, and what the submission has to prove.

Trash screens: when a council will refuse one

A trash screen is a flood risk as well as a mitigation. Authorities refuse screens where the blockage case is not modelled or the maintenance duty has no owner.

Flood, Drainage and SuDS8 min read

Rainwater harvesting and the runoff calculation

Attenuation volume has to be empty when the storm arrives and a harvesting tank is trying to stay full. Here is how the two calculations differ and how to design one tank that serves both.

Flood risk where only ordinary watercourses affect a site

National flood mapping models main rivers, so a ditch or brook can present real risk that the published zones do not show. Here is the local evidence that carries a small site.

Piling and ground works over a principal aquifer

A pile can connect contaminated made ground to clean water below, and that cannot be undone. Here is what a piling risk assessment has to cover and where the Environment Agency objects outright.