Flood, Drainage and SuDS
Flood risk assessments, sequential and exception tests, drainage strategies and SuDS design.
Latest in Flood, Drainage and SuDS
290 articlesThe residual risk section of a flood risk assessment
Residual risk is what remains once the mitigation is built and working. Breach, overtopping, blockage, exceedance and mechanical failure each need naming, quantifying and managing.
Water supply capacity: what a developer must evidence
Four documents carry the water supply case, none of which the applicant can write. Why the enquiry belongs alongside transport and drainage scoping, not at drafting stage.
Rainwater harvesting and the drainage credit
Attenuation needs empty volume and a supply tank is designed to be full. How a credit can be earned through a two-zone tank, and why harvesting belongs in the water efficiency case.
Who maintains the SuDS after the houses are sold
Adoption, highway transfer or a management company: the route has to be fixed at concept drainage stage, because each one constrains where components can sit.
Surface water sewer capacity and the undertaker's position
The right to connect under section 106 of the Water Industry Act 1991 survives, subject to limited refusal grounds. Why the capacity enquiry has to come before the layout is fixed.
Floodplain compensation: level for level volume calculations
Compensatory storage has to be replaced within the same elevation band it was lost from, not just in the same total volume. The calculation method, the siting rules and the drawings that get it approved.
Property flood resilience works and planning permission
Internal resilience works fall outside the definition of development, and demountable barriers are not building operations. Listed buildings, conservation areas and raised external levels are the exceptions.
Drainage that puts water onto a neighbour's land
Hard surfacing can be lawful under the GPDO and still leave a neighbour with a claim in nuisance. How the Class F condition works, what evidence is needed and how to design the problem out.
Setting a discharge rate into a combined sewer
Where surface water has to reach a combined sewer, the rate offered depends on the sewerage undertaker's capacity position and on how much existing impermeable area you can disconnect.
Rain gardens and the highway adoption problem
Street bioretention usually fails at technical approval over maintenance liability rather than hydraulics. The adoption objections highway authorities raise, and the design and land arrangements that answer them.
Trash screen blockage and the scenario the model must test
A trash screen is both a safeguard and the component most likely to fail during a storm. The flood model has to test the blocked condition and show where the water goes.
Infiltration drainage where the ground is contaminated
Clean water entering contaminated soil becomes a flushing mechanism towards groundwater. That is why the drainage hierarchy and groundwater protection pull in opposite directions on brownfield sites.
Separating foul and surface water on a conversion
Councils and sewerage undertakers expect foul and surface water to be separated on a conversion. On an older building served by a combined drain, partial separation with a stated reason is usually the honest answer.
Attenuation beneath a car park: loading and access
Below ground attenuation under a car park has to carry vehicle loads and stay maintainable for decades. Most failures on these schemes are maintenance problems created at design stage.
Raising ground levels and the displacement objection
Raising a building platform above the flood level removes storage the river still needs. Here is when that is acceptable, what conditions follow and the alternatives.
Floodplain compensation storage and level for level
Balancing volumes is not enough. Floodplain compensation has to replace storage at the same elevations, connected to the same reach, and be secured on land you control.
Wet pond or dry basin: choosing between them
A retention pond and a dry detention basin do the same hydraulic job. The choice turns on treatment performance, water balance, aerodrome safeguarding and who maintains it.
Choosing a flow control device for a small development
On a small site the permitted discharge rate is often so low that the physical outlet becomes the problem. Here is how vortex devices, orifice plates and weirs differ, and what head you need for each to work.
Which rainfall dataset the drainage model should use
The rainfall input changes a drainage answer more than the model does. Here is how the older and newer national rainfall datasets differ, how to agree one with the lead local flood authority and what a reviewer checks first.
Drainage for a single plot: what is proportionate
Major development drainage standards get applied to single plots far too often. Here is what one new house has to show, and when that changes.
What a linked 1D-2D flood model shows, and what it does not
A linked channel and floodplain model answers questions published mapping cannot. It also introduces assumptions a reviewer will test one by one.
Lead local flood authority pre-application advice
The lead local flood authority sets your discharge rate and decides which drainage components it will accept. Asking before you lay out the plots saves redesign.
Proving the surface water discharge hierarchy
The discharge hierarchy puts infiltration first and a combined sewer last. Stepping down requires dated evidence: BRE 365 percolation results at the design depth, riparian correspondence and the undertaker's written position.
Finished floor levels, freeboard and the right datum
Finished floor level is the one flood mitigation an officer can verify with a tape measure. Set it from the design flood level plus an agreed freeboard, state it in metres above Ordnance Datum, and plan the verification early.
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