Flood, Drainage and SuDS – Page 11
Flood risk assessments, sequential and exception tests, drainage strategies and SuDS design.
Flood, Drainage and SuDS articles – page 11
290 articlesReservoir inundation mapping as a planning constraint
A site inside a reservoir inundation extent is not in a flood zone, and the two are often confused. The distinction changes what the assessment has to do.
Basements and habitable rooms in flood risk areas
A basement bedroom and a basement plant room are treated completely differently in flood risk policy. The vulnerability classification, not the construction method, decides whether the scheme is arguable.
Flood warning and evacuation plans: who writes and approves them
A flood warning and evacuation plan is an emergency planning document, not a hydraulic one. Knowing who approves it saves weeks at determination.
Critical drainage areas and what changes inside one
A critical drainage area lowers the threshold at which a flood risk assessment is required and tightens discharge rates. Most applicants find out at validation.
Attenuation storage, interception and half drain time
Discharge rate and storage volume are separate calculations. This is how interception, long term storage and half drain time change the volume a site has to hold.
Reading the surface water flood risk map for a site
Two national flood maps exist and they show different things. Mistaking one for the other is why some applications arrive with no surface water evidence at all.
Flood resistant and resilient construction: what to specify
Below about 600 mm you can try to keep water out. Above that, the sensible strategy is letting it in and designing so the house dries out and comes back into use quickly.
The flood sequential test for a single plot in Flood Zone 2
One new house in Flood Zone 2 is not minor development, so the sequential test applies. The argument is almost always about the area of search and what counts as reasonably available.
Basement drainage: pumped systems and surcharge risk
Basement drainage fails in two directions. Water has to get out against gravity, and sewer water has to be stopped from coming in when the public sewer surcharges.
Overland flow routes and exceedance design
Every drainage system has a capacity. Exceedance design decides where water goes once that capacity is passed, and it is resolved on the levels drawing rather than in the hydraulic model.
Rainwater harvesting in a drainage and water strategy
A harvesting tank cuts mains water use. It does not reliably attenuate, because on the day of the design storm it may already be full. Present it accordingly.
Foul sewer capacity: making a pre-development enquiry
Submit the pre-development enquiry before the planning application. The undertaker answers on its own timetable, and a no-capacity response reshapes the whole programme.
Sequential Test: agreeing the area of search
The area of search decides the outcome of a sequential test more often than the flood zone does. Define it from the development's function, and evidence every discount from the council's own list.
Flood warning and evacuation plans: what councils expect
The flood risk assessment sets the levels. The evacuation plan answers what the occupants do when the water arrives, and it is judged on lead times rather than modelling.
Permeable paving on clay: designing for zero infiltration
Tanked permeable paving keeps the interception and treatment benefits and replaces infiltration with a controlled discharge. The sub-base depth and the outfall invert drive the design.
Blue roofs: design, loading and maintenance
A blue roof turns the deck into an attenuation tank. The storage depth drives the structural design, and the flow control is the part that fails.
Discharging surface water to a highway drain
The pipe under the road belongs to the highway authority and was sized for the carriageway. How to seek consent, what flow restriction to design for and how to evidence that infiltration was tested.
Groundwater flooding where the maps show nothing
Flood zones cover rivers and the sea, not a rising water table. Where the evidence for groundwater risk comes from, how to monitor it and what lead local flood authorities expect to see.
Reservoir inundation areas and what they mean for a site
A mapped reservoir inundation extent does not change the flood zone or the sequential test. What it does change is the residual risk chapter and, for vulnerable uses, the evacuation strategy.
Water quality treatment in a SuDS management train
A basin sized correctly is only half a drainage design. The simple index approach balances pollution hazard against mitigation per sub-catchment, and reviewers should see the arithmetic.
Package treatment plants and the general binding rules
The general binding rules let a package treatment plant discharge without a permit within set volume limits. Percolation results and the foul drainage hierarchy decide most rural applications.
Building over or near a public sewer: the agreement
Building over or within three metres of a public sewer needs the undertaker's agreement, separately from planning permission and building control. Here is the process and the timings.
Greenfield runoff rates and the methods used to derive them
The greenfield runoff rate caps what a developed site may discharge and drives the whole attenuation design. Here is how the estimation methods work, why small sites are a problem, and how to agree the rate with the lead local flood authority.
Infiltration testing to BRE 365 and reading the result
A BRE Digest 365 infiltration test needs a trial pit at the soakaway location, dug to full depth, filled three times. Here is how the rate is calculated, why the slowest cycle governs, and what to present in a drainage strategy.
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