Flood, Drainage and SuDS – Page 8
Flood risk assessments, sequential and exception tests, drainage strategies and SuDS design.
Flood, Drainage and SuDS articles – page 8
290 articlesSustainable drainage on a steeply sloping site
On a steep site drainage is about slowing water down. Here is how velocity, terraced storage, infiltration risk and exceedance routing come together.
Drainage strategy for a single infill plot
On one plot the drainage evidence is a short note, a percolation test and a levels plan. Here is what to include, in the order a case officer reads it.
Flood risk vulnerability classification of uses
Vulnerability classification decides whether the exception test applies. Get it wrong and the whole flood risk case is built on the wrong test.
Drainage for a change of use with no new hard surface
A change of use with no external works cannot increase surface water run-off. Officers still ask for a drainage strategy, so answer the question they should have asked about foul flows.
Discharge to a highway drain: the authority's position
A road gully looks like the easy answer for surface water. The highway authority owns that pipe for the road, not for your plot, and consent is discretionary.
Detention basins against permanently wet ponds
Only the volume above top water level counts in a wet pond, which is where most submitted strategies go wrong. This compares basins and ponds on attenuation, treatment, safety, land take and maintenance.
Swale conveyance capacity against what is drawn
A swale on a layout and a swale that carries the design flow are often different things. This covers the gradient and roughness assumptions, an illustrative capacity calculation, and the checks a reviewer should make.
Geocellular storage tanks: design life and adoption
Geocellular tanks find attenuation volume where nothing else fits, but nobody adopts them. This covers the design life argument, the silt regime, the uplift check and how to present the system so a condition is discharged.
Blue roofs as attenuation and the maintenance case
Blue roofs buy attenuation volume where there is no room for a tank. The hydraulics are the easy part. Winning the argument means showing the blocked-outlet case, the drawdown time and who maintains the flow control.
Tidal and fluvial flood risk together: joint probability
On a tidal river the peak flow and the peak sea level rarely coincide, so design levels come from joint probability rather than from stacked worst cases. This sets out the method and the checks a reviewer should make.
Flood defences and the residual breach scenario
Flood zones ignore defences, so a defended site is judged on what happens when the defence fails. This covers breach modelling, hazard rating and the design response.
Modelling a small watercourse with no published model
On an unmodelled ordinary watercourse the answer is to estimate flows, check what the channel carries, and model only as much as the decision needs.
Half drain time and consecutive storm testing
Peak storage volume tells you nothing about the second storm. Half drain time and consecutive event testing are where attenuation designs are found out.
Finished floor levels: setting and justifying them
A finished floor level has to clear the design flood level plus freeboard and still allow a step-free approach. Resolving that conflict is a design job, not a calculation.
Climate change allowances: choosing epoch and percentile
The epoch and allowance category in a flood risk assessment both follow from the lifetime of the development. Here is how to fix that lifetime and apply the Environment Agency figures without tripping a consultee objection.
Permeable paving: where it works and where it fails
It appears on drawings as a hatch with a storage volume claimed and no section to support it. The design decision is which of the three construction types you are using.
Sewer connection rights and capacity refusals
An undertaker cannot refuse a connection simply because its network is busy. It can refuse on two statutory grounds, and it can tell the council to phase your occupation until improvements land.
Sequential test for a windfall housing site
A windfall site is unallocated, so nobody applied the sequential test for you at plan-making. This is how to define the area of search and evidence the alternatives an officer will accept.
SuDS treatment trains for a car park
Attenuation and water quality are separate questions. How to build a treatment train from permeable paving, bioretention and swales, and where a separator actually belongs.
Attenuation storage sizing and the assumptions that move it
Two engineers can size the same site and differ by half, because the assumptions do the work. Design events, allowances, long term storage and the inputs a drainage reviewer tests first.
Exceedance routing: designing for the storm beyond design
Attenuation sizing does not tell you what happens when the system surcharges. Exceedance routing is a separate drawing, and its absence is a common drainage objection.
Infiltration testing that fails: what to do next
A failed soakaway test changes the drainage strategy and often the layout. How to work out why it failed, whether to retest, and how to justify the next option down the hierarchy.
Setting a discharge rate with no greenfield record
A brownfield site has no greenfield record and often no documented existing rate. Here is how to derive both, and how to get a limiting rate agreed before design freeze.
Residual flood risk and property flood resilience measures
Residual risk is what is left after the design has done its work. Here is how to describe it in a flood risk assessment, and where resilience measures stop being enough.
Not sure which documents your council will ask for?
Enter your postcode and PlanScope recommends the right reports for your project, then drafts them in minutes.
Enter your postcode on PlanScope