Sizing Storm Attenuation and SuDS Storage with the Environment Agency Peak Rainfall Climate Change Allowances

The standards the design must satisfy

Defra’s 2015 non-statutory technical standards (NSTS) are the baseline that planning conditions and LLFA guidance reference. S2 requires greenfield developments to discharge no more than the greenfield peak rate for the 1 in 1 year and 1 in 100 year events; S3 allows brownfield sites to discharge no more than the pre-development rate, but as close to greenfield as is reasonably practicable. S4 and S5 control volume for the 1 in 100 year, 6 hour event, and S6 allows a controlled slow release where volume cannot be matched. S7 bars flooding anywhere on site in the 1 in 30 year event, S8 bars flooding of buildings and vulnerable plant such as pumping stations and substations in the 1 in 100 year event, and S9 requires exceedance routes above that. Local guides tighten these: Suffolk’s LLFA, for example, prefers discharge at Q_BAR rather than the event-specific greenfield rate, asks for a minimum 30 percent betterment on brownfield sites for the 1 hour storm, adds 10 percent for urban creep on residential impermeable areas, sets a 100 mm minimum orifice on small sites, requires 5 mm interception storage, and asks for storage to half-drain within 24 hours after the 1 in 30 year plus climate change event.

Planning policy background, including the NPPF sequential tests, is covered in the surface water runoff and SuDS guide; this article concentrates on the numbers.

Where the climate change allowances come from

Environment Agency guidance on flood risk assessment climate change allowances was updated on 10 May 2022 to replace the former national peak rainfall uplifts (20 to 40 percent) with values derived from the Met Office UKCP Local 2.2 km convection-permitting projections and the Newcastle University FUTURE-DRAINAGE ensemble. Allowances are given for each of around 100 management catchments in England, for the 3.3 percent and 1 percent annual exceedance probability events, for two epochs (2050s covering 2022 to 2060 and 2070s covering 2061 to 2125) and at two percentiles of the high emissions ensemble: central (50th percentile) and upper end (95th percentile). Guidance is to use the central allowance for development with a lifetime to 2100 and the upper end for lifetimes to 2125, with the upper end also used for essential infrastructure and where a credible maximum scenario is required.

Two points catch designers out. First, the 3.3 percent AEP allowances are often the same or 5 percentage points lower than the 1 percent values, so the S7 check on the 1 in 30 year event needs its own uplift from the table rather than the 1 in 100 year figure. Second, the allowances are uplifts to rainfall intensity across all durations, applied to the FEH depth-duration-frequency estimate; they are not applied to the greenfield discharge rate, which is calculated without climate change. Because the allowable outflow stays fixed while the inflow grows, the storage volume grows by more than the percentage uplift, as the worked example shows. Suffolk’s 2023 guide adopts 45 percent for residential development with a 100 year lifetime, the upper end 2050s value for its catchments, and notes that the online table takes precedence whenever it is updated.

Rainfall, runoff and the critical duration

Design rainfall comes from the FEH web service as a depth-duration-frequency (DDF) model for the site centroid, using the FEH13 model for return periods of 2 years and above and, by convention, the older FSR profile for the 1 in 1 year check because FEH13 is less reliable for frequent events. Hydraulic models apply summer and winter storm profiles across a range of durations, typically 15 minutes to 48 hours, and the critical duration is whichever produces the largest storage requirement for the chosen outflow. Short storms govern where the outflow is a large fraction of peak inflow; long storms govern where the outflow is small relative to the site, which is the usual case once discharge is throttled to greenfield rates. Greenfield rates for sites under 50 hectares are estimated by the IH 124 or ReFH2 methods on the positively drained area only, without any climate uplift:

Runoff from impermeable area is taken at a volumetric coefficient of 1.0 in most LLFA guides, so a hectare of roof or hardstanding yields 10 m³ per millimetre of rain. Permeable surfaces are either excluded from the network or given a runoff coefficient from the soil type.

Worked tank sizing

Site: 2.0 ha greenfield in the East Suffolk catchment, residential, 100 year lifetime; 1.2 ha impermeable plus 10 percent urban creep = 1.32 ha; Q_BAR for the whole site at 1.5 L/s/ha = 3 L/s, raised to 5 L/s by the 100 mm minimum orifice (a hydro-brake would allow the lower value). Allowance: 40 percent (2070s upper end, 1 percent AEP), the value for a lifetime to 2125; a 2050s central design at 20 percent is shown for comparison. FEH13 1 in 100 year depths for the site centroid are taken as 45, 55, 72, 85, 100 and 120 mm for 1, 2, 6, 12, 24 and 48 hour durations. A first-pass storage estimate ignores drain-down during the storm beyond the controlled outflow:

Critical duration is 24 hours at this low outflow, and the tank needs about 1,420 m³ at the 40 percent allowance against 1,150 m³ at 20 percent and 890 m³ with no uplift. A 40 percent rainfall uplift has therefore produced a 59 percent increase in storage, because the outflow term does not grow. Half-drain check: Suffolk asks that storage half-empties within 24 hours after the 1 in 30 year plus climate change storm. With a 24 hour 1 in 30 year depth of 75 mm uplifted to 105 mm the stored volume is 954 m³, and 477 m³ at 5 L/s takes 26.5 hours: a marginal failure, resolved by splitting the outflow between the throttle and an infiltration trench, or by accepting a modest increase in discharge rate with the LLFA. Long-term storage under S4 is also needed because the site discharges above Q_BAR: the 1 in 100 year 6 hour developed volume of 1,331 m³ less the greenfield volume of about 600 m³ (2 ha at 101 mm and 30 percent runoff) leaves roughly 730 m³ to be infiltrated or released at 2 L/s/ha. Finally the 1 in 30 year on-site flooding check (S7) and the 1 in 100 year building check (S8) are run in the model with their own allowances, and exceedance routes mapped.

A full hydraulic model with FEH13 summer and winter profiles and 15 minute to 48 hour durations will trim these hand figures by 10 to 20 percent, because outflow continues through the recession limb and the design storm profile peaks rather than being uniform. Hand calculation remains worthwhile as a check: a model result far from it usually means an area, coefficient or allowance has been entered wrongly.

Attenuation for treatment plant and construction sites

Sewage and water treatment sites attract the S8 condition explicitly, since pumping stations, MCCs and chemical stores are named as plant susceptible to water. Hardstanding fractions are high, so impermeable area approaches the whole site and storage volumes per hectare are large; roof water from packaged plant rooms and kiosks should be collected separately from process areas so that it can go to infiltration or a clean outfall rather than through the works drainage. Contaminated-yard drainage is a separate system with interceptors and, where chemicals are stored, bunded or valved isolation, and its attenuation must still satisfy the same rate limits.

Temporary works are often overlooked. A construction site stripped of topsoil runs off at close to impermeable rates for two or three years, and silt-laden runoff is the commonest pollution incident on new developments. Temporary lagoons sized on the same 1 in 30 year plus allowance basis, with the long storm governing, and with a silt treatment step before discharge, meet both the flood and the water quality obligations under the Environment Agency regulatory position statements for construction water. Where a tide-locked or surcharged outfall is possible, storage must cover the coincident 1 in 30 year pluvial event with the outfall closed, which can double the volume again.

Pitfalls

• Wrong epoch or percentile. Residential lifetimes are taken as 100 years, which now pushes designs into the 2070s epoch and, for lifetimes beyond 2100, the upper end allowance. Commercial units with 60 year lifetimes can use the 2050s central value and save a third of the storage.
• Applying the uplift to the greenfield rate. Greenfield rates are calculated without climate change; uplifting them inflates the allowable discharge and understates storage.
• Short duration bias. Models run only to 6 hours miss the critical duration at low outflows; run to 48 hours, or longer where tide locking applies.
• Orifice blockage. Below 100 mm the throttle blocks; vortex controls deliver a lower rate at a larger opening and are worth the cost on sites where Q_BAR is under 5 L/s.
• Neighbouring catchment. Where the dominant source of flood risk lies in an adjacent management catchment, the EA expects that catchment’s allowance to be used.
• Silt and void ratios. Geocellular crates are 95 percent void, stone-filled trenches 30 percent; add 10 percent storage for silt in tanks and verify the half-drain time with the final geometry.