Case Study: Replacement Dwelling on a Surface Water Flow Route — Drainage Strategy Praised by the Council

Written by Antony Riozzi BEng (Hons) — Director & Principal Consultant, Flood Risk Environmental. 17+ years in flood risk & drainage, with experience at a Lead Local Flood Authority, a national housebuilder and in consultancy. About the author →

Location

North Heath, Pulborough, West Sussex

Development

Demolition of an agricultural barn and erection of a 1.5-storey dwelling (alternative to a Class Q prior approval)

Authorities

Horsham District Council (drainage consultee) · West Sussex County Council (LLFA)

What we delivered

Combined FRA and foul & surface water drainage strategy · BRE 365 testing · Causeway Flow+ modelling · three-sheet drawing set · long-term maintenance plan

Outcome

No Objection from the council's drainage consultee, no conditions recommended — the report described as “an excellent piece of work”

The existing agricultural barn at the farm near Pulborough with the trial pit spoil in the foreground — The existing agricultural building to be replaced, with the BRE 365 trial pit in the foreground. Site photograph from the June 2026 BRE 365 infiltration testing (Simon Harvey Designs Ltd), reproduced with permission.

The challenge

The site is in Flood Zone 1, so a planning FRA was not formally required — but the Environment Agency's surface water map shows a flow path running through the eastern part of the farmyard alongside an ordinary watercourse, and the council's drainage consultee picked it up. Rather than deal with drainage by condition, the officer asked for a full foul and surface water drainage strategy and a site-specific assessment of flood risk before the application could be determined, and set out a long list of what it had to contain.

The ground did not help. The site sits on Weald Clay, there is no public sewer within or adjacent to the site, and the only outfall is a small watercourse that can surcharge in wet weather. Every step of the drainage hierarchy had to be tested with evidence, not assumed.

What we did

We began with the flood risk, because it fixed the floor level. Interpolating the Environment Agency's surface water extent against the topographical survey gave a design flood level of 27.45 mAOD beside the proposed dwelling in the 1 in 100 year event with climate change. We recommended a finished floor level of 27.75 mAOD — 300 mm freeboard. The existing barn thresholds sat at or just below the flood level, so the replacement dwelling is a clear improvement on both the current building and the consented Class Q fallback.

Infiltration came next. BRE 365 testing in the Weald Clay returned 4.29 × 10^-7 m/s — soakaways were not viable — and the water company's asset records confirmed there were no sewers to connect to. That left a controlled discharge to the watercourse, which we designed from the ground up.

Water standing in the BRE 365 trial pit 70 hours after filling — Seventy hours after filling, the water in the trial pit had fallen just 0.7 m — 4.29 × 10^-7 m/s in the Weald Clay. Site photograph from the June 2026 BRE 365 infiltration testing (Simon Harvey Designs Ltd), reproduced with permission.

FRE drainage strategy drawing 1048_1001 for the replacement dwelling showing the tanked permeable paving, flow control chamber, outfall headwall, catchment areas, exceedance routing and package treatment plant — FRE drawing 1048_1001 — Detailed Drainage Strategy. Roof and driveway drain into a fully tanked permeable paving sub-base; discharge is limited by a flow control chamber to a flap-valved headwall on the watercourse, and foul flows go to a package treatment plant. Catchment areas, exceedance routing and levels are all on the sheet.

All of the impermeable area — 234.8 m² of roof and drive, plus 10% urban creep — drains into a fully tanked (CIRIA C753 System C) permeable paving construction with a 600 mm sub-base, giving about 19.7 m³ of storage. Discharge is restricted to 1.0 l/s, the minimum practically achievable against a greenfield QBAR of 0.1 l/s, through a 25 mm orifice in a proprietary flow control chamber, to a precast headwall with a factory-fitted flap valve. Causeway Flow+ modelling with FEH22 rainfall showed the 1 in 100 year plus 45% climate change event contained wholly within the sub-base with 172 mm of headroom, half-emptying in 144 minutes.

Because the consultee had flagged that the outfall may be submerged during events up to the 1 in 30 year, we assessed the surcharged case against surveyed channel geometry: the orifice invert sits above the design tailwater, and the flap valve is rated to 1.3 m head against a maximum design tailwater of 1.16 m. Foul drainage goes to a 5PE package treatment plant discharging under the Environment Agency's general binding rules.

FRE construction details sheet 1048_1002 for the permeable paving, flow control chamber, headwall and pipework — FRE drawing 1048_1002 — Construction Details for the tanked permeable paving, pipe bedding, inspection chambers and the filter inlets that take the roof water into the paving.

The report closed with a long-term maintenance plan and a Statement of Compliance table that took every requirement in the consultee's letter — site-specific flood risk, existing and proposed flow paths, pre- and post-development areas, greenfield rates, the drainage hierarchy, BRE 365 evidence, the watercourse assessment, the three containment tests — and pointed to the paragraph or drawing that answered it.

The outcome

A single document that answered the consultee's request in full, ahead of determination, without a further round of questions. The council's drainage officer returned a formal response of No Objection with no conditions recommended, and said this:

“The Flood Risk and Drainage Strategy report written by Flood Risk Environmental, on behalf of the applicant, is an excellent piece of work. All concerns raised about flood risk on site have been addressed and the design of the strategy has been carefully considered, providing justification for the design and meeting industry standard.”
— Horsham District Council — Drainage consultee response, 27 July 2026 (No objection, no conditions)

Horsham District Council drainage consultee response for DC/26/0597 recommending no objection and describing the Flood Risk and Drainage Strategy as an excellent piece of work — The council's drainage consultee response in full — recommendation: No Objection, no conditions. Open the response (PDF, 1 page) →

The applicant got a scheme that is buildable as drawn, a floor level that protects the house, and a drainage system that can be maintained by the householder for the life of the building.

If you are in the same position. Drainage consultees increasingly refuse to leave drainage to condition on small sites with a mapped flow path or a difficult outfall. The way through is a strategy that tests the hierarchy with real evidence — infiltration testing, sewer records, a surveyed watercourse — and answers the officer's letter line by line.

Ask about a drainage strategy for your site →

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Frequently Asked Questions

Why did a Flood Zone 1 site need a flood risk assessment?

Flood Zone 1 only describes river and sea flooding. The site had a mapped surface water flow path through the yard, so the council asked for a site-specific assessment of flood risk from all sources even though a planning FRA was not formally triggered. The assessment fixed the finished floor level for the new dwelling.

What happens when infiltration fails and there is no sewer?

You move down the drainage hierarchy to a watercourse, with the discharge restricted as close to greenfield as is practicable — here 1.0 l/s — and enough storage to hold the 1 in 100 year plus climate change event. The outfall then has to be designed for the case where the watercourse itself is high, which is why the flap valve and orifice level mattered.

How much storage did the scheme need?

Around 19.7 m³ gross, provided entirely within the 600 mm sub-base of a tanked permeable paving driveway — so no separate tank. The 1 in 100 year plus 45% climate change event peaked 172 mm below the block surface.

What is a Statement of Compliance?

A table that lists every requirement in the consultee's response and states where in the report or drawings it is answered. It lets the officer check the strategy in minutes and removes the usual second round of questions.

Replacement Dwelling on a Surface Water Flow Route — Drainage Strategy Praised by the Council