Case Study: Six-Storey Mixed-Use Scheme on a Surface Water Flow Path — FRA and Drainage Strategy, Elland

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

Elland, Calderdale, West Yorkshire

Development

Demolition of a vacant former pharmacy; six-storey building with two commercial units, ten apartments and twelve parking spaces — outline application

Authorities

Calderdale Metropolitan Borough Council (LPA and Lead Local Flood Authority)

What we delivered

Site-specific FRA · outline drainage strategy with two options · 14-plan open-data map series · two drainage strategy drawings · completed Calderdale surface water drainage pro forma

Outcome

Submitted with the outline application with the flow path, levels and exceedance routes resolved in principle for the reserved matters design

The challenge

The site is 20–25 m above the River Calder and comfortably in Flood Zone 1, so the sequential test is passed without argument. Surface water is another matter. The Environment Agency's surface water map shows a flow path entering from the higher St Mary's Gate side, crossing the existing undrained car park and reaching the footprint of the proposed building, at depths of 600–900 mm in the 1 in 1,000 year event and the 1 in 100 year event with climate change. On a 0.0535 ha plot, with ten apartments above two ground-floor commercial units and a car park to fit in, there was nowhere to hide from it.

FRE plan of the Environment Agency Flood Map for Planning around the Elland site showing Flood Zone 1 at the site and the River Calder floodplain to the north-west — Flood Map for Planning: the site is in Flood Zone 1, with the River Calder floodplain 250–400 m to the north-west and 20–25 m below. Flood data © Environment Agency (OGL); basemap © MapTiler © OpenStreetMap contributors.

Calderdale, as Lead Local Flood Authority, also requires its surface water drainage pro forma to be completed at outline stage, and there was no topographical survey, ground investigation or infiltration testing yet.

What we did

FRE plan of Environment Agency surface water depth for the 1 in 100 year event with climate change across the Elland site, showing the modelled flow path crossing the car park to the building footprint with depth bands and sample points — Surface water depth, 1 in 100 year event with 2061–2125 climate change, from our open-data plan series. The flow path crosses the car park and reaches the building footprint — this plan, and eleven others like it, set the design problem. {OSM}

We produced a 14-plan open-data map series — flood map for planning, surface water extents and depths for three return periods with climate change, historic flooding, warning areas, geology — so that every claim in the report could be seen rather than asserted. Then we dealt with the flow path by design.

The car park becomes permeable paving draining to on-site storage. A linear drainage channel along the rear face of the building intercepts residual flow before it reaches the elevation. Proposed levels grade the car park to the southern boundary, and the overland flow routes on the drawings direct any exceedance onto the lane to the south and down to Huddersfield Road, away from the building. Finished floor levels sit a minimum of 150 mm above external ground, and all habitable accommodation is at first floor and above — so the ground-floor commercial units are the only floor space in contact with the flow path, and they are protected by levels, interception and routing.

FRE outline drainage strategy drawing for the Elland scheme, Option 2, showing the permeable car park draining to a lined attenuation tank with a restricted discharge to the combined sewer, proposed levels and overland flow routes — Outline drainage strategy — Option 2, attenuation and restricted discharge to the combined sewer at 1.0 l/s, for use if infiltration testing rules out Option 1. Both options carry the same levels, interception channel and exceedance routes.

The drainage strategy follows the hierarchy with two options carried in parallel. Option 1 is infiltration: BGS mapping shows permeable Rough Rock sandstone at or near the surface with no clay cover, so a geocellular soakaway is the preferred route, to be confirmed by BRE 365 testing at detailed design. Option 2 is a lined attenuation tank with a vortex flow control restricting discharge to 1.0 l/s to the combined sewer, using Yorkshire Water's georeferenced asset records. Green or blue roofs, rainwater harvesting and the permeable car park provide source control and treatment in both. The design allows for the Environment Agency's upper-end +45% peak rainfall allowance for the Aire and Calder catchment over a lifetime to 2125, and the pro forma was completed line by line from the strategy.

The outcome

An outline submission that turns the surface water flow path from a reason for refusal into a set of design commitments the reserved matters scheme has to honour: levels, an interception channel, permeable parking, exceedance routes and a drainage hierarchy with the evidence for each step. The applicant knows exactly what testing is needed before detailed design, and the LLFA has its pro forma answered from the outset.

If you are in the same position. A surface water flow path across a tight urban plot is a design problem, not a reason to abandon the scheme. It is solved with levels, interception, permeable surfaces and an honest exceedance route — and by giving the LLFA the evidence it asks for in the format it asks for it.

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Frequently Asked Questions

The site is in Flood Zone 1 — why was an FRA needed at all?

Because a flood risk assessment is also required where a site is affected by other sources of flooding, and here the Environment Agency's surface water map showed a flow path crossing the plot to the building footprint. Most councils will also expect a drainage strategy and their LLFA pro forma with any outline application of this size.

How do you deal with a surface water flow path that reaches the building?

Three things together: proposed levels that grade the site away from the building, an interception feature — here a linear channel along the rear face — and a drawn overland flow route that takes exceedance flow to a safe place, in this case the lane and the road. Finished floor levels are then set with freeboard above the surrounding ground.

Why carry two drainage options at outline stage?

Because infiltration cannot be confirmed without BRE 365 testing, and there was no ground investigation yet. Presenting the preferred infiltration option alongside a fully worked attenuation-and-discharge alternative shows the LLFA the scheme is deliverable either way and removes the risk of a second round of questions.

What is the Calderdale surface water drainage pro forma?

A standard form the Lead Local Flood Authority asks applicants to complete, summarising site areas, existing and proposed runoff rates, storage, the drainage hierarchy and the SuDS proposals. We complete it directly from the drainage strategy so the two documents agree.

Six-Storey Mixed-Use Scheme on a Surface Water Flow Path — FRA and Drainage Strategy, Elland