Drainage Strategy for Business Park prevents pollution from Fire-fighting event

7 11 2017

Toulouse Plant Limited proposed redevelopment of a 5,600m2 brownfield site at Littlebrook Business Park, north of Dartford.  The proposal comprised a materials recycling building, installation of weighbridges, offices, and ancillary buildings.  Their architects, March Design, submitted a Consultation to Dartford Borough and Kent County Councils.

The Stilwell Partnership were appointed to carry out a Flood Risk Assessment and develop a Drainage Strategy to submit with the Consultation.  The final Drainage arrangement had to prevent potential sources of pollution entering the groundwater, not just from normal operations, but to include catastrophic events such as a serious fire on the site.

Littlebrook Business park Schematic Drainage Layout

A Drainage Strategy considers all the many interrelated flooding and drainage issues associated with a development: Highway and Land drainage, Groundwater and Watercourses, Foul Water drainage, Surface Water drainage and Sustainable Drainage (SuDS).

The Environment Agency mapping service indicated the site is over a Minor Aquifer and is adjacent to a Ground Water Source Protection Zone.  Treatment measures would be included in the drainage network as appropriate to ensure that potential sources of pollution are prevented from entering the groundwater.

The Drainage Strategy considered the SuDS hierarchy and the best option was selected based on the underlying geology.  In this case, based on MicroDrainage calculations, buried attenuation tanks were proposed to take run-off from intensive rainfall events.  The tanks would gradually empty at a controlled rate into an existing private surface water sewer.

On site gradients and crossfalls were developed to accommodate the complex drainage arrangements.  Rainwater run-off from the roof areas would pass through a catchpit to trap the sediment content.  Surface water run-off from the parking/weighbridge area would pass through a bypass oil separator to remove pollutants and debris.

The Environment Agency requested that surface water run-off from the main yard passes through a full retention separator, to remove the potentially high silt content and debris with large sediment particles from the run-off, before discharging to the main foul water sewer.

Vehicle wash down effluent would be expected to contain high levels of pollutants. A specialised vehicle wash catchpit and full retention separator would be provided to treat the effluent before discharging to the main foul water sewer.

The Flood Risk Assessment revealed that the site is presently protected by existing off-site Environment Agency flood alleviation measures and is therefore categorised as within Flood Zone 3 (Defended)

The site is not expected to be subject to flooding provided those flood defences remain intact.  The report considered a breach of the flood defences and recommended additional flood protection (flood gates and doors) to the office and ancillary buildings, equivalent to the defence breach level plus climate change provision.  These additional measures would provide adequate time for any occupants to evacuate the site safely.

At a late stage in the process, Kent County Council raised the question of protecting the surface water drainage system in the event of a fire.  They required measures to be included that would prevent pollution bypassing the interceptors and washing into the sustainable drainage system.

It was proposed that in the event of a fire, shut-off valves would isolate the yard areas from the surface water drainage systems.  These areas have been “dished” to increase their standing water capacity.  Following a fire event, the client would use a temporary pump system to over-pump the fire water into the foul sewer before opening the shut off valves to their normal position.

In September 2017, Kent County Council accepted this proposal and lifted their objection to the development proceeding.

The Stilwell Partnership also produced a Transport Statement and Travel Plan for this proposed development.

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