West Sussex maps drains to calculate flood risk

22 October 2008

West Sussex County Council is identifying the contribution of its drainage assets to surface water flooding problems by mapping all pipes and chambers across its highway network.

West Sussex has surveyed more than 122,000 gullies, identifying their exact location using the KOREC GPS, and recorded silt levels, as a first step to producing a 10 to 20-year strategy for maintaining its drainage assets.

The authority is also on track to have recorded, and mapped, the size, depth and condition of all manhole chambers and pipes across its 3,900km network by next April.

The priorities for more frequent maintenance, or work to increase drainage capacity, will be influenced by fresh data on areas more prone to surface water flooding due to urban topography, provided by the Environment Agency, following a Pitt review recommendation.

The data collection started before the Pitt review, or the floods of last summer, with the approval of £500,000 additional funding for 2007/08, West Sussex emphasised.

Stuart Smith, operations manager, said: ‘The drivers for this were our asset-management plan, and to further improve management of highway works. Once we’ve analysed the data, we’ll put together a costed maintenance plan.’

The gulley survey revealed that 94% were less than 50% full with silt which, Smith said, suggested that the majority of the drainage network was functioning adequately. However, he stressed to local politicians that this still meant that, in as many as 6,500 locations, inadequate drainage was resulting in localised flooding.

‘At some locations, gullies need to be emptied more frequently than annually, as now, but others don’t need to be emptied every year,’ he said. A capital programme to enlarge pipes – to at least 252mm in diameter – costing more than £1M a year would be approved, with funds diverted from carriageway maintenance because ‘there’s no point improving carriageways that aren’t adequately drained,’ said Smith.

The EA advised councils on areas at risk of surface water flooding, using a model JBA Consulting developed.

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