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Climate outstrips flood mapping

Changing weather patterns could make existing flood mapping techniques irrelevant, a senior consultant said this week.


Flood mapping: could be outdated

“Short bursts of rainfall will cause flooding through the existing drainage systems via a mix of river and surface water flooding, and that can happen in more local settings - for example in small catchments that were previously “safe” from flooding,” said WSP head of water Ola Holmstrom.

“A lot of flood mapping has in the past not looked very closely at catchments less than 40km2.

“We are good at mapping catchments for big rivers, but very bad at looking at small catchments,” he added.

“This hasn’t been given a high priority because not as many people are being affected as are affected by major catchments like the Trent or the Thames. But in future there will be far more people coming forward saying ‘why didn’t you tell us?’”

Holmstrom said it is “almost impossible” to put out flood warnings for these localised intense flash floods based on current mapping methods, which are based on the rate of rise in river systems and flow velocities.

“What we need to move towards is looking at rainfall volumes and patterns, which it will be possible to use for storm mapping,” he said. “That is a very different way of looking at it and might be able to help small catchments.”

Readers' comments (1)

  • I agree that small catchment areas present particular challenges when localised downpours occur. In my recent experience ( October 2012), The rainfall was too great for the local combined system pumping station resulting in sewage backing up into the houses. A storm overflow onto the local beach would have saved the houses but from an environmental perspective this might not have been very popular. I would maintain that the dilution of the sewage in such cases would be great enough not the cause a significant impact.

    Rod Cooper
    Sutton, Dublin

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