WIREDGOV

A pioneering natural flood risk management scheme reduced river peak flow by around 15-20% during the December floods, a briefing note published yesterday (13 April) reveals.

The natural flood risk management scheme provided important protection for the town of Pickering in Yorkshire when a total of 50 mm of rainfall fell over a 36-hour period over Christmas 2015.

Pickering’s ‘Slowing the Flow’ partnership was set up after the town saw four serious floods in 10 years, with floods in 2007 estimated to have caused damage of around £7 million.

The scheme combines conventional upstream flood storage with a wide range of measures designed to work with nature to hold water on the land and slow the speed at which it enters the river system. In addition to the construction of a flood storage reservoir: 40,000 trees were planted, local heather moorland restored and over 300 leaky dams built in forest and moorland drains and streams in the upper catchment.

The new analysis, conducted by Slowing the Flow Partnership, concludes that these measures reduced the flow of flood water between 15% and 20% and prevented the flooding of a number homes and the town’s museum. Based on the extent of inflows to the flood storage area, it is estimated that around half of the reduction was due to the upstream land management measures and half due to the flood storage area

The investigation involved:

• Quantifying the volume of rain that fell during the Boxing Day event by analysing the records from local rain gauges

• Determining if the amount of rainfall was sufficient to generate a flood by comparing this with other events in previous years

John Curtin, Environment Agency Executive Director of Flood Risk Management, said:

Natural flood risk management measures when used alongside more traditional flood defences can make an effective contribution to reducing flood risk, as demonstrated in Pickering. They can also deliver more benefits than just reducing flood risk – such as improving water quality, preventing erosion and in some cases storing carbon.

Jeremy Walker, ‘Slowing the Flow’ Partnership Chairman, said:

We wanted to understand fully whether our efforts in recent years in trying to hold back and store water in the catchment prevented flooding in Pickering in the days after Christmas. This analysis by our hydrologists confirms that some flooding was avoided, although the measures were not fully tested. The key finding for us is that they appear to be working as expected and reducing the peak flood flow by up to 20%.

This is good news for the town, although we need to remember that the measures installed have their limits and would not be enough to prevent flooding in the event of rainfall on the scale experienced for example in 2007.