Slowing the flow of water as it leaves the hills

Over the last few winters we’ve seen the impact of major flooding on communities and the landscapes across the UK. Nigel Hester, project manager at the National Trust, reflects on some of the lessons from a major flood demonstration project in north Somerset:

The weather this winter has been characterised by a series of storms battering the UK with gales, mountainous seas and record amounts of rainfall, causing misery, damage and disruption to homes, businesses, infrastructure and the landscape.

 

Allerford 4

Water flowing through the village of Allerford on the River Aller; one of two villages that have been at risk of flooding as the waters head down from the hills of Exmoor

In recent years, there has been a shift in focus in flood risk management recognising that, in addition to conventional flood measures, more can be achieved by allowing the land to function more naturally. This natural flood management is at the core of an exciting demonstration flood project at the National Trust’s Holnicote Estate in Exmoor.

The project, core funded by Defra, has been running for 6 years and shows how working with nature, introducing some careful natural flood management interventions, and working in partnership, can contribute to reducing local flood risk and, importantly, provide a range of other benefits for the environment and local communities.

The target area for the work is based on the whole catchment approach, working from source of the rivers Horner and Aller high on Exmoor down to the Bristol Channel, using natural features to slow down or store flood water before it reaches the downstream villages of Allerford and Bossington.

East from bund top

One of the bunds, helping to store the water when the river levels rise, helping to reduce the risk of flooding in the villages of Allerford and Bossington

Since 2011 a range of natural flood management measures have been undertaken including moorland drainage interventions, woody debris dams, woodland creation, leaky weirs and flood storage areas on the floodplain.

Partnership work with farmers has also focused on improved soil management to reduce run-off and soil loss during rainfall events. In addition, it has been critical to have an extensive hydrological monitoring network in place to provide high quality rainfall and flow data to capture the effects on any land management changes made.

The extreme weather events in recent years have been a good test of the natural flood management measures implemented and the key outcomes are very positive. During a severe storm in late December 2013, when the ground was already waterlogged, there was a 10% reduction in the flood peak reaching the downstream villages.

In the extremely wet winter of 2013/14, there was no flooding in the vulnerable catchment villages that have experienced regular flooding in the past. The insurance value of the properties at risk is estimated at £30 million yet the capital costs of constructing the flood storage area were £163,000, a small cost in comparison.

Despite the high rainfall experienced so far in 2016, people’s homes have remained dry and the river has remained in its channel. The work has not finished at Holnicote; there are still lots of opportunities to slow the flow even further by encouraging the land to act as a natural sponge and the National Trust is committed to finding ways to continue this work into the future.

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