Strategic Review of Drainage and Flood Management for the Lower Otter Valley

The Clinton Devon Estates commissioned Haycock Associates in late 2009 to undertake a review of the current drainage and flood management of the Lower River Otter from Otterton to the discharge point of the River Otter into the sea. We looked at the long term options for the drainage and flood management of this section of the Valley, and how the underlying hydrological, geomorphological and climatic change regimes may impact the farming, business and conservation operations on this land as well as local amenities such as the cricket club. The review generated options for the strategic management of the valley.

Floods in October 2008 provided an insight into the dynamics of the site, but this flood event was extreme (>220 year return period) and so we focussed on smaller storms that had resulted in 21 flood events in the Lower Otter Valley since 1959.

Although part of the East Devon Area of Outstanding Natural Beauty, the Lower Otter has been subject to a series of management regimes for as long as man has used the river and valley.  The historical development of the floodplain, from the leating of the channel for the mill prior to 1066, the need for navigation to Otterton until 1540 and then the embankment and drainage of the floodplain in 1812 for farming, plus the railway embankments created in 1903, have all complicated the natural flood and geomorphological regime of the river. Further complications arise in the form of a major storm surge in the decade of 1480 that created the pebble bar, and also recent sea level adjustments, that have accelerated the siltation of the estuary and the geomorphology of the coastal sediment supply.

In reviewing the current hydrology, hydraulics and landscape operations within the Lower Otter, it is apparent that the impact of poor drainage and flooding is becoming worse and more damaging to the economics of the Estate and users of the landscape. The poor performance of the sea outfalls, the reduced hydraulic capacity of the River Otter and the siltation of the estuary since 1812 are all conspiring to increase the flood frequency of a number of assets, most visibly at South Farm Lane and the Cricket Club.

"It seems that the flood protection that has worked for years and the stability of the surrounding farmland is now unzipping itself.  Facing a future where sea levels and climate change will create more unpredictability now is a good time to look for long term solutions - solutions dictated to us by nature - not the other way round.", Nick Haycock, Haycock Associates.

Leaving the Valley to its own devices is not really an option. The future performance of drainage will become increasingly challenging, with increased fluvial runoff and higher tides, plus the aggressive sedimentation of the sea outfall. The situation would become progressively worse, farming of the floodplain to become less viable, conservation management more demanding and recreational access more difficult. At some point, either due to fluvial flooding or tidal flooding the current embankments, which are nearly 200 year old, would fail and the resulting impact would be extensive loss of terrestrial land and access to businesses.

It is proposed that there may be a series of options to phase the retreat of agriculture from the marshes in some locations, upgrade the functioning of assets in other locations, and enhance the life of some of the embanked lands through small modifications of the current land levels and linkages of the river to its floodplain. The options, in combination, will allow the long term retreat of agriculture from the marshes, but allow the controlled formation of designated habitats. Currently Devon Clinton Estates are consulting with a wide range of stakeholders to explore the different options proposed.

To read more about this project see links below:

BBC News - Devon estate considers valley flooding retreat plan

Clinton Devon Estates | News | Latest News | A future for land use in the Lower Otter Valley - by John Varley