Boscastle 2004 revisited

It seems unbelievable that it was nearly 10 years ago when our TV’s were screening live footage of cars and bits of buildings travelling down a narrow valley in the Cornish coastal village of Boscastle. It was the same year that on Boxing Day the Indian Ocean earthquake and tsunami tore through south and south-east Asia, claiming over 230,000 lives. I took the opportunity to visit Malaysia and Thailand between January and November of 2005 to see the extent of the destruction, although at that time blogging was the last thing on my mind. I thought the comparably insignificant disaster at Boscastle could wait, and I was right; I went to Boscastle this week.

Timeline of the event

13:30 – Heavy rain begins to fall in the River Valency and adjacent catchments – the Met Office begins to monitor rainfall – 200mm of rain will fall over 24 hours, with 15mm over a five-minute period at peak rainfall

15:15 – The River Valency, which flows through the village, reaches bankfull height – The Environment Agency open an Incident Room

15:30 – The River Valency begins to experience overbank flows

16:00 – The River Valency is out bank and swelling, with a 3m high wall of water reported at the National Trust Visitor Centre

16:30 – Cars and buildings are beginning to form part of the river debris

17:00 – Peak discharge, believed to be approximately 180 cumecs, is reached – there was a 75 minute time to peak (the time between maximum rainfall and maximum river discharge)

18:00 – Large trees and woody debris that have joined the flow following rapid vertical incision and associated bank destabilisation now begin to enter the village, exacerbating damage to buildings and causing a back-water effect at pinch-points

19:00 – Flood waters begin to recede – the swift response of local emergency services and other agencies prevent any loss of life – 100 people are airlifted to safety – 60 properties are damaged or destroyed

The 2004 flood of Boscastle was a 1 in 400 year event. It occurred as a result of heavy rainfall, impermeable geology and a responsive catchment. Orographic uplift combined with Carboniferous sediment deposits and a highly-coupled fluvial system to deliver 180,000 litres of water per second to a popular Cornish village.

Geomorphology

But what of the geomorphology of the event? Well, it was impressive – reminiscent of the forms and processes I observed on a geography field-trip to the Welsh town of Llandudno in 2007. Highlights include:

Rapid bedrock incision into fractured slates and siltstones with catastrophic bed substrate failures in existing channels

A location where a large piece of bedrock was fractured and transported downstream during the flood

A location where a large piece of bedrock was fractured and transported downstream during the flood.

Channel widening due to river bank instability caused by downcutting

The river banks at this location were removed during the flood event.

The river banks at this location were removed during the flood event, with the current flow occupying only a fraction of the channel.

The formation of new river channels in locations where rivulets, rills and gullies already existed

Rills like this developed into active river channels in August 2004.

Rills like this transformed into active river channels in August 2004.

Bedload transport of seemingly alien granite cobbles and boulders

These granite boulders stand out in an area dominated by dark flints. It is hard to imagine how much greater the discharge would have been during the flood compared to the trickle in this image.

These granite boulders stand out in an area dominated by dark flints. It is hard to imagine how much greater the discharge would have been during the flood compared to the trickle in this image.

The deposition of fine sediment and coarse gravels in tributary deltas and lateral bars

The gravel-sized sediments in this image are also made of flint

The gravel-sized sediments in this image are also made of flint.

Aftermath

Today the village is far better prepared for flooding than it was in 2004. Within the village itself a widened river channel capable of containing a 1 in 75 year event has been constructed.

The new channel for the River Valency

The new channel for the River Valency.

In addition, a new gauging station has been built at New Mills on the River Valency and the Environment Agency has produced internal guidance on rapid response catchments. Upstream of the village a secondary channel has been dug (to improve conveyance and capacity), whilst the bridge on the river that was damaged during the flood has been replaced and reinforced to allow emergency vehicle access.

There is, of course, still room for improvement. Land-use in the catchment is predominantly low-intensity pastoral agriculture, which probably contributed to the magnitude of the flood event. Nothing has been done to attenuate the flood peak, with the five tributaries (all of which are approximately the same length and size) that feed into the River Valency remaining as they were previous to 2004.  An afforestation scheme, or the creation of flood storage areas, could have been incorporated within the defences – addressing the cause of the flooding rather than its effects.

Silver linings?

Walking around the village, you wouldn’t know there’d ever been such a devastating flood. The only real indications are the infrequent markers on some of the buildings that show the maximum height of the water in 2004 – they come around 2m up on most of the buildings.

The floods may have put Boscastle on the map, but now it's defining it's own future

The 2004 flood put Boscastle on the map, and now it’s defining its own future.

Despite the loss at the time, modern Boscastle is thriving, as explained in this article on The Ecologist website. Tourist visitor numbers have increased, tangible ecological benefits have been recorded and the village has not experienced a significant flooding since 2007.

If you are even vaguely interested in geomorphology and river management then I’d recommend you go and see it to judge for yourself. You can have an ice-cream if it’s warm or a sweet and savoury pasty if it’s cold. If you’re really lucky you might see some seals frolicking in the harbour.

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About Trevor Bond

A Geomorphology Technical Officer at the Environment Agency. All opinions expressed herein are my own and do not necessary reflect the views of my employer.
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