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Environment Agency Boscastle Flood Findings

Previous Study

• The flood of August 16 2004 was amongst the most noteworthy experienced in England and Wales, with clearly the potential for loss of life.

• The Environment Agency has commissioned a thorough and independent review of the flood.

• A consortium of leading experts has undertaken this work, to a tight timescale, comprising

HR Wallingford (leading)
Met. Office
CEH Wallingford
Halcrow
Posford Haskoning

The Findings

• Rainfall 200 mm at Otterham in 24 hours

• Rainfall 185mm at Lesnewth in 24 hours

• However most of the rain fell in a five hour period

• Peak intensities were in excess of 300mm/hr (5mm per minute)

• However 4 of the nearest 10 gauges recorded less than 3mm in total

(at Tresmeer, Lower Moor Works, St Breward and Michaelstow)

• The probability each year of the heaviest 1 hour of rainfall is about 1 in 400

• The probability each year of the heaviest 3 hour rainfall is about 1 in 1300 (note that rainfall probability is not the same a flood probability)

• The storm is classified as extreme, in accordance with joint Environment Agency/Defra R&D.

• Comparable with storms at Camelford (1957), Martinstown, Dorset (1955) and Lynmouth (1952). (The 1950’s saw 9 extreme rainfall events somewhere in the UK).

• However the meteorological factors were not like Lynmouth

• Compared with previous known floods at Boscastle we can confirm this is the worst on record (includes 1847, 1957, 1958, 1963)

Flows/Flood:

• The peak flow was about 140 m3/sec (tonnes), between 5:00pm and 6:00pm BST.

• The annual chance of this (or a greater) flood in any year of about 1 in 400

• A flow of 77 m3/sec has an annual chance of about 1 in 100

• A flow of 64 m3/sec has a chance of about 1 in 75

• Water speed was in excess of 4m/sec (10mph), more than sufficient to cause structural damage

• 2 million tonnes of water flowed through Boscastle that day.

What this means:

• The Environment Agency advises the planning authority, North Cornwall District Council in this case, on flood risks.

• We follow policy guidance, in particular Planning and Policy Guidance Note 25, Development and Flood Risk.

• Our aim is to ensure no increase in flood risk, and reduction where this is reasonably achievable.

• We may object to proposals where we consider risks to be unreasonable, such as where risk has been increased or development is inappropriate.

• The planning authority is guided to seek flood risk assessments relating to planning proposals in the floodplain.

• An annual chance of 1 in 100 of river flooding is considered significant. We still need to be able to manage the flood risks associated with less frequent flooding.

• The area affected in Boscastle was known to be floodplain before the flood of 16 August 2004. There is a history of less severe (less deep) flooding.

• The flood of 16 August 2004 was more extreme than 1 in 100 and therefore our advice to the planning authority will be based upon consideration of a lesser event. This will not necessarily affect significantly fewer properties but it will be less deep and less fast.

• In this case, rather than seek individual flood risk assessments, the Environment Agency is working with North Cornwall District Council on a Strategic Assessment that will inform all the relevant planning issues. This will save individual applicants from needing to undertake this work, and will ensure a consistent approach. This work has started.

• Every case will be considered on its merits but, given the findings, we can now advise the planning authority that in most cases the replacement of damaged buildings should not be prevented on flood risk grounds.

• We will also be using the findings of the report to inform investigations into the need for further flood risk measures. The Environment Agency is already undertaking improvements on the River Jordan in Boscastle.

• If the need for further improvement is identified then this will be considered by the South West Regional Flood Defence Committee.

11 January 2005

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