Boscastle, United Kingdom,

Drainage basin flooding

By Amy and Beatriz

Location

Boscastle is a rural village located on the North coast of Cornwall, in the South West of England.

Boscastle

There are two rivers which meet at Boscastle. The Valency River and its tributary the Jordan River, which come from high hills of Bodmin Moor. These rivers were the main cause of the flood.

Background - physical causes

The confluence of two rivers at Boscastle.

The location of the village, within the Valency valley.

The steep valley sides ensured a high amount of surface run-off.

The geology and the soil type of the area.

Impermeable rocks with clay soils.

Impermeable rocks do not allow water to pass through them and so they produce more surface runoff.

Clays have small pore spaces which reduces infiltration and through flow, but encourages surface runoff and increase the risk of flooding.

The event• Monday, 16 August 2004

• Heavy rains fell over the course of eight hours that afternoon

• 200 mm of rain in four hours of rain fell over the high ground just inland of Boscastle. The average monthly rainfall for the area in August is 100-120 mm over the high ground

• 90 millimetres of rain fell in one hour. The average is 70-90 mm at lower levels around Boscastle itself.

• The torrential rain led to a 2 m (7 ft) rise in river levels in one hour

• Water speed was in excess of 4 m/s

Primary effectsFlash flood: it is a sudden rise of water in a river

Videohttp://news.bbc.co.uk/player/nol/newsid_4150000/newsid_4155600/4155688.stm?bw=bb&mp=wm&news=1&bbcws=1#

Secondary effects

• Effect on tourism: about 90% of Boscastle’s economy is dependent on tourism. The town was closed to tourists after the flood, causing significant loss of revenue.

• Damage to homes, businesses and land

• Environmental damage to local wildlife habitats

• Coastal pollution caused as debris and fuel from cars flowed out to sea.

• Cars, walls and even bridges were washed away.• (There were no deaths)

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

Monday 16th August 2004

• 12.15 Rain gauge at nearby Lesnewth some 4km (2½ miles) up the valley, shows no rainfall

and it is dry in Boscastle’s harbour area, yet there are torrential showers at Camelfordand at the top of Boscastle.• 12.30 Rain gauge at Lesnewth records just over 3mm of rain.• 12.45 Rain gauge at Lesnewth records just over 12mm of rain fallen within the last

15 minutes.• 13.15 Rain gauge at Lesnewth records 15mm of rain in 15 minutes.• 14.15 Rain still persists, but seems to have eased. Rain gauge at Lesnewth records

7mm of rainin 15 minutes.• 15.00 First of several power cuts caused by lightning. Start of persistent, heavy rain.

Rain gaugeat Lesnewth records 13mm of rain in 15 minutes.• 15.30 The River Valency begins to breach its banks.• 15.45 Rain gauge at Lesnewth records almost 25mm of rain in 15 minutes.• 17:00 The Boscastle floods are approaching peak level. The waters, some 2½ m (8ft)

deep, are now lapping the top step of the attic ladder.

Tuesday 17th August 2004

• 09.00 Boscastle 88mm in 60 minutes and 112mm in 90 minutes

Human causes

• Construction of small bridges; blocked by cars and vegetation, so water had to flow around the bridge, flooding properties.

• There wasn’t any type of flood defences

• The river had been narrowed down to allow the construction of bridges, so there wasn’t enough space to carry a such amount of water.

Lessons learnt?

£800,000 flood defence scheme:

- incorporating drainage and sewage systems

- The river bed has been deepened by six feet to accommodate more water volume in times of flood

- Boscastle car park has been raised in height. This will stop the river from bursting it’s banks so easily.