978-0-00-812422-9 COLLINS CAMBRIDGE AS AND A LEVEL GEOGRAPHY

Why does the river flood?
The River Harbourne is a small river tributary of the River Dart, in Devon. There
are a number of reasons for flooding.
Physical factors
• There has been an increased frequency in the number of intense rainfall events.
• The river starts 350 m above sea level on the impermeable granite bedrock
of Dartmoor.
• Dartmoor receives 2020 mm of rainfall annually, twice as much rain as lower
surrounding coastal areas.
• From the moor the river cuts through steep narrow valleys on to slate
bedrock descending 300 m in 12 km.
• For the size of catchment the river has a high drainage density.
• The village of Harbertonford lies at the confluence of three rivers – the River
Harbourne, the Harberton Stream and the Yeolands Stream.
Human factors
• Many properties are built on the low-lying floodplain in the central area of
the village.
• The A381 road has been widened over the years to cope with traffic pressures,
thereby increasing the amount of runoff flowing directly to the river.
• Traditionally some water was extracted along mill leats to power the local
mills, which have since closed.
How is the river managed?
Harbertonford is designated as a Conservation Area and several listed
structures, including the village bridge, are contained within it. Atlantic salmon,
bullhead, sea trout and brown trout occur in the river and protected species are
also present within the catchment, including otter and common dormouse.
With this in mind it was important that any flood management works must be
sensitive to the environment.
The river is managed using a variety of hard and soft engineering techniques.
The aim of the scheme was to provide a range of flood defence measures
whilst enhancing the local environment. As a result it was decided that the
scheme should use natural local materials where possible in keeping with the
surroundings with minimum need for maintenance.
The scheme has two main features – an upstream flood storage reservoir,
and flood defence works through the village. This option has reduced the risk of
flooding from one in three years to a minimum of once in 40 years.
Upstream
• Wetland area and flood storage area: 1 km upstream from the village of
Harbertonford a wildlife area was created containing flood-resistant trees and
shrubs. The area directly upstream from the dam will become a 41 000 sq m
water storage area in times of flood. Local schoolchildren will monitor the
afforested area as part of an ongoing partnership.
• The Palmer Dam: Built to control the flow of the river, this earthen mound
was constructed using locally excavated materials. The dam gates can be
controlled to restrict river flow in times of flood. A culvert was created to
allow the free movement of fish up and downstream of the dam, whatever
the flood conditions.
figure 1.40 Students measuring the channel at
Harbertonford village green.
Through the village
• Bed-lowering: In order to keep the aesthetic quality of the central village
green, the riverbed was lowered to increase the river’s carrying capacity
without the need for flood walls.
• Channelisation: Throughout the lower sections of the village, along Bow
Road, a 200 m wall has been created to protect the residential area from
overtopping. The river is now twice as wide. The wall on the bend of the river
is reinforced to reduce erosion.
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Hydrology and fluvial geomorphology