27-9-07Flooding

What is a River FloodA flood occurs whenever a river overflows its banks (exceeds its “bankfull discharge”).

However, a flood becomes a problem when the water rises to a level where it threatens property and/or life.

Why do rivers flood?Rivers usually flood due to a range of physical factors. These physical factors canbe divided into climatic factors and drainage basin characteristics. Human intervention can also make flooding worse.

A major flood devastated the village of Lynmouth in Devon in August 1952. 229mm of rain fell in 24 hours. The impact of this was increased because narrow bridges across the River Lyn acted as dams and stopped the water flowing away.

Snowmelt-especially in springtime, this contributes a sudden excess amount of discharge to some rivers.

Little vegetation cover-Little or no interception and so the water reaches the ground more quickly.

Heavy rainfall-if rainfall is very heavy, there is not enough time for water to infiltrate, surface run-off is immediate.

Prolonged rainfall-The ground will become saturated and so little infiltration can occur.

Steep valley sides-Steep gradients increase the speed at which water can travel into the river channel.

Small river basin-If the river basin is not very big, water reaches the channel more quickly.

Low evapotranspiration-In cooler conditions there will be less water lost from the drainage basin system and so more water available to the river channel.

Low drainage density-Fewer surface channels to transport high discharges means greater risk of flood water.

Impermeable rock-There is no percolation of water therefore greater surface run-off and throughflow.

Coastal storms-Flooding near the mouth of a river can be made more severe.

What caused the Boscastle FloodBoscastle is situated on the North Cornwall coast. For a hundred years the village was a thriving port but the coming of the railways soon saw its decline as a trading port. About 90% of Boscastles economy is now reliant on tourism.

On Monday 16th August 2004 major flooding occurred in North Cornwall. The small village of Boscastle was devastated.

What were the causes of the Boscastle Flood?Climatic conditions were the main reason of the floods in Boscastle. 200mm fell over Ottersham Moor, to the east of Boscastle, within a 4 hour period. This was combined with coastal winds and a rising tide. The ground was also already saturated from previous rainfall events. It occurred at the confluence of the River Valency and the River Jordan

What Factors increase the risk of flood?- A small drainage basin causes run-off and throughflow to reach

the channel more quickly.- Steep valley sides causes surface run-off to travel very quickly- R.Jordan passes through 100m underground culvert so it cannot

cope with high discharge- Boscastle is at the confluence of the Jordan and Valency so a

large volume of water can collect quickly- Building on the flood plain means that water cannot infiltrate the

ground- Bridges narrow the river channel- Low drainage density means there are not enough channels to

carry away the water- Clearing vegetation from the valley means that water was not

intercepted.

What were the effects of the Boscastle Floods?Clean up operation The Village Hall and the community hall were used to organise the clean-up operation.Damage The flood came so quickly businesses could not prepare, This meant losses on damaged or destroyed stock were substantial.

Tourist Industry The tourist industry is vital to the economy of Boscastle. People are concerned that it will take months to repair the damage and tourists may choose to avoid the area next season. Cars More than 50 cars were carried out to sea by the 3m high wall of water that swept through the village. Roads were damaged and two months later there was still no public car parking area in the village. Properties Three homes and a shop were destroyed. About 80 properties suffered varying degrees of damage. Structural engineers estimated the coast of repair could reach £500 million. Harbour The harbour area was severely damaged and residents could not return home for more than 10 days. They then had to wear protective clothing in case of contamination from broken sewers.

How can the flood risk be managed?How river management techniques affect the characteristics of the river and its channel.Increase Capacity Increase Velocity Reduce Discharge

Deepen Channel Straighten Channel Plant vegetationWiden Channel Concrete Channel Build dams

Dams on the RhoneDams can be multi-purpose:

- They hold back the spring melt water and so stop the floods.- They release this water in the dry summer to irrigate the

intensive fruit and vegetable growing areas. The water is also used for cooling in the two nuclear power plants near Genissiat-Beugy and Creys-Malville.

- In addition, the dams produce hydroelectric power.

Key Ideas- A river floods when it exceeds its bankfull discharge- Flooding can be a result of both physical and human factors.- Physical factors include climatic conditions and drainage basin

characteristics- The effects of flooding are short-term and long-term- Flood management schemes can aim to prevent flooding or

reduce its impact- Flood management schemes can ave both positive and negative

effects.