1

Why do we get floods?Why do we get floods?

Where did all the water Where did all the water come from?come from?

2

That’s rightThat’s right

If there is not enough room in the river, then the water overflows the banks.But what is the most usual reason why this happens – what event can occur to make the water in the river overflow?

3

Right again!Right again!

Heavy rain is the most usual event that happens right before a river floods.But not all the rain that floods out of the river, fell in it from the sky in the first place. Most of the water in the river came from the land either side and flowed into the river.

What kind of rivers flood the

most easily?

4

It is not really the rivers it is what It is not really the rivers it is what goes on around them that mattersgoes on around them that mattersRemember most of the water lands on the sides of the river. How much of this ends up in the river depends on a number of things

Steep or flatter valley sides?

Grassy or wooded valley sides?

Recent weather: sunshine or rain?

Town or country?Rocks are like cheese – some have

holes – some don’t. Which is better to have under you in a flood?

5

So we decided that floods are more So we decided that floods are more likely iflikely if

A river is running through a steep valley

the water more likely to run down hill rather than soak in

with grass rather than trees, less of the water is trapped in the vegetation

a place where there had been recent rainthe ground would be like a full up sponge that can take in no more

A place with impermeable rockWater cannot soak into it – it stays ion the surface

past a townThe concrete and tar stops the water soaking in

6

Where is Boscastle?Where is Boscastle?

7

So what happened in So what happened in Boscastle on 16th Boscastle on 16th

August 2004?August 2004?

It started It started off nice off nice enoughenough

8

But look what But look what happened later than happened later than

day!day!

9

10

11

12

The biggest peacetime rescue in The biggest peacetime rescue in 50 years!50 years!

13

No-one diedNo-one died

And the best bit …And the best bit …

14

Why did it happen?Why did it happen?

Surrounding vegetation includes agricultural land with a bit of woodland. (So conditions would have been even worse, without these patches of woodland!) The rain coincided with high tide in the bay. This slowed the rate of exit of floodwater into the harbour.

The three river valleys are very steep and narrow. The steep valley sides mean that soils are thin, with limited water storage capacity when heavy rain comes.The rock is old, hard sandstone which is impermeable. The rivers here are naturally ‘flashy’.

15

The first signs of danger came at 2 p.m. on 16th August, when cast iron manhole covers in the town’s streets began to shudder, indicating that forceful storm waters were gathering in the sewers beneath. By mid-afternoon, the Valency and Jordan had over-topped their. At 5 p.m. the Clovelly Clothing shop was dragged 50 metres away from its site by floodwater flowing down the main street before later collapsing.

As evening fell, soaked locals and holidaymakers huddled together in the village sports hall for warmth while helicopter winches lifted stranded home-owners off the roofs of their own houses

16

Effects:Effects: Around 1000 residents and visitors are believed to have been affected in this devastating event. Miraculously, there were no fatalities, with only 1 reported casualty - a broken thumb.7 helicopters airlifted 100 people (including 6 firefighters) to safety.29 out of the 31 Cornwall County Fire Brigade stations were involved in the incident. They remained at Boscastle for 7 days, assisting in the clean-up operation.58 properties were flooded, 4 of which were demolished including the Visitor Centre. A further 40 properties were flooded in other villages4 footbridges along the Valency Valley were washed away.84 wrecked cars were recovered from Boscastle’s harbour and streets, 32 could still be out at sea.The infrastructure damage to buildings and services, could cost up to £2 million.

17

The environment Agency identified The environment Agency identified several major problemsseveral major problems

Because the river valleys were narrow and the rock impermeable, these river valleys would have difficulty coping with flash rainfall – very heavy rain in a short time.So measures needed to be taken to make it as easy for the water pass through as quickly as possible

E.g narrow stone bridges cut down the flowAs it the narrow bricked river banksAnd overhanging trees which could fall block the river

18

The environment Agency identified The environment Agency identified several major problemsseveral major problems

The carpark was right beside the river and just about level with it and was one of few places the water could go when it over flowed.The River Jordan, which passed under the town in a culvert before joining the Valency just above the harbour, was too small to take.

19

20

21

So what did they do?So what did they do?

Widen the river bed and the bridges to prevent material blocking themWiden and deepen the river bed to allow more space for extra water to flow throughRaise the river bank for the new car park so it would not flood in futureBuild in features upstream from the town to catch debris brought downstream in a floodMake sure that overhanging vegetation was either cut back or removed

22

Now for the new footbridgeNow for the new footbridgeOption A was the preferred one by the community as it was similar to the old one and in-keeping with surroundingsBut it would have to be double the width of the old one so stone was not suitable for an arch that sizeHence the 2 modern designs were looked at. Which do you think would be best?