CAN MY PROPERTY FLOOD?

A guide and manual for the

Property owner

With examples from England

John C Doornkamp

M.Sc., Ph.D.

Consultant

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First published in Great Britain in 2012

All text and images Copyright © John C Doornkamp (2012)

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the prior written consent of the copyright holder.

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PLEASE NOTE

IT IS IMPORTANT TO NOTE THAT ONLY A FULL EXAMINATION OF THE SITE AND SITUATION OF A PROPERTY CAN DETERMINE (WITH ANY DEGREE OF CERTAINTY) THAT THERE IS NO FLOOD RISK.

THIS BOOKLET, AND THE GUIDANCE THAT IT CONTAINS, CANNOT AND SHOULD NOT BE THE BASIS FOR ANY FINANCIAL DECISIONS REGARDING A PROPERTY. THE AUTHOR ACCEPTS NO RESPONSIBILITY OR LIABILITY FOR ANY DECISIONS MADE BY ANYONE AS A RESULT OF READING THE INFORMATION OR GUIDANCE PROVIDED IN THIS BOOKLET.

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TABLE OF CONTENTS

Can this property flood?

Introduction

Part A: The nature, history and impact of flooding in England

Introduction

Flooding by standing water

Flooding by rivers

Flooding by the sea

Part B: Assessing flood risk

Coastal flood risk

Assessing coastal flood risk

Analysing the 1:25,000 O.S. maps

What to do if you are vulnerable to coastal flooding

River flood risk

Assessing river flood risk

Analysing the 1:25,000 O.S. maps

What to do if you are vulnerable to river flooding

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Flooding by failure of storm water drains

Flooding as a result of rising groundwater

What to do if you are vulnerable to flooding from rising groundwater or drains

The extent of flooding

Part C: Dealing with flood risk

The role of “The Authorities”

Areas blatantly unsuitable for property

Development

Areas set aside for flood alleviation

Flood protection work

Protecting your property

Keeping flood water out

Dealing with water that gets in

Coping through flood insurance for buildings and contents

PART D: Prospects for the future

Change is with us

The role of global warming

PART E: A practical agenda for the property owner

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The short-cut version

After the flood

You and your insurer

A final note

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CAN THIS PROPERTY FLOOD? Let’s go straight in.

Answer the following questions:

Does the property stand on or near the floor of a valley? Does the property stand at or near sea-level? Has the site previously been flooded? Do the maps produced by the Environment Agency (EA) show that the property stands in a flood hazard area? Is the property at the lower end of a sloping street or road? Is the property backed by higher land?

If the answer to any of these questions is “Yes” then you need to read on.

It is a good idea to read the introductory pages before attempting to assess the risk of flooding (as described in Part B).

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INTRODUCTION

In the autumn of 2000 (I could have picked any one of a number of years) heavy rainfall brought flooding to more than 700 locations in England and Wales, causing flood damage to around 12,000 properties.

When we read a statistic like this our automatic reaction is to say: “Poor souls, hit by a flood”.

Perhaps our reaction should have been: “Poor souls, fancy building a property there”.

Or even: “Poor souls, though it is their fault for buying a property there”.

This booklet will try to help you to avoid the same mistake.

Before you get too certain that this booklet is not for you, remember that more than 40% of people at risk from flooding do not realize it – yet.

Flooding in Great Britain is not new. We have a long history of catastrophic coastal and river flooding. What is comparatively new is that more and more properties are being built on sites that are vulnerable to flooding.

It is estimated that 17% (5.2 million) of all properties in England are now standing on flood risk sites. Of

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these about 46% are liable to flooding from rivers or the sea, and 54% are liable to flooding from standing surface water. This last statistic will come as a surprise to many, and indicates the importance of being able to recognize a site on which water can pond as it accumulates.

We would all feel better if we were not part of that 17%. None of us want to experience the distress caused by flooding. Nor do we want the financial losses that flooding can inflict.

Therefore, the more we know about the nature of flooding, and the more we understand how a potential flood site can be recognized, the better.

It is helpful if we agree what some words mean. I have used the words “vulnerability”, “hazard” and “risk” to mean the following:

“Vulnerability” – is used in the sense “is the property vulnerable to flooding” i.e. does the property stand within an area that could be flooded.

“Hazard” or “peril” – is the flood itself

“Risk” – identifies the likelihood that a property will be hit by a flood.

There are two groups of people who know most about flood risk. These are the Environment Agency and domestic insurance companies. The former has the task of defining flood risk areas and bringing about

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some measure of protection or alleviation. The latter has to foot the bill if those properties affected by flooding carry flood risk insurance.

It is a matter of concern to both these groups (as well as the rest of us) that the cost of flooding could rise from £1.2billion a year (on average) to £12billion by the 2080s. That is why insurance companies are asking for central government to bear the cost of major catastrophic flooding (but more on that later).

The increasing extent of housing, especially on flat land close to sea level, means that about five million people and about two million homes are now vulnerable to flooding. As the population grows in size, so will the numbers put at risk.

The combined value of at-risk-properties is estimated to be in excess of £250 billion (at 2012 values).

The estimated annual average cost of flooding is about £1.2 billion. In the absence of flood protection works this figure might well be £3 billion (or more) per year.

More than half of those at risk occur within the Thames Region (as defined by the Environment Agency). This includes nearly 600,000 homes and 45,000 commercial properties.

If a property is liable to flooding (and especially if it has a history of flooding) it may be difficult to obtain affordable insurance for either or both buildings and

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contents. Furthermore, it may become impossible to obtain a mortgage. When this happens not only does the market value of the property fall, but also it may be impossible to sell.

Properties that stand on sites vulnerable to flooding, but as yet without a history of flooding, may be trading at prices well above those that would exist if their vulnerability to flooding was realized.

Things, of course, are never clear-cut. A great deal of money has been spent by governments over the years on protecting land from being flooded.

There is a tendency to assume that once these works are completed protection is assured. It may not be so. The individual homeowner needs to know the size and nature of the flood for which these defences were designed.

Climate change (probably through global warming) is likely to have a dramatic impact on both the size and the frequency of flooding.

In the case of rivers if the predicted increase in the number and intensity of rainstorms should occur, there will be more floods.

Coastal sites are also predicted to experience more frequent intense storms that will cause the sea to flood adjacent low land. This situation will also get worse if there is a further rise in sea-level.

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It is by no means certain that insurance companies will continue to provide compensation for flood damage. Nor is it certain that governments and their agents will be able to meet the challenges that the future may bring.

Fortunately we can all try to understand the flood risk facing our own property or one that we would like to buy.

Once we understand those risks we can take action to lessen the possible impact that flooding may have upon our welfare or our property.

The rest of this book is all about how this can be achieved.

PART A describes the general background to flooding in England.

PART B deals with the way in which you can recognize a property that is standing on a flood risk site.

PART C providers some guidance as to what you can do about any flood threat that exists.

PART D looks at the prospects for the future, including the role of global warming.

PART E provides you with a practical agenda of what you can do to make things better.

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If you want to skip PART A that is fine, though you may want to return to it later as it contains some very useful background information.

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PART A

THE NATURE, HISTORY AND IMPACT OF FLOODING IN

ENGLAND

Introduction

Flooding in England is brought to our homes by four dominant means: direct rainfall, along rivers, by the sea, or by human action.

The driving force behind a flood usually involves one or more of the following:

- Heavy and/or prolonged rainfall,

- Rapid snowmelt (especially if the ground is frozen),

- And coastal storms accompanied by high tides.

And flood damage to properties occurs because properties have been placed in flood-prone areas.

Flooding by standing water

In the Introduction I identified the fact that more properties are at risk from standing (ponding) water than from any other source.

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Sometimes it is difficult to separate this precise cause from others simply because they may be closely related.

For example, when river levels are high, groundwater is also high. In areas of flat land, or within depressions, this groundwater will appear at the surface. If deep enough, this same groundwater can cause water damage to properties. It is these same sites that form collecting places for heavy and prolonged rainfall. Drains and culverts become overwhelmed and surface water collects.

This is one of the flood types not covered by the flood risk maps on the environment-agency’s web site (see later).

It is already clear that there is an advantage in having a property that stands on a natural slope from which water can drain away. This is one of the first things that you should check out for yourself.

Flooding by rivers

River floods are usually a response to excessive rainfall within the catchment of that river. They may be greatest when these heavy rains are accompanied by frozen ground and rapid snowmelt. Thus, the highest floods in Nottingham and Derby have been at a time when the hills of the southern Pennines have received heavy rainfall in late winter, whilst the ground is still frozen and the lying snow is thawing.

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River flooding is also related to the size and shape of the catchment that drains into the river. If the catchment is in the form of a large basin within an upland area, and if this basin drains to a single river flowing down a narrow valley, then flooding is concentrated by the form of the land. This is precisely what happened at Lynmouth (1952) and Boscastle (2004).

Sites that may be particularly vulnerable to flooding are those just below the junction of two streams/rivers. They are in danger of receiving floodwaters from either river, or worse still, from both rivers if they are both under the same rainstorm.

In recent years the most memorable river floods have been those of the River Derwent in Cumbria (2005 and 2009), January 2008 when flooding was widespread across England and Wales (e.g. Grimsby, Worcester, Oxford, and Tewkesbury), January 2003 (e.g. Billericay, Tilbury, Canvey Island, and Yalding), and especially November 2000 when an estimated 10,000 homes and businesses were flooded at over 700 locations. The main rivers responsible were the Thames, Trent, Severn, Wharfe, Ouse (Yorkshire) and Dee.

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River Derwent, Chatsworth Park, Derbyshire. Peaceful under normal conditions, but with a catchment in the

Pennines it is capable of extensive flooding.

The driving force behind such events is, of course, the weather. As far as we are concerned it is unlikely that the precise location and timing of floods in the future can ever be achieved. Flood-generating weather can occur anywhere in Britain, and we have to look for those places that will be affected, rather than worrying about when it will happen.

The purpose of this Manual is to show you how to recognize such places.

We also have to recognise that over the years engineering work has taken place in order to reduce the risk of flooding.

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Chatsworth House (Derbyshire) is carefully located on a river terrace above the floodplain of the River

Derwent.

Such engineering work includes: the building of sluice gates in order to control river level, the widening and deepening of the river channel, the creation of an additional channel to carry water in times of flood (a flood relief channel), a retaining dam to hold back flood water in an area where there are no properties that can be damaged, and the raising of river levees in order to contain higher than normal river levels.

Any river within its flood plain creates levees naturally. These are created out of river sediment left against the river edge when a flood is falling.

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It is always tempting for flood protection reasons to raise the height of these natural banks and so providing an even larger channel to contain flood flows.

Unfortunately the construction of this extra protection may lead to an unjustified sense of security, the building of properties on the floodplain, and a devastating loss when higher than expected river levels occur and even the raised levees are over-topped by flood waters.

It is also important to realize that there is no such thing as a single flood boundary. Any sequence of floods may reach different places according to the magnitude of the flood and the local topography.

It is convenient for flood mapping purposes to assume a flood of a particular size, and to draw an imaginary line on a map to show where those floodwaters might reach. However, there may never be a flood that exactly fits the line drawn.

In your searches for information on past flooding check to see how many times the location in which you are interested has been flooded in the past 200 years. The rest of your analysis can go from there.

For example, have those historical floods led to the construction of flood defences? If so, has there been an occasion when high water levels tested them and if so has flooding ceased since they were built?

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Be aware, however, that flood records are notoriously incomplete, or incorrectly recorded. In addition you must remember that the controls on flooding (e.g. the weather, the sea level, the built environment) can all influence flooding in the future. There is no reason to assume that any of these are the same as they were in the past.

Engineers need data on river levels before they are able to design flood protection structures. What they have to do is to calculate is the water levels that will be reached on average (say) once in every 250 years – and create a structure that will not be over-topped by such a flood.

In general, the higher the flood level the less frequently it will occur. So for example, a rise in river level of 3 metres might occur on average once in every 10 years. This would be called the 1 in 10-year flood. A rise in level of 5 metres in that same river at the same location might re-occur on average every 100 years, and would be known as the 1 in 100 year flood. This numeric or statistical expression of frequency is known as the recurrence interval of flooding.

The numbers I have used do not relate to anywhere in particular. However, they serve to illustrate the hydrological jargon. It is a common mistake, however, to assume that this number reflects regularity as well as frequency. A 1 in 100 year flood does NOT occur habitually on each 100-year anniversary of the previous such flood. In fact a 1 in

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100 year flood could be achieved two years in succession and then not again for over 100 years.

These frequency statistics (the return period, also known as the recurrence interval) is not an event predictor but a measure of the likelihood of the frequency with which a flood of a certain magnitude is likely to occur.

This statistical number could be expressed in another way. Instead of saying that a flood of a certain size only happens with a recurrence interval of 1 in 100 years, we can say that the chances of that flood level being reached this year is 1 in 100.

It should be remembered that the amount of damaged that flood waters do is related to how long they linger in a property. The longer they stay the greater the final cost.

It is very difficult to predict the depth that floodwaters will reach; in fact it cannot be done. In the case of both river and coastal flooding, it is obvious that the higher you are above the river/sea the shallower the floodwaters are likely to be.

Flooding by the sea

The most damaging flood in living memory to hit England was the East Coast flood of 1953. This was part of a catastrophic flood event that caused damage to the countries bordering the North Sea. In the Netherlands the death toll was around 1800

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compared with about 300 in eastern England. In England more than 30,000 people were evacuated from the 24,000 (or so) properties that were flooded.

PLACES WHERE FLOODING TOOK PLACE

31st JAN – 1st FEB 1953

Aldeburgh Kings Ferry Bridge

Anderby Lossiemouth

Andely Creek Lowestoft

Bacton Mablethorpe

Banff Macduff

Birchington Margate

Canvey Island Mundesley

Chapel St. Leonards Musselburgh

Clacton-on-Sea Overstrand

Cleethorpes Poplar

Cley-next-the-Sea Portgordon

Colchester Portobello

Covehithe Purfleet

Coverton Reculver

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Cromer Saltfleet

Crovie Salthouse

Dartford Sandilands

Felixstowe Sandwich

Foulness Island Sea Palling

Gardenstown Sheerness

Gedney Drove End Skegness

Gravesend Snettisham

Great Wakering Southwold

Great Yarmouth Spalding

Grimsby Sutton Bridge

Harlepool Sutton-on-Sea

Heacham Swalecliffe

Herne Bay Theddlethorpe St. Helen

Hollesley Trusthorpe

Horsey Island Walcott

Hunstanton Walton

Ingoldmells Wells-next-the-Sea

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Isle of Grain Whitehills

Isle of Sheppey Whitstable

Jaywick Wiveton

King’s Lynn

Note: Flood protection has increased since the 1953 flood.

The coastal floods of 31st January and 1st February 1953 were the most recent to have hit the shores of England with such devastation. They seem to have blotted out the fact that severe storms have previously caused flooding in this area (e.g. in 1897, 1906, 1928, and 1949). Widespread flooding and damage also occurred in 1978.

Cley-next-the-Sea (Norfolk), one of the coastal sites along The Wash hit by the 1953 flood.

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For a catastrophic flood to occur along the east coast several factors have to take place at the same time.

These are:

A deep low-pressure system (especially if it is tracking from north to south over the North Sea) A high tide (especially an equinoctial spring tide) A strong on-shore wind (which would be the case with a deep low pressure over the North Sea) A sea surge superimposed on the high tide.

It is relevant to note that surges by themselves do not generate a catastrophic flood. There are around 10 surges along the North Sea coast every year, although these may bring some high water levels that are a nuisance, they do not generate a catastrophic flood.

Existing flood defences can cope with such events.

The route taken by a low-pressure system is also important. Normally, within the area of the North Sea they usually come in from the south-west, and move away in a north-easterly direction. If the system is deep it can generate strong winds, even intense rainfall, but it does not generate a catastrophic coastal flood.

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Flood defences and warning systems have improved dramatically since the 1953 flood. Our hope, as homeowners, is that these defences will save us from the next flood. Experience has shown that such hopes have not always been realized. Such defences can, and usually do, keep out the more frequent minor floods. They are not designed for, and cannot cope with, the larger less frequent (i.e. usually those with a statistical frequency (return period) of 1:1000 or 1: 500 years).

A special note about The Fens of East Anglia: The following data were available in 1990 – At the Boston Grand Sluice extreme high water at spring tides is 4.60 metres O.D. and the highest recorded water level is 5.63 metres O.D. This can be compared with the minimum defence requirement for the western shore of The Wash of 6.0 metres O.D. Any analysis that you make about flooding within The Fens of land below 5 metres O.D. must assume that the flood protection schemes have failed, have been breached, have been by-passed or have been over-topped. The fact that such areas lie below recorded high water levels is not in question, nor is the fact that since 1953 flood defences have been sufficient to prevent a catastrophe within the area of The Fens.

SOME RECORDED COASTAL FLOODS IN

WESTERN BRITAIN

PLACE DATE PLACE DATE

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Clevedon 25/02/1990 Portmadog 28/10/1929

30/01/1846 28/12/1924

Severn Beach 25/02/1990 Criccieth 28/10/1929

Weston 25/02/1990 28/12/1924

Hinkley Point 25/02/1990 Llanarber 02/11/1925

Uphill 13/12/1959 28/12/1924

Ilfracombe 17/12/1957

Burnham on Sea 09/01/1936 Bangor 18/10/1987

Porthcawl 27/12/1979 11/08/1957

09/01/1936 13/11/1923

Swansea --/07/1973 16/12/1852

Amroth 08/01/1939 Menai Bridge

11/08/1957

23/11/1938 Towyn 28/02/1990

Aberavon 16/01/1938 15/01/1938

Langhain 16/01/1938 Kinmel Bay 28/02/1990

Magor 01/12/1935 Morecambe 01/02/1985

Ystrad Mynach 28/12/1924

Cardiff 17/10/1883 Blackpool 04/09/1967

Barmouth 28/10/1992 28/12/1924

13/11/1964 23/10/1872

15/01/1938 Llandudno 08/01/1939

10/10/1936 28/12/1924

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05/11/1926 Litherland 30/05/1938

31/12/1925 Rhyl 15/01/1938

28/10/1925 28/12/1924

25/02/1925 Fleetwood 28/10/1929

28/12/1924 28/01/1927

13/11/1923 Colwyn Bay 28/12/1924

Aberystwyth 24/06/1967 Conwy 28/12/1924

13/11/1964 Annan 21/04/1947

23/11/1938 Dornock 21/04/1947

01/02/1938 Glasgow 10/12/1942

16/01/1938 Dumfries 09/11/1811

25/06/1935 Dalswinton 09/11/1811

28/12/1924 Saltcoats 05/01/1991

Aberdaron 24/04/1947 Rothesay 05/01/1991

15/01/1938 04/02/1945

Ynyslas 23/11/1938 Prestwick 03/10/1976

Abereiddy 02/02/1938 16/07/1939

New Quay 16/01/1938 Drummore 08/03/1962

Llangranog 16/01/1938 Greenock 12/07/1961

Pwllheli 15/01/1938 Dippen Head

17/10/1954

25/02/1925 Troon 16/07/1939

28/12/1924 06/06/1935

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Porthmadog 15/01/1938 --/11/1913

10/01/1936 Girvan 09/12/1934

Note: Some of these storms brought about the will to create sea defences that have provided a degree of protection against more recent high water levels.

Along the west coast floods are generated by surges in the Irish Sea when on-shore winds can produce high waves on top of an already high tide.

Although storms of any particular magnitude can be defined with a statistical frequency (e.g. 1 in 1000 years for widespread catastrophic storms, or 1 in 20 years for less damaging but still costly storms) the truth is that storms of any magnitude can happen at almost any time.

A further truth is that the statistical frequencies that apply to storms in the past may very well not apply to storms in the future. In fact, the consensus is that with global warming such storms will occur more frequently in the future.

The history of coastal flooding has shown that if your property is within 5 metres of sea level it may well be exposed to a risk of flooding by the sea within the next 50 years (and it could be tomorrow).

Today about one million homes and business are vulnerable to flooding by the sea.

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As time goes by the vulnerability to flooding increases in England for, in general, sea levels have been and still are, rising. Measurements have shown a rise of 8 mm/year between Portsmouth and Eastbourne, in excess of 4 mm/year between Weymouth and Folkestone, and around 2mm/year in the Thames Estuary and in the Mersey Estuary.

The way in which attempts have been made to keep coastal floodwaters at bay include:

Building sea walls Building embankments along the water channels in estuaries Providing beach nourishment by adding sand, grave, and/or stone as is appropriate for that beach Creating breakwaters to prevent the loss of sand and/or gravel from beaches that protection to the land behind from flooding Raising the height of the ground before properties are built so that they can be artificially raised above the height of expected floods.

Do any of these apply to your area?

The obvious alternative to such engineering works is to have planning regulations that prevent property from being built within areas vulnerable to flooding.

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Unfortunately, even when the risk of flooding has been known to exist, planning permission in some cases has still been granted.

Sea walls along the coast at Southwold.

These walls are designed both as a defence against erosion and as a protection from flooding.

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Groynes on the shore of The Wash designed to “catch” sediment to help to prevent inland flooding.

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Beach nourishment with gravel has helped to protect the centre of Sidmouth from coastal flooding.

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Shingle banks result from storms piling coarse sediments into a long ridge at the land-sea boundary. These natural embankments protect the lower ground

behind them from being flooded (except by catastrophic floods).(Blakeney Point, Norfolk)

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PART B

ASSESSING FLOOD RISK

This section shows you how you can make an initial flood risk assessment for yourself.

There are only a limited number of situations in which flooding is likely to occur (and I am ignoring situations such as leaving the tap running into the bath when the plug is in place). These can be divided into:

Flooding by the sea Flooding by a river Flooding by the failure of storm water drains Flooding by rising ground water.

Each of these will be considered in turn.

If you live more than 10 metres above sea level you can skip the section on coastal flood risk and move straight on to river flooding.

COASTAL FLOOD RISK

Locations that we would all recognise as prone to flooding in coastal areas are:

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Those immediately adjacent to a sea front or beach Those in areas lying close to sea level (in general this is land less than 5 metres above sea level).

We can place these and other locations on a scale of increasing hazard as we go from an inland location some 10 kms from the shore (e.g. up a large estuary), perhaps having some protection from flooding, to a high exposure to flooding at the coast, especially if there is little or no flood protection in place.

Large areas of England lie below 5metres O.D. (Ordnance Datum). These include much of East Anglia and Lincolnshire, the Thames Basin, many parts of the south coast between Dover and Weymouth, land adjacent to the Bristol Channel, and extensive areas of northwest England.

The North Sea floods at the end of January 1953, demonstrated just how extensive such low ground is, and how vulnerable it is to extreme flood events. This flood covered more than 200,000 acres (81,000 hectares), claimed 307 lives and made about 21,000 people homeless.

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Many readers will think that 1953 is a long time ago and we have moved on since then. Flood defences have improved, and in any case 1953 was a freak

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To read the rest of this Guide please follow this link:

http://www.amazon.com/dp/B00ABGPJNQ