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An assessment of the additional flood losses

associated with groundwater flooding: a report

to Hampshire County Council and Winchester

City Council

Colin Green, Theresa Wilson, Trevor Masterson

and Neil Boothby

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An assessment of the additional flood losses

associated with groundwater flooding: a

report to Hampshire County Council and

Winchester City Council

September 2006

Colin Green, Theresa Wilson, Trevor

Masterson and Neil Boothby

Flood Hazard Research Centre Middlesex University

Queensway Enfield EN3 4SA FHRC1@mdx.ac.uk 020 8411 5359

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Contents

Executive Summary ...3

Study Objectives ...5

Introduction ...6

What is economic loss?...6

Flood losses ...7

Potential causes of the divergence ...7

Discussion of the individual mechanisms...10

The sampling problem...10

Property characteristics...11 Groundwater flows ...11 Flow velocity...12 Duration of flooding ...12 Survey results...13 Findings ...20

Implications for the damages caused by groundwater flooding across Hampshire...23

Losses and insurance...26

Conclusions...26

Recommendations ...27

References ...28

Annex 1 ...29

Annex 2 ...30

Annex 3 Report by Neil Boothby and Trevor Masterson ...31

Acknowledgements:

The photograph on the cover is reproduced with the permission of Hambledon Parish Council for whose help in undertaking this study we are very grateful.

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An assessment of the additional flood losses

associated with groundwater flooding: a report to

Hampshire County Council and Winchester City

Council

Executive Summary

• The Flood Hazard Research Centre (FHRC) has explored the extent to which the damages experienced from groundwater flooding in Hampshire have been underestimated when using Middlesex depth-damage data. In doing so, it has been necessary to examine the reasons why the building fabric

damages from groundwater flooding might be higher than from other forms of flooding. A number of mechanisms that might result in higher losses were considered: duration, underground flows, and water flow velocities above ground.

• In seeking to determine whether groundwater flooding can result in higher than anticipated flood losses, it has been necessary to rule out other reasons why the losses in a particular village might be higher than those predicted using Middlesex depth-damage data. Flood losses are generally a function of the characteristics of the property, the characteristics of the occupiers of the property, and the characteristics of the flood. It was therefore necessary to rule out the first two groups of characteristics as a cause of higher than anticipated damages. In particular, to exclude higher than average incomes and forms of building construction that are unusually susceptible to flood damage. These alternative causes of differences were ruled out.

• The conclusion of the study is that the extended duration of groundwater flooding results in substantially higher flood losses than would be predicted using standard Middlesex depth-damage data. It was concluded that a flood duration of 1 week resulted in losses which are 240% of the building fabric damages expected using Multi-Coloured Manual (MCM) data; for a flood lasting three months, the building fabric damages increased to 360% of those estimated using MCM data.

• Four sets of data were compared in the study: 1. The most recent MCM depth-damage data sets.

2. The Flood Loss Assessment Information Report (FLAIR) data as used in some of the engineering studies of flood risk management options for groundwater flooding in some of the villages in Hampshire.

3. The damage data collected by Hambledon Parish Council from some 100 households in the village.

4. A data set prepared specifically for this study by Masterson and Boothby, the surveyors who have been responsible for the estimates of the building fabric damages that are incorporated in the FLAIR, MCM and earlier FHRC data sets. This data set was prepared for a pre-1919 detached house as

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being the dwelling type most commonly found in Hambledon and other villages affected by groundwater flooding in Hampshire.

• In addition, the availability of insurance data was reviewed. There are two main problems with insurance data. Firstly, insurance is a luxury good; the poor are much less likely to take out insurance than those with higher

incomes. The result is that insurance losses are an upwardly biased sample of flood damages. Secondly, it is not possible to separate out flood losses from one source of flooding rather than another.

• The original intention was to carry out an interview survey in another village or villages which could be compared to that undertaken by Hambledon Parish Council. In order to confidently establish that the losses from groundwater floods are higher than anticipated, the ideal sample is one of identical properties occupied by people with the same characteristics and flooded in exactly the same way. Given a sufficiently large such sample, it is possible to test statistically whether the reported losses are significantly different from those anticipated on the basis of the MCM data. The intention had to be abandoned because it was not possible to find a sufficiently homogenous and large sample of properties that had been flooded by groundwater. Hence, the results of any sample would have shown up differences in the flood

characteristics, building type, and occupants but the sample size would have been far too small to separate out these effects. In turn, it would have been only happenstance if the losses from such sample showed any statistically significant difference to the MCM values.

• The proposal rather speculatively proposed to examine the health and other so-called ‘intangible’ effects of flooding and compare these to results we have obtained from previous studies of those effects from other types of flooding. It was a speculative proposal in that the results of some 3,000 interviews to date have shown that the inter-relationships between the different factors are complex, and large samples are necessary if these are to be disentangled. The relevant questions would have been included in the proposed interview survey in another village and consequently the study was not undertaken.

• The clear conclusion of the study is that extended duration floods, such as those which groundwater flooding typically produces, result in significantly greater damages to building fabric than the values provided in the Middlesex depth-damage tables. Those higher losses should be taken into account in economic assessments of such groundwater flooding. Those higher losses should also be taken into account in other floods where the duration of the flood exceeds one week.

• We have made a number of recommendations resulting from this work, the principal ones being:

1. It is essential to take the extra damage effects of long duration flooding into account when undertaking cost-benefit analyses of relevant flood alleviation schemes, for example the floods of long duration that are commonly

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2. Investment appraisals for such flood alleviation schemes should be able to use the results of this research. To this end Defra should accept the data contained herein, and require its use where appropriate.

3. At FHRC we will develop a new dataset within our Multicoloured Manual to allow users to have easy access to this data.

Study Objectives

(1) To investigate the damage costs of groundwater flooding in Hampshire, using the Hambledon property damage cost data and that from another village to be established affected by flooding. In particular:

To what extent the estimates of the losses are increased when the most recent ‘Multi-Coloured Manual’ (MCM) data is used rather than FLAIR data.

The extent to which the much longer duration of the flooding than that upon which both the FLAIR and MCM data sets are based affects the losses. The differences between the financial losses to individual households and economic losses to the country where it is the latter that are used when considering applications for grant aid.

(2) To examine the ways in which the use of FLAIR may result in the

underestimation, or omission, of some categories of economic losses, as well as the extent in this instance of the differences between economic and financial losses. (3) To explore whether the Association of British Insurers (ABI) can provide any information as to flood losses with respect to groundwater flooding. In particular, whether (a) it is possible to separate out flood losses as a result of groundwater flooding or (b) flood losses from properties in Hampshire during a given time period. (4) To establish the impact of groundwater flood damage to building fabric as advised by specialist construction consultants under subcontract. The construction consultants will be asked to evaluate (a) increased damages as a result of water pressure below ground and flows below ground level and (b) increased damages as a result of the prolonged duration of flooding.

(5) To determine from the Hampshire Highways Department whether there is any apparent increase in the formation of potholes or other problems with the roads associated with long duration groundwater flooding events.

(6) To examine the health and other so-called ‘intangible’ effects of flooding over a considerable period of time. The results will be compared with the different health and other effects established for flooding from rivers, sewers and the sea.

(7) To review the analyses undertaken by Posford Duvivier and Halcrow in order to determine the extent to which estimates of flood losses are increased when the most recent depth-damage data is used.

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(8) To determine those modifications necessary to MCM data in order to take account of the additional economic losses caused by groundwater flooding. In making those modifications, regard will be taken of the requirements of the Treasury’s ‘Green Book’”

Introduction

It is generally true that until the widespread flooding across Hampshire in late 2000, groundwater flooding did not feature in any assessment of the risk of flooding in the UK.

The scoping study prepared as background to the consultation over ‘Making space for water’ made a very preliminary assessment that that the number of properties at risk from groundwater flooding could reach 1.7 million, including some 130,000 on flood plains (Jacobs 2004), as compared to the 1.72 million considered to be at risk from surface water flooding (Defra 2004). Of those 1.7 million additional properties at risk of flooding, some 380,000 properties on the exposed chalk aquifers of southern England were considered to be most at risk.

In particular, the groundwater flooding in Hambledon in 2000/2001 was estimated by the Parish Council to have resulted in financial losses of some £1.1 million. This sum exceeded the estimates of the economic value of the flood losses that were prepared by consultants, which estimates were based upon the Middlesex University (‘FLAIR’) depth-damage curves. This divergence raised questions as to whether the difference was solely due to the difference between financial and economic losses or whether groundwater flooding results in higher losses than does surface water

flooding.

What is economic loss?

What counts as a loss depends upon from whose viewpoint the situation is

analysed. Clearly, from the flood victim’s perspective then it is the money they had to pay out or lost as a result of the flood that counts, together with all the things that affected their lives adversely even though those things did not involve a monetary loss or cost. But conventionally in an economic analysis, the country is treated as if it is a single person so money which is, in effect, simply transferred from one pocket to another is excluded. Hence, indirect taxes, like excise taxes and VAT are

excluded. What does not simply move from one pocket to another are the resources that are lost; if a television is destroyed or a house has to be repaired, that television and those materials are lost to the country. But if the television is a few years old, what we lose is an old television even though the household who lost it will probably buy a new one.

One reason for using this second approach is that it is the taxpayers in general who pay for the costs of any flood alleviation scheme. There is then potentially a third perspective: how much is the individual taxpayer prepared to pay to reduce the risk that someone else will be flooded? It might also be asked: what are the priorities of

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the taxpayer, which floods to whom do they consider it most important to reduce the risk, and how much are they prepared to pay to reduce those risks? Because

orthodox economics focuses exclusively upon the individual as an individual and not at all upon the individual as a member of society, this question is totally ignored. In fact, conventionally, the question that the economist would ask is: how much is the person at risk prepared to pay in order to reduce the risk? That in fact the general taxpayer bears the costs is a reflection of a general social acceptance of the belief that it would be unfair if those who suffer from floods had to bear the whole burden of paying to reduce those risks from flooding.

Flood losses

Flood losses arise to the extent to which the elements and structure of a building, and its contents, are susceptible to the different characteristics of a flood. The physical mechanisms whereby contact with water can cause damage to materials are discussed later. There are a number of different characteristics of a flood which could cause damage; conventionally, the most important characteristic is understood to be the depth of flooding. Hence, it is usual to estimate flood losses by reference to a depth-damage curve (Figure 1). Other characteristics of floods that can compound flood losses include the velocity of flow, the duration of the flood, and materials entrained in the flood water.

Potential causes of the divergence

There is a number of potential different reasons why the losses estimated by the two different approaches, the survey by Hambledon Parish Council and using the

Middlesex depth-damage curves, might be different. Those other reasons have to be ruled out if some characteristic of groundwater flooding is to be determined to be the primary reason for the difference in losses, and before a revised set of depth-damage curves can be deemed to be appropriate for groundwater flooding. The most obvious potential difference is that the survey data collected by Hambledon Parish Council, summarised in Table 1, is financial loss data whereas the values used in cost-benefit analyses are economic values. The major difference between the two series of costs will be that indirect taxes, including VAT and exercise duty, are netted out of losses in order to derive economic loss figures.

Table 1 Flood losses from the 2000/01 flood as compiled by

Hambledon Parish Council

Factor Loss

Costs of vacating homes £34,000

Insurance claims £799,000

Changes at householder’s cost £195,000

Car and mileage costs £38,000

Lost time at work etc £48,000

Costs of pumps and electricity £26,000

Other costs £10,000

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A second difference between the Parish Council data and the analysis undertaken by the consultants was that the consultants did not use, or have access to, the most recent MCM depth-damage curves (Penning-Rowsell et al 2003). Those latter curves incorporated substantial revisions to the previous depth-damage curves and the increases in anticipated losses are most marked at the shallow depths of

flooding, precisely those experienced in groundwater flooding (Figure 1). In these curves, clean-up costs are included with the inventory rather than the fabric losses and the sharp rise in losses between flooding to floor level and flooding to 5 cms above floor level is largely accounted for by cleanup costs. Reassessing the flood losses using the consultants estimates of the depths of flooding but the most recent depth-damage curves yielded an estimate of the losses of £1.8 million inclusive of dehumidifier costs but exclusive of the costs of extra heating and for temporary accommodation.

Figure 1 Middlesex depth-damage curves for a pre-1919 detached house

house type 111 0 10000 20000 30000 40000 50000 60000 70000 -0.3 0 0.05 0.1 0.2 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3 depth of flooding lo s s ( £ )

TOTAL BUILDING FABRIC TOTAL INVENTORY

Two further potential sources of divergence are, firstly, in the land use coding of individual property, since the depth-damage curves refer to specific house types (defined by the date of construction and building form e.g. semi-detached). Since the Middlesex land use classification does not discriminate within the pre-1919 age band between properties of widely different ages, the scope for mis-coding was therefore low. A second potential source of error is in the estimate of the areas of the individual properties. Again, there are no obvious errors.

More generally, there are six mechanisms whereby flood losses from groundwater flooding might be greater than those predicted using FLAIR data. We should only

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expect any losses to occur in terms of damage to the fabric and not from damage to the contents since furnishings and other contents can be expected to be removed before flooding and certainly not to be left in the flood water for the duration of the flood. These six mechanisms are:

Income: the residents of the properties flooded may have higher incomes than

those of the average in the social classes for which FLAIR data is compiled. Whilst this would primarily manifest itself through increased values of contents, some structural elements, notably kitchens and floor finishes might translate into higher values and susceptibility to flood damage. The revised edition of the Treasury’s ‘Green Book’ (H M Treasury 2003) introduced income weightings, the losses from high income households being heavily downweighted. Census data does not give details of income, but only of social class which is only a proxy for income at the general level. Actual household incomes are only available at regional and sub-regional levels; the income per capita (gross domestic

disposable income) is estimated at £13,060 for Hampshire and the Isle of Wight. This is marginally higher than most other parts of the country (average for

England is £12,952) but lower than the average for the South-East as a whole (National Statistics 2005). The level of aggregation means that it is impossible to determine whether household incomes in the areas flooded are above or below the average for Hampshire/Isle of Wight as a whole.

• The statistical problem of drawing a sample from a population; the smaller the

sample, the less likely it is that the sample mean will coincide with the population mean. This is discussed further below.

Property characteristics: structural damage is associated with the form of

construction and the standard house types included in FLAIR cannot take

account of every possible form of construction, particularly those adopted prior to the mid-nineteenth century. FLAIR data should not be expected to give very good approximations for buildings constructed in the Elizabethan period for example. A significant number of properties in Hambledon are listed buildings, some with features such as wood panelled walls, timber frames or flint walls which might result in higher than average costs to restore. The majority of the buildings in Hambledon, however, are not described as having features

(http://www.imagesofengland.org.uk/) that might be expected to have a strong

influence on the losses experienced.

Groundwater flows might result in the washing out of fine material; for example,

from beneath utility runs.

High velocity flows can cause additional damages to those caused by the depth

of flooding. The problem of estimating those additional damages is that of predicting the flow velocities as obstructions can both divert flow and increase local flow velocities in the narrowed section and suffer from scour around those obstructions. In turn, those obstructions may then collapse, reducing flow velocities across the flooded area.

Duration: the FLAIR data sets are provided for flooding of less than 12 hours

and over 12 hours, with no further discrimination within the greater than 12 hour class. Flooding from groundwater can last for months and hence duration of flooding is a primary suspect for any increase in flood losses.

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Discussion of the individual mechanisms

The sampling problem

There are a number of different means of estimating the losses from flooding. The central problem is a sampling problem; how far can the losses experienced in any one property be generalised to any population of properties? The converse of that problem is: how far will the estimate of the flood losses for any single property calculated on the basis of a population estimate differ from the actual losses for that property?

The larger the sample, the greater should be expected to be the reliability of the population estimate as applied to that sample; this is the ‘regression to the mean’ effect. If the average height of a population is 1.9 metres, then with a sample of two people the mean of their height is likely to be some way away from 1.9 metres. But if the sample is of 10,000 people, then the sample average should be close to the population average of 1.9 metres. The greater the variance within the population, the greater the difference is likely to be between the population mean and the sample mean. This is obviously most marked when the sample is one i.e. a single property. Hence it should not be expected that the population estimate will

accurately estimate the losses to a single property but will provide increasingly accurate estimates of the sample mean as the sample size increases.

One answer to the problem of the relationship of the sample to population is to

evaluate the actual flood losses to each property individually. This is the basis of the assessment by Hambledon Parish Council. The obvious disadvantage is that it can only be used for past floods i.e. only applied where there has been a flood.

Secondly, it only refers to a flood which had specific characteristics for each property; it does not provide a simple basis for estimating the losses from either more extreme or more minor flooding. Undertaking detailed assessments of individual properties is also expensive and the money spent in that way is not

available for other purposes, and, in particular, not for constructing a flood alleviation scheme.

In dealing with the sample-population problem, there are two separate issues:

• Reducing the variance within the population;

• Improving the estimate of the population mean.

If we took the relevant population as being all the buildings in the country then the actual flood loss for any single building in the country would be expected to diverge markedly from the population mean. Hence, to reduce variance in flood losses, we want to group buildings of similar potential flood loss together through some form of land use classification.

Any land use classification is something of a compromise. From the perspective of assessing flood losses, the ideal is a classification system within each category of which there is very little variance in the flood losses experienced by properties under similar conditions and the maximum variance in flood losses between properties in different categories. Unfortunately, this will not be known until after data for different properties has been collected and the approach also requires a very large data set.

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Secondly, the classification system must be related to the classification systems used for secondary source data when that secondary data must be used as part of the appraisal process. Thirdly, the system must be useable; for example, it might be expected that structural damages will depend upon the form of the construction of the building. It is, for instance, known that houses with chipboard floors and lightweight timber frames are particularly susceptible to flood damage; however, there is no readily available means of identifying properties built in this way. For older buildings, the form of construction of the external walls is often hidden by a render or other coating. Hence, whilst a land use classification system that was based upon the constructional form of the building would probably be a superior means of yielding good predictions of flood losses, it would be impractical to use. The Middlesex land use classification uses age as a crude proxy for structural form but the oldest land use class necessarily covers a multiplicity of construction forms as traditional building forms show wide regional differences.

The second problem is to derive a population estimate. The problems of using losses in actual floods have already been noted. In practice, attempts to derive any usable transferable functions, in the form of a depth-damage curve, by this means have generally, with some exceptions, proved to be very problematic. A plot of flood losses against the depth of flooding generally looks like a cloud rather than showing any clear pattern. Any attempt to fit a regression curve through that cloud of data leaves most of the variation in flood losses unexplained. The number of properties affected in a particular flood is also a very small fraction of the number of properties at risk; the national autumn 2000 floods affected around 10,000 properties out of the around 3 million properties now estimated to be at risk of some form of flooding. Since those properties that are flooded in any period are not randomly selected from the population of 3 million properties, the sample is also biased.

Losses in those events may be estimated either by those who are flooded or by expert assessors. The two limitations of the first approach are that latent damage may not be observed and income limitations may restrict the scope or extent of repair works undertaken by flood victims. Therefore, the preferred option for

dwellings has been to have expert assessments made of the damages that would be done to different house types under different flood conditions, notably depth.

Property characteristics

Except in that a significant number of the properties in the village are more than two hundred years old, there is no obvious special characteristic that will make the properties differentially vulnerable. Generally the properties have been extensively modified over the period since they were built.

Groundwater flows

Most structures and underground utilities are laid on beds of material containing fine material (e.g. sand). The trenches cut in order to emplace the foundations or utilities may also act as underground flow channels. In turn those underground flows may wash out the fine material leaving the structures or utilities only partially supported and surface loads, particularly from traffic, may then cause damage to those utility runs. In addition, since water is incompressible, ground pressures over utility runs

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which are laid in material where the voids are filled with water will be transmitted directly to the utility ducts, cables or pipes.

Roads are similarly constructed on a base containing fine material. Here, an additional problem is that if there are any surface cracks, traffic will force water under pressure into the substructure of the road surface. If this water then freezes and expands, further damage will be done to the road structure.

In extreme cases, where water tables are near the surface for extended periods of time, water may rise to quite considerable heights above ground level, in some cases 2-3 metres (Green 2003) when walls lack damp proof courses and the walls are absorbent. The damage is particularly severe when the water contains soluble salts.

Flow velocity

It is well established that at velocities greater than about 4 metres/second, a water head of greater than about 1 metre across a wall can result in the complete or partial structural failure of buildings (e.g. Kelman 2002). Debris borne by a flood may also cause battering damage and quite shallow water can cause scour damage to the ground surface.

The problem is to estimate where that damage will occur because obstructions, by channelling the water, create local velocities. In addition, considered as an

obstructed plain, there is a negative feedback loop between the built form

obstructions and flood velocity: removal of obstructions as a result of failure caused by the velocity of flow, can reduce the velocity of the flow across the remainder of the flood plain. This is most often seen by the failure of boundary walls, the failure of which allows flood water to spread out over a wider area.

The survey conducted by Masterson and Boothby records a number of instances of damage which they attributed to high flow velocities.

Duration of flooding

Flooding can cause damage through four broad mechanisms or combinations of those mechanisms:

◊ physical (e.g. capillary action, swelling of wood)

◊ mechanical (e.g. washing away of fines, scour, collision damage)

◊ chemical (e.g. rust, formation of crystals)

◊ biological (examples of biological effects are mould and other forms of fungal damage).

We should expect damages to rise with duration in so far as any of these four mechanisms are time dependent. Unfortunately, there is no real scientific literature that discusses these mechanisms in any detail but it is reasonable to expect all to have some degree of dependency upon time. In addition, the drying out period is related to the quantity of water absorbed into the fabric of the property; hence it is plausible to expect that the longer the flood, the longer will be the drying out period required and hence the longer it will take to restore the property.

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Survey results

The building surveyors (Trevor Masterson and Neil Boothby) who have been responsible for preparing the data used to assemble the FLAIR and MCM depth-damage curves for the fabric of dwellings were commissioned to examine whether either or both the extended duration of the flooding in Hambledon, or that it was groundwater flooding, resulted in more damage than would be expected on the basis of the FLAIR depth-damage curves.

Hambledon was chosen because a relatively large number of properties which were reasonably homogenous in terms of built form were found in one place. A review of the reports prepared by the Halcrow Group in 2002 for the Environment Agency, plus discussions with Hampshire County Council, suggested that only relatively small numbers of properties would be found in each of the other Hampshire villages. Looking at properties in those other villages would tend to increase the variance as a result of differences in property types whilst simultaneously increasing the costs of the survey.

It was envisaged that it might be necessary to excavate some trial pits to investigate the soil structure; this did not prove necessary in practice. Masterson and Boothby adopted the following approach:

“With these facts in mind, we considered the most accurate way to assess likely costs of damage to structure and fabric was to build a ‘typical Hambledon’ property model. This we have done, taking an average sized property as a base. We have then adjusted the figures up or down to reflect the exact size of each property.

Over the years, road levels have been raised so that the floor levels of a number of properties are now below road levels and we have anticipated slightly higher levels of cost in these instances.

We have adjusted the costs to reflect damage to properties with no

basement, albeit that we understand that basements are not supposed to be made watertight, and the damage is generally restricted to pumping out and cleaning where foul water has entered.

Where properties have been ‘affected only’, we have assumed only minor repairs are required in respect of sundry joinery warping, minor decorative repairs and the like, and have inserted a moderate repair figure, based on house size, in these cases.”

The ‘Hambledon house’ was based upon a pre-1919 detached house so as to enable comparison between the assessed damages from the flood of 2000/2001 and FLAIR data. The bill of works for the ‘Hambledon house’ is shown in Table 2.

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Table 2 Bill of works for the ‘Hambledon House’ – based upon a pre-1919 detached house 2005 Cost per unit Cost Typical Hambledon House or Cottage Descr

Aggregated over the properties which were flooded in 2000/2001 at Hambledon, the direct losses are estimated to have amounted to some £2.7 million (Table 3). The duration of the flood was estimated to increase the losses that were experienced by a factor of 3.6 over those which would be experienced in a short duration flood. For various reasons, clean-up costs, including the use of dehumidifiers, is included under contents losses rather than under damage to the property itself. So, for those properties which were flooded, the additional costs shown in Table 4 should be included. If this value is applied to all 129 properties reported to have been flooded in 2000/2001, the cost is £961,050. Annex 1 contains more detail of anticipated cleanup costs.

iption Quantity 12Hr 1 week 2 weeks 3 month 12Hr 1 week 2 weeks 3 month

9.90 82.50 82.50 82.50 0.00 216.30 216.30 216.30 0.00 19.20 28.80 48.00 0.00 0.00 11.60 46.40 0.00 23.52 23.52 58.80 0.00 159.89 213.18 373.07 0.00 1202.50 1443.00 2405.00 0.00 560.00 672.00 1120.00 0.00 203.13 203.13 507.83 5.02 30.09 40.12 65.20 6.00 18.00 18.00 36.00 0.00 14.42 21.63 43.26 282.42 423.63 988.47 1412.10 326.03 326.03 326.03 326.03 0.00 250.00 250.00 250.00 294.68 294.68 294.68 294.68 68.97 275.88 275.88 275.88 0.00 90.28 112.85 180.56 5.83 116.60 116.60 116.60 175.00 175.00 175.00 175.00 3.26 16.30 81.51 0.00 0.00 75.24 75.24 75.24 0.00 490.93 490.93 981.85 0.00 275.88 275.88 275.88 210.72 526.80 1053.60 2107.20 595.92 2085.72 4171.44 5959.20 98.70 296.10 315.84 394.80 0.00 0.00 619.55 3097.76 1053.24 1316.55 1316.55 1316.55 10.54 0.00 0.00 131.78 148.96 931.00 931.00 1862.00 0.00 2502.50 2502.50 2502.50 65.12 108.54 108.54 108.54 331.88 553.14 553.14 553.14 1.36 10.19 10.19 67.95 0.00 2708.56 2708.56 2708.56 0.00 175.56 175.56 175.56 0.00 225.48 225.48 225.48 4500.00 5850.00 6750.00 9000.00 49.14 368.55 368.55 491.40 0.00 198.22 198.22 396.43 0.00 113.52 113.52 227.04 194.50 194.50 194.50 194.50 666.00 333.00 333.00 666.00 510.60 255.30 255.30 510.60 335.96 167.98 167.98 335.96 0.00 540.00 900.00 1800.00 219.44 131.66 219.44 438.88 0.00 150.48 150.48 150.48 1159.90 2319.80 2319.80 0.00 0.00 0.00 0.00 6375.00 _________ _________ _________ _________ 11329.09 27403.14 33099.58 51163.47

RAKE GRAVEL, ROLL 30.00 0.12 1.00 1.00 1.00 AVING AND RE-BED 15.00 0 1.00 1.00 1.00

PHALT FINISH 15.00 0 0.20 0.30 0.50

E FINISH 10.00 0 0.00 0.05 0.20

SPHALT BASE & FILL 15.00 0 0.10 0.10 0.25

TURAL STONE 10.00 0 0.15 0.20 0.35

/STONE WALL REPAIR 37.00 0 0.50 0.60 1.00 cm BRICK WALL REPAIR 32.00 0 0.50 0.60 1.00 CE WITH ARRIS RAIL AND BOARD 45.00 0 0.10 0.10 0.25 N SHED - REPAIRED 1.00 0.05 0.30 0.40 0.65 NHOUSE - REPAIRED 1.00 0.05 0.15 0.15 0.30

E 1.00 0 0.10 0.15 0.30

RICK AND GLASS CONSERVATORY 0.25 0.1 0.15 0.35 0.50

UMP OUT BASEMENT 1.00 1 1.00 1.00 1.00

UMP OUT GROUND FLOOR 1.00 0 1.00 1.00 1.00 LEAN BASEMENT FLOOR 1.00 1 1.00 1.00 1.00 AN GULLIES ROD AND FLUSH DRAINS 4.00 0.25 1.00 1.00 1.00 NDERING TO BRICK 20.00 0 0.20 0.25 0.40 XTERNAL RENDER PAINT 20.00 0.05 1.00 1.00 1.00

AN SILT 1.00 1 1.00 1.00 1.00

PAIR EXTERNAL DOOR & FRAME 2.00 0.02 0.10 0.50 0.00

PLACE GLASS 25.00 0 0.05 0.05 0.05

W SASH WINDOW 29.00 0 0.05 0.05 0.10

AINT EXTERNAL DOORS INSIDE AND OUT 4.00 0 1.00 1.00 1.00 NTI-FUNGAL WALL TREATMENT 240.00 0.1 0.25 0.50 1.00 MOVE PLASTER, REPLACE AND SKIM 240.00 0.1 0.35 0.70 1.00 TUD PARTITIONS REDO 20.00 0.25 0.75 0.80 1.00 PAIR AND REPLACE MOULDED CORNICE 76.00 0 0.00 0.20 1.00 RDWOOD FLOOR AND TILING 15.00 0.8 1.00 1.00 1.00 ND & REPAIR BOARDS 70.00 0.02 0.00 0.00 0.25 IMBER MOULDED SKIRTINGS 76.00 0.08 0.50 0.50 1.00

R BOARDS REDO 70.00 0 1.00 1.00 1.00

R TILING STRIP 18.00 0.6 1.00 1.00 1.00 INYL FLOOR TILES 18.00 0.6 1.00 1.00 1.00 RPET GROUNDS 45.00 0.02 0.15 0.15 1.00 NEW INTERNALDOOR AND FRAME 8.00 0 1.00 1.00 1.00

NK 2.00 0 0.25 0.25 0.25 ROOM/UNDERSTAIRS CUPBOARD 1.00 0 1.00 1.00 1.00 OD QUALITY KITCHEN 1.00 0.5 0.65 0.75 1.00 ASH DOWN 260.00 0.1 0.75 0.75 1.00 AINT WINDOWS 29.00 0 0.50 0.50 1.00 AINT DOORS 24.00 0 0.50 0.50 1.00 MEWASH CELLAR 50.00 1 1.00 1.00 1.00

MULSION PAINT TO PLASTER 148.00 1 0.50 0.50 1.00 TRIP PAPER PREPARE 148.00 1 0.50 0.50 1.00 NG GOOD QAULITY WALLPAPER 148.00 1 0.50 0.50 1.00

PAIR PLUMBING 1.00 0 0.30 0.50 1.00

RHAUL BOILER 1.00 0.5 0.30 0.50 1.00

EST HOUSE ELECTRICS 1.00 0 1.00 1.00 1.00 RHAUL ELECTRICS 1.00 0.5 1.00 1.00 0.00 PLACE ELECTRICS 1.00 0.00 0.00 0.00 0.75

NTICIPATED TOTAL REPAIR COST E 2.75 14.42 6.40 23.20 15.68 106.59 65.00 35.00 45.14 100.31 120.00 144.20 11296.80 326.03 250.00 294.68 68.97 22.57 5.83 175.00 81.51 60.19 338.57 68.97 8.78 24.83 19.74 40.76 87.77 7.53 24.50 35.75 6.03 30.73 1.51 338.57 351.11 225.48 9000.00 1.89 13.67 9.46 3.89 4.50 3.45 2.27 1800.00 438.88 150.48 2319.80 8500.00 P AS CONCRET A NA FLINT 23 FEN GARDE GREE GAT B P P C CLE RE E CLE RE RE NE P A RE S RE HA SA T FLOO FLOO V CA RE SI B GO W P P LI E S HA RE OVE T OVE RE A NOT

ypical Hambledon house assessed to have ground floor area of 70 square metres (excluding conservatories etc)

susceptibility Repair Cost

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Table 3 Estimated losses from the 2000/2001 flood in Hambledon (2000 depth and 2005 cost levels)

12 hr duration

1 week 2 weeks 3 month

Properties (£) 740,650 1,741,350 1,953,500 2,667,500

Ratios of higher to base `figure

2.4 2.6 3.6

Table 4 FLAIR depth-damage curves – cleanup costs (£)

Average across all

depths

0.05 0.10 0.20

Domestic cleanup 5,953 5,953 10,444 7,450

The detailed bill of works for the ‘Hambledon House’ (Table 2) is analysed in more detail in Figure 2. It can be seen that there are three major elements of costs: replacement of the kitchen, replacement of plasterwork, and damages to the electricity system.

Figure 2 Breakdown of structural losses

0.00 1000.00 2000.00 3000.00 4000.00 5000.00 6000.00 RAK E G RAVE L, R OLL CO NCRE TE FI NIS 7000.00 8000.00 9000.00 10000.00 H FLIN T/STO NE W ALL REPA IR GAR DEN SHE D - R EPAI RED BRIC K AN D GL ASS CON SERV ATO RY CLEA N BA SEM ENT FLO OR EXTE RNAL REN DER PAI NT REP LAC E G LAS S ANTI -FUN GAL WAL L TR EATM ENT REP AIR AND REP LACE MOU LDED COR NIC TIM BER MOU LDED SKI RTI NGS VINY L FL OOR TILE S SINK WASH DO WN LIM EW ASH CELL AR HANG GO OD QAUL ITY WAL LPAP ER TEST HO USE ELEC TRIC S 12Hr 1 week 2 weeks 3 month

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One major category of cost for which data is not given in the FLAIR tables, and it is not given because it is so locally specific, is the cost of alternative accommodation. The Defra guidelines (Defra 1999) specify that the allowable cost is the cost of alternative accommodation of the same standard as that which is flooded during the time the flooded property is uninhabitable. Here, the loss is the decrease in the quality of life in the home which can perhaps be best proxied by the cost of providing equivalent accommodation. With short duration floods, this period is that taken to repair the property so as to return it to a habitable condition. This cost depends upon the local cost of equivalent accommodation. The survey by Hambledon Parish Council found that, at the date of the survey, the costs of alternative accommodation had been relatively low and likely to be underestimated.

In addition to the individual properties, the infrastructure in Hambledon was also examined.

Masterson and Boothby noted the following problems:

1. Flint roadside and garden walls. The jointing in the flint walls is weak mix and will erode causing collapse and structural problems, subsequent water

penetration and frost action can cause progressive damage.

2. Kerbs at the side of the road can be undermined by the washing out of fine material through fissures.

3. Surfaces of pavements and roads will be disturbed and there will be breaking up of surface asphalt material. Potholes and cracks will form and edges will degrade quickly by wear from subsequent traffic.

4. Electrical Mains will short out and require replacement.

5. Sewage systems will back up and pollution will occur that requires additional attention and specialist cleaning.

6. Subsidence can occur to pavements, paths and boundary walls.

7. Some subsidence can affect water mains, gas mains, and electric cables. 8. Saturation of brickwork on a frequent basis and subsequent frost action

causes spalling of the Brick surfaces.

9. Rot can form in low timbers again through saturation.

10. Edges of paths and shingle drives will be affected by the flow of water. Table 5 Damages to infrastructure

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Hambledon Infrastructure Hambledon Infrastructure Hambledon Infrastructure

Description Quantity Units COST/UNIT £

55.00 35.00 680.00 410.00 8.00 19.00 0.75 8.50 31.50 1.10 9.50 16.50 95.00 800.00 2.20 1.35 30.00 40.00 900.00 60.00 1250.00 30.00 400.00 6000.00

12 Hour 1 week 2 weeks 3 months 12 Hour 1 week 2 weeks 3 months

REPAIR FLINT WALLS 1450 Lin M 1.00% 5.00% 20.00% 55.00%

REPAIR BRICK WALLS 685 Lin M 1.00% 4.00% 15.00% 40.00%

REBUILD FLINT WALLS 1450 Lin M 0.00% 0.00% 2.00% 7.50%

REBUILD BRICK WALLS 685 Lin M 0.00% 0.00% 1.50% 6.00%

REBED KERBS 3450 Lin M 2.00% 10.00% 12.00% 20.00%

REPLACE KERBS 3450 Lin M 0.00% 0.00% 0.00% 10.00%

CLEAN ROAD SURFACES 17785 Sq M 50.00% 100.00% 100.00% 100.00%

REPAIR ROAD SURFACES 17785 Sq M 0.00% 5.00% 8.00% 15.00%

RENEW ROAD SURFACES 17785 Sq M 0.00% 0.00% 0.00% 6.00%

CLEAN PAVEMENT 3620 Sq M 50.00% 100.00% 100.00% 100.00%

REPAIR TARMAC PAVEMENT 3620 Sq M 0.00% 2.50% 7.50% 25.00%

REPLACE TARMAC PAVEMENT 3620 Sq M 0.00% 2.00% 5.00% 15.00%

INSPECT/REPAIR ELECTRIC MAINS 221 House 15.00% 100.00% 100.00% 100.00%

REPLACE ELECTRIC MAINS 221 House 0.50% 2.50% 4.00% 9.00%

FLUSH OUT 225 DIAMETER DRAIN 2965 Lin M 10.00% 100.00% 100.00% 100.00% FLUSH OUT DRAINS (GENERAL) 3500 Lin M 10.00% 100.00% 100.00% 100.00%

REPAIR DRAIN RUNS 3500 Lin M 0.00% 2.50% 5.00% 10.00%

REPAIR MANHOLE 100 No 5.00% 50.00% 50.00% 100.00%

REBUILD MANHOLE 100 No 0.00% 0.00% 2.50% 7.50%

INSPECT/REPAIR WATER MAINS 221 House 15.00% 100.00% 100.00% 100.00%

REPLACE WATER MAINS 221 House 0.50% 2.50% 4.00% 9.00%

INSPECT/REPAIR BT MAINS 221 House 15.00% 100.00% 100.00% 100.00%

REPLACE BT MAINS ETC 221 House 0.50% 1.25% 2.00% 4.50%

DE-SILT EXISTING DYKES 1 No 10.00% 50.00% 70.00% 100.00%

__________ __________ __________ __________ TOTAL 797.50 3,987.50 15,950.00 43,862.50 239.75 959.00 3,596.25 9,590.00 0.00 0.00 19,720.00 73,950.00 0.00 0.00 4,212.75 16,851.00 552.00 2,760.00 3,312.00 5,520.00 0.00 0.00 0.00 6,555.00 6,669.38 13,338.75 13,338.75 13,338.75 0.00 7,558.63 12,093.80 22,675.88 0.00 0.00 0.00 33,613.65 1,991.00 3,982.00 3,982.00 3,982.00 0.00 859.75 2,579.25 8,597.50 0.00 1,194.60 2,986.50 8,959.50 3,149.25 20,995.00 20,995.00 20,995.00 884.00 4,420.00 7,072.00 15,912.00 652.30 6,523.00 6,523.00 6,523.00 472.50 4,725.00 4,725.00 4,725.00 0.00 2,625.00 5,250.00 10,500.00 200.00 2,000.00 2,000.00 4,000.00 0.00 0.00 2,250.00 6,750.00 1,989.00 13,260.00 13,260.00 13,260.00 1,381.25 6,906.25 11,050.00 24,862.50 994.50 6,630.00 6,630.00 6,630.00 442.00 1,105.00 1,768.00 3,978.00 600.00 3,000.00 4,200.00 6,000.00 21,014.43 106,829.48 167,494.30 371,631.28

They estimated that the damages as shown in Table 5 would occur. Some of these costs are associated with the cleaning of drainage channels but the majority of the costs fall into one of three categories: damage to walls, roads and utilities. In turn, the damage to walls and roads is considered to be primarily the result of the high velocity flows, although these damages are also regarded as being influenced by the duration of the flooding (Figure 3). The damage to the utilities is anticipated as a result of the fine material upon which the services are laid being washed out as a result of the groundwater flooding, this loss of support resulting in an increased risk of failure under ground pressures.

Figure 3 Estimated damage to infrastructure, Hambledon 2000/2001

0.00 50,000.00 100,000.00 150,000.00 200,000.00 250,000.00 300,000.00 350,000.00

12 Hour 1 week 2 weeks 3 months damage to infrastructure

utilities roads walls

As compared to the damage to buildings, the damage to infrastructure contributes a relatively minor fraction of the total losses. Taken together (Figure 4 and 5), flood losses rise sharply as the duration of flooding increases from 12 hours to one week, and again when flood duration increases to three months.

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Figure 4 Estimated losses by variation in flood duration 2000 DEPTH, 2005 PRICES 0 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 12 hr duration

1 week 2 weeks 3 month

DURATION

LO

SS properties

infrastructure TOTAL

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Figure 5 Flood losses including professional fees. 2000 DEPTH, 2005 PRICES 0 500,000 1,000,000 1,500,000 2,000,000 2,500,000 3,000,000 3,500,000 4,000,000 12 hr duration

1 week 2 weeks 3 month

DURATION

LO

SS propertiesinfrastructure

TOTAL

Masterson and Boothby carried out a further analysis in which the estimated depths of flooding in each property were varied by plus or minus 100mm. For relatively shallow depths of flooding, damage estimates are very sensitive to variations in depths (Figure 6). In addition, with shallow flooding, the depth of flooding in

particular properties can vary markedly by small variations in local topography (e.g. kerb heights).

Figure 6 Variations in estimated losses by variation in depth of flooding

0 1,000,000 2,000,000 3,000,000 4,000,000 5,000,000 6,000,000 7,000,000 8,000,000 9,000,000 12 hr duration

1 week 2 weeks 3 month

duration lo ss 20 00 flo od 2000 less 100mm plus 100mm

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Figure 7 Comparison of losses to properties as reported in the Parish Council survey as estimated by the specialist survey

Finally, it is worth comparing the results from the Parish Council survey with those using the Masterson and Boothby analysis. This comparison is shown in Figure 7. The overall regression line, as it happens, has a slope which is not statistically significantly different from one and an intercept which is not statistically significantly different from zero. What is more interesting are two classes of property which are markedly away from the regression line (explained variance is around 30%). These two classes are:

1. Those properties which have markedly greater damages than is predicted from the Masterson and Boothby analysis.

2. Those properties which were reported in the Parish Council survey to have zero damages whereas the Masterson and Boothby analysis expects damages, in some cases quite significant damages.

DC survey 0 10000 20000 30000 40000 50000 60000 70000 0 10000 20000 30000 40000

Revised FLAIR estimates

Pa ri s h C o u n c il Su rv e y

The first form of difference may simply be examples of the ‘sample-population’ problem; there is also the possibility that they represent listed buildings, with associated higher repair costs. But it did not prove possible to reliably identify the listed buildings with the Parish Council data set.

The cases where the Masterson and Boothby analysis expects more damage than the Hambledon Parish Council survey reported are partly those where the Parish Council survey reported no response or where the occupier did not yet know what the costs would be. These cases were left in for the purposes of the statistical analysis because the effect would be to reduce the fit between the two sets of data. But in some instances, the survey reported a zero loss when a non-zero loss would be expected.

Findings

The clear conclusion is that groundwater flooding can result in substantially higher losses than is usual from surface water flooding. For buildings, the primary driver behind these increased losses is the longer duration of the flooding.

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For the property damage, the longer duration floods experienced in groundwater flooding are estimated to result in significantly greater damages to the fabric of properties. As compared to a flood of 12 hours duration, a flood of 1 week duration is expected to result in losses which are 2.4 times greater; a flood of 2 weeks duration in losses which are 2.6 times greater; and a flood of 3 months duration in losses which are 3.6 times as great as those from a flood of 12 hours duration. Table 6 Direct damages estimated for Hambledon

(2000 depth, 2005 cost levels)

12 hr duration

1 week 2 weeks 3 month

Properties (£) 740,650 1,741,350 1,953,500 2,667,500 ratio 2.4 2.6 3.6 infrastructure 21,014 106,829 167,494 371,631 ratio 5.1 8.0 17.7 FEES ETC 114,250 277,277 318,149 455,870 TOTAL 875,914 2,125,456 2,439,143 3,495,001 RATIO 2.4 2.8 4.0

Since floods of approximately one week’s duration can also be experienced on major rivers such as the Thames, flood losses in those events are also likely to be under-estimated when FLAIR data is used.

These results are, however, indicative and should not be generalised to all floods of duration longer than 12 hours. Firstly, in order to derive them, a typical ‘Hambledon’ house was used; the results may not be accurate for other types of property.

Secondly, it is plausible to expect that the increased damages which result from long duration flooding as compared to flooding of short duration fall with increased depths of flooding. In part this is because at extreme depths of flooding there is not much more damage that could be done. But in addition some of the additional damage that may be caused by long duration floods is as the result of water travelling

through materials, such as by capillary action, to parts of the building above the flood level. In more extreme floods, those areas would instead be below the flood level. To these losses should be added:

• The costs of clean-up; FLAIR data, based upon advice from professional disaster recovery firms, leads to an estimate of £961,050.

• The costs of alternative accommodation during the period for which properties were uninhabitable either during the flood or during the restoration period. Here, the Parish Council survey data is likely to result in an underestimate.

• A number of other relatively small items of loss (e.g. damage to cars).

Following the new Treasury guidance, the resultant figure should be weighted to take account of income.

The survey of Hambledon raises a further question with respect to properties constructed prior to 1919. FLAIR data only covers a generic class of ‘pre-1919’

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houses and national data does not discriminate in any greater detail as to the

proportions of the building stock which are of different ages. For housing, the ODPM statistics report that 23% of the housing stock was built prior to 1919. A large

proportion of that 23% was constructed in the Edwardian and Victorian periods of masonry construction and heavy timber floors and hence is not especially

susceptible to damage in flooding. But a proportion of that stock will be of other forms of construction which will be particularly susceptible to flood damage. For example, the English Heritage report (2004) on flood damages to historic properties shows the interior of an early seventeenth century timber framed house, with interior infills of painted plaster, located on a flood plain. It might be expected that other traditional forms of buildings such as those of cob, clunch or lathe and plaster would also be especially susceptible to flood damage.

Whilst the stock statistics do not differentiate within the pre-1919 age band, OPDM do give the historical series for housing stock at different dates

(http://www.odpm.gov.uk/index.asp?id=1156006); the figure for all dwellings for 1801

was 1.5 million dwellings. The Welsh Assembly gives a similar historic series with some 118,000 dwellings being inhabited in 1801. These figures are upper limits for housing since some will have been demolished since 1801; but they exclude non-residential properties. The best estimate of the number of buildings which might be more susceptible to flood damages than the pre-1919 category of FLAIR is the number of buildings listed by English Heritage; some 372,000 (

www.english-heritage.org.uk/server/show/nav.1430). Since Listed Building Consent will be

required for some actions to dry out and repair listed buildings (English Heritage 2004), the costs of restoring listed buildings are, in any case, likely to be greater than for a non-listed building. Therefore, FLAIR data may well underestimate the damages to listed buildings. In turn, there are a significant number of listed buildings in Hambledon.

Nationally, some 13% of scheduled monuments are believed to lie on flood plains (Defra 2004). Similar considerations also apply to buildings in Conservation Areas; no figures are available as to the number of buildings in Conservation Areas (Bottrill 2005) but Bottrill estimates that around 1.1 million dwellings are likely to be included in the 13,922 Conservation Areas in England and Wales.

In the Hambledon Parish Council survey, residents were also asked whether they considered that the value of their property had been reduced. As is usual with victims of flooding, many felt that the value had been reduced. Whether or not this amounts to an economic loss is problematic; if any reduction in property value reflects the capital value of anticipated future flood losses then it would be double-counting to include both those expected losses and the capital value. But if that reduction in property value, at least in part, is a result of factors other than the losses already counted then it would be correct to include that part of the reduction in

property value. For example, if the reduction in property value reflects the

anticipated disruption to the household’s life as a result of flooding in the future and the stress caused by that flooding. Obviously, distinguishing between that fraction of any reduction in property value which reflects flood losses already included and that fraction which reflects the other impacts of flooding is problematic. Hence, by convention, anticipated reductions in property value are not included in economic assessments of the benefits of flood alleviation.

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For infrastructure damages, there is evidence of additional losses as a result of groundwater flooding and also from the velocity of the flood flows experienced in Hambledon.

Implications for the damages caused by groundwater flooding

across Hampshire

The detailed analysis was undertaken of one house type; the obvious question is: to what extent can the result, that long duration flooding will result in higher losses than anticipated from Middlesex depth-damage data, be extended to other villages in Hampshire?

There are two ways of extending the result to other villages:

1. If Hambledon is typical of all villages in Hampshire in having a high proportion of houses which are pre-1919 detached properties.

2. The increased damages to pre-1919 detached houses can reasonably be extended to other house types.

Across the country as a whole, pre-1919 detached houses constitute only 3% of the housing stock (Table 7).

Table 7 Housing stock in England

note: these statistics do not differentiate between bungalows and other buildings

However, David Richardson’s detailed analysis1 of the housing composition of twenty villages in Hampshire shows the proportion of pre-1910 detached houses in those villages is considerably higher than the national average. The proportion of pre-1910 detached houses for the sample as a whole averages 19%, with the proportion in individual villages varying from 2% to 48%. The proportion of such properties in Hambledon (18%) is thus very close to sample mean. The sample of villages was selected from the 38 settlements for which groundwater was the only or principal cause of flooding in 2000-01. The proportion of pre-1910 detached houses

1

Environment Department 2006 House Type Study of a Typical Hampshire Village Prone to Groundwater Flooding, Hampshire County Council

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was determined by comparing the 3rd edition Ordnance Survey 25 inch County series map to the current edition.

The alternative approach is to consider how far it is legitimate to extrapolate from the difference in damages by duration of flooding from one house to another. Since we are only concerned with damages to the fabric of the building, it is differences in the form of construction that are significant. Since the floor areas of the different house types which are used in the Middlesex depth-damage data differ significantly, it is more appropriate to compare losses per square metre. This does not totally remove the effect of differences in floor areas of the different house types for two reasons. Firstly, the sizes of individual rooms, such as the kitchen, are not necessarily constant as a proportion of the ground floor. Secondly, as the size of an individual wall increases, the ratio of the walls to the floor area falls. For example, for a three by three square, the ratio of the perimeter to the area is 1.33 but this falls to 0.67 for a square of six by six.

In principle, any given quantum of damages which result from a long duration flood could also be achieved by depth of flooding, greater than that experienced. In short, if the average damages to a house are 2.4 times those experienced in a short

duration flood, those higher damages could result from either a longer duration flood or a deeper flood. The x axis in Figure 8 is a plot of the losses, as described above, from floods of different durations. Against the y axis are plotted the depths of flooding necessary to yield the same losses. There appears to be some form of relationship between the two, although probably not a linear relationship.

Figure 8 pre-1919 detached house: depth of flooding required to result in equivalent damages 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 0 5 10 weeks de pt h ( m e tr e s ) 15

The following equation was fitted by eye: depth = 0.33.weeks0.625 – 0.01

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This equation fits the data almost exactly but since there are only four data points, no very great weight should be given to this fit. But, it therefore seems not

unreasonable to argue that the relative increase in damages as duration increases are likely to be approximately related to the relative increase in damages as depth increases.

Figure 9 is a plot of damages at different depths, for different house types, as a ratio

to the damages at 0.05 metres. The slopes therefore show how sensitive damages are to the depth of flooding over this limited range. Two points are significant:

1. The flattish slopes are for those house types which have a higher than average loss at shallow depths of flooding; hence, there is less additional damage to be caused by increasing the depth of flooding.

2. The ratio is highest for pre-1919 detached houses. Hence, it is reasonable to argue that the increase in losses for pre-1919 detached houses sets an upper bound on the degree to which using the Middlesex depth-damage tables understates flood damages for extended duration floods for other house types.

Figure 9 Relative increase in damage by depth of flooding

ratio of loss for 0.3 metres as compared to 0.05 m 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0.1 0.2 depth P RE 1919 DETA CHED 1919 - 1944 DETA CHED 1945 - 1964 DETA CHED 1965 - 1974 DETA CHED 1975 - 1985 DETA CHED 1975 - 1985 UTILITY DETA CHED

P OST 1985 DETA CHED P RE 1919 SEM I-DETA CHED 1919 - 1944 SEM I-DETA CHED 1945 - 1964 SEM I-DETA CHED 1965 - 1974 SEM I-DETA CHED 1975 - 1985 SEM I-DETA CHED 1975 - 1985 UTILITY SEM I-DETA CHED

P OST 1985 SEM I-DETA CHED P RE 1919 TERRA CE 1919 - 1944 TERRA CE 1945 - 1964 TERRA CE 1965 - 1974 TERRA CE 1975 - 1985 TERRA CE 1975 - 1985 UTILITY TERRA CE P OST 1985 TERRA CE

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Losses and insurance

It is only relatively recently that insurance companies began to separate claims data for floods from other natural hazards, notably from storm damage. The first problem is that there is a clear association between income and insurance cover; those with higher incomes are more likely to have building and contents cover than those with low and fixed incomes. This means that insurance claims for flood losses are a biased sample of flood losses, and in so far as there is any positive association between income and flood loss, insured losses are biased upwards.

Once data has been separately compiled for floods, the problem is that floods arise through a multitude of mechanisms which include: localised rainfall that cannot escape; runoff down hill sides; water which is unable to discharge into a sewer or other drainage channel; surcharges from sewers which are full or because the sewer is blocked or has collapsed or because a pump has failed; groundwater; overflow from minor watercourses; overflow from main rivers; and flooding from the sea. The responsibility for managing the risk varies and so it is not unknown for the different organisations to debate by which mechanism a particular property was flooded and who, therefore, is responsible for managing that particular flood risk. Ascribing an insurance claim to a particular flood mechanism is not likely therefore to be

particularly reliable. The Association of British Insurers has in turn produced various figures for the proportion of flood claims which arise in areas outside of recognised flood plains.

A different problem, which was discovered in the course of the study, is the legal definition of a ‘flood’ which definition is not particularly clear. The following summary of the current position was provided by Professor David Crichton. The Court of Appeal provided the following definition: “something large, sudden and temporary not naturally there such as a river overflowing its banks” (Young v Sun Alliance, 1976). Young v Sun Alliance also

held that flood does not include seepage of water from an underground watercourse. However, the Court of Appeal also held in Computer & Systems Engineering plc v John Lelliott (Ilford) Ltd (1989) that a flood had to be the result of a natural

occurrence and that water damage caused by accidental damage to a sprinkler system was not a flood. The morerecent decision of the Court of Appeal in Rohan Investments Ltd v Cunningham (1998) held that abnormally heavy rainfall over a period of some days did constitute a flood when it entered the plaintiff's property. In another case it was decided that “heavy rainfall is not a storm”(Anderson v Norwich Union 1977). Rising groundwater is a hazard that is specifically excluded from standard domestic residential property insurances; it would seem that to be an insured risk, the groundwater must first flow across the surface of the land before entering the building that is insured.

Conclusions

The effect of duration on flood losses has largely been neglected to date. This study has shown that flood durations of one week or more add substantially to damages to building fabric.

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1) The clear conclusion from the study is that groundwater flooding can result in substantially higher losses than is usual from surface water flooding. For buildings, the primary driver behind these increased losses is the longer duration of the

flooding.

(2) For the property damage, the longer duration floods experienced in

groundwater flooding are estimated to result in significantly greater damages to the fabric of properties as follows:

• Compared to a flood of 12 hours duration, a flood of 1 week duration is expected to result in losses which are 2.4 times greater;

• A flood of 2 weeks duration in losses which are 2.6 times greater; and

• A flood of 3 months duration, losses which are 3.6 times as great as those from a flood of 12 hours duration.

(3) To these losses should be added:

• The costs of clean-up;

• The costs of alternative accommodation during the period for which properties were uninhabitable either during the flood or during the restoration period.

• A number of other relatively small items of loss (e.g. damage to cars).

(4) Following the new Treasury guidance, the resultant figures should be weighted to take account of income.

(5) For infrastructure damages, there is evidence of additional losses as a result of groundwater flooding such as from the velocity of the flood flows.

Recommendations

Our research has shown that flood durations of one week or more add substantially to flood damages. We recommend therefore that:

1. It is essential to take flood duration and the extra damage that it causes into account when undertaking cost-benefit analyses of relevant flood alleviation schemes.

2. Floods of long duration are commonly experienced from groundwater floods but may also occur on large lowland rivers, or when polders are flooded from which the water must then be evacuated. As recommended above,

investment appraisals for such flood alleviation schemes should therefore be able to use the results of this research. To this end Defra should accept the data contained herein, in the absence of better information, as the most appropriate for the assessment of relevant scheme benefits and require local authorities and the Environment Agency and their agents to use this data, again where appropriate.

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3. For ourselves, we will develop a new dataset within our Multicoloured Manual to allow users to have easy access to this data (and provide this data on our MCM CD in 2007 when that dataset is updated).

References

• Bottrill C 2005 Homes in Historic Conservation Areas in Great Britain: Calculating the Proportion of Residential Dwellings in Conservation Areas, Oxford:

Environmental Change Institute

• Defra (Department for Environment, Food and Rural Affairs) 1999 Flood and Coastal Defence Project Appraisal Guidance 3: Economic Appraisal, London: DEFRA

• Defra (Department for Environment, Food and Rural Affairs) 2004 Physical Drivers behind flood and coastal erosion risks, London: Defra

(www.defra.gov.uk/environ/fcd/policy/strategy.htm)

• English Heritage 2004 Flooding and Historic Buildings, Technical Advice Note

(www.english-heritage.org.uk)

• Green C H 2003 Handbook of Water Economics, Chichester: John Wiley

H M Treasury 2003 The Green Book: Appraisal and Evaluation in Central Government, London: H M Treasury

• Halcrow Group 2002 Winter 2000-2001 Flooding in Hampshire, Hambledon, Swindon: Halcrow Group

• Jacobs 2004 Strategy for Flood and Coastal Erosion Risk Management: Groundwater Flooding Scoping Study (LDS 23), London: Defra

• Kelman I 2002 Physical flood vulnerability of residential properties in coastal, eastern England, unpublished PhD, Cambridge: University of Cambridge

• N’Jai A, Tapsell S M, Taylor D, Thompson P, and Witts R C 1990 Flood loss assessment information report (FLAIR) Flood Hazard Research Centre, London

• National Statistics 2005 Regional household income, London: National Statistics

(http://www.statistics.gov.uk/pdfdir/rhi0405.pdf)

• Penning-Rowsell E, Johnson C, Tunstall S, Tapsell S, Morris J, Chatterton J, Coker A and Green C 2003 The Benefits of Flood and Coastal Defence: Techniques and Data for 2003, Enfield: Flood Hazard Research Centre

• Penning-Rowsell E, Johnson C, Tunstall S, Tapsell S, Morris J, Chatterton J and Green C 2005 The benefits of flood and coastal risk management: A manual of assessment techniques Middlesex University Press, London

• Posford Haskoning 2001 Hambledon Flooding Investigation Study Report, Haywards Heath: Posford Haskoning

(30)

Annex 1

Average clean-up costs at flood depths below 0.1m (Source: ARK Ltd.)

Clean-up component Unit cost (£) No. units

No. days Other Costs (£) Total (£)

Pressure washer 30 p/d 1 1 30

Aquavac and transformer 30 p/d 1 1 30

Decontamination 100 p/d 2 2 200

Skip 176 p/w 1 7 176

Storage cabin 220 p/m 1 28 195 Delivery - 195 Collection 610

Blower heater 112 p/m 2 28 224 Air mover 180 p/m 2 28 360 Dehumidifier 340 p/m 4 28 1360 Labour costs Pressure washer 195 p/d 1 195 Aquavac 195 p/d 1 195 Decontamination 195 p/d 2 390 Carpet removal 195 p/d 2 390 Flooring removal 195 p/d 2 390 Skip loading 195 p/d 1 195 Dehumidifier maintenance 35 p/d 28 980 Total £ 5725

(31)

Annex 2

Average clean-up costs at flood depths above 0.1m (Source: ARK Ltd.)

Clean-up component Unit cost (£) No. units No. days Other costs (£) Total (£)

Pressure washer 30 p/d 2 2 60

Aquavac and transformer 30 p/d 2 2 60

Decontamination 100 p/d 3 3 300

Skip 176 p/w 2 7 352

Storage cabin 220 p/m 1 56 195 Delivery - 195 Collection 830

Blower heater 112 p/m 2 56 448 Air mover 180 p/m 2 56 720 Dehumidifier 340 p/m 4 56 2720 Labour costs Pressure washer 195 p/d 2 390 Aquavac 195 p/d 2 390 Decontamination 195 p/d 3 585 Carpet removal 195 p/d 2 390 Flooring removal 195 p/d 2 390 Skip loading 195 p/d 2 390 Dehumidifier maintenance 35 p/d 56 1960 Total 9985 UK depth-damage 0.00 500.00 1,000.00 1,500.00 2,000.00 2,500.00 3,000.00 3,500.00 4,000.00 4,500.00 -0.3 0.05 0.2 0.3 0.6 0.9 1.2 1.5

FLAIR - all dwelling UK -survey data

Figure

Table 1  Flood losses from the 2000/01 flood as compiled by  Hambledon Parish Council
Figure 1  Middlesex depth-damage curves for a pre-1919 detached house
Table 4  FLAIR depth-damage curves – cleanup costs (£)
Figure 3  Estimated damage to infrastructure, Hambledon 2000/2001
+7

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