Annex D

Flood Risk in Hampshire risk assessment methodology

Annex D

D.1 Introduction D.1.2 Data Sources D.1.3 Calculating risk D.2 Detailed methodology D.2.2 The cost of surface water flooding to residential property D.2.3 The cost of surface water flooding to non-residential property D.2.4 The cost of surface water flooding affecting critical infrastructure D.2.5 The cost of groundwater flooding D.2.6 The cost of surface water flooding of roads D.2.7 Total economic damage D.3 Summary of key assumptions D.3.1 Every property flooded internally has the same cost of flooding (for its

property type) regardless of the depth of flooding or return period of that flood.

D.3.2 No allowance has been made for confidence of modelled results in the initial assessment. Therefore reported flood incidents have the same confidence as the modelled flood extent.

D.3.3 The 2000/2001 groundwater event has been used as the only complete and fully analysed information on groundwater flooding.

D.3.4 An average flood duration of 1 week has been applied across all flooded properties in the 2000/01 groundwater event

D.3.5 An event return period of 25 years has been applied to the 2000/01 groundwater event.

D.3.6 The impact of groundwater flooding on receptors other than residential properties has not been included.

D.3.7 The Hampshire County Council data set has not been used to assess the impact of flooding on roads.

D.3.8 Traffic disruption of 1 hour duration has been assumed D.3.9 Length of flooded road is used as a proxy for the additional distance

travelled to avoid flooding D.3.10 The total annualised cost of damages per ward has not been normalised

against either the number of properties in the ward, or the area of the ward. D.4 Recommendations

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D.1 Introduction D.1.1.1 This annex outlines the risk assessment undertaken to identify areas within

Hampshire which have a high risk of local flooding (from groundwater, surface water and ordinary watercourses).

D.1.1.2 The aim of the assessment is to provide a consistent means to compare the risk of flooding between areas in Hampshire. This can then act as a guide to indicate where investment in flood risk management may be most effectively focused.

D.1.1.3 For the purpose of this assessment, the risk of flooding has been calculated in relation to: o The number of residential properties which flood internally o The number of non residential properties (such as shops and factories)

which flood internally o The number of properties which are critical infrastructure which floods

(such as schools, hospitals, electricity sub stations) o The length of motorway and A-roads which flood

D.1.1.4 It is not possible to directly compare the relative importance of these different receptors1 directly. For example, what length of flood road has the same impact as a flooded house? Therefore to estimate a value of risk that combines the individual risks to these different receptors a common means to compare between them is needed. To do this an economic value of the damages caused by flooding to these receptors is estimated and applied. This allows the risk to different features to be added together and compared between areas in terms of economic cost in pounds sterling. This economic value does not represent the actual cost of flooding; rather it is a means by which relative risks can be compared between areas.

D.1.1.5 The scale at which to carry out and view the comparative assessment of risk was discussed and agreed with the key stakeholders and steering group of the Strategy. It is important to carry out the risk assessment at a scale which is a

1 A receptor is anything which could be affected by flooding such as a house, road, school or park. In this assessment the receptors

of interest are residential properties, non residential properties, motorways, a-roads and critical infrastructure.

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balance between a number of different considerations. The analysis needs to be undertaken at a scale that: o is detailed enough to reflect differences in flood risk due to characteristics of

an area o is not so detailed that it could risk ‘property blight’ i.e. effect the ability of

people to get insurance or sell a property o is not so detailed as to suggest more certainty in the outcome than exists

(due to inherent uncertainties in the data used and assumptions made in the method)

o has some meaning to the public o is applicable to the whole County Council area (e.g. Parishes do not exist in

some parts of Hampshire)

D.1.1.6 Of the possible scales considered (River Catchment, District, Parish, Ward, 1km National Grid Square and Post code), undertaking the assessment at a ward scale was deemed the most appropriate based on the criteria outlined above.

D.1.1.7 The outcome of the assessment is a map which expresses the risk of flooding as an economic value for each ward in the County Council area of responsibility. To produce the map publicly available guidance is used to establish standardised costs of flooding of different features. The map does not and should not be used to express the actual cost of flooding, it is merely a means by which to standardise the comparison of the risk of flooding between wards in Hampshire.

D.1.2 Data Sources

D.1.2.1 Two forms of data are required for the assessment: information on what is susceptible to flooding (location, building type, road type, floor area) and information on flooding (location, frequency, depth).

D.1.2.2 For data about properties and roads at risk from flooding the Environment Agency’s National Receptor Dataset (NRD) and Mastermap data produced by Ordnance Survey have been used. Data about critical infrastructure have been provided from Hampshire County Council records.

D.1.2.3 Information about flooding was gathered from the following sources: o The Environment Agency Flood Map for Surface Water o Hampshire County Council flooding records o Report on the groundwater flooding in the winter of 2000-2001

D.1.2.4 These three sources of data can be thought of as two types each with different strengths and weaknesses. The Environment Agency Flood Map for Surface Water is modelled data. This data set is complete (it covers the whole country), it shows where flooding could occur but not where flooding has occurred in the

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past. It is important to remember that modelled data is inherently uncertain because it is based on generalised assumptions. Therefore some of the areas shown as flooding in the Environment Agency Flood Map for surface water may never flood.

D.1.2.5 The second set of data (Hampshire County Council flooding records and report of the groundwater flooding) are records of flood events which have occurred in the past. Unlike the modelled data, it is known that flooding occurred at the locations shown in these datasets and therefore could probably occur there again. However all datasets of real events have gaps; they do not record all flooding that has ever occurred. In addition, these data sets do not provide information about whether an area is at risk from flooding if it has not flooded in the past.

D.1.2.6 Therefore this flood risk assessment uses both types of data (modelled and recorded) to develop an understanding of the locations at risk both because they have flooded in the past and they may flood in the future.

D.1.3 Calculating risk

D.1.3.1 Risk is a product of likelihood and consequence. Risk = Consequence x Likelihood

D.1.3.2 To establish the consequence of flooding the economic cost of a flood to each type of receptor flooded (e.g. Property, road, infrastructure) is identified.

D.1.3.3 The economic cost of flooding is not the actual cost of damages to the receptor. Numerous factors combine to produce the actual cost of damages, many of which can only be determined in detail after an event. The economic cost of flooding, in contrast, uses a number of assumptions to produce a comparative value that can be used for strategic assessments.

D.1.3.4 Using the Environment Agency Flood Defence Grant in Aid (FDGiA) a value of £30,000 cost per residential property which is flooded internally is applied and used to calculate equivalent costs for other types of receptors.

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D.1.3.5 The other component of risk, likelihood, is expressed in terms of a frequency, probability, or a return period. For example a flood which is said to have a 1 in 200 year return period, has a 1 in 200 chance of occurring in anyone year, or a 0.5% probability of occurring in any one year.2

D.1.3.6 The assessment needs to compare and combine the costs of floods of different frequencies. To do this the potential cost of flooding to a ward is expressed on an annual basis, by dividing the cost by the return period or frequency (or multiplied by the probability). For example if 1 house is flooded by a 1 in 200 year flood, the total cost is £30,000 over that two hundred year period. The annual cost of damage caused by that flood is actually £150 (£30,000 divided by 200).

D.1.3.7 Some properties which are flooded by an infrequent or severe floods (e.g. 1 in 200), may also be flooded by more frequent floods. Therefore it is not possible to just sum all of the costs of different frequencies of floods to establish the annual cost of flood damage: this would lead to an overestimate of damages. To combine the costs of different probability events a graph must be plotted that plots the probability (likelihood) of different floods against the damage (consequence) each of those floods cause. The area under the curve is then equal to the annual damages of flooding. This is shown in Figure D.1.

2 It is important to remember that a 1 in 200 year return period flood can occur more often than once every 200 years.

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Figure D.1: Calculating annual cost of flood damage

D.2 Detailed methodology D.2.1.1 A property is assessed to flood internally if:

o It is specified within the HCC database as having done so

o It is listed within the groundwater report as having done so

o Any part of its floor plan (taken from Ordnance Survey Mastermap) lies within 1 metre of the ‘deep’ (flooding greater than 0.3m depth) extent of the Environment Agency Surface Water Flood Maps3.

D.2.1.2 For this study a road is deemed to flood if it is covered by either the shallow or deep extent in the Environment Agency Surface Water Flood Maps. Under the method applied within this assessment, information about the length of flooded road is necessary to undertake an assessment of the cost of flooding to roads. This information was not consistently available within the Hampshire County

3 The Environment Agency Surface Water Flood Map is buffered by 1 meter to overcome issues with the precision of the flood

extents as a result of differences in grid sizes used when developing the map.

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Council dataset or the groundwater flood report. Therefore the data was used to assess the impacts of flooding on properties only.

D.2.1.3 Using geospatial analysis the number and types of properties and the length of a-road or motorway that lie within the flood extents are identified. These are then categorised by the type of property and by the ward in which they are located.

D.2.2 The cost of surface water flooding to residential property

D.2.2.1 The cost of internal flooding from surface water to residential property is based on standard assumptions used by the Environment Agency when populating the FDGiA calculator which values the damages to a residential property caused by internal flooding as £30,000. This value is applied to all residential property irrespective of location, size or type.

D.2.3 The cost of surface water flooding to non-residential property

D.2.3.1 The cost of internal flooding from surface water to a non-residential property is based upon its ground area and the bulk class of the property as defined in the Multi-coloured manual4. The cost per square metre relative to residential property has been calculated by determining the average ratio of Weighted Annual Average Damage (WAAD) for each bulk class (type) of non-residential property against that for residential properties. This factor is then applied to the £30,000 cost of flooding to a residential property. Table D.1 illustrates the resulting cost per square metre of floor area.

Table D.1: Cost (£) per m2 of flooding for non residential property by Bulk class (assuming cost of flooding to residential property is £30,000)

Factory Bulk class

Retail Bulk Class

Warehouse Bulk Class

Office/other Bulk Class

Non-Bulk

Cost (£k/m2)

0.23 0.27 0.22 0.21 0.21

4 The Benefits for Flood and Coastal Risk Management: A Manual of Assessment Techniques, Flood Hazard Research Centre, 2005;

The Benefits for Flood and Coastal Risk Management: A Manual of Assessment Techniques 2010

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D.2.4 The cost of surface water flooding affecting critical infrastructure

D.2.4.1 Critical infrastructure is contained within the non-residential property bulk classes, so these costs are included within the non-residential property calculations. However, these are costs to the physical fabric of the buildings or structure and do not account for indirect costs or those relating to failure of service. These costs are not readily available, and much of the data behind these costs will be commercially sensitive. Therefore we have not directly accounted for these indirect costs in the consequence assessment. The impact of flooding on critical infrastructure will therefore be assessed qualitatively following the quantitative cost assessment.

D.2.5 The cost of groundwater flooding

D.2.5.1 The number of flooded properties during the 2000-2001 groundwater flood event are shown in section 4.1 of Environment Agency report (September 2002)5. The report states that the estimates of the return period for total rainfall for the year (April 2000 – March 2001) range between 1:50 to 1:200. Most groundwater level data for the winter exceeded previous records.

D.2.5.2 The return period or frequency of the rainfall responsible for causing a flood and the return period of the flood itself often vary. This is because the occurrence of a flood is not just dependent on rainfall, other factors also affect its, such as water levels in the ground before the rainfall event. The known occurrence of repeat flooding from groundwater in some locations indicates that groundwater flooding occurs more frequently than the rainfall return period indicated in the report into the 2000-2001 flooding. To allow for this, and in the absence of further information, the 2000-2001 groundwater flooding is assumed to have a return period of 1 in 25, or annual probability of 4%.

D.2.5.3 A report prepared the Flood Hazard Research Centre at Middlesex University6 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)

5 Environment Agency – Southern Region. Hampshire and Isle of Wight Area. Winter 2000-2001 Flooding in Hampshire Final Overview Report September 2002.

6 Colin Green, Theresa Wilson, Trevor Masterson and Neil Boothby (2006) An assessment of the additional flood losses associated with groundwater flooding: a report to Hampshire County Council and Winchester City Council FHRC.

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data; for a flood lasting three months, the building fabric damages increased to 360% of those estimated using MCM data.

D.2.5.4 For the purposes of this strategic assessment an average flood duration of one week for all groundwater flooded properties has been assumed. Whilst there is a risk that a 1 week duration will underestimate the duration of groundwater flooding events it is deemed that on average this is a more representative duration for groundwater flooding in Hampshire than 3 month duration. If a 3 month duration been assumed the costs of groundwater flooding would have increased.

D.2.5.5 To calculate the economic cost of groundwater flooding the same base cost per receptor type used in the surface water flooding consequence assessment, has been used and multiplied by a factor of 2.4.

D.2.5.6 Flood extents for the 2000/01 event are not available; therefore it is not possible to apply a groundwater flood consequence to any receptors other than properties.

D.2.6 The cost of surface water flooding of roads

D.2.6.1 The Multi-Coloured Manual indicates that the cost of a road flooding is a function of the number of vehicles delayed, the cost of that delay and the length of time the disruption lasts:

Cost = Number of vehicles delayed x additional cost per vehicle x number of hours that the flood disruption lasts.

D.2.6.2 Where the value for the additional cost per vehicle is dependent upon the additional distance that is travelled to avoid flooding.

D.2.6.3 This assessment has focussed on major traffic disruption to A-roads and motorways. Minor roads have not been considered at this stage. Although flooding to minor roads can be a major inconvenience and cause considerable expense, we did not consider that the data sources available to us were sufficiently robust to assess the impact of local road flooding.

D.2.6.4 For this study it is assumed that all modelled surface water flooding causes 1 hour of disruption. This is consistent with the Flood Map for Surface Water used for the assessment of disruption to roads, which assumes a storm duration of 1 hour. Whilst some roads will be affected for more than an hour the use of this as a consistent value allows comparison throughout the county. If a longer storm duration is used, the cost of flooding to roads would increase relative to the cost to properties.

D.2.6.5 The length of road that is flooded is used as a substitute for the additional distance travelled to avoid flooding. The distance travelled to avoid flooding is not directly related to the length of flooded road and is highly variable.

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However at this strategic scale it is not possible to directly calculate the distance travelled to avoid flooding. Therefore the length of flooded road is used as a substitute which can be applied consistently throughout the county to indicate the amount of disruption caused.

D.2.6.6 The tables from the Multi-Coloured Manual (MCM) chapter 6 have been simplified into two categories of road types; Motorway, A road. No distinction has been made between urban and rural roads. Rather, where the multi-coloured manual distinguishes values for urban and rural A-roads an average value is taken for this assessment. At this strategic scale it is not feasible to distinguish urban and rural A-roads. The tables below show the values taken from the MCM tables and highlight assumptions made.

Table D.2: Average hourly motor vehicle flow (from table 6.2 – South East)

Motorway A Road*

3900 759

* The values are an average of rural and urban values given in the MCM table.

Table D.3 Average speed per road type km/h (from table 6.3)

Motorway*1 A Road*2

95 73

*1 This is the average speed for the motorway for all time periods

*2 This is the average speed for ‘A-roads All’ for all time periods.

Table D.4 The percentage of road traffic by type of vehicle and road class (adapted from MCM table 6.4)

Cars LGV OGV1 OGV2 PSV

Motorways 74.8 12.4 4.1 8.1 0.6

A roads*1 80.7 12.7 3.2 2.3 1.1

*1 Average of rural A-roads All and urban A-roads

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Table D.5 Total cost of travel as function of speed (pence) (from MCM table 6.6)

Speed km/h

80 100

Car 17 15

LGV 23 20

OGV1 29 27

OGV2 40 37

PSV 124 106

D.2.6.7 The speeds provided within table D.5 do not match those within Table D.3, therefore 80km/hr for A-roads and 100 km/hr for motorways has been assumed.

D.2.6.8 Combining the information and assumptions listed above, the cost of disruption per hour, excluding the length of flooded road can be estimated and is shown below.

Table D.6 The cost of disruption per hour (excluding length of flooded road) (£)

Motorway A-road

719.16 150.68

D.2.6.9 To estimate the economic cost of road flooding the appropriate value in Table D.6 is multiplied by the length of road.

D.2.7 Total economic damage

D.2.7.1 To determine a total economic damage, the sum of the annualised damages for each receptor is calculated and expressed by ward, using the method outlined in section D.1.3

D.2.7.2 These calculated annualised costs of flooding values are then displayed on maps, with the ward with the highest damages having the darkest colour. The maps are presented in Appendix E of the Strategy.

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D.3 Summary of key assumptions D.3.1 Every property flooded internally has the same cost of flooding (for its property type)

regardless of the depth of flooding or return period of that flood.

D.3.1.1 The purpose of the assessment is to provide a means to compare the risks to wards within Hampshire by using approximate economic costs to compile risk to a variety of different types of receptor. The assessment is not designed to calculate the financial cost of damages to individual properties, nor accurately reflect the overall economic cost of damage of flooding across Hampshire. Therefore whilst the actual cost of damages caused to a property are likely to vary depending on factors such as how deep the flooding is (deeper flooding could potentially cause more damage), given the strategic level of the assessment and the available data, this assumption is reasonable. . In addition introducing information about the depth and cost of flooding would add precision to the results which was not appropriate to the level of accuracy achieved.

D.3.2 No allowance has been made for confidence of modelled results in the initial assessment. Therefore reported flood incidents have the same confidence as the modelled flood extent.

D.3.2.1 Two types of data about location and likelihood of flooding have been used for this assessment: modelled data and reported data. Each type of data has its own strengths and weaknesses.

D.3.2.2 Modelled data (Environment Agency Flood Map for Surface Water) – this is a complete data set, it shows all areas which could be at risk from surface water flooding given the assumptions followed. However this modelled data is inherently uncertain as it has been developed at a national scale and is based on a series of assumptions. It provides no indication of areas that have flooded in the past and may show areas that may never flood.

D.3.2.3 Recorded data (Hampshire County Council data set and data on the 2000-2001 groundwater flooding) – this data shows areas which have flooded in the past. However, uncertainties in the data result because it is not be a complete record of all the locations that have flooded in the past. In addition this recorded data provides no indication of where may flood in the future.

D.3.2.4 To combat the uncertainties in these two forms of data both have been used within the assessment. Although we can be more certain of the risk of flooding at locations shown within the recorded data (it has flooded in the past so could flood again) we do not wish to bias against the possibility of flooding in other locations. Therefore we have assumed the same level of confidence in both types of data.

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D.3.2.5 The modelled data shows greater areas at risk from flooding than the recorded data; therefore it dominates the risk assessment, with the result that some wards which only contain a modelled risk of flooding are shown as at greater risk than those with a recorded risk. We have considered this when deciding upon which wards to produce detailed action plans for, so that in wards where there is a known risk of flooding (from recorded data) activities to combat this flooding are suggested, whilst in wards deemed at high risk from modelled data only awareness raising activities are suggested.

D.3.3 The 2000/2001 groundwater event has been used as the only complete and fully analysed information on groundwater flooding.

D.3.3.1 The groundwater data used within this analysis was limited by data availability. Whilst groundwater flooding events are known to have occurred relatively frequently within Hampshire (in recent years 1994/95, 2000/2001, 2002/2003) data about the impact of this flooding is limited. Therefore only the 2000-2001 data has been used. This will have an impact on the risk assessment if it does not represent the locations and extent of groundwater flooding in Hampshire. Comparison of the locations of the 2000-2001 flooding indicates that the more locations are shown as flooding in 2000-2001 than in 1994/95 or 2002-2003. It appears that the 2000-2001 data is unlikely to underestimate the risk of groundwater flooding.

D.3.4 An average flood duration of 1 week has been applied across all flooded properties in the 2000/01 groundwater event

D.3.4.1 The cost of groundwater flooding is increased (over that for surface water flooding) to account for the generally longer duration of groundwater flood events. Using reported data (see section D.2.5 of this Annex) the costs of groundwater flooding are assumed, for the purposes of this study) to be 240% greater than surface water flooding. This is based on the assumption that groundwater flooding lasts for 1 week. Alternative figures are available based on the assumption that groundwater flooding lasts 3 months. Whilst there is a risk that a 1 week duration will underestimate the duration of groundwater flooding events it is deemed that on average this is a more representative duration for groundwater flooding in Hampshire than 3 month duration. Had a 3 month duration been assumed the costs of groundwater flooding would have increased.

D.3.5 An event return period of 25 years has been applied to the 2000/01 groundwater event.

D.3.5.1 As discussed in section D.2.5 of this Annex a return period of 1 in 25 years (4% annual probability) has been applied to the 2000-2001 groundwater flood event based on expert judgement. This is an assumption made in the absence of data providing an estimate of flood return period.

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D.3.5.2 Had a higher probability been applied (e.g. 10% annual probability or 1 in 10 year return period) the annualised cost associated with the risk of groundwater flooding would increase (as the total cost to flooded properties would be spread over less years).. Had a lower probability been assumed the annualised cost would decrease (the same total cost would be spread over less years).

D.3.6 The impact of groundwater flooding on receptors other than residential properties has not been included.

D.3.6.1 Limited information about the impact of groundwater flooding was available for this analysis. Available information included details of properties flooded, but did not include information about roads affected or the types of non residential properties. As a result it was only possible to include the costs to residential properties. This may underestimate the impact of groundwater flooding in terms of the economic costs calculated for this study.

D.3.7 The Hampshire County Council data set has not been used to assess the impact of flooding on roads.

D.3.7.1 Under the method applied within the assessment, information about the length of flooded road is necessary to undertake an assessment of the cost of flooding to roads. This information was not consistently available within the Hampshire County Council dataset. Therefore the data was used to assess the impacts of flooding on properties only.

D.3.7.2 The Environment Agency Flood Map for surface water was used to assess risk of flooding to roads. This dataset shows a much larger area at risk from flooding than is held within the Hampshire County Council database. Therefore it is considered that despite the exclusion of roads from the Hampshire County Council database, the risk of flooding to roads is adequately covered within the assessment. It is considered that the inclusion of the Hampshire County Council assessment of roads would be extremely unlikely to impact on the ultimate ranking of wards by risk.

D.3.8 Traffic disruption of 1 hour duration has been assumed

D.3.8.1 For this study it is assumed that all modelled surface water flooding causes 1 hour of disruption. This is consistent with the Flood Map for Surface Water used for the assessment of disruption to roads, which assumes a storm duration of 1 hour. Whilst some roads will ne affected for more than an hour the use of this as a consistent value allows comparison throughout the county. If a longer storm duration is used, the cost of flooding to roads would increase relative to the cost to properties.

D.3.9 Length of flooded road is used as a proxy for the additional distance travelled to avoid flooding

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D.3.9.1 The length of road that is flooded is used as a substitute for the additional distance travelled to avoid flooding. The distance travelled to avoid flooding is not directly related to the length of flooded road and is highly variable. However at this strategic scale it is not possible to directly calculate the distance travelled to avoid flooding. Therefore the length of flooded road is used as a substitute which can be applied consistently throughout the county to indicate the amount of disruption caused.

D.3.9.2 Due to availability of reported incident data, only primary public highway routes have been considered, and urban and rural A-roads have not been considered separately. In future revisions of the strategy it is our aspiration to include non primary and rural routes.

D.3.10 The total annualised cost of damages per ward has not been normalised against either the number of properties in the ward, or the area of the ward.

D.3.10.1 The risk for each ward (expressed as an economic cost) is independent of the size of the ward. This means that areas at risk are highlighted purely on the number (and type) of properties at risk and the length of road rather than the costs relative to size of ward. So if two wards each have 2 houses at risk from a 1 in 200 year flood, the risk for those wards will be equal, regardless of whether one ward has 1000 houses and the other 20.

D.4 Recommendations D.4.1.1 This section outlines recommendations for the update to the assessment of

flood risk as part of the planned reviews of the Strategy. If the current methodology is followed, the following should be considered:

o Updated modelled data on surface water data should be used if it is considered to provide a better reflection of surface water flooding in Hampshire than the existing Environment Agency Flood Map for Surface Water

o Modelled data of risk due to groundwater data should be considered for use if it is thought to provide a reasonable indication of risk of groundwater flooding in Hampshire

o Any additional information on recorded flood events should be included within any future assessments. Consideration should be placed on the type and detail of data recorded, estimates of the types of property flooded (internally and externally), lengths of road and frequency would be useful for future applications of the method described in this annex.

o The method used to calculate risk of flooding to roads could be updated to distinguish between urban and rural roads, include cost to minor roads, and estimating distance travelled to avoid flooding.

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Table D.7: Summary of data sources and associated risks

Data source Data risk Mitigation applied

The Environment Agency Flood Map for Surface Water

Modelled data identifies where it may flood, not where it has flooded. The data series is a national scale dataset therefore suitable for strategic assessment over wide areas, but may not be accurate at a local scale.

Consultation on flood risk areas

The Environment Agency Flood Map for Surface Water

Only two return periods modelled Consultation on flood risk areas

The Environment Agency Flood Map for Surface Water

Only shallow and deep flooding identified, therefore not suitable for quantifying differential damage of deeper flooding.

Consultation on flood risk areas

The Hampshire County Council flooding records

Reported data only includes where it has flooded and where people have reported. Does not provide any indication of where it may flood in future.

Consultation on flood risk areas

The Hampshire County Council flooding records

Data record is short, therefore reliable for frequent and regular events, unreliable for high return period, low probability events. Difficult to estimate the frequency (short record).

Consultation on flood risk areas

The Hampshire County Council flooding records

Reviewed data set, removed anything which is shallow or does not flood properties internally. Have had to ‘clean’ data set to categorise it into the number of flooded houses and the frequency of the event. Risk of this data cleansing mis-reporting reality

Data cleansing undertaken in partnership with HCC officers

Groundwater flooding records Shows the location of groundwater flooding in one of the most severe events. Consultation on flood risk areas

Groundwater flooding records May not show all areas at risk from groundwater Consultation on flood risk areas

Groundwater flooding records Difficult to provide a return period for groundwater, so have assumed 100 year (1% probability) Consultation on flood risk areas