Local Flood Risk Management Strategy

A flood is formally defined within the Flood and Water Management Act, 2010, as occurring “where land not normally covered by water becomes covered by water”.

Flood risk is defined as the combination of the probability of a flood occurring and the consequences or harm should that flooding occur.

The probability (or likelihood) of flooding is described as the chance that a location will flood in any given year. This is usually expressed as an annual chance of occurring, or by the interval (in years) over which an event is expected to occur. For example, we may refer to a location being at risk of:

• a 1 in 50 chance of flooding in that location in any given year; or
• a 2% chance of flooding in that location in any given year.

It is important to understand that this does not mean that a location will flood only once every 50 years. We can think of each consecutive year as a new roll of a dice – with each roll, (or with each year), the chance of that storm happening remains the same: a 1 in 50 chance, or 2% each and every year. Therefore, there is a chance that an event of this magnitude could happen again the same year or in subsequent years, interspersed with any number of smaller or larger flood events.

Receptors are properties, people, agricultural land, infrastructure assets and environmentally or culturally significant sites which may suffer harm should flooding occur. The consequences of flooding can be:

• Economic - damage to property and infrastructure, lost business, insurance costs
• Social - displacement of residents, damage to community assets, mental health impacts
• Environmental - harm to habitats and designated environmental sites
• Cultural - damage to sites with historical and heritage value

We have traditionally sought to reduce the risk of flooding through hard engineering techniques, for example through building flood defences such as flood walls. Traditional engineering continues to be an important part of the approach, but it is not possible to provide such schemes for every community at risk, due to a range of environmental, technical and funding limitations.

There are a range of other ways to manage flooding in response to a changing climate and improve awareness of communities, environment, and economics. As the climate changes we need to use a broader range of measures to increase our resilience. Adopting wider catchment and community resilience actions, alongside the capital programme, can help to increase flood resilience of communities and is vital as climate change increases flood risk.

The national Flood and Coastal Management Strategy defines four aspects to resilience, around a cyclical plan to adapt, which we have integrated into this Strategy:

Improve placemaking: Making the best land use and development choices to manage flooding and coastal change. Communities, planners and land managers making the best land use and design choices for development and infrastructure to manage the damages from flooding and coastal change. This includes making space for water to manage risk and support wider environmental benefits.

Better protect: Building and maintaining defences and managing the flow of water. Sustained and long-term investment in building and maintaining flood and sea defences ensuring they provide an appropriate standard of protection, operate reliably and perform as expected when exceeded. Better protection includes nature-based solutions that manage the flow of water to reduce the risk of flooding and coastal change.

Recover quickly: Getting back to normal and building back better. Helping people and local economies recover more quickly by clearing up the damages, returning water and power supplies or draining floodwaters from farmland. Recovery should also include building back better so that properties and infrastructure are more resilient to future events.

Ready to respond: Preparing for and responding effectively to incidents. Organisations and communities working together to prepare for and respond to flood and coastal incidents through timely and effective forecasting, warning and evacuation.

The increase in vulnerability and exposure to flood risk due to climate change, infrastructure development and land use changes brings unprecedented challenges to current and future flood risk management in Buckinghamshire.

The UK has experienced changes in average rainfall patterns over recent decades. Seasonal rainfall is highly variable but overall, the broad trends show a decrease in total summer rainfall, but with more intense rainfall events, and an increase in total winter rainfall. While the impacts of climate change on the intensity and frequency of extreme rainfall events are still emerging, the changes already being observed in annual rainfall patterns are projected to persist and intensify.

Both climate change adaptation and mitigation measures will be necessary to improve our capacity to effectively respond to future flood events. Adaptation requires a focus on anticipating future impacts, to not just minimise potential damages but also to capitalise on opportunities that can provide a range of benefits.

We draw our understanding of changes to future flood risk across Buckinghamshire from the latest scientific advice and guidance available on climate change and apply this knowledge across our flood risk management activities. The impacts of climate change on flood risk are estimated through a scenario-based approach. The UK Climate Projections 2018 (UKCP18) are the most recent probabilistic projections developed for the UK for rainfall.

Based on these projections, the Environment Agency published revised climate change guidance for peak river levels and peak rainfall uplifts. The new guidance provides risk allowances for management catchments, which allow anticipated changes in peak flow and rainfall to be accounted for at a catchment scale. The guidance provides a range of allowances, based on the percentage of possible scenarios that fall below the peak flow or rainfall estimate. The central, higher central and upper end allowance represent the 50th, 70th and 95th percentile respectively. Allowances are also provided for three future time frames, these are the 2020s (from 2015 to 2039), 2050s (from 2040 to 2069) and the 2080s (2070 to 2125).

Residential developments typically have a minimum planned lifetime of a 100 years. For developments with an anticipated lifetime up to 2100, the central or higher central allowance should be considered in the planning process. The higher central allowance is used when development is classified as being more vulnerable or is located in areas of higher flood risk. The following section provides the currently advised higher central allowances for river and surface water flooding.

The impacts of climate change on different sources of flood risk are described in more detail here.

Flooding in a particular location can be caused by the complex interaction of a number of different sources. Understanding the sources of flood risk and their interactions helps better management and identification of appropriate responses.

This section summarises the characteristics of flooding which are managed by Buckinghamshire Council, other Risk Management Authorities, and partners.

Find out more about the risk of flooding from different sources in Buckinghamshire on our map.

The quality of data and mapping available varies depending on the source of flood risk. The scale of fluvial and surface water flood risk in Buckinghamshire for a 1% annual chance event is set out below.

Across Buckinghamshire, 3,357 properties are at risk of fluvial flooding, and a further 4167 properties are at risk in the future due to climate change. 18,380 properties are at risk of surface water flooding in Buckinghamshire, this increases by 28,702 properties under future climate change. Numbers of properties at risk have been calculated by intersecting the Risk of Flooding from Surface Water and Flood Map for Planning with the Ordnance Survey MasterMap. There may be properties at risk from both surface water and fluvial flooding (and other sources) counted in both datasets.

There are also significant numbers of properties at risk from groundwater in areas underlain by Chalk and gravels, particularly in the Chilterns and River Thames corridor, as well as other sources such as reservoir, canal and sewer flooding. Reliable comparable data is not available across the county for these sources so they have not been displayed on the graph.

The different sources of flood risk in Buckinghamshire, and how they are likely to be impacted by climate change, are described in more detail below.

Main River or 'fluvial' flooding typically occurs when the riverbanks are overtopped, inundating the floodplain, usually in response to intense or prolonged rainfall in the catchment. In many instances, overtopping riverbanks is not a significant problem if the floodplains are in a natural state without buildings or infrastructure. Buckinghamshire contains the Thames and Great Ouse catchments; the natural geography and topography of these catchments strongly influence their susceptibility to large-scale fluvial flooding.

The Thames basin is characterised by rolling hills with a wide floodplain, which means that for the larger rivers, there is a considerable time lag before river levels respond to rainfall. In areas of chalk where river flows are dependent on groundwater levels, low flows occur when groundwater stores are reduced due to inadequate rainfall.

The Great Ouse catchment contains several extensive rivers with multiple tributaries, many of which have experienced large fluvial events. The most recent flood event that affected several rivers in the Great Ouse catchment was in December 2020, when heavy, prolonged rainfall fell on an already saturated catchment resulting in a rapid fluvial response.

Climate change is predicted to increase peak river flows. Buckinghamshire covers five main management catchments: Upper and Bedford Ouse; Cherwell and Ray; Colne; Thames and South Chilterns; and Maidenhead and Sudbury. The most recent Environment Agency fluvial climate change allowances published in July 2021, indicate that indicate that peak river flows are likely to increase by between 25% and 47% for the 'Higher Central' scenario by the 2080s (2070 to 2125) across the five management catchments.

Watercourses designated as Main River are managed by the Environment Agency.

All watercourses that convey a flow and that are not designated Main River, are referred to as 'Ordinary Watercourses'. Some of the ordinary watercourses which have recently flooded in Buckinghamshire are the Tonne Brook and Cowerde Brook in Thornborough, the River Leck in Leckhampstead, and the culverted watercourses through Tingewick and Gawcott.

Flooding from ordinary watercourses, or 'fluvial' flooding, typically occurs when the banks of a river or stream are overtopped, or culvert capacity is exceeded. Urban watercourses are often culverted over long sections and the entrances to these culverts, even though screened, can often be flooding ‘hotspots'.

Debris, both natural and man-made often accumulates in urban watercourses which not only constricts the watercourse but can accumulate at culvert screens and cause bottlenecks or blockages.

Climate change is predicted to increase peak ordinary watercourse flows. Buckinghamshire covers five management catchments (as defined by the Environment Agency): Upper and Bedford Ouse, Cherwell and Ray; Colne, Thames and South Chilterns, and Maidenhead and Sunbury. The most recent Environment Agency fluvial climate change allowances published in July 2021, indicate that peak river flows are likely to increase by between 25% and 47% for the 'Higher Central' scenario by the 2080s (2070 to 2125) across the five management catchments.

It should not be assumed that all Main Rivers are large, and all Ordinary Watercourses are small – the distinction between the two relates only to the responsibility for overseeing the management of these watercourses.

The risk management authority for overseeing the management of ordinary watercourse is Buckinghamshire Council or Internal Drainage Boards.

Surface water or 'pluvial' flooding can occur when intense rainfall causes water to run off or pond on the surface of the ground. Impermeable surfaces such as roads may become direct pathways for surface water. Surface water flooding may be made worse when the capacity of local drainage (both natural and man-made) is overwhelmed so additional flood water cannot enter the drainage system.

Where slopes are steep, high velocity flows along roads and streets can also be a hazard to pedestrians and traffic. Ponding in low spots in the ground surface can occur rapidly and be a particular risk to underground assets.

In Buckinghamshire, the most extensive surface water flow paths are found in the low-lying Aylesbury Vale catchments of the River Thame and Ray as well as in the steeper Chess, Misbourne and Wye River catchments located in the east of the county. The areas of Chesham and High Wycombe have been designated as national significant Flood Risk Areas for surface water flooding.

Climate change projections predict increases in average summer temperatures accompanied by a decrease in average rainfall but with an increased likelihood of intense localised summer storms. These types of rainfall events are likely to result in more extreme surface flooding due to incapacity of the existing drainage systems.

Environment Agency climate change guidance published peak rainfall intensity allowances in 2022. By the 2080s (2061 to 2125), rainfall is expected to increase by 25% across Buckinghamshire for the 'Higher Central' scenario.

The risk management authority for overseeing the management of Surface water flood risk is Buckinghamshire Council.

Groundwater flooding is caused by water which is usually contained below the surface of the ground. The risk of groundwater flooding is highest where the water is held in permeable rocks, called aquifers, and where the water table is relatively close to the ground surface. The Chalk of the Chilterns forms extensive aquifers, which drives groundwater flood risk in several areas of Buckinghamshire.

A rise in the water table may lead to groundwater emerging at the surface and ponding or flowing over the surface. This may happen during unusually wet winters with long periods of above-average rainfall. Emerging groundwater does not always cause flooding, however once raised, levels can remain high for weeks or months and cause severe damage and disruption. Amersham, Chesham, the Chalfonts and Monks Risborough are some of the areas that have experienced notable winter groundwater flooding from the Chalk aquifer.

Groundwater flooding can also be tied to high river levels, especially when the floodplain is underlain by gravel deposits. This type of flooding has occurred in low lying areas of Marlow. Once groundwater has emerged, it often follows surface water flow paths so it sometimes difficult to distinguish the source of flood waters.

At present, the effects of climate change on groundwater flood risk and groundwater dominated river systems are poorly understood. The effects of an increase in winter rainfall on the frequency of groundwater flood incidents may be dependent on the flood mechanism, underlying geology, and historical susceptibility. The increased incidence of prolonged winter rainfall events may increase the likelihood of groundwater flooding in susceptible areas due to catchments being saturated for long periods. Conversely, hotter drier summers may result in greater decreases in groundwater storage in the summer months.

The risk management authority for overseeing the management of Groundwater flood risk is Buckinghamshire Council.

Sewer flooding can occur when sewers are overwhelmed by heavy rainfall, seepage of groundwater or when they become blocked. The likelihood of flooding depends on the characteristics of the local sewerage system and the local hydrology.

Land and property can be flooded with water contaminated with raw sewage as a result of sewer flooding. Rivers can also become polluted by sewer overflows.

Sewer flood risk is managed by Water and Sewerage Companies. Buckinghamshire is covered by two Water and Sewerage Companies: Anglian Water and Thames Water.

Reservoirs are artificially created ponds or lakes typically formed by either damming a river (on-line) or raised (off-line). Reservoirs retaining more than 25,000m3 are regulated under the under the Reservoirs Act (1975) as amended for England by the Flood and Water Management Act (2010).

The likelihood of flooding from reservoirs is rare, however overtopping of the dam crest or embankment can happen when the spillway is too small or becomes blocked. Flood risk is increased if the overspill is unable to cope with excess volumes of water. If the flow is too large or overtopping lasts too long, water can erode the surface of the dam which can lead to the unintended release of large volumes of water in a short time period.

There are 19 reservoirs located within Buckinghamshire, and 22 located outside the county that may impact Buckinghamshire if they were to overtop or fail.

Arrangements for reservoir safety are based on risk rather than the size of the reservoir, meaning that risks are carefully managed.

Reservoir flood risk is managed by the Environment Agency.

Canals are artificially created waterways which were historically used for freight. The likelihood of flooding is dependent on their hydrological inputs. Canals in the UK are supplied by groundwater, river abstraction or by direct inflow. Water levels are usually controlled by weirs or sluices. Uncontrolled loss of water from the canal can result from overtopping or breaching.

The Grand Union Canal flows north through eastern Buckinghamshire, with the Aylesbury Arm branching off at Marsworth and Slough Arm at Iver. The Aylesbury Arm is mainly raised above the surrounding ground level.

Canal water levels are managed by the Canal and River Trust. The Trust is a navigation authority, not a Risk Management Authority as defined by the Flood and Water Management Act (2010).

The hydrology, geology and terrain of Buckinghamshire means that the key sources of flooding and how they interact varies across the county.

Major watercourses: River Great Ouse, River Ray and numerous smaller watercourses

Geology: Underlying limestone

Hazards: Groundwater flooding, surface runoff, fluvial flooding

Major watercourses: River Thame and numerous smaller watercourses

Geology: Underlying combination of limestone, chalk, clay and alluvium

Hazards: Groundwater flooding, surface runoff, fluvial flooding

Major watercourses: River Thame, River Wye, River Chess, River Misbourne, River Thames

Geology: Underlying chalk

Hazards: Groundwater flooding, fluvial flooding

Major watercourses: River Colne, River Misbourne, River Thames

Geology: Underlying clay

Hazards: Groundwater flooding, surface runoff, fluvial flooding

Flooding is a natural phenomenon, the adverse consequences of which can be exacerbated by poor management of the landscape and the environment.

Flooding is generally driven by natural weather events such as:

• Heavy rainfall and thunderstorms over a short period
• Prolonged, extensive rainfall
• Snow melt

Flood risk may be changed by other factors including:

• Insufficient capacity in drainage infrastructure
• Inadequate maintenance of watercourses and flood defences
• Inappropriate development in floodplains and on natural surface water or groundwater flow paths.
• Groundwater abstraction being carried out in areas at risk of groundwater flooding.
• Building on land in a way that prevents rainfall from draining away naturally, such as roads and car parks that are impermeable to water without adequate mitigation such as sustainable drainage systems.
• Deforestation can reduce interception of precipitation by trees and therefore increases surface runoff.
• Intensive grazing and heavy farm machinery can increase soil compaction and reduce infiltration, leading to greater surface water runoff and soil erosion.
• Sediment-laden runoff entering rivers reduces their capacity to transport water.
• Straightening of watercourse channels increases the flow rate, increasing sediment transport and potentially accentuating flood peaks downstream.

Buckinghamshire has a history of flooding in many different locations from river, surface water, sewer, and groundwater sources. Flood records have been collated from selected sources, in particular the Environment Agency’s recorded flood outline dataset and Buckinghamshire Section 19 flood investigations and were assessed to understand historic flooding in the district.

Section 19 investigations are especially valuable in capturing the complex interdependence of drainage infrastructure failure or exceedance and other flood pathways. The Buckinghamshire Level 1 Strategic Flood Risk Assessment also contains more extensive information on historic flooding from a range of sources. All flood events that have occurred in Buckinghamshire have not been captured, the timeline below only considers key flood events in the county from the past two decade.

Localised, intense rainfall on the 11, 12, and 20 June, resulted in high volumes of surface water runoff flowing over urban surfaces across the county. Highways drainage systems were overwhelmed, which meant that water was unable to drain away quickly enough. This caused internal flooding to residential properties and businesses in several towns and villages across the county.

A very localised storm on 17 May at Ley Hill resulted in a significant amount of hail, some of which washed along with surface water flows and blocked the drainage system along Kiln Lane. The hail also melted slowly in situ and contributed to surface runoff volumes. Surface water formed rapidly over the impermeable surfaces and flowed overland towards properties. At least seven properties are known to have flooded internally during the event.

Across the sub-catchment of the Upper Bedford Great Ouse, the three months until the end of November 2020 had brought notably high rainfall. 112mm of rainfall was received through December as a whole, nearly twice the average monthly rainfall for December. On the day of the flood event (23 December), 52mm of rainfall fell within sixteen hours at Brackley onto an already saturated catchment. Very little of the storm’s rainwater could be absorbed and retained within the soil, so water rapidly arrived into drainage systems and watercourses. This led to significant flooding on the 23 and 24 December across the north of the county, including internal property flooding in Buckingham, Leckhampstead, Thornborough, Thornton, Gawcott and Tingewick.

Storm Alex brought heavy frontal rainfall over a three-day period between 2 and 4 October 2020, at least three months’ worth of rain fell in about two and a half days. On 4h October itself, up to 109mm of rain is recorded to have fallen in 60-hours. A dry summer preceded the event; therefore, surface water runoff is expected to have formed rapidly on the dry ground and impermeable urban surfaces. The storm caused various flooding incidents across the county, including in the Aylesbury area, Ickford and Quainton.

Flooding of the High Street in Chalfont St. Peter resulted from heavy rainfall overnight on 23 September 2019. Surface water runoff from the road and hard surfaces entering the highway drainage system and foul sewer system caused the foul sewer to become overwhelmed and reach maximum capacity. This caused sewage to flow out of a manhole located in the middle of the carriageway and flow into the highway drainage system causing it to become backed up. Ponding in the road and rising water levels then encroached into the shops along the High Street.

In the early hours of 27 May 2018 an intense localized thunderstorm hit the village of Great Missenden causing internal surface water flooding to four commercial and two residential properties.

On the evening of 18 July 2017, there was a two-hour period of heavy rainfall over the High Wycombe area. Several properties in the Desborough Road, Sands, Booker and Cressex areas of High Wycombe reported flooding in and off the road and five residential properties and two business reported internal property flooding.

Localised, intense rainfall resulted in high volumes of surface water runoff on urban surfaces causing flooding of homes and businesses. Due to the nature of the event, there was very little warning or time to prepare. In total across all the areas affected, nine residential and 20 commercial properties flooded internally.

A flood event in Leckhampstead occurred due to a month’s worth of rainfall falling on already near-saturated ground. Water levels in the River Leck rose rapidly through the morning of 9 March 2016, overtopping the banks through Church End. High depths and velocities of floodwater across the main highway access route led to the village being inaccessible for some hours.

A prolonged period of above average rainfall caused groundwater levels in the underlying aquifer to rise to unusually high levels and the surrounding land to become saturated in Princes Risborough. Subsequent rainfall caused flooding where the flow was unable to pass through restricted culverts and drains. Groundwater rose to the surface through both widespread seepage and through springs. Runoff and groundwater flowed off the Molins sports ground southwards, flooding the rear gardens and garages of properties on Mill Lane (east of the railway) and Crowbrook Road.

An intense rainfall event caused surface water runoff as well as increased flow in the River Chess and its tributary the Vale Brook. At least 34 properties were flooded internally in Chesham with 20 to 50mm flood depths.

Extensive flooding occurred following six weeks of above average rainfall that saturated soils and raised river levels. At Medmenham, overtopping of the River Thames combined with impeded drainage and saturated soils, caused inundation in some areas to a depth of 0.5m. Extensive groundwater flooding was experienced in the Wycombe and Chiltern areas, exacerbating the impact of surface water flooding. In Old Amersham a combination of high groundwater and a riverbank collapse resulted in flooding to four residential properties and five businesses.

A prolonged period of above average rainfall saturated soils throughout the region. Across Buckinghamshire, a combination of high groundwater levels, surface water runoff and unmaintained river features led to extensive inundation. In Bishopstone, constrictions on Standall's Ditch caused overtopping and flooding of the highway.

Following heavy rain and a sustained period of high groundwater, extensive flooding affected roads in southern Buckinghamshire (Marlow, Chorleywood, Seer Green and Little Chalfont).

An intense rainfall event on 20 July 2007 followed many weeks of wet weather. Widespread flooding occurred from both surface water and flooding from the River Thames in Marlow and the River Great Ouse in Buckingham. A total of 96 properties were affected by the flooding.

A sequence of rainfall events caused several localised surface water flood events in the Chalfonts and in parts of Aylesbury, including sewer surcharging in Manor Park.

A combination of groundwater flooding from the Chalk aquifers and high river levels followed an exceptionally wet winter. Surface water flooding occurred in several towns in the Aylesbury area. The River Thames flowed out of the bank and flooded southern parts of Marlow.

An unusually wet winter caused groundwater levels to rise across the Chalk aquifer in Buckinghamshire. The high groundwater levels caused high river flows (particularly on the River Misbourne) and widespread groundwater flooding in the valleys of the Chiltern Hills. Groundwater levels remained high for many months and caused extensive flooding of properties, roads and public areas.