Flood Investigation Report summer 2023

Brighton and Hove City Council are the designated Lead Local Flood Authority for Brighton and Hove. Therefore, under Section 19 of the Flood and Water Management Act 2010, they are required to investigate flooding incidents that occur within the council’s administrative area.

The objective of this report is to:

• support Brighton and Hove City Council in its role as Lead Local Flood Authority
• investigate the flooding incidents reported in Brighton and Hove, as a result of heavy rainfall events in June 2023

The LLFA, as the Risk Management Authority, has permissive powers to:

• investigate flooding incidents
• carry out works to reduce flood risk, as deemed appropriate, based on the scale and nature of the risk

Flooding issues

Heavy rainfall was experienced within Brighton and Hove on 20th June 2023, which resulted in reports of flooding in several locations around Brighton and Hove, both commercial and residential.

Overland and surface water flow occurred quickly following the onset of heavy rainfall, causing localised surface water flooding.

During the 20 June 2023 flood event, 11 incidents of flooding were reported to Brighton and Hove City Council.

East Sussex Fire and Rescue Service have identified that they attended 17 locations as a result of this flood event. Flooding was also reported in social media and news articles.

As a result of the above flooding, this Section 19 investigation was commissioned by Brighton and Hove City Council to:

• consider the flooding events
• identify the mechanism for flooding
• summarise the impacts and any remedial actions required to address the issues

The flood incidents align well with the predicted surface water flooding indicated on the Environment Agency’s Risk of Flooding from Surface Water maps.

Surface water flooding tends to be routed along the highway network and follows the steeply sloping ground terrain of Brighton and Hove.

Recommended actions

As a result of the Section 19 investigation, the following recommendations should be explored further:

1. Opportunities to improve local detention of surface water in suitable public spaces.
2. Where possible, impermeable areas should be disconnected from the public sewer network, or flows reduced, for instance by the provision of extensive green roofs, and permeable paving of car parks and other hardstanding areas.
3. According to the Environment Agency’s Detailed River Network dataset, there are no rivers indicated within Brighton and Hove, so the drainage network of Brighton and Hove is reliant on the capacity of the drainage infrastructure.
4. Options for community flood wardens to proactively clear road gully inlets before flood events. 

1. Introduction

Royal HaskoningDHV has been commissioned by Brighton and Hove City Council, the Lead Local Flood Authority (LLFA) for Brighton and Hove, to undertake a flood investigation under Section 19 of the Flood and Water Management Act 2010[1] (FWMA).

This report has been produced in response to the flood event that occurred on 20 June 2023. The flood event resulted in reports of flooding in several locations around Brighton and Hove, both commercial and residential.

1.1 Section 19 requirements

Brighton and Hove City Council, as the Lead Local Flood Authority (LLFA), have the following responsibilities, under Section 19 of the Flood and Water Management Act 2010.

Section 19 Local Authorities: investigations

1. On becoming aware of a flood in its area, a lead local flood authority must, to the extent that it considers necessary or appropriate, investigate –
   1. Which risk management authorities have relevant flood risk management functions, and;
   2. Whether each of those risk management authorities has exercised or is proposing to exercise, those functions in response to the flood.
2. Where an authority investigates subsection (1) it must –
   1. Publish the results of its investigation, and;
   2. Notify any relevant risk management authorities.

Therefore, under Section 19 of the Flood and Water Management Act 2010, it's the responsibility of Brighton and Hove City Council, in its LLFA role, to deliver the flood investigation for the June 2023 event.   

Brighton and Hove City Council, as the LLFA, commissioned this report to fulfil its flood risk management responsibility of investigating flood incidents.

1.2 Background studies

To provide context to the Section 19 investigation report, a review of the available background studies and reports was completed.

The draft Surface Water Management Plan for Brighton and Hove City Council (SWMP, 2022) identified local flood issues and potential options for mitigating these risks. This followed the completion of the Strategic Flood Risk Assessment in 2012, which identified that the main risk to Brighton and Hove was surface water flooding.

The SWMP indicated that flooding incidents from surface water sources were scattered across the City (records up to 2017). The most affected areas were identified as central Brighton and Hove, specifically close to the coast.

Other areas affected by surface water flooding include:

• Southern Cross
• Portslade Village
• Preston
• Patcham
• Roundhill
• Hollingbury

Reports of flooding in residential areas surrounding Brighton and Hove Centre include:

• Saltdean
• Ovingdean
• Woodingdean
• Black Rock
• Kemp Town
• Whitehawk
• Hollingdean
• Moulsecoomb
• Coldean Aldrington
• Mile Oak   

The SWMP identified 8 priority catchment areas, based on the areas of Brighton and Hove at the most risk of surface water flooding. Within these catchment areas was a list of hotspots (specific areas or roads frequently affected by flooding).

The hotspot areas and priority catchment areas within Brighton and Hove are provided in Figure 1, extracted from the SWMP.

Table 1 below provides a list of the priority catchments and relevant hotspots. These catchment areas were then used to inform mitigation measures. 

Table 1: List of hotspots identified through the Brighton and Hove SWMP

The SWMP provides a high-level assessment for potential mitigation measures, across the study area. This includes improving data and evidence on:

• surface water flood risk
• flood warning systems
• flood resilience for communities
• SuDS retrofit options
• maintenance
• capital schemes

The SWMP provides catchment-specific recommendations on how to mitigate flood risk, to the 8 priority catchment areas. This includes opportunities for SuDS retrofits and flood storage schemes.

The potential mitigation measures considered for the whole study area and the specific catchment areas were assessed and scored, based on benefit, practical and viability considerations. These findings were used to formulate an Action Plan for Brighton and Hove.

This Section 19 investigation has been developed based on the reported flood incidents set out in Section 3, however, the context and understanding of the hydrological and flooding interactions that exist within Brighton and Hove have been developed considering the conclusions of the SWMP.

1.3 Purpose of report

To meet the above requirements, this report will discuss the following:

• section 1: introduction, including Section 19 requirements, background studies and data sources
• section 2: Brighton and Hove Study Area, including location, topography, geology, historic flooding and hydrological background to the area
• section 3: June 2023 Flood Incident, including discussion of online sources and evidence from locals
• section 4: Hydrological Assessment, including analysis of Environment Agency rainfall gauge data for the 20 June 2023 event.
• section 5: responsibilities of Risk Management Authorities, based on the Flood Water Management Act
• section 6: recommendations, based on findings of this assessment, as well as mitigation measures from local planning documents
• section 7: summary and conclusions of the findings

1.4 Data sources

This flood investigation has been informed by evidence from the June 2023 flood event and data provided by the following stakeholders:

• Brighton and Hove City Council
• Environment Agency
• East Sussex Fire and Rescue
• National Highways

The following sources of data have been reviewed during the production of this report:

Table 2: Data sources

Footnotes

[1] Flood and Water Management Act 2010 (legislation.gov.uk).

2. Brighton and Hove Study Area

Brighton and Hove City Council is a unitary authority with City status, located in East Sussex.

The authority area is bound by:

• the South Downs National Park to the north
• the English Channel to the south

The total area of Brighton and Hove is 87.54 km2. Much of Brighton and Hove consists of urban areas, with urban parks located across the city.

According to Ordnance Survey data, the city consists of 22 electoral wards covering a total area of 8,431 Ha. These are shown in Figure 2.

Figure 2: Electoral wards in Brighton and Hove (source: Ordnance Survey data)

A summary of the ward areas is presented in Table 3.

Table 3: Electoral Wards of the City of Brighton and Hove and the areas covered

2.1 Topography

Based on the Environment Agency’s LiDAR data, the topography of the City varies considerably. The northern part of the City is located at ground levels of up to 150m Above Ordnance Datum (AOD), while the coastal edge to the south lies at approximately 10m AOD.

This change in ground elevation occurs over approximately 7 km (furthest point), which would give an average ground slope of 1 in 50. Ground levels from LiDAR are presented in Figure 3.

Figure 3: Topography and hydrological Setting (LiDAR and Detailed River Network) contains data © Ordnance Survey and © Environment Agency

2.2 Geology, Soils and Groundwater Source Protection Zones

According to the British Geological Survey (BGS) 50k mapping[1], Brighton and Hove is underlain with the following geology:

• bedrock: primarily chalk, including the Lewes Nodular Chalk and Seaford Chalk Formations
• superficial deposits: head deposits, beach deposits and tidal flat deposits, formed of sands, gravels, silts and muds

2.3 Mechanisms of flooding

Soilscapes[2] identifies 5 different types of soils in the Brighton and Hove area. These are mostly freely draining lime-rich soils.   

British Geological Survey (BGS) data indicates numerous groundwater source protection zones within the Brighton and Hove area, including zones 1, 2 and 3. 

A summary of flood risk to Brighton and Hove is provided below. This is based on the SFRA and SWMP findings.

Fluvial flooding

There are no rivers within the City of Brighton and Hove. This is based on the Environment Agency’s Detailed River Network dataset, LiDAR data and Ordnance Survey mapping, as indicated in Figure 2 above. Therefore, there is no risk of fluvial flooding in Brighton and Hove.

Tidal flooding

Areas along the coastline are in tidal Flood Zones 1, 2 and 3. Most areas at risk from tidal flooding are protected by tidal defences. There is, however, a residual risk of overtopping or defence failure during storm events.

Surface water flooding

Local documents (including SFRA and draft SWMP) identify that surface water flooding is the most frequent cause of flooding in Brighton and Hove.

Generally, surface water flow routes drain from the South Downs to the sea, along dry valleys, including major roads.

Surface water runoff from agricultural land has previously led to flooding in Brighton and Hove, known as ‘muddy flooding’.

Groundwater flooding

The most notable and largest groundwater flood event was in 2000 to 2001 which resulted in extensive flooding along A23.

Most of Brighton and Hove has low susceptibility to groundwater flooding.

Sewer flood risk

There are areas across the city prone to sewer flooding, according to Southern Water’s records of sewer flooding dataset. This is discussed in both the SFRA and SWMP.

Flood risk from artificial sources

There are no reservoirs, canals or other artificial sources within Brighton & Hove, so there is no flood risk from artificial sources.

Based on the above, the main risk of flooding in the Brighton and Hove area is from surface water sources.

2.4 Analysis of HR Wallingford

According to the HR Wallingford SuDS Tools[3], the underlying soil within the City of Brighton and Hove would result in low rates of surface water run-off. However, due to the urbanisation of large portions of the city, the amount of rainfall converted to run-off would be anticipated to be higher than the Greenfield equivalent.

Based on calculations undertaken using the HR Wallingford Surface Water Attenuation Calculator, to restrict the rate of surface water run-off to the greenfield rate for each impermeable hectare of land, 1,650m3 of surface water attenuation would be required, to accommodate the 1 in 100-year rainfall event.

This includes a 10% allowance for urban creep and a 40% allowance for climate change. This volume of attenuation equates to a storage depth of 165mm per square metre area of hard landscaping.

It should be noted that there is a natural catchment area north of the northern boundary of the city, and consequently, surface water run-off from that area would also be expected to drain into the city.

According to the HR Wallingford SuDS Tools, the QBAR rate per Ha is 0.18l/s.

Calculations are included in Appendix A.

Footnotes

[1] BGS Geology Viewer.

[2] LandIS Soilscapes Viewer.

[3] Tools for the design and evaluation of Sustainable Drainage Systems (SuDS).

3. June 2023 flood incident

The city of Brighton and Hove was impacted by flooding on 20 June 2023, due to heavy rain and thunderstorms.

To understand which areas of the city were affected by flooding, the following sources of information have been reviewed:

• online sources of evidence including news articles, and social media
• recorded incidents of flooding provided by:
  • Brighton and Hove City Council
  • National Highways
  • East Sussex Fire and Rescue Service

3.1 Online sources of evidence

These records of flooding incidents have been collated in a historic flood map of Brighton and Hove.

Due to the wide use of camera phones, footage has been reviewed from several online sources such as newspapers and YouTube.

The records have been accepted as being accurate. No verification of the data has been undertaken.

A summary of the flooding records is presented in Table 4. Imagery associated with the flood events is provided in Appendix B.

Table 4: Evidence of flooding in Brighton, based on online sources

3.2 Brighton and Hove Council reported incidents

Brighton and Hove Council have provided correspondence, including photos and videos, from 7 local stakeholders, regarding properties affected by flooding in this event. Evidence is included in Appendix C.

This correspondence indicates 11 reported incidents of flooding during this event.

1. Carden Avenue (1).
2. 175 Carden Avenue.
3. 63 Hartfield Avenue.
4. 87 Rotherfield Crescent.
5. Valley Road.
6. 82 Valley Road (and 2 neighbouring houses), North Portslade.
7. No. 12, 14, 18, 19 and 20b Warmdene Road.

3.3 National Highways

An information request was sent to National Highways, and they have confirmed that they do not hold any records of flooding on their assets in June 2023 in the Brighton and Hove areas.

Correspondence with National Highways is included in Appendix D.

3.4 East Sussex Fire and Rescue Service

East Sussex Fire and Rescue Service have identified that they attended several locations as a result of flooding on 23 June 2023 (see Appendix D). These include:

• Batholomews, Brighton
• Beaconsfield Villas, Brighton, BN1 6**
• Bristol Road, Brighton, BN2 1**
• Brunswick Place, Hove, BN3 1**
• Chates Farm Court, John Street, Brighton, BN2 9**
• Hove Business Centre, Fonthill Road, Hove, BN3 6**
• Jackson Street, Brighton, BN2 9**
• John Street, Brighton, BN2 0**
• London Road, Brighton, BN1 4**
• Milner Road, Brighton,  BN2 4**
• North Road, Preston, Brighton, BN1 6**
• Oriental Place, Brighton, BN1 2**
• Preston Village Mews, Middle Road, Brighton, BN1 6**
• The Drive, Hove, BN3 3**
• Upper Gardner Street, Brighton, BN1 4**
• Wellend Villas, Springfield Road, Brighton, BN1 6**
• Windlesham Road, Brighton, BN1 3**

3.5 Mapping of historic flooding

Mapping of the locations identified by East Sussex Fire and Rescue is limited to the road name. They're unable to provide exact addresses.

Similarly, flooding incidents reported to Brighton and Hove Council are a mixture of exact addresses and road names only.

The locations are indicated on the recorded flooding map, in Appendix E.

4. Hydrological assessment

4.1 Records of flooding in the June 2023 event

The June 2023 rainfall event resulted in 28 recorded flooding incidents from Brighton & Hove City Council and East Sussex Fire and Rescue Service.

These records were spread across the Brighton and Hove authority area. Most recorded incidents were located within central Brighton and Hove, Preston Park and Patcham areas, with a few outliers in Portslade and Woodingdean.

4.2 Rainfall gauge data for the June 2023 event

Data obtained from the Environment Agency as part of this investigation identified only one rainfall gauge close to Brighton and Hove. This is a tipping bucket rain gauge (TBR) located at Ditchling Road (reference: E8743), opposite the Surrenden Road junction. This is located within the Brighton and Hove City Council administrative area.

The 15-minute rainfall data has been obtained for the period 18 to 22 June 2023. From this data, it's possible to identify the key rainfall event that occurred on 20 June 2023.

The data indicates that the rainfall event occurred within 1 hour, with a total of 26.8mm of rainfall measured at the rainfall gauge (maximum 14mm in 15-minute intervals). The rainfall gauge data for June 20 is represented in Figure 4 below.

Figure 4: Rainfall data for the June 2023 event (source: EA Ditchling Road rainfall gauge data)

The Met Office’s classifications on rainfall have been used to understand the type of rainfall event that occurred, which have been reproduced in Table 5. The definition of showers is also useful: showers are often characterized by short duration and rapid fluctuations of intensity.

Based on the Met Office classifications, the rainfall event on 20 June 2023 could be classed as a heavy shower event.

Table 5: Met Office classifications for rain and shower events

4.3 Average monthly rainfall data

The Met Office’s UK climate averages data has been used to understand local and regional rainfall averages (based on data from 1991 to 2020). Based on the Met Office climate station map[1], the nearest station to Brighton and Hove is in Shoreham.

Therefore, data from this station has been used to represent local rainfall data, whilst regional data is represented using England South East and Central South, and also England South.

Average rainfall for Shoreham, South East and Central South of England, and Southern England is presented in Figure 5 below.

For June, the average rainfall amounts for the nearest recorded local and regional areas to Brighton are as follows:

• Shoreham: 46.68 mm
• South East and Central South of England: 53.66 mm
• South of England: 59.07 mm

Figure 5: Average monthly rainfall based on data from 1991 to 2020

Source: Met Office.

4.4 Estimated return period for the June 2023 flood event 

To understand the estimated return period for the June 2023 flood event, a comparison has been made between the observed rainfall gauge data and FEH rainfall points data. Point data has been used as opposed to catchment data, due to the urbanised nature of the study area.

Point rainfall data was obtained from the FEH web service, based on a pre-determined centroid located in the centre of the Brighton and Hove authority area.

The centroid (Coordinates: 531477, 106640) is in the centre of most recorded flood incidents and is approximately 1km south of the Ditchling Road rainfall gauge station.

Figure 7 below provides the location of this centroid in relation to the June 2023 flood incidents and authority area.

Figure 6: Location of the centroid for FEH points data

The FEH22 Depth, Duration, and Frequency growth model data for the centroid point indicate that, for the observed rainfall of 26.8mm, the return period would be an estimated 1 in a 12-year event.

The rainfall growth curve data for this point was then exported and translated into a hyetograph on the ReFH2 model. The results of the analysis indicate that the observed rainfall event, as taken from the local rainfall gauge, would have an expected 150-year return period.

The hyetograph for the 150-year design rainfall event is shown in Figure 8. This is based on a rainfall event with a 1-hour duration (as per the observed June 2023 event), and a total of 26.62mm.

Figure 7: ReFH2 hyetograph for a 150-year return period

Footnotes

[1] UK climate averages.

5. Responsibilities of Risk Management Authorities

Within the Flood & Water Management Act (2010), a number of risk management authorities (RMAs) were defined as having flood risk management functions.

A summary of the key RMAs within Brighton and Hove, specifically in relation to the flooding incidents, has been outlined in the following section including their risk management function.

5.1 Environment Agency

Under the Flood & Water Management Act 2010, the Environment Agency has a number of flood risk management functions including but not limited to national strategy as well as a local operational role.

A review of the flood warning and flood alert data from the Environment Agency identifies that there are 2 areas in Brighton and Hove City which are served by the Environment Agency flood warning and flood alert system; notably, coastal areas of Brighton Marina Village including the Strand and areas at risk of groundwater flooding in Patcham.

The areas affected by flooding as a result of the June 2023 event were affected by surface water flooding. Therefore, no assets or structures which fall within the Environment Agency’s remit in the local area would have required intervention during the June 2023 events.

Based on the above information the Environment Agency does not appear to have any specific flood risk management functions that are directly relevant to the flooding events occurring in June 2023.

5.2 Southern Water

Southern Water has responsibility for the surface water and foul water sewer network within Brighton and Hove.

In the absence of any watercourses within the Brighton and Hove City Council area, all the city's drainage is directed into the Public Sewer Network.

5.3 Lead Local Flood Authority (Brighton and Hove City Council)

Brighton and Hove City Council has a number of key roles that are likely to affect its flood risk management function.

Under the Flood and Water Management Act (2010), it's designated as the Lead Local Flood Authority, which means it has a flood risk management and land drainage function as well as being the Highway Authority (see below).

As previously indicated Brighton and Hove City Council, as the Lead Local Flood Authority, has commissioned this report to fulfil its flood risk management function regarding the investigation of flooding incidents.

The incidents that led to the commissioning of this investigation are reported incidents from local residents.

5.4 Highway Authority (Brighton and Hove City Council and National Highways)

Brighton and Hove City Council and the National Highways have a responsibility to ensure:

• the highway's drainage and assets are maintained to limit the risk of flooding
• new or proposed drainage schemes do not result in increased flood risk

National Highways are responsible for the A-roads servicing Brighton, notably the A27 and A23. A Freedom of Information request to the National Highways indicated no reported flooding incidents on the highways roads servicing Brighton. 

During the rainfall events in 2023, Brighton and Hove City Council, as the Highway Authority, responded to several emails from the public regarding concerns that gullies and drains were either blocked or the flood water was not draining away.

The Highway Authority is required to respond to flooding incidents affecting the highway regardless of whether they also directly affect properties.

6. Recommendations

The Brighton and Hove catchment represents a unique set of challenges:

1. Although the city has established parks and mature trees, the catchment is predominantly urban.
2. There are no recorded open surface water channels within the city.
3. The city is formed on many hills, with distinct valleys that rainfall routes along.
4. The city has an established network of highways which are predominantly hard paved and fully utilised from the back of the kerb to the back of the kerb.
5. Due to the absence of watercourses, in high rainfall conditions, the highway network acts as a canal routing the flood waters from high ground to low ground.

The following recommendations are based on an understanding of the rainfall event that occurred and the wider hydrological response of the Cities Catchment.

The use of Sustainable Drainage Systems (SuDS) retrofits within the existing urban environment is likely to be limited, due to:

• the existing pressure on the urban landscape
• concerns regarding maintenance and accessibility of the features, especially on the main arterial routes within the city

However, where possible, the following opportunities should be considered:

1. All Planning Consents to include a pre-construction and post-construction CCTV survey of the Highway and other Public Drainage assets 100m upstream and downstream of the site boundary, to determine if construction activities have resulted in additional debris entering the drainage system.
2. All Planning Consents to require Surface Water Attenuation to Qbar or 2l/s (whichever is greater), for rainfall events to the 1 in 100 + Climate Change event. Where geo-cellular storage is proposed, this should be dual use, rainwater harvesting and storm attenuation.
3. All new public open spaces to include rain gardens.
4. Use of extensive green roofs to be promoted on new and refurbished buildings where acceptable within the urban landscape.
5. All new car-park areas to be formed of attenuating permeable paving or permeable asphalt.
6. Meaningful interventions within the Highway will require coordination of the public realm, and transport planners to optimise junctions to reduce the total area of hard standing. This is likely to require a city-wide, integrated public realm, SuDS, and transport strategy.

Consideration of the formation of local Flood Wardens, formed of community volunteers who, on receipt of a Met Office Yellow Weather Warning for rainfall, would clear the road gullies and inlets, when safe to do so.

Working in Partnership with the parks department to make sure all clippings or removed vegetation are collected and disposed of into contained areas, to prevent leaf litter etc becoming mobilised into the surface water drainage network.

The use of on-street waste storage presents an issue if excessive waste is deposited on the street and breaks down due to weather or other mechanisms. This material can then become mobilised and enter the drainage network. This is a particular problem if the waste material has a high fat or grease content, as this can lead to blockages and reduced drainage capacity.

7. Summary and conclusions

The objective of this report is to:

• support Brighton and Hove City Council in its role as Lead Local Flood Authority
• investigate the flooding incidents reported in Brighton and Hove, as a result of heavy rainfall events in June 2023

Heavy rainfall in Brighton and Hove on 20 June 2023 resulted in reports of flooding in several locations, both commercial and residential. Overland and surface water flow occurred quickly following the onset of heavy rainfall, causing localised surface water flooding.

During the 20 June 2023 flood event, 11 incidents of flooding were reported to Brighton and Hove City Council. East Sussex Fire and Rescue Service have identified that they attended 17 locations as a result of this flood event. Flooding was also reported in social media and news articles.

As a result of the Section 19 investigation, the following recommendations should be explored further, to mitigate the surface water flood risk:

1. Opportunities to improve local detention of surface water in suitable public spaces.
2. Where possible, impermeable areas should be disconnected from the public sewer network, or flows reduced, for instance by the provision of extensive green roofs, and permeable paving of car parks and other hardstanding areas.
3. According to the Environment Agency’s Detailed River Network dataset, there are no rivers indicated within Brighton and Hove. This means that the drainage network of Brighton and Hove is reliant on the capacity of the drainage infrastructure.
4. Options for community flood wardens to proactively clear road gully inlets before flood events.