Flood investigation reports

1.0 Introduction

1.1 Instructions

1.1.1 Martin Andrews Consulting Ltd (MAC) have been appointed to undertake this Section 19 Flood Investigation on behalf of North Northamptonshire Council (NNC).

1.1.2 A flood report was submitted to NNC for an incident at XX Priory Road, Wollaston on the 21 September 2024. This warrants a flood investigation because internal flooding of a building has been experienced on more than one occasion in the last five years.

1.1.3 The approximate location of the flood incident is shown below in Figure 1.1.

Figure 1.1: Incident Location Plan

1.2 What is a Section 19 Flood Investigation

1.2.1 As set out below, under Section 19 of the Flood and Water Management Act (2010), NNC as Lead Local Flood Authority (LLFA), have a duty to conduct flood investigations.

1.2.2 The Act states that:

(1) on becoming aware of a flood in its area, a lead local flood authority must, to the extent that it considers it necessary or appropriate, investigate:
(a) which risk management authorities have relevant flood risk management functions, and
(b) 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 carries out an investigation under subsection (1) it must:
(a) publish the results of its investigation, and
(b) notify any relevant risk management authorities.

1.2.3 It is not the responsibility of NNC to resolve the flooding issues, however, they do need to investigate the cause of the flooding and notify any relevant Flood Risk Management Authorities (RMA) and associated landowners.

1.3 Method

1.3.1 A site visit was undertaken by MAC who conducted interviews with those affected by the flooding to ascertain the mechanisms of flooding involved with this specific flood event. In addition, a desk based review of mapped drainage infrastructure was conducted. Our report was prepared based on these findings and is accurate based on our understanding of what happen during the flood event.

1.3.2 NNC provided MAC with copies of the flood incident report, historical flood records, water company asset plans, and highway drainage plans which provides locations of gullies and culverts.

1.4 Summary of Investigation Findings

1.4.1 A significant rainfall event resulted in the capacity of the highway drainage being exceeded resulting in overland flows towards XX and XX Priory Road resulting in internal flooding of XX Priory Road.

2.0 Flood Investigation - Background Information

2.1 Key Information

2.1.1 The flood investigation was conducted due to an incident that occurred on 21 September 2024.

2.1.2 The flooding affected: XX Priory Road, Wollaston

2.1.3 A site visit was conducted on: 14 October 2024

2.1.4 In attendance at the flood investigation were:

• Property owner of: XX Priory Road, Wollaston
• Martin Andrews from MAC

2.2 History of Flooding

2.2.1 The LLFA have no records of historical flooding in this area.

2.2.2 Historical flood information has been provided by the resident of the building affected by the flooding. These are summarised in Table 2.1 below.

Table 2 1: Historical Flooding - Residents

2.3 Mapped Flood Maps

2.3.1 Fluvial (river) and Pluvial (surface water) flood maps produced by the Environment Agency are shown below in Figure 2.1 and Figure 2.2. The approximate area of the flooding is marked in red.

2.3.2 Dark blue shows Flood Zone 3 (high risk) the area that could be affected by flooding, either from rivers or the sea, if there were no flood defences. This area could be flooded:

• from the sea by a flood that has a 0.5% (1 in 200) or greater chance of happening each year
• or from a river by a flood that has a 1% (1 in 100) or greater chance of happening each year

2.3.3 Light blue shows the extent of the Extreme Flood Outline Flood Zone 2 (medium risk), which represents the extent of a flood event with a 0.1% chance of occurring in any year, or the highest recorded historic extent if greater.

2.3.4 These two colours show the extent of the natural floodplain if there were no flood defences or certain other manmade structures and channel improvements. Sites outside the two extents, but behind raised defences, may be affected by flooding if the defences are overtopped or fail.

Figure 2.1: Fluvial / Tidal Flood Risk - Flood Map for Planning - gov.uk - 20 November 2024

2.3.5 The Environment Agency’s flood map shows that the investigation area is located within Flood Zone 1 (Low Probability) and as such, the site is at a low (less than 1 in 1000 years) risk of flooding from rivers or the sea.

Figure 2.2: Surface Water Flooding - Technical Flood Map - gov.uk - 20 November 2024

2.3.6 The dwelling affected by flooding is shown to be in an area of low to high surface water flood risk whilst Priory Road is at a high risk of surface water flooding.

2.4 Description of Investigation Area Catchment

2.4.1 The description below describes the catchment as understood to be relevant to the investigation.

2.4.2 The site affected by the flooding is located in a ‘suburban’ residential development with roads bound by footways with front and rear gardens reflective of a development built in the 1960s/70s. Some driveways within the catchment remain at their original size although some have been clearly extended resulting in urban creep.

2.4.3 From visual inspection Priory Road falls to the north towards The Maltings which is near adjacent to the affected property. Land then falls away slightly to the west along the Maltings. North of The Maltings Priory Road to the north falls towards the south. Hence, without any drainage features (e.g. gullies) any surface water upstream of the flooded site would drain towards the affected property and along The Maltings.

2.4.4 Photographs of the investigation area are enclosed in Appendix A.

2.5 Drainage Assets

2.5.1 This section describes drainage systems and watercourses in direct proximity to the flood incident. The wider catchment is drained by numerous systems, all of which have some impact on the effective management of surface water flowing to and from the area of the flood incident. The description of drainage assets is based on our observations and mapped records as provided by NNC.

2.5.2 Historically drainage wasn’t designed for a return period and overtime the standard of drainage design has increased such that modern drainage, serving multiple properties is typically designed to accommodate events up to and including the 1 in 100 year event plus an allowance for climate change. This means that most drainage regardless of maintenance regime is not designed to accommodate extreme rainfall events.

Natural Watercourses

2.5.3 There are no watercourses within close proximity to the investigation area or had any impact on the flood. Wollaston Book is located approximately 400m to the west of Priory Road and is lower than the site.

Drains / Sewers

2.5.4 Anglian Water asset plans are enclosed in Appendix C. These show adopted sewers only. Sewers which were previously private in 2011 and transferred to water companies in 2011 may not be shown on water company asset plans. Water company asset plans do not show private sewers or drains (pipes which serve only a single property). As such water company asset plans do not show all sewers or drains.

2.5.5 The investigation area is shown to be served by a foul and surface water sewers. Surface water sewers drain southwards towards Cobb’s Lane.

Highway Drainage

2.5.6 The public highway is drained by road gullies it is understood these drain into the adopted sewer.

Private Drainage

2.5.7 Private residential properties generally have their own drainage systems to collect surface water and convey this to an outfall. These comprise roof gutters and downpipes, and gullies/channels for external areas.

2.5.8 The outfall is usually a soakaway, watercourse or public sewer. In this location we expect that the private drainage systems will drain to the surface water public sewer /a watercourse / via infiltration.

3.0 Description of Flood Event

3.1 The Event

3.1.1 Flooding occurred on 21 September 2024 and twice again over the weekend.

3.2 Rainfall Analysis

3.2.1 The LLFA have provided rain gauge data for Wellingborough for the 20 September 2024 through to 23 September 2024. A summary of the rainfall data for the is shown below Table 3-1 with graphs showing the peak hourly rainfall on the 22 September 2024 for both gauges as Figure 3.1.

Table 3 1: Rainfall data from the nearest rain gauge

3.2.2 The rainfall gauges show a high level of daily rainfall on the 22nd and 23rd September but not a high level of daily rainfall on the 21st. Peak intensity was not especially high on the 21st but from my own relatively local observations there were some very short intense bursts of rain throughout this event which could have overloaded the capacity of the gullies. On the 22nd the data shows two periods of approximately 1 hour where the peak rainfall intensity peaked at around 14-16mm/hr between 0745-0900 and 1800-1915.

3.2.3 The rainfall data shows that there was a significant amount of rainfall within a short period of time.

Figure 3.1: Hourly Rainfall - 22 September - Wellingborough gauge

3.3 Description of Flood Event

3.3.1 The flooding is based on descriptions taken from those affected during our site visit. All depths mentioned are an approximation based on the descriptions provided.

3.3.2 The capacity of the highway drainage system was exceeded during an extreme rainfall event. This resulted in flood water ponding on the highway until such point as it overflowed and drained across the front gardens of XX and XX Priory Road resulting in internal flooding of XX Priory Road with water entering the property through the front door, front wall panel and integral garage. Flooding effected part of the lounge which extends from front to rear, the hall, and the kitchen located at the rear of the property.

3.3.3 The resident managed to remain in the property during and after the flooding.

3.3.4 Flood depths were in the order of 300mm externally, 200-300mm within the integral garage and 50-75mm within the kitchen.

3.3.5 A plan showing the approximate extent of flooding and flow routes is enclosed in Appendix D.

3.4 Maintenance Records

3.4.1 The road gullies were last cleaned in 2021 inline with NNC’s 3 year maintenance programme for this location.

3.5 Suspected Cause(s) of the Flooding Event

3.5.1 In any flood incident there can be numerous factors which could have resulted in the flooded witnessed. From the information we have received this is the only factors in this incident:

• Capacity of the highway drainage was exceeded

3.6 Additional Factors

3.6.1 The rainfall that occurred was extreme and is likely to have exceeded the capacity of the road gullies in the short term in any event. However, there has been some urban creep in the area which will have increased the volume and rate of surface water runoff.

3.7 Response to Flood Incident

Person(s) flooded

3.7.1 The occupier has installed temporary barriers to prevent possible future flood incidents.

Lead Local Flood Authority

3.7.2 The Lead Local Flood Authority (NNC) were notified after the incident, as the incident met their investigation threshold an investigation has been instructed.

Fire Brigade

3.7.3 The fire brigade attended the site but there wasn’t enough water for them to pump the water away.

Highway Authority

3.7.4 The Highway Authority had met with the flooded resident and Anglian Water to discuss a previous flood event approximately 18 months prior to this incident.

3.7.5 The Highway Authority did not attend the site due to this incident.

3.7.6 Others

3.7.7 No other parties provided help during the flood incident.

3.8 Flood Risk Management Authorities

3.8.1 The following bodies have flood management duties in this area:

• Highway Authority - maintenance of highway drainage
• Anglian Water - maintenance of adopted sewers
• Owners of private property - maintenance of private drainage

3.9 Recommendations

3.9.1 It is fully expected that the capacity of the highway drainage would have been exceeded in any circumstances, but we would recommend that both the highway drainage and adopted surface water sewers are checked to ensure they are working to the best of their ability.

4.0 Conclusion

4.1 A flood investigation was conducted by MAC on the instruction of NNC following the report of a flood incident which met the threshold for investigation XX Priory Road on 21 September 2024 and days following.

4.2 The property previously flooded in 22 June 2023.

4.3 Based on Environment Agency flood maps the area is at a low risk of flooding from rivers and sea. The area is identified as being at a high risk of surface water flooding.

4.4 The flood incident occurred as follows: The capacity of the highway drainage system was exceeded during an extreme rainfall event. This resulted in flood water ponding on the highway until such point as it overflowed and drained across the front gardens of XX and XX Priory Road resulting in internal flooding off XX Priory Road with water entering the property through the front door, front wall panel and integral garage. Flooding effected part of the lounge which extends from front to rear, the hall, and the kitchen located at the rear of the property.

4.5 The following are suspected to be factors affecting the flood:

• Capacity of the highway drainage was exceeded

4.6 Our recommendations are:

• It is fully expected that the capacity of the highway drainage would have been exceeded in any circumstances but we would recommend that both the highway drainage and adopted surface water sewers are checked to ensure they are working to the best of their ability

Appendix A

Photographs of Flood Investigation Area

Priory Road from the north

Priory Road, at The Maltings

Priory Road looking southwards at flood area

Priory Road looking northwards at flood area

Appendix B

Appendix C

1.0 Introduction

1.1 Instructions

1.1.1 Martin Andrews Consulting Ltd (MAC) have been appointed to undertake this Section 19 Flood Investigation on behalf of North Northamptonshire Council (NNC).

1.1.2 Several flood reports were submitted to NNC for an incident at Main Road and Blackmile Lane, Grendon starting on the 22 September 2024. This warrants a flood investigation on two counts due to internal flooding of a building being experienced on more than one occasion in the last five years and internal flooding of five buildings in close proximity has been experienced during a single flood incident.

1.1.3 The approximate location of the flood incident is shown below in Figure 1.1.Both Gibbards Arm and Grendon Brook are Environment Agency main rivers

Figure 1.1: Incident Location Plan

1.2 What is a Section 19 Flood Investigation

1.2.1 As set out below, under Section 19 of the Flood and Water Management Act (2010), NNC as Lead Local Flood Authority (LLFA), have a duty to conduct flood investigations.

1.2.2 The Act states that:
(1) on becoming aware of a flood in its area, a lead local flood authority must, to the extent that it considers it necessary or appropriate, investigate:
(a) which risk management authorities have relevant flood risk management functions, and
(b) 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 carries out an investigation under subsection (1) it must:
(a) publish the results of its investigation, and
(b) notify any relevant risk management authorities.

1.2.3 It is not the responsibility of NNC to resolve the flooding issues, however, they do need to investigate the cause of the flooding and notify any relevant Flood Risk Management Authorities (RMA) and associated landowners.

1.3 Method

1.3.1 A site visit was undertaken by MAC who conducted interviews with those affected by the flooding to ascertain the mechanisms of flooding involved with this specific flood event. In addition, a desk based review of mapped drainage infrastructure was conducted. Our report was prepared based on these findings and is accurate based on our understanding of what happen during the flood event.

1.3.2 NNC provided MAC with copies of the flood incident reports, historical flood records, water company asset plans, and highway drainage plans which provides locations of gullies and culverts.

1.4 Summary of Investigation Findings

1.4.1 The capacity of Grendon Brook was exceeded resulting in water overtopping the banks in several locations. This water flowed north towards Main Road and Blackmile Lane causing flooding of multiple properties.

2.0 Flood Investigation - Background Information

2.1 Key Information

2.1.1 The flood investigation was conducted due to an incident that occurred on 22 September 2024.

2.1.2 The flooding affected: Main Road and Blackmile Lane

2.1.3 A site visit was conducted on 8 October 2024

2.1.4 In attendance at the flood investigation were:

• Property owners of:
  - XX and XX Main Road
  - X, X, XX, X, X, XX, XX, XX, XX and XX Blackmile Road
  those in bold experienced internal flooding
• Others
  - Parish Council were invited but did not attend, their attendance was not required
• Martin Andrews from MAC

2.2 History of Flooding

2.2.1 Relevant historical flood information provided by residents is summarised in Table 2 1 below.

Table 2 1: Historical Flooding - Source from Residents

2.3 Mapped Flood Maps

2.3.1 Fluvial (river, Flood Map for Planning) and Pluvial (surface water, Technical Flood Map) flood maps produced by the Environment Agency are shown below in Figure 2.1 and Figure 2.2. The approximate area of the flooding is marked in red it is likely that flooding occurred further to the east, west and south of Grendon Brook but we do not have details of this.

Figure 2.1: Fluvial / Tidal Flood Risk - Flood Map for Planning - gov.uk - 8 October 2024

2.3.2 The Environment Agency’s flood map shows that the investigation area is located within. Flood Zones 1, 2 and 3. Grendon lies in areas of low to high flood risk.

2.3.3 Dark blue shows Flood Zone 3 (high risk) the area that could be affected by flooding, either from rivers or the sea, if there were no flood defences. This area could be flooded:

• from the sea by a flood that has a 0.5% (1 in 200) or greater chance of happening each year, or
• from a river by a flood that has a 1% (1 in 100) or greater chance of happening each year

2.3.4 Light blue shows the extent of the Extreme Flood Outline Flood Zone 2 (medium risk), which represents the extent of a flood event with a 0.1% chance of occurring in any year, or the highest recorded historic extent if greater.

2.3.5 These two colours show the extent of the natural floodplain if there were no flood defences or certain other manmade structures and channel improvements. Sites outside the two extents, but behind raised defences, may be affected by flooding if the defences are overtopped or fail.

Figure 2.2: Pluvial / Surface Water Flooding - Technical Flood Map gov.uk - 8 October 2024

2.3.6 The investigation area is located in an area of very low to high surface water flood risk.

2.4 Description of Investigation Area Catchment

2.4.1 Grendon Brook (a main river) runs from east to west. Upstream of the flood incident three different watercourses join to form Grendon Brook including arms from Bozeat and Yardley Hastings.

2.4.2 Grendon Brook is at the bottom of the valley which to the south rises relatively steeply towards Grendon. To the north the valley is marginal, and the land is relatively flat to the north.

2.4.3 West of the flood incident the watercourse heads north and joins the River Nene south of Earls Barton.

2.4.4 To the north of Blackmile Lane is another main river called Gibbards Arm. Gibbards Arm flows to the north and ultimately into the River Nene.

2.4.5 The Environment Agency, using its permissive powers, can carry out maintenance, improvement, or construction work on main rivers to manage flood risk and protect the environment.

2.4.6 Photographs of the investigation area are enclosed in Appendix A and the catchment area is shown on Figure 2.3.

Figure 2.3: Catchment Map of Grendon Brook

2.5 Drainage Assets

2.5.1 This section describes drainage systems and watercourses in direct proximity to the flood incident. The wider catchment is drained by numerous systems, all of which have some impact on the effective management of surface water flowing to and from the area of the flood incident. The description of drainage assets is based on our observations and mapped records as provided by NNC.

2.5.2 Historically drainage wasn’t designed for a return period and overtime the standard of drainage design has increased such that modern drainage, serving multiple properties is typically designed to accommodate events up to and including the 1 in 100 year event plus an allowance for climate change. This means that most drainage regardless of maintenance regime is not designed to accommodate extreme rainfall events.

Natural Watercourses

2.5.3 Grendon Brook is an Environment Agency main river and was the source of the flooding. Another main river Gibbards Arm is located adjacent to Main Road and begins at the Main Road / Blackmile Lane junction.

2.5.4 Within the fields and adjacent to Main Road there are ditches within Ripiarian (owned by the adjacent land owner) ownership.

Drains and Sewers

2.5.5 Anglian Water asset plans are enclosed in Appendix B. These show adopted sewers only. Sewers which were previously private in 2011 and transferred to water companies in 2011 may not be shown on water company asset plans. Water company asset plans do not show private sewers or drains (pipes which serve only a single property). As such water company asset plans do not show all sewers or drains.

2.5.6 The investigation area is shown to be served by foul water sewers only and no surface water sewers.

Highway Drainage

2.5.7 The public highway is drained by road gullies and grips it is understood these drain into the ditch and watercourses.

Private Drainage

2.5.8 Private residential and commercial properties generally have their own drainage systems to collect surface water and convey this to an outfall. These comprise roof gutters and downpipes, and gullies and channels for external areas.

2.5.9 The outfall is usually a soakaway, watercourse or public sewer. In this location we expect that the private drainage systems will drain to a watercourse.

3.0 Description of Flood Event

3.1 The Event

3.1.1 Flooding occurred within the dwellings at approximately 20:50 on 22 September 2024.

3.2 Rainfall Analysis

3.2.1 The Environment Agency have provided rain gauge data for two locations Quinton and Wellingborough for the 20 September 2024 through to 23 September 2024. A summary of the rainfall data is shown below in Table 3-1 with graphs showing the peak hourly rainfall on the 22 September 2024 for both gauges as Figure 3.1 and Figure 3.2.

Table 3 1: Rainfall data from the nearest rain guages

3.2.2 The rainfall gauges show a raised level of daily rainfall on the 22 and 23 September. At the Wellingborough gauge the data shows two periods of approximately 1 hour where the peak rainfall intensity peaked at around 14-16mm/hr between 0745-0900 and 1800-1915. Similar patterns of high intensity rainfall also occurred at the Quinton gauge with a higher peak around 15-22mm/hr approximately 30 minutes earlier.

3.2.3 The rainfall data shows that there was a significant amount of rainfall within a short period of time.

Figure 3.1: Hourly Rainfall - 22 September - Wellingborough gauge

Figure 3.2: Hourly Rainfall - 22 September - Quinton gauge

3.0 Description of Flood Event

3.3.1 This flood investigation is based on descriptions taken from those affected during our site visit. All depths mentioned are an approximation based on the descriptions provided. A plan is enclosed in Appendix C showing the approximate extent of flooding and key features whilst some photos provided of the flood are enclosed in Appendix A.

3.3.2 The channel capacity of Grendon Brook was exceeded around 20:50 and water escaped the brook in several locations but from what can be seen on the ground, it would appear that water left the brook:

• At the eastern side of Main Road at the road crossing
• Near the public right of way (PRoW) bridge across Grendon Brook on Bridleway TF12
• To the west of Bridleway TF12 near the overhead power lines

3.3.3 Water then flows across adjacent fields on the northern side of Grendon Brook ultimately following contours and flowing east and north east towards Main Road and the Main Road and Blackmile Lane junction and the rear of properties at the western end of Blackmile Lane.

3.3.4 We are told that at approximately 2am the water level in Grendon Brook by Main Road was approximately 7ft lower than the top of the bank.

3.3.5 The water flowed from these fields in two main directions. Route 1 saw flood water find a route(s) through the hedgerow onto Main Road. Water then flowed northwards along Main Road as far as The Ark approximately 195m north of the Main Road and Blackmile Lane junction. This flow route resulted in the primary source of the internal flooding of the Main Road properties.

3.3.6 The second flow route (Route 2) was across the field flowing northwards into the rear gardens of the properties on Blackmile Lane. It is understood that this was the main route of internal flooding for properties on Blackmile Lane.

3.3.7 The flooding was described as happening very quickly and surging into the properties in a matter of minutes from no flood water to depths of 1m. This would suggest that there was a sudden and significant overtopping of the river bank causing this surge of flooding.

3.3.8 Residents raised concerns that ‘flood gates’ in Yardley Hastings may have been opened causing a sudden surge of flood water. We have checked with the Environment Agency who say:

“The reservoirs operated as designed. They exceeded their capacity which meant water flowed over the spillways leading to a rapid increase in water levels downstream. The flood storage reservoirs are designed so there is no active intervention required.“

3.3.9 Water depth in the fields is unknown but peak water depth was approximately 1m at the Main Road and Blackmile Lane junction with water depths ranging from 10 to 300mm deep within flooded properties.

3.3.10 Walking around the area of flooding there are numerous “tide marks” left by straw stubble which was washed into the watercourse from overland flows off adjacent fields. Examples of this can be seen by on the photos enclosed in Appendix C. Comparing high water level marks on 80 Main Road it would appear that the water depth in this event exceeded the depth of water in the 2012 event.

3.3.11 A number of properties were flooded during the event details of the extent of flooding at each property is set out in the below table.

Table 3 2: Resident Responses to Survey

3.4 Suspected Cause(s) of the Flooding Event

3.4.1 In any flood incident there can be numerous factors which could have resulted in the flooding witnessed. From the information we have received we believe this to be the cause of the flooding.

3.4.2 It is clear that the capacity of the Grendon Brook was exceeded during the rainfall event. The flood water overtopped the banks resulting in downstream drainage features being overwhelmed as the volume of water will have exceeded any design. There are no features or flood defence assets raised above the natural ground level which have failed.

3.5 Additional Factors

3.5.1 It is not possible to comment on the condition of the watercourse before the flood as this has not been witnessed and post flood it can be clearly seen that vegetation has been removed from the watercourse so the post flood view of the watercourse is very different. Based on anecdotal evidence from the residents it understood that the watercourse hasn’t been maintained to a level that they deem to be acceptable.

3.5.2 It was observed that in some locations there were fallen trees and vegetation extending across the watercourse which when in combination with the high level of straw in the watercourse could have led to some capacity restriction within the watercourse. It should be noted that the areas where the overtopping of the natural banks occurred did not correlate with any manmade flow restriction, such as culverts or bridges, within the watercourse except where there was a degree of overtopping of the bank by the bridge on Main Road. This might suggest there was one or more blockages in the watercourse caused by vegetation and possibly aggravated by the straw, however, it is not possible to be certain in this regard without a detailed level survey of the river bank it is not possible to say where the bank low spots are and where we would expect water to have overtopped. It may be the low spots in the bank were where the capacity of the channel was exceeded. Although we would note that except by the bridge there was no evidence that the watercourse overtopped the banks elsewhere within the field adjacent to Main Road a length of some 550m.

3.5.3 The extent of flood water includes large areas of Flood Zone 3 (high risk of fluvial flooding) as predicted by the Environment Agency mapping. The flood water did also extend to areas of Flood Zone 1 and 2 which might suggest that the other factors such as watercourse maintenance have caused the watercourse to act in ways not predicted by the mapping although we do not know if the flood maps are based on flood modelling information.

3.6 Response to Flood Incident

Person(s) flooded

3.6.1 Property owners took some action to stop water entering their properties, but the speed and depth of flooding was such that their actions had limited effect.

3.6.2 Some residents sought to try and clear the watercourse and ditch culverts of debris to help the water flow away.

Environment Agency

3.6.3 The Environment Agency were notified of the incident through telemetry alarms and NIRS reports but did not attend the site. A flood warning was issued at 3:22am on the 23 September 2024 which is after the first reports of flooding by the residents.

3.6.4 The Environment Agency had cleaned all trash screens before the incident.

Lead Local Flood Authority

3.6.5 The Lead Local Flood Authority (NNC) were notified after the incident, as the incident met their investigation threshold an investigation has been instructed.

Fire Brigade

3.6.6 The fire brigade attended to help anyone who wished to evacuate their house.

Highway Authority

3.6.7 The highway authority attended site and is understood to have cleared some road gullies of debris post flood and closed the road during the flood. It was observed during the site visit that some road gullies had a significant amount of debris in them. Highway drainage did not directly affect the flood.

3.6.8 Others

3.6.9 No other parties provided help during the flood incident.

3.7 Flood Risk Management Authorities

3.7.1 The following bodies have flood management duties in this area:

Involved with this flood event

• Owners of private property - maintenance of private drainage
• Riparian Owners - land owners adjacent to non-controlled watercourse have a duty of maintenance. Owning a watercourse - GOV.UK and Your watercourse: rights and roles guide
• Environment Agency - using its permissive powers, can carry out maintenance, improvement, or construction work on main rivers to manage flood risk and protect the environment

Not involved with this flood

• Highway Authority - maintenance of highway drainage
• Anglian Water / Severn Trent - maintenance of adopted sewers

3.8 Recommendations

3.8.1 The flooding was caused as the capacity of the watercourse was exceeded.

• Review the maintenance of Grendon Brook to ensure any maintenance guidelines are accorded with and any issues are rectified - this should include all drainage features in the upstream catchment including ordinary watercourses and ditches and drains
• Flooding to Grendon Brook has happened in 2012, 2020 and 2024 consideration might be given to whether any mitigation could be implemented which would stop or limit the flow of water across the fields away from the watercourse - this could include upstream measures such as natural flood management to slow down the rate of water entering the watercourse or measures where the flood water overtopped the banks to direct flows away from dwellings and business premises

3.8.2 We understand the Environment Agency has undertaken a maintenance review of Grendon Brook and raised the need for some vegetation (bushing) management from What Three Word reference points ///website.earplugs.deprives to ///jobs.erupt.ourselves. The schedule of identified works is as follows:

• Remove all blockages from the channel between ///website.earplugs.deprives to ///jobs.erupt.ourselves
• Tunnel the watercourse to the height of the opposite bank by only removing extending limbs (i.e. do not fell)
• All trees on the face of the embankment (1m up from the toe) should be retained unless they have been pictured and highlighted in the work plan
• These works aim to aid conveyance at the points at which overtopping and wracking were observed
• Our current routine maintenance (both frequency and extent) will remain the same, as grass cutting of the banks and weed cutting of the channel has been deemed sufficient as long as the upstream vegetation management is carried out

4.0 Conclusion

4.1 A flood investigation was conducted by MAC on the instruction of NNC following the report of a flood incident which met this threshold for investigation in Main Road and Blackmile Lane, Grendon on 22 September 2024.

4.2 There has been a history of flooding in the area with previous flood events pre 1980s, 2012 and 2020. A Section 19 investigation was also completed for the 2012 event.

4.3 Based on Environment Agency flood maps the area is at risk of fluvial and surface water flooding. The incident is located in Flood Zones 1, 2 and 3. The incident has a very low to high risk of surface water flooding.

4.4 The flood incident occurred as follows:

• The capacity of Grendon Brook was exceeded which resulted in overtopping of the watercourse’s banks in several locations - this caused water to flow north across the fields towards Main Road and the rear of properties on Blackmile Lane - this caused significant flooding to a number of properties and standing water to a depth of approximately 1m at the Main Road / Blackmile Lane junction

4.5 The following are suspected to be factors affect the flood:

• Capacity of the watercourse was exceeded
• Poor maintenance of the watercourse has been reported by residents, however, we do not know the condition of the watercourse was pre-flood - poor maintenance of the watercourse could have increased the likelihood of the channel capacity being exceeded

4.6 Our recommendations are:

• Review the maintenance of Grendon Brook to ensure any maintenance guidelines are accorded with and any issues are rectified
• Flooding to Grendon Brook has happened in at least 2012, 2020 and 2024, consideration might be given to whether any mitigation could be implemented which would stop or limit the flow of water across the fields and away from the watercourse.

4.7 We understand the Environment Agency have identified maintenance works which will be undertaken on Grendon Brook.

Appendix A

1241-FRA12-Photograph locations

Photo 1: Indication of High Water Mark on XX Main Road / X Blackmile Lane

Photo 2: Indication of High Water Mark on Stump at Main Road / Blackmile Lane junction

Photo 3: Example of straw washed off fields - Main Road

Photo 4: Example of Straw washed off fields - Main Road

Photo 5: Example of straw washed off fields – Main Road

Photo 6: Straw on footbridge on PRoW TF7/14

Photo 7: Straw on footbridge on PRoW TF12

Photo 8: Overtopping Location 1 (approx.) - Main Road Bridge

Photo 9: Overtopping Location 2 - PRoW TF7/14

Overtopping Location 3 - not photographed off publicly accessible areas

Photo 10: Main Road - hole through hedge likely to be one of the locations where water flowed from field to Main Road

Photo 11: Main Road - Damaged Culvert Head Wall - not thought to have been a significant factor in the flood incident

Photo 12: Blackmile Lane – date and time unknown

Photo 13: Blackmile Lane near Main Road - 22:52 22 September 2024

Photo 14: Blackmile Lane near Main Road - 07:55 24 September 2024

Photo 15: Main Road - 04:59 - 27 September 2024

Appendix B - Grendon Asset plan_Sewers 01

Appendix B - Grendon Asset plan_Sewers 02

Appendix C - 1241-FRA11A-Flood Extent Drawing

1.0 Introduction

1.1 Instructions

1.1.1 Martin Andrews Consulting Ltd (MAC) have been appointed to undertake this Section 19 Flood Investigation on behalf of North Northamptonshire Council (NNC).

1.1.2 A flood report was submitted to NNC for an incident at XXXXX XXXXX, Huxley Close, Park Farm Industrial Estate, Wellingborough Northamptonshire NN8 6AB on 21 and 22 September 2024. This warrants a flood investigation because internal flooding of a building has been experienced on more than one occasion in the last five years and internal flooding of five buildings in close proximity has been experienced during a single flood incident.

1.1.3 The approximate location of the flood incident is shown below in Figure 1.1.

Figure 1.1: Incident Location Plan

1.2 What is a Section 19 Flood Investigation

1.2.1 As set out below, under Section 19 of the Flood and Water Management Act (2010), NNC as Lead Local Flood Authority (LLFA), have a duty to conduct flood investigations.

1.2.2 The Act states that:
(1) on becoming aware of a flood in its area, a lead local flood authority must, to the extent that it considers it necessary or appropriate, investigate:
(a) which risk management authorities have relevant flood risk management functions, and
(b) 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 carries out an investigation under subsection (1) it must:
(a) publish the results of its investigation, and
(b) notify any relevant risk management authorities.

1.2.3 It is not the responsibility of NNC to resolve the flooding issues, however, they do need to investigate the cause of the flooding and notify any relevant Flood Risk Management Authorities (RMA) and associated landowners.

1.3 Method

1.3.1 A site visit was undertaken by MAC who conducted an interview with those affected by the flooding to ascertain the mechanisms of flooding involved with this specific flood event. In addition, a desk-based review of mapped drainage infrastructure was conducted. Our report was prepared based on these findings and is accurate based on our understanding of what happen during the flood event.

1.3.2 NNC provided MAC with copies of the flood incident report, historical flood records, water company asset plans, and highway drainage plans which provides locations of gullies and culverts.

1.4 Summary of Investigation Findings

1.4.1 A significant rainfall event resulted in water draining through weep holes in a retaining wall with significant force and flow to cause flooding to the investigation site. It is unknown why this happened but it could be because surface water drainage from a third party site failed in some manner during a significant rainfall event or a mechanisms associated with previously open watercourse on the site which has been built over. How this watercourse was treated within the development of the site is unknown.

2.0 Flood Investigation - Background Information

2.1 Key Information

2.1.1 The flood investigation was conducted due to an incident that occurred on 21 and 22 September 2024.

2.1.2 The flooding affected: XXXXX XXXXX, Huxley Close, Park Farm Industrial Estate, Wellingborough Northamptonshire NN8 6AB

2.1.3 A site visit was conducted on: 10 October 2024

2.1.4 In attendance at the flood investigation were:

• Management company for the site of XXXXX XXXXX, Huxley Close, Park Farm Industrial Estate, Wellingborough Northamptonshire NN8 6AB
• Occupier of XX Huxley Close
• Martin Andrews

2.2 History of Flooding

2.2.1 The LLFA have no records of historical flooding in this area.

2.2.2 Historical flood information has been provided by the occupiers of the buildings affected by the flooding. Two of the properties which provided details of flooding were new tenants so were unsure of any flooding history. These are summarised in Table 2 1 below.

Table 2 1: Historical Flooding - Source from Occupiers

2.3 Mapped Flood Maps

2.3.1 Fluvial (river) and Pluvial (surface water) flood maps produced by the Environment Agency are shown below in Figure 2.1 and Figure 2.2. The approximate area of the flooding is marked in red.

Figure 2.1: Fluvial / Tidal Flood Risk - Flood Map for Planning - gov.uk– 10 October 2024

2.3.2 The Environment Agency’s flood map shows that the investigation area is located within Flood Zone 1 (Low Probability) and as such, the development is at a low (less than 1 in 1000 years) risk of flooding from rivers or the sea.

Figure 2.2: Surface Water Flooding - Technical Flood Map gov.uk - 6 February 2025

2.3.3 The investigation area is located in an area of low and high surface water flood risk. The upstream areas of the surface water flood risk are a managed surface water drainage area comprising a yard and building which would reduce the likelihood of surface water flooding happening as shown on the plan.

2.4 Description of Investigation Area Catchment

2.4.1 The flood incident occurred within a commercial development where the majority of areas are impermeable. The affected site is approximately 1.2-2m lower than a neighbouring large commercial site to the north. A retaining wall separates the two sites. Surface water from the neighbouring site to the north drains to the south passing through or below the retaining wall in the north western corner of the flood investigation site.

2.4.2 Photographs of the investigation area are enclosed in Appendix A.

2.5 Drainage Assets

2.5.1 This section describes drainage systems and watercourses in direct proximity to the flood incident. The wider catchment is drained by numerous systems, all of which have some impact on the effective management of surface water flowing to and from the area of the flood incident. The description of drainage assets is based on our observations and mapped records as provided by NNC.

2.5.2 Historically drainage wasn’t designed for a return period and overtime the standard of drainage design has increased such that modern drainage, serving multiple properties is typically designed to accommodate events up to and including the 1 in 100 year event plus an allowance for climate change. This means that most drainage regardless of maintenance regime is not designed to accommodate extreme rainfall events.

Natural Watercourses

2.5.3 There is a tributary of Swanspool Brook located at the western end of Huxley Close. This watercourse had no connection to the flood incident.

2.5.4 Our attention has also been drawn to a watercourse which does or did cross the site. We are unsure what has happened to this watercourse it may be buried (culverted) or diverted or stopped up entirely. The route of the watercourse in context of the modern layout can be seen in the plan (Figure 2.3) below. For a watercourse to be mapped it has to reach a certain size and often the actual watercourse will extend beyond the mapped extents.

Figure 2.3: Watercourses within the vicinity of the S19 Investigation

Drains and sewers

2.5.5 Anglian Water asset plans are enclosed in Appendix B. These show adopted sewers only. Sewers which were previously private in 2011 and transferred to water companies in 2011 may not be shown on water company asset plans. Water company asset plans do not show private sewers or drains (pipes which serve only a single property). As such water company asset plans do not show all sewers or drains.

2.5.6 The investigation area is shown to be served by a foul and surface water sewers. There is a surface and foul water sewer located between the retaining wall and the building, the surface water sewer which drains to the east whilst the foul water sewer drains to the west. Further surface and foul water sewers are located to the south of the flood incident site in Huxley Close.

Highway drainage

2.5.7 Huxley Close is a mix of private and adopted carriageway. The highway is drained by road gullies. It is likely that these either drain into the adopted sewer or watercourse.

Private drainage

2.5.8 Private residential and commercial properties generally have their own drainage systems to collect surface water and convey this to an outfall. These comprise roof gutters and downpipes, and gullies or channels for external areas.

2.5.9 The outfall is usually a soakaway, watercourse or public sewer. In this location we expect that the private drainage systems will drain to the surface water public sewer and or a watercourse.

2.5.10 We have been provided with a copy of drainage plans for the third party site to the north, see plan enclosed in Appendix B. This shows a surface water sewer passing through the retaining wall in the north west corner.

3.0 Description of Flood Event

3.1 The Event

3.1.1 Flooding occurred on 21 and 22 September 2024 no details of the time of the flooding have been provided because the incident happened at a commercial premises at the weekend.

3.2 Rainfall Analysis

3.2.1 The Environment Agency have provided rain gauge data for Wellingborough for the 20 September 2024 through to 23 September 2024. A summary of the rainfall data for the is shown below in Table 3-1 with graphs showing the peak hourly rainfall on the 21 and 22 September 2024 for both gauges as Figure 3.1.

Table 3 1: Rainfall Data

3.2.2 The rainfall gauges show a high level of daily rainfall on the 22 and 23 September. At the Wellingborough gauge the data shows two periods of 1 hour 15 minutes where the peak rainfall intensity peaked at around 14-16mm/hr between 0745-0900 and 1800-1915.

3.2.3 The rainfall data shows that there was a significant amount of rainfall within a short period of time.

Figure 3.1: Hourly Rainfall - 21 and 22 September - Wellingborough gauge

3.3 Description of Flood Event

3.3.1 Flooding occurred within a rear access area behind XX-XX Huxley Close and then flowed eastward towards and into XX Huxley Close and properties on the southern side of Huxley Close were also affected. A plan showing the approximate extent of flooding and flow routes is enclosed in Appendix C.

3.3.2 Flood water entered XX, XX, XX, XX and XX, and XX-XX Huxley Close and is understood to have covered approximately half to the whole of the ground floor of the buildings to a relatively shallow depth of 20-30mm. Externally the depth of water is thought to be around 75-80mm deep for properties on the northern side of Huxley Close increasing to 1200mm in loading docks of properties on the southern side of Huxley Close.

3.3.3 Flooding occurred through two mechanisms in both instances flood water has flowed from the third-party site to the north southwards onto the flood affected property.

3.3.4 The two mechanisms comprised a very significant flow out of the retaining wall weep holes, possibly through the timber retaining wall and some overtopping of the retaining wall.

3.3.5 It is believed that water came out of the weep holes first and this can be considered the primary mechanism with water overtopping of the retaining wall being the secondary mechanism.

3.3.6 We have been provided drainage plans off the third party land to the north. This shows a surface water sewer passing through the retaining wall in the north western corner of the flood investigation site. With this and the historical watercourse, we consider there to be three options for this sudden flow of water through the weep holes.

3.3.7 Option 1: the pipe passing through or below the retaining wall has failed in some way and is discharging surface directly into the free draining material behind the retaining wall which is then causing the water to leave via the weep holes.

3.3.8 Option 2: The drainage on this site could not cope with the intensity of the rainfall event leading to water flowing offsite. If this is the case this shouldn’t be surprising given the intensity of the rainfall event that caused this flooding. Based on the dates of the planning application for the third party site we would imagine the building was built in the late 1980s or early 1990s. At this time surface water drainage would not have been designed to cope with storms with an intensity of greater than 50mm/hr/m2 and would not have included attenuation. If this was the case, we would have expected overtopping to be the primary source of flood water as we would expect it to take longer for the water to drain behind the rear of the retaining wall such that it could primarily flood from the weep holes.

3.3.9 Option 3, flooding has occurred within the vicinity of the onsite watercourse. We are unsure of the history and the current status of this watercourse. This could be source of the flooding. It could also be that this watercourse was spring fed and the flooding is coming from this spring, the flow of which is restricted by the retaining wall.

3.3.10 A number of properties were flooded during the event details of the extent of flooding at each property is set out in the below table.

Table 3 2: Responses to Questionnaire

3.4 Maintenance Records

3.4.1 All drainage aspects involved with this flood were private, as such we do not have any drainage records.

3.5 Suspected Cause(s) of the Flooding Event

3.5.1 In any flood incident there can be numerous factors which could have resulted in the flooding witnessed. From the information we have received these are the key factors:

• If mechanism 1 is correct we would expect the surface water drainage which passes through or below the retaining wall to have failed in some way such that the free draining material behind the retaining wall has become surcharged and water leaves the site via the weep holes or through the timber retaining wall
• If mechanism 2 is correct the overtopping of the retaining wall is likely to have been caused by a failure of the drainage to accommodate the intensity of the rainfall event - based on the dates of planning application we would imagine the building was built in the late 1980’s or early 1990s - at this time surface water drainage would not have been designed to cope with storms with an intensity of greater than 50mm/hr and would not have included attenuation
• If mechanism 3 is correct it is difficult to know what is happening without knowing the history of the watercourse. It could be that the culverting has failed in some way - it could be that if the watercourse is spring fed this now has no where to go - details of what happened to the watercourse would need to be looked into

3.6 Response to Flood Incident

Person(s) flooded

3.6.1 No action was taken.

Lead Local Flood Authority

3.6.2 The Lead Local Flood Authority (NNC) were notified after the incident, as the incident met their investigation threshold an investigation has been instructed.

Fire Brigade

3.6.3 Not notified.

Highway Authority

3.6.4 Not notified.

Others

3.6.5 No other parties provided help during the flood incident.

3.7 Flood Risk Management Authorities

3.7.1 The following bodies have flood management duties related to this flood incident:

• Owners of private property - maintenance of private drainage

3.8 Recommendations

3.8.1 We would recommend that the third party undertake a survey of their surface water drainage to ensure it is not damaged or blocked in any way and working as per expectations of a drainage system of its age.

3.8.2 An investigation into the watercourse within the site should be undertaken to establish what happened to it and if it has been culverted does it still operate as designed.

3.8.3 If the surface water drainage system is working as expected the owner(s) of the flood affected land should look to see if external levels can be altered to direct flood water away from buildings.

4.0 Conclusion

4.1 A flood investigation was conducted by MAC on the instruction of NNC following the report of a flood incident which met this threshold for investigation in Huxley Close, Wellingborough on 21 and 22 September 2024.

4.2 There has been one known previous event which occurred in a similar fashion to this event. The date of this incident is not known.

4.3 Based on Environment Agency flood maps the area is at risk of surface water flooding.

4.4 The flood incident occurred as follows:

4.4.1 A significant rainfall event resulted in water draining through weep holes in a retaining wall with significant force and flow to cause flooding to the investigation site. It is unknown why this happened but it could be because surface water drainage from a third party site failed in some manner during a significant rainfall event or a mechanisms associated with previously open watercourse on the site which has been built over. How this watercourse was treated within the development of the site is unknown.

4.5 The following are suspected to be factors affect the flood:

• If mechanism 1 is correct we would expect the surface water drainage which passes through or below the retaining wall to have failed in some way such that the free draining material behind the retaining wall has become surcharged and water leaves the site via the weep holes or through the timber retaining wall
• If mechanism 2 is correct the overtopping of the retaining wall is likely to have been caused by a failure of the drainage to accommodate the intensity of the rainfall event - based on the dates of planning application we would imagine the building was built in the late 1980’s or early 1990s - at this time surface water drainage would not have been designed to cope with storms with an intensity of greater than 50mm/hr and would not have included attenuation
• If mechanism 3 is correct it is difficult to know what is happening without knowing the history of the watercourse - it could be that the culverting has failed in some way - it could be that if the watercourse is spring fed this now has no where to go - details of what happened to the watercourse would need to be looked into

4.6 Our recommendations are:

• We would recommend that the third party undertake a survey of their surface water drainage to ensure it is not damaged or blocked in any way and working as per expectations of a drainage scheme of its age
• If the drainage scheme is working as expected the owner(s) of the flood affected land should look to see if external levels can be altered to direct flood water away from buildings
• An investigation into the watercourse within the site should be undertaken to establish what happened to it and if it has been culverted does it still operate as designed

Appendix A

Photographs of Flood Investigation Area and Flood Incident

Photo 1: Land to the rear of XX-XX Huxley Close

Photo 2: Land to the rear of XX-XX Huxley Close

Photo 3: Land to the rear of XX-XX Huxley Close

Photo 4: Land to the rear of XX-XX Huxley Close

Photo 5: Land to the rear of XX-XX Huxley Close

Photo 6: Land to the rear of XX-XX Huxley Close – Unknown date and time

Photo 7: Land to the rear of XX-XX Huxley Close – 23 September 2024 - 11:17am

Photo 8: Rear and Eastern Elevation of XX-XX Huxley Close - 12 September 2023 - 1:36pm

Appendix B

Appendix B1 - Coop drainage plan

Appendix B2 - STATS Asset Plan Huxley Close

Appendix C

Appendix C - 1243-FRA10A-Flood Extent Drawing

1.0 Introduction

1.1 Instructions

1.1.1 Martin Andrews Consulting Ltd (MAC) have been appointed by North Northamptonshire Council (NNC) to undertake a survey of properties located along the route of the East Brook Culvert (as understood) to establish the extent of any flooding along this culvert along with basic details of the incident.

1.2 What is the East Brook Culvert?

1.2.1 The East Brook Culvert is a buried watercourse which runs through Kettering beginning near Brambleside to the north of Kettering and continuing southwards until it becomes an open watercourse again at the allotments accessed off Spring Rise.

1.2.2 The route of the culvert, as best understood, is shown in Appendix A.

1.2.3 From a review of historical OS mapping the culverting of East Brook Culvert began between 1884 and 1899 and will have continued as Kettering expanded.

1.3 Survey methodology

1.3.1 NNC provided MAC with a plan which showed the route of the culvert, as they have been able to identify it, within Kettering. From this MAC identified 607 properties which are in some way adjacent to the culvert and may have been affected by flooding as a result. Drawings showing the houses which were included within the survey are enclosed in Appendix B.

1.3.2 A questionnaire was produced which was mostly hand delivered (others were posted) to all of the houses identified. This provided the resident with an opportunity to complete a paper survey to be returned by post or complete the form electronically. A copy of the questionnaire is included in Appendix C. We understand some residents may not have received a copy of the survey, however, best endeavours were undertaken to include all possibly affected properties across a large area with an unknown scope of impact. It is also noted that the local press published an article which included a link to the online survey.

1.3.3 The purpose of the survey was to identify where flooding had occurred and the approximate number of properties which had been flooded and the extent of this flooding.

1.3.4 The results of the survey will be used to inform any further work and to support bids for additional funding which may be need.

1.4 Mapped flood risk

1.4.1 The route of the culvert is located within Flood Zone 1 as per the Environment Agency’s Flood Map for Planning (Rivers and Sea). Extracts from the Environment Agency’s surface water flood risk map are shown below, see Figure 1.1 and Figure 1.2. This shows that the route of the culvert (indicated in red) is located predominantly in an area of high-risk surface water flooding. The high-risk category would fit with what is a local valley running through this area of Kettering, hence, the historical watercourse which has now been
buried.

Figure 1.1: Surface water flood risk map - extract 1

Figure 1.1: Surface water flood risk map - extract 2

2.0 Results of the Survey

2.1 How many responded?

2.1.1 Of the 607 surveys which were distributed 89 (15%) were returned of which 45 (7%) reported they had been flooded and 44 (7%) reported no flooding.

2.1.2 Of the 45 responses which responded yes to flooding, 25 reported internal and external flooding and 20 reported external flooding only.

2.1.3 Whilst the flooding is impactful based on the results of the survey a relatively small number of properties of those adjacent to the East Brook Culvert have been affected although of those affected some have been affected multiple times.

2.1.4 It is possible that not all flood incidents have been recorded for a number of reasons including a change of homeowner, however, the results included within this report represent the entirety of the survey responses received.

2.1.5 A summary table of all responses is included in Appendix D, this has been anonymised to remove house numbers and contact details.

2.2 Where did the flooding occur?

2.2.1 Of those that responded yes, the flood incidents occurred on the streets set out in Table 2.1:

Table 2.1: Location of flood incident

2.2.2 For anonymity purposes we have not provided a map to show the location of all respondents, however, whilst there have been flood incidents along the length of the culvert’s route the majority fall into three broad locations.

• Location 1: Kipling Road / Browing Avenue
  Whilst also affecting other streets this area at the head of the culvert in the north of Kettering reported 17 flood incidents or 38% of the total returned responses.
• Location 2: Byron Road
  Slightly south of location 1 there is a small cluster of 4 properties which flooded
• Location 3: Waverley Road / Boddington Road
  Whilst also affecting other streets this area at the downstream end of the culvert reported 11 flood incidents or 24% of the total returned responses. Waverley Road has been subjected to a previous S19 investigation by North Northamptonshire Council

2.2.3 The three locations are indicated in Appendix D and Figure 2.1, Figure 2.2, and Figure 2.3. Figure 2.2 below.

Figure 2.1: Location 1

Figure 2.2: Location 2

Figure 2.3: Location 2

2.3 When did the flooding occur?

2.3.1 Not all respondents to the survey provided date information or dates for all of the floods, however, a summary of the date details with the number of properties affected during each date window is summarised below in Table

2.2. Due to the historical nature of this assessment, dates of flood incidents are summarised by month or year only. On some occasions more than one significant rainfall event has occurred in any one month, hence, there will be a small amount of double counting of properties affecting by a single rainfall
event in some time periods.

Table 2.2 Date and number of incidents

2.3.2 Based on the survey responses the majority of flood incidents affected only a single dwelling although the further back in time the less accuracy there will be in this data as people move houses etc.

2.3.3 During 2022 and the first 8 months of 2023 there were 63 flood incidents with two particularly significant events affecting multiple properties in August 2022 (17 reports) and June 2023 (31 reports). For ease of reference the responses which include reference to flooding in either of these events are summarised in Appendices E and F respectively.

2.4 What might have caused the flooding?

2.4.1 The purpose of this survey was not to try and understand the full mechanisms behind each flood or even how they flooded, however, the survey did include a question to ascertain what the residents’ thought was behind the flooding. The responses are summarised below in Table 2.3.

Table 2.3: Residents reasons for flooding

2.4.2 The main reason cited for the flooding by residents affected was a lack of drainage capacity / maintenance of the drainage system.

2.4.3 At this stage further investigations into drainage capacity have not been undertaken.

2.5 How deep was the flooding?

2.5.1 The survey asked residents to estimate the depth of flooding during the worst event both internally and externally. They were asked to provide an estimate based on one one of four depth bands. The results of the survey are summarised below in Table 2.4.

Table 2.4: Estimate of flooding depth

2.5.2 Internally typical flood water depths were typically up to 150mm whilst external flooding depths tended to range from 150mm to 600mm.

3.0 Other Reports of Flooding

3.1.1 In addition to the survey we have reviewed information available within the public domain and reported directly to the LLFA to identify any other locations of flooding. The following addresses also have historical reports of flooding.

• Bath Road
• Cedar Road
• Willow Road

5.0 Conclusion

5.1 A survey of 607 properties located along the route of the East Brook Culvert was completed in August 2023. Of these 45 responded to say that they had experienced historical flooding of their property. Many of those affected by the flooding have been flooded multiple times.

5.2 The route of the East Brook Culvert lies in a natural valley as such is the located in an area with a high risk of surface water flooding.

5.3 Properties along the route have experienced flooding, however, there are three broad clusters around Kipling Road / Browing Avenue, Byron Road, and Waverley Road / Boddington Road.

5.4 From the responses we received flooding seems to have always occurred in some locations along the route of the culvert although there were two significant events which occurred in August 2022 and June 2023 with 16 and 31 reports of flooding.

Appendix A - Route of East Brook Culvert

Extend of Survey - Page 1 of 4

Appendix B - Extend of Residents Survey

Extend of Survey - page 2 of 4

Extend of Survey - page 3 of 4

Extend of Survey - page 4 of 4

Appendix C - Residents questionnaire

1. Address
2. Name (not mandatory)
3. Contact details (not mandatory)
   • Phone
4. Contact details (not mandatory)
   • Email
5. Has your property flooded from external sources? i.e not because of an internal plumbing failure
   • Yes
     • If yes please complete the below questions
   • No
   • If no please return the form. No further questions need to be provided.
6. How many times has your property flooded internally?
7. How many times has your property only flooded externally?
8. Can you provide approximate dates for the flooding?
9. What do you think caused the flooding?
10. Where did your property flood from? i.e front or back, if different each time please provide details.
11. How much of your property was affected?
12. Did you have to leave the house for more than 24 hours due to the flood incident(s)?
13. In the worst event approximately how deep was the flooding externally?
    • 0-150mm (0-6 inches)
    • 150-300mm (6-12 inches)
    • 300-600mm (12-24 inches)
    • greater than 600mm
14. In the worst event approximately how deep was the flooding internally?
    • 0-150mm (0-6 inches)
    • 150-300mm (6-12 inches)
    • 300-600mm (12-24 inches)
    • greater than 600mm

Appendix D - Summary of all survey responses

Appendix E - Summary of all August 2022 survey responses

Appendix F - Summary of all June 2023 survey responses

1.0 Introduction

1.1.1 Martin Andrews Consulting Ltd (MAC) have been appointed to undertake this Section 19 Flood Investigation on behalf of North Northamptonshire Council (NNC) for a flood incident which occurred on Rockingham Paddocks, Kettering. The properties had flooded more than once in the specified period so warranted an investigation.

1.1.2 This investigation was undertaken following an incident which occurred on 23 October 2022, with the previous and subsequent events occurring in June 2021, 20 October 2022, June 2023 and September 2023.

1.2 This document comprises an anonymised summary of the full Section 19 Flood Investigation for the publication.

2.0 Description of Investigation Area Catchment

2.1.1 The description below describes the catchment as understood to be relevant to the investigation.

2.1.2 Rockingham Paddocks comprises a metalled surface serving a number of residential dwellings. The carriageway falls from east to west towards Rockingham Road. The properties which flooded are downstream and to the north of Rockingham Paddocks. They are served off a shared driveway area where the carriageway edge of Rockingham Paddocks is defined by dished and bullnosed kerbs with the aim of directing water away from the two properties.

2.1.3 Rockingham Paddocks has two road gullies upstream of the flood incident. Both were observed to be blocked to some degree.

2.1.4 Ground levels on the shared driveways fall towards the two properties.   

3.0 Description of Flood Event

3.1.1 The flooding is based on descriptions taken from those affected during our site visit. All depths mentioned are an approximation based on the descriptions provided.

3.1.2 Upstream of the affected properties a rainfall event exceeded the capacity of the highway drainage. During the 2023 site visit two road gullies were found to be blocked to some degree and in their current state of maintenance would not have been operating at their expected performance level. As a result, surface water flowed down Rockingham Paddocks bypassing the dished kerb and followed natural flow routes towards Property 1 and Property 2 rather than continuing down Rockingham Paddocks towards Rockingham Road.

3.1.3 At Property 1 water entered the small courtyard area which has two doors and watered entered the property via these openings. Water reached depths of around 125mm externally and caused internal flooding.

3.1.4 Property 2 has an external stepped access into a basement. Water flowed down the steps and into the basement. External flooding reached depths of around 300mm deep outside the below ground access into the basement and then enters.

3.1.5 Residents did not leave either dwelling.

3.1.6 A plan of the flood incident showing the approximate location of the event and flow routes is enclosed in Appendix A.

4.0 Suspected Causes of the Flooding Event

4.1.1 In any flood incident there can be numerous factors which could have resulted in the flooded witnessed. From the information we have received these are the key factors:

• Poorly maintained private drainage caused overland flows along Rockingham Paddocks and ultimately into both properties. 

5.0 Recommendations

5.1.1 The road gullies are cleaned of all silt. An inspection of the drain (sewer) which serves the highway drainage may be required to confirm it is not blocked and operating effectively.

1.0 Introduction

1.1.1 Martin Andrews Consulting Ltd (MAC) have been appointed to undertake a Section 19 Flood Investigation on behalf of North Northamptonshire council (NNC) for a flood incident which occurred on Benefield Road, Oundle. The property had flooded twice in 2 years so warranted an investigation.

1.1.2 This investigation was undertaken following an incident which occurred on 18th June 2023, with the previous event occurring in July 2020.

1.2 This document comprises an ammonised summary of the full Section 19 Flood Investigation for the publication.

2.0 Description of investigation area catchment

2.1.1 From visual observation and videos of flooding Benefield Road in front of the properties appears to be broadly flat but has a fall from west to east. On the southern side of Benefield Road there is a footway which at its maximum is approximately 500-750mm lower than the carriageway. Although the depth varies from west to east with the maximum depth broadly by the affected property(ies). South of the footway is a row of terrace houses which include the flooded property which itself is approximately 150mm lower than the footway.

2.1.2 To the north the footway is of a standard arrangement. North of this and opposite the property is the Drill Hall car park which is managed by Oundle Town council. The car park is relatively steep falling from north to south towards Benefield Road and also but to a lesser extent falls from east to west towards the access point. On its southern boundary the car park is accessed via a crossover with a stonewall along the rest of the boundary.

3.0 Description of flood event

3.1.1 This section is based on descriptions taken from those affected during our site visit. All depths mentioned are an approximation based on the descriptions provided.

3.1.2 A similar flood occurred in July 2020 with broadly identical mechanisms. The event in 2020 caused more extensive damage than the 2023 event.

3.1.3 A significant rainfall event occurred during both events. During the event, rainwater ponds on Benefield Road in front on the impacted dwellings. The ponded water then overtops the retaining wall upstand either due its depth or due to cars splashing the water over the upstand. This water then ponds on the lower footway entering the affected property through the door and wall. Additionally, water is also seen flowing from the west along the existing fall in the footway towards the dwelling.

3.1.4 There are road gullies on Benefield Road in this location, however, these were unable to cope with the volume of water. There is also a linear drainage channel on the lower footway, this was also unable to cope with the water during this incident.

3.1.5 Video evidence seen during the investigation provided by the residents shows that there is water flowing off the Drill Hall car park on to the highway. As the Drill Hall car park is served by isolated gullies rather than a linear drain it is possible that this is a normal occurrence during an extreme rainfall event. Based on our observation and knowledge of drainage design we would expect water to flow off the car park during significant rainfall events regardless of the maintenance schedule of the Drill Hall car park drainage.

3.1.6 Whilst it was not mentioned by the resident it is noted that rainwater pipes from the terraced houses discharge directly onto the lowered footway. The significant rainfall event would have led to additional volumes of water, compared to a typical event, draining onto the footway which may have exceeded the capacity of the linear drainage without the addition of any water overtopping the upstand of the retaining wall.

3.1.7 Flooding has occurred as the highway drainage was unable to cope with the rainfall which was generated during the event. Additional water from the Drill Hall car park has increased the volume of the water on the highway but based on the historical drainage design used on the car park this would be expected to occur.

3.1.8 Since the 2023 event, the residents have installed a flood barrier to both external doors and use sandbags to prevent water flowing along the footway.

3.1.9 A plan showing the approximate extent of flooding and flow routes is enclosed in Appendix A.

4.0 Suspected cause(s) of the flooding event

4.1.1 In any flood incident there can be numerous factors which could have resulted in the flooded witnessed. From the information we have received these are the key factors:

• Excess rainfall exceeded the capacity of the drainage infrastructure resulting in flooding.
• Possible blockage of surface water drainage– although with the exception of possibly one Drill Hall car park gully there were no obvious signs of blocked inlets during the visit. The highway drainage is also maintained annually.
• Drainage arrangement on Drill Hall car park allows water to flow onto the highway. This will have increased the volume of water on the highway and exacerbated the inundation of the highway drainage beyond its designed capacity. It should be noted that the car park will have existed for many years and the drainage arrangement has not been altered recently.
• Rainwater pipes from properties directly discharge onto the lowered footway. The rainfall event may have overloaded the linear drain without the addition of highway water.

5.0 Recommendations

5.1.1 Drill Hall car park and highway drainage should be inspected to ensure there are no blockages or damage and it is able to operate at its designed capacity.

5.1.2 Consideration should be given to whether the Drill Hall car park drainage could be improved to reduce the volume of water which is discharged onto the carriageway.

5.1.3 Highway drainage on the lower footway to be inspected to ensure it is operating as expected to ensure that it has as much capacity as possible for flows from the rain water pipes and the footway.