Flood investigation reports

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.