Flood investigation reports

1.0 Introduction 

1.1 David Smith Associates (DSA) have been appointed to undertake this Section 19 Flood Investigation Report on behalf of the former Northamptonshire County Council (NCC). Internal flooding has been reported at a residential property on more than one occasion in the last five years, warranting this investigation.

1.2 This investigation was undertaken following the flood incident at Manor Court, Brackley that occurred on Tuesday 1st October 2019.

1.3 This document comprises and anonymised summary of the full Section 19 Flood investigation Report for the publication.

2.0 Description of catchment area 

2.1 Manor Court is located in the centre of Brackley, Northamptonshire. It is a small cul-de-sac serviced by an adopted road.

2.2 The surface water catchment area to the flood incident extends to a high point approximately 300m to the west at Antelope Hill. The majority of surface water flow from high ground is intercepted by Manor Road and flows north. Therefore, the contributing area to the affected property is localised to Manor Court itself.

2.3 The catchment is urban in nature, however approximately 50% of the area comprises grass playing fields.

3.0 Description of the Flood Event 

3.1 Rainfall 

3.1.1 Rainfall during September 2019 exceeded the national average, which would provide conditions for saturated ground to be present.

3.1.2 There was significant rainfall in the two days prior to the flood incident. This provided conditions for saturated ground to be present.  

3.1.3 This was then followed by a reasonably heavy rainfall event that led to the flooding.

3.2 Surface Water Flow 

3.2.1 The reported flow of surface water appears to follow paths through the catchment.

3.2.2 The water flowed east down Manor Court, collecting, and puddling in the carriageway at the front of the affected property.

3.2.3 This water deepened then flowed over the top of the kerb edge and down the driveway towards the garage threshold. Here it increased in depth through the garage causing internal flooding.  

3.2.4 Surface water flow was described as fast flowing along the road and deepening around low points adjacent to the property.  

3.2.5 The flow of surface water was beyond the capacity of road gullies, and the ultimate outfall for surface water was ineffective leading to the build-up of surface water at the property.  

3.2.6 Runoff from private roofs and driveways contributed to the flow of surface water at the affected property.

3.3 Standing Water 

3.3.1 Flood water in the garage of the property reached a depth of approximately 200mm.

4.0 Cause of Flooding  

4.1 In any flood incident there can be multiple factors which result in the flooding witnessed. From the information received, the key factors are:

• Surface water followed the local topography of the carriageway overtopping into the private driveway of the affected property. The driveway to the adjoining neighbouring property, has a raised threshold which prevents the Highway drainage from entering this property.  
• In previous occasions the surface water that flowed into the driveway of the affected property also overflowed to the adjoining neighbour’s property. However, the neighbour has subsequently constructed a brick wall along the boundary line that now prevents this and directs the flow wholly towards the garage threshold of the affected property.  
• There is no overland flood flow route away from the properties. 
• The driveway of Property 1 is wholly covered with impermeable blockwork with a fall back to the property. The only drainage evident is the channel drain across the garage threshold.  

5.0 Recommendations 

5.1 The flooding to Manor Court was caused by above average rainfall during previous days and weeks, and heavy rainfall on the day of the flood incident. Drainage systems did not have capacity to collect and dispose of surface water effectively. There is no overland flood flow route to direct surface water away from vulnerable areas.

5.2 To prevent the same levels of detriment in the future, the following should be undertaken:

• LLFA to continue work with the community and Flood Risk Management Authorities to: 
  • Manage surface water to provide a better standard of protection to Highway infrastructure, and the community. 
  • Identify further surveys, investigations and studies required to locate and record existing drainage systems. 
  • Identify all legal responsibilities for drainage and watercourse maintenance, reminding relevant parties of these responsibilities.  
  • Assess the requirement and viability of engineering schemes to ensure the existing infrastructure operates as intended, and to provide a better standard of protection if required.  
• Increase their preparedness to flood events through implementing: 
  • Emergency Plans 
  • Business Community Plans 
  • Property Level Resilience 

1.0 Introduction 

1.1 David Smith Associates (DSA) have been appointed to undertake this Section 19 Flood Investigation Report on behalf of the former Northamptonshire County Council (NCC). Internal flooding has been reported at a residential property on more than one occasion in the last five years, warranting this investigation.

1.2 This investigation was undertaken following the flood incident at Francis Row, Upper Stowe that occurred on Tuesday 1st October 2019.

1.3 This document comprises and anonymised summary of the full Section 19 Flood investigation Report for the publication.

2.0 Description of catchment area 

2.1 Upper Stowe is located in the southwest of the county, approximately 11km west of Northampton. Francis Row is located at the southwest of the village. This is a narrow road leading to residential properties, including converted farm buildings.

2.2 To the north and east are residential areas of the village, and to the south and west are agricultural fields. The land falls from north to south, with a high point midway through the village.

2.3 The surface water catchment area to the flood incident extends approximately 70 metres to the north, east and west. This is within the catchment of the River Nene.

2.4 The catchment comprises the paved asphalt road of Francis Row, paved private driveways, and a relatively small area of agricultural land.

3.0 Description of the Flood Event 

3.1 Rainfall 

3.1.1 Rainfall during September 2019 exceeded the national average, resulting in the ground being saturated.  

3.1.2 Rainfall was particularly heavy between 11:00 to 20:00 on the 1st October 2019. Over 30 mm of rain fell in this period which is considered significant.

3.2 Surface Water Flow 

3.2.1 As rainfall fell, surface water flowed to the southwest bend of Francis Row from the north, east and west. This was a combined flow from the carriageway of Francis Row, roofs and driveways of residential property, and a relatively small area of agricultural land.

3.2.2 It was reported that surface water on Francis Row bypassed the offset road gully on the west of the road, and therefore all water flowed to the low point at the southwest bend.

3.2.3 Drainage systems at the low point could not drain the volume and intensity of rainwater. This increased in depth until it overtopped the edge of the highway and flowed into the driveway of the affected property.

4.0 Cause of Flooding  

4.1 In any flood incident there can be multiple factors which result in the flooding witnessed. From the information received, the key factors are:

• Blockage or breakage of the outlet pipe from road gullies 
• The ground levels of the driveway to the affected property are marginally lower than ground levels to adjoining land.

5.0 Recommendations 

5.1 The flooding at Francis Row was caused by above average rainfall during previous days and weeks, and heavy rainfall on the day of the flood incident. Drainage systems did not have capacity to collect and dispose of surface water effectively. There is no overland flood flow route to direct surface water away from vulnerable areas.

5.2 To prevent the same levels of detriment in the future, the following should be undertaken:

• LLFA to continue work with the community and Flood Risk Management Authorities to: 
  • Manage surface water to provide a better standard of protection to Highway infrastructure, and the community. 
  • Identify further surveys, investigations and studies required to locate and record existing drainage systems. 
  • Identify all legal responsibilities for drainage and watercourse maintenance, reminding relevant parties of these responsibilities.  
  • Assess the requirement and viability of engineering schemes to ensure the existing infrastructure operates as intended, and to provide a better standard of protection if required.  
• Increase their preparedness to flood events through implementing: 
  • Emergency Plans 
  • Business Community Plans 
  • Property Level Resilience