Mill Hill Circus Drainage

MQT on 2012-10-17
Session date: 
October 17, 2012
Question By: 
Andrew Dismore
Labour Group
Asked Of: 
The Mayor


Further to your response to Question No: 2604/ 2012 on Mill Hill Circus Drainage what representations in detail on what dates have TfL made to Thames Water to press them take action over the capacity of the Thames Water surface water drainage with what response and will you publish all correspondence and emails on the issue?


Answer for Mill Hill Circus Drainage

Answer for Mill Hill Circus Drainage

Answered By: 
The Mayor

There are two main causes of flooding at Mill Hill circus which impact on both Transport for London's road network and the surrounding area. There is a brook that flows through parkland belonging to London Borough of Barnet which discharges into a Thames Water culvert to the north of Mill Hill Circus. The brook has a grill over it that in times of heavy rain can block causing the brook to overflow across the park into the carriageway. TfL has engaged with officers of Barnet Council who now attend site every time there is significant rainfall to ensure the grill is clear and the brook is free flowing. The second cause of flooding is the Thames Water surface water sewer which appears to have a flow restriction around The Broadway area, to the south of Mill Hill Circus. At times of heavy rainfall the sewer surcharges back through the gullies around Mill Hill Circus and also back through the culvert causing the brook to the north of Mill Hill Circus to overflow.

TfL keeps its gullies clear and has also carried out some gully alterations from Apex Corner to Mill Hill Roundabout to reduce the flow from the highway gullies entering the Thames Water surface water sewer to help reduce the likelihood of the sewer surcharging. TfL also approached Thames Water last year to carryout a CCTV survey and cleaning of the culvert to improve its flow. This was carried out in June 2011 with TfL in attendance. Whilst cleansing is likely to have improved the sewer flow, the survey failed to identify the underlying cause of the problem.

Following further flooding in January 2012, TfL met with Thames Water and requested that they extend the extent of their surveys to include the area to the south of Mill Hill to determine whether there is a capacity constraint downstream. To TfL's knowledge this survey has not yet been undertaken but TfL has been assured that a survey is programmed.

TfL has considered whether any further action can be taken to improve its drainage system, but without resolution of the Thames Water capacity constraint, further modifications will not be effective. TfL now believes that unless Thames Water can identify any capacity constraints which can be remedied in the short term, the likely solution is for Thames Water to install a retention tank at Mill Hill. This would require significant investment by Thames Water and may take some years to design and implement such a solution. TfL's engagement with Thames Water and the London Borough of Barnet has mainly been verbal and through site meetings, but TfL has now written to Thames Water to urge that they take robust action to address the issue. TfL will also continue to attend site investigations with Thames Water and work with them to seek an operational solution.