We're working to reduce the risk of sewer flooding and pollution in Brent and Harrow.
What's the problem?
Sewer flooding and resulting restricted toilet use are the worst results of heavy rainfall and are often linked with surface water flooding.
Examples of what causes specific areas to flood would be that they are low-lying or, are close to a river. However the reason that flooding occurs right across the region is due to factors such as loss of river flood plains, high-density development, paving over of gardens and more besides.
Pollution in watercourses can be linked to property misconnections, blockages within the sewer network, and excessive inflow into the sewers during rainfall events.
So the causes of the flooding and pollution are multiple and they fall under the responsibilities of many organisations. Identifying ways to reduce the risk of flooding and pollution calls for a detailed understanding of all the root causes and all the potential solutions.
What we're doing
A partnership between LB Harrow, LB Brent, Thames Water and the Environment Agency has been carrying out an investigation into the root causes of flooding.
We are are jointly investigating the drainage networks in the area, to identify ways to reduce the risk of future flooding. Due to the scale and complexity of the area, the work has so far taken 2 years and will be completed in 2017. Once complete, the report will identify potential ways to reduce the impact of flooding across the region.
Work so far
We have reviewed previous studies, data capture and projects completed in this catchment, and have completed a plan going forward for the study.
We have collected customer questionnaires in historical flooding areas, to understand the root causes and frequency of flooding. This information will play a vital role to help build our business case for future funding to combat sewer flooding and loss of service.
We have developed a computerised hydraulic model of the foul, surface water and river network in the Brent and Harrow catchment. We are using this model to understand the root causes of the flooding in the area, and to analyse potential solutions.
We have completed impermeable area survey in the area, to understand any large hard-standing areas where rainwater flows directly to the foul system.
We have analysed the locations of historical pollution reports and created a prioritised plan for data capture in the catchment, to trace pollution and identify if sewage enters the watercourses. This work is ongoing and will continue through autumn 2016.
We have used the model to simulate the rainfall event on August 26th 2015 and June 8th 2016, and understand why flooding occurred. We have identified specific flooding clusters, which are linked by a common root cause. In December 2015, the partnership met to go through the model results and seek agreement on the source and pathways of flooding. The source of the flooding in the catchment is excessive surface water that overwhelms the capacity of the river, watercourses and sewerage during rainfall events. The pathway depends on the cluster. In some cases, further investigation is required before details of source and pathway can be confirmed. This further investigation includes survey, review of CCTV records, confirmation of the connectivity of certain areas, and flow monitoring. At the workshop we agreed on the required investigations and who would be responsible for each.
We are seeking to formalise the partnership between the different stakeholder organizations, and are creating a Memorandum of Understanding which will outline the objectives of the partnership and the responsibilities of each organisation.
In March 2016, the partnership met to discuss and agree possible solutions for each flooding cluster. These will be analysed using the hydraulic model, to determine the impact on flooding.
In May 2016, the partnership met to update progress on the modelling, and to agree a timeline for remaining work.
June 8 rainfall event
On June 8 2016, flooding occurred at several locations across Harrow and Brent following a period of intense rainfall.
We, together with the Environment Agency and the London Boroughs of Harrow and Brent, are using the hydraulic model to understand the root causes of the flooding that occurred, and will use this information to help in the development of potential solutions.
If you experienced flooding but have not let us know, please advise us using the links in the below ‘How you can help’ section. This will help us understand the locations and causes of the flooding. For any more immediate concerns, please see the numbers on the ‘Contact Us’ page.
We have planned an extensive program of data capture in the catchment, including property and asset surveys, and pollution tracing, to confirm how rainwater is entering the foul system and how sewage flow enters the watercourses. This data capture will be carried out in the second half of 2016.
The preferred solution options for each of the main flooding and pollution clusters in the catchment will then be confirmed. We will meet with the other catchment partners in October 2016 to discuss and agree the preferred solution strategy.
A Drainage Strategy Plan for the catchment, summarising the preferred solution options and overall catchment strategy is currently being developed. This will be made made publicly available on this website once all internal governance and QA checks are complete.
How you can help
We may wish to contact you to gather information about your experiences of flooding and would greatly appreciate your input to the investigation.
If you have been flooded by our public sewer or experienced restricted toilet use but have not let us know, please download and complete our sewer flooding questionnaire and return to us at the address on the form.