Integrated action is the solution

Repeated flooding in North Brent prompted Defra to step in and include the London suburb in a pilot study on integrated urban drainage. MWH's principal engineer, Chris Digman, and senior engineer Tony Bamford reveal what they learnt from the findings about the scourge of urban flooding

Urban flooding, which can be caused by a wide range of factors, is becoming a real challenge for the bodies responsible for picking up the pieces.

Sewer flooding, stemming from a lack of system capacity, is probably the most common cause of urban flooding, and water companies do have programmes in place to reduce the risk of property flooding from this source.

However, when other sources of flooding – including groundwater, private drainage, watercourses, river, sea and overland flow – come into play they create particular challenges both for those who are affected by, and those responsible for tackling, flooding.

Those property owners affected can often be at a loss as to who to turn to for help because of the large number of bodies responsible for managing the risk of urban flooding.

At the same time, those who may be responsible may not fully understand the problem. A further difficulty may be lack of funds.

These challenges have been recognised by Defra – 15 integrated urban drainage pilot studies were set up in 2006. These pilots aim to provide guidance on how flood risk from more than one cause could be better managed in urban areas, and how different bodies can work together.

One of the pilots commissioned by Defra is based in North Brent, north-west London. Thames Water, the lead stakeholder for this project, employed MWH to investigate because of its specialist capability in the environmental and water sectors and, in particular,its flood experience.

North Brent has a history of flooding, so it was unsurprising that floods happened during the summer storms of 2007. The catchment has developed substantially over the years, but the capacities of the drainage systems have not kept pace.

A particular feature of the area is the extent to which individual properties have been developed with elements like extensions and patios. Paving over front gardens, in particular, has added to the flow of water entering the sewer system. In an attempt to relieve the surface water sewer system, cross-connections to the foul system have been made over the years. This now also floods during heavy rain.

A key element enabling the Defra pilot to take place was the formation, in 2003, of the Brent North Flood Working Group. This brought the key stakeholders together.

Previous approaches had relied on investigating the performance of the different drainage systems individually. While such an approach avoids complexity, it can severely underestimate flood risk where there is strong interconnectivity between the different drainage systems.

To gain a complete understanding of the flood risk in North Brent, a fully integrated model of the watercourse, sewers and surface flood pathways was needed. Integrated modelling meant that:

  • The complex interactions between the different drainage systems were fully replicated
  • Locations where one drainage system adversely affected the performance of another were correctly identified
  • The location of different sources of flooding were presented together, making it easy to view and analyse
  • The conveyance capacity of the different drainage systems was quantified
  • Overland flow routing from all sources of flooding could be accurately modelled
  • Integrated solutions could be tested and a future strategy for flood risk management readily developed

The results of the integrated modelling provided a good match between the predicted and reported flood events.

The flooding in 2007 showed substantial overground flood flow in parts of North Brent, and a good agreement with observed data was achieved relatively quickly.

Much of the flooding occurred from overloading of the surface water sewerage system. However, this, in part, was due to the high water levels in the watercourse.

The modelling showed that simply upsizing the surface water sewerage system, without improving conditions in the watercourse, would not deliver the necessary flood protection. Historic cross-connections between the surface water and foul-sewer systems further complicated the situation.

The modelling showed that, if these cross-connections were removed, then there was adequate capacity in the foul-sewer system. However, removal without other improvements elsewhere would further exacerbate flooding from the surface-water system.

In common with most urban areas when surface flooding occurs, the resulting overland flood flow is not actively managed and this results in the indiscriminate flooding of property. Being able to accurately model such flood flow opens the door for providing measures to make urban communities resilient to such flooding in extreme events.

The integrated model enables the responsible stakeholders to be identified, and provides the opportunity for a joint approach to developing a solution. Importantly, the integrated approach enables the appropriate contributions from each stakeholder to be identified for such a joint solution.

Integrated modelling has shown that, overall, there is a lack of drainage capacity in North Brent that arises from progressive urbanisation of the area.

An integrated approach to solution development is therefore being moved ahead by the stakeholder group.

As the North Brent area is towards the top of the catchment, any increased conveyance capacity is likely to lead to additional flooding downstream. An alternative option of storage attenuation would require large volumes of storage to be provided.

The North Brent Pilot IUD Project clearly demonstrates why we need an integrated approach to urban drainage across the country. Flooding is complex and responsibilities are spread across a wide range of stakeholders. The public needs flooding solutions that are robust, environmentally sensitive, and affordable.

Predictions for climate change indicate rainfall events will become even more intense. This means we are likely to experience an increased frequency of flooding from a variety of sources with increasingly extreme results.

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