All three roads are shut on my way home from work. There are severe delays on the trains. My phone is bleeping with messages from rain gauges and river sensors across Enfield. After five months of little or no rain the sky has decided to wring out a storm.
The flooding is a stark contrast to the previous months of drought that saw our team nervously monitoring the drying watercourses. Our recent freak weather only amplifies the effects of the urban water cycle.
London Borough of Enfield, like most of the UK’s urban areas, has separate surface-water and foul-water sewer systems. When it rains all the runoff from the streets and rooftops drains into gullies and flows into the nearest watercourse.
Urban watercourses are disconnected from the natural water cycle and therefore are more vulnerable to pollution, flooding and drought. The rivers react quickly to rainfall: they flood when it rains and dry out when it hasn’t.
Sustainable drainage (SuDS) measures can address these impacts by mimicking the natural water cycle. They slow the flow, hold water, treat it and enable infiltration. Well-distributed SuDS, utilised across a whole catchment, can reduce flood risk, improve water quality, and top up the baseflow of rivers – a key part of integrated water management.
We know that SuDS can make our infrastructure more resilient to climate change. The question is why are they not yet a common sight on our streets, in our parks and for our buildings? We can’t reverse the impact of the traditional drainage system by taking a piecemeal approach to delivering SuDS. So how can councils deliver SuDS on a meaningful scale?
Learning from nature
The best way to mimic nature is to use nature. Our preferred SuDS measures lean towards rain gardens, tree pits, swales, wetlands, and ponds. SuDS are still widely considered to be features that solely reduce flood risk – it’s why most of the funding for these features comes from Lead Local Flood Authorities (LLFAs), the Environment Agency and, increasingly, the water companies.
Historically there has been more focus on delivering SuDS in areas that are subject to flood risk – surface water or fluvial – than on considering the catchment as a whole. However, the London Strategic SuDS Pilot demonstrates that distributed SuDS across a catchment can shift the hydrological characteristics back towards a natural state, delivering flood benefits locally and further downstream.
The study also concluded that the additional benefits of green infrastructure SuDS, such as rain gardens, go far beyond managing flood risk – improving water quality, biodiversity and air quality.
One of the key benefits of rain gardens was found to be traffic calming and improving people’s health – mental as well as physical. The quality of our public spaces can determine whether people take up active travel. Understanding and accounting for these benefits may help improve funding opportunities for SuDS.
In Enfield, we are opportunistic about delivering rain gardens. Collaborative working – whether through different teams within the council or with charities and even utility companies that are digging up roads, parks or gardens – can highlight opportunities to deliver SuDS. What if we replace that bit of tarmac with a rain garden when you’ve dug it up.
There are many missed opportunities to integrate good SuDS measures in schemes to improve public spaces, especially where we encounter new raised planters and mounded planting next to roads. There are some incredibly obvious and simple ways that these areas could have been rain gardens.
A large proportion of the rain gardens delivered in the public realm are through active travel or greener travel projects with our Highways and Traffic & Transport colleagues.
Rain gardens have been used as green buildouts, installed as horizontal traffic-calming in designated Quieter Neighbourhoods and School Streets. They can help separate cycleways from the carriageway and mark safer crossing points outside shops.
Collaborative working isn’t easy. The principle of rain gardens is. They can be flexible and diverse –planted holes in the ground that draw in runoff from adjacent hardstanding areas.
On a larger scale, Enfield Council has delivered several successful constructed wetlands within urban parks. These are large SuDS features that capture runoff, usually from a diverted surface-water sewer or underground watercourses, treat it naturally using wetland vegetation such as reeds, and slowly release the runoff to the nearest watercourse.
Our wetlands and fens are declining fast. Constructed wetlands are little pieces of nature that can transform a park into more interesting and valuable spaces for people and wildlife.
Of course, delivering green SuDS is not without its challenges. Difficulties retrofitting rain gardens can discourage authorities from installing them. But we can easily overcome many of the challenges:
Quite often we are not able to design these features to manage large storm events (such as the 1 in 100 year event), but SuDS can still add capacity and resilience to the sewer system, if only to manage small, day-to-day storms.
An opportunity to install a rain garden is an opportunity to realise multiple benefits – every bit of green space matters. Existing road buildouts and large, dead spaces within streets can be perfect places to retrofit a rain garden.
Our roads can conceal a chaotic network of utilities. It is rare to not encounter utilities when you excavate a rain garden in the street. If the utilities are well protected, sit within the gravel layer and the rain garden is well drained it should not cause problems to have utilities sit within a rain garden.
Sometimes the ground is incredibly busy with multiple utilities. Here, we can adjust the designs and excavate the rain garden deeper where there are fewer or no utilities and shallower where the space is occupied. This avoids compromising rain garden capacity.
Laying the topsoil and planting a rain garden is critical to its functionality. Freeboard is the depth between the hardstanding area and the top of the topsoil layer. This provides storage space for water as it slowly drains through the rain garden topsoil and gravel layers.
If the topsoil is too high, it compromises the storage capacity of the rain garden and ultimately rainwater will not be able to flow into the rain garden. To account for the silting up that affects rain gardens over the years, we ensure that the freeboard is at least 200mm deep. This often needs close site supervision as landscapers’ old habits of mounding topsoil die hard.
Rain gardens are so well drained that they are dry most of the time. They need hardy drought-tolerant plants, not water-loving plants. The depth of the gravel and sandy topsoil layers can be altered to suit different planting schemes, including trees, shrubs, and wildflowers.
What this shows us is that green SuDS measures are valuable and versatile. There are many inspiring SuDS projects across the UK. For now, we need to jump over the hurdle of delivering SuDS as a habit, a requirement for all public realm projects.
Enfield Council has delivered more than 200 rain gardens and 9 wetland schemes. We plan to keep going.
The team return to some of the flooding hotspots after the rain, looking for evidence of ponding. The rain gardens have sucked in all the water.
A highway maintenance officer calls me up. “There’s always flooding on this road. Shall we put a rain garden here?”
We’ll go to have a look, but the likely answer is yes.
Jamie Kukadia is a senior engineer with Enfield Borough Council’s watercourses team
This story was first published in The Environment magazine in Dec 22/Jan23
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