7 minute read
Urban raingarden design
Diagram of a typical bioretention raingarden anatomy. © Images courtesy Robert Bray Associates
The concept of directing runoff from roofs and hard surfaces to sunken planted areas that can collect the runoff and allow it to soak into the ground has been around for millennia. It is now essential both as a drainage function and to passively irrigate trees and plants.
The modern concept of a raingarden was originally developed in Maryland, United States, in the 1990s as a simple means of managing relatively unpolluted runoff from roofs and domestic hard surfaces within property gardens, as part of wider neighbourhood-scale water management systems that typically included other more integrated approaches such as bioswales to manage more polluted highway runoff.
Since then, the City of Portland, Oregon, has pioneered the use of raingardens retrofitted into existing urban streetscapes, carefully adapted to deal with the increased pollution and silt loads associated with highway runoff. Typically referred to as ‘bioretention raingardens’ in the UK, Portland has installed over 3,600 around the city and estimates that it reduces annual runoff volume by around 80%, as well as delivering the water quality benefits that were the initial driver for the initiative.
Whilst the UK lags behind parts of the US in the uptake of integrated, nature-based rainwater management practices, there has been a recent surge of interest in installing raingardens adjacent to highways. Initially this was driven by councils wanting to explore more effective solutions to local flooding and river water quality with multiple benefits, often with co-funding from water companies, exploring their use to help reduce Combined Sewer Overflow (CSO) events. Councils are increasingly appreciative of the benefits that bioretention gardens offer in helping to tackle numerous aspects of climate resilience, the biodiversity crisis and health and wellbeing. They can also be deployed in new development where they form valuable ‘source control’ functionality – managing the most polluted smaller rainfall events close to where the rain lands, whilst protecting the remaining SuDS landscape from pollution and regular wetting.
So far, uptake of bioretention raingardens in the UK has been in a pattern of hotspots where there are proactive local champions, as well as an increasing interest in other areas.
Particular hotspots are:
– Sheffield, where the city council design team have implemented a number of transformative infrastructure projects that integrate rainwater management
– London Borough of Enfield, where the Watercourses Team has been responsible for scores of retrofit raingardens in residential streets delivering both urban greening and traffic management/active travel benefits
– Cardiff, where bioretention raingardens have been retrofitted into a range of street typologies
– London Borough of Hammersmith and Fulham, where the council highways team hope to ‘disconnect’ the entire White City estate from the main drainage system
– London Borough of Haringey, where several highway SuDS schemes have now been delivered by the council in a range of street typologies, increasingly including bioretention raingardens.
More recently the huge scale of the combined sewer overflow problem in the UK, as well as more awareness around highway pollutants such as microplastics, has finally reached the public consciousness and water companies are beginning to fund retrofit SuDS projects to very significant degrees with the blessing of industry regulator, Ofwat. This has resulted in proposals for thousands of new raingardens to be installed in coming years across the country. To meet this ambition and ensure that investment delivers benefits to local communities, all sectors involved in public realm delivery, particularly landscape architects, are going to have to quickly up-skill.
Unlike raingardens for roof runoff and non-highway hard surfaces –which are simple planted depressions in the landscape that should not normally require specialist highly draining soils, gravel drainage layers or ‘under-draining’ – bioretention raingardens must respond to the specific challenges of highway runoff: collecting runoff from the highway; and increased silt and pollution inputs.
Through layering, we can provide both pollution treatment functionality temporary storage of rainwater. With planted space for stilling water, alongside bioremediating soils and drainage layers in the base, raingardens can effectively provide the function of multiple SuDS features in one. This makes them particularly useful in constrained urban environments where space for multiple SuDS features in series is not available.
The requirement to manage higher levels of silt and pollution within a small footprint comes with compromises and challenges making the technical detail critical. We have seen many examples that will certainly fail to perform or dramatically underperform due to some basic design flaws. Some pointers to avoid these include:
– Create large and/or multiple inlets with a drop from the highway to allow intense runoff to reach the raingarden at the surface – rather than via underground pipes
– Be mindful of erosion protection and a silt collection area at inlet points to reduce the deleterious impacts of silt on the hydraulic conductivity (ability to percolate water) of soils
– Include a dropped soil level relative to the highway sufficient to collect intense runoff and spread it over the soil surface to allow enough time for it to filter through the soil layer
– Avoid long falls to the soil surface without baffles, which result in collected runoff flowing to the lowest point and overflowing before it has had time to pass through the soil layer
– Avoid a mismatch between soil type and planting typologies that results in poorly performing planting
– Be mindful of placing overflows too close to inlets or at too low a level resulting in rapid bypassing and insufficient pollution control
– Design out high rates of positive drainage to the base of the construction, which can result in suboptimal pollution treatment and/ or flood mitigation, or in some cases the bypassing of the raingarden entirely.
Bioretention raingardens (and arguably well-integrated nature-based SuDS as a whole) are still in their infancy in the UK and this presents some issues around their design, soil specifications, experienced construction, and funding and skills for maintenance. The UK is still experimenting and theorising with details and soil specifications. This has led to some confusion in the industry where individuals’ approaches and experiments in sand-, grit- and aggregate-based growing media and planting typologies have led to regional ‘standards’ being replicated locally without an understanding of what other options are available or more context-relevant: such as the Portland-proven standard of sandy loam topsoil with 30% organic matter capable of supporting conventional planting typologies.
The installation of raingardens, as well as other nature-based SuDS features, has the potential for significant benefits for people and the natural world. We would like to see landscape architects leading their roll-out to transform all our urban landscapes, so that we can move towards a future where working with nature on our doorsteps to manage the water that falls out of the sky is the norm.
Kevin Barton FLI is Managing Director at Robert Bray Associates and a passionate advocate for landscape-led, nature-based SuDS.
