In the southeast corner of the London Borough of Waltham Forest, a little project is nearing completion which is designed to alleviate a local flooding problem.
Esther Road is an unremarkable street in the north of Leytonstone, a community which was severed by the A12 in the 1990s as part of the M11 Link Road project, one of the last large road schemes in London of that era and which was bitterly protested against.
What makes the street more interesting than the others in the area is some sections of the car parking on both sides have been repurposed for surface water management using a Sustainable Drainage Systems (SuDS) feature commonly known as "rain gardens" (above).
The project covers three streets in the area which were hit by floods in 2021 and which are in a bit of a valley circled in red above - taken from the London Topographic Map. It's a problem which is only going to get worse with climate change and the project demonstrates that we're going to need lots of these little localised interventions in order to adapt to the warmer wetter conditions the UK faces.
In terms of design, the layout is pretty common whereby the rain garden areas are bounded by kerbs to retain the adjacent carriageway and there are regular slots to take the flows from it. In theory, the kerb could be flush with the road surface, but usual practice is to keep an upstand to dissuade drivers and to provide a detectable edge should someone walking require it (above).
A closer look along the rain garden shows the existing kerbline (left) and new kerbline (right) supported with concrete backing, and what is nice here is the edge of this concrete has been neatly formed and faced with stones set into it. These stones not only look nicer than formed concrete, they'll also do a little bit to dissipate the energy of water entering the rain garden.
Elsewhere, we can see chamber covers set into the rain gardens and curious red domes (above). I'm not entirely sure how this all fits together for this project, but the covers will provide maintenance access to chambers which provide the ultimate connection to the sewer network.
The red domes are overflows (above) with a pipe extending downwards. Designs vary, but the general arrangement of a rain garden like this will be that water enters at the surface and soaks into the soil and then down into a stone layer and the ground itself.
The stone layer is specially graded to provide 30% voids and so provides storage for flood water. If the inundation exceeds the ability of the surrounding soils to absorb, then the stone layer fills with water, then the soil layer fills and then it finally overflows to the sewer. The soil layer is also specially selected for the job to be free-draining, and in fact, rain gardens need plants which can cope with long dry spells!
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| © City Infinity |
In ground where water infiltration is likely to be very slow, the bottom of the stone layer will have a pipe (called an underdrain) with lots of little holes to allow water to percolate into and drain away after the storm event, otherwise the stone layer will stay saturated and so be useless if another storm event occurs in a short time. Above is a typical detail of how that would look in long section - there will also be a layer of geotextile around the stone layer (a cloth-like material) which stops fine particles getting in and clogging the voids needed for water storage.
The project isn't just about the rain gardens, it includes a trial of rainwater planters for households as a way of keeping even more water away from the sewers, but there is also something in plain sight that unless you knew about it, you might miss. Have a look at this little video.
Yes, the water is soaking into the asphalt surface! The product is "SuperDrain" and is designed to allow water to drain through; and like the rain gardens, the substructure of the road has been rebuilt with stone layers designed to retain flood water with outfall structures designed to slowly release the water into the sewer system - I don't know the exact details, but I assume there is an underdrain of some sort.
The porous carriageway is an expensive thing to build, but it could be used for new-build developments. For Esther Road, it has been used to deal with an existing problem and in an urban area, we don't have the space to build ponds and other land-hungry solutions so we need to use existing streets. That to one side, rain gardens are a cost-effective measure which can be built all over and it is this kind of diffuse solution that can provide resilience if deployed routinely.
For more technical information on rain garden design, it is well worth looking at Designing Rain Gardens: A Practical Guide from Urban Design London, although if used outside of London, you need to use appropriate local storm intensity information - one for designers!
Also great for replenishing ground water and preventing subsidence in clay soil areas, soakaways also prevent runoff entering rivers, and sewers are spared some of the storm flow that results in sewage discharge into rivers.
ReplyDeleteSuper draining asphalt (or at least ZOAB, Zeer Open Asfalt Beton, litterally very open asphalt concrete, which looks and works the same) is commonly used in the Netherlands because of the much improved drainage making streets and highways safer during downpours, but also because it really helps to reduce the noise from motor-vehicle tires.
ReplyDeleteWith better drainage, improved braking & better sight during heavy rain, and less noise pollution, it's proven to be worth the cost of material and earlier replacement on near-urban highways and streets.
Hanneke
Indeed - the UK imported it from France originally for it's quieter properties and the reduction of spray from rain, but I'm not sure how widespread it is as it is more costly. It seems to me using it in smaller areas as a drainage approach might be more useful.
DeleteI think that's used on bits that flood easily,
DeletePeople probably drive faster !
Interesting piece - I was not familiar with the "overflow" aspects of this design, though I am very familiar with drainage swales and outflow control; a new 110 dwelling estate I was involved with required a buffer swale covering half an acre with an intelligent outflow.
ReplyDeleteIt's worth a note that these essentially constitute thousands of large ponds / bog gardens across the country that do not seem to be included in the count of new ponds required for nature habitats - just in my immediate are I have scores of them.
I'm comparing the "underdrain" with "French drains" that I use in conjunction with domestic soakaways. I have one question - in the design above how is the graded stone with 30% voids prevented from becoming clogged up with soil over time, which would both reduce capacity and slow down drainage over the design?
(In my simple soakaways I line the pit with weed membrane to catch the soil dust before it gets into the clean stone gravel.)
The key to it is having geotextile filter membranes surrounding the stone layer to stop fines from the surrounding soil as well as from the planting media on top - the planting media is a ratio of approximately 50% sand, 30% topsoil and 20% compost so the sand helps filter too - eventually the very top will get clogged with dust/ fines, but there needs to be vegetation removal and tidy up anyway and the cycle for this is many years - it's good low-tech maintenance!
DeleteThe underdrain is essentially the same as a french drain really and you'd usually wrap the stone in geotextile too, although with the top being open - I've certainly seen the stop of a french drain capped with a permeable layer though to try and keep the weeds out.