I Come From A Land Down Under

Sustainable Urban Drainage Systems (SuDS) is an area of civil engineering I've only had a peripheral involvement with, but when putting together my programme of Continuing Professional Development this year, it was something I wanted to learn more about.

SuDS is a branch of drainage engineering which has been around for many years and essentially looks at managing storm water flows from development by slowing the rate at which water is released into the ground, sewers and watercourses. SuDS tools don't just apply to urban areas, it's just they are generally associated with urban development. The idea is to prevent flash flooding and to reduce the burden downstream of any given point. From what I know of the subject (and I am happy to be put right), the best systems are those generally dealing with water where it falls, rather than having to deal with it further downstream.

So, at the small (and important) end of the scale, we have "source control" which is dealing with storm water flows at source. At its most basic, we can let the water run into the ground to soak away (infiltrate), although once the underlying soil becomes waterlogged, then water will run on the surface. We can use "green roofs" which have plants which use the water falling onto them (thus water is used for growing and the transpiration process) and the growing media to slow the flows down. We can also store water in tanks during times of rain with the water being let out during dry periods. Gardeners are familiar with this in the form of the good old fashioned water butt. 

The photo above is from my garden which features a 1000 litre tank repurposed from palletised fruit juice concentrate tank. Very simply, the gutter down pipe from the shed goes into the tank. Over the winter (or in summer storms) the tank fills up and the water is used to the garden or to top up the pond during a dry spell. I put the tank in to compensate for the paved area on which the shed sits. There is a second down pipe which current runs into the ground where the water soaks away.

We can replicate this kind of low-tech (and therefore resilient) technology to downpipes around a house and in some developments, rainwater is harvested to flush toilets and to provide water for washing machines. At a larger scale, we can consider how we manage surface water runoff from our streets and this brings me to the main subject of this post (but I will return to this fascinating and practical subject again).

A few weeks ago, I went on a site visit with Urban Design London to have a look at some SuDS schemes in the flesh, but I want to concentrate on one scheme at Australia Road which is in the White City area of West London. I want to talk about this scheme in particular as it has outcomes beyond surface water management. First, have a look at what the site used to be like, courtesy of Google Streetview;

It's a typical city street with parking and traffic calming, but on the south side, there is an early years centre and on the north side, there is a school and playgrounds. The wider estate has modal filters (the area is between the A40 Westway and A219 Wood Lane) but on the visit we were told that the school drop offs were busy and cars used to take over the space. 

The area is at risk from surface water and sewer flooding and so the London Borough of Hammersmith and Fulham are proactively looking to find locations for potential SuDS interventions with Australia Road being a fairly large intervention. At a cost of some £900k, it's a very expensive scheme (it works out at £11.3m per mile, about the same as widening a motorway by a single lane per mile) and this is down to having to move buried utilities as much as it did building the scheme. It is entirely possible to retrofit existing streets far more cheaply and the council does have other, more modest projects underway. So, let's look at what has been done.

The new space has been named "Bridget Joyce Square" after a local children's worker with the Council and it is very clear on entering the space that this is not the domain of the driver. There are signs up which state "no motor vehicles, except access" and those needing vehicular access are very few. This section of the street is only open to motor vehicles at its western end. This is essentially a pedestrianisation scheme into which there is access of off street parking for a few cars and one is allowed to cycle through.

The western half of the scheme is dominated by a large retention basin, that is an area designed to flood, but to let water our slowly. The lush planting is just over a year old and as well as utilising water for growth, the leaves from the plants will transpire water into the atmosphere.

The paving around the square is permeable concrete block paving. The uneven edges creates a gaps around each unit which are filled with grit to give pathways into the underlying bedding material which is a similar grit and all generally a single size. Normally, permeable paving is designed to allow water through into the ground below, but at Australia Road, the underlying material is thick reinforced concrete which would have been too costly to remove. Instead, the water seeps towards and into the retention basins (there are several of differing sizes).

The retention basis is divided into cells which take time to fill up before emptying into the next one. The central wall provides the cells and something for kids to walk along. In fact, the wall also replaces an old low wall along the edge of the street which generations of kids walked along and the local community wanted to capture it in the new scheme.

This shows a slot between cells.

The main detention basin has a timber deck across it's middle to provide permeability through the space and further interest.

At one end of the retention basin, there is an overflow to the storm water sewer, but the size of the basin means that this is very rarely used.

A view to the western end of the square. The building on the right is the early years centre and it's entrance has been incorporated into the space.

A smaller basin with a transition from a paved "wall" route turning into an actual wall in the basin. As well as the paved area, roof drainage from nearby buildings has been disconnected from the local drainage system and diverted into the basin and smaller "rain gardens".

Another view of the smaller basin. The granite "plinth" provides a hard edge to the basin and somewhere to sit; but it was quite expensive!

The western approach to the square is gated and only really for emergency and maintenance access. It's a let down from a cycling and accessibility point of view, the gate needs to be replaced by removable bollards.

Australia Road is an expensive scheme to be sure, but it admirably showcases some SuDS concepts. Just as importantly, it has created a new public space and brought the uses on both sides of the street together. It has been referred to as a "shared space" by some, but I challenge that (it's a divisive and misleading term in my view). In reality, it's a scheme which puts the pedestrian first, provides a through route for cycling (notwithstanding the gate issue) and permits vehicle access to those who need it; and of course, it's wonderful community space which builds in resilience to the local surface water drainage system.