September 30, 2015

Helping us manage water and heat in towns and cities: A vital role for Green Space

Water shortages and floods as well as heat build up are becoming more common; partly because of climate change that is resulting in more extreme weather patterns but also, because of the way we use and manage the land. As hedges, woodlands and natural wetlands have been removed the countryside has become far less absorbent. (In England alone, 90% of wetlands have been lost since the industrial revolution).   In urban areas, something similar has happened, the buildings and paving over of land has meant that water cannot soak into the ground like it once would have done. I don’t know about you, but the scale of effect of the sun and the rain always strikes me in our cities. For me, that city is Manchester and my thoughts of city centre streets are either of  the water running off the pavements and roads like a river, or the searing heat coming off the sides of buildings. This isn’t surprising! Car parks, roofs and roads all serve to speed up the rate of rainwater run-off and increase the risk of flash flooding as well as heating up the environment, and there isn’t enough green to address these impacts.  By greening and making the area more attractive, there are also huge economic and place-making values as well as benefits to the environment and to the people, so it seems to me that we are missing a trick!

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Public and private green space and its role

For a long time now, environmentalists including landscape architects, have been making a robust, logical case for more water-retentive towns, cities and countryside as a way of managing water more smartly. Given predictions of climate change delivering even more extreme weather in the future, these arguments for more intelligent land management are clear. Sustainable Drainage systems (SUDS) are now widely accepted as the way forward with the Flood and Water Management Act 2010 hopefully encouraging SUDs as an essential underpinning element of all developments rather than a token afterthought.  The best types of SUDS schemes double up to be not only good for water management but also for wildlife through the creation of habitats and increase in biodiversity. In the countryside, peat bogs, heather moorland, broad-leafed woodland, meadows and reed beds can serve as natural sponges whilst in urban areas, public green space and gardens offer the same water and heat absorbing opportunity, with well designed SUDs landscapes offering a range of opportunities that are good for water management, wildlife and people.

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Here are my 10 of the best of these water management approaches: different ones are suited to different circumstances and requirements:

  1. Rain gardens: These are shallow depressions that are planted such that rainwater run off is slowed. Rainwater can be received from down pipes or paved areas ( not car parks). So-called rain gardens are planted with a variety of species able to tolerate short periods of inundation with water  (including shrubs like Cornus, perennials like Christmas rose and some ferns) and are able to reduce flooding, provide clean water and mitigate the urban head island effect. Rain gardens can be integrated with adjacent hard and permeable surfaces and are suitable for private domestic gardens, e.g. front gardens as well as in public spaces. If water overflows, then excess water needs to flow to an existing drain or another rain garden. Larger gardens are better than smaller ones and soil needs to be permeable.

 

  1.  Bioretention areas: These are shallow depressions of landscaped area similar to rain gardens but these bioremediate polluted run-off from roads and car parks. They can be formally landscaped with shrubs and herbaceous plants with nectar for insects and are under-drained. They are ideally suited to high-density housing, commercial and retail areas and reduce run-off and localized flooding. In addition, they also enhance urban areas that might otherwise be devoid of green space; creating elements of green-infrastructure that can aid with reduction of the urban heat island effect as well as intercepting and filtering pollutants. 2015-09-29_0004
  2. Green Roofs: These include both so-called intensive garden roofs (see some of our projects here and blogs here) with a mix of native and non-natives, ‘gardens in the sky’ for people to use require more maintenance such as pruning and cutting of trees and shrubs as well as management of flowers. Extensive green roofs are generally not for main access and include grasses, wildflowers and sedums. Both these types of roofs can slow and reduce water run off, particularly during heavy storms. Other benefits include supply of filtered water for wildlife, reduction in the urban head island effect through evaporative cooling and providing for leisure (intensive) in high-density urban areas. Of course, as shown, below, there are several creative ways of using these approaches, for example for sheds, bin stores and other structures.
  1. Green living walls: These can take several forms including climbers on trained wires or trellis as well as planters with soil, hydroponic modules or textile blankets. These need to go hand-in hand with rainwater harvesting (see no. 10 below) (or grey water which requires special planting). Benefits include reduction in heat within the local environment and noise buffering, both very useful in urban areas! Planting needs to be adjusted to suit aspect and can include features such as nest boxes and insect hotels.

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  1. Filter strips: These are vegetated, usually grassy areas of broad, flat and gently sloping land, 1-2 m plus wide and can be used anywhere except over aquifers; they act to filter rainfall producing run-off into a swale or other suds feature. They can be planted with native plants to create wildflower meadows and should be located alongside adjacent hard and impermeable and permeable surfaces. Filter strip creation can be an opportunity to create wildflower grasslands with facilities for people to enjoy wildlife e.g. with tables, benches, etc.

 

  1. Permeable surfaces: These include surfaces such as block pavers which allows water to drain through vertical holes or gaps between units, although there isn’t much selection at present. They can also be concrete or recycled plastic cellular blocks with space for soil and vegetation e.g. Grasscrete. Gravel in the form of bound gravel or loose but packed into honeycomb structures also enables run-off to percolate naturally into the ground or collection chamber. These surfaces are also a first line of defense against pollution because the pollutants can be retained in the subsurface matrix. Although they don’t provide the same wildlife benefits, gravel can be planted with nectar rich plants, tolerant of drought, food and vehicle damage such as thyme and chamomile.  The image on the right below, shows how hard impermeable surface can be limited to the area where the tyres of the cars  go, leaving permeable and planted areas to aid water percolation and provide aesthetic and wildlife value.

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  1. Soakaways: Soakaways are one type of infiltration device that offers a simple way of dispersing surface and storm water and are one of the key technologies for SUDS. They enable storm water to be dealt with at source rather than being diverted into over-burdened sewer systems. However, there are limitations, soakaways rarely work on heavy clay soils . The basic principle is that of a hole in the ground that looses water rather than collecting water and there are a number of ways of constructing them. Older soakaways can be little more than a hole in the ground filled with old bricks or gravel but most recently, much more sophisticated modular storage cells have been introduced. They are usually manufactured from recycled polypropylene and give incredible strength for very little weight. Also, as they are modular, units can be linked together to give whatever capacity is required.
  1. Detention basins: These are features within or at the edge of developments that provide a second or third treatment stage that has been conveyed from other structures like green roofs or rain gardens. They are vegetated depressions which temporarily hold water and are generally present in suburban and rural locations. They detain water so as to allow gradual infiltration into the soil and removal of pollutants through bioremediation. They can also have much amenity and wildlife value.  
  1. Swales: When storage of water run-off cannot be easily accommodated within a development, it may be possible to convey excess volume out into the wider public open space. Swales are the most common conveyencing features used in SuDS and bring ecological and amenity benefits. They link other components such as filter strips and permeable surfaces to collect the water and convey the run off. They are wide, shallow grassed elements that slow down run off and trap sediments. They can be under-drained where a dry surface is needed or become permanently wet to create a linear wetland habitat.   
  1. Water collection and water reuse for irrigation: Last, but certainly not least, rainwater harvesting. Not only does this approach reduce the flow and volume of water run-off (every little helps!) but it also reduces the demand upon mains water during times of drought.  If installing a green living wall, then these should really only be considered if using harvested rainwater or grey water sources. The most common is the simple garden water butt (although its amazing how much water you can collect from even a shed roof!) which is very easy to install and maintain. Larger systems are available for commercial use but care needs to be taken in their placement if placed above ground and coming up with creative ways of hiding these often less than inspiring storage tanks!  The image below (left) shows water tanks on an allotment, which, whilst in need of hiding, have a very important function for  those growing food!

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I am sure that in the areas where we live, there are many opportunities for these more resourceful and smarter ways of managing our water and heat. Lets hear it for these creative strategies that enable us to start to deal with these water and heat issues in our cities and towns . As citizens, we should be able to influence public bodies, including planners and councillors to create greener public areas and of course we should be taking  considered and creative approaches to our own front gardens, rather than covering them entirely with impermeable surfaces!

Here are a selection of useful links:

http://www.susdrain.org/delivering-suds/

http://www.uksuds.com

http://www.landscapeinstitute.co.uk/knowledge/SustainableDrainageSystemsSuDS.php

http://www.wwt.org.uk/news/all-news/2013/01/wwt-news/wwt-news-conservation/flooding-new-guidance-could-help-protect-homes-and-benefit-wildlife/

 www.raingardens.info

www.pavingexpert.com/suds

I am a landscape architect with experiences in private and public design and green space management. My practice includes the design of interesting urban spaces including roof terraces, courtyards, kitchen gardens and other green oases. I provide both design services as well as project management and green space consultancy across a wide range of urban projects.

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