Ecology – SP Civil Design

Sustainable Drainage – A Brief Introduction

April 23, 2019 Steve ParkerLeave a comment

What Is Sustainable Drainage?

Sustainable Urban Drainage Systems (or SUDS for short) mimic the natural drainage patterns in a development as far as possible, to alleviate flooding and reduce associated pollution during heavy or prolonged rainfall.

To illustrate this, let’s compare how the drainage system serving a hard paved surface responds compared to that of a natural green space.

Firstly, when rain falls on a green space, such as a field, woodland or even garden, it stays in situ for a while. It soaks into the ground, it evaporates into the atmosphere or gets absorbed by vegetation. If it ends up flowing into a river or stream, its passage is slowed down and the volume draining to the river is far lower than the volume that originally fell as rain. Plus, the vegetation also helps to filter out pollutants so the runoff is cleaner.

Conversely, when rain falls onto a hard surface, it flows away much more quickly. Drainage systems of gullies and pipes collect this water and transport it away as quickly as possible to the nearest river or stream. It doesn’t soak into the ground, or get absorbed by vegetation, so the volume leaving the site is far greater. It is also untreated, so any pollution in the water is carried with it.

We can compare the runoff from a hard surface to that from a green surface diagrammatically as follows:

Runoff Comparison Hard Paved Surface vs Green Surface

If Q is the flow running off the site, t is time, peak flow is the highest point on the curve and the total volume of runoff is the area under the curve. Then we can see that both the peak flow and the total volume of runoff for a hard surface tend to be higher than for a green surface.

Why Do We Need Sustainable Drainage?

As our towns & cities have grown over the years, more & more pipes pass more & more flow into our streams & rivers, many of which cannot cope with these additional flows. The result is flooding downstream, erosion, destruction of delicate river habitats and pollution. It also reduces the amount of water soaking into the ground & recharging the underground aquifers, which means less water is available to supplement the rivers in times of drought and leads to water shortages.

Which is where sustainable drainage comes in. As I said earlier, SUDS mimic the natural drainage patterns in a development as far as possible, to alleviate flooding and associated pollution during heavy or prolonged rainfall. Essentially, we are trying to turn the blue line on the curve above into the red line.

Key Concepts Behind Sustainable Drainage

The key concepts behind SUDS are that:

Rainwater should be encouraged to soak into the ground wherever possible;
Surface water run-off from a development should be as free from pollution as possible;
Rate of flow of surface water run-off should not be greater than that for an equivalent undeveloped area.

Remember, we are trying to turn the blue line on the curve above into the red line.

Sustainable Drainage Techniques

SUDS systems use a variety of techniques to control surface water run-off. We can provide any number or combination of the following:

Soakaways and Other Infiltration Devices – these are essentially underground structures, sized to store water from a rainfall event that is then allowed to soak into the ground naturally;

Permeable Surfaces or Paving – these allow the rainwater to pass through the surface into the subbase beneath, where it is stored and again allowed to soak into the ground, unlike traditional paving which is impervious and needs a system of gullies and pipes to channel flow into the sewers. Permeable paving allows these gullies & pipes to be eliminated or much reduced. It also removes pollution by filtering out fine particles in the water;

Filter Strips and Swales – these are areas of ground, covered in vegetation, that absorb runoff from adjacent hard surfaces. Filter strips are flatter, while swales resemble long shallow ditches, and are often used to carry the collected water elsewhere. They slow down the flow of surface water and remove pollutants as the flow is filtered by the planting;

Above Ground Storage & Flow Balancing – is provided by basins and ponds. They can be either dry or wet, with the water level fluctuating to store runoff and flows going out controlled with a specially engineered device to reduce flooding. By retaining water and use of planting, they can remove pollution. They can also be an important amenity feature on a site, as well as providing much-needed habitat and biodiversity;

Below Ground Storage & Flow Balancing – is provided by large pipes, culverts or tanks, with the outflow again controlled to reduce flooding. They don’t provide any amenity value or reduce pollution as the above ground features do, but they take up much less space and so are often the preferred solution for developers.

Tank Sewers Awaiting Construction at Parade, Leamington Spa

Further Information

That is a very brief introduction to sustainable drainage and some of the techniques that can be used to reduce the negative impact of a development on the surrounding environment. Much more information can be found on the Susdrain website.

SP Civil Design’s specialism is water and environmental civil engineering, with a focus on sustainability. If you would like to discuss a potential project with us, or have any comments or questions about this article, then please get in touch via our Contact section or email steve@spcivildesign.co.uk.

Can Development Really Go Hand In Hand With Nature?

January 16, 2018January 16, 2018 Steve ParkerLeave a comment

Development versus nature. Construction versus conservation. Economic progress versus fluffy bunnies and tree huggers. These are the traditional conflicts that come to mind when considering construction projects and how they impact on the natural world around them, particularly when large projects are proposed that could destroy huge areas of natural landscape and habitat. I remember some of the major infrastructure development projects of the 1980’s & 1990’s which had colossal stand-offs between construction companies and environmental protesters (anyone else remember Swampy and the Newbury Bypass?)

The Development Paradox?

That the UK needs more new homes and other infrastructure is beyond doubt: for example, the Government has a target to build 300,000 new homes a year. Also beyond doubt is the need to protect our environmental infrastructure because nature is in trouble: in 2014 it was estimated that over 10% of local wildlife sites had been lost or damaged in the previous five years and, once lost, these areas are incredibly difficult to re-establish.

I think it is fair to say that, in the past, neither side has covered itself in glory at times, with often entrenched “us & them” views. But does it really have to be this way? Can we really only provide development and progress at the expense of the environment? Can we really only protect our valuable habitats by preventing development?

A New Way To Build

Not everybody thinks so. The Wildlife Trusts have recently published new guidelines on how to build in a way that is also friendly to nature. Although focussed on housing, the principles contained in these guidance notes can be extended to many other forms of development.

WWT Image Green Infrastructure — Green Infrastructure – Image courtesy of the Wildlife Trusts

This bold vision shows that development doesn’t have to squeeze out nature. It can be done sympathetically, retaining and even enhancing many features of the natural environment. Yes, there is a cost to providing such measures, but the benefits they provide can be enormous, to wildlife, to the local community, to the economy and to the developers. For example:

Important wildlife sites are retained not lost, new habitats can be created, buildings can be made more wildlife-friendly, habitats can be connected with wildlife corridors, all of which gives wildlife more space to thrive;
People can enjoy nature as part of their daily lives, they feel more connected, with improved physical & mental health, children grow up with wildlife integral to their surroundings rather than something separate;
Green spaces can provide safer transport routes, space to grow local food and community areas to be shared;
The local environment can be protected in a cost-effective way, such as by reducing surface water run-off to surrounding areas;
Green developments are more desirable places to work & live, attracting higher market prices and enhancing developers’ environmental credentials.

WWT Image Nature-Rich Development — Benefits of a Nature-Rich Development – Image courtesy of the Wildlife Trusts

According to the Wildlife Trusts’ guidelines, these benefits can be achieved as long as the development follows two overarching guiding principles:

It must result in a measurable improvement for wild species & habitats – for example, designing around existing habitats to avoid losing or damaging them, creating new habitats and ensuring that any habitat that is lost is more than compensated for;
Residents must have lasting access to nearby nature – with wildlife “on the doorstep”, well-managed green spaces and local communities encouraged to get involved.

Building With Wildlife In Mind

The overall masterplanning of a development should be done with regard to the existing ecological networks on the site, understanding the limits of the environment’s ability to cope with the development. This is best done by using local knowledge and expertise. Many of the 47 local Wildlife Trusts in the UK have their own Wildlife Consultancies who can advise on all ecological issues relating to developments.

When looking at the detailed design of a site, there are lots of measures that can be put in place to help follow the guiding principles above. Just some simple features that developments can include are:

Sustainable drainage features such as permeable paving, swales and ponds to reduce flood risk;
Open green spaces, trees, hedgerows, water features and wildflower verges, especially when inter-connected to form wildlife corridors. Such corridors can also provide safe, pleasant transport routes for pedestrians & cyclists;
Bat roosts, bird boxes, etc. built into buildings (for example, here in Cornwall, Green & Blue produce a range of bee bricks that can be built into walls, as well as a range of other products);
Wildlife-friendly green roofs and walls;
Wildlife-friendly plants in gardens and open spaces, with more permeable boundaries to encourage wildlife to migrate (more hedges, fewer walls);
Renewable energy & water-efficiency measures, such as solar panels, wind turbines, heat pumps, greywater recycling, etc.;
Street lighting that doesn’t disrupt wildlife;
Allotments & communal orchards to encourage local food production.

There is no doubt that the Wildlife Trusts’ vision for the future is challenging, but many believe (me included) that it is a challenge that must be embraced if we are to meet the needs of our modern & growing society without destroying more of the natural environment as we have done in the past.

Thanks go to the Wildlife Trusts’ Living Landscape Development Manager Rachell Hackett for her kind permission to use the images and links to the Wildlife Trusts’ material in this article. Disclaimer: All view & opinions expressed herein are my own. My wife & I are members of Cornwall Wildlife Trust and keen supporters of their work. Although I have no commercial connection with Green & Blue and do not endorse their products, I think they have some excellent products that can really help our beleaguered urban wildlife.

Blue Moor frog (Rana arvalis) in breeding habitat.

Save the frogs! How to Prevent Pollution on Construction Sites

June 28, 2017 Steve ParkerLeave a comment

Many construction activities on site can cause watercourse pollution and damage to local wildlife. This article briefly explains how construction activities lead to pollution, the effects this can have on the aquatic environment and how pollution prevention can be achieved on site through proper planning and control measures.

How does construction cause pollution?

Just excavating or breaking ground on site exposes loose soil, which is then prone to be mobilised by subsequent rainfall. Movement of construction plant creates gullies and drainage channels, which increases exposure of this loose soil and provides a pathway for pollutants within the soil to be washed into the aquatic environment.

Mobilisation of large amounts of sediment can also cause local drains to become blocked, which can cause flooding of the local area.

Excavations often need to be pumped out or de-watered to allow work to be carried out in them. In clay or fine silty soils this water is often laden with sediment suspended in the water which, if not properly treated, will result in pollution.

Spillages of oil and diesel from construction plant can also cause pollution off-site, as can washout from concrete pours.

What effects does this pollution cause?

When sediment enters watercourses, it causes pollution and puts stress on the wildlife. Typically it increases turbidity, causing the water to appear cloudy. High levels of turbidity over time kill fish, which of course impacts the whole aquatic ecosystem.

Oil and diesel, even in very small volumes, is a major pollution risk on site as hydrocarbons are extremely toxic to aquatic life.

When still wet, concrete can cause a huge amount of damage to the environment as it is highly alkaline.

Fish kills can result in large fines from the Environment Agency.

How can pollution from construction sites be prevented?

It is easy to ignore where site run-off ends up because it is often remote from the site and not instantly obvious. However, causing a pollution event is illegal and, as already stated, can result in very large fines.

Effective planning and monitoring are essential in controlling the sources of pollution on site and should be included in the site’s environmental management plan. Control of pollution AT SOURCE is key. It will always be cheaper and easier to prevent or control a pollution event on site rather than allowing it to spread off-site and then trying to manage the impact once the pollution has been released into the environment.

Silt fences are often used to capture sediment, but they must be installed properly and maintained frequently, otherwise the sediment will still be carried off-site and cause pollution. Similarly, natural straw, jute or coir barriers can be used to trap the sediment.

Surface water drains are a major pathway for pollutants to enter watercourse and so must be protected against pollutant entry.

Storage of materials and waste on site, both hazardous and non-hazardous, is also very important in minimising environmental impacts. For example, skips should always be covered to prevent waste getting into lakes & rivers. Storage of oil and diesel should always be in bunds so that any spillages are contained.

Spillages from construction plant can be absorbed by special plant “nappies”. More serious and extensive spills can be cleaned up using spill kits, which should always be present on site.

Water from excavations should be passed through a settlement tank or stilling pond, which can also be filtered. If necessary, flocculants can be added to make the suspended solids settle out of the water more quickly.