September 1, 2010 - March 31, 2010

Pilot Study to Maximise the Role of Trees in Flood Control

Locations:
Falkirk, Stirling
Types:
Urban Greening, Water Environment
Funds:
CSGN Development Fund
Contacts:
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Lead Partners:
EnviroScience
Partners:
Scottish Environmental Protection Agency (SEPA), Forestry Commission Scotland (FCS), Stirling Council
Download Case Study

Context

Increasing urbanisation has amplified the risk of flash flooding. As areas of vegetation are replaced by impermeable concrete and tarmac they lose their ability to absorb rainwater. This rain is then directed into surface water drainage systems, often overloading them and causing floods. The cost of flooding to Scotland is approximately £350 million per annum and the costs of flood controls are escalating.

There is evidence that the use of trees as part of Sustainable Drainage Systems (SUDS) can double the soil moisture deficit as compared to grass cover and that rain interception by the canopy and soil leaf litter can also help to reduce the risk of flooding.

However, there is some question as to the effectiveness of trees in flood control as the infiltration of rain water into natural soil is often too slow to control floods.

Project background

EnviroScience sought to enhance the role of trees in flood control through the installation of specially engineered boreholes. Their study demonstrates that during storm events these facilitate deep infiltration, enabling storage of water and minimising run off. The boreholes also increase soil surface area, soil air and encourage deep rooting, with further benefits to tree stability, structural damage control and drought resilience.

How the project was delivered

With the support of FCS, SEPA and Stirling Council, experimental boreholes were installed in both urban and forest environments in Stirling, Falkirk and Aberfoyle.

It is believed that the trees in these selected areas will begin to root towards the boreholes and that, in time, fibrous roots will emerge through the borehole perforations.

The success of the boreholes depends on design detail and lessons have been learned about limitations of glacial soils, drilling techniques and road infrastructure constraints.

Project outcomes

The pilot project has shown that the storm water capacity of each engineered tree is approximately 7m3. It has also found that a network of boreholes can typically take 50mm of storm rainfall from street surfaces, protecting sewers from overflow and river pollution.

The project has the added benefit of increasing carbon capture, thus reducing the levels of CO² in the atmosphere. Furthermore, the excess wood produced in the project can be used as wood fuel, reducing the consumption of fossil fuels and helping to reduce energy costs.

The pilot project has shown that enhancing the role of trees as part of a SUDS scheme is cost effective as a flood solution, thus adding real economic value to urban trees against the escalating cost of flood control. Furthermore, urban tree installation work is estimated to take two days per borehole and could be expected to stimulate employment in a civil engineering/arboricultural subsector.

Quotes from participants and beneficiaries

Alan Lascelles, Principle Adviser, Business Link – "Councils should be biting your hand off for this."

A Council Floods Officer – "In a storm we have manhole covers popping all over the place. We could use this."

Ron Melville, Region Director, Forestry Commission - "...very sound, both technically and practically"

CSGN Support and Learning

This project highlights the potential and cost effectiveness of tree enhanced flood control in urban spaces where there is little space to sacrifice to flood waters. It encourages woodland creation, the creation of habitats for wildlife, urban greening and the use of biomass energy in the heart of urban communities, thus helping to create urban sustainability.

Future development of project

Building on the success of the CSGN funded pilot study, EnviroScience conducted further research focussing on Forestry Commission roads in the River Forth catchment. This study shows that Forest Enhanced Flood Control has the potential to control millions of cubic metres of flood water at a fraction of the cost of conventional flood controls.

With flood prevention costs increasing each year, the proposed method of Forest Enhanced Flood Control could provide a much cheaper soft engineering solution, saving 90-99% of flood control cost as compared to conventional civil engineering solutions. It supports commercial forestry, vital for sustainable building timber into the future and has more precise flow control and repeat storm performance in enhancing natural forest flood management.

The Forest Enhanced Flood Control method also includes a new drainage product from recycled plastic. This product is proposed for manufacture in Scotland and will take up to 10,000m³ low grade plastic waste from landfill.

The massive savings demonstrated in this research greatly enhances the economics of existing forest and reforestation, for which Scotland has a world leading reputation. Several Local Authorities have also expressed interest in applying the model.

Funding Details:

Year Fund Value
2010 CSGN Development Fund 12,800

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