Reducing urban diffuse pollution and surface water flooding

by | Jun 9, 2016 | Featured Slider, Features

City of Trees looks at a groundbreaking new project in the City of Salford to study how trees can aid the management of urban stormwater.

Nature-based solutions for the management of urban stormwater have been growing in popularity, but there is a lack of empirical performance data for field-scale installations, especially in a UK context.

To address this, a novel retrofit street tree demonstration project was commissioned in the City of Salford, near Manchester (UK), in June 2015. The ground-breaking study, which will run for three years until 2017, is a partnership between the Environment Agency, The University of Manchester, City of Trees, United Utilities, Urban Vision and Salford City Council. It aimed to capture the impact that trees had on both cleaning polluted water from road run off and managing levels of surface water, which can lead to flooding.

The project is part of the Irwell Catchment Partnership’s initiative to improve the quality of our rivers, lakes and canals.

The Trees

Three fifteen year old London Plane trees were planted in the specially designed roadside tree trench in Howard Street, Salford, Greater Manchester in June 2015 using DeepRoot Silva Cell, a modular suspended pavement system was used to maximise soil volume, avoid compaction and support large tree growth.

The trees were chosen to fit in with the local street landscape and due to their ability to tolerate pollution.

Capturing road runoff

Using specialist equipment, a team from The University of Manchester has been monitoring the quantity and quality of the rainwater as it enters and leaves the trench.

Road runoff is directed to the tree trench via a 13m length section of ACO Slot Kerbs. Water is then distributed evenly throughout whole system via a perforated uPVC pipe. Excess water is conveyed out of the system via a uPVC underdrain pipe, which is subsequently connected to the sewer network.

The tree trench is lined with an impermeable HDPE membrane liner.  This membrane ensures that water within the tree trench is solely from road runoff – thus facilitating accurate hydrological monitoring.


Access chambers are positioned on the inflow and outflow of the tree trench to enable hydrological and water quality monitoring. Monitoring equipment consists of an ISCO 6712 Automatic Water Quality Sampler with ISCO 750 Area Velocity Module for measuring flow.

Flow measurements are automatically recorded at 5 minute intervals, whilst water quality sampling is undertaken at variable intervals to capture high flow and low flow conditions.

All monitoring work was undertaken by Dr James Rothwell, a Senior Lecturer in Geography at the University of Manchester. His research is focused on how soils and vegetation mediate the transfer of pollution within hydrological systems, and he has 15 years’ experience in hydrological monitoring.

Dr Rothwell presented initial results from the study at the European Geosciences UnionGeneral Assembly 2016 in Vienna in April 2016, which brought together 13,650 scientists from 109 countries

Utilising University laboratories

Water sample analysis was undertaken within the Geography laboratories at the University of Manchester, amongst the top Geography facilities in the UK.

Analysis consists of the determination of nitrate and phosphate by a Metrohm 882 Compact IC+ ion chromatograph. Heavy metal (e.g. Pb, Cu, Zn) analysis is undertaken by a Perkin Elmer NexION 350D Inductively Coupled Plasma Mass Spectrometer which is equipped with a Dynamic Reaction Cell and S10 Autosampler.

Initial results

The project has already produced promising initial monitoring results which reveal that the average water volume retention by the tree pit system was approximately 40% and the average storm peak reduction was 50%. Storm waters were also slowed by the system by up to 2 hours.

Innovative Solutions

As the population here in the UK continues to increase, more green space is being built on which means a reduction in the number of places for water to drain naturally into the ground.

The loss of green spaces means that increasingly more rainwater drains into our sewer systems which were not designed to cope with the rising number of storms and as a consequence we are seeing a greater frequency and severity of flooding in our towns and cities.

Pete Stringer, Technical and GI Planning Manager, from the City of Trees team comments; “Research projects such as these provide innovative and cost effective solutions to complex environmental problems faced in urban areas.”

He adds; “With many experts predicting further radical changes to our climate, leading to increased rainfall and greater instances of flooding, these challenges are likely to become even more of an issue in the future”.

As well as managing rainwater it is hoped that this project will demonstrate how trees can clean pollutants, providing a natural alternative to the expensive physical, biological and chemical processes that are employed by water utility companies at their sewage treatment plants.

A natural solution

Peter Stringer, comments; “Trees are often viewed as a ‘nice to have’ but they provide a whole range of significant benefits to both people and the environment.

This project demonstrates how important trees are to urban areas – why we need to plant more of them, and protect the ones we have”.

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