Ms Helen Gibbs
PhD student

School of Geography
Queen Mary, University of London
Mile End Road, London E1 4NS

PhD Title
Urban river restoration: some potential problems associated with sediment accumulation.

Restoration of urban rivers often involves the removal of bank and bed reinforcement, exposing natural bank materials to erosion, and the creation of a more complex channel form. As a result of bank and bed sediment exposure and increased hydraulic complexity, sediment is readily mobilised, no longer flushed through the system and begins to accumulate.

There may be negative effects from the accumulation of sediment, particularly fine sediment (silts and clays), as these are associated with contaminants. Runoff from urbanised surfaces and municipal and industrial discharges result in increased loading of metals, nutrients, organic contaminants (petroleum products, PCBs, PAHs) and pesticides, all of which may accumulate within sediments of restored reaches.

This research will look at paired restored and un-restored reaches within the Thames catchment. Sedimentation patterns will be compared to understand the process, rate and patterns in both reaches. Sediments will be sampled seasonally, using metals as a marker of contamination, to draw comparisons between reach pairs and their seasonal performance and to establish whether there is any enhanced level of metal accumulation within restored reaches. Since some plants are able to uptake contaminants (phytoextraction) macrophyte sampling will be undertaken to establish any metal uptake from the accumulated sediments.

Key words: river restoration, contaminated sediments, phytoextraction

PhD Research

Background

Rivers are increasingly being restored to enhance biodiversity, provide sustainable flood risk management, and, in urban areas, contribute to regeneration through provision of amenities for local people. Restoration often involves removal of channel bed and bank reinforcement, exposing natural bank materials to erosion, and it almost always involves creating a channel that has a form which is more complex than that which pre-dated restoration. As a result of bank and bed sediment exposure and increased hydraulic complexity, sediment is readily mobilised, leading to deposition in some channel areas and frequently a net accumulation of sediment as a result of a decreased channel slope and increased flow resistance of the restored channel. Thus, sediment is no longer flushed straight through the river system, but it begins to accumulate in areas of low shear stress or high flow resistance.


Restored river stretches on the Wandle (left), Ravensbourne (centre) and Quaggy (right) rivers.

Accumulation of sediment is beneficial to the extent that it builds natural geomorphological features that increase channel heterogeneity, and support habitat creation and the establishment of varied plant species and communities, and thus, it enhances the aesthetic appeal of the channel. There may, however, be negative effects of increased sedimentation, particularly the accumulation of fine mineral (<64 μm silts and clays) and organic particles, since these are often associated with contaminants. Runoff from urbanised surfaces, municipal and industrial discharges result in increased loading of metals, nutrients, organic contaminants (petroleum products, PCBs, PAHs) and pesticides, all of which may accumulate within sediments of restored reaches.

Aims

to compare sedimentation patterns between restored and un-restored stretches;
to analyse the quality of sediment within restored and un-restored stretches;
to undertake biological surveys to assess the diversity of plants; and
to undertake biological surveys to assess if any contaminant uptake is occurring by plants.

Methods

My PhD will look at rivers within the Thames catchment that have had stretches restored. Paired stretches will be identified so that sedimentation patterns within restored stretches can be compared to adjacent un-restored stretches to understand the process, rate and pattern of sedimentation in both stretches. Sediments will be sampled seasonally to draw comparisons between stretch pairs and their seasonal performance and to establish whether there is any enhanced level of accumulation within restored reaches. Since some plants uptake contaminants and therefore decrease contamination within the sediment (phytoextraction), biological surveys and sampling will be undertaken to establish plant diversity, potential uptake of pollutants, and whether pollution tolerant species are dominant.


Adjacent restored (left) and un-restored (right) stretches on the Ravensbourne River at Cornmill Gardens, Lewisham.

Supervisors
Professor Angela Gurnell
Dr. Kate Heppell
Dr Kate Spencer

Funding
Queen Mary, University of London Studentship

Background
MSc Aquatic Resource Management, King’s College London
BSc (Hons) Geography, University of Edinburgh

Ms Helen Gibbs PhD student

Ms Helen Gibbs
PhD student