Bigger cities lead to higher river flows

With all the recent heavy rainfall and flooding in many parts of the country the impacts of urban growth on river flows has again been a topic of debate. The video below shows the dramatic impact, in this case in August, of intense rainfall in an urban area on river flow. Contrast this with the same stretch of river during a dry spell (image on the right).

Impact of intense rainfall on an urban stream 
Urban river near Bracknell town centre

Recently two papers have been published both shedding light on the existence of trends in runoff, floods and water quality in rivers downstream of growing urban areas. The papers use similar statistical approaches, identifying whether there has been long-term change, and if so identifying whether it is attributable to rainfall trends or indicators of urban development or a combination of both.

Development of Bracknell (The Cut river catchment) in last 50 years

Much urban growth has taken place since the 1960s. Particularly striking is the rapid growth of towns in the south-east such as Bracknell and Swindon, the subject of the Science of the Total Environment paper. The maps of Bracknell show how the urban area has increased. Urban areas tend to be built with non-permeable materials, which do not allow water to infiltrate the ground during rainfall events. Unlike meandering natural watercourses, artificially built channels in towns are generally straight and route the rainfall through the river system much faster. The paper studied the rivers flowing through Bracknell and Swindon, collating historic information on water flows and water quality to investigate whether there have been changes in line with the urbanisation. In contrast it also looked at two nearby rivers that have catchments that are predominantly rural and have seen little land-use change in that time.

The paper investigated annual and seasonal trends in averages in rainfall, runoff and some key aspects of water quality. There has been no trend in rainfall in any of the study catchments. Upward trends in runoff, dissolved oxygen and water temperature are apparent in the urbanising catchments but not in the others.

The trends in runoff are explained by a combination of urban extent and rainfall. The discernible, yet subtle, impact of urban growth is masked by large variability of rainfall from year to year and from season to season.

Trends in temperature and dissolved oxygen are apparent in the urban catchments, particularly in winter. These cannot be explained by rainfall or runoff, but are however related to change in urban extent.

Dissolved oxygen levels can be low in rivers influenced by sewage effluent rich in organic matter that is readily broken down. Although still below the levels seen in the two rural rivers, increases in dissolved oxygen are thought to be related to substantial improvements in sewage treatment in recent decades, despite the rising effluent loads from the growing population.

In terms of river flow, the paper in Water Resources Research came to a similar conclusion. The focus was slightly different, addressing change in extreme flows in two rivers in the north-west. Whilst significant increases in magnitude and frequency of high flows were seen in an urbanising catchment (River Lostock, including the town of Leyland) and these were particularly apparent in the summer, change was not identified in a nearby, hydrologically-similar, rural catchment (River Conder). The trends were still present also when the climatological variability was taken into account. The paper also discusses the importance of using the most appropriate statistical tools and the most relevant information possible when investigating the effects of a changing environment on flood risk. 

The first author of the Science of the Total Environment paper was an MSc student, Banyu Putro, who studied Environmental Diagnosis and Management at Royal Holloway University of London. Many students from this course have worked in collaboration on CEH project-work over the last five to ten years and it’s proving a very fruitful way of taking our research forward. Banyu is from Malaysia where the average annual rainfall is as much as ten times that generally seen in SE England, so the video clip would represent nothing out of the ordinary. I helped supervise Banyu’s summer dissertation alongside Thomas Kjeldsen (formerly of CEH, now at University of Bath). It can be accessed here:

PUTRO, B., KJELDSEN, T. R., HUTCHINS, M. G. & MILLER, J. D., 2016. An empirical investigation of climate and land-use effects on water quantity and quality in two urbanising catchments in the southern United Kingdom. Science of the Total Environment, 548-549, 164-172. 

The Water Resources Research paper:

PROSDOCIMI, I., KJELDSEN, T. R. & MILLER, J. D., 2015. Detection and attribution of urbanization effect on flood extremes using nonstationary flood-frequency models. Water Resources Research, 51, 4244-4262.

Both papers used long-term gridded estimates of daily rainfall developed at CEH:

Tanguy, M.; Dixon, H.; Prosdocimi, I.; Morris, D. G.; Keller, V. D. J. (2014). Gridded estimates of daily and monthly areal rainfall for the United Kingdom (1890-2012) [CEH-GEAR]. NERC Environmental Information Data Centre.

Much of the work was supported by the POLLCURB project funded under the NERC Changing Water Cycle programme

Photographs and maps courtesy of James Miller

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