More pavement, more problems 

There may be a hidden cost to urban expansion – more flooding. A new study by John Hopkins University finds that for every percentage point increase in pavement and impervious surfaces, annual floods increase by 3.3 per cent.

_{An increase in urban development has shown a corresponding increase in the frequency of annual flooding, according to new research which has looked at the cause-and-effect relationship between impervious surfaces and flood magnitude. Image: hkeita/123rf}

This means that if an undeveloped river basin increases the amount of impervious surfaces from zero to 10 per cent, scientists would expect, on average, a 33 per cent increase in annual flooding.

"With recent major floods in heavily urbanised cities like Houston and Ellicott City, Maryland, we wanted to better understand how much urbanisation is increasing flood flows," says Annalise Blum, a former postdoctoral fellow in Johns Hopkins University's Department of Earth and Planetary Sciences and the paper's first author.

While previous studies have tried to estimate how much impervious surfaces affect flooding, those studies used smaller datasets that looked only at one stream or a small set of streams at a single point in time, but these were not generalisable across the country. These studies also could not isolate the cause-and-effect relationship between impervious surfaces and flood magnitude, Blum says, because they were not able to effectively control for other factors such as climate, dams and land use. These other factors make it difficult to say how more impervious cover affects flood magnitude.

Blum worked with Paul Ferraro, a Bloomberg Distinguished Professor. Together, the researchers employed mathematical models not often used in the study of water or floods.

"Inferring cause and effect in the environment around us is difficult," Ferraro says. "However, in the last few decades, fields like economics and biostatistics have made great advances in methods that can isolate cause and effect. By bringing these methods to hydrology, we hope that we can spur advances in hydrological science, as well as in the urban policies and programs that depend on that science."

Blum and colleagues created a data set that allowed them to leverage differences across both time and space to isolate the effect of impervious surfaces on floods. The research team analysed 39 years of data (1974-2012) from more than 2,000 US Geological Survey stream gauges, which measure the amount of water flowing through a stream. The team then merged the stream data with data on the growth of impervious surfaces in the basins upstream of the gauges.

The authors estimate that annual flood magnitude, defined as annual maximum streamflow, increases by an average of 3.3 per cent for every one per cent increase in the area of patios, garages, pavement or other impervious surfaces.

"Due to the large variability in annual flooding, it is difficult to isolate the effect of urbanisation. Combining these large datasets with both time and space dimensions allowed us to tease out and calculate the magnitude of the effect," Blum says.

Blum hopes that researchers will apply the methods to other environmental challenges and use the results to prepare for the unintended consequences of urbanisation.

"If you're looking at a basin that you expect will urbanise in the next five years, these findings will give you a ballpark estimate of additional flooding to expect due to that urbanisation," she says.

The study was published in Geophysical Research Letters.