Stormwater runoff in Miami and other cities in southeast Florida could swell by about 80% to nearly 120% within the next several decades, scientists have reported, indicating that existing sewers and canals will be inundated without substantial upgrades.

The researchers modeled how changes in precipitation and development will affect runoff under two climate change scenarios, and found the bulk of the increase will occur within the next 30 years. The team published the findings Feb. 23 in Science of the Total Environment. 

"Climate change is by far the most dominant driver of stormwater runoff changes in this urban coastal environment," said Omar Abdul-Aziz, an associate professor of civil and environmental engineering at West Virginia University and last author of the study. "That means a lot of water, and the current infrastructure of stormwater runoff drainage systems is not designed for this."

Climate change is expected to increase rainfall over many areas. Most predictions for the impacts on coastal cities have focused on storm surge, the abnormal rise in seawater caused by intense storms, Abdul-Aziz says. Less attention has been paid to how coastal urban areas such as Houston and Miami will be affected by stormwater runoff, the freshwater that flows over the ground instead of soaking into it after a storm.

However, stormwater runoff can cause serious problems. In 2017, Hurricane Harvey dumped 60 inches of rainfall on parts of Texas. Because this water could not drain quickly enough, it accumulated and caused flash floods. Runoff also carries nutrient pollution into rivers and bays, leading to harmful algal blooms.

The groundwater generally lies close to the surface in southern coastal areas. This means the soil quickly becomes waterlogged, making these areas particularly vulnerable to flooding from stormwater runoff.

Abdul-Aziz and his coauthor Erfanul Huq, also of WVU, were interested in how coastal Florida might fare as climate change alters rainfall and more land is converted into paved surfaces that don't absorb water. They focused on the Florida Southeast Coasts Basin, an area of about 7,117 square kilometers that is mostly covered by houses, apartment complexes, shopping centers and industrial areas, along with some farms and wetlands. 

The researchers used 40 projections for monthly rainfall and evapotranspiration, or evaporation from the land and plants, from two scenarios based on relatively more and less extreme greenhouse gas emissions developed by the Intergovernmental Panel on Climate Change. The team also drew on land use predictions from the Environmental Protection Agency's Integrated Climate and Land-Use Scenarios project, which created a dataset of expected housing density across the United States through the 21st century. 

Abdul-Aziz and Huq fed this information into a mathematical model they developed to predict how stormwater runoff would change by the periods of 2044 to 2053 and 2076 to 2085. As a baseline for comparison, the team also computed how much runoff the area saw from 2004 to 2013 based on records from meteorological stations.

The researchers estimated that, compared with the 2010s, the annual amount of runoff across the drainage basin would increase by around 106% by the 2050s and 118% by the 2080s under the less extreme scenario, and 86% by the 2050s and 80% by the 2080s under the more extreme scenario.

One reason why the more extreme scenario appeared to cause smaller projected increases in stormwater runoff is that some of the climate projections expected an increase in rainfall while others predicted decreasing rainfall for the area, Abdul-Aziz says, and partly canceled each other out.

He and Huq also found that development would likely play a less important role than rainfall and evaporation because the area is already so urbanized. 

"Land cover change will basically be the icing on the cake," Abdul-Aziz said. 

To cope with the coming change, southeast Florida should add more wetlands around urban neighborhoods to absorb the excess water, he says. Also important will be widening or deepening existing sewers and canals such as the Miami River and West Palm Beach Canal that carry stormwater to the sea, as well as building new ones.

"You have to retrofit [the] drainage system; otherwise I don't think these places will be livable," Abdul-Aziz said.

He is now investigating whether sea level rise could also enhance flooding across the region by boosting the water level in coastal rivers, making them more likely to overflow their banks when it rains.

The study, "Climate and land cover change impacts on stormwater runoff in large-scale coastal-urban environments," published Feb. 23 in Science of the Total Environment, was authored by Erfanul Huq and Omar I. Abdul-Aziz, West Virginia University.