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Do cities address the hazards of rainwater floods equitably?

As cities expand, more of the surface area of the ground becomes covered with materials that are impervious to rainwater. In cities across the US, roads, rooftops, parking lots, sidewalks and driveways increased by an average of 326,000 hectares per year between 2012 and 2017.

These surfaces do not allow rainwater to percolate through and reach the soil underneath. Instead, water that falls on these surfaces usually flows away into storm drains and sewer systems. If the downfall is excessive, the rainwater can cause local flooding that affects residents and may cause damage and destruction. In many cities, green infrastructure (GI) such as bioswales or channels with permeable surfaces, or interconnected networks of green space that conserve natural ecosystems, have been introduced into the worst affected urban areas to help with storm water drainage.

Although municipalities now can invest in green infrastructure to mitigate the effects of pluvial floods, they have to select the most appropriate areas in which to employ this technology. Cities including Portland, Phoenix and Atlanta have developed plans and invested in implementing GI but there is a perception that the deployment of the GI may not be equitable in terms of the socio-economic status, race or risk of the population. 

A recent study by researchers from Portland State University considers demographic data obtained from the 2011 census along with an identification of the geographic areas prone to rainwater flooding in three US cities, namely Portland, Phoenix and Atlanta. This information is used to assess whether the GI is distributed in these cities in an equitable and appropriate manner in order to mitigate flood risks. 

The researchers divided pluvial flood zones into four categories – Vulnerable, ManagedPrepared, and Low Concern – based on the potential flood area and the density of GI infrastructure. They then investigated how GI, racial groups, and economically disadvantaged groups are distributed with respect to the areas at risk of pluvial flooding. 

Study lead author Arun Pallathadka is a PhD student in the Earth, Environment and Society program at Portland State University. According to Pallathadka, the research team found that the placement of GI in each of the cities did not consistently overlap with areas prone to rainwater flooding. Furthermore, non-white and low-income populations were found to be more vulnerable to flood risk. The results, published recently in the journal Landscape and Urban Planning indicated inequalities and the potential for discriminatory flood-risk management measures.  

For example, in Phoenix and Portland, low-income and non-white populations were more likely to be exposed to pluvial flooding than were high-income and white populations. However, there has been increased investment in GI in these two cities, particularly in neighborhoods with higher populations of non-white and low-income residents, marking a transition to more equitable flood risk management. 

In Atlanta, the risk of rainwater flooding was relatively low for non-white and low-income residents. In fact, the relationships were reversed, with white populations tending to be more exposed to pluvial flooding. However, GI usage is more frequent where the at-risk white population lives, indicating that there may be active efforts to manage flood risk in these communities. The researchers also found that GI investment increased over time in areas with predominantly white populations. 

“Flooding is the costliest natural hazard,” said Pallathadka. “But when we talk about flooding, the focus is often on the floodplain, on rivers. But with climate change, we’re expecting an increase in flooding associated with rainfall events. We wanted to know where the hotspots in cities are associated with a risk of flooding from rain, who are the people living in those communities, and where are cities placing infrastructure to help reduce the hazards.”

The researchers found that in Phoenix and Portland, non-white and low-income populations more often lived in areas classified as Vulnerable or Managed, in terms of flood risk, than white and high-income populations. In contrast, white and high-income populations more often lived in areas that were Prepared or of Low Concern. In Atlanta, non-white and low-income populations were more likely to live in Low Concern areas than were white and high-income populations, but white and high-income populations were more likely to live in Prepared and Managed areas.

In the past, urban planning has often been used as a tool, wielded by whites and the wealthy, for discrimination against ethnic minorities and impoverished groups. The results of the current study both reinforce and challenge the argument that urban resources are unequally shared. 

The study authors have developed a system to categorize neighbourhood flood risk levels that can be applied by researchers, city planners and policymakers to other cities. This tool can be used to help guide future investments in GI such that neighborhoods most at risk of flooding from rainfall events will receive the required infrastructure. 

The methods used by the team provide cities with a novel approach to assessing the need for GI in neighborhoods prone to rainwater flooding while also planning for the equitable distribution of those resources as they move ahead under the threat of increasing extreme precipitation events associated with a warming planet.

By Alison Bosman, Staff Writer

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