As more cities across the country, and especially across the globe, adopt nature-based solutions to manage flooding, city and storm water engineers can apply their system data and evaluate alternative approaches within their communities
Cities across the United States face ever-increasing threats from climate change—whether from fires, droughts, or heavier and more frequent storm events.
The latter—more rain, more often—has dramatic impacts on urban storm, sewer and coastal systems. Approximately 3.5 million miles of storm sewers underlie cities across the country. Many of them are legacy systems subject to aging and added stress. In addition to experiencing more storm events, urban areas continue to expand, resulting in additional impervious surfaces: more development means more pavement. Excessive rainfall and reduced infiltration combine to generate substantial storm water runoff – in other words, urban flooding.
Given increased urbanization, the capacity of existing storm water systems is commensurately reduced relative to their original design requirements. Statistical storm events have increased as well, triggering additional demands on the systems, and further limiting capacity.
Mapping Flood Risk
Understanding the factors contributing to increased flooding, and how those factors can vary so widely across a community, becomes critical as cities determine how and where to spend precious resources. Typical risk indices apply a series of physical parameters, like elevation and proximity to water. However, incorporating socioeconomic parameters, such as the age of a neighborhood, income, population density and land use, can provide additional insight into which neighborhoods are at greatest risk of flooding. StormSensor developed the Stormwater Urban Flood Risk (SURFR™) Index to do just that (Figure 1).
Monitoring Key Problem Areas
Flooding isn’t the only issue that cities are grappling with, especially in areas with combined systems.
StormSensor began monitoring combined sewage overflows in Jersey City in 2019 as part of validating the efficacy of their long-term control plan. The data show some interesting trends that are relevant to city planning as we move into a period with heavier rains.
Take one outfall in Jersey City, for example. The charts below show a correlation between storm events and overflows over a period of one month (Figure 2). Because of capacity issues, the system often overflows during even relatively small storms.
For reference, Storm 2 had the least accumulation of 0.24 inches over 14 hours; Storm 1 had 1.1 inches of accumulation over 20 hours.
Of course, the longer and heavier the storm, the longer and more significant the overflow. What becomes more interesting however—and in the case of climate change, more relevant—is that, with longer and heavier storms, we also see slower infiltration rates and a longer time to return to baseline. And as the frequency of the storms increases, as well as the duration and intensity, we will see more overflows and—very likely—more flooding.
The longer it takes to return to baseline, the lower the available capacity (Figure 3). This exacerbates the combined effects of heavier storms, old pipes and increased demand from urbanization.
Overlying capacity on the original SURFR Index maps average available capacity—not high tide or heavy storms—across the locations monitored in Jersey City (Figure 4).
Yes but…What about Monitoring During Hurricanes?
Urban storm water and sewage monitor systems play a decisive role in providing city officials and maintenance teams with critical insights regarding how their systems function on both good days and bad. But then a hurricane hits and the monitoring systems become overwhelmed. How could a networked storm system provide useful insights under those circumstances?
Hurricane Ida was a massive storm; estimates range from a 50-year to a 200-year storm event. Flooding covered much of the northeast, and the sensor network was fully submerged once the combined system surcharged.
But what we can see is the point of the storm at which the system surcharges, the point at which it begins to recover, and the time it takes to return to baseline, which, in the case of Jersey City’s system, occurred more than three days following the hurricane (Figure 5).
We saw a similar pattern across the city, in both combined and separate systems, both downstream and upstream. The data can be used to prepare for future storm events, track the likelihood of flooding in real time, and design capital improvements that can handle—if not a 200-year event—at least large storms more frequently.
Comparing this data to storm surge—or, in some cases, sea level rise—allows cities to connect multiple climate and system impacts across town and, in the case of Ida, demonstrate that heavy rain, not storm surge (measured at 0.23 feet; shown in red in Figure 6, below), caused the flooding seen across Jersey City in September.
Climate Adaptation Options for Cities
Four components are ultimately required to comprehensively address climate change: mitigation (i.e., undo it), adaptation (i.e., live with it), resilience (i.e., thrive in it), and education (i.e., making climate change data/feedback loops accessible, understandable and logical for those outside the climate-focused community). Each component, if successfully executed, feeds back into the success of the next component. Therefore, the success of one component enhances the next, and together they create true resilience, building human and natural systems that thrive.
And each of these components encourages economic growth in the communities in which they are implemented by creating new jobs, embracing solutions that make our cities and our actions more sustainable and regenerating a degree of equilibrium with the natural planet that helps us all truly thrive.
While mitigation is critically important to stop or reduce ongoing impacts of human activities on carbon content of the atmosphere, it is too late to go backwards. Climate change is clearly happening, and its impacts are more devastating year over year. In 2020, the U.S. saw 22 separate weather and climate disasters—more than double the average—that each cost more than $1 billion in damages, shattering the previous annual record of 16 such disasters.
So we cannot undo climate change or eliminate its impacts. The goal of adaptation, therefore, is to reduce communities’ and citizens’ vulnerability to the harmful effects of climate change (e.g., more extreme weather events, such as hurricanes and storms, and their subsequent impacts like flooding). Adaptation also means that we actively embrace any potential beneficial opportunities associated with climate change (e.g., longer growing seasons or increased agricultural yields in northern regions).
Once a city understands its component risk factors, including social, economic, and geographic, a number of adaptation options can be implemented; implementation is often best done when combined with several alternatives for a more comprehensive solution.
For coastal cities, dikes and levees are frequently used as a cost-effective method to protect dense populations and industries. However, the initial financial investment required to build or expand a dike is significant, and dikes pose no benefit whatsoever to inland cities.
Storm and sewer monitoring systems allow cities to track variability and performance of those systems given different weather conditions. Utilizing open spaces to capture storm water runoff; sponge cities are an incredible example of integrating open space within urban areas to mitigate flooding. Protecting, expanding, and mitigating wetlands provides a similar benefit with a greater overall value to a community. In fact, wetlands reduced damages from Hurricane Sandy by approximately $625 million.
Natural systems evolved to be ideal climate management systems, and the ecosystem values of these systems also provide an economic benefit to the communities that embrace them. However, most communities continue to focus on gray infrastructure instead of green: bigger pipes, larger storage tanks, injection systems, etc. are widely accepted because they are, frankly, what we’ve always done and they are what we know.
Incorporating the data from monitoring networks within these systems, and overlaying those data risk maps and master plans, provides cities with the insights related to how their systems function now, and how they are likely to function in the future given changing climate conditions. As more cities across the country, and especially across the globe, adopt nature-based solutions to manage flooding, city and storm water engineers can apply their system data and evaluate alternative approaches within their communities.