Protecting Communities from Growing Flood Concerns
Do you feel like every spring goes down as the wettest on record? Doesn’t it seem that you’re always hearing about another 100-year flood?
It’s not your imagination—the weather is changing. Rainfall patterns are shifting. Spring floods are becoming more severe. Images of flood-damaged transportation infrastructure, commercial properties, and residential neighborhoods are common on the daily news.
So, why is this happening, and what can your community do to protect itself? According to Mark Land, PE, Vice President of Snyder & Associates, community resilience to flooding and other weather-related concerns begins with a comprehensive approach to planning and communication.
Climate Changes Require Additional Flood Protection Planning
While we may not experience record-breaking rainfall every year, historical data indicates that average annual rainfall rates are trending upward. Understanding how rainfall patterns are changing and the impacts that may result in terms of stormwater management is the first step in community preparation.
According to the most recent NOAA Climate Report, the average annual precipitation in the U.S. during 2018 was 4.69 inches above the long-term average. It was also the sixth consecutive year with above-average precipitation and the third wettest year on record. Spread out over an entire year, that might not seem like much to be concerned about. But, the change in rainfall patterns is about more than just the average amount of precipitation that falls each year. It’s about how and when storm events occur.
“Extreme weather events that produce quick bursts of heavy precipitation over a short timeframe can overwhelm urban systems and regional watersheds, which leads to flooding,” explains Steve Klocke, PE, Civil Engineer for Snyder & Associates.
In the Central U.S., summer precipitation has become more variable, with intense thunderstorms that produce high amounts of rain and runoff.
For example, in late August 2018, over 10 inches of rain fell over a 24-hour period in Madison, Wisconsin. Parks and playgrounds became lakes. Streets became rivers. Flood damage to homes and infrastructure exceeded $154 million, according to Dane County officials. With total precipitation exceeding 50.64 inches, 2018 went down on record as Madison’s second-wettest year since record-keeping began over 150 years ago.
Generally speaking, Klocke says summer storms move parallel with the direction of river basins, setting up the potential for a thunderstorm to lineup and track along a particular basin. When these circumstances occur, a single storm can dump massive amounts of rain on a single basin, inundating the drainage system and causing widespread flooding. However, flooding isn’t specific to one season.
“In spring, storms typically move southwest to northeast, which is perpendicular to river basins. Despite the fact that a storm may not track a single basin, flooding is still possible because frozen ground inhibits stormwater infiltration.”
During a bomb cyclone in late March 2019, between one and three inches of rain fell across Iowa, Nebraska, and Missouri. The combination of snowmelt, frozen ground, and rain led to intense and widespread flooding along the Missouri River. With more spring storms expected, NOAA predicts flooding could continue through May.
Updating Public Infrastructure Design Standards to Withstand Climate Change
In response to changing weather and rainfall patterns, regulatory agencies and communities are being challenged to prepare and make proactive adjustments to reduce the risk of current and future flooding.
“Drainage structures built 75, 50, or even just 10 years ago may not have been designed to handle the higher precipitation amounts of today,” states Klocke.
As an example, Klocke highlights neighborhood development in the 40s and 50s. At the time, he says there wasn’t much data on how big pipes should be. It was also common for creeks to be filled in. When these neighborhoods become inundated with the heavy, intense storms of present day, it’s evident that pipes and drainage structures from decades ago are undersized.
“Large rain events overwhelm undersized pipes. Water flows overland where creeks once were, putting residential homes at risk for flooding,” he adds.
The data used to develop design standards has been largely based on data from the past 100 years. However, Klocke says that’s changing because planning for flood resiliency is about more than the data we have. SUDAS design standards are evolving, and the focus is turning towards taking into account what science is predicting, rather than what’s already occurred.
In light of changing weather patterns and the new design standards that result, communities may find that the level of service and protection their drainage infrastructure provides may no longer be sufficient.
Common Flood Types & Stormwater Management Best Practices
For communities experiencing drainage system concerns, Klocke and his colleagues begin by analyzing the performance of the system and how it functions within its watershed. Hydrologic and hydraulic modeling are used to provide insight on runoff volume as well as how water flows and the paths it will take. In the Midwest, three types of flooding are common:
- River Flooding
The most common type of flood event in which a body of water exceeds its capacity. This type of flooding typically results from high amounts of precipitation over a long period of time.
- Urban Flooding
Excessive runoff and inadequate drainage to sewer systems, rivers, lakes, and streams in an urban area can lead to urban flooding. Water-covered streets and neighborhoods are common with urban flooding.
- Sewer Backup
Blocked sanitary mains, aging sewer systems, the intrusion of tree roots, and other problems can push wastewater back up into homes and businesses through pipes from the sewer or drainage system. Properties in low-lying areas are at the highest risk of sewer backup, which is unpleasant and harmful to human health.
Improving Flood Resiliency: Options for Communities & Property Owners
If a community drainage system is determined to be insufficient, recommendations are provided to improve stormwater management and strengthen community resilience. Potential suggestions for stormwater management and flood mitigation may include:
In areas with high flood risk, a community may want to explore strategic buyouts of affected properties to help relieve the burden on residents. This option is often less expensive than infrastructure improvements and it provides opportunities for stormwater detention basins or public space improvements.
To prevent sewer backup, it’s often wise for property owners in low-lying areas to install a sewer backflow valves. Installed on the sewer service entering a home, a sewer backflow valve prevents sewage from pushing back up into a property.
In place of traditional grass and shrub landscaping, native plants are a smart alternative. The use of native plants helps promote stormwater infiltration due to robust root systems that creating space within the ground. Additional benefits of native landscapes include improved water quality, biodiversity, and livability, as well as reduced maintenance and associated costs.
Flood Control & Infrastructure Improvements
Oftentimes, the most appropriate way to address rising floodwaters is to provide additional conveyance capacity or storage. The Hamilton Drain project in Polk County, Iowa is a great example of this option. In this instance, a combination of stormwater wetlands and upsized culverts provided a dramatic shift in flood protection for the neighborhoods involved. This area, which previously experienced flooding on nearly an annual basis, is now able to withstand extreme flood events, such as the June 2018 storm that brought over 10 inches of rain.
Preventing Future Flooding through Comprehensive Stormwater Management
Understanding what options are right for your community can be a challenge. And as we look towards the future, Klocke says growing communities have special concerns. While addressing the increase in runoff that results from land development, you must also ensure that infrastructure is designed, built, and maintained to handle the strength and intensity of the changing climate.
“When it comes to helping a community determine the proactive approach that’s right for their unique circumstances, we take a comprehensive approach that accounts for future forecasts because the actions we take today are essential to prepare us for what’s ahead.”