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Designing Resilient Stormwater Systems

Image (c) Random Sky Studio
Image (c) Random Sky Studio

This article by Rucker Simon, PE, ENV SP, originally appeared in the June 2023 issue of Civil+Structural Engineer magazine.

The earth’s weather patterns have turned chaotically unpredictable and even dangerous due to climate change and are contributing to a greater risk of increased flooding. As rainfalls intensify, stormwater runoff has become more severe and has caused flooding resulting in significant damage to structures and substantial impacts to daily commerce.

In recent years, hurricane season in the United States has produced more intense hurricanes, bringing more significant rainfall and higher storm surges to coastal areas. In September 2022, Hurricane Ian made landfall on Cayo Costa, Florida. The Category 4 hurricane slammed the area with 155 mph winds, and 10-15 feet of storm surge, according to the National Oceanic & Atmospheric Administration (NOAA).

As the storm moved across the state, heavy rains and powerful wind caused catastrophic flooding in its wake. After the storm crossed over Florida and was over open water, it regained strength and made a second landfall in South Carolina. It was the first hurricane South Carolina experienced in six years, resulting in heavy rain, high winds, and flood ing along the coastline.

A total of 149 deaths were reported due to the hurricane, and insured losses range from $50 billion to $65 billion.

Future weather pattern predictions have indicated more intense rain events are expected. According to NOAA’s Atlas 14 rainfall analysis, rainfall intensities have increased within established recurrence intervals, and increases are expected to continue as global temperatures rise. An example of more intense rainfall occurred in April 2023 where more than 26 inches of rain fell in 24 hours in Fort Lauderdale, Florida. This storm was a record rainfall for Florida that shut down the city and closed the airport, causing in excess of 800 flights to be cancelled.

It is critical the design approach to infrastructure in flood prone urban areas be considerate of potentially catastrophic rain events to minimize damages due to flooding. Many current stormwater systems are based on outdated design criteria that are incapable of handling future expected greater rainfall intensities and the resulting extreme flood events. Alternative strategies, such as low impact development (LID) may be a key element of an effective solution.

Urban Flooding Issues

As development occurs, areas that once were able to infiltrate stormwater become increasingly impervious. The new development increases the volume and speed of stormwater runoff over the predevelopment conditions during a similar rain or flood event. When combined with the effects of climate change, the impact is compounded and points to a future with an even greater increase in urban flooding.

Furthermore, existing drainage facilities such as storm sewers, creeks, rivers, and bayous located in urban areas can only manage the lower intensities that have guided the design of stormwater systems in the past. The more intense the rainfall, the higher the peak runoff and the greater the volume of runoff. As the amount of impervious cover increases, so does the pace at which runoff can accumulate and travel.

In the past, the expansion and paving of drainage facilities, such as the Los Angeles River and paved sections of bayous in Houston, were the norm. This method of stormwater management sends the water downstream much faster, which helps with urban flooding in the areas immediately adjacent and upstream.

“This is simply passing the problem downstream to create an even bigger problem,” says Edwin Friedrichs, senior advisor and managing principal in Walter P Moore’s Infrastructure Group. “Too much water sent downstream at one time adds to flooding in the downstream areas and can also create backwater flooding that eventually reaches the areas upstream if the rain event is severe.”

Aging public drainage systems also create an increasing burden to the public on the cost of maintenance and repair, further reducing drain age capacity when the systems are not regularly maintained in a fully operational manner.

Read the entire article in the June 2023 issue of Civil+Structural Engineer magazine

Rucker Simon, PE, ENV SP, is a senior associate and team director in Walter P Moore’s Infrastructure Group.

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