Designing Resilient Stormwater Systems 

By Rucker Simon

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 flooding 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 Many current stormwater systems are based on outdated design criteria that are incapable of handling future expected greater rainfall intensities and the resulting extreme flood events.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. June 2023 csengineermag.com 25 

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 drainage capacity when the systems are not regularly maintained in a fully operational manner. 

Low Impact Development 

In most urban areas, the increase in imperviousness and runoff is offset by designing detention ponds to accommodate the excess stormwater runoff due to development. This requires a storm sewer system to send water downstream to the detention pond—typically the detention pond is nearby—where it is slowly filtered and released at a prescribed predevelopment rate. 

The core philosophy of LID is conscious site planning. This means maintaining the natural environment’s ecological systems throughout the construction process and for the lifetime of a development. 

Alternatively, LID can provide a similar effect of reducing peak runoff flows and volumes. Through a distributed network of LID features, detention volume can be provided while slowing down and, in some cases, infiltrating runoff into the soil, reducing the land area and storage volume needed for the downstream detention system. These LID systems also improve stormwater quality by providing vegetated filter and filtration zones that clean the water removing floatables, pollutants, and nutrients from the stormwater runoff. 

Aspects of LID include minimizing land disturbances, conserving natural features, reducing impervious cover, and incorporating distributed natural drainage systems to attenuate runoff. LID projects have increased recently because of the lack of space for engineered structural drainage controls in highly developed, dense urban areas, and to offset the negative impact of development in combined sewer service areas. 

LID is an effective approach to managing stormwater runoff, combining green and gray infrastructure to reduce peak runoff and urban flooding. Municipalities are incentivizing LID design through expedited permitting, tax breaks, and alternative favorable stormwater development requirements, making it innovative, cost-effective, and faster to build. 

Low Impact Development Applications 

LID can be used in street and roadway applications to capture, treat, attenuate, and convey stormwater runoff. This approach can reduce the cost of storm sewer systems, detention facilities, and treatment devices, while providing aesthetic benefits. 

The Bagby Street Reconstruction Project in Houston is an example of LID utilizing bioretention for stormwater treatment, detention, attenuation, and conveyance. The project includes rain gardens that provide a buffer to traffic and thereby enhance pedestrian safety and add to the community’s visual appeal. 

“The installation of rain gardens along Bagby Street treats and captures 33 percent of the stormwater that falls within the right-of-way,” says Marlon Marshall, director, engineering, and construction at Midtown Houston. “Before being discharged into local bayous, stormwater along Bagby Street is now collected in rain gardens, which use native plants, trees, and mulch to filter pollutants that have accumulated on surfaces between rains.” 

According to Marshall, Bagby Street’s LID elements have been valuable flood mitigation assets during Houston’s heavy rains and hurricanes. 

“The green stormwater infrastructure has performed effectively to prevent flooding during major weather events including Hurricane Harvey in 2017. Midtown Redevelopment Authority has been able to successfully leverage low impact development resources to attract development and positively impact the quality of life in Midtown,” Marshall says. 

Additionally, it is the first Greenroads certified project in Texas, highlighting the potential for sustainable and efficient infrastructure design.

Another example of how LID adds value involves master planned developments which offer a greater opportunity for large-scale reuse of stormwater by incorporating LID features. The LID systems can treat the runoff, capture the excess water, and then use the captured water to reduce the potable water needed for irrigation and other potential non-potable water uses such cooling tower makeup water. This approach is essential to minimize the environmental impact of the project. 

City Place, formerly Springwoods Village, in the Houston area employs natural streams, vegetated drainage swales, bioswales, sedimentation basins, and wet ponds to treat stormwater runoff. The treated stormwater is then made available as a non-potable water utility for non-potable demands such as cooling towers and irrigation. By incorporating LID into the overall design and infrastructure systems, master planned communities can create sustainable and efficient non-potable water supply systems that benefit both the environment and community. Captured and reused stormwater can help to alleviate flooding downstream by diverting excess runoff to reduce the demand for potable water treatment and distribution, and to improve the quality of the water released into the downstream systems. 

Finally, LID can be applied to small-scale development projects in innovative ways as well, despite limited space available to accommodate the features. Midtown Park in Houston is an example of this approach, using linear rain gardens to capture, detain, and treat stormwater runoff. 

At Midtown Park, an underground tank captures additional runoff, which is then reused for park irrigation. A bioswale provides overflow capacity for further detention and treatment of runoff on-site. The use of LID reduces the need for above ground detention, allowing the space to be repurposed for a revenue-generating parking facility. Reimagining the surface detention area as a LID park feature adds to the enjoyment of the space. 

“LID was integral in Midtown’s ability to address the dilemma of being ‘under-parked,’” Marshall says. “In the case of Midtown Park, ‘under-parked’ had a dual meaning in that the neighborhood did not have enough park space nor enough parking to meet the demands of its residents and visitors.” 

The “under-parked” dilemma presented an opportunity to include creative LID elements in the Midtown Park design to provide site detention as a park feature while allowing for the development of a 400-space underground parking facility. 

According to Marshall, Midtown Park’s signature LID feature is the “bayou” which serves as the site detention system for the project. The “bayou” is a constructed water channel which mimics the natural bayous, swamps, bottomland hardwood forests, and wetlands of Houston. The site’s LID rainwater collection system stores water from the exposed portion of the top of the underground parking garage and other impervious surfaces in the park in a 70,000-gallon subsurface cistern to be reused on-site for irrigating plant material. 

“Working together with bioswales and rain gardens, the LID features at Midtown Park have proven to enhance economic and environmental resiliency by protecting against flooding while improving water quality,” Marshall says. “During extreme rainfall events, like Hurricane Harvey, the ‘bayou’ has detained stormwater to prevent local flooding and property damage.” 

The successful integration of highly complex green stormwater infrastructure systems has helped to offset operational costs and led Midtown Park to become Houston’s first SITES-certified project, a rating system designed to protect ecosystems. 

LID can also expand to a larger scale with the re-greening and expansion of drainage facilities. This includes the Harris County (Texas) Flood Control District’s Brays Bayou project, which has transformed flood damage reduction projects into multi-use parks and landscapes. 

Additionally, the Los Angeles River Ecosystem Restoration, which involved restoring 11 miles along the river and includes the reintroduction of ecological and physical processes, such as a more natural hydrologic and hydraulic regime that reconnects the river to historic floodplains and tributaries, reduced flow velocities, increased infiltration, improved natural sediment processes, and improved water quality. 

Much of the initial attention from infrastructure funding was focused on broadband and electric vehicle deployment. But now, other funding sources are getting lots of attention and bridge repair and replacement is occurring throughout the country. 

The bipartisan infrastructure bill provided $5 billion per year until 2026 just for bridges. That funding is encouraging state and local officials to reimagine how they deliver bridge projects. 

In 2021, it was reported that U.S. bridges had accrued a $125 billion backlog of critical maintenance needs. That was because 42 percent of them had been in service for more than 50 years. The same report found that it could possibly take 50 years to complete the backlog of critical bridge maintenance projects that would be required. That sobering news most likely encouraged elected officials to prioritize expediency related to repairing or replacing bridges in America. 

State transportation departments are now promoting design/build approaches to rehabilitating and replacing bridges because of delivery speed. This past summer, the state transportation departments for Ohio and Kentucky agreed to partner on the $2.8 billion design/build improvement of an interstate bridge between the two states. Earlier in 2022, the state of Missouri established a firm precedent for design/ build delivery of historic bridge projects. The Missouri Department of Transportation requested proposals for the design/build of its $243 million Chester Bridge project and a contract to a design/build team could be awarded as early as March 2023. 

A $75 million bridge project in Washington’s Clark County will call for a design/build team to replace the East Fork Lewis River bridge. Proposals will be requested in March of 2023. Despite being categorized as structurally deficient, the steel truss bridge services 38,000 vehicles—including nearly 7,000 large commercial freight vehicles—daily. The bridge was constructed in 1936 and mounting maintenance costs and disruptions to service have heightened the need to replace the structure. 

Historic bridge projects are being reimagined not only because of available funding but also because of incredible new technological developments. Accelerated Bridge Construction is changing how state transportation departments plan, design, and construct bridges without interrupting traffic. 

In Cape Cod, Massachusetts, an accelerated bridge is projected to cost more than $1 billion. Construction is forecasted for 2026.The Cape Cod peninsula juts into the Atlantic Ocean and its connectivity to mainland Massachusetts depends almost entirely on two bridges that were built in 1935. Those bridges—the Sagamore and the Bourne—are no longer capable of meeting modern transportation demands. The project will include prefabricating and building components off-site as the existing bridge is removed. 

A $400 million project in New York, slated to launch in 2023, will replace a bridge that has served as a lynchpin for rail travel along what has been one of the country’s most heavily used transportation corridors for over a century. The environmental review period has ended and the final design work for replacing the Livingston Avenue Bridge will proceed. Currently, the bridge’s deterioration has caused trains to cross in single-file and at speeds no greater than 15 miles per hour. 

A $1.8 billion bridge project has been announced for Connecticut. The plan is to select a contracting partner in 2023 for work on the Connecticut River Bridge. Within Connecticut, almost all passenger rail and freight services inevitably converge on the Connecticut River Bridge. The 115-year-old structure represents a critical juncture along Amtrak’s widely used Northeast Corridor as it supports freight trains and forms a foundational segment of passenger rail service. The proposed replacement will entail a two-track movable bridge structure with electrification and resilience components that include new tracks, improved signaling, sustainable power supplies, enhanced communication systems and other security features. 

The Federal Lands Highway Program has become yet another source of funding for bridges. As an example of this program, an allocation of $25 million has been announced to repair and preserve Gardner River High Bridge, located in the Wyoming-based portion of Yellowstone National Park. This bridge was built in 1939 and federal funding will be available for a project launch in late 2023 or early 2024. Another $13.1 million allocation will support a project to replace the Sun River Bridge in Montana’s Lewis and Clark County. The project will focus on a 105-year-old, 225 foot long, single lane, structurally deficient bridge that has historically provided access to numerous state and federal public lands. 

These projects are just a small sampling of the type of upcoming opportunities that will be available in 2023 and 2024. A lack of funding has held public officials back from addressing critical infrastructure needs in past years, but that obstacle has been removed. Not only is funding available, new technology and construction methods are being embraced because of the speed and sustainability that they offer. America is moving quickly now to enhance its infrastructure, and bridge repair is at the top of the priority list. 

Now is the time for interested contractors to approach public owners to ask for more detailed information about projects of interest. Planning documents, design outlines and cost projections are available for the asking. The public at large and especially the citizens who travel across the bridges are the primary benefactors, but many regions will also benefit from job creation and economic vitality. America’s global competitiveness will be enhanced, and taxpayers will benefit because the country’s infrastructure assets will be upgraded and preserved for another century if this type of work continues.