Contributors: Peter Yen & Tam Nguyen
As part of Bechtel’s commitment to contribute 100 ideas to support the United Nation’s 2030 Sustainable Development Goals (SDGs), we examine the opportunity of utilizing permeable concrete and other associated materials to help control flooding in developed cities.
SDG Target 11.b: By 2020, substantially increase the number of cities and human settlements adopting and implementing integrated policies and plans towards inclusion, resource efficiency, mitigation and adaptation to climate change, resilience to disasters, and develop and implement, in line with the Sendai Framework for Disaster Risk Reduction 2015-2030, holistic disaster risk management at all levels.
As cities continue to expand and develop, the possibility of severe flooding increases. The expansion of sidewalks, roads, parking lots, and other areas decrease the amount of land able to absorb rainfall, and increases the amount of rain sent to one runoff site. This presents the problem that it’s not always about the rainfall, it’s about where the rain goes – or doesn’t go in this case.
In 2017, Houston was hit with 60.58 inches or 33 trillion gallons of rain over the course of five days (August 25 – August 30). That’s a little over five times the amount of rain from Hurricane Katrina (6.5 trillion gallons). In a concrete jungle like Houston, the rainfall had nowhere to go but up. Heavy amounts of rain were dumped so quickly, the city’s storm drains struggled to keep up; and likely would have caused similar damage to any urban area.
Throughout Houston’s flat geography exists a low density development1 which has limited the city’s natural ability to absorb heavy rainfall. The lack of land and agriculture near the city, coupled with the city’s reservoir from the 1940s created the perfect storm for Harvey to wreak havoc.
In other major cities, like New Orleans, flooding can begin after only 10 minutes of rainfall. Because of this, New Orleans has begun to experiment with permeable concrete. The use of permeable concrete captures stormwater and allows it to seep into the ground, ultimately reducing stormwater runoff and potentially hazardous flooding. Permeable concrete creates more efficient land use by eliminating the need for retention ponds, swales, and other stormwater management systems.
Although permeable concrete has been around for more than twenty years, the application is fairly new to New Orleans. City officials have teamed up with green-builders Make It Right Foundation to experiment with the pavement and apply it to its latest projects. Working with civil engineering students to design the work needed to pave a project’s single city block, the team found that the use of permeable concrete only added 10 percent of costs compared to the cost of using conventional concrete. However, the project didn’t have the gutters, curbs, underground utilities, and sidewalk that are included on a typical city street. In addition to eliminating expenses for drains and underlying drainage, Brent Magee, president of Mandala Concrete, LLC in New Orleans, commented that permeable parking lots remove the need for water-retention ponds that are often required at lots of big box stores and suburban shopping malls2.
Pierre Moses, a project manager at Make It Right Foundation noted that for parking lots or streets with oily discharge from vehicles, the mixture drops into the permeable material, where it’s trapped as food for natural bacteria and fungi. The oil biodegrades into simpler components that are released into the atmosphere, reducing the flow of containments into waterways2.
To have permeable concrete be widely accepted in developed cities across the globe, there should be a substantial amount of models and testing completed to understand the full impact.
For cities like New Orleans, the concept of permeable concrete is a growing application for updating existing and newly developed areas. The city has provided a permeable parking lot at LSU in Baton Rouge, Mary Bird Perkins Cancer Center in Houma, Dillard University, New Orleans BioInnovation Center on Canal Street, and various Housing Authority of New Orleans sites in Uptown and Mid City New Orleans.
Likewise, other major cities like Chicago have found that a lack of focus on infrastructure has created flooding issues that cause more damage in the long run. Much of Chicago’s 1,900 miles of public alleys was originally unpaved, lacked drainage structures or connection to sewer systems which forced rainwater to drain through surfacing4. Long ago, the City of Chicago paved over alleys with traditional concrete, however many of the city’s surfaces are deteriorating and leading to flooding issues.
To alleviate the flooding issues, the City of Chicago Department of Transportation teamed up with the project team of Hitchcock Design Group, Knight E/A, Inc., and Hey and Associates, Inc., to develop four pilot approaches for future alleyway improvements4:
1. Green Pavement Materials with Conventional Drainage: a properly graded and pitched alley surface directs stormwater towards the center of the alley, into adjacent streets and finally into the existing sewer system. Optional inlet structures connected to underground drainpipe assist the system during heavy rain.
2. Full Alley Infiltration Using Permeable Pavement: the entire alley surface integrates permeable pavement materials such as permeable concrete.
3. Center Alley Infiltration Using Permeable Pavement: the alley way is pitched to direct stormwater towards permeable pavement materials at the center of the alley. Optional inlet structures connected to under drain pipe assist the system during heavy rain.
4. Green Pavement Materials with Subsoil Filtration System: the concrete alley way is pitched to inlet structures with perforated sides which allow stormwater to seep into an infiltration trench.
IV. KEY LEARNING
Bechtel has utilized pervious concrete on various projects, including the Oak Ridge National Laboratory (ORNL), the largest US Department of Energy (DOE) open science laboratory. The campus’s new parking lot outside of the recently built cafeteria was built with an early use of pervious concrete, after the American Concrete Institute (ACI) developed a guide document for its use. The pervious concrete was used to conform to stormwater best practices by allowing percolation in an area with surrounding buildings and hardscape. The Oak Ridge area has experienced severe flooding in several months over the past few years, leading to thousands in damage across the community. With the addition of the pervious concrete outside of the laboratory, and its work utilizing 3D models to gauge flood impacts, ORNL will gain a better understanding of the pros and cons of the pervious concrete addition.
With a small number of case studies, there lacks an urgency within the engineering and construction industry to further explore permeable concrete – which may be attributed to the 10 percent costs previously stated above. However, with the impact of Hurricane Harvey fresh on everyone’s mind, an opportunity is presented to further investigate the advantages of New Orleans, Chicago, and other developed cities.