In recent years, climate change has affected Earth’s weather patterns, which have turned unpredictable and often dangerous. Rising temperatures are accompanied by a higher risk of intensified flooding, as heat domes and cold fronts collide, creating more extreme rain events. As a result, stormwater runoff becomes more severe, leading to destructive flood events that cause significant damage to structures and disrupt daily commerce. The urgency to address these challenges is paramount to safeguarding our communities and the environment.
Furthermore, future weather-pattern predictions indicate that more intense rain events are expected. According to NOAA’s Atlas ‘14 rainfall analysis, rainfall intensities have increased within established recurrence intervals, and increases in extreme rainfall are expected to continue as global temperatures rise. One example of intense rainfall occurred in April 2023 when more than 26 inches of rain fell in 24 hours in Fort Lauderdale, Fla., shutting down the entire city and its infrastructure for several days.
In the face of mounting climate-change impacts, the degradation of stormwater quality has become an alarming reality. As rising temperatures and altered precipitation patterns have intensified storm events, it causes drainage systems to become overwhelmed and leads to a surge in pollution levels. The heightened storm intensities not only exacerbate the transport of pollutants and suspended solids from urban and industrial areas into water bodies but also amplify nutrient runoff from residential and agricultural lands.
These nutrient pollutants, primarily nitrogen and phosphorus, originate from fertilizers and contribute to harmful algal blooms and oxygen-depleted “dead zones” in aquatic ecosystems. As heavy rains wash away these nutrients, they find their way into storm drains, compromising water quality and ecosystem health.
Addressing this issue necessitates a multifaceted approach encompassing climate-resilient infrastructure, sustainable land-use practices and proactive pollution-control measures. By acknowledging the intricate interplay between climate change and stormwater quality, engineers and infrastructure professionals can better formulate strategies to mitigate the deteriorating state of the vital water resources.
It’s critical that the design approach to stormwater infrastructure be considerate of increasingly intense storm events and address the repercussions. Many current stormwater systems are based on outdated design criteria that are uncapable of handling future expected greater rainfall intensities and the resulting extreme flood events. Low-impact development (LID), or green infrastructure, is an effective solution that can address these new stormwater quantity and quality problems.
Stormwater Quantity and Quality Issues
As development occurs, areas that previously were able to infiltrate stormwater become increasingly impervious. New development increases the volume and speed of stormwater runoff compared to predevelopment conditions during a similar rain or flood event. With the increase in runoff and developed area, the amount of nutrients and pollutants in stormwater also increases. 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 as well as a larger impact on the health of our waterbodies.
Furthermore, existing drainage facilities such as storm sewers, creeks, rivers and bayous located in urban areas can only manage the lower intensities that previously guided the design of stormwater systems. 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 drainage capacity when the systems are not regularly maintained in a fully operational manner.
Low-Impact Development (LID)
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’s slowly released at a prescribed predevelopment rate.
The core philosophy of LID is conscious site planning to maintain 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 by attenuating and treating stormwater at the source. 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 advanced, dense urban areas as well as 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 improve stormwater quality. 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.
LID Features and Functionality
Bioretention systems (also called rain gardens) are shallow, vegetated depressions in the ground used to slow and infiltrate stormwater runoff to native soils or filter through drainage media to a nearby storm sewer. They extend the timing and reduce the volume of stormwater runoff while adding a landscaped amenity that can treat stormwater quality at the source. Bioretention basins should be designed to infiltrate stormwater wherever possible. Where native soils don’t allow for infiltration, a layer of soil can be designed as drainage media to provide filtration to a subsurface drainage system. This concept of biofiltration can exist in many forms, including rain gardens and green roof systems.
Green roofs are layers of growing medium and vegetation installed on top of a conventional roof and function to reduce stormwater runoff, filter pollutants, reduce carbon footprint, reduce energy costs and add aesthetic quality. Green roofs can provide nature-based stormwater retention where there otherwise would be impervious roof surface, and they serve to slow down and filter runoff.
Maximizing stormwater infiltration helps minimize runoff, replace soil nutrients and recharge groundwater. However, in many areas, in-situ soils don’t allow for significant infiltration of stormwater. Soil amendments can be added to native site soils to improve stormwater management and plant health. Amendments are made using soil organic matter, which comes from various sources, including compost, composted woody material, biosolids and forest product residuals. Soil amendments seek to increase water storage and infiltration.
Maintaining and enhancing the population of trees intercepts rainfall, increases evaporation, increases plant use of stormwater, improves air quality and reduces the urban heat-island effect. According to Purdue University’s Landscape Report, mature trees can absorb up to 150 gallons of water from soil per day, further enhancing the capacity of soils to infiltrate and retain stormwater runoff. Incorporating native trees into urban developments is a straightforward way to add aesthetic value and mitigate the effects of increasing impervious cover.
Rainwater harvesting uses cisterns, rain barrels and other storage tanks to capture and store stormwater runoff for non-potable water uses such as irrigation, toilet flushing or industrial processes. Rainwater capture is ideal for urban areas as it reduces stormwater runoff and unnecessary potable water use. There are numerous sizes, styles and levels of treatment required to meet different goals. Runoff is channeled from surfaces to cisterns and rain barrels, where it’s stored. Release of the stormwater typically is controlled by valves or pumps and based on the water’s end use. Rainwater harvesting systems can be used in conjunction with other LID techniques, such as bioretention systems, to improve infiltration and remove runoff pollutants.
LID 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, enhance pedestrian safety and add to the community’s visual appeal.
“The installation of rain gardens along Bagby Street treat and capture 33 percent of the stormwater that falls within the right-of-way,” says Marlon Marshall, director of 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,” says Marshall. “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.”
Another example of how LID adds value involves master-planned developments that offer a greater opportunity for large-scale reuse of stormwater by incorporating LID features.
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 then is 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 the environment and community. Captured and reused stormwater can help alleviate flooding downstream by diverting excess runoff to reduce the demand for potable water treatment and distribution as well as improve the quality of the water released into the downstream systems.
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 then is reused for park irrigation. A recirculating bioswale provides overflow capacity for further detention and treatment of runoff onsite. According to Marshall, Midtown Park’s signature LID feature is this “bayou,” which serves as the project’s site-detention system. The “bayou” is a constructed water channel that mimics the natural bayous, bottomland hardwood forests and wetlands of Houston. The site’s LID rainwater collection system stores runoff from the exposed impervious surfaces in the park in a 70,000-gallon subsurface cistern that’s reused onsite 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,” adds Marshall. “During extreme rainfall events, like Hurricane Harvey, the ‘bayou’ has detained stormwater to prevent local flooding and property damage.”
The successful integration of complex green stormwater infrastructure systems has helped offset operational costs and led Midtown Park to become Houston’s first SITES-certified project, a rating system designed to protect ecosystems.