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. 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 stor- age 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, pollut- ants, and nutrients from the stormwater runoff. Aspects of LID include minimizing land disturbances, conserving natural features, reducing impervious cover, and incorporating distrib- uted natural drainage systems to attenuate runoff. LID projects have in- creased 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 permit- ting, 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, attenu- ation, 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 Hurri- cane 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, high- lighting the potential for sustainable and efficient infrastructure design.
25
June 2023 csengineermag.com
Made with FlippingBook Annual report