THE LOWDOWN ON LOW IMPACT DEVELOPMENT
Stormwater management is an important design features that engineers must account for in land development. When development occurs, the quantity of stormwater runoff increases due to the removal of pervious surfaces (which water can infiltrate) and construction of new impervious surfaces and structures. In addition, the water quality of stormwater runoff from developments can decrease due to the accumulation of sediments, metals, nutrients, and pathogens after flowing over the impervious surfaces. Therefore, if appropriate stormwater controls and best management practices are not implemented by the civil engineer, flooding and/or discharging of polluted runoff into a water body can occur. This pollution can then adversely affect aquatic wildlife and even contaminate human water supply. In this post, we’ll explain the benefits of Low Impact Development (LID) practices for stormwater management and dispel the myths that may deter engineers from using these methods.
Before we discuss LID, it’s helpful to understand traditional stormwater management practices. The most common method for both new development and re-development sites is installing storm catch basins/inlets to capture storm runoff. After runoff is collected in these structures, it is then piped to an above ground stormwater pond for storage and water quality treatment. The water is detained in the pond for a certain period of time until it is discharged out of the pond via an outlet control. An example of traditional stormwater management for a land development project is shown below.
Stormwater management strategy has changed over time from transporting runoff away as quickly as possible, either off-site or to a pond on-site, to now handling/retaining as much water as possible near its source. One way in which stormwater management has evolved is the implementation of LID techniques. LID, commonly referred to as green infrastructure, is a form of stormwater management for land development that aims to model the natural hydrology of the watershed by retaining runoff on-site by infiltrating, filtering, storing, or transpiring it via soils, vegetation, and other natural processes. The GA Coastal Stormwater Supplement defines LID as “small scale stormwater management practices that are used to disconnect impervious and disturbed pervious surfaces from the storm drain system and reduce post-construction stormwater runoff rates, volumes, and pollutant loads.” LID practices essentially treat stormwater runoff as a resource for the site rather than as a disposable waste. Numerous benefits of LID practices have been documented, including reducing erosion from stream banks after peak storm flows, visual aesthetics, more flexible site development layouts, pollutant removal, reducing the size of a stormwater detention/flood control systems, and lower costs. A schematic of how LID practices can be used for stormwater management is shown below.
There are numerous types of LID practices, and they are commonly used in combination. Some common examples of LID/Green Infrastructure practices include:
Bioretention: a shallow landscaped area in which native or engineered soil captures and treats stormwater runoff.
Infiltration trenches/basins: excavated areas filled with stone and soil that stores and treats runoff temporarily until it infiltrates into the soil.
Pervious concrete pavement and permeable pavement: the use of pavement in developments that allows runoff to infiltrate into the underlying soil.
Stormwater planter: planter boxes for landscaping, commonly in urban areas, that temporarily store stormwater runoff.
Bioswales: vegetated ditches that convey and treat runoff and are commonly found in medians or along the sides of roads/streets.
Green roof: the use of waterproof fabric, soils, and vegetation on building roofs to capture rainfall to use for plant growth.
Despite the benefits, there are some myths regarding LID that may make engineers hesitant to incorporate the components into their design. These include:
Higher Cost: A common argument against the use of LID techniques is that it comes at a much higher construction cost than traditional stormwater controls. In a report completed by the EPA in 2007 to compare conventional stormwater controls with LID controls, this argument was found to be incorrect, as 16 out of the 17 case studies reported lower costs with implementing LID controls over traditional stormwater methods.
Unreliable and unproven: This is not true as LID practices have achieved success throughout the country and in different climates. LID techniques are continually monitored and assessed each year by different agencies, such as the UNH Stormwater Center.
More maintenance than conventional stormwater practices: Both LID practices and conventional stormwater methods require approximately the same amount of maintenance and inspection.
Each civil engineering project has its own specific regulations and restrictions it must take into consideration. Therefore, implementing LID practices in a land development or redevelopment project might not always be feasible. Nonetheless, LID practices are quickly becoming more popular in stormwater design. Jurisdictions are not only advocating for them, but also incorporating them into their ordinances after seeing the social, environmental, and economic benefits they can produce. Even the GA Stormwater Management Manual and the GA Coastal Stormwater Supplement both acknowledge the economical and social benefits that can come with LID implementation. It may not be long before it is a requirement, rather than a choice, to use LID methods for stormwater management in land development.
About Will Avant, PE
Will Avant, PE, is a Project Manager for the Land Development Division in Peachtree Corners, Georgia. Will graduated from the University of Georgia with a Bachelor’s Degree in Environmental Engineering in 2013. He has experience in both residential and commercial land development and has a special interest in stormwater management design.