Low Impact Development
Mechanisms for Stormwater Managment
Over the past several years, there has been significant momentum towards using “Low Impact Development” (LID) methodologies for stormwater management. Historically, most approaches to stormwater management focused on designing and installing a system of swales and pipes to route stormwater to a single point of discharge — generally either directly or indirectly to a surface water body. Today, LID methodologies relate to avoiding concentrated points of discharge, and even eliminating them, by promoting infiltration and/or treatment with natural and native vegetation.
Embracing LID techniques
Recognizing that traditional stormwater Best Management Practices (BMPs) that focus solely on the conveyance of stormwater have failed to protect the receiving waters to which they drain, many New England regulatory agencies have embarked on a process of updating stormwater regulations with an eye to encouraging LID techniques. In an effort to meet the four primary stormwater management objectives — effective pollutant removal, cooling, channel protection, and flood control — guidance has been given towards improved methods of stormwater management, including the use of LID techniques, which seek to sustain four important principles: minimize impervious areas; limit areas of clearing and grading; minimize directly connected impervious areas; and, manage stormwater at its source.
Rain gardens – design and consideration
Rain gardens, or bioinfiltration areas, are an attractive mechanism for reducing runoff from developed areas. Done properly, they add visual interest, provide limited detention and treatment of surface runoff, and contribute to habitat values for birds and wildlife. In New England, their effectiveness is most evident between March or April and November. In northern areas, the subsoil may freeze to a depth of several feet, which greatly limits their ability to dispose of the precipitation and snow melt from that occasional late February rainstorm. Rain garden design in northern climates should consider the consequences of inability to discharge to groundwater during the winter months and include provisions to manage such flows. Bear in mind that the big open space next to your parking lot that looks like a great spot for a rain garden in the summer, may in fact be your snow storage area in the winter. The piling of snow, coupled with use of winter maintenance sand and deicing chemicals are important considerations in locating and designing a rain garden.
Infiltration – pros and cons
Infiltration of surface runoff is an effective mechanism for removing pollutants that might otherwise be discharged to adjacent surface waters. However, the filtering effect of the topsoil and vegetation is short-circuited when the runoff is directed to a highly permeable subsoil. Promoting the use of infiltration from developed areas to groundwater should take into account the presence of aquifers or nearby water supply wells. In areas where subsoils may contain contaminants from past industrial uses, promoting infiltration may actually enhance transport of these substances, ultimately increasing their concentration within adjacent surface water bodies.
Porous pavements reduce runoff
Use of porous pavements constitutes an excellent mechanism for reducing runoff from parking areas and other paved surfaces that are not typically subjected to heavy vehicle loadings. The jury is still out as to whether use of porous pavements on highways will result in reduced pavement life, due to the lower density of the bituminous courses, or reduced support values of base materials through the introduction of surface water. It is also important to consider compatibility with winter maintenance. Most experts agree that use of winter maintenance sand will dramatically impair the ability of that porous pavement to function as designed.
Conclusion
In short, the commonly used range of LID methodologies present a number of excellent tools for addressing stormwater management challenges for both developers and municipalities, but like any other tools, they are most effective when one contemplates their use in the full context of the application. As always, consideration must be given to the benefits and pitfalls of LID stormwater management solutions as one seeks opportunities to enhance the sustainability of our developments and infrastructure systems.
Through our role in assisting both municipalities and developers alike, Wright-Pierce has maintained a staff experienced in all aspects of stormwater management. We pride ourselves in “staying ahead of the curve” with respect to emerging technologies and regulations. Like many others, we have embraced the use of LID techniques for the many benefits they offer, and have incorporated them into many of our projects over the past several years. At the same time, we recognize that our role as consultants includes assisting our clients in understanding both the benefits and the limitations associated with key design decisions related to LID technologies. Project specific decisions with respect to pre-treatment requirements, the need to address certain priority pollutants, and consideration of long-term operation and maintenance requirements, which are different for each BMP, can be expected to have a meaningful impact on the longevity and effectiveness of a stormwater management system.