Low Impact Development (LID) for a Sustainable Built Environment within Public Infrastructure

Low impact development features minimize runoff, maximize retainage, and reduce demands on public infrastructure. KCI has designed and inspected bioretention areas, grassed swales, and permeable pavements and understands proper construction techniques and methods.

Over the years, the desire and the need for sustainable development has pushed communities and members of the construction industry to come up with or adopt ways, means and methods to study and analyze the building process and its effects. Especially since the early 1990s, the building sector has been very active in promoting sustainability.

The term “sustainable development” was first introduced in the Our Common Future report of the World Commission on Environment and Development in 1987, and the concept has since been adopted as a policy principle by the UN, the EU, numerous countries, companies, business councils, political parties, NGOs, etc. The culmination point of these early efforts was the Rio Earth Summit in 1992, where sustainability became an important factor in planning of construction projects and developments in general, covering various aspects of the process, namely technical, environmental, economic, social, cultural, and individual.

Low Impact Development (LID) addresses the site development step of the building process. In essence, it means producing a design and construction sequence that will have the least effect on the existing landscape. Though its reach can address a wide variety of site development issues, the focus of low impact development efforts have largely focused on stormwater management. The amount of urbanized land in the U.S. continues to increase every year, which means an increased rate of conversion of natural cover to impervious land, and increased impacts to flora and fauna and to our streams and lakes. Generally, conventional methods have taught industry professionals to manage rainfall in the pipe, i.e. collect and convey the rainfall runoff in a development as quickly as possible. LID aims to manage the rainfall where it makes its first ground contact, providing treatment and management options for the water, spreading it, slowing its travel through the site, encouraging its absorption, and making the most of this resource before releasing the excess flow into the conventional piped systems nearby. This approach provides many benefits, including improved water quality, reduced runoff, enhanced ecosystem diversity, and among other things, increased property values.

There are many ways to implement structural (engineered systems) and non-structural (planned design improvements) LID BMPs (Best Management Practices) on a development, whether it is a new site or a retrofit project on existing land. Non-structural BMPs aim to reduce or eliminate the impact of the construction process through smarter planning, design and material selection. Examples of these BMPs are:

  • Reducing or minimizing land disturbance through design and by eliminating partial or complete clearing of land.
  • Protecting natural site characteristics with regards to drainage patterns.
  • Minimizing the areas where soil compaction is needed.
  • Limiting the amount of proposed impervious surfaces through smarter design.

Structural BMPs include improvements such as vegetated strips, bio-swales, rain gardens, stormwater planters and permeable pavement.

Vegetation
When designing a new project, trees, shrubs, grasses, and perennials are used to create a diverse landscape suitable for the site conditions and neighborhood. Plants should be chosen based on the level of care expected at the facility. For projects consisting of roadway elements, planting design must be done to ensure sight lines are preserved for pedestrians and vehicles on the street. Existing mature trees should be protected, as they capture stormwater, provide shade and cool pavement.

KCI developed Green Street Concept Studies for several roadways in Prince George’s County, Maryland. For each location, our team developed concept alternatives to explore Green Street improvements to enhance the community and pedestrian safety of urban streets through “Road Diet” concepts, parking changes and additional green space. Alternative solutions and concepts were developed that consisted of adding and/or removing street parking, street trees, protecting mature existing street trees, traffic calming measures such as street narrowing, improving drainage features such as catch basin locations, bio-retention features, bioswales, permeable pavement, landscaping features to promote retention and numerous other green infrastructure and LID practices.

Stormwater
LID-conscious design places a much higher emphasis on preserving and improving upon the living components of projects and the ability of the improvement features to maximize the retention of stormwater runoff. The proper construction of the bio-retention features, the tree spaces and other LID and green infrastructure features starts with the understanding of the need for improving the quality and maximizing the retention of stormwater.

Proper construction of LID features is as important, if not more so, than designing it. For example, in order to successfully construct bio-retention cells and bio-swales, the following factors must be taken into consideration:

  • Construction of the bio-retention areas should begin only after the drainage area is stabilized, limiting the amount of sediment entering the features. Temporary controls such as diversion dikes and/or silt fencing may be needed to divert storm-water away from the bio-retention area until its construction is completed.
  • Excavation and other construction equipment should not be allowed to operate within the bioretention area to minimize compaction of the subgrade soils.
  • Permeability testing of the subgrade soil is required prior to the installation of the stone storage layers and other materials. Ripping or scarification of the subgrade soils within the bioretention area may be required to promote infiltration.
  • The “bioretention soils” placed within the BMPs must conform to local and national standards and should consist of “base loam, organic material and sand.”

As part of a design-build team, KCI managed, designed and built a showcase bioretention facility at the Abingdon Branch of the Harford County Library in Maryland. This location serves as a central hub for community programs and offered a high visibility location—the perfect venue to provide water quality treatment and raise public awareness. The bioretention facility was designed as a centerpiece teaching tool around which an educational campaign including public workshops were implemented.

Pavement
For permeable and porous pavement projects, the quality of materials, proper application, and soil conditions are critical to the long-term performance of the pavements and/or pavers (hardscape surfaces). In addition to proper pavement mixes and construction methods, other elements of the design critical to the function of filtering and retaining stormwater are subgrade soil permeability, proper bedding aggregate size and thickness, and the use of underdrains. The aggregate layers required below porous pavement sections for both asphalt and concrete consist of a filter layer, a reservoir layer and a choker layer overlain by the porous pavement layer(s). Subgrade permeability is of upmost importance for the permeable pavements to function properly. Infiltration rates of the subgrade soils must be confirmed by performing field permeability testing after excavation, but prior to placing aggregates or filter fabric.

In Washington, DC, KCI provided construction management services for the reconstruction of Permeable “Green” Alleys.  Work for this project included the removal of existing alleys made with non-permeable material and replacing them with permeable concrete, which provided the area with a more environmentally friendly method to control runoff. At the time, DC was only the second city in the nation to develop green alleys. New storm drain piping and connections to existing structures were included in order to eliminate excess water not readily absorbed into the ground water table. The pervious pavement sections were eight-inch sections on compacted gravel, with a centrally located perforated drainage system to carry off any excess runoff.