Low Impact Development: Installing Drainage at a Condominium Building Complex
Low impact development (LID) has emerged as an effective approach to storm water management. Its basic principle is modeled after nature: Manage rainfall at the source by using uniformly distributed, decentralized, micro-scale controls. In mimicking a site’s pre-development hydrology, LID uses design techniques that infiltrate, filter, store, evaporate and detain runoff close to its source. Instead of conveying and managing or treating storm water in large, costly end-of-pipe facilities located at the bottom of drainage areas, the LID approach to storm water management consists of small, cost-effective landscape features at the lot level.
LID employs several principles, such as preserving and recreating natural landscape features and minimizing impervious surfaces to create a functional and aesthetically pleasing drainage site. It has been used in a variety of ways — bioretention facilities, rain gardens, vegetated rooftops, rain barrels and permeable pavement — and offers three benefits — simplicity, cost-efficiency and flexibility — for civil engineers, landscape architects and contractors.
LID allows for the integration of treatment and management measures into urban site features without large investments in complex and costly centralized infrastructure. This involves strategic placement of distributed lot-level controls that can be customized to more closely mimic a watershed’s hydrology. The result is a landscape that generates less runoff, pollution, erosion and overall damage to lakes, streams and coastal waters.
LID designs also feature fewer pipes and belowground infrastructure requirements. Space once dedicated to storm water ponds can be used for additional development or left for conservation. By offering variety of techniques to provide for runoff benefits, LID works well in urbanized, constrained areas, as well as open regions and environmentally sensitive sites. Opportunities to apply LID principles and practices are nearly unlimited, because any feature of the urban landscape can be modified to control runoff or reduce the introduction of pollution, resulting in a truly customized design for watershed management.
One project that is a good example of LID principles is a recent installation of NDS EZflow at a condominium site in Denver. NDS, Inc. manufactures a wide range of flexible and scalable stormwater drainage and stormwater management products that all help manage storm water as close to its source as possible. By providing runoff reduction and quality enhancement at the site scale, NDS products can be configured to meet a wide variety of residential and commercial applications and are engineered for high performance while meeting today’s regulations for onsite stormwater management.
In Denver, the former Lowry Air Force Base was given new life, reconstructed into a modern development of residential and commercial buildings with parks, public art projects, an ice arena, a golf course and restaurants.
Four complexes of high-end condominiums were constructed in a section of the site called Siena at Lowry. Drainage specifications for these buildings initially included the traditional method of 4-in. pipe, surrounded by ¾-in. stone, wrapped in geotextile cloth. The earthwork and utility subcontractor on the project suggested an alternative drainage method as part of the value engineering process to reduce costs while providing an effective solution for draining water away from the foundations of the buildings.
When constructing its own headquarters, the contracting team chose the EZflow drainage system instead of pipe and stone. Due to site conditions, project planners made modifications to the drainage design for the Siena at Lowry project. To assure adequate water movement into the system, a geotextile cloth and course sand backfill were utilized. Replacement of native soil fill with sand is a common practice in highly restrictive, fine-grained soils. Even with these additions, the cost of the project was reduced by 40%. To address drainage, contractors dug a trench approximately 18 to 24 in. wide. The system was placed around the buildings in 10-ft sections at a 0.5% to 1% slope that drains into a sump pit at the north end of the buildings. The sand backfill was placed to cover the system to a depth of 1.5 to 2 ft depending on the slope of any given area.
This method provided a new solution for an old problem, keeping water away from building foundations.
