In addition to the emergence of LEED Certification over the last number of years, municipalities and conservation agencies have set sustainable planning objectives for new developments with a push towards “Low Impact Development” (LID) requirements. LID strategies emphasize the use of natural or engineered stormwater controls to mimic the natural hydrological cycle. The main goals of the LID initiative is to reduce stormwater runoff volumes and rates, reduce pollutant loading to downstream municipal sewers and receiving waters, recharge groundwater and to provide water conservation.
There are several LID practices available which range in both cost and effectiveness. It is also important to note that some practices provide more than one benefit. For example, permeable pavers will reduce peak flows, reduce runoff volumes and trap sediments which improve the quality of stormwater runoff. A bio-swale and infiltration pit feature would provide the same benefits more cost effectively, but may not be appropriate due to space limitations or soils conditions. Selecting the appropriate LID practices can speed up approvals, and often reduce site servicing costs by eliminating portions of the traditional site drainage features (catchbasins and storm sewers). Combining LID practices and incorporating them into the overall stormwater design by working back from the most economical practice will reduce the overall cost associated with the servicing component of a project.
A recent Fire Station project completed by MGM included a centralized bio-swale surrounded by a strip of permeable pavers within the parking lot with an infiltration pit constructed below. These three separate LID practices constructed together achieved multiple stormwater objectives within a single footprint. The upper bio-swale area provided both a portion of the required storage volume during a 100 year storm event as well as a quality component for sediment and phosphorus removal. The infiltration pit below the bio-swale provided the majority of the required water balance component. The small strip of permeable pavers constructed along the edge of the bio-swale was installed to achieve the remaining water balance requirement. The philosophy was to maximize the design value of the less expensive infiltration pit and bio-swale while minimizing the required area of the more expensive permeable pavers to achieve the municipality and conservation authority objectives.
In some situations where site conditions are favourable, implementing LID practices can result in a more economical approach than the traditional site servicing designs that were required before sustainable planning objectives were introduced. These economies are only realized when the proper engineering principles are applied to optimize the design. For example, drainage swales were traditionally required to be graded with a minimum of a 2% slope while bio-swales and enhanced swales can be graded as low as 0.5%. The reduction in slope allows for a longer contact time for runoff which assists with pollutant removals. The slope reduction also provides more latitude in the grading design which could reduce earthwork costs as well as servicing costs by reducing the number of storm structures.
MGM recently reviewed a functional engineering report at the request of a developer due to unanticipated site work costs. The functional design was for a 17 hectare industrial development which included a traditional storm system for drainage conveyance. LID practices were not required for this particular development and as a result were not employed into the functional design. The report indicated that an extensive amount of engineered fill would be required due to grading constraints at the perimeter of the site. MGM redesigned the site by incorporating a bio-swale with an overflow storm system along the rear of the property. The redesign allowed for the majority of the rear parking lot to drain via sheet flow to a perimeter bio-swale therefore eliminating a large portion of the storm conveyance system. The decreased bio-swale grades reduced the grading constraints which allowed for the finished floor to be lowered by 1.0m eliminating the requirement to import fill. Incorporating LID strategies into this particular project drastically reduced servicing and grading costs, increased the marketability of the development and was supported by the approval authorities which assisted in expediting permits.
For more information on this topic and how it could relate to your project, contact MGM directly.
