CONVENTIONAL VS LOW IMPACT DEVELOPMENT STORMWATER MANAGEMENT CASE STUDIES

摘要

Low Impact Development (LID) Stormwater Management (SWM) techniques is the appropriate approach to be applied to site developments in an attempt to mimic a site's natural hydrologic cycle to store and treat stormwater runoff close to where it falls. In 2014, the UTRCA conducted a survey of member municipalities to gauge a familiarity with SWM LIDs to help UTRCA effectively target their efforts regarding SWM LID during a development and planning process at the Municipal level. Several issues were highlighted from the survey results, including the presence of institutional, technical, and physical barriers, as well as a general lack of awareness and knowledge about LID. The survey helped UTRCA effectively target efforts on the implementation SWM LIDs in the watershed. The UTRCA initiated pilot projects in the watershed with the help of proponents. This paper discusses two case studies, implementing the new LID approach, a private condominium development in Ingersoll and a brownfield site in Chatham Ontario. Both projects were designed comparing conventional SWM and SWM LIDs from project inception and design to final implementation. The two projects are within the Thames River watershed which has been identified as one of the watersheds contributing to the Lake Erie Algae Bloom. The SWM LIDs in the Thames River watershed will reduce phosphorous loading of the Thames River and will improve the quality of runoff. The site in Ingersoll is approximately 1.31 ha (3.24 Ac.) in area which is being developed as a private condominium development consisting block townhouse structures totaling 41 units. SWM LID and over control of the sites stormwater runoff under the 250- year post-development conditions to the 2-year pre-development conditions was managed using SWM LIDs features known as bioswales and raingardens. The cost savings were approximately 41%, in addition two additional units that were able to be built as a result of the additional area made available by eliminating a conventional stormwater management pond on the site. The brownfield site in Chatham is approximately 0.43 ha (1.06 Ac.) in area and was converted to a 27 stall parking lot with rain-garden type presentation. SWM LID and control of the sites stormwater runoff under the 100- year post-development conditions to the 5-year pre-development conditions was managed using SWM LIDs features known as bioswales and raingardens. The SWM LIDs considered, on the sites, will slow down the runoff, reducing its velocity and providing appropriate plantings for nutrient uptake whereby helping improve the water quality while controlling the runoff quantity and mimicking the hydrological cycle on the site as much as possible. In both cases the Municipalities had revised intensity-duration curves to address the effects of climate change on new and existing infrastructure. At the Ingersoll site the external storm sewer, providing a stub for servicing to the site and downstream conveyance sewers, were designed many years earlier without the current knowledge and impact of climate change. The result is existing sewers typically are unable to meet the capacity requirements. The SWM LID design option was able to minimize the input onto the existing sewer system eliminating the need to increase the size of the conveyance sewers. The existing LTVCA parking lot in Chatham had its parking lot catch basins and collection storm sewer discharging to an existing 150mm combined sewer service into the city sewer. The new parking lot design eliminated this storm connection and separated it to discharge directly to the Thames River after filtering through proposed LID features. This reduces the amount of water that discharges to the combined sewer providing greater capacity and reduces the I & I to be treated at the wastewater treatment plant. The cost comparison between conventional SWM and SWM LID approaches for the projects highlight the benefits SWM LIDs can have as a potential savings. SWM LIDs offer many benefits when compared with conventional such as environmental benefits through water quality and quantity control, a potential reduction on the capacity requirement on existing infrastructure. New construction typically results in a loss of vegetative cover and drainage capacity. The conventional systems use storm sewers and detention ponds which can impact water quality and water quantity. Also, the conventional conveyance and disposal disrupts the natural hydrologic cycle. The pilot projects in the Thames River will help in educating the public with a hopeful result of policy change and will improve quality of runoff in the Thames River watershed.