by Alex Nichols, P.Eng.
Every year there is an increasing push by approval agencies to step up our efforts to achieve some kind of green or sustainable objective. With each project we push the limits of what is acceptable and is possible. From the perspective of stormwater management, in most projects we are asked to try and achieve a reduction in runoff quantity, an improvement in runoff quality and an improvement in runoff retention through what are known as low impact development techniques (LIDs). Sometimes trying to achieve these objectives using LIDs pushes the limits of what is actually possible. This paper is intended to be a case study in the reconstruction of a parking lot whose reconstruction could have a significant impact on the surrounding environment and to highlight what can be done to achieve a more green or sustainable parking lot.
In its existing condition, it is an asphalt covered parking lot, which is noticeably higher than the surrounding grade (particularly along the east side and at the south end) and drains via sheet flow to the east. This sheet flow has lead to a noticeable amount of erosion along the east side. The parking lot also has a number of limitations. It is immediately adjacent to a sensitive watercourse to the east, existing trees close to the parking lot had to be maintained to the west, and there is no site storm sewer service nor is there a municipal storm sewer nearby to which any runoff could be conveyed.
The reconstruction of the parking lot was supposed to showcase a proactive and sustainable approach to stormwater management. As a result, permeable pavers were selected instead of conventional asphalt as they had the potential to make a positive impact on stormwater management. Permeable pavers offer a number of benefits, all of which are a function of their ability to capture runoff. When compared to conventional asphalt, this ability enables them to significantly reduce runoff quantity by either detaining or retaining runoff and by acting as a filter thereby improving runoff quality. The granular joint material has the ability to screen out larger suspended particles held in runoff. If included in the permeable pavement design, permeable pavers also can improve runoff quality and retention by encouraging runoff to be infiltrated into the subgrade. While this behaviour is dictated by the permeability of the soils, some benefit is still obtained even if the soils have poor permeability. It should also be noted that in most cases runoff that is unable to be retained is picked up by a subdrain and conveyed to a conventional storm sewer system, unfortunately as indicated previously, this parking lot is not one of those cases.
It should also be noted that a geotechnical investigation was performed in order to determine the permeability of the native soils, thereby determining the effectiveness of using permeable pavers as part of the parking lot’s reconstruction. The geotechnical investigation indicated that the site had low permeability soils. However, even soils with low permeability still offer some opportunity to retain runoff and obtain some benefit from a stormwater management perspective.
As the soils were not ideal thereby limiting their ability to retain runoff and that there was no storm sewers available to drain the granular base, a method of conveying this excess runoff had to be determined. Due to the fact that the site is higher than adjacent grade along its east and south sides, there was an opportunity to provide an exposed “granular seam” along the east side slope of the parking lot that could be used to drain the granular base. The “granular seam” was positioned to allow for a certain volume of runoff to be retaining and allowed for any that could not be retained to be discharged to grade. Not only did this “granular seam” eliminate the need for a conventional storm sewer system but it also had the ability to eliminate the erosion that was occurring along the east side slope of the parking lot. The erosion was being caused by frequent events that discharged at various concentrated points along the east side of the parking lot. Permeable pavers will capture more frequent events, conveying them into the granular base where they would either be retained or slowly conveyed through the granular material to the “granular seam” where they would be slowly discharged to grade in a distributed manner along the south portion the east side slope.
As a result, all of the components of stormwater management have been improved. Larger storm events will be attenuated and reduced while the entire volumes of smaller storms have the potential to be entirely retained, having no runoff leaving the site. Runoff will also be prevented from causing erosion and will have a higher level of quality. This is something that is not usually nor easily achieved through the use of more conventional storm sewer infrastructure. Normally this would require a considerable investment in infrastructure that improves runoff quality and infrastructure that helps to reduce peak discharge rates from the site. This unconventional parking lot offers a significant benefit to the environment through improvements in stormwater management practices and relies solely upon the continued functioning of the permeable pavers and their granular base.