Greening the Stormwater Infrastructure for Cost Effectiveness

Urban development within developed areas poses risks to the existing stormwater and drainage systems. These risks include increase in peak runoff and stormwater runoff volumes, stream erosion, and deterioration in water quality. Providing retrofitting options to mitigate impact of such impacts require consideration of several factors, such as space, budget and local O&M practices.

Green Infrastructure such as Low-Impact Development (LID) techniques mimic the natural hydrological cycle. These promotes natural processes such as evaporation and infiltration, adds aesthetic value and can bring many environmental benefits such as reduction in heat island effect and groundwater recharge. These can be integrated with landscape as well.

Application of LID and common BMPs (such as a storage pond) together can help not only mitiaget impact of the development but it can also reduce costs of the retrofits when compared with common BMPs alone. In this example, a combination of LID and common SWM BMPs (i.e., a hybrid approach) have been applied to provide stormwater quantity and quality control. A large existing area needs to be developed for future residential and commercial use. The goal of the is to find economical cost-effective ways to mitigate impact on the SWM, based on a combination of common and LID based BMPs. The entire area is tributary to an existing creek. Only one small pond exists on in the area which was found to lacks surplus capacity for both the required stormwater quantity and quality control.

The proposed development area is to be developed in stages. First phase requires construction of the proposed road, which provides ditches for stormwater runoff on both the sides. Based on the PCSWMM simulation, a wetpond (Option 1) of total 6500 m3 storage is required for this purpose. This will comprise a footprint of 3000 m2. But this will need to dedicate a costly and large piece of land for SWMF use only. This land could otherwise be used for some development purpose and can be source of revenue instead. An OGS unit was proposed to be included as well to facilitate O&M and enhance the SW quality control for the site. Overall proposed SWM system is estimated to cost C$ 290,000.

As an LID-based GI alternative (Option2) , the proposed roadway ditches were proposed to provide the required SWM quantity and quality control. Several low-height storage zones were formed by use of check dams. Low flow discharge was provided at a controlled rate by installing flow control orifices. Multiple zones of the proposed roadway ditch were converted to grass swale to provide stormwater quantity control. Grass was planted on sides of these ditches to provide sediment filtration. This system meets the required water quantity and quality objectives. The additional cost to develop this system base on the propsoed ditch has been estimate to be C$ 110,000 only. This system also provides all the above-mentioned environmental benefits of a GI system as well. Monetary value of these GI benefits is substantial, but it has been ignored as current standards and guidelines lack a unified criteria at this time. This way, no piece of land has to be dedicated fo exclusive use of SWM retrofits.

It has been demonstrated that the above-mentioned proposed LID-based GI alternative offer demonstrated multiple advantages over the commonly used SW BMPs such as a wetpond.

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