New tool improves Low Impact Development stormwater systems

By Yuestas David

The aim of sustainable stormwater practices, also referred to as Low Impact Development (LID), Green Infrastructure (GI), or Best Management Practices (BMPs), is to treat stormwater as close to the source as possible. This can be done by either delaying or reducing the stormwater runoff, and by removing pollutants from it before conveying it downstream. Common goals for these practices include having post-development infiltration volumes and peak flow rates match pre-development values, or creating the capacity to retain runoff from a 12.5 – 25 mm precipitation event.

LID comprises a set of site design strategies that minimize runoff by means of distributed, small-scale structural practices that mimic natural or predevelopment hydrology through the processes of infiltration, evapotranspiration, harvesting, filtration, and detention of stormwater. Usually, because one structure cannot treat the volume or the variety of pollutants for the entire drainage area, stormwater is conveyed through a treatment train. These practices can effectively remove nutrients, pathogens and metals from runoff, while reducing the volume and intensity of stormwater flows.

Click here to download and learn more about the LID Treatment Train Tool.

The purpose of the LID Treatment Train Tool (LID TTT) is to analyze whether sustainable stormwater management goals can be achieved through the implementation of LIDs. The tool is used to compare hydrology and pollutant loading for the pre- and post-development (with LIDs) scenarios, using annual and event based simulations.

What is the LID TTT?

Free for users, the LID TTT was developed by Sustainable Technologies Evaluation Program (STEP Water), a partnership between Lake Simcoe Region Conservation Authority, Credit Valley Conservation, and Toronto and Region Conservation Authority. The goal was to streamline the planning and approval process by selecting and organizing results of the model simulations and comparisons in a way that is clear and aligned with Ministry of the Environment and Climate Change guidelines and that eliminates the need for additional downloads or computations.

The LID TTT, conceived as a preliminary design tool, can be used for designing and running a scenario very quickly. To support this, the tool includes a number of default settings, which can be altered, depending on a user’s experience and monitoring studies. Also built into it are annual rainfall series, design storm time series, and temperature time series for quicker model setup. The tool also provides preliminary water budget analysis (i.e., surface evapotranspiration, surface runoff, infiltration to soil) and pollutant load removal estimates that can be compared side by side for pre- and post-development scenarios.

It is built upon the widely used and accepted open source US EPA SWMM5 model. While there are a number of models that do this already, the LID TTT provides a very user-friendly interface for novice modelers. Because it is built upon the SWMM5 engine, a preliminary design can be exported directly into SWMM5 for detailed design. Support for users is available through the STEP website where they can access FAQs and contact technical advisors.

Who would benefit from using the LID TTT?

The LID TTT can assist stormwater developers, designers, water resource engineers, municipal staff, landscape architects and planners understand and implement more sustainable stormwater management planning and design practices in their watersheds.

Yuestas David is with the Toronto and Region Conservation Authority. This article appears in ES&E Magazine’s February 2018 issue.