Application of Continuous Modeling to Assess Hydraulic Performance of Proposed Soil Cell LID Practices in Ottawa, ON

The City of Ottawa aims to expand the range of Low Impact Development (LID) methods it uses to meet stormwater retrofit objectives including capture and retention of “first flush” rainfall events. Urban trees are municipal amenities of public interest for their aesthetic, environmental, and recreational merits – however, increasingly congested municipal right-of-ways can limit soil volume available for urban trees and therefore impede development of a healthy urban tree canopy. Application of engineered soil cell systems provide an opportunity to both achieve stormwater retrofit objectives through LID while also protecting soil volume required to grow larger, healthier urban trees.

Conventional design of soil cell LID systems relies on spreadsheet-based approaches to determine the required volume of the practice and evaluate hydraulic performance. This approach involves simplified calculations that do not consider natural unsteady rainfall patterns or routing effects as runoff is treated by the practice. Conservatism that could result from these assumptions may impact willingness to consider application of soil cell LID systems where subsurface volume is limited and spreadsheet-based approaches show that this volume is insufficient to achieve treatment objectives. It was hypothesized that soil cell systems designed using hydrodynamic methods may show a more realistic level of performance when compared to conventional spreadsheet-based approaches.

This project investigated the application of continuous modelling of ~20 years of rainfall in PCSWMM to evaluate the performance of proposed soil cell LID systems in Ottawa, ON. Performance of proposed soil cell systems in response to design storms was also reviewed. Modelling results were interpreted to characterize hydraulic performance including storage utilized, high water level, and drawdown duration / operational time, and compared across all methods. Findings of significance include the conclusion that the proposed soil cell systems have capacity to capture 30% to 75% more runoff volume than predicted through spreadsheet-based methods. Areas of divergence in the characterization of hydraulic performance across each method were noted and recommendations for application for future projects were identified. Overall, it was confirmed that (i) conventional spreadsheet calculations are a conservative approach to the design of soil cell LID systems and may underestimate the volume retained by a proposed soil cell system, and (ii) continuous modelling can be used to provide more realistic depictions of the true hydraulic performance of the system, including the volume of runoff captured and retained by the system, to demonstrate achievement of stormwater retrofit objectives where subsurface constraints limit storage volume available.