Seeking More Cost-Efficient Design Criteria for Infiltration Trenches

For many jurisdictions, the current design criteria for low-impact development practices (LIDs) including infiltration trenches require LIDs to provide enough storage capacity to store catchment runoff from the location’s 90th-percentile storm. The 90th-percentile storm used in Ontario, Canada, has a depth of approximately 25 mm. This study examines the performances and costs of infiltration trenches built in Ontario but sized to accommodate alternative storm depths ranging from 5 to 50 mm. Analytical equations are used to determine the runoff reduction ratios of infiltration trenches, and a cost estimation tool specifically developed for LIDs is used to estimate their overall costs. Results indicate that the current 90th-percentile storm criterion is probably too high and not cost efficient. An evidence-based methodology for selecting more appropriate design criteria is proposed. Using this methodology, it was found that the economically more efficient design criterion for Ontario averages about $20–22\mathrm{mm}$ for different design cases. Significant savings can be realized if a lower design criterion is implemented. The proposed methodology is therefore recommended for jurisdictions seeking more cost-efficient design criteria.