Drainage time

Storm inter-event times for Thunder Bay and Toronto

The recommended drawdown times for all storage and infiltration facilities are based upon rainfall statistics and likelihood that the facility will have had time to regenerate storage capacity before the arrival of the next storm event. A summary analysis of 14 locations across Ontario found that storms were most frequent in Thunder Bay and least frequent in Toronto.
The bar graph shows that there is a ~50% chance of rainfall occurring again within 48 hours (Thunder Bay) to 72 hours (Toronto).

Darker teal locations have more frequent rainfall; darker yellow have the least frequent rainfall. This image links to a Tableau page, where the data may be viewed in more detail.

The analysis was conducted using the recommended minimum inter-event time of 12 hours from the MOECC, and earlier rainfall probability distribution parameters from 1960's-1980's . It is not anticipated that recent incremental changes to our climate will have significantly impacted this broad window of recommended drawdown times. Locations considered included:

• Brantford
• Etobicoke
• Hamilton
• Kingston
• London
• Niagara Falls
• Oshawa
• Ottawa
• Peterborough
• Sault Ste Marie
• Thunder Bay
• Toronto
• Trenton
• Windsor