|New design standards are tigers.
Old ones are lambs.
|Short duration Southern Ontario rainfall intensities trends are mostly insignificant (randomly up and down) and are decreasing for short durations linked to urban flash flooding in overland drainage and sewer systems.
|More statistically significant DECREASES in rainfall intensity are observed than increases.
|Annual maximum rain intensities are DECREASING for all durations of 5 minutes to 24 hours at the long term Toronto climate station. Decreasing trends are statistically significant for the 6, 12 and 24 hour durations.
|Design standards increase level of service over time significantly - about 250% for old, pre-1980's 2-year minor-only systems to 1980's dual drainage 100-year systems, including overland flow component.
|Dual drainage design standard incorporating minor sewer and major overland system capacity to avoid flooding into basements during 'heavy storms'.
Sanitary Drainage / Wet Weather Stress Reductions
|Extraneous inflow and infiltration to wastewater system sanitary sewers decreased dramatically with complete sewer separation (foundation drains connected to large storm instead of smaller sanitary sewers).
|Fully separated sanitary sewer systems have high level of service.
|Toronto Watermain Installation Dates (Estimate of Drainage Design Standard Era) and Historical Flood Reports
More details: runoff increases with intensification, with the amount of impermeable, high runoff surfaces more than doubling over a 50 year period in some municipalities:
Rainfall intensity increases have been been reported by insurance media, confusing theoretical shifts with actual Environment Canada historical data: