Anatomy of an Extreme Event: The July 14–15, 2004 Peterborough Rainstorm

Abstract

The July 14−15, 2004 rainstorm in Peterborough, Ontario, produced the largest 24 hour total rainfall depth (>220 mm) on record for southern Ontario. The storm was localized over the city for much of this period and rainfall depths and peak rainfall intensities dropped markedly with distance from the city. Peak intensity for 0.5 hour duration at the Trent Weather Station (TWS) in the north end of the city was 93 mm h−1, while peak intensity for one hour duration was 87 mm h−1. Even greater intensities may have occurred in other parts of the city. These intensities exceeded the corresponding 100−year return period values (81 and 53 mm h−1, respectively) estimated from the 1971 to 2002 record at the Peterborough Airport (PA) station, southwest of the city. Another critical characteristic of the July 14−15, 2004 storm was the distinctive shape of its intensity−duration−frequency curve relative to that for the PA. The TWS intensities for the July 14−15, 2004 storm showed a more protracted decline with increasing duration compared to the PA 100−year return period values. The storm produced the largest instantaneous peak flow on record for Jackson Creek, which flows through Peterborough's downtown area. Peak rainfall intensities exceeded the infiltration capacities of artificial surfaces as well as many nominally−pervious surfaces in Peterborough and the ensuing surface runoff contributed to widespread flooding in the city. These results strongly suggest that such storm properties, and their influence on surface runoff generation in urban areas, need to be considered when planning engineering structures such as storm sewer networks and storm water retention ponds in the Peterborough region.