A comparison of the OSG and the 2007 IDF curves revealed that for short durations the intensities from the 2007 IDF Curves are less than the OSG IDF Curves on average by 5% and 10% for the 5 year and 100 year curves respectively. However, for the longer durations the intensities for the 2007 IDF Curves are actually greater than the OSG IDF Curves on average by 11% and 17% for the 5 year and 100 year curves respectively. The intensities at the shorter durations would influence storm sewer sizing while the longer durations will influence stormwater pond sizing. The lower intensities at the short durations will tend to result in smaller storm sewer sizes while the larger intensities at higher durations will tend to increase stormwater management pond requirements.
Given that the percentage differences in intensities between the IDF curves is within the margin of error associated with data collection and hydrologic assessments, it was ISD’s opinion not to update the OSD IDF curves. As part of the Stormwater Levels of Service review, the need to revisit the IDF curve selection was identified."
Because stormwater ponds are not designed with IDF curve statistics alone, i.e,. they are designed by simulating temporal patters of storms in hydrologic/hydraulic models, it is questionable if an IDF shift in long durations alone would alter the design of a stormwater management pond. That is the design storm (or hyetograph) pattern would influence the pond performance more than the input IDF used to adjust storm volume. Also, some municipalities assess pond performance assuming the outlet is completely blocked, as an operational worst case scenario, such that all collected runoff is routed through the emergency spillway. In this extreme operating condition, a small change in rainfall volume may not affect overall performance (i.e., spillways operate efficiently as weirs, able to accommodate additional flow release with limited increase in operating level (pond water surface elevation).