|IDF curves describe rain intensity that defines|
runoff rates, flood flows, and flood damage
potential in urban drainage systems.
Analysis shows that Environment Canada's annual rainfall maxima trends, the data revealing increasing or decreasing storm intensities over time, can be used as indicators of whether IDF curve updates for design are necessary.
Environment Canada does not update IDF curves frequently, but a new data product in the Engineering Climate Dataset can be used to determine if an upward or downward trend in rainfall severity is occurring at a climate station. These trends can guide as to whether IDF curve revisions are needed to reflect more extreme weather, based on observed storm characteristics.
The image below shows long term Toronto maximum rainfall observations and trend lines (climate station 6158355) from idf_v2-3_2014_12_21_615_ON_6158355_TORONTO_CITY_t.pdf. The 't' in the Environment Canada file name indicates this is the trend file in their dataset. This and '*_t.png' format trends plots are provided with the version 2.3 data.
The trends in the nine Toronto station graphs above correspond to the nine colour coded table cells below, i.e., the first six short duration graphs (5 minute to 1 hour rainfall durations) correspond to the six light green cells in the table below. These mean decreasing rainfall intensity and no need to increase IDF curves based on more extreme storms. The last three longer duration graphs (2 hour to 24 hour durations) correspond to the three dark green cells in the table - these demonstrate strong decreasing intensity trends observations and, again, no need to update IDF curves based on more extreme weather - in fact IDF curves could be decreased based on the data.
Trends at all Canadian climate stations, including those with shorter records and non-active sites, are available in a previous post for 15 minute durations (includes table and interactive map). An interactive map showing trends for all durations for all Canadian climate stations is available as well (all data is in map pop-up attributes, and station symbols are colour coded based on 5 minute duration trends).
Why is rainfall intensity important? Urban flooding on table lands, beyond the river systems subject to riverine flooding, is often correlated with high intensity summer convective storms. This is so because such urban catchments are 'flashy' or fast responding, meaning the runoff and flow in infrastructure that causes flooding peaks shortly after the peak rainfall intensity occurs. In hydrological design terminology, this flashiness is expressed as the catchment's 'time of concentration', defining how long it takes for runoff from the most remote portion of the catchment to contribute flow to the outlet. Just as a crystal glass resonates with sound wave frequency, the flow from urban catchments resonates with (responds to) rainfall frequency defined on IDF curves.
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