Rainfall Study Brings Up New QuestionsPosted on Monday, September 16th, 2013 by Cliff Speegle
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I recently attended a seminar about updated National Oceanic and Atmospheric Association (NOAA) precipitation frequency estimates (how much rain we get) put on by the Johnson County Stormwater management Program. KU professors Dr. Bruce McEnroe and Dr. Bryan Young gave an overview of their comparisons between the old data and new data. So what did we learn?
This data from the rainfall study replaces other sources of data which included Technical Paper 40 (TP-40) which came out in 1961. There is probably an old tattered and yellowed copy of TP-40 on the shelves in most civil engineering firms. When compared, the new rainfall study does show a general increase in rainfall for the Kansas City metropolitan area. Depending on the duration, frequency, and location in the metro the increase is as much as 25%. Lee’s Summit actually has a local high for rainfall now, meaning that Lee’s Summit gets more rain than the areas around it, which didn’t exist in previous data. So what do you do with this new information?
That is a good question… and if you ask around you may get different answers to this question. Most stormwater design in the metro area uses the APWA standards which uses one rainfall frequency table for the region. It is likely that it will be some time before this table gets updated with NOAA’s new data. The quick answer is to use the accepted standards and make it clear which standards you are designing to. Don’t ignore the new data but compare to it and then use engineering judgment on how to proceed.