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<br />2. Efficient Pel:formance: Manage the appropriate amount of runoff volume to maintain <br />reasonable efficiency at the same time that facility costs are reduced. More storage is not <br />always better, and can impact feasibility and greatly increase construction costs. <br /> <br />3. Easy Implementation: An event standard is understandable, relatively easy to calculate <br />with CUtTent methods and hydrologic models, and is workable over a range of <br />development conditions and densities. <br /> <br />INFILTRATION BMP EFFICIENCY A1~ALYSIS <br /> <br />Since infiltration practices are capable of removing 100% of the TP for the runoff volume that is <br />retained and infiltrated, at least 60% of a site's anllual runoff volume must be captured and <br />infiltrated to meet cutTent District Rules. To equate this 60% runoff volume back to a specific <br />rainfall event, a rainfall frequency analysis was conducted using 1938-2004 Mim1eapolis/St. Paul <br />Intemational Airport data. From this analysis, it is shown that 60% of the ammal precipitation <br />volume is generated by stonns that are 0.8 inches or less. This is equivalent to capturing <br />approximately 90% of the rainfall events in a given average year (Figure 1). This approach keys <br />into the impervious cover in the subwatershed and uses runoff volume from impervious surfaces <br />as the goal for treatment of small ston11S. <br /> <br />Figure 1. Daily Precipitation Frequency for Minneapolis/St. Paul Airport, 1938-2004. <br /> <br />10 <br /> <br />I I I <br /> <br />-' Rainfall :reque~~~ <br /> <br />- Rainfall Volume ! <br />I <br /> <br />+ <br /> <br />9 :_~ <br />I <br /> <br />B - <br />, <br /> <br />Ul <br />" <br />J:: 7 <br />" <br />" <br /> <br />_~_~-L-",_.,,_,_,~ <br /> <br />J:: 6 <br />~ <br />C- <br />" <br />Cl 5 --- <br />" <br />o <br />~ 4 <br />~ <br />.is. <br />.;::; <br />" <br />~ <br />c.. <br /> <br />I <br />.--L---J <br /> <br />I <br />---" <br /> <br />L--.1- <br /> <br /> <br />~_. <br />i <br />, <br /> <br />0.0% <br /> <br />10.0% <br /> <br />20.0% <br /> <br />30.0% <br /> <br />40.0% <br /> <br />50.0% <br /> <br />60.0% <br /> <br />70.0% <br /> <br />80.0% <br /> <br />90.0% <br /> <br />100.0% <br /> <br />Cumulative Probability (%) <br /> <br />For comparison, as part of EOR's analysis for the 2005 MPCA Stormwater Manual, a teclmical <br />analysis was conducted on various pollutant removal capabilities for different water quality <br />volume criteria. One criteria evaluated was the 90% rainfall frequency capture rule. The 90% <br /> <br />Emmons & Olivier Resources, Inc. <br /> <br />Page 3 of7 <br />