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<br />disturbance threshold (e.g. 4,000 SF) should also be considered. It is also recommended that if <br />50% or more of the site is disturbed, then stormwater management for the entire site is required. <br /> <br />Stormwater Management Criteria for Linear Projects <br />Stormwater management design criteria for linear projects (street, utility and trail projects not <br />associated with a development plan) are not explicitly addressed in the existing rules. It is <br />recommended that requirements for all types of linear projects including new construction, <br />expansion/reconstruction, rural to urban improvements (addition of curb and gutter), and mill & <br />overlay projects be defined. Again it is recommended that the criteria focus on infiltration, <br />where feasible, since infiltration practices are capable of removing 100% of the TP for the runoff <br />volume that is retained and infiltrated. <br /> <br />To meet current District water quality treatment requirements via infiltration BMPs, at least 60% <br />of a site's annual runoff volume must be captured and infiltrated. To equate this 60% runoff <br />volume back to a specific rainfall event, a rainfall frequency analysis was conducted using 1938- <br />2004 Minneapolis/St. Paul International Airport data. From this analysis, it is shown that 60% of <br />the annual precipitation volume is generated by storms that are 0.8 inches or less. This is <br />equivalent to capturing approximately 90% of the rainfall events in a given average year. <br /> <br />Figure 4. Daily Precipitation Frequency for Minneapolis/St. Paul Airport, 1938-2004. <br />10 <br /> <br />2 <br /> <br />- '=-- -~ - := <br />-- .- " <br /> - Rainfall Frequency I <br /> I I <br /> - Rainfall Volume <br />- <br />- <br />- <br />- <br />- <br />- <br />- <br /> ;~.8" / <br /> /- .-/ / <br /> io'" <br />....................................................'1'........... ........ ..... <br />. <br />. <br /> I . <br /> <br />9 <br /> <br />8 <br /> <br />u;- <br />., <br />~ 7 <br />u <br />~ <br />~ 6 <br />- <br />Co <br />., <br />o 5 <br />c <br />o <br />~ 4 <br />- <br />'5. <br />'(j 3 <br />., <br />~ <br />ll. <br /> <br />o <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 ('!oj <br /> <br />This approach keys into the impervious cover in the subwatershed and sets the runoff volume <br />from impervious surfaces as the standard for treatment of small storms. Summarized below are <br />the recommended stormwater standards for linear projects. <br />Table l. Proposed Stormwater Management Standards for Linear Projects <br /> <br />Emmons & Olivier Resources. Inc. <br /> <br />Page 5 of 8 <br />