310 W CASINO RD 2017-08-11 - Laserfiche WebLink

�— . • Common misconceptions re�ardin�flow control for small proiects in the Citv of Everett: The impervious area threshold is 5,000 square feet in the Citv of Everett, not 10,000 square feet: The City's threshold is different than many other municipalities because the City's previous stormwater management manual had a threshold of 5,000 square feet, and "backsliding" is not allowed under the Clean Water Act (and the City's Phase II NPDES Municipal Stormwater Permit is issued by Ecology under the EPA's delegated authority under the Clean Water Act). The 0.1 cfs "threshold": The 0.1 cfs increase "threshold" is irrelevant if either of the preceding two thresholds are crossed– either the amount of net new impervious surface, or the specified conversions of pervious surface. The 0.1 cfs increase follows the other thresholds and is separated by an "or", not an "and" in the City's Municipal Stormwater Permit. Although the word "or" is not used to separate the thresholds in the SWMMWW,the permit requirements override that oversight. As the permit says,the 0.1 cfs increase is intended to capture the situation where a combination of new impervious and converted pervious creates results in the 0.1 cfs discharge, even though neither(net new impervious or converted pervious) meets the square footage thresholds. The 0.1 cfs increase is intended to capture those sites that are just below the above two thresholds in terms of impervious created or converted pervious, but the combined effect(of the effects of both created impervious and converted pervious) results in a 0.1 cfs increase or more. Or_i_ n other words, if a project will create 5,000 square feet or more of net new impervious arPa �n thP r�t�,of Everett, it doesn't matter what the calculated increase in the 100-year -------- -- runoff is–the City's flow control standard must be met. - – -- --� � - Continuous simulation and pre-European settlement land cover conditions (i.e., forested) are not required for proiects that will disturb less than an acre of land: See the discussion of flow discharge requirements contained mostly on page 2-13 of Volume I in the City's 2010 Stormwater Management Manual... For small sites, the flow duration standard does not have to be matched. WWHM2012 can still be used, but it must only be shown that the peak flows meet the lesser standard, instead of having to get all "passes"for the flow durations. In addition, hydrological software that utilizes a unit-hydrograph method–such as Water Works–may be used instead of WWHM2012. Alderwood till soils are NOT tvpe B soils for the purpose of hydrological modelin�: They are type C to account for hardpan at a relatively shallow depth. ' ��/ � �