1815 13TH ST SEPA 016 - 013 - PRMC NEW CLINIC BUILDING Land Use Decision Documents 2025-04-21

MAGN USSON KLEMENCIC ASSOCIATES ■ - During a September 26, 2012 community meeting regarding the project, neighbors of the project site mentioned periodic flooding of the residential yards nearby (which are significantly higher in elevation than the project site). It is likely that the yard flooding is caused by poor infiltration of local soils. No correlation could be made between the flooding of these lots and a lack of capacity of the combined system proposed to be used for this project. PRF- DF.\LFI OPED HYDROLOGY Rainfall coefficients "m" and "n" are taken from Stormwater Management Manual Volume 1, Table 5.2 for purposes of creating an Intensity-Duration-Frequency curve for site storm event calculations for both the pre-developed and developed conditions. The pre-developed site is considered to be "light forested" land without a true drainage system for conveyance. Pre-developed hydrology analysis assumes a runoff coefficient of 0.15 for the entire impacted site (from Manual Volume 1, Table 5.3). The area of 3.95 acres correlates to a total flow for a 2-year storm event of 0. 9 cf S. For the 50-year storm event,the flow equates to 2. 1 cf s. In neither event does this runoff leave the site through a dedicated conveyance. The calculation of these flows is included in Figures 4 and 5. DEVELOPED HYDROLOGY The full drainage calculations for the developed hydrology are included in Figures 6 and 7. A runoff coefficient of 0.90 is used for areas of new impervious asphalt concrete paving, and 0.45 is used for areas to be vegetated (including the Swale, rain garden, and ponds). For the 2-year event,the resultant flow is 3. 9 cf s,while for the 50-year event the resultant flow is 9. 2 cf S. As the pre-developed condition does not allow runoff to depart the site,the detention ponds serve to detain runoff with an orifice restriction to provide a slow release meeting the City requirements of matching existing flow(no flow leaving the site). In order to ensure that the proposed drainage system has adequate capacity for its 10-year design life. . and to avoid pond slope failures and combined system impacts,the proposed system is designed for the 100-year event,which generates a runoff of 10. 3 cf s. (vs. 2.3 cfs for the pre-developed condition). More specifically,the ponds are designed such that the 100-year event flow can be contained within the volume of the ponds. However, during an event over the 50-year event the ponds reach a detained depth of water at elevation = 115.70, at which point detained flow will have backed up the gravity pipe system and would escape through the grated rims of the lowest proposed inlet at the south end of the parking lot(and higher pond water elevations would escape through the higher grated inlets). Water bubbling up through the grated inlets would then surface flow to the south of the parking lot and out of the paved PEMC CUP parcel entrance road to the northerly curb and gutter line of 131h Street where it will travel east and re-enter the existing public combined system offsite through street inlets. The pond system has been designed with this 50-year overflow as a way to provide an emergency release. The ponds are designed with floor elevations set at 109.20 and 108.50 to meet the topographical constraints of the site to meet minimum pipe slopes. These finished floors provide the ponds with volumetric capacity to contain the flow from a 100-year event. The only time that a 100-year event would actually be contained in the ponds would be the extremely unlikely condition that a 100-year Storm Drainage Design 6 Utility Plant Expanded Parking Facility, Everett, Washington