6411 BROADWAY 2016-01-01 MF Import

Regency 4-Plex Dra�nage Plan 1. Purpose and Scope The purpose of this report is t� address any changes to the existing drainage that will be caused by the construction of this building and associatcd driveway, tumaround, and sidewalks. Development will cha�ge the runofl characteristics of the site. The addition of impervious surfaces could increase the rate and volume of stormwater runoff. Without the provision for mfigating measures downstream drainage's could be adversely impacted. In addition, during construction, on-site erosio� could result in sediment being transported downstream with associated negative impacts. Temporary and permanent water quality measures are proposed to mitigate potential impacts. Interim erosion centrol measures will be proposed during constructi�n, and pertnanent measures are propased for long term water quality. A detention facility is proposed to insure that stormwater from the �ite will not be increased. This will include two -type 2 catch basins and 80' of 36" CMP with a controlled release to Lexington. Pre{leveloped and post developed stormwater runoff hydrographs were determined by the modified SCS TR-20 method(SBUH). SCS Type 1A 24 hour storm curves were use�i to generate the hydrographs. The 2 yr., 10 yr., and 100 year storms were be routed through the stortn conveyance system . The storm water conveyance system was sized for the 100 yeFr- 24 hour storm event. The stormwater facility is sized to the City of Ever.ttlDOE criteria of: 50% of 2 yr.24 hr event release from the site—Tho orifice size for this criteria is calculated at 0.55". This has been increased to the City of Everett minimum of 1.0" 100% of the 10 and 100 yr.-24 hr events—See below for proposed flows. The system is sized for the 100 yr storm with 35% additional capacity as required. The analysis was done using computer software called Hydraflow - Hydrographs for Windows by Intelisolve. 2. Existing Site Conditions � The existing site conditions of the proposed site consist of a gently sloping iot with an area of 0.21 acres, covered with grass and blackberries fronting o�� Broadway and Lexington. Both streets are paved without sidewalks.