1730 112TH ST SW STONEBROOK APARTMENTS 2019-12-13

Gordon Witchcr Octobcr 2R, 199G Pagc 2 Westerly Aree: 1.8 Acres Ground Cover: Dense Trees and Underbrush, CN=76 (Alderwood-Urban, Soils Group C) Time of Concentration: 300' of 4.3% Sheet Flow through forested underbrush, n=0.40, Tc=55.61 minutes As depicted in the attached preliminary site plan, the proposed development consists of 93 total units. There will be one detention vault followed by a single water quality wet vault (or wetpond if possible). We propose to set the project beck from the westcrly wetland and to maintain its base flows with an appropriate amount of roof drainage. Therefore, post-developed conditions for the single basin are summarized as follows: Developed Area: 4.5 Acres Undisturbed Land (n� CN=7G 4.G Acrcs Ruildings/I'arkin�;/Othcr lmpervious Surfaccs a CN=98 Time of Concentration: Tc=10 minutes (assumed) You also asked that we revicw Ihc drainagc system downstrcam of thc sitc. 1 completed thc review on October 25, 1996 (overcast, not raining) and have ettached a sketch with the following corresponding comments: A Drainage discharges from the Site into the edja�ent creek Gom the easterly wetland area at the point labeled "start". The creek flows south and then east for a total of approximately 400' where it enters a 36"0 concrete culvert. At the head water of the culvert there is a 1 acre depression that appears to back up during large tlows. The creek along this reach slopes at 1% to 2%, is approximately 1.0' to 1.5' deep, is approximately 8' wide, has side slopes beyond the [low path with thick vegetation, and a potential depth of over 6' on hoth sides; B The 36"0 concrete culvert is approximately 350' long and conveys the Flow through the 2 commercial sites (an RV sales lot and an auto wrecking lot). The Flow was approximately 0.75' to I.0" deep as it en;ered the culvert. Approximately 120' south of the entrance the pipe passesthrough a catch basin receiving drainage from the graveled RV sales lot. The pipe along this reach slopes at I% to 2%; C From the end the culvert the creek conlinues with the same characteristics as Reach A for approximately 250'; D The creek enters a 4' x 4' concrete box culvert under Highway 99 approximately 125' long. The Ilow was approximetcly OJS' to 1.0" dcep as it entered the culvert. The culven along this reach slopes et I% to 2%;