10412 ROSEWOOD AVE ROSEWOOD PARK WHOLE SITE DEVELOPMENT 2017-04-07

V. Upstream/Downstream Analyses Upstream• Note: Additional detailed information pertaining to the upstream drainage system/basins is provided on a drawing on the next page. The two upstream drainage basins for this site consist of (1) Offsite basin "A" - a 9.6 acre basin located along the east side of Airport Road between 100th St SW and 103rd St SW, and (2) Offsite basin "B" - a 23.1 acre basin !ocated north and west of the development sile, but south of 103rd St SW. The 100-year flows generated by Offsite Basins "A" and "8" were each modeled, and then the two individual offsite hydrographs were added to produce a resultant hydrograph. (A printout of the individual offsite basin hydrographs and the resultant combined hydrograph is shown on the second page following this page.) Results showed that the developed upstream 100-year flow is 4.65 cfs. The total proposed on-site 100-year release is 0.37 cfs. Therefore, the totai 100-year discharge from the developed site will be 5.02 cfs. The pair of existing 12" culverts located at the downstream end of the subdivision was checked for adequacy. The calculations show that the two 12" culverts will handle the proposed 100-year flow at a depth of 0.76 feet in each pipe. It should be pointed out, that the Chief Engineering Officer (following his review of the initial drainage report whi�e considering a waiver request for this project) reported that he and the Senior Drainage Plans Examiner observed flooding at the site location on January 18, 1986. He reported that the observed water lev�l was approximately 3" over the driveway above the twin 12" culverts located just south of the development site. He also reported tl�at the water level was approaching, but still somewhat below, the level of the house located in the southeast corner of the property. From this information, we have concluded that the high water level they observed on January 18, 1986 was at approximately the 534+/- elevation level. We had made numerous inquiries about possible flooding on the subject property when we were preparing our initial drainage report, and we had been informed in each case that the property had not been flooding. We have been unable to determine to what extent flooding may have occurred on the property between 1966 and the present time, but we have concluded that the reported 1986 flooding probably represents a"worst-case-scenario". The magnitude of the storm event associated with the 1986 flooding is unknown. Also, it is unknown whether or not the upstream drainage system andlor detention areas have significantiy changed since 1986. Finally, it is unknown whether or not the twin 12" pipes at the south side of the subject property were partially plugged or if t�