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HomeMy WebLinkAboutWPO201500033 Certified Engineer's Report 2019-08-26PROJECT MANAGEMENT SHIMCIVIL ENGINEERING LAND PLANNING ENGINEERING' August 26t", 2019 John Anderson Albemarle County Department of Community Development Engineering Department APPROVED by the Albemarle County Community D v lop ent Departmert Date File c ► 6 -� l % Regarding: WPO 201500033 Hillbrook Subdivision SWM As -Built Analysis Dear Mr. Anderson, Please find attached a summary of the SWM Plan for the As -Built storm system at Hillbrook Subdivision. Revisions from approved SWM plan: 1. Revision to the undetained sheet flow area to the east of the site: the area of sheet flow has been increased in the northeastern portion of the site near lots 2 & 3 because the ditch in the back of lot 3 was not installed. Instead of being routed to the underground detention, this undetained area sheetflows to the wooded area at the rear of the lot. This is shown as Drainage Area B and includes 0.03 acres of impervious disconnection within this area. The SWM Maps, HydroCAD calculations, and the VRRM spreadsheet have been updated to reflect this revised drainage. 2. Revision to the underground detention at the east of the site near lot 3: This detention now has a 3" drain that outlets into the open space parcel for the subdivision. This drain outlets to a level spreader, thereby maintaining the sheet flow condition originally proposed for this BMP. Items that have not changed, and still comply with the approved SWM plan: 1. The concentrated runoff that flows to the northwest of the site: the as -built condition of this concentrated runoff still complies with the energy balance equation 2. Simple disconnection where specified in the original plan: downspouts where simple disconnection was specified have simple disconnection. Please note that it is the individual lot owners responsibility to maintain this simple disconnection per SWM maintenance agreements 3. The function of the detention pipes at the northwest portion of the site: the detention system functions as designed. 4. The required total phosphorous reduction: the original plan required treatment for 1.02 Ibs TP. This amount was met with a nutrient credit purchase. The as -built VRRM requirements remain the same. The as -built SWM analysis methodology has been kept from the original SWM Plan. The rear portion of the site which outlets into woods had been designed to sheetflow in compliance with 9VAC25-870-66-D. The as -built conditions achieve this design strategy. The as -built 10-yr runoff to the wooded area is less than the pre -development 10-yr runoff. Thus, flood protection requirements have been met. The as -built 1-yr runoff to the wooded area is 0.21 cfs more than the pre -development 1-yr runoff. This increase in runoff is acceptable because the runoff is entirely sheet flow and does not concentrate within the wooded area. This runoff will not cause erosion due to the even slopes and heavy groundcover over which the runoff sheetflows. Since flood protection requirements have been met, and since the physical condition of the site ensure sheetflow, this 0.21 cfs increase in sheetflow runoff from the 1-yr storm is acceptable per standards set forth in 9VAC25-870-66-D. Therefore the as -built conditions meet the approved design requirements for SWIM analysis. Runoff Flow Rates (cfs) 10-yr (cfs) j5heetflow Post- Dev Pre- Dev Post- Dev 0.53 3.29 2.62 If you have any questions please do not hesitate to contact me at michael@shimp-engineering.com or you may contact Justin Shimp at Justin@shimp-engineering.com or by phone at 434-227-5140. iviicnaei unanaier Shimp Engineering, P.C.