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.