HomeMy WebLinkAboutWPO201700031 Certified Engineer's Report 2019-09-27TIMMONS GROUP
YOUR VISION ACHIEVED THROUGH OURS.
September 27, 2019
Mr. David James
Community Development
County of Albemarle
401 McIntire Road
Charlottesville, VA 22902
608 Preston Avenue
Suite 200 P 434.295.5624
Charlottesville, VA 22902 F 434.295.8317
www.timmons.com
RE: UVA Foundation Golf Facility As -Built Certification (W P0201700031)
Dear Mr. James:
Upon our review of the design documents (seal date 11/8/2017), as built survey (seal date 6/5/19), and field
inspection, we find the construction of the biofilter and underground detention to be in conformance with the
requirements of the Virginia Stormwater Management Handbook.
Review of Bioretention Facility
A field investigation yielded deficiencies that have now been remedied by the contractor, including additions
of mulch and rip rap, as well as repair of areas that were featuring erosion.
Additionally, a review of the design calculations has been done, verifying that the slightly smaller
bioretention facility, that was constructed, meets all requirements of the project, per state and local
guidelines and requirements.
While Sheet C6.03, of the design documents, notes a volume of 4,586 cubic feet required (or a surface area
of 1,872 sf), this is in fact not the required volume for two reasons: first, the associated area in the
calculation is noted as 2.66 acres, which does not agree with any other area within the approved set of
documents per our review (we believe this should have been the 2.22 acres of disturbance). It is also
important to note that neither of these numbers equates to the full drainage area, including offsite area,
which means overflow of non -treated area was anticipated to pass through the bioretention facility. Second,
in the Virginia Runoff Reduction Method spreadsheet tab for BMP Selections (also shown on Sheet C6.03),
0.47 acres of managed turf and 0.53 acres of impervious have been input. Therefore, the level 2
bioretention area should be sized for 1.00 acres, with a BMP Treatment Volume of 2,203 cubic feet.
The as built investigation that was performed by Roudabush, Gale, & Associates verified the 36" depth of
the bioretention soil media using a probe to measure down to the gravel layer. Additionally, their survey
demonstrated a 1,260 square feet bottom of the primary cell of the bioretention facility (see Image 1), with
an average ponding depth of 0.7' to the top of structure S8. Using a surface ponding factor of 1.0, soil
media factor of 0.40, and a gravel layer factor of 0.25 a total volume of 2,709 cubic feet (1,260 x ((0.7x1.0) +
(3.04.40) + (1.00.25))) is available, exceeding the 2,203 cubic feet required.
CIVIL ENGINEERING I ENVIRONMENTAL I SURVEYING I GIs I LANDSCAPE ARCHITECTURE I CONSTRUCTION SERVICES
Image 1: Surface area calculation per Blubeam review of Roudabush, Gale & Associates as -built survey
Review of Underground Detention Facility
Per field investigation and review of the Roudabush, Gale & Associates as -built survey, the underground
detention facility has been installed per plan and will operate as designed. During the field investigation
structures S6 and S3 were opened and observed. The weir wall, along with orifices and trash rack were
observed in structure S3.
Please accept this as the Engineering Certification of the stormwater management structures installed for
the UVA Foundation Golf Facility. If there are any questions, please let me know.
Sincerely,
Craig Kotarski, PE
Principal
Cc: Chris Schooley
Annette Tamblyn
2 1 P a g e