HomeMy WebLinkAboutWPO201600074 Correspondence 2021-06-22f 608 Preston Avenue
P 434.295.5624
Suite 200 IF434.295.1800
T I M M O N S GROUP
Charlottesville, VA 22903 www.timmons.com
June 21, 2021
John Anderson
Albemarle County Community Dev.
401 McIntire Rd
Charlottesville, VA 22902
RE: Woodbrook ES — VSMP Plan Review — WP0201600074 - Comment Response
Letter
Dear Mr. Anderson:
We have reviewed all of your emailed comments from May 27, 2021 and made the
necessary revisions. Please find our responses to the comments below in bold lettering.
1. Revise C0.0 to include reference to Amendment #2.
The cover sheet has been revised to read: "VSMP Plan — Amendment #2."
2. With respect to revised Calculations, May 3, 2021 (and C0.0 revision cloud, text below),
please:
a. Revise p. 3, Calc. packet, Project Narrative to explain change in 72" CMP
storage facility, SWM B, from 170 LF to 155 LF. Narrative is silent on topic of
change to SWM B. It is atypical for large diameter detention systems to provide
8.8% excess storage capacity, but the alternative 155 LF design appears to result
in Qp=-de , 1_year = 2.17cfs, which Is <2.19Q post/1-yr allowable -
Additional explanation has been added to the project narrative. Note,
the weir plate, and specifically the lower orifice was revised to still allow
for compliance with the I and 10-year post -developed flow requirements.
The pipe reduction does, however, result in less "extra" capacity beyond
the 10-year storm event. Grated inlets are provided on the facility and
positive drainage away from buildings is maintained in the event of a
large storm event exceeding storage capacity.
b. Please revise plan sheet C 10 to provide CPos,_d�/,_year Actual; that is, 2.17cfs, and
reference Calculation booklet hydrograph.
Sheet C 10 has been revised to correspond correctly to calculations.
ENGINEERING I DESIGN I TECHNOLOGY
c. Please send .PDF of Calculation packet d. April 6, 2017 for comparison of initial
vs. As -built routings (hydrographs). Initially approved (print/digital) copy of 4/6/ 17
calculation booklet is not immediately or easily located. Request .PDF to
complete review.
A PDF of the 4/6/ 17 calculations have been provided.
d. Please confirm SWM B As -built condition has no effect on removal rate,
efficiency, or total phosphorus removed by the on -site cartridge filtration
system. Engineering does not anticipate any effect or increase in quantity of
phosphorus released from the site due to change in SWM B design, from 170' to
155 LF 72" CMP storage facility.
The Bayfilter Cartridge system is sized based on incoming flowrates from
I -year and smaller events. The orifice on the revised SWM B weir plate
was designed to ensure that the flow rate from the I -year storm
remained at or below the original design flowrate of 2.19cfs.
Image below from LOR3, SDP201600069 (identical with VSMP Revision Narrative):
LETTER OF REVISION NARRATIVE:
THIS PLAN PROPOSES THE FOLLOWING REVISIONS TO THE APPROVED MAJOR
SITE PLAN AMENDMENT BY TIMMONS GROUP WITH REVISION DATE OF 4/27/17
REVISIONS ASSOCIATED WITH THIS LETTER OF REVISION ARE CLOUDED
1. REVISION TO THE 72- CMP STORAGE FACILITY. SWM B. FROM 170 LF TO 155
LF TO MATCH AS -BUILT CONDITION. REVISIONS TO THE SIZE AND ELEVATION
OF THE ORIFICES ON THE SWM B STEEL WEIR PLATE UPDATED WATER
QUANTITY CALCULATIONS SUMMARY. THE UPDATED CALCULATION BOOKLET
IS PROVIDED UNDER SEPARATE COVER. NOTE, THE POST DEVELOPED
DRAINAGE AREA AND LAND COVER CONDITIONS ARE UNCHANGED FROM THE
APPROVED PLAN
2 UPDATED STORM SEWER PLAN AND PROFILES FOR STRUCTURES 208-200
AND 211-204 BASED ON AS -BUILT SURVEY DATA
3. ADDITION OF STRUCTURES 206E AND 206D. HORIZONTAL AND VERTICAL
LOCATIONS ARE BASED ON AS -BUILT SURVEY DATA PLAN AND PROFILES
INCLUDED
We have included PDF copies of the plans and calculations for your review. If you have any
questions or comments, please feel free to give me a call at 434.295.5624 or email at
clint.shifflett(cct_�timmons.com .
Sincerely,
Clint Shifflett, PE
Project Manager