HomeMy WebLinkAboutWPO201300032 Correspondence 2021-06-18®ROUD"US11, GALE &c ASSOC.
A PROFESSIONAL CORPORATION Serving Virginia Since 1956
June 18th, 2021
Community Development Department
401 McIntire Rd
Charlottesville, VA 22902
RE: WP0201300032 — Cascadia Blocks 4-7, Final ESC and SWM Plan Amendment#2
Dear John,
We have received your comments dated 5/6/2021 and have provided the following
responses:
Now is an appropriate time to provide geotechnical inspection reports for construction of the
permanent wet pond embankment. Engineering seeks to confirm embankment construction
(compaction, suitable material, cutoff trench, impervious core, etc.). Please submit relevant
permanent geotechnical reports for the existing 13.3' high embankment with next submittal of the
WPO Amendment application. (Rev. 1) Not addressed. Applicant: `This is an Amendment
submittal. We are not asking for final as -built review or trying to get off of any bonds. I believe
this is a little early for inspection reports at this time and not required for amendment approval.'
As follow-up: This Amendment plan proposes modification to existing conditions that already
deviate from design; request for As -built information for a SWM facility that does not obtain
design parameters before applying or approving additional modification is reasonable. For
example, equipment mobilized to modify the emergency spillway could at the same time effect
adjustments that As -built information may identify are needed. Please respond to initial comment
for embankment construction /other information.
RESPONSE: The contractor is currently building the sediment forebay and will provided
embankment inspection reports for the sediment forebay and the wet pond embankment
concurrently. This submittal is intended to address all remaining comments except for this
one which will be handled as the construction is completed and the inspections are done.
Since construction is underway it is imperative that we receive technical approval minus this
last comment.
999 Second Street SE. Charlottesville. VA 22902. www.Roudabush.com 434.977.0205
Cascadla Pond tt2 VSMP Amendment
6/18/2021
4. Emergency spillway outfall:
d. Revise emergency downslope riprap spillway design to accommodate a threefold
increase in weir width, from 16' to 48'. Ref. images, below: (Rev. 1) Partially addressed.
Applicant: 'The emergency downslope riprap spillway design has been revised to include the 40'
increase in the emergency spillway. A permanent diversion will be used at the new end of the
emergency spillway to create a non -erodible channel down to the original 16' spillway near the
outside toe of the dam.' As follow-up: Please add section arrows, notes, or labels to plan view to
clearly indicate placement and purpose of modified permanent diversion dike. Proposed grade
must redirect flow in the spillway; provide spot elevations or 0.5' proposed contours to
emergency spillway so that grade redirects flow in the spillway. Design may rely on a modified
permanent diversion dike to redirect flow, but without grade change, proposed modified diversion
dike may experience inertial force not yet analyzed, or force it cannot withstand.
RESPONSE: Proposed grade has been shown to redirect flow in the spillway with spot
shots. The proposed design decreases the velocity flowing over the emergency spillway into
the downhill channel to the toe of slope. See the comparison calculations for the original
approved flow through the emergency spillway and the updated flow through the emergency
spillway and you will see that the velocity has actually decreased. I would be more
concerned if this were a vegetative spillway but am not concerned about a reduction of
velocity making that turn in a rip rap spillway.
h. Provide and label existing and revised proposed grades in vicinity of
emergency spillway, to property line (PL) and beyond. Ref. approved plan image, below: (Rev.
1) Partially addressed. As follow-up: Please see follow-up at item 5.d., above.
RESPONSE: Proposed grade has been shown to redirect flow in the spillway with spot
shots. The proposed design decreases the velocity flowing over the emergency spillway into
the downhill channel to the toe of slope.
Provide design rationale as riprap outfall narrows by Y to 2/3 the 48' width at
weir. Consider velocity, momentum, and scour potential as riprap narrows, 48'
to 16' (or other dimension). (Rev. 1) Partially addressed. As follow-up: Please
see follow-up at item 5.d., above.
RESPONSE: Proposed grade has been shown to redirect flow in the spillway with spot
shots. The proposed design decreases the velocity flowing over the emergency spillway into
the downhill channel to the toe of slope.
999 Second Street SE. Charlottesville. VA 22902. www.Roudabush.com Page 2 of 3
Cascadia Pond #2 VSMP Amendment
6/18/2021
Email comment 5/27/2021
Comment request for barrier around perimeter of the retention basin is withdrawn. From a water
quality perspective, since As -built volume of the pond appears less than design volume, please
confirm there is no effect on storm water quality. Specifically, please consider Table. 3.06-1 (p. 7,
Attached .PDF), and whether decreased WQ volume shifts retention basin type from higher target
phosphorus removal efficiency to a lower phosphorus removal efficiency. I missed this, but joint
review picked it up. If type changes, please provide remedy —nutrient purchase, for example. Thanks
for your help and patience. -take care, best, J. Anderson
RESPONSE: Email sent back on 5/28/21 to John Anderson "The new water surface
elevation was chosen to maximize the storage for the 1.5yr, 2yr, 10yr and 300yr storm
events. The updated SWM-2 Stage Storage 4*WQV and aquatic bench still provide the
required 65% removal rate for the drainage area. See page 4 of the H&H Calculation packet."
No response from Engineering on this and it is assumed that this comment has been satisfied.
There has been no change to the calculation packet at this time so there is resubmittal of the
H&H calculation packet.
We trust the above adequately addresses your comments. Please let me know if you have any questions
or require additional information.
Best Regards,
Dustin Greene, PE
999 Second Street SE. Charlottesville. VA 22902. www.Roudabush.com Page 3 of 3