HomeMy WebLinkAboutWPO201300032 Review Comments Engineering Approval 2021-08-11V
401 McIntire Road, North Wing
County of Albemarle Charlottesville, VA 22902-4579
COMMUNITY DEVELOPMENT DEPARTMENT Telephone:434-296-5832
WWW.ALBEMARLE.ORG
VSMP Permit Plan Review - Amendment #2
Project: Cascadia Blocks 4-7, Final ESC and SWM Plan Amendment #2
Project file#: WPO2013-00032
Plan preparer: Jimmy Taggart, PE, Dustin Greene, PE —Roudabush, Gale & Associates, Inc.
fJTaegart(a),,roudabush.com. dgreene(o)roudabush.com ]
172 S. Pantops Drive, Charlottesville, VA 22911
Owner or rep.: Cascadia Development LLC [170 S. Pantops Drive, Charlottesville, VA 22911]
Charlie Armstrong(crmstrongAsouthem-development.com )
Received date: 19 Feb 2021
(Rev. 1)
23 Apr 2021
(Rev. la)
25 May 2021—Applicant email (C. Armstrong to county, May 25, 2021 10:08 AM)
(Rev. 2)
18 Jun 2021 [ ref. Applicant email, 7/1/2021 8:42 AM]
(Rev. 3)
2 Aug 2021 [ ref. Applicant email, 8/2/2021 1:50 PM]
(Rev. 4)
11 Aug 2021 .PDF attachments to email, 8/11/2021 8:48 AM
Date of comments:
2 Apr 2021
(Rev. 1)
6 May 2021
(Rev. la)
27 May 2021—County email to Applicant /RGA (5/27/2021 6:09 PM)
(Rev. 2)
e-comments: 7/30/2021 1:12 PM, 7/30/2021 2:25 PM
[Also, county email to Applicant, 7/l/2021 8:17 AM]
(Rev. 3)
10 Aug 2021—Approval pending rev..PDF
[Also, county email to Applicant, 8/6/2021 8:58 AM]
(Rev.4)
8/11/21—droved
Also, email: 8/10/2021 4:14 PM, 8/10/2021 4:16 PM
Reviewer:
John Anderson
SWPPP, ESOP, Mitigation Plans not reviewed with this correspondence —NA (SWM Plan Amendment)
WPO201300032 —Amendment 2, explained in cloud text image, below
Ensure proposed Amendment wet pond design and elevations yield post -developed Q1.5, Q2.0, and Q10
rates less than or equal to the approved post -developed release rate summary shown on pg. 7 of Calculation
report d. 2/18/21 (PE -seal 2/19/21). Reasons: 9VAC25-870-488 includes specific requirements concerning
"no increase in the volume or rate of runoff." [9VAC25-870-48.A.1. - httl)s:Hlis.virginia.gov/cgi-
bin/legp604.exe?000+reg+9VAC25-870-48 ]. Also, significance of this SWM facility: it provides water
quantity and quality control for 60.77 acre Cascadia subdivision, yet proposal reduces pond volume.
Quoting VSMP Amendment 42 Narrative, sheet SW L `The northeastem portion of the pond bottom and
the eastern 2:1 pond upland slope was located in cut with depths of up to 30. Significant rock was
encountered in these locations at the pond elevations, which was fairly hard rock, but was also fairly
rippable. The contractor ripped as much of the rock as feasible using a hoe ram but encountered refusal at
bedrock. For this reason, the provided pond volume is approximately 90% of the originally approved
plans.' This is first notification of decision or circumstance concerning sediment basin excavation issues,
which now affect permanent SWM facility wet pond design, we are aware of. There are implications,
foremost of which: Any approved amendment to this SWM facility pond design cannot propose predictive
/modeled Q1.5, Q2.0, or Q10 flows higher than flows approved with WP0201300032, 9/16/14, else it loses
9VAC25-870-48 (Grandfathering) eligibility. (Rev. 1) Addressed. Applicant response (letter d. 4/23/21):
`An exception to the orifice size has been requested with this submittal. We are showing that a 2.8" orifice
provides flows that are less than originally approved post -developed flows. A 3.0" orifice has been
Engineering Review Comments
Page 2 of 6
provided. See the narrative for the differences in these flows.' Note: A separate exception request
approval will follow.
SW24 Storm Sewer Calculations v V
SW25 Storm Sewer Calculations
N
A
VSMP AMENDMENT /2 FOR WP02013ODD32
THIS AMENDMENT IS TO CONVERT THE SEDIMENT BASIN FACIUTY IMMEDIATELY ADJACENT TO ROUTE 20
NORTH INTO THE PERMANENT WET POND FACILITY THAT WILL PROVIDE STORMWATER MANAGEMENT WATER
QUANTITY AND OUAUTY CONTROL FOR THE 60.77 ACRE CASCADIA SUBDIVISION. THE NORTHEASTERN
PORTION OF THE POND BOTTOM AND THE EASTERN 2A POND UPLAND SLOPE WAS LOCATED IN CUT WITH
DEPTHS OF UP TO 30. SIGNIFICANT ROCK WAS ENCOUNTERED IN THESE LOCATIONS AT THE POND
ELEVATIONS, WHICH WAS FAIRLY HARD ROCK, BUT WAS ALSO FAIRLY RIPPABLE. THE CONTRACTOR RIPPED
AS MUCH OF THE ROCK AS FEASIBLE USING A HOE RAM BUT ENCOUNTERED REFUSAL AT BEDROCK. FOR
THIS REASON, THE PROVIDED POND VOLUME IS APPROAMATELY 90% OF THE ORIGINALLY APPROVE PLANS.
THE PERMANENT POOL HAS BEEN LOWERED AND THE OUTLET STRUCTURES HAVE BEEN UPDATED SO THAT
THE STORMWATER FACILITY PERFORMS WITH THE INTENT OF THE ORIGINAL APPROVED PLANS AND
CALCULATIONS. PLEASE SEE SHEET 14 FOR THE UPDATED DESIGN DETAILS AND THE H&H CALCULATIONS
FOR COMPLIANCE WITH THE ORIGINAL APPROVED PLAN.
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SHEET
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2. Additional (Rev. 1) Supportive: see previous item. No design revision required
a. P. 2, Calc. report
Predeveloped, Allowable Flows:
Q1.5=0.52cfs
2.0=0.67cfs
Ql .98 cfs
Postdeveloped ted Flows:
Q1.5 = 0.cfs
Q2.0 = 0.60 60 cfs
Q10=8.91cfs
-, r-^Ic report
Allowable Release Rate:
Q1.5 (allowable) = 3.16(42,253 / 256.568) = 0.520 cfs
Q2 (allowable) = 3.83(47,0451269.636) = 0.668 cfs
Q10 (allowable) = 27.83(172,498 / 534,481) = 8.982 cfs
Post -Development Release Ra Summary:
Q1.5 Post Development Routed - 0.48 cfs < 0 20 cfs
02 Post -Development Routed 0.52 cfs <0 68 cfs
Q10 Post -Development Routed 8.83 cfs 8.982 cfs
c. Please note that Amendment proposes Q1.5 that is 0.04cfs >Approved Q1.5 (0.48cfs), and Q2.0,
Q 10 peak flow rates that are each 0.08 cfs higher than 9/16/14 approved post -developed release
rates. These increases, if not eliminated, void eligibility for Amendment review under SWM
design technical criteria IIC. [ Grandfathering] (Rev. 1) Supportive: see item 1., above.
3. 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.' Asfollow-uj2: This Amendment plan proposes modification to existing
Engineering Review Comments
Page 3 of 6
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. (Rev. 4) Withdrawn. Applicant response (letter, 6/18/21): `The contractor
is currently building the sediment forebay and will provided (sic) 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.'
4. Emergency spillway elevation:
a. Compare approved emergency spillway elevation, 356.10', with proposed, 356.00'.
b. Compare approved 10-yr elevation 355.92' with proposed 10-yr elevation, 356.03'
c. Revise design to ensure emergency spillway is not activated during the Q10 event. (Rev. 1)
Addressed.
d. Provide at least 0.18' freeboard between ]0-yr elevation and emer. spillway elevation. (Rev. 1)
Addressed,
5. Emergency spillway outfall:
a. Approved 16' emergency spillway width at weir elevation 356.10 outfalls into a riprap channel
with same section (16').
b. Approved riprap outfall emergency spillway design includes proposed grading intended to bend
discharge away from adjacent property line, and toward a culvert beneath site access to Broadus
Memorial Baptist Church.
c. Approved design shows proposed and existing grades, to PL and beyond.
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. (Rev. 2) Partially addressed. Ref. Applicant
email, 7/l/2021 8:25 AM (Applicant comment response letter d. 18-Jun 2021): `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.' As ollow-up:
County email to Applicant, 7/30/2021 1:12 PM.: `Although p. 3.03-2 of VSMH, 1999, states `that
an armored emergency spillway over the top of an embankment should be designed by a qualified
professional,' Engineering maintains that the hydraulic design guidance that follows at item p.
3.03-6 applies; namely: `The exit channel should have a straight alignment and grade and, at a
minimum, the same cross-section as the control section.' Please identify the control section of the
revised emergency spillway, then, provide exit channel width that, at a minimum, is the same
cross-section as the control section. Please find VSMH, 1999, Min. Std. 3.03 Attached. Also,
since the proposed revised emergency spillway width is increased from 16' to 40' at the spillway
inlet channel, yet narrows to 16' at the toe, Engineering requests Amendment provide an
emergency spillway section (detail) at the spillway exit channel, to ensure Q100-yr does not leave
confines of the revised emergency spillway at the toe. Last, the county engineer offered
Engineering Review Comments
Page 4 of 6
perspective that a berm (proposed grade shown on plan sent with email 7/l/2021 8:25 AM) is
inappropriate, since berms, if breached, fail. Engineering requests conventional tie-in grading
along west edge near inlet channel of emergency spillway, instead of a berm. A berm suggests the
emergency spillway may be perched too high relative to immediately adjacent embankment
grade/s. Failure and erosion are primary concerns with a berm at the edge of an emergency
spillway.' (Rev. 3) Addressed; ref. Applicant email, 8/2/2021 1:50 PM. As follow-up, ref county
email request for revised .PDF (rev. to 6-18-21 print, 7/l/2021 8:25 AM e-attachment). County
email, 8/10/21 4:14 PM; 8/10/2021 4:16 PM. (Rev. 4) Addressed, ref. Applicant email
(8/11/2021 8:48 AM) with rev. WPO Amendment #2, .PDFs.
e.
f. With revised emergency spillway riprap outfall design, provide detail to specify emergency
spillway riprap keyed into embankment. Label riprap class and specify filter cloth underlay. (Rev.
1) Addressed.
g. Provide Note to clear and maintain the emergency spillway free of vegetative growth. See 6/l/20
satellite image, below. Vegetation covers lower reaches of Ex. sediment basin stone outfall. (Rev.
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 seefollow-up at item 5.d., above. (Rev. 3,4) Addressed. Ref. item 5.d.
above.
Engineering Review Comments
Page 5 of 6
i. Provide design rationale as riprap outfall narrows by '/2 to /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 ollow-W: Please seefollow- u at item 5.d., above. (Rev. 3, 4)
Addressed. Ref. item 5.d., above.
6. Ensure primary spillway barrel slope is listed correctly, both approved and proposed principal riser pipe
slopes appear incorrect. (Rev. 1) Addressed.
7. Revise Detail 3/SW 14 since label reading `outfalls into rip -rip channel with same section' appears
inaccurate. (Rev. 1) Addressed. [Image removed with Rev. 1 comments.]
8. SWM facility wet pond sediment forebay does not yet appear to be constructed. Ref. 6/l/20 satellite
image, below: [Image removed with Rev. 1 comments]
No rationale is presented for change to SWM-1 forebay design details (proposed lower floor elevation, 10'
aquatic bench vs. 15' approved aquatic bench), these changes are not approved. Please ensure SWM-1
forebay is constructed per approved SWM-1 forebay detail, below [Image removed with Rev. 1
comments]: (Rev. 1) Addressed,
9. New: Propose barrier with waming on HOA property at top of steep 2:1 cut slope that falls to SWM wet
pond. There are multiple risks: 2:1 slope 44' vertical interval between elevation 400f and 356f is not
traversable. Design proposes permanent pool depth of not less than 5.60' ft. with depth approaching 13'
during event that raises pond elevation to that of emergency spillway (355.93'). At top of 2:1 slope, single-
family residences stretch from the lot south of and adjacent to 1849 Marietta Drive to 1891 Marietta. For
the moment, satellite imagery indicates eight occupied SF dwellings: 1849, 1855, 1861, 1867, 1873, 1879,
1885, and 1891 with back lawns immediately adjacent to 44' vertical 2:1 cut slope ending in 5.6-13' deep
permanent pool. Barrier must be permanent and include placard warning (text/symbols) of hazardous risk
to residents. Not required with the initial plan, comment is justified, then and now, to ensure reasonable
safety and to protect public welfare. Cascadia BOA is not required to fund this expense, rather, this is a
development expense. Landscaping alone is not sufficient at the top of the slope immediately adjacent to
residential lawns. Elsewhere, for the entire perimeter of the wet pond, provide either hard structure barrier
(fencing subject to ARB approval) or impenetrable thorn -bearing landscaping to discourage human trespass
with all but minimal gap or break in hard structure /landscaping at pond embankment elevation/s. There
shall be no gap in barrier at top of slope immediately adjacent to residential back lawns for the currently
built eight, and future -assumed ninth SF structure that may be built on parcel south of 1849 Marietta Drive.
(Rev. la) Comment withdrawn. Ref. county email, 5/27/2021 6:09 PM.
C. N JMI': MN PPP: erosion Control Plan —NA
Engineering Review Comments
Page 6 of 6
D. Mitigation Plan —NA
WPO201300032_Caseadia blocks 4J_VSMP_Amendment 2_rev4_Approv081121