HomeMy WebLinkAboutWPO201200047 Review Comments Stormwater Management Plan 2012-07-27ALg�,��
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: WPO- 2012 - 00047, Foothill Crossing Phase H & III ESC and SWM Plans
Plan preparer: Mr. Scott Collins, PE; Collins Engineering
Owner or rep.: Foothills Crossing Inc.
Date received: 10 May 2012
11 July 2012
Date of Comment: 18 June 2012
27 July 2012
Engineer: Phil Custer
Michael Koslow
The 2 "d submittal of E &SC and SWM plans for Foothill Crossing Phase II & III, received on 11 July
2012, has been reviewed. This review does not include comments on the road plans (SUB2012- 00057).
The plans cannot be approved as submitted and will require the following changes /corrections prior to final
approval.
C. Stormwater Management Plan Review Comments (WPO- 2012 - 00047)
1. The applicant might not be fully mindful of how the county will review this property with regard
to Stormwater Treatment. For all properties that drain directly to the Lickinghole Regional
Stormwater Management Facility, stormwater quality is taken care of by the payment of pro -rata
fee. This fee, for the entire property, was already paid in August of 2009. Therefore, the county
considers stormwater quality treatment for this property already addressed. The applicant may
simply convert these facilities to dry detention basins. However, if this application continues to
propose water quality facilities, they must be designed according to the standards of the VSMH.
The following comments have to do with review of the water quality facilities currently proposed
by the applicant (the remaining comments have to do with just detention):
a. This parcel is within a water supply protection area and the removal rate computation must
refer to this row, not the development area row. This puts the target removal rates for
facilities 1 and 2 as 70% and 57 %, respectively. This causes Facilities 1 and 2 to
undertreating their watersheds.
b. To receive a 35% removal rate, the orifice on the extended detention facility must be sized
to drain the 2xWQV in 30 hours using the equation in Chapter 5 of the Virginia
Stormwater Management Handbook. The current proposal is not satisfactory.
c. The forebay in the extended detention facility is not effective. Most discharges will flow
through the forebay with very little settling or other treatment.
(Rev]) Comment has been addressed.
2. Overall, the setup of the detention calculations is not correct. The following comments are
provided for Sheets SWM -1, SWM -2, and the overall outline presented in the detention
calculations.
a. On SWM -2, the northeast corner of DA- lA is not accurate; Park Ridge Road north of
Cardinal Crest Court is captured in the pipe system to Stormwater Facility 1.
(Rev]) Comment has been addressed.
b. On SWM -2, the applicant assumes that 50ft south of the ROW for Brookstone Court is
redirected to Stormwater Facility 1 which is impractical and not even exhibited by the
current grading plan.
(Rev]) There are too many variables associated with the projected build -out of future
phases of this project to approve a final build -out condition with this submittal. Please
remove the Post - Development inset at Final Build Out references from sheet SWM -2.
As future phases are developed, the routing and detention calculations for SWM #1 will
need to be updated. Detention compliance for Phases II & III have been confirmed as
satisfactory with this plan.
c. Stormwater Facility 2 must compensate for the free release of the increased discharge from
Phase I, half of lots 9 -17, and the uncaptured portion of Park Ridge Road. This must be
clearly demonstrated in the Detention Compliance Tables.
(Revl) Comment has been addressed.
d. Stormwater Facility 1 must compensate for the free release of the increased discharge from
most of lots 76 -84, 39 -53, 27 -32, and the rest of the Eastern Connector that has not
currently been accounted for in this stormwater management detention analysis (south of
post -dev 1B). Except for a Drainage Area 1B, these free release areas seem to be ignored.
Because water quality requirements are covered by the Lickinghole basin, many lots and
roadways can be released freely but this increase in post - development discharge must be
accounted for and mitigated by overdetention of the areas that are directed to a detention
facility. This calculation must be clearly exhibited in the Detention Compliance Tables.
(Rev]) See comment 2B.
e. In the pre - development condition, rain falling on the lagoons does not runoff.
(Rev]) Comment has been addressed.
The following modifications to the routings are required:
a. The routing of Stormwater Facility 2 does not match the design of the lowest orifice.
(Rev]) Comment has been addressed.
b. The antivortex devices should be routed "in riser" not "through dam ".
(Revl) Comment has been addressed.
c. The invert used for outlet barrel in the routing should be the invert out of the riser, not
invert out of the pipe.
(Revl) Comment has been addressed.
d. Please update the routings as each facility is modified. Additional comments may be
required if new errors are introduced to the routing calculations.
(Revl) Comment has been acknowledged.
4. All dry facilities need low flow channels from the pipe outlet directly to the riser that is sized to be
non - erosive during the 2 year storm and have a minimum slope of 0.5 %.
(Revl) Comment has been addressed.
5. Please provide more detail on what needs to be done to the riser in stormwater facility 1. There are
also no orifice or elevations specified. Without the routing, which is hard to read without a
familiarity with program, it would be difficult to determine all characteristics of the riser.
(Rev]) Comment has been addressed.
6. The details for both facilities show a impervious core and cut off trench, but facility 1 is already
constructed. Does the contractor need to reconstruct this facility to provide the impervious core on
the plan?
(Revl) Comment has been addressed.
7. Emergency spillways must be proposed in cut. For Stormwater Facility 1, it seems a deeper
spillway will likely be necessary because a 75ft spillway in cut would be impractical. The spillway
to Stormwater Facility 2 simply needs to be rotated slightly.
(Rev]) Comment has been addressed.
8. Please replace all references to Trashrack Technical Bulletin #7 with a reference to the exact
trashrack parameters within Chapter 2 of the Virginia Stormwater Management Handbook.
(Rev]) Comment has been addressed.
9. The perforated pipe in the extended detention facility is not an acceptable trashrack.
(Rev]) Comment has been addressed.
10. What kinds of seepage control have been constructed or proposed for Stormwater Facility 1?
(Rev]) Comment has been addressed.
11. Access to Stormwater Facility 1 must also be provided from Phase II since it will be the sole route
to this facility for an indeterminate amount of time.
(Rev]) Comment has been addressed.
12. Direct vehicular access to the forebay and pipe system outlet into Stormwater Facility 1 is needed.
(Rev]) Comment has been addressed.
13. The 90 degree turn just after the 10% slope on the grass access path is dangerous and unnecessary.
Please put more of a horizontal curve on this road with a consistent grade from the pipe (from
Brookstone) crossing.
(Rev]) Comment has been addressed.
14. Stormwater Facility Maintenance Agreements are needed for all parcels with a stormwater facility
proposed on it. Please submit these completed documents to Ana Kilmer after reviewing the
information online.
(Rev]) Comment has been acknowledged.
15. It is recommended that the embankment of Stormwater Facility 2 is designed for a full buildout
scenario of its watershed for the 100 -year storm to avoid any increase in liability for the HOA
when the adjacent property is developed.
(Rev]) Comment has been addressed.
16. Once the plans are approved, please provide a completed Bond Estimate Request Form to the
County Engineer to receive a SWM bond computation.
(Rev]) Comment has been acknowledged.
17. (Rev]) Easement label for SWM Access Road on sheet S -3 is proposed with a 20' ACSA Access
Easement. Was this easement intended to provide access for SWM Maintenance and ACSA
access?
D. Erosion and Sediment Control Plan Review Comments (WPO- 2012 - 00047)
1. Please refer to Comment A.2. The ESC plan cannot be approved without all road plans approved.
Considering the county's 9month deadline for permanent seeding, it seems breaking up the
development into constructible chunks is prudent.
(Rev]) Comment has been addressed.
2. Please provide all state and federal permits for disturbances to streams and wetlands.
(Rev]) Comment has been addressed.
3. ESC -3 is essentially a blank sheet. ESC -2 contains the entire ESC plan crammed into one sheet at
a scale that is very difficult to read and review. Please disperse the ESC information in two
phases: one with existing grades and perimeter measures and the second one with final proposed
contours and road infrastructure in place. Show the drainage area to each sediment trap and
sediment basin for each phase and make sure each facility is sized for the phase when its drainage
area is the largest. A smaller scale, no greater than 50, must also be used for each of the phases.
Once the plan is clearer, additional comments may be provided.
(Revl) As designed, runoff from Park Ridge Dr. would divert past Structure 38 to ST2. This is
unacceptable. Please revise the plan so ST2 is not blown out by providing a sump with
sufficient freeboard around Structure #38 so flows are directed to SB2 which is adequately
designed to handle this drainage area.
4. Many of the diversion dikes at the perimeter of the construction site are too flat to be constructed
properly, shown going uphill, or proposed perpendicular to existing gullies which will result in a
blow out. Please add more sediment trapping measures, where necessary, and adjust problematic
diversions so that they can be constructed with a reasonable downhill slope. The diversions west
of sediment basin 5 to the western corner of the site and north of sediment trap 2 are the biggest
concerns. Sediment traps or basins will likely be needed around MH 32 and MH 24.
(Rev]) Comment has been addressed.
5. Please remove the internal diversion from the site.
(Rev]) Comment has been addressed.
6. The edge of the existing pavement is a few feet after Raven Stone Road and this is where the
limits of construction must begin. However, providing a construction entrance here with the
existing houses on either side of the road would be difficult to achieve. Therefore, the CE(paved
wash rack) must be moved farther south into the watershed of Sediment Trap 1 with a ROW
diversion directing runoff across the travelway. Construction road stabilization from the edge of
existing pavement to this ROW diversion will be needed for the entire width of the stream
crossing. This area must remain clean from mud.
(Rev]) Please show the RWD proposed near the north end of the construction entrance in ESC
Phase I in Phase II as well.
7. The construction of the stream crossing for the Eastern Connector must comply with County
Engineer Commentary #5. Sediment Basin 4 can be used as one of these flanking sediment
trapping measures if it is lowered considerably.
(Rev]) Comment has been acknowledged.
8. Please detail and provide typical VESCH references for how the construction of the stream
crossing will occur. For example, will a temporary stream crossing be necessary? If so, where and
what size is the crossing?
(Revl) Comment has been acknowledged.
9. Sediment Basin 4 becomes isolated from half its drainage area when the construction of the
Eastern Connector occurs because it will act as a diversion keeping dirty water on the eastern side
of the road. Please provide another measure adjacent to the stream on the eastern side of this road.
(Rev]) Comment has been acknowledged.
10. There are critical issues with the design of Sediment Basin 1. First of all, the construction of the
northern end of the Eastern Connector cannot be treated by the sediment basin until the storm pipe
has been placed, which is unacceptable. Secondly, structure 106 becomes the low point in the
facility once pipe 105 is constructed. The riser and structure 106 will be about the same elevation
and water will escape out the barrel and downstream of structure 106, which is a problem. Lastly,
the outlet of pipe 105 will be placed below the water elevation in the sediment basin and will be a
risk of clogging. Please move the invert out of pipe 105 to at least the wet storage elevation so
flushing out of the pipe can occur. As a result of these comments, Sediment Basin 1 will likely
need to be moved closer to the stream.
(Revl) Comment has been acknowledged.
11. In the construction sequence, there is a mention of construction of a greenway through the wooded
area. Please clarify. If a greenway trail is to be constructed, please identify what standard it must
meet, show all improvements for it, and include all necessary ESC measures in the ESC plan. If a
greenway trail is not to be constructed by the applicant, please remove any reference to it from the
plan set.
(Rev]) Comment has been addressed.
12. Please provide Inlet Protection (IP) on all inlets proposed in Phase II of the ESC plan.
(Rev]) Comment has been addressed.
13. Inlet protection for curb inlets on grade will simply cause the runoff to pass on to the next structure
until a sump or basin is encountered. This would cause the drainage area to Sediment Trap I to
increase dramatically in Phase I1. Please convert this facility to a basin sized for its drainage area
from Park Ridge Road and the area to the north or design an alternative method that addresses this
concern.
(Rev]) see response D3
14. Sediment traps have a maximum height of 5ft. Please adjust the design of sediment trap to meet
this requirement.
(Rev]) Comment has been addressed.
15. A diversion is needed west of Sediment Trap 1 at the edge of the limits of disturbance since the
topography shows some of the disturbed area sheet flowing directly to the stream.
(Rev]) Comment has been addressed.
16. The soil boundaries are not clear on the ESC plan. It may be best to show the soil information on
sheet E -1 instead.
(Rev]) Comment has been addressed.
17. The disturbance southwest of Sediment Basin 2 is not protected and seems unnecessary. Please
clarify.
(Rev]) Comment has been addressed.
18. Please provide more information on Sediment Basin 3. It does not appear there is a pipe or riser in
the current proposal.
(Rev]) Comment has been addressed.
19. Please remove the 15" pipe from Sediment Basin 2, if it is no longer needed, rather than plugging
it in place and it being a potential maintenance issue. Replace with a metal drain valve if one is
desired by the applicant.
(Revl) Comment has been addressed.
20. Please remove details from the previous design if the details contain conflicting information as the
current proposal.
(Revl) Comment has been addressed.
21. Please provide dewatering orifice calculations. Each seems to be oversized. The calculations
should use required dry volume and the height of required dry volume rather than provided.
(Rev]) Comment has been addressed.
22. Please move the note regarding the regrading of Stormwater Facility 1 from the SWM sheets to
Phase 1 of the ESC plan
(Rev]) Comment has been addressed.
23. Please show Permanent Seeding (PS) symbols throughout the plan.
(Revl) Comment has been addressed.
24. Please show staging, stockpile, and parking areas on the plan.
(Rev]) Comment has been addressed.
25. An analysis of all downstream channels per MS -19 must be provided. The applicant has shown a
few cross - sections downstream of detention facilities in the plan but has not provided any analysis.
Also, the cross - sections are not provided to the point that the 1% rule is reached.
(Rev]) Following comments relate to the MS -19 analysis:
a. Xsec 4 -9 and 20 -25 indicate water velocity over the stream banks in a 2 year storm
event. This violates MS 19.b.2.a in VESCH. Please conrirm stream capacity at these
cross sections.
b. Xsec 5 -8, 22, and 23 indicate channels cut into rock. Photos do not show a rock bed
(instead stream cut into soil with rocks). Please update nl's to "Channel in rock and
earth" = 0.22 for these cross sections.
c. Xsecs 4 -9 and 20 -25 indicate appreciable effect of obstructions. Please clarify as photos
do not appear to indicate this condition.
26. Once the plans are approved, please provide a completed Bond Estimate Request Form to the
County Engineer to receive a ESC bond computation. All parcels disturbed with this plan must
sign this bond request form and be party to the bond unless easements are recorded.
(Rev]) Comment has been acknowledged.
E. Mitigation Plan Review Comments
1. As was required with the previous construction plan, a mitigation plan for all non - exempt
disturbances to the buffer is required. Please provide a sheet showing all proposed stream buffer
disturbances (include those disturbed by Phase I as well as possible exempted disturbance),
proposed mitigation planting, and a table comparing the disturbed area, minus the exempted areas
(to be reviewed by the Engineering Department), and the newly planted areas. Because of changes
to staffing levels, the county has not setup a program to accept dedication of stream buffers for
perpetual agreements as expected shortly after 2007. That option is no longer available to the
applicant.
(Rev]) Comment has been addressed.
File: C: \Users\mkoslow\Documents \Current Reviews \WP0201200047 Foothills Crossing Phase II and
III\E2_MAK_esc swm WP0201200047 Foothill Crossing Ph II and III.doc