HomeMy WebLinkAboutWPO201400032 Correspondence 2014-06-03 McKEECARSON
LANDSCAPE ARC-iCTECTS •CONSULTING ENGII I=ERS • LAND PLANNERS
03 June 2014
John Anderson
Civil Engineer
Albemarle County Community Development
A. Stormwater Management and Mitigation Plan (WP0201400032)
2.These plans should include BMP/s for phase 1 that reflect ultimate build-out.With change to Master Plan and
prior applications that eliminate certain features(bio-filters, cistern, swale), it is important that SWM design
reflect overall development plans. May 1, we met to discuss need to retain semblance of biological treatment
since bio-filters were incorporated (as exhibit drawings) in the 2009 special permit.Since bio-filters and a cistern
were elements of prior design, please ensure that water quality requirements are met using proposed rain
gardens, or another approach. Please furnish details that demonstrate that phase 1 SWM meets quality and
quantity requirements.Compare pre-and post-development impervious areas; show that selected SWM BMP/s
meet required (pollutant) removal rates.
Duly Noted:The current plan incorporates a total of 6 biofilters (5 rain gardens and a dry swale), as well
as permeable pavement and multiple infiltration galleries. These features provide stormwater quantity
and quality control above and beyond the original design and County standards. Additional information
regarding the treatment volumes, impervious areas, and pre- & post-development flows has been
incorporated into the project plans.
Rev 1.)Comment has been addressed, yet design details require clarification. Total rain garden design
volume appears sufficient to meet detention requirements, but significant post-developed storm runoff
does not reach any rain garden. Hydrocad modeling shows that post-developed 2-and 10-year peak
runoff values are less than pre-developed, but the Short Version BMP Computations spreadsheet, which
calculates volume for water quality purposes, assumes runoff from developed areas reaches SWM
facilities. Only 3 of 9 defined drainage areas reach rain gardens. Sub-area 3 routes to Rain Garden RG-5;
Sub-area 4 routes to Rain Garden RG-4;Sub-area 7 routes to Rain Gardens RG-1, RG-2& RG-3. Please
present calculations that show minimum removal efficiency of 33%is met with current Rain Garden and
permeable paver subgrade storage design. Specifically, statement that "the project BMPs remove a total
of 72.7 cubic yards of stormwater(at 100%efficiency), resulting in a total pollutant removal of 4.9 times
the minimum required amount"may be misleading. For example, a 100%efficiency rate applied to half
an affected area equates to 50%efficiency for the entire area. Please evaluate runoff from areas reaching
rain gardens against runoff from impervious areas of the 1.26 Acre site. Restate removal efficiency as it
relates to post-developed impervious areas. Re-stated efficiency should include areas routed to rain
gardens and impervious areas that simply discharge to existing inlets. Further,please note that
statement of 100%efficiency presumes no overflow from any rain garden, yet unless Hydrocad Software
predicts 100%detention during design year storms for DAs that route to the gardens (DA-3, 4, 7), then
removal efficiency is<100%. Please consider runoff areas that do not reach the rain gardens, and
overflow, and re-state efficiency with respect to site impervious areas. Sheet 16:plan-profile views
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require a scale to aid review and construction. Label bottom of wall elevation for each wall shown in rain
garden/sub-grade storage profile view. Show dimensions for RG-1, 2, 3, 4, and sub-grade SW storage
(beneath permeable pavers;plan view). Furnish RG-5 details/dimensions, including connection with inlet
83. Please compare dimensions against table SWM treatment volumes (as a check). Confirm ponded
volumes(cf)for RG-1, 2, 3 (as a check).
Duly Noted: Stormwater Quality Treatment calculations have been revised and corrected to reflect the
standard "Blue Book" removal efficiencies. These calculations take the impervious areas into account for
each drainage subarea, and achieve the overall site required pollutant removal. Dimensions have been
shown for subgrade storage, for RG 1-5 plan view dimensions are located on Sheet 04 to eliminate
duplication. BOW elevations have been added to section views for individual RGs. Scale has been added
to sections to aid review and construction.
3. Water Quantity: Please demonstrate compliance with AC Code 17-314, Control of Peak Rate and Velocity of
Runoff.
Duly Noted: The proposed design reduces the post-development flows below the current pre-
development rates for the 2-year and 10-year design frequencies. Information regarding each drainage
sub area and the general stormwater runoff modeling approach has been added to the plans.
(Rev. 1)Comment has been partially addressed. Please furnish 2-and 10-yr design storm qpeak for rain
gardens. Confirm that pre-and post-development 2-and 10-yr design storm release rates shown on sheet
15 graphic reflect qpeak for the entire 1.26 Ac. site; that is, that the graph includes storm runoff that does
not reach rain gardens but drains instead directly to the existing inlet near the intersection with Rolkin
Drive.
Duly Noted:The stormwater peak flows shown on Sheet 15 are for the total site. The runoff
hydrographs from each subarea are attached, as requested. Please note that these individual peak flows
do not show the runoff reductions/changes in peak timing that result as the flows are routed through
the various LID practices. I have attached the HydroCAD reports for pre-and post-development flows for
your information (note that the post-development sub areas have been run as two separate files, due to
the limitation on active nodes for my HydroCAD software. This doesn't impact the overall flows).
4. Furnish profiles for new sanitary sewer and storm drain lines. Furnish pipe and inlet computations for existing
lines or inlets that receive additional flow, including capacity and spread calculations for inlets on Rolkin Road
that receive increased runoff with change from gravel to asphalt parking surface. Consider IN-A2 to MH-B1, and
discharge through this line. Ensure that discarded design profiles (IN-1 to IN-2 to MH-3,for example) and labels
(MH-3, OF-3,for example) are removed from sheet 7. Show plan/profile and furnish computational details for
the parking lot rain garden island if this is a SWM structure. How will runoff reach the parking lot rain garden?
Ensure that labels and notes on sheet 16 are readable scale (font pitch).
Duly Noted: Profiles have been furnished for the new sanitary and storm lines. Stormwater pipe flow
calculations and capacities have been added to the plans. The existing curb inlet located along the
,:`.r:'nce drive to the school ha been evaluated. Stormwater :.ows this structure as a result ;i the
proposed development will increase front 2.2 CFS to 3.0 CFS(10-Yea.). The resulting increase in spread
at the inlet is shown on the attac hed chart. This increase is minor, and should not present any significant
issue. Additional information regarding the rain garden in the parking lot island (Pk: 5) have been added
to the plans. Pow into this rain garden will be via sheet flow into the uphill s rle of tie rain garden. Font
scales . ,1 Shc e ., irae' been cerre:teci.
Nowy vie
(Rev. 1) Comment has been partially addressed. Response states that "stormwater pipe flow calculations
and capacities have been added to the plans... [and that]the resulting increase in spread at the inlet is
shown on the attached chart."These items are missing;please provide.
Duly Noted: Pipe flow capacities, 10 year flows and velocities have been added to the piping table. The
flow spread at the curb inlet on the entrance drive (near Rolkin Rd.) is attached. Note that the increased
flow will result in an increase in spread at the inlet from 4.2' to 5.3'. This should not create any safety
issues, since the access drive has limited traffic flow and a driver could pull around the "flooded" area if
needed. Traffic in this location would be travelling at less than 5 MPH, due to the intersection.
5. 2014,Stormwater Narrative-Additional Stormwater Measures (sheet 15) relate to temporary ESC, not
permanent SWM measures—revise/furnish Stormwater Narrative-Stormwater Measures for Phase 1. This is
critical.
Duly Noted: The Stormwater Narrative has been updated to reflect the permanent SWM measures
incorporated into this project.
(Rev. 1) Comment has been partially addressed. Revise statements concerning pollutant removal
efficiency. Ref. item 2., above.
Duly Noted:The comment regarding pollutant removal efficiencies has been removed and corrected
stormwater treatment data has been incorporated.
6. Restore Nyoplast Dome Grate detail—ref sheet 13, 3/31/14 plans.
Duly Noted: Nyloplast detail is shown on sheet 13.
7.Sheet 3, Note 8, please eliminate reference to "sediment which accumulates in the existing pond," a possible
carryover from earlier design.
Duly Noted: review error, no change required.
B. Erosion Control Plan (WPO201400032)
4. Furnish safety fence to prevent incidental trespass. None is shown.All controls must be shown and detailed on
the plan, and within the limits of disturbance.
Duly Noted: Safety fencing has been added to the site ESC plan and details,
(Rev 1) Comment partially addressed. Please show SAF immediately adjacent to sidewalks on Rolkin Drive
and Richmond Road(Rte. 250). This SAF should remain up at all times except as needed to allow
construction access.
Duly Noted:Additional safety fencing has been added to the ESC plan.
5. Indicate access (vehicle travel path)to stockpile area, sheet 3. Stockpile area needs a construction entrance.
Also, there is an apparent conflict between the construction entrance and storm drain line from IN-A2 to MH-B1
(sheet 5).
Duly Noted: The proposed soil stOr_kpile area has been relocated (as weli :> ti:c c.: :'r°..r.tion eIItr:A e) to
minimize the o.'eIail disturbed
(Rev. 1)Comment has been addressed. Minor—relocate CE label to correspond with new CE location,
sheet 03.
Duly Noted: Construction Entrance symbol has been relocated.
6. Furnish sediment trap details and dimensions(profile, perspective, cross section of outlet) sufficient to
evaluate against VESCH; see Plate 3.13-2 (VESCH)—ST floor length and width, especially.
Duly Noted: Additional data on the proposed sediment trap has been added to the plans.
(Rev. 1)Comment has been addressed. Minor—relocate CE label to correspond with new CE location,
sheet 03
Duly Noted:Volume data for the proposed sediment trap has been added to the ESC Plan.
If you have any questions or comments, please do not hesitate to contact me.
Sincerely,
Christopher L. Mantle, LA
RECEIVED
JUN 0 i► 2014
COMMUNITY
DEVELOPMENT
Chapter 9- Storm Drains
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From: John Anderson
Sent: Friday, May 30, 2014 10:28 AM
To: chris @sustainable-sites.com; 'Christopher Mantle'; 'csonne @mckeecarson.com'
Cc: Glenn Brooks; Wendy Fisher
Subject: WPO201400032- Review comments (Montessori)
Re.Review comments d. 28 May 2014
Chris,Chris,
SWM, comment#7. Please respond`review error;no change required.' My mistake-sorry!
SWM,comment#6. Please also restore Nyoplast Drain Basin with Dome Grate detail(ref. sheet 13, 3/31/14 plans).
Thank you for your help,and patience.
John
John Anderson, PE
Community Development
Albemarle County
434-296-5832,ext. 3069
1
McKEECARSON
LANDSCAPE.ARCHITECTS • CONSULTING ENGII JEERS • LANG PLANNERS
19 May 2014
John Anderson
Civil Engineer
Albemarle County Community Development
A.Stormwater Management and Mitigation Plan (WP0201400032)
1. Separate out the WPO plans.They cannot be approved as part of a site plan set.
Duly Noted:The WPO plans have been separated from the site plan set.
2.These plans should include BMP/s for phase 1 that reflect ultimate build-out. With change to Master Plan and
prior applications that eliminate certain features (bio-filters, cistern, swale), it is important that SWM design
reflect overall development plans. May 1, we met to discuss need to retain semblance of biological treatment
since bio-filters were incorporated (as exhibit drawings) in the 2009 special permit. Since bio-filters and a cistern
were elements of prior design, please ensure that water quality requirements are met using proposed rain
gardens, or another approach. Please furnish details that demonstrate that phase 1 SWM meets quality and
quantity requirements. Compare pre-and post-development impervious areas; show that selected SWM BMP/s
meet required (pollutant) removal rates.
Duly Noted:The current plan incorporates a total of 6 biofilters(5 rain gardens and a dry swale), as well
as permeable pavement and multiple infiltration galleries. These features provide stormwater quantity and
quality control above and beyond the original design and County standards. Additional information regarding
the treatment volumes, impervious areas, and pre-& post-development flows has been incorporated into the
project plans.
3.Water Quantity: Please demonstrate compliance with AC Code 17-314, Control of Peak Rate and Velocity of
Runoff.
Duly Noted:The proposed design reduces the post-development flows below the current pre-
development rates for the 2-year and 10-year design frequencies. Information regarding each drainage sub area
and the general stormwater runoff modeling approach has been added to the plans.
4. Furnish profiles for new sanitary sewer and storm drain lines. Furnish pipe and inlet computations for existing
lines or inlets that receive additional flow, including capacity and spread calculations for inlets on Rolkin Road
that receive increased runoff with change from gravel to asphalt parking surface. Consider IN-A2 to MH-B1, and
discharge through this line. Ensure that discarded design profiles (IN-1 to IN-2 to MH-3,for example) and labels
(MH-3, OF-3, for example) are removed from sheet 7. Show plan/profile and furnish computational details for
the parking lot rain garden island if this is a SWM structure. How will runoff reach the parking lot rain garden?
Ensure that labels and notes on sheet 16 are readable scale (font pitch).
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Duly Noted: Profil have been furnished for the_ ew sanitary and storm lines. ormwater pipe flow
calculations and capaciti have been added to the plans,yhe existing curb inlet located along the entrance
drive to the school has been_ealuated. Stormwater flows to this structure as a result of the proposed
developmeiltwitlincrease fro 2.2 CFS to 3.0 CFS (10-Year). The resulting increase in spread at the inlet is
shown on the attached chart.this increase is minor, and should not present any significant issue. Additional
information regarding the rain garden in the parking lot island (RG-5) have been added to the plans. Flow into
this rain garden will be via sheet flow into the uphill side of the rain garden. Font scales on Sheet 16 have been
corrected.
5. 2014,Storm water Narrative-Additional Storm water Measures (sheet 15) relate to temporary ESC, not
permanent SWM measures—revise/furnish Storm water Narrative-Storm water Measures for Phase 1. This is
critical.
Duly Noted:The Stormwater Narrative has been updated to reflect the permanent SWM measures
incorporated into this project.
B. Erosion Control Plan (WPO201400032)
1. Separate out the WPO plans.They cannot be approved as part of a site plan set.
Duly Noted: The WPO plans have been separated from the site plan set.
2. Provide clear limits of disturbance, including areas near amphitheater and geodesic dome if these areas will be
graded.
Duly Noted: Only the area within the limits of disturbance indicated shall be disturbed. The plans have
been modified to clarify the work to be done as part of this first phase.
3. Provide perimeter controls that include utility demolition and installation of new sanitary sewer line.The
sediment trap is upslope of utility work and appears to release to a steep slope.There is potential for sediment
trap discharge to wash across the sidewalk and onto Rolkin Road. Consider measures that limit this potential.
Please clearly delineate sidewalks at Rolkin and Richmond Roads.
Duly Noted: The sediment trap discharge conditions have been modified to spread out flows and
eliminate erosion of the downstream bank. This shall be done using a level spreader, followed by compost bank
protection. Information regarding this approach (from the Ohio ESC manual) has been attached. Erosion control
for the utility lines being installed outside the perimeter of the sediment trap capture area is being provided by
silt fencing. These disturbed areas will be very small, and the duration of disturbance will be short.
4. Furnish safety fence to prevent incidental trespass. None is shown.All controls must be shown and detailed on
the plan, and within the limits of disturbance.
Duly Noted: Safety fencing has been added to the site ESC plan and details.
5. Indicate access (vehicle travel path)to stockpile area, sheet 3. Stockpile area needs a construction entrance.
Also,there is an apparent conflict between the construction entrance and storm drain line from IN-A2 to MH-B1
(sheet 5).
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Duly Noted: The proposed soil stockpile area has been relocated (as well as the construction entrance)
to minimize the overall disturbed area.
6. Furnish sediment trap details and dimensions (profile, perspective, cross section of outlet)sufficient to
evaluate against VESCH; see Plate 3.13-2 (VESCH)—ST floor length and width, especially.
Duly Noted: Additional data on the proposed sediment trap has been added to the plans.
C. Road plans
NA
D.Site Plan (SDP201400032)
Parking: Increase parking lot rain garden island radius from 1.8-ft to 3-ft, minimum. Lengthen curb (bulb-out) at
parking lot entrance so that parking space at entrance is bordered by 18-ft length of curb, minimum.
Duly Noted: Radii have been increased to 3ft, curb at bulb-out has been lengthened to 18ft.
E. Final Plat
NA
If you have any questions or comments, please do not hesitate to contact me.
Sincerely,
Christopher L. Mantle, LA