HomeMy WebLinkAbout20181662 Ver 1_Stormwater_20181207
robinsondesignengineers.com
March 12, 2018
Mr. McCray Coates
Stormwater Services Manager
City of Asheville, NC
(submitted via hand-delivery through development services center)
Reference: Oakley Springs Low-Impact Development Subdivision (16-06312PZ)
Stormwater Narrative
Mr. Coates,
This letter represents a concise written summary of our stormwater management concept and calculations to support
a Special Exemption in accordance with Code of Ordinances section 7-12-2(f)(1). Herein, we describe our overall
stormwater management concept, provide data from our calculations and modeling, and describe how we believe this
approach satisfies the spirit of the ordinance.
This letter accompanies design drawings and an exhibit that describe the overall approach within the context of the
neighborhood site plan. The project and proposed approach are unique simply because stormwater quantity and
quality controls will be accomplished within a small creek / floodplain that flows through the site. The heavily-
degraded creek will be restored, and an existing pond will be replaced with a constructed wetland. We believe that
this approach fits squarely within the spirit of the City stormwater ordinance. Furthermore, our extensive hydrology
and hydraulic modeling supports the concept and demonstrates stormwater abatement requirements will be met.
Importantly, all stormwater runoff from developed land and proposed impervious surface will be routed through and
temporarily ponded within a treatment device containing hydrophilic vegetation.
APPROACH
Our conceptual framework for the proposed stormwater management approach follows the central tenets of “low-
impact development”: slow, spread, and soak stormwater runoff as close as possible to its source, using vegetation
as natural water quality treatment.
Existing Conditions
The project site is a narrow, relatively steep valley within the Oakley community. A 2nd order perennial creek bisects
the site; two 1st order ephemeral streams join on-site to create the main creek, which is a tributary of the Swannanoa
River. Within the project site, the creek is impounded by a small pond and low-head dam. Downstream of the dam,
the channel is eroded and denuded of riparian vegetation. Upstream of the pond, the channel has aggraded with
gravel and cobble sediments, and is overgrown with invasive species. The pond itself is nearly filled with sediment.
Water levels within the pond are controlled by two dilapidated, corrugated metal pipes. During storm events the pond
overflows the embankment, causing erosion of the embankment itself and surrounding valley floor. In addition, the
pond causes significant backwater even during small storm events, which further destabilizes the creek and valley.
Stormwater Management Concept
In the proposed scheme, the pond will be demolished and the existing dam will be removed. In its place will be built a
constructed wetland that allows creek base flows and bed sediments to flow freely through a low-flow channel. During
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storm events, the wetland control structure will regulate outflows so that excess runoff will accumulate in the wetland,
and upstream into the undisturbed reach. The water surface of the wetland will stage up to create temporary shallow
ponding in a floodplain wetland. Downstream of the constructed wetland, the creek channel will be re-shaped to a
more stable form, and native riparian vegetation will be planted to promote bank stability. At the edge of the riparian
buffer, shallow linear depressions will be created to capture any sheet flow or concentrated runoff before it reaches
the creek. These linear depressions will mimic the function of natural, vernal, floodplain wetlands and will promote
shallow infiltration and water quality improvement.
RAINFALL-RUNOFF & HYDRAULIC MODELING
The SCS Runoff Curve Number (CN) Method as described in USDA-NRCS Technical Release 55 (TR-55) was used
to analyze the proposed site hydrology. A stormwater management software, EPA-SWMM, was used to represent
existing hydrologic conditions and channel hydraulics for various storm events. For these models, design storms were
created according to a Type II SCS Rainfall Distribution and include 2 year-, 5 year-, 10 year-, 25 year-, 50 year-, and
100 year-24 hour storm events. As it pertains to water quality requirements, all stormwater runoff from proposed
impervious surface will be routed through and temporarily ponded within a treatment device. This impervious surface
runoff will filter through grass strips, and be temporarily impounded based on the fine grading plan shown on the
plans. The fine grading includes heavily vegetated linear depressions, and a super-elevated floodplain within the
constructed wetland.
Table 2: peak off-site discharge rates for various design storms under existing conditions, and
proposed conditions. As seen in the table, the post-development peak discharges for the 2 year-
and 10 year-24 hour storms are lower than pre-development conditions.
2-year 10-year 25-year 100-year
Existing Conditions 37.43 cfs 72.72 cfs 96.46 cfs 112.81 cfs
Proposed Conditions 35.58 cfs 70.50 cfs 80.47 cfs 104.69 cfs
CONCLUSION
This submittal has been developed concurrently with 401/404 regulators Zan Price (NCDEQ-DWR) and Amanda
Fuemmeler (USACOE). The preconstruction notification to the USACOE will be verified under NWP27 for Aquatic
Habitat Enhancement and NWP18 for Minor Discharges. Thank you for your assistance with this unique and exciting
project.
ROBINSON DESIGN ENGINEERS
Philip Ellis, PE & Joshua Robinson, PE
ENCLOSURES
• Engineering plans (x4)
• Stormwater Exhibits
2040 2
0
5
0
2
0
6
02070 2070
2040
2060
BlackDB 2689, PG 1259658119246McJunkinDB 1763, PG 2899658119069WangDB 2200, PG 0019658109948KellerDB 2868, PG 1509658109836WilliamsDB 4181, PG 339658109608WolfDB 4221, PG 1094PB 95, PG 1619658108581Bell & WardenDB 4726, PG 500PB 117, PG 1909658109453FauerbachDB 4897, PG 4779658203348FreemanDB 2362, PG 276PB 32, PG 1019658202575TensiDB 2245, PG 8489658202666BG Propertiesof WNC, LLCDB 2233, PG 547PB 69, PG 149658202762ThompsonDB 4803, PG 1712PB 151, PG 789658203800ColeDB 1027, PG 629PB 35, PG 11760' EasementDB 5156, PG 246PB 81, PG 102SpringNorfolk Southern100' R/WPer DB 81, PG 102Norfolk Southern100' R/WPer DB 81, PG 1022
3
1
9
10
11
12
13
4
7
8
14
6
5
2054205620582058205620542054205820602062206420662068207020802078207620742074C200
STORMWATER PLANN
sheet
PERMITTING
PROJECT: Blalock Neighborhood
RDE REF:16196
SUBMITTAL:Utilities Permit
DATE:7/08/2016
REVISIONS:
03/24/17 per MSD and City of Asheville
comments
06/30/17 per Asheville Water Dept
comments
07/22/17 per full-build-out, and MSD /
City of Asheville comments
11/30/17 prelim comments from CoA
03/06/18 per MSD & CoA comments14 Church StreetAsheville, NC 28801828 545 3514www.robinsondesignengineers.comOAKLEY SPRINGSJ. D. Land Holdings, LLCCity of Asheville, NC c Copyright
These drawings are the property of the
engineer and may not be reproduced without
the express, written consent of
Robinson Design Engineers.FINAL DRAWING - FOR AGENCY REVIEWTHESE DRAWINGS ARE NOT RELEASED FOR CONSTRUCTIONUNTIL AUTHORIZED BY THE ENGINEERENGINEER
Robinson Design Engineers C-3863
Philip Ellis, PE NC # 039870
Site
Thom
p
s
o
n St.East
St
.WindingRd.FirstSt.Fo
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St.ThirdSt.LoweAve.CrestviewDr.MainSt.RidgeAve.VistaSt.WanokaAve.River
vi
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w MulvaneyDr.SimpsonOldPattonRd.Gl
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Av
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.SteeleSt.ShannonDr.Bl
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St.FradyLn.GlendaleAve.Fairview
Av
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.DogwoodLn.Sp
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Railwa
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d.
PIPE 1
PIPE 4
PIPE 2
PIPE 3
PIPE 8
PIPE 7
PIPE 6
PIPE 5
PIPE SCHEDULE
NAME LENGTH (LF)IN OUT SIZE & TYPE SLOPE (%)
PIPE #1 36 2046.5 2044.5 15" RCP 5.6
PIPE #2 30 2051.5 2050.7 15" RCP 2.7
PIPE #3 21 2050.5 2050.0 15" RCP 2.4
PIPE #4 37 2054.0 2050.5 15" RCP 9.5
PIPE #5 30 2050.3 2049.5 15" RCP 2.7
PIPE #6 19 2050.7 2050.5 15" RCP 1.1
PIPE #7 95 2058.6 2050.5 15" RCP 8.5
PIPE #8 19 2059.0 2058.8 15" RCP 1.1
LINEAR DEPRESSION 170LFD
C801
A
C823
B
C823
C821DETAILS - STORMWATERsheetPERMITTINGPROJECT: Blalock NeighborhoodRDE REF:16196SUBMITTAL:Utilities PermitDATE:7/08/2016REVISIONS:03/24/17per MSD and City of Ashevillecomments06/30/17per Asheville Water Deptcomments07/22/17per full-build-out, and MSD /City of Asheville comments11/30/17prelim comments from CoA03/06/18per MSD & CoA comments14 Church Street
Asheville, NC 28801
828 545 3514
www.robinsondesignengineers.com
OAKLEY SPRINGS
J. D. Land Holdings, LLC
City of Asheville, NC c CopyrightThese drawings are the property of theengineer and may not be reproduced withoutthe express, written consent ofRobinson Design Engineers.FINAL DRAWING - FOR AGENCY REVIEW
THESE DRAWINGS ARE NOT RELEASED FOR CONSTRUCTION
UNTIL AUTHORIZED BY THE ENGINEER
ENGINEERRobinson Design EngineersC-3863Philip Ellis, PENC # 039870LINEAR DEPRESSIONDNTSUNDISTURBED NATIVE SUBGRADE2.5H:1V SIDE SLOPES, TYPSOIL AMMENDMENTEC8211 FT PONDING DEPTHSTEP-POOL STRUCTURE (bedforms)CNTSSHALLOW WATER ZONEjuncus effususiris virginicusTEMPORARY INUNDATION ZONEtaxodium distichumcarex cherokeensisilex glabraleucothoe fontanesianailex verticillataitea virginica PLANTING LIST FOR STORMWATER CONTROL MEASURESE*REFER TO PLANTING PLAN BY EQUINOX ENVIRONMENTALCLASS B RIPRAPINTERSTITIAL FILLGRADE VARIES1V:2HTOP OF BANKBIO D-BLOCK for SOIL LIFTSBSCALE: 1" = 5'CHANNEL IMPROVEMENTS TYPICAL SECTIONANTSBC821
10+0011+0012+0013+0014+002,0402,0502,0452,0552,0352,06015+0016+00BlackDB 2689, PG 1259658119246McJunkinDB 1763, PG 2899658119069WangDB 2200, PG 0019658109948FauerbachDB 4897, PG 4779658203348FreemanDB 2362, PG 276PB 32, PG 1019658202575TensiDB 2245, PG 8489658202666BG Propertiesof WNC, LLCDB 2233, PG 547PB 69, PG 149658202762SpringNorfolk Southern100' R/WPer DB 81, PG 1022319101112134781465C822DETAILS - STORMWATERsheetPERMITTINGPROJECT: Blalock NeighborhoodRDE REF:16196SUBMITTAL:Utilities PermitDATE:7/08/2016REVISIONS:03/24/17per MSD and City of Ashevillecomments06/30/17per Asheville Water Deptcomments07/22/17per full-build-out, and MSD /City of Asheville comments11/30/17prelim comments from CoA03/06/18per MSD & CoA comments14 Church Street
Asheville, NC 28801
828 545 3514
www.robinsondesignengineers.com
OAKLEY SPRINGS
J. D. Land Holdings, LLC
City of Asheville, NC c CopyrightThese drawings are the property of theengineer and may not be reproduced withoutthe express, written consent ofRobinson Design Engineers.FINAL DRAWING - FOR AGENCY REVIEW
THESE DRAWINGS ARE NOT RELEASED FOR CONSTRUCTION
UNTIL AUTHORIZED BY THE ENGINEER
ENGINEERRobinson Design EngineersC-3863Philip Ellis, PENC # 039870UNNAMED TRIBUTARY PROFILEAUNNAMED TRIBUTARY PLANBCC821NVERTICAL SCALE: 1" = 3' HORIZONTAL SCALE: 1" = 30'PLAN SCALE: 1" = 30'CREEK RESTORATIONBC823
108C823
DETAILS - STORMWATER
sheet
PERMITTING
PROJECT: Blalock Neighborhood
RDE REF:16196
SUBMITTAL:Utilities Permit
DATE:7/08/2016
REVISIONS:
03/24/17 per MSD and City of Asheville
comments
06/30/17 per Asheville Water Dept
comments
07/22/17 per full-build-out, and MSD /
City of Asheville comments
11/30/17 prelim comments from CoA
03/06/18 per MSD & CoA comments14 Church StreetAsheville, NC 28801828 545 3514www.robinsondesignengineers.comOAKLEY SPRINGSJ. D. Land Holdings, LLCCity of Asheville, NC c Copyright
These drawings are the property of the
engineer and may not be reproduced without
the express, written consent of
Robinson Design Engineers.FINAL DRAWING - FOR AGENCY REVIEWTHESE DRAWINGS ARE NOT RELEASED FOR CONSTRUCTIONUNTIL AUTHORIZED BY THE ENGINEERENGINEER
Robinson Design Engineers C-3863
Philip Ellis, PE NC # 039870
OUTLET STRUCTUREC NTS
4'
SIDE VIEW FRONT VIEW
TOP VIEW
CONSTRUCTED WETLAND SECTIONA
CREEK RESTORATION SECTIONB
STONE BAFFLE C
PLAN VIEW
SCALE: 1" = 30'
WEIR CROSS SECTION
NTS
FLOW
BAFFLE PROFILE
NTS
D
C821
D
C821
A
C821
2H:
1
V
2H:1V
A
C821