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Home My WebLink About20181662 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 robinsondesignengineers.com 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 r t h St.ThirdSt.LoweAve.CrestviewDr.MainSt.RidgeAve.VistaSt.WanokaAve.River vi e w MulvaneyDr.SimpsonOldPattonRd.Gl e n d a l e Av e .SteeleSt.ShannonDr.Bl a l o c k St.FradyLn.GlendaleAve.Fairview Av e .DogwoodLn.Sp r i n g d a l eAv e . Railwa y Swa n n a n n o a R 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