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HomeMy WebLinkAboutSW5210302_Green Hill Subdivision_Stormwater Management Report_2021-02-23_20210316 STORMWATER MANAGEMENTREPORT FOR GREEN HILL SUBDIVISION PHASE I LOUISBURG, FRANKLIN COUNTY, NC FEBRUARY 23, 2021 PREPARED BY: FLM Engineering, Inc. PO Box 91727 Raleigh, NC 27675 919.423.8975 NC License No. C-4222 Table of Contents 1.0 Introduction ........................................................................................................................ 1 2.0 Site Description .................................................................................................................. 1 3.0 Proposed Development ...................................................................................................... 2 4.0 Stormwater Management ................................................................................................... 2 4.1 Design Data and Methodologies .................................................................................... 2 4.2 Pre-development Analysis .............................................................................................. 3 4.3 Post-development Analysis ............................................................................................ 3 4.4 Stormwater Management Summary ............................................................................... 4 4.5 Nutrient Reduction .......................................................................................................... 5 5.0 Limitations .......................................................................................................................... 5 Tables Table 4-1 Pre-development Hydrologic Model Results Table 4-2 Post-development Hydrologic Model Results Table 4-3 Stormwater Management Summary Table 4-4 Nutrient Loading Summary © 2021 FLM Engineering, Inc. Page ii of iii February 23, 2021 Appendices Appendix A FEMA Firm Map Appendix B USGS Map Excerpt Appendix C Soils Map Excerpt Appendix D NRCS Soil Report Appendix E NOAA Rainfall Data Appendix F Pre-Development Watershed Map Appendix G Pre-Development Hydrologic Model Appendix H Post-Development Watershed Map Appendix I Post-Development Hydrologic Model Appendix J Discrete NRCS Curve Number Model Appendix K SCM Supplemental Form Appendix L Draw Down Calculations Appendix M Rip Rap Energy Dissipator Calculations Appendix N Operations & Maintenance Agreement Appendix O Tar-Pamlico Accounting Tool © 2021 FLM Engineering, Inc. Page iii of iii February 23, 2021 1.0Introduction Holden Development, LLC is proposing to develop 48 single-family residential lots and associated site improvements as the first phase of Green Hill Subdivision. This report details the stormwater management design for the project. 2.0Site Description Green Hill Subdivision is a proposed 148-lot single-family residential subdivision on approximately 64 acres located on NC Highway 39 in Louisburg, North Carolina (Franklin County PIN 2805-70- 1959). The existing 64-acre site is currently the Green Hill Country Club, consisting of an 18-hole golf course with a clubhouse and associated site improvements. The site is bordered on the west by Burnette Road, the north by vacant land, the east by the Tar River, and the south by rural residential and agricultural land. The site was investigated for the presence of wetlands and streams by Soil & Environmental Consultants, PA. Jurisdictional streams and associated wetlands exist along the western and northeastern borders along with the aforementioned Tar River along the eastern border. An Approved Jurisdictional Determination (AJD) documenting that the existing pond is non- jurisdictional is in progress with the United States Army Corps of Engineers (USACE). The site is located within the Town of Louisburg and is subject to Phase II Post-construction Stormwater requirements. Based on a review of Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) number 3720280500K, dated April 16, 2003, portions of the site lie within FEMA designated 100-year and 500-year floodplains. The FEMA FRIM is included as Appendix A. and USDA NRCS Soil Survey Map are included within Appendices B and C, respectively. The property generally drains from the south to the west, north, and east. Topography on the site ranges from elevation 246186Hydrologic Soil Group (HSG) B as described in the National Resources Conservation Service (NRCS) soils report in Appendix D. © 2021 FLM Engineering, Inc. Page 1 of 6 February 23, 2021 3.0Proposed Development The overall proposed development will consist of 148 single-family residential lots and associated site improvements, the first phase of which will consist of 48 lots on approximately 23 acres. However, for the purposes of stormwater management, the site will drain to two stormwater control measures (SCMs). The first measure, which is included in this report, will serve the 48 Phase I lots along with 80 additional lots in remaining phases. The total onsite proposed impervious surface area for the project is approximately 819,120 square feet, or 18.80 acres, which is 29% of the site. Because the contains more than 24% built-upon area, the project is considered high density by the Phase II Post-construction Stormwater standards. The impervious area tributary to the first SCM will be approximately 679,520 square feet, or 15.60 acres. 4.0Stormwater Management 4.1 Design Data and Methodologies The design of the proposed stormwater management facilities was performed in accordance with the North Carolina Department of Environmental Quality (NCDEQ) Stormwater Design Manual and the Phase II Post-construction Stormwater requirements. Per the Phase II standards, high density projects must use structural stormwater management systems that will: Control and treat runoff from the first one inch of rain; Draw down the treatment volume no faster than 48 hours, but no slower than 120 hours; Discharge the storage volume at a rate equal to or less than the pre-development discharge rate for the 1-year, 24-hour storm; and Remove an eighty-five percent (85%) average annual amount of Total Suspended Solids (TSS). HydroCAD Version 8.50, a computer modeling software package, was used for the analysis of stormwater routing and hydrology of the existing and proposed watersheds. The hydrology calculations were performed using the Natural Resources Conservation Service (NRCS) Soil Conservation Service (SCS) Technical Release 55 (TR-55) methodology. Franklin County lies within the Type II rainfall distribution. The rainfall amounts for the site were obtained from the National Oceanic and Atmospheric Administration (NOAA) and are included in Appendix E. © 2021 FLM Engineering, Inc. Page 2 of 6 February 23, 2021 4.2Pre-development Analysis The pre-development watershed was approximated based on the surveyed topographic data. The peak flow rate for the 1-year, 24-hour storm event is based on the topography, land use cover (such as open space, grass, woods, etc.), and soil type. The 55.08-acre existing watershed was modeled in HydroCAD as one subcatchments as described below. Refer to Appendix F for the pre-development watershed map. Subcatchment 1S (Pre-development DA-1): Subcatchment 1S is 49.08 acres and drains from south to north. A time of concentration of 11.6 minutes was calculated for the subcatchment. The pre-development hydrologic model is included in Appendix G. Table 4-1 summarizes the pre- development hydrologic model results. Table 4-1 Pre-development Hydrologic Model Results 1-year, 24-hour Storm Event Pre- Subcatchment development Peak Flow (CFS) Pre-development DA-1 35.09 4.3 Post-development Analysis An analysis of the post-development runoff was performed using the same methods, parameters, and assumptions as described in the pre-development analysis above. Based upon the hydrologic calculations, the project does not create an increase in the peak runoff rate for the 1-year, 24- hour storm event. Refer to Appendix H of this report for the post-development watershed map. The 55.08-acre watershed was modeled in HydroCAD as two subcatchments as described below. Subcatchment 3S (Post-development to SCM DA-1A): Subcatchment 3S is 38.66 acres and includes the area draining to the proposed stormwater wetland. A conservative time of concentration of six minutes was used for the subcatchment. © 2021 FLM Engineering, Inc. Page 3 of 6 February 23, 2021 Subcatchment 4S (Post-development Bypass DA-1B): Subcatchment 4S is 10.42 acres. This subcatchment is bypass areas not treated by the proposed stormwater wetland. A conservative time of concentration of six minutes was used for the subcatchment. The post-development hydrologic model is included in Appendix I. Table 4-2 summarizes the pre- development hydrologic model results. Table 4-2 Post-development Hydrologic Model Results 1-year, 24-hour Storm Event Post- Subcatchment development Peak Flow (CFS) Post-development DA-1 24.05 4.4 Stormwater Management Summary This project will not increase peak flow rate for the 1-year, 24-hour storm event. Stormwater facilities that do not increase the peak flow rate help prevent adverse downstream impacts from flooding and erosion. Table 4-3 compares the pre-development and post-development peak flow rates for the design storm event. Table 4-3 Stormwater Management Summary 1-year, 24-hour Storm 1-year, 24-hour Storm Event Post- Subcatchment Event Pre-development % Difference development Peak Peak Flow (CFS) Flow (CFS) DA-1 35.09 24.05 -31.5% The stormwater control measure (SCM) was sized to control and treat runoff from the first one inch of rain and to provide peak rate of runoff attenuation. The minimum required treatment volume was determined using the Discrete NRCS Curve Number Method as shown in the hydrologic model of Subcatchments 3S-A and 3S-B in Appendix J. Subcatchment 3S-B represents the impervious surface directly linked to the SCM and produces a runoff volume of © 2021 FLM Engineering, Inc. Page 4 of 6 February 23, 2021 0.136 acre-feet, or 5,924 cubic feet, and subcatchment 3S-A represents the balance of the drainage area to the SCM and produces a runoff volume of 0.301 acre-feet, or 13,111 cubic feet. The total minimum required treatment volume is 19,035 cubic feet, and the provided treatment volume is 20,150 cubic feet. The supplemental form for the proposed stormwater wetland is included in Appendix K. The proposed stormwater wetland is anticipated to draw down the design volume within two to five days. Draw down calculations are provided in Appendix L. The inlet and outlet velocities are reduced to non-erosive velocities through the use of rip rap energy dissipators, the calculations for which are included in Appendix M. An operations and maintenance agreement for the SCM is included in Appendix N. 4.5 Nutrient Reduction The Tar-Pamlico accounting tool was used to calculate the pre- and post-development nutrient loads and is included in Appendix O. The spreadsheet indicated that nitrogen loading in the post- development with SCM condition is below the threshold of 4.0 pounds per acre per year. However, the phosphorus loading in the post-development with SCM condition is above the 0.4 pounds per acre per year threshold. Thus, the phosphorus loading requirement will be achieved through buy-down methods. As shown in the Development Summary of the tool, Table 4-5 summarizes the nutrient loads for the pre-, post-, and post-development with SCM conditions. Table 4-4 Nutrient Loading Summary Post Development Nutrient Pre-development Post-development with SCMs Total Nitrogen 1.08 5.21 3.30 Loading (lbs/ac/yr) Total Phosphorus 0.21 0.74 0.47 Loading (lb/ac/yr) 5.0Limitations This work was performed in a manner consistent with the level of care and skill ordinarily exercised by other members of FLM Engineering (FLM) profession practicing in the same © 2021 FLM Engineering, Inc. Page 5 of 6 February 23, 2021 locality, and under similar conditions, as of the date any services were provided. opinions and recommendations were necessarily based on a limited number of data and observations. It is possible that actual conditions could vary beyond the data evaluated. Therefore, FLM makes no guarantee or warranty, express or implied, regarding any services, communications, reports, opinions, or instruments of service provided. © 2021 FLM Engineering, Inc. Page 6 of 6 February 23, 2021 APPENDIX A FEMA FIRM MAP APPENDIX B USGS MAP EXCERPT APPENDIX C SOILS MAP EXCERPT APPENDIX D NRCS SOIL REPORT APPENDIX E NOAA RAINFALL DATA APPENDIX F PRE-DEVELOPMENT WATERSHED MAP APPENDIX G PRE-DEVELOPMENT HYDROLOGIC MODEL Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" APPENDIX H POST-DEVELOPMENT WATERSHED MAP APPENDIX I POST-DEVELOPMENT HYDROLOGIC MODEL Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" Type II 24-hr 1-Year Rainfall=2.90" APPENDIX J DISCRETE NRCS CURVE NUMBER MODEL Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" Type II 24-hr 1-Inch Rainfall=1.00" APPENDIX K SCM SUPPLEMENTAL FORM SUPPLEMENT-EZ COVER PAGE FORMS LOADED PROJECT INFORMATION Project Name Green Hill Subdivision - Phase 1 1 2Project Area (ac)23.02 3Coastal Wetland Area (ac) 4Surface Water Area (ac)0.65 5Is this project High or Low Density?High Does this project use an off-site SCM?No 6 COMPLIANCE WITH 02H .1003(4) 7Width of vegetated setbacks provided (feet)124 8Will the vegetated setback remain vegetated?Yes 9If BUA is proposed in the setback, does it meet NCAC 02H.1003(4)(c-d)?N/A Is streambank stabilization proposed on this project?No 10 NUMBER AND TYPE OF SCMs: 11Infiltration System 12Bioretention Cell 13Wet Pond 14Stormwater Wetland1 15Permeable Pavement 16Sand Filter 17Rainwater Harvesting (RWH) 18Green Roof 19Level Spreader-Filter Strip (LS-FS) 20Disconnected Impervious Surface (DIS) 21Treatment Swale 22Dry Pond 23StormFilter 24Silva Cell 25Bayfilter Filterra 26 FORMS LOADED DESIGNER CERTIFICATION 27Name and Title:Jon Frazier, PE / Principal 28Organization:FLM Engineering, Inc. 29Street address:PO Box 91727 30City, State, Zip:Raleigh, NC 27675 31Phone number(s):919.610.1051 Email:jfrazier@flmengineering.com 32 Certification Statement: I certify, under penalty of law that this Supplement-EZ form and all supporting information were prepared under my direction or supervision; that the information provided in the form is, to the best of my knowledge and belief, true, accurate, and complete; and that the engineering plans, specifications, operation and maintenance agreements and other supporting information are consistent with the information provided here. Designer ______________________________________________ Signature of Designer ______________________________________________ Date Seal DRAINAGE AREAS 1Is this a high density project?Yes 2 If so, number of drainage areas/SCMs1 3Does this project have low density areas?No 4 If so, number of low density drainage areas0 Is all/part of this project subject to previous rule 5versions?No FORMS LOADED Entire Site1 DRAINAGE AREA INFORMATION 4Type of SCM Wetland 5Total drainage area (sq ft)2,147,0781694675 6Onsite drainage area (sq ft)2,147,0781694675 7Offsite drainage area (sq ft) 8Total BUA in project (sq ft)679520 sf679520 sf New BUA on subdivided lots (subject to 9permitting) (sq ft)374500 sf374500 sf New BUA not on subdivided lots (subject to 10permitting) (sf)305020 sf305020 sf 11Offsite BUA (sq ft) 12Breakdown of new BUA not on subdivided lots: - Parking (sq ft) - Sidewalk (sq ft)66299 sf66299 sf - Roof (sq ft) - Roadway (sq ft)198721 sf198721 sf - Future (sq ft) - Other, please specify in the comment box below (sq ft) 40000 sf40000 sf New infiltrating permeable pavement on 13subdivided lots (sq ft) New infiltrating permeable pavement not on 14subdivided lots (sq ft) Existing BUA that will remain (not subject to 15permitting) (sq ft) 16Existing BUA that is already permitted (sq ft) 17Existing BUA that will be removed (sq ft)50346 sf50346 sf 18Percent BUA32%40% 19Design storm (inches)1.0 in 20Design volume of SCM (cu ft)20150 cf Calculation method for design volumeDiscrete CN 21 ADDITIONAL INFORMATION Please use this space to provide any additional information about the 22drainage area(s): Other BUA is pool/clubhouse. STORMWATER WETLAND 1Drainage area numberDA-1A 2Minimum required treatment volume (cu ft)190357 cf GENERAL MDC FROM 02H .1050 3Is the SCM sized to treat the SW from all surfaces at build-out?Yes 4Is the SCM located away from contaminated soils?Yes 5What are the side slopes of the SCM (H:V)?3:1 Does the SCM have retaining walls, gabion walls or other 6No engineered side slopes? Are the inlets, outlets, and receiving stream protected from erosion 7Yes (10-year storm)? Is there an overflow or bypass for inflow volume in excess of the 8Yes design volume? 9What is the method for dewatering the SCM for maintenance?Pump (preferred) 10If applicable, will the SCM be cleaned out after construction?Yes 11Does the maintenance access comply with General MDC (8)?Yes 12Does the drainage easement comply with General MDC (9)?Yes If the SCM is on a single family lot, does (will?) the plat comply with 13Yes General MDC (10)? 14Is there an O&M Agreement that complies with General MDC (11)?Yes 15Is there an O&M Plan that complies with General MDC (12)?Yes 16Does the SCM follow the device specific MDC?Yes 17Was the SCM designed by an NC licensed professional?Yes STORMWATER WETLAND MDC FROM 02H .1054 18Design volume of SCM (cu ft)20150 Are the inlet(s) and outlet located in a manner that avoids short- 19Yes circuiting? 20Are berms or baffles provided to improve the flow path?Yes Does the orifice drawdown from below the top surface of the 21Yes permanent pool? Does the wetland minimize impacts to the receiving channel from the 22Yes 1-yr, 24-hr storm? 23Is a trash rack or other device provided to protect the outlet system?Yes 9ƌĻǝğƷźƚƓƭ Elevation, peak attenuation above temporary pool (if applicable) 24196.24 (fmsl) Elevation, temporary pool (top of the temporary inundation zone) 25194.25 (fmsl) 26Elevation, permanent pool (top of the shallow water zone) (fmsl)193.00 27Elevation, bottom of shallow water zone (fmsl)192.25 Elevation, bottom of forebay deep pool at deepest point (at forebay 28189.67 entrance) (fmsl) Elevation, bottom of forebay deep pool at shallowest point (at 29191.00 forebay exit) (fmsl) 30Elevation, bottom of non-forebay deep pool at deepest point (fmsl)190.00 tƌğƓƷźƓŭ œƚƓĻƭ Area, total surface area of the SW wetland at temporary pool elev. 3117,518.00 (sq ft) 32Area, temporary inundation zone at temporary pool elev. (sq ft)6,131.00 33Area, shallow water zone at temporary pool elev. (sq ft)6,131.00 34Area, forebay at temporary pool elev. (sq ft)2,628.00 35Area, non-forebay deep pool at temporary pool elev .(sq ft)2,628.00 Percent area provided, temporary inundation zone (should be 30- 3635% 45%) 37Percent area provided, shallow water zone (should be 35-45%)35% 38Percent area provided, deep pool (forebay) (should be 10-15%)15% 39Percent area provided, deep pool (non-forebay) (should be 5-15%)15% 5ĻƦƷŷƭ ğƓķ hǒƷƌĻƷ 40Peak attenuation depth above temporary inundation zone (inches)24 in Temporary inundation zone depth (temporary pool to permanent 4115 in pool) (inches) Shallow water zone depth (permanent pool to bottom of wetland) 429 in (inches) Depth, forebay at entrance (permanent pool to bottom of forebay 4340 in entrance) (inches) Depth, forebay at exit (permanent pool to bottom of forebay exit) 4424 in (inches) Depth, non-forebay deep pools (permanent pool to deep pool 4536 in bottom) (inches) 46If there is an orifice, diameter (inches)2 in 47If there is a weir, weir height (inches) 48If there is a weir, weir length (inches) 49Drawdown time for the temporary pool (days)3.4 Wetland39:07 PM 2/4/2021 STORMWATER WETLAND {ƚźƌ ğƓķ tƌğƓƷƭ 50Soil amendment depth (inches)12 in 51Has a soil amendment specification been provided?Yes Has a landscaping plan that meets SW Wetland MDC (12) been 52Yes provided? 53Number of plants per 200 square feet (#) in the shallow water zone: Does the temporary inundation zone planting comply with SW 54Yes Wetland MDC (14)? Are the dam structure and temporary fill slopes planted in non- 55Yes clumping turfgrass? 56Will cattails be planted in the wetland?No ADDITIONAL INFORMATION Please use this space to provide any additional information about 57 the stormwater wetland(s): Wetland49:07 PM 2/4/2021 APPENDIX L DRAW DOWN CALCULATIONS Orifice Drawdown Calculations 3 ft Volume provided at 15''13851 Orifice Equation Outlet Diameter2in Cd =0.6 Ho / 3 =0.42ft 2 ft / sec g =32.2 A =0.022sf Q =0.068cfs 203561.266seconds Drawdown Time56.5hours APPENDIX M RIP RAP ENERGY DISSPATOR CALCULATIONS DESIGN OF RIPRAP OUTLET PROTECTION WORKSHEET ProjectGreen Hill SDDate Project No.20020Designer Outlet IDFES-001 Outlet flowrate113.26cfs Pipe diameter48inches Number of pipes1 Pipe separation0feet Outlet pipe slope0.78percent Figure 8.06.b.1 25 Zone 7 20 Zone 6 ) c e 15 s / t f ( y t i Zone 5 c 10 o l e 9.0 ft/sec Zone 4 V 5 Zone 1 Zone 3 Zone 2 0 0246810 Pipe diameter (ft) Zone from graph above =1 Outlet pipe diameter48 in.Length =16.0 ft. Outlet flowrate113.3 cfsWidth =12.0 ft. Outlet velocity9.0 ft/secStone diameter =3 in. Material =Class AThickness =9 in. ZoneMaterialDiameterThicknessLengthWidth 1Class A394 x D(o)3 x D(o) 2Class B6226 x D(o)3 x D(o) 3Class I13228 x D(o)3 x D(o) 4Class I13228 x D(o)3 x D(o) 5Class II232710 x D(o)3 x D(o) 6Class II232710 x D(o)3 x D(o) 7Special study required 1. Calculations based on NY DOT method - Pages 8.06.05 through 8.06.06 in NC Erosion Control Manual 2. Outlet velocity based on full-flow velocity Green Hill_Riprap Outlet Protection.xls 2/4/2021 DESIGN OF RIPRAP OUTLET PROTECTION WORKSHEET ProjectGreen Hill SDDate Project No.20020Designer Outlet IDFES-SCM 1 Outlet flowrate58cfs Pipe diameter30inches Number of pipes1 Pipe separation0feet Outlet pipe slope1.14percent Figure 8.06.b.1 25 Zone 7 20 Zone 6 ) c e 15 s / t f ( y 11.8 ft/sec t i Zone 5 c 10 o l e Zone 4 V 5 Zone 1 Zone 3 Zone 2 0 0246810 Pipe diameter (ft) Zone from graph above =1 Outlet pipe diameter30 in.Length =10.0 ft. Outlet flowrate58.0 cfsWidth =7.5 ft. Outlet velocity11.8 ft/secStone diameter =3 in. Material =Class AThickness =9 in. ZoneMaterialDiameterThicknessLengthWidth 1Class A394 x D(o)3 x D(o) 2Class B6226 x D(o)3 x D(o) 3Class I13228 x D(o)3 x D(o) 4Class I13228 x D(o)3 x D(o) 5Class II232710 x D(o)3 x D(o) 6Class II232710 x D(o)3 x D(o) 7Special study required 1. Calculations based on NY DOT method - Pages 8.06.05 through 8.06.06 in NC Erosion Control Manual 2. Outlet velocity based on full-flow velocity Green Hill_Riprap Outlet Protection.xls 2/4/2021 APPENDIX N OPERATION & MAINTENANCE AGREEMENT Operation & Maintenance Agreement Project Name:Green Hill Subdivision Project Location:NC Hwy 39, Louisburg, NC Cover Page Maintenance records shall be kept on the following BMP(s). This maintenance record shall be kept in a log in a known set location. Any deficient BMP elements noted in the inspection will be corrected, repaired, or replaced immediately. These deficiencies can affect the integrity of structures, safety of the public, and the pollutant removal efficiency of the BMP(s). The BMP(s) on this project include (check all that apply & corresponding O&M tables will be added automatically): Bioretention CellQuantity:Location(s): Dry Detention Basin Quantity:Location(s): Grassed Swale Quantity:Location(s): Green Roof Quantity:Location(s): Infiltration Basin Quantity:Location(s): Infiltration Trench Quantity:Location(s): Level Spreader/VFSQuantity:Location(s): Permeable PavementQuantity:Location(s): Proprietary System Quantity:Location(s): Rainwater Harvesting Quantity:Location(s): Sand Filter Quantity:Location(s): Stormwater WetlandSee Plans Quantity:1Location(s): Wet Detention Basin Quantity:0Location(s): Disconnected Impervious Area Present:NoLocation(s): User Defined BMP Present:NoLocation(s): I acknowledge and agree by my signature below that I am responsible for the performance of the maintenance procedures listed for each BMP above, and attached O&M tables. I agree to notify NCDENR of any problems with the system or prior to any changes to the system or responsible party. * Responsible Party: Title & Organization: Street address: City, state, zip: Phone number(s): Email: Signature: Date: I,, a Notary Public for the State of County of, do hearby certify that personally appeared before me thisday of and acknowledge the due execution of the Operations and Maintenance Agreement . Witness my hand and official seal, _____________________________________________. STORM-EZ10/5/2020 SealMy commission expires O&M Manual Version 1.4 Page 1 of 3 Stormwater Wetland Maintenance Requirements Important maintenance procedures: Immediately following construction of the stormwater wetland, bi-weekly inspections will be conducted and - wetland plants will be watered bi-weekly until vegetation becomes established (commonly six weeks). No portion of the stormwater wetland will be fertilized after the first initial fertilization that is required to establish - the wetland plants. Stable groundcover will be maintained in the drainage area to reduce the sediment load to the wetland. - Once a year, a dam safety expert should inspect the embankment. - After the stormwater wetland is established, it shall be inspected monthly and within 24 hours after every storm event greater than 1.0 inches (or 1.5 inches if in a Coastal County). Records of operation and maintenance will be kept in a known set location and will be available upon request. Inspection activities shall be performed as follows. Any problems that are found shall be repaired immediately. BMP element:Potential problem:How I will remediate the problem: Trash/debris is present.Remove the trash/debris. Entire BMP Areas of bare soil and/or Regrade the soil if necessary to remove the gully, and then plant a The perimeter of the BMP erosive gullies have formed.ground cover and water until it is established. Provide lime and a one- time fertilizer application. Vegetation is too short or too Maintain vegetation at a height of approximately six inches. long. Sediment has accumulated in Search for the source of the sediment and remedy the problem if Forebay the forebay to a depth that possible. Remove the sediment and dispose of it in a location where it inhibits the forebay from will not cause impacts to streams or the BMP. Erosion has occurred.Provide additional erosion protection such as reinforced turf matting or riprap if needed to prevent future erosion problems. Weeds are present.Remove the weeds, preferably by hand. If a pesticide is used, wipe it on the plants rather than spraying. The pipe is clogged.Unclog the pipe. Dispose of the sediment off-site. The inlet device The pipe is cracked or Replace the pipe. otherwise damaged. Erosion is occurring in the Regrade the swale if necessary to smooth it over and provide erosion swale.control devices such as reinforced turf matting or riprap to avoid future problems with erosion. Stone verge is clogged or Remove sediment and replace with clean stone. covered in sediment (if applicable). STORM-EZ10/5/2020 O&M Manual Version 1.4 Page 2 of 3 Stormwater Wetland Maintenance Requirements (Continued) Algal growth covers over 50% Consult a professional to remove and control the algal growth. Deep pool, shallow water of the deep pool and shallow and shallow land areas water areas. Cattails, phragmites or other Remove invasives by physical removal or by wiping them with invasive plants cover 50% of the deep pool and shallow Shallow land remains flooded Unclog the outlet device immediately. more than 5 days after a storm event. Plants are dead, diseased or Determine the source of the problem: soils, hydrology, disease, etc. dying.Remedy the problem and replace plants. Provide a one-time fertilizer application to establish the ground cover if necessary. Best professional practices Prune according to best professional practices. show that pruning is needed to maintain optimal plant Sediment has accumulated Search for the source of the sediment and remedy the problem if and reduced the depth to 75% possible. Remove the sediment and dispose of it in a location where it of the original design depth of will not cause impacts to streams or the BMP. A tree has started to grow on Consult a dam safety specialist to remove the tree. Embankment the embankment. An annual inspection by Make all needed repairs. appropriate professional shows that the embankment Evidence of muskrat or Consult a professional to remove muskrats or beavers. beaver activity is present. Sediment has accumulated Search for the source of the sediment and remedy the problem if Micropool and reduced the depth to 75% possible. Remove the sediment and dispose of it in a location where it of the original design depth.will not cause impacts to streams or the BMP. Clogging has occurred.Clean out the outlet device. Dispose of the sediment off-site. The outlet device The outlet device is damagedRepair or replace the outlet device. Erosion or other signs of Contact the local NC Department of Environment and Natural The receiving water damage have occurred at the Resources Regional Office. outlet. STORM-EZ10/5/2020 O&M Manual Version 1.4 Page 3 of 3 APPENDIX O TAR-PAMLICO ACCOUNTING TOOL Last Modified 2/4/2021 Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Piedmont of the Tar-Pamlico River Basin: Includes Oxford, Henderson, Rocky Mount and Tarboro as well as Franklin, Nash and Edgecome Counties Total Nitrogen and Total Phosphorus Loading Calculation Worksheet (Automated) Project Name: Green Hill Subdivision Phase 1 Date:2/4/2021 By:Jon FrazierChecked By: Directions (same for pre-development and post-development tables): > Enter the acres of each type of land cover in the green boxes. The spreadsheet will calculate all of the values in light blue. > Compare total areas of development in pre- and post- tables for consistency (bottom of column (2)), and also for consistency with the site plans. If all of these values are not the same, there is an error that must be corrected. > Unless drainage onto the development from offsite is diverted around or through the site, offsite catchment area draining in must be included in the acreage values and treated. Pre-development: (1)(2)(3)(4)(5)(6)(7) Type of Land CoverArea S.M. Formula Average EMC Column Average EMC Column (acres)(0.46 + 8.3I)of TN (mg/L)(2) * (3) * (4)of TP (mg/L)(2) * (3) * (6) Transportation impervious 1.670.742.603.230.190.24 Roof impervious 0.000.741.950.000.110.00 Managed pervious 47.410.741.4250.000.289.86 (lawn/landscaped) Managed pervious 0.000.744.230.001.230.00 (cropland) Managed pervious 0.000.742.040.000.620.00 (pasture) Wooded pervious 0.000.740.940.000.140.00 TN Loading TP Loading Fraction Impervious (I) = 0.0353.2310.10 (lb/yr) =(lb/yr) = TN Exp. Coeff. TP Exp. Coeff. Total Area of Development = 49.081.080.21 (lb/ac/yr) =(lb/ac/yr) = Post-development: (1)(2)(3)(4)(5)(6)(7) Type of Land CoverArea S.M. Formula Average EMC Column Average EMC Column (acres)(0.46 + 8.3I)of TN (mg/L)(2) * (3) * (4)of TP (mg/L)(2) * (3) * (6) Transportation impervious 7.003.102.6056.440.194.12 Roof impervious 8.603.101.9552.010.112.93 Managed pervious 33.483.101.42147.440.2829.07 Wooded pervious 0.003.100.950.000.140.00 TN Loading TP Loading Fraction Impervious (I) = 0.32255.8936.13 (lb/yr) =(lb/yr) = TN Exp. Coeff. TP Exp. Coeff. Total Area of Development = 49.085.210.74 (lb/ac/yr) =(lb/ac/yr) = Note: The nutrient loading goals are 4.0 lb/ac/yr for TN and 0.4 lb/ac/yr for TP. If the post-development nutrient loading is below these levels, then no BMP is necessary. Otherwise, the next worksheet calculates post-development TN and TP loadings after BMPs are installed. Last Modified 5/23/03 Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Piedmont of the Tar-Pamlico River Basin: Includes Oxford, Henderson, Rocky Mount and Tarboro as well as Franklin, Nash and Edgecome Counties BMP Removal Calculation Worksheet (Automated) Project Name:Green Hill Subdivision Phase 1 Date: 2/4/2021 By:Jon FrazierChecked By:Jon Frazier Directions: > It may be advantageous to split the development into separate catchments to be handled by separate BMPs. The tables below allow the development to be split into as many as three catchments, and can be copied for greater than three. NOTE: Unless runoff flowing onto the development from offsite is routed separately around or through the site, the offsite catchment area draining in must be included in the acreage values of the appropriate land use(s) and treated. > Above each table: Enter the catchment acreage in the top green blank. Based on a comparison of the post-development TN and TP export coefficients you calculated above to the rule requirements of 4.0 lb/ac/yr TN and 0.4 lb/ac/yr TP, select BMP(s) from the list for treating the catchment runoff. Enter the chosen BMP(s) nutrient removal rates in the green blanks. If more than one BMP is to be used in series, the combined removal rates will be calculated automatically in the blue blanks. > Catchment Tables: Enter the acres of each type of land cover in the green boxes. The spreadsheet will calculate all of the light blue boxes. NOTE: Compare the Total Catchment Acreage for the Development (final table) to the value you established in the pre-BMP worksheet tables, and also to the site plans, for consistency. All of these values need to be the same TNTPDesign Standard Wet Detention Pond NC BMP Manual 2540 BMP Stormwater Wetland NC BMP Manual 4035 Nutrient Sand Filter NC BMP Manual 3545 Removal Bioretention NC BMP Manual 3545 Rates Grass Swale NC BMP Manual 2020 Vegetated Filter Strip w/ NC BMP Manual 2035 Level Spreader Dry Detention NC BMP Manual 1010 Catchment 1: Total acreage of catchment 1 =ac 38.66 First BMP's TN removal rate =%First BMP's TP removal rate =% 4035 Second BMP's TN removal rate =%Second BMP's TP removal rate =% Third BMP's TN removal rate =%Third BMP's TP removal rate =% TOTAL TN REMOVAL RATE =%TOTAL TP REMOVAL RATE =% 4035 (1)(2)(3)(4)(5)(6)(7) Type of Land CoverCatchment S.M. Formula Average EMC of Column Average EMC of Column Acreage(0.46 + 8.3I)TN (mg/L)(2) * (3) * (4)TP (mg/L)(2) * (3) * (6) Transportation impervious 7.003.812.6069.400.195.07 Roof impervious 8.603.811.9563.950.113.61 Managed pervious 23.063.811.42124.870.2824.62 Wooded pervious 0.003.810.940.000.140.00 Area taken up by BMP 0.003.811.950.000.110.00 Pre-BMP TN Pre-BMP TP Fraction Impervious (I) = 0.40258.2133.30 Load (lb/yr) =Load (lb/yr) = Pre-BMP TN Pre-BMP TP Total Area of Development = 38.666.680.86 Export (lb/ac/yr) =Export (lb/ac/yr) = Post-BMP TN Post-BMP TP 154.9321.65 Load (lb/yr) =Load (lb/yr) = Post-BMP TN Post-BMP TP 4.010.56 Export (lb/ac/yr) =Export (lb/ac/yr) = Last Modified 5/23/03 Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Catchment 2: Total acreage of catchment 2 =ac 10.42 First BMP's TN removal rate =%First BMP's TP removal rate =% Second BMP's TN removal rate =%Second BMP's TP removal rate =% Third BMP's TN removal rate =%Third BMP's TP removal rate =% TOTAL TN REMOVAL RATE =%TOTAL TP REMOVAL RATE =% 00 (1)(2)(3)(4)(5)(6)(7) Type of Land CoverCatchment S.M. Formula Average EMC of Column Average EMC of Column Acreage(0.46 + 8.3I)TN (mg/L)(2) * (3) * (4)TP (mg/L)(2) * (3) * (6) Transportation impervious 0.000.462.600.000.190.00 Roof impervious 0.000.461.950.000.110.00 Managed pervious 10.420.461.426.810.281.34 Wooded pervious 0.000.460.940.000.140.00 Area taken up by BMP 0.000.461.950.000.110.00 Pre-BMP TN Pre-BMP TP Fraction Impervious (I) = 0.006.811.34 Load (lb/yr) =Load (lb/yr) = Pre-BMP TN Export Pre-BMP TP Export Total Area of Development = 10.420.650.13 (lb/ac/yr) =(lb/ac/yr) = Post-BMP TN Post-BMP TP 6.811.34 Load (lb/yr) =Load (lb/yr) = Post-BMP TN Post-BMP TP Export 0.650.13 Export (lb/ac/yr) =(lb/ac/yr) = Catchment 3: Total acreage of catchment 3 =ac First BMP's TN removal rate =%First BMP's TP removal rate =% Second BMP's TN removal rate =%Second BMP's TP removal rate =% Third BMP's TN removal rate =%Third BMP's TP removal rate =% TOTAL TN REMOVAL RATE =0%TOTAL TP REMOVAL RATE =0% (1)(2)(3)(4)(5)(6)(7) Type of Land CoverCatchment S.M. Formula Average EMC of Column Average EMC of Column Acreage(0.46 + 8.3I)TN (mg/L)(2) * (3) * (4)TP (mg/L)(2) * (3) * (6) Transportation impervious 2.600.19 Roof impervious 1.950.11 Managed pervious 1.420.28 Wooded pervious 0.940.14 Area taken up by BMP 1.950.11 Pre-BMP TN Pre-BMP TP Fraction Impervious (I) = Load (lb/yr) =Load (lb/yr) = Pre-BMP TN Export Pre-BMP TP Export Total Area of Development = (lb/ac/yr) =(lb/ac/yr) = Post-BMP TN Post-BMP TP Load (lb/yr) =Load (lb/yr) = Post-BMP TN Post-BMP TP Export Export (lb/ac/yr) =(lb/ac/yr) = Last Modified 5/23/03 Tar-Pamlico Stormwater Rule 15A NCAC 2B .0258 Weighted Average of Nutrient Loadings from the Catchments: Post-BMP Post-BMP Catchment AcreageTN Loading TP Loading (lb/ac/yr)(lb/ac/yr) Catchment 1 38.664.010.56 Catchment 2 10.420.650.13 Catchment 3 0.000.000.00 TOTAL FOR DEVELOPMENT 49.083.300.47 Note: The nutrient loading goals are 4.0 lb/ac/yr for TN and 0.4 lb/ac/yr for TP. If the post-development nutrient loading is below these levels, then the BMPs planned are adequate. Otherwise, additional BMPs and/or modifications in development plans are required.