Table of Contents Project Description... 1 Summary of Existing Conditions... 1 Summary of Proposed Conditions... 1 Stormwater Compliance... 2 Collection... 2 Treatment... 3 Storage... 3 Disposal... 4 Soils... 4 Calculations... 4 Summary and Conclusions... 4 Appendices Appendix A Pre and Post Drainage Area Maps Appendix B - On-site Soils Report and Memo Appendix C State Stormwater Calculations Appendix D Currituck County Stormwater Calculations
Project Description The following narrative will detail the stormwater management plan for the proposed Foxglove Acres subdivision located in Moyock, Currituck County, NC. The subject property is located along Tulls Creek Road, Moyock, NC approximately 0.6 miles eastward from the intersection of Panther Landing Road and Tulls Creek Road. The property is located along the southern side of Tulls Creek Road and previously known as Lots 1 & 2, Ward Acres Subdivision. The site development proposed the construction of a street, sidewalks, 13 residential lots, open space that includes stormwater control measures (SCM s) and associate utilities such as power, watermain, and fire hydrant assemblies. The project will also include two parking spaces (one of which will be a van accessible ADA space) and a cluster mailbox unit near the front of the subdivision. The following narrative, application, and calculations will demonstrate the parameters of the proposed subdivision design, which will illustrate an effective stormwater management system in compliance with all State regulations. Summary of Existing Conditions The existing 20.01 acre project area is currently vacant and consists of agricultural fields and woods. There are three agricultural ditches that run through the project, a major drainage ditch that follows the southeastern property line and a roadside ditch along Tulls Creek Road. There are no wetlands within this project area. Existing ground elevations range between 5 to 8 with an average slope of 0.3%. Existing stormwater runoff is via sheet flow to the agricultural ditches that run through the properties. These ditches direct runoff to the major drainage ditch along the southeastern property line to Tulls Creek Road which then leaves the property eventually making it to the Northwest River. The site does not currently have any existing built upon area and only has one drainage area (DA 1). The boundary for DA 1 is provided within Appendix A on the Drainage Area Maps. Summary of Proposed Conditions The project proposes the construction of a street, sidewalk, 13 residential lots, open space that includes stormwater control measures (SCM s) and associated utilities such as power, watermain, and fire hydrant assemblies. The subdivision will propose 248,260 sq.ft. of coverage including a wet pond permanent pool area of 52,072 sq.ft. Upon completion of the proposed improvements, the sites overall BUA will be 34% of the drainage area. The proposed Built Upon Area (BUA) summary table can be seen on Table 2 below.
Table 2: Proposed Built Upon Area Summary Land Cover Description Drainage Basin 1 (sq.ft.) (acre) Drainage Area = 739,270.00 16.97 Open Space 491,010.00 11.27 Lot BUA = 161,700.00 3.71 Right of Way BUA = 33,927.00 0.78 Open Space BUA = 561.00 0.01 Pond = 52,072.00 1.20 Total Impervious = 248,260.00 5.70 Stormwater management improvements will be provided to treat runoff from all proposed impervious construction. The primary method of stormwater treatment will be via a wet detention basin located on the northeastern of the site. The wet detention basin has been sized to manage an overall impervious area of 248,260 sq. ft. All of these impervious areas are located within Drainage Area 1 on the post-development drainage area map. Stormwater Compliance Per 15A NCAC 02H.1005 (a) (3) (B) High Density Coastal Development is required to meet particular criteria. This development is proposed to have 34% of impervious coverage within the entire lot. The proposed wet detention basin onsite is designed in accordance with NCDEQ and Currituck County Stormwater Manual Requirements and is designed to store, control, and treat the stormwater runoff from all surfaces generated by the one and one-half inch of rainfall event (State Requirements) and route the 10-year post-development back to the 2-year predevelopment, wooded conditions (Currituck County Requirements). In addition to these requirements, a minimum 50 vegetative buffer from surface waters is provided. The High Density Residential Subdivisions Deed Restrictions & Protective Covenances Form will be supplied with the State Stormwater Permit Application and will include the table below for allowable Built Upon Area for each lot: Lot # BUA (sf) Lot # BUA (sf) Lot # BUA (sf) Lot # BUA (sf) 1 15,000 5 12,030 9 12,030 13 12,030 2 12,030 6 12,030 10 12,030 3 12,030 7 13,200 11 12,030 4 12,030 8 13,200 12 12,030 Collection Runoff from the proposed impervious area is to be collected and conveyed to the wet detention basin via sheet flow to the lot line / roadside vegetated swales. The vegetated swales will then deliver the runoff to the proposed forebay of the wet detention basin.
Treatment The proposed system will offer several methods of treatment prior to release. Runoff from the residential lots will be directed to the lot line swales and roadside swales via sheet flow. The vegetated swales will provide the first level of treatment by the filtration of small particulates and nutrients prior to entering the wet detention basin. Riprap will be provided at the inlet to the pond from the stormwater conveyance connections. Stormwater entering the forebay will be deenergized by the riprap as to reduce inflow velocities to the basin. Larger debris and pollutants will settle on the riprap aprons as the stormwater enters the forebay. The primary treatment of runoff will be provided within the wet detention basin. Several modes of treatment are available in the wet detention basin. Runoff first entering the basin will lose velocity and large particulates and sediment will settle out primarily in the forebay. The vegetated side slopes and vegetated shelf will also provide filtration of runoff and nutrient uptake through natural biological processes. Storage The proposed wet detention basin has been sized to allow for a local requirement of routing the 10-year post developed condition back to the 2-year pre developed wooded condition. This storage capacity is in excess of the State required 1.5 inch and provides approximately 7.8 inches of storage in the basin. The storage required to completely capture the first 1.5 inch of rainfall is 32,600 cubic feet. The proposed wet detention basin will have a temporary storage capacity of 169,183 cubic feet. The temporary storage capacity has been calculated between the permanent pool elevation of 2.0 and the overflow spillway invert at 5.0. For NCDEQ calculations, the anticipated permanent pool and surface areas referenced in the application documents and attached calculations are measured at 2.0 ft. in elevation. Utilizing permanent pool average depth equation 2 from Section C-3 Wet Pond in the NCDEQ Stormwater Design Manual, the average depth was calculated to be 4.3 feet. Table 2: Coastal SA/DA Table from this Manual using a 35% impervious area and a 4.0 foot depth states that the minimum surface area to drainage area percentage is 1.96% (linear interpolation). The proposed surface area to drainage area for this project is 5.82%, far exceeding the requirements. NCDEQ calculations are shown in Appendix C. As stated above, Currituck County Stormwater Manual requires the stormwater design to route the 10-year post-development storm back to the 2-year pre-development storm in wooded conditions. The peak runoff of the 10-year post-development storm is 51.8 cubic feet per second (cfs) and the peak runoff for the 2-year pre-development storm in wooded conditions is 16.3 cfs. The peak discharge from routing the 10-year post-development storm through the proposed stormwater detention basin is 12.9 cfs equating to less than the 2-year predevelopment storm in wooded conditions, thus satisfying the Currituck County Stormwater Manual. The Currituck County Stormwater Calculations are shown in Appendix D. The attached soils analysis memorandum in Appendix B states that the seasonal high water table (SHWT) was approximately 30 inches below the existing ground surface at SB-1 identified
on the Drainage Area Map. Existing grade at SB-1 was surveyed at 4.53 feet in elevation, which puts the SHWT at an elevation of 2.03 feet. A rounded elevation of 2.0 was used in the calculations for the SHWT. The basin s 6 foot wide, 6:1 (H:V) vegetated shelf is specified to be constructed between the elevations of 1.5 ft. and 2.5ft. Disposal The wet detention basin s primary mode of disposal for elevations between 2.0 and 4.0 ft. is through a 6 drawdown orifice on a structure located inside of the main pool. The invert elevation of the 6 drawdown orifice is proposed to be at an elevation of 2.0 ft. Elevations between 4.0 and 5.0 feet will utilize the 2.0 by 2.0 stormwater spillway structure with a trash rack on top as well as the 6 drawdown orifice. The invert elevation of the rim of the spillway structure is proposed to be 4.0 feet in elevation. The total drawdown time from an elevation of 5.0 ft. is 2.54 days. Supporting calculations for the drawdown time and storage of the proposed wet pond have been provided within Appendix C. Larger storms that will cause the wet pond to exceed the elevation of 5.0 ft. will overflow through the 10 wide, riprap-lined spillway into the downstream stormwater system. Soils The USDA NRCS Soil Survey lists the soil in the vicinity of the proposed stormwater detention basin as described below: Ro Roanoke fine sandy loam This soil typically has 0 to 2 percent slopes. Roanoke fine sandy loam has a very high runoff class and is poorly drained. This soil is categorized in Hydrologic Soil Group D. To Tomotley fine sandy loam This soil typically has 0 to 2 percent slopes. Tomotley fine sandy loam has a very high runoff class and is poorly drained. This soil is categorized in Hydrologic Soil Group D. Quible & Associates, P.C. conducted a soil boring test in the vicinity of the wet pond. The soils observed were consistent with the NRCS soil description. The results of this boring are available in Appendix B. Calculations A copy of the Drainage and Storage Calculations are provided in Appendix C and Appendix D of this narrative. Summary and Conclusions The proposed stormwater management plan for this site provides stormwater treatment in excess of the requirements for the State and Currituck County. The proposed system will offer primary and secondary methods of treatment. This proposed design will adequately serve the stormwater management requirements of the site.
Appendices
Appendix A - Pre- and Post-development On-site Drainage Area Maps
PRELIMINARY NOT FOR CONSTRUCTION
PRELIMINARY NOT FOR CONSTRUCTION
Appendix B - On-site Soils Map and Data
Soil Map Currituck County, North Carolina 4042400 4042470 4042540 4042610 4042680 4042750 4042820 76 8' 52'' W 4042400 4042470 4042540 4042610 4042680 4042750 4042820 76 8' 22'' W 36 31' 31'' N 36 31' 31'' N 397250 397320 397390 397460 397530 397600 397670 397740 397810 397880 397950 36 31' 16'' N 397250 397320 397390 397460 397530 397600 397670 397740 397810 397880 397950 76 8' 52'' W 76 8' 22'' W 36 31' 16'' N Soil Map may not be valid at this scale. N Map Scale: 1:3,360 if printed on A landscape (11" x 8.5") sheet. Meters 0 45 90 180 270 Feet 0 150 300 600 900 Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 18N WGS84 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/28/2017 Page 1 of 3
MAP LEGEND MAP INFORMATION Soil Map Currituck County, North Carolina Area of Interest (AOI) Area of Interest (AOI) Soils Soil Map Unit Polygons Soil Map Unit Lines Soil Map Unit Points Special Point Features Blowout Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip Sodic Spot Spoil Area Stony Spot Very Stony Spot Wet Spot Other Special Line Features Water Features Streams and Canals Transportation Rails Interstate Highways US Routes Major Roads Local Roads Background Aerial Photography The soil surveys that comprise your AOI were mapped at 1:20,000. Warning: Soil Map may not be valid at this scale. Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale. Please rely on the bar scale on each map sheet for map measurements. Source of Map: Natural Resources Conservation Service Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Currituck County, North Carolina Survey Area Data: Version 15, Sep 19, 2016 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Dec 31, 2009 Mar 8, 2017 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/28/2017 Page 2 of 3
Soil Map Currituck County, North Carolina Map Unit Legend Currituck County, North Carolina (NC053) Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI Ro Roanoke fine sandy loam 20.0 96.7% To Tomotley fine sandy loam 0.7 3.3% Totals for Area of Interest 20.7 100.0% Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/28/2017 Page 3 of 3
Map Unit Description: Roanoke fine sandy loam---currituck County, North Carolina Currituck County, North Carolina Ro Roanoke fine sandy loam Map Unit Setting National map unit symbol: 3rp1 Elevation: 0 to 20 feet Mean annual precipitation: 42 to 58 inches Mean annual air temperature: 61 to 64 degrees F Frost-free period: 190 to 270 days Farmland classification: Farmland of statewide importance Map Unit Composition Roanoke, drained, and similar soils: 80 percent Roanoke, undrained, and similar soils: 10 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Roanoke, Drained Setting Landform: Depressions on marine terraces, flats on marine terraces Down-slope shape: Linear Across-slope shape: Linear Parent material: Clayey marine deposits and/or fluviomarine deposits Typical profile Ap - 0 to 8 inches: silt loam Btg - 8 to 58 inches: clay Cg - 58 to 80 inches: fine sand Properties and qualities Slope: 0 to 2 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Poorly drained Runoff class: Very high Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately high (0.00 to 0.20 in/hr) Depth to water table: About 0 to 12 inches Frequency of flooding: Rare Frequency of ponding: None Available water storage in profile: High (about 9.0 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 3w Hydrologic Soil Group: C/D Hydric soil rating: Yes Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/6/2017 Page 1 of 2
Map Unit Description: Roanoke fine sandy loam---currituck County, North Carolina Description of Roanoke, Undrained Setting Landform: Depressions on marine terraces, flats on marine terraces Down-slope shape: Linear Across-slope shape: Linear Parent material: Clayey marine deposits and/or fluviomarine deposits Typical profile A - 0 to 8 inches: silt loam Btg - 8 to 58 inches: clay Cg - 58 to 80 inches: fine sand Properties and qualities Slope: 0 to 2 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Poorly drained Runoff class: Very high Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately high (0.00 to 0.20 in/hr) Depth to water table: About 0 to 12 inches Frequency of flooding: Rare Frequency of ponding: None Available water storage in profile: High (about 9.0 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 4w Hydrologic Soil Group: C/D Hydric soil rating: Yes Data Source Information Soil Survey Area: Currituck County, North Carolina Survey Area Data: Version 15, Sep 19, 2016 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/6/2017 Page 2 of 2
Map Unit Description: Tomotley fine sandy loam---currituck County, North Carolina Currituck County, North Carolina To Tomotley fine sandy loam Map Unit Setting National map unit symbol: 3rp4 Elevation: 0 to 30 feet Mean annual precipitation: 42 to 58 inches Mean annual air temperature: 61 to 64 degrees F Frost-free period: 190 to 270 days Farmland classification: Prime farmland if drained Map Unit Composition Tomotley, drained, and similar soils: 75 percent Tomotley, undrained, and similar soils: 10 percent Minor components: 6 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Tomotley, Drained Setting Landform: Depressions on stream terraces, flats on marine terraces Down-slope shape: Linear Across-slope shape: Linear Parent material: Sandy and loamy fluviomarine deposits and/or marine deposits Typical profile Ap - 0 to 7 inches: fine sandy loam Btg1-7 to 12 inches: fine sandy loam Btg2-12 to 42 inches: sandy clay loam BCg - 42 to 50 inches: sandy loam Cg - 50 to 80 inches: loamy sand Properties and qualities Slope: 0 to 2 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Poorly drained Runoff class: Very high Capacity of the most limiting layer to transmit water (Ksat): Moderately high to high (0.20 to 1.98 in/hr) Depth to water table: About 0 to 12 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Moderate (about 8.1 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 3w Hydrologic Soil Group: B/D Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/6/2017 Page 1 of 3
Map Unit Description: Tomotley fine sandy loam---currituck County, North Carolina Hydric soil rating: Yes Description of Tomotley, Undrained Setting Landform: Depressions on stream terraces, flats on marine terraces Down-slope shape: Linear Across-slope shape: Linear Parent material: Sandy and loamy fluviomarine deposits and/or marine deposits Typical profile A - 0 to 7 inches: fine sandy loam Btg1-7 to 12 inches: fine sandy loam Btg2-12 to 42 inches: sandy clay loam BCg - 42 to 50 inches: sandy loam Cg - 50 to 80 inches: loamy sand Properties and qualities Slope: 0 to 2 percent Depth to restrictive feature: More than 80 inches Natural drainage class: Poorly drained Runoff class: Very high Capacity of the most limiting layer to transmit water (Ksat): Moderately high to high (0.20 to 1.98 in/hr) Depth to water table: About 0 to 12 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Moderate (about 8.1 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 4w Hydrologic Soil Group: B/D Hydric soil rating: Yes Minor Components Nimmo, undrained Percent of map unit: 3 percent Landform: Depressions on marine terraces, flats on marine terraces Down-slope shape: Concave Across-slope shape: Linear Hydric soil rating: Yes Arapahoe, undrained Percent of map unit: 3 percent Landform: Depressions, flats Down-slope shape: Linear Across-slope shape: Concave Hydric soil rating: Yes Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/6/2017 Page 2 of 3
Map Unit Description: Tomotley fine sandy loam---currituck County, North Carolina Dragston, undrained Percent of map unit: 0 percent Landform: Marine terraces Down-slope shape: Linear Across-slope shape: Linear Hydric soil rating: No Data Source Information Soil Survey Area: Currituck County, North Carolina Survey Area Data: Version 15, Sep 19, 2016 Natural Resources Conservation Service Web Soil Survey National Cooperative Soil Survey 9/6/2017 Page 3 of 3
Appendix C - State Stormwater Calculations
NCDEQ Stormwater Calculations Foxglove Acres Subdivision Moyock, Currituck County, North Carolina Drainage Area Calculations Drainage Basin A (sq.ft.) (acre) Drainage Area = 739,270.00 16.97 Open Space 491,010.00 11.27 Lot BUA = 161,700.00 3.71 Right of Way BUA = 33,927.00 0.78 Open Space BUA = 561.00 0.01 Pond = 52,072.00 1.20 Total Impervious = 248,260.00 5.70 Runoff generated by 1.5" Rainfall Event (NCDEQ Simplified Method) Ia = Impervious Percentage = Impervious Area/Drainage Area Rv= Runoff Coefficient, 0.05+0.9Ia Rd= Rain fall depth (1.5 in.) V= Runoff Volume, 3630*Rd*Rv*A Area 1 Ia = 33.6% Rv= 0.35 Rd (in.)= 1.5 A (ac.) = 16.97 V (cf.)= 32,528 Total Storage Required by NCDEQ = 32,600.00 cf Permanent Pool Volume Provided In Wet Detention Basin 1 Volume Elev Area (sf) Avg area (sf) (cf) Cum Vol. (cf) -3 32882 0 33682.5 33683-2 34483 33683 35297.5 35298-1 36112 68981 36941 36941 0 37770 105922 38613.5 38614 1 39457 144536 41224 41224 2 42991 185760 l Permanent Pool Storage (cf.) Provided in Basin 1: 185,760
Storage Above Permanent Pool for Basin 1 Elev Area (sf) Avg area (sf) Volume (cf) Cum Vol. (cf) 2 42991 49793 49793 3 56595 49793 58138 58138 4 59681 107931 61252 61252 5 62823 169183 Total Storage (cf.) Provided in Basin 1: Rainfall Equavalent: 169,183 7.80 in Volume in Forebay for Basin 1 Elev Area (sf) Avg area (sf) Volume Cum Vol. (cf) (cf) -3 4573 0 4906.5 4907-2 5240 4907 5588 5588-1 5936 10495 6298 6298 0 6660 16793 7036.5 7037 1 7413 23830 8247 8247 2 9081 32077 Total Storage (cf.) Provided in Forebay for Basin 1: 32,077 17.3% A bot_shelf = 48112 sf (1.5') A perm_pool =42991 sf (2.0') A bot_pond = 32882 sf (-3.0' Main Pool only) V perm_pool =185760 cf (2.0') Depth = 5 ft Main Pool Volume Req'd Option 1 Dav = 4.3 feet Option 2 Dav = 4.7 feet Percent Impervious Coverage: 33.6% Surface area of mainbay 42991.0 sf Drainage area 739270.0 SA/DA 5.82% Min. per Table 2 1.96% OK (for 35% Impervious @ 4.0' depth)
Orifice Drawdown Calculations Stage 5.0' to 4.0' Orifice Outlet Equation Q = C D A(2gH o ) 0.5 C D = 0.6 Orifice Diameter = 0.5 ft. Orifice Area, A = 0.20 sq.ft. H o = 3.25 ft. H o /3 = Q = Volume = Drawdown Time = Weir Structure EquationQ = C w LH 1.5 1.08 ft. 0.98 cfs 61,252 cf. 62247 sec. 0.72 days C w = 0.30 L = 8.00 ft. (2.0' x 2.0' Grate) H = 1.00 ft. Q = 2.40 cfs Volume = 61252 cf. Drawdown Time = 25522 sec. 0.30 days Stage 4.0' to 2.0' Orifice Outlet Equation Q = C D A(2gH o ) 0.5 C D = 0.6 Orifice Diameter = 0.5 ft. Orifice Area, A = 0.20 sq.ft. H o = 2.25 ft. H o /3 = Q = Volume = Drawdown Time = 0.75 ft. 0.82 cfs 107,931 cf. 131823 sec. 1.53 days Total Drawdown Time 2.54 days
Appendix D Currituck County Stormwater Calculations
Watershed Model Schematic Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 1 2 3 Legend Hyd. Origin Description 1 SCS Runoff P17088 - Post-Development 2 SCS Runoff Pre-Development 3 Reservoir Wet Pond Routing Project: P17088 - Currituck County Stormwater Model.gpw Wednesday, 06 / 27 / 2018
Hydrograph Summary Report Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph No. type flow interval Peak volume hyd(s) elevation strge used Description (origin) (cfs) (min) (min) (cuft) (ft) (cuft) 1 SCS Runoff 31.54 2 742 176,077 ------ ------ ------ P17088 - Post-Development 2 SCS Runoff 16.25 2 754 111,618 ------ ------ ------ Pre-Development 3 Reservoir 2.537 2 912 171,153 1 104.13 337,975 Wet Pond Routing P17088 - Currituck County Stormwater Model.gpw Return Period: 2 Year Wednesday, 06 / 27 / 2018
Hydrograph Report Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Wednesday, 06 / 27 / 2018 Hyd. No. 2 Pre-Development Hydrograph type = SCS Runoff Peak discharge = 16.25 cfs Storm frequency = 2 yrs Time to peak = 12.57 hrs Time interval = 2 min Hyd. volume = 111,618 cuft Drainage area = 16.970 ac Curve number = 77 Basin Slope = 0.0 % Hydraulic length = 0 ft Tc method = TR55 Time of conc. (Tc) = 49.90 min Total precip. = 4.00 in Distribution = Type III Storm duration = 24 hrs Shape factor = 484 Q (cfs) Pre-Development Hyd. No. 2 -- 2 Year 18.00 18.00 Q (cfs) 15.00 15.00 12.00 12.00 9.00 9.00 6.00 6.00 3.00 3.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Time (hrs) Hyd No. 2
TR55 Tc Worksheet Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Hyd. No. 2 Pre-Development Description A B C Totals Sheet Flow Manning's n-value = 0.100 0.011 0.011 Flow length (ft) = 300.0 0.0 0.0 Two-year 24-hr precip. (in) = 4.00 0.00 0.00 Land slope (%) = 0.30 0.00 0.00 Travel Time (min) = 32.59 + 0.00 + 0.00 = 32.59 Shallow Concentrated Flow Flow length (ft) = 198.00 0.00 0.00 Watercourse slope (%) = 0.30 0.00 0.00 Surface description = Unpaved Paved Paved Average velocity (ft/s) =0.88 0.00 0.00 Travel Time (min) = 3.73 + 0.00 + 0.00 = 3.73 Channel Flow X sectional flow area (sqft) = 2.90 15.90 0.00 Wetted perimeter (ft) = 6.30 12.80 0.00 Channel slope (%) = 0.40 0.20 0.00 Manning's n-value = 0.035 0.035 0.015 Velocity (ft/s) =1.60 2.20 0.00 Flow length (ft) ({0})581.0 1001.0 0.0 Travel Time (min) = 6.05 + 7.58 + 0.00 = 13.63 Total Travel Time, Tc... 49.90 min
Hydrograph Summary Report Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Hyd. Hydrograph Peak Time Time to Hyd. Inflow Maximum Total Hydrograph No. type flow interval Peak volume hyd(s) elevation strge used Description (origin) (cfs) (min) (min) (cuft) (ft) (cuft) 1 SCS Runoff 51.80 2 742 295,354 ------ ------ ------ P17088 - Post-Development 2 SCS Runoff 31.61 2 754 214,315 ------ ------ ------ Pre-Development 3 Reservoir 12.90 2 782 290,266 1 104.82 380,631 Wet Pond Routing P17088 - Currituck County Stormwater Model.gpw Return Period: 10 Year Wednesday, 06 / 27 / 2018
Hydrograph Report Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Wednesday, 06 / 27 / 2018 Hyd. No. 1 P17088 - Post-Development Hydrograph type = SCS Runoff Peak discharge = 51.80 cfs Storm frequency = 10 yrs Time to peak = 12.37 hrs Time interval = 2 min Hyd. volume = 295,354 cuft Drainage area = 16.970 ac Curve number = 89 Basin Slope = 0.0 % Hydraulic length = 0 ft Tc method = TR55 Time of conc. (Tc) = 33.00 min Total precip. = 6.00 in Distribution = Type III Storm duration = 24 hrs Shape factor = 484 Q (cfs) P17088 - Post-Development Hyd. No. 1 -- 10 Year 60.00 60.00 Q (cfs) 50.00 50.00 40.00 40.00 30.00 30.00 20.00 20.00 10.00 10.00 0.00 0.00 0 2 4 6 8 10 12 14 16 18 20 22 24 26 Time (hrs) Hyd No. 1
TR55 Tc Worksheet Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Hyd. No. 1 P17088 - Post-Development Description A B C Totals Sheet Flow Manning's n-value = 0.010 0.011 0.011 Flow length (ft) = 300.0 0.0 0.0 Two-year 24-hr precip. (in) = 6.00 0.00 0.00 Land slope (%) = 0.00 0.00 0.00 Travel Time (min) = 21.69 + 0.00 + 0.00 = 21.69 Shallow Concentrated Flow Flow length (ft) = 103.00 0.00 0.00 Watercourse slope (%) = 0.10 0.00 0.00 Surface description = Unpaved Paved Paved Average velocity (ft/s) =0.51 0.00 0.00 Travel Time (min) = 3.36 + 0.00 + 0.00 = 3.36 Channel Flow X sectional flow area (sqft) = 6.00 0.00 0.00 Wetted perimeter (ft) = 12.60 0.00 0.00 Channel slope (%) = 0.80 0.00 0.00 Manning's n-value = 0.035 0.015 0.015 Velocity (ft/s) =2.32 0.00 0.00 Flow length (ft) ({0})1101.0 0.0 0.0 Travel Time (min) = 7.92 + 0.00 + 0.00 = 7.92 Total Travel Time, Tc... 33.00 min
Hydrograph Report Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Wednesday, 06 / 27 / 2018 Hyd. No. 3 Wet Pond Routing Hydrograph type = Reservoir Peak discharge = 12.90 cfs Storm frequency = 10 yrs Time to peak = 13.03 hrs Time interval = 2 min Hyd. volume = 290,266 cuft Inflow hyd. No. = 1 - P17088 - Post-DevelopmentMax. Elevation = 104.82 ft Reservoir name = Wet Pond Max. Storage = 380,631 cuft Storage Indication method used. Wet pond routing start elevation = 102.00 ft. Q (cfs) Wet Pond Routing Hyd. No. 3 -- 10 Year 60.00 60.00 Q (cfs) 50.00 50.00 40.00 40.00 30.00 30.00 20.00 20.00 10.00 10.00 0.00 0.00 0 6 12 18 24 30 36 42 48 54 60 66 72 Time (hrs) Hyd No. 3 Hyd No. 1 Total storage used = 380,631 cuft
Hydraflow Rainfall Report Hydraflow Hydrographs Extension for AutoCAD Civil 3D 2018 by Autodesk, Inc. v12 Wednesday, 06 / 27 / 2018 Return Intensity-Duration-Frequency Equation Coefficients (FHA) Period (Yrs) B D E (N/A) 1 0.0000 0.0000 0.0000 -------- 2 69.8703 13.1000 0.8658 -------- 3 0.0000 0.0000 0.0000 -------- 5 79.2597 14.6000 0.8369 -------- 10 88.2351 15.5000 0.8279 -------- 25 102.6072 16.5000 0.8217 -------- 50 114.8193 17.2000 0.8199 -------- 100 127.1596 17.8000 0.8186 -------- File name: SampleFHA.idf Intensity = B / (Tc + D)^E Return Intensity Values (in/hr) Period (Yrs) 5 min 10 15 20 25 30 35 40 45 50 55 60 1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 5.69 4.61 3.89 3.38 2.99 2.69 2.44 2.24 2.07 1.93 1.81 1.70 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5 6.57 5.43 4.65 4.08 3.65 3.30 3.02 2.79 2.59 2.42 2.27 2.15 10 7.24 6.04 5.21 4.59 4.12 3.74 3.43 3.17 2.95 2.77 2.60 2.46 25 8.25 6.95 6.03 5.34 4.80 4.38 4.02 3.73 3.48 3.26 3.07 2.91 50 9.04 7.65 6.66 5.92 5.34 4.87 4.49 4.16 3.88 3.65 3.44 3.25 100 9.83 8.36 7.30 6.50 5.87 5.36 4.94 4.59 4.29 4.03 3.80 3.60 Tc = time in minutes. Values may exceed 60. Precip. file name: Q:\2017\P17088\Documents\County Stormwater\P17088 - Moyock Rainfall Data.pcp Rainfall Precipitation Table (in) Storm Distribution 1-yr 2-yr 3-yr 5-yr 10-yr 25-yr 50-yr 100-yr SCS 24-hour 0.00 4.00 0.00 0.00 6.00 7.05 8.20 9.47 SCS 6-Hr 0.00 2.66 0.00 0.00 3.95 4.81 5.66 6.51 Huff-1st 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-2nd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-3rd 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-4th 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Huff-Indy 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Custom 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00