SW5230902_Stormwater Report_20240103

Stormwater Management Report
FOREST HILLS
Louisburg, North Carolina
August 8, 2023
Revised January 2, 2024
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Prepared By:
The Nau Company, PLLC
PO Box 810
Rolesville, North Carolina, 27571
[phone]
[e-mail]
NCBELS License # P-0751

Stormwater Management Report
January 2, 2024
Forest Hills
INTRODUCTION
This report presents the stormwater management design for the proposed improvements at the
Forest Hills Subdivision in Franklin County, North Carolina. The site is within the Town of
Louisburg's ETJ and will be annexed into the Town of Louisburg. The site is located on Hawthorn
Street at the intersection with NC Highway 561 northeast of the Town of Louisburg.
BACKGROUND
Forest Hills is a proposed 50 acre development that will consists of 64 single family lots. The project
includes construction of the water, sewer, storm drainage, road network and other infrastructure to
support the development of the single-family lots. Water and sewer services will be provided by
the Town of Louisburg. The proposed subdivision will have a stormwater control measure (SCM)
that will meet State requirements for stormwater quality. The roads within the subdivision will be
built to the Town of Louisburg standards and will ultimately be owned and maintained by the Town
of Louisburg. Stormwater runoff will be conveyed to the stormwater control measure by utilizing a
storm sewer piping network.
REQUIREMENTS
Stormwater Permits for the Town of Franklinton are issued by the North Carolina Division of
Environmental Quality (NCDEQ).
The relevant NCDEQ requirements for high density projects found in 15A NCAC 02H .1003 (3) are
summarized below:
• Treatment requirements -SCMs shall be designed, constructed, and maintained so that the
project achieves either"runoff treatment" or "runoff volume match" as those terms are
defined in Rule .1002 of this Section.
• On-site stormwater- Stormwater runoff from off-site areas and existing development shall
not be required to be treated in the SCM. Runoff from off-site areas or existing
development that is not bypassed shall be included in the sizing of on-site SCMs at its full
built-out potential.
• MDC for SCMs -SCMs shall meet the relevant MDC set forth in Rules .1050 through .1062 of
this Section except in accordance with Item (6) of this Rule.
The relevant MDC for wet ponds in 15A NCAC 02H .1053 are summarized below:
• Main Pool Surface Area and Volume -The main pool of the wet pond shall be sized using
either:
o the Hydraulic Retention Time (HRT) Method; or
o the SA/DA and Average Depth Method.
• Main Pool Depth -The average depth of the main pool shall be three to eight feet below the
permanent pool elevation. The applicant shall have the option of excluding the submerged
portion of the vegetated shelf from the calculation of average depth.
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Stormwater Management Report
January 2, 2024
Forest Hills
• Sediment Storage -The forebay and main pool shall have a minimum sediment storage
depth of six inches.
• Location of inlet(s) and outlet-The inlet(s) and outlet shall be located in a manner that
avoids short circuiting.
• Forebay-A forebay that meets the following specifications shall be included;
o Forebay volume shall be 15 to 20 percent of the volume in the main pool;
o The forebay entrance shall be deeper than the forebay exit;
o The water flowing over or through the structure that separates the forebay from the
main pool shall flow at a nonerosive velocity; and
o If sediment accumulates in the forebay in a manner that reduces its depth to less
than 75 percent of its design depth, then the forebay shall be cleaned out and
returned to its design state.
• Vegetated shelf-The main pool shall be equipped with a vegetative shelf around its
perimeter. The minimum width of the vegetated shelf shall be six feet and the slope shall be
no steeper than 6:1 (horizontal to vertical).
• Drawdown time -The design volume shall draw down to the permanent pool level between
two and five days.
• Protection of the receiving stream -The wet pond shall discharge the runoff from the one-
year, 24-hour storm in a manner that minimizes hydrologic impacts to the receiving
channel.
• Trash rack-A trash rack or other device shall be provided to prevent large debris from
entering the outlet system.
• Vegetation -The following criteria apply to vegetation in and around the wet pond:
o The dam structure, including front and back embankment slopes, of the pond shall
be vegetated with non-clumping turf grass; trees and woody shrubs shall not be
allowed; and
o The vegetated shelf shall be planted with a minimum of three diverse species of
herbaceous, native vegetation at a minimum density of 50 plants per 200 square
feet of shelf area.
The design storm for this project is the one inch 24-hour rainfall.
It should be noted that limiting post-development runoff to pre-development rates is not required
by NCDEQ. However, this report presents the results of pre-development and post-development
runoff calculations for each analysis point for reference.
This project receives offsite runoff that is not bypassed around the proposed SCM. Therefore,
NCDEQ requires the entire offsite area that flows into the SCM be assumed to have 100% BUA or
have all upstream offsite properties enter into a legal agreement to limit the impervious. This
design assumes all upstream offsite areas that drain to the SCM have 100% BUA.
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Stormwater Management Report
January 2, 2024
Forest Hills
PROPOSED STORMWATER CONTROL MEASURE
One wet pond is proposed to meet the State stormwater requirements. Refer to the construction
drawings for details of the SCM.
METHODOLOGY
Drainage Areas and SCS Curve Numbers
Drainage areas were delineated based on proposed and existing topography using CAD software.
Land use types were measured using CAD software and SCS curve numbers were applied to each land
use type within the drainage area to calculate a weighted, composite SCS Curve Number. Curve
numbers for the various land use types were taken from NRCS TR-55 and are included as an appendix
to this report.
Times of Concentration
The minimum time of concentration used for calculation of peak flows was 5 minutes. For times of
concentration more than 5 minutes, a simplified TR-55 method was used. The simplified method
calculates the time of concentration only using sheet flow time and channel flow time—shallow
concentrated flow time is disregarded. Since channel dimensions vary along the channel flow path,
channel parameters were selected for the entire length of channel flow that yield a flow velocity of
approximately 4.5 feet per second. A flow velocity of 4.5 feet per second is greater than or equal to
actual flow velocities expected in the pipes or open channels along the flow path and therefore will
yield a runoff higher that what would be expected at the analysis point.
Runoff and Pond Routing
Runoff flowrates and volumes for the proposed SCM were calculated using the Hydrology Studio
software program. The software utilizes the SCS Methodology to determine peak flow rates and
performs routing calculations to determine detention results by the Storage-Indication method.
Rainfall Depths
The following rainfall depths used in the calculations for this report were taken from the NOAA
Precipitation Frequency Data Server(PFDS). A printout of this rainfall data is included as an
appendix to this report.
Rainfall event Rainfall Depth(inches)
Design storm 1.00
1-year,24-hour 2.83
2-year,24-hour 3.42
10-year, 24-hour 4.94
25-year, 24-hour 5.84
100-year,24-hour 7.27
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Stormwater Management Report
January 2, 2024
Forest Hills
Pre-Development and Post-Development Drainage Areas
Drainage area maps showing the pre-development and post-development drainage areas are
included as an attachment to this report. Note that the drainage areas are based field topography
where available and GIS topography outside the limits of field topography.
ANALYSIS POINTS
The analysis point for the project was selected based on the location where surface flow leaves the site.
The analysis point for this project is located on the far eastern corner of the parcel where the stream
crosses under Highway NC561.
RESULTS
Runoff Volume
Two runoff volumes were calculated as part of this design. One runoff volume the Water Quality
Volume which is for the entire area that drains to the SCM and the other volume is the Runoff
Treatment Volume which is the volume from the net increase in BUA
The runoff volumes for this project were calculated by the simple method and are summarized below.
Runoff Volume Drainage area Impervious area Impervious% Runoff volume
Water Quality 39.25 ac 20.25 ac 51.6% 73,282 CF
Volume
Runoff Treatment 8.26 ac 8.26 ac 100% 28,470 CF
Volume
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Stormwater Management Report
January 2, 2024
Forest Hills
Peak Flow Attenuation
Although peak flow attenuation is not required for this project, pre-development and post-
development flows were calculated for several storm events for reference. The table below
summarizes the pre-development and post-development data for selected storm events. Detailed
routing information can be found in the appendix to this report.
ANALYSIS POINT DRAINAGE AND RUNOFF DATA
Development Condition Drainage area SCS CN Time of Q(1) Q(2) Q(10)
concentration
Pre-Development 53.68 acres 73 21.3 minutes 40.8 cfs 64.1 cfs _132.5 cfs
Post-Development to 39.25 acres 86 17.4 minutes 72.2 cfs 96.3 cfs 159.7 cfs
SCM
Post-Development SCM 14.43 acres 76 10.0 minutes 20.5 cfs 30.2 cfs 57.6 cfs
bypass
Post-Development SCM 25.7 cfs 37.5 cfs 139.0 cfs
outfall flowrate
Post-Development 29.0 cfs 45.2 cfs 161.9 cfs
Pond Surface Area
The required pond surface area was determined based on NCDEQ Stormwater Design Manual for
the Piedmont and Mountain area. The impervious percentage of the onsite drainage area was used
to determine the required surface area. The pond surface area data is summarized in the table
below.
SCM1
Drainage area 26.29 acres
Impervious area 8.11 acres
Impervious percentage 27.7%
Pond average depth 3.0 feet
Required surface area 12,531 SF
Surface area provided 18,833 SF
*Pond surface area calculations are included in the Appendix to this report.
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Stormwater Management Report
January 2, 2024
Forest Hills
SCM Dewatering Time
The required pond dewatering time was based on the NCDEQ Stormwater Design Manual and was
calculated using the falling head equation assuming vertical pond sides. The pond dewatering time
is summarized in the table below.
SCM1
Required dewatering time 2 to 5 days
Design storm 1 inch
Calculated dewatering time 3.1 days
Pond dewatering time calculations are included in the Appendix to this report.
Pond Average Depth and Forebay Volume
Pond average depth was calculated using equations 2 and 3 in the NCDEQ Stormwater Design
Manual for Wet Ponds. The interior of the ponds were graded so that the forebays are between
15%and 20%of the main pool volume. The results of the average depth and forebay volume
calculations are summarized below. Calculations are included in the appendix to the report.
Pond Average Depth
Pond ID Average Depth
SCM1 3.3 feet (by equation 2)
SCM1 4.1 feet (by equation 3)
Pond Forebay Volumes
Pond ID Forebay Volume
SCM1 19.5% (total of both forebays)
Erosion at Pond Inlets and Outlets
Flows into and out of the stormwater control measures should not produce erosion for the 10-year
storm. Riprap energy dissipaters are used to eliminate erosion at the pond inlets and outlets. Data
for the riprap dissipaters are summarized below. Calculations are included in the appendix to the
report.
Location Length Width Thickness Riprap size
Inlet pipe on north 20.0' 7.5' 22" Class I
side of pond
Inlet pipe on south 28.0' 10.5' 22" Class I
side of pond
Pond outlet 12.0' 6.0' 22" Class B
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Stormwater Management Report
January 2, 2024
Forest Hills
Pond Freeboard
The pond is required to have at least 1 foot of freeboard between the 100-year water surface
elevation in the pond and the top of the embankment. The table below summarizes the freeboard
for each pond.
Pond ID Top of berm elevation 100-year WSEL Freeboard
SCM1 220.0 218.9 1.1 feet
Riser Anti-Flotation Device
An anti-flotation device was designed to counteract the buoyant forces on the pond riser. The riser
anti-flotation block was sized to provide a factor of safety for buoyancy of at least 1.5. Calculations
for the anti-flotation block are included in the Appendix to the report.
Nutrient Loading
Nutrient loading calculations for this project were performed with the SNAP Tool v4.2 provided by
NCDEQ. The output from this tool is included in the appendix to this report.
The output calculated by the spreadsheet for nutrient loading is summarized in the table below:
Post-BMP TN Loading 2.27 Ib/ac/yr
Post-BMP TP Loading 0.46 lb/ac/yr
CONCLUSION
Based on the results of this analysis and design data contained in the construction drawings, we
believe that the NCDEQ requirements will be met for the proposed development. Additional details
can be found in the Appendix.
7

APPENDICES
Appendix A Drainage Area Maps
Appendix B Hydrology Studio Output
Appendix C Wet Pond Supporting Calculations
Appendix D Energy Dissipater/Riprap apron Calculations
Appendix E SNAP Tool Output
Appendix F Supporting Documentation

APPENDIX A
DRAINAGE AREA MAPS

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APPENDIX B
HYDROLOGY STUDIO OUTPUT

Basin Model Project Name:
Hydrology Studio v 3.0.0.29 01-02-2024
Pre AP1 Post DA1.1 Post DA1.2
17171
Post SCM
+I ► `'t ' PostAP1

Hydrograph by Return Period Project Name:
Hydrology Studio v 3.0.0.29 01-02-2024
Hyd. Hydrograph Hydrograph Peak Outflow(cfs)
No. Type Name 1-yr 2-yr 3-yr 5-yr 10-yr 25-yr 50-yr 100-yr
1 NRCS Runoff Pre AP1 40.79 64.06 132.5 249.7
2 NRCS Runoff Post DA1.1 72.21 96.26 159.7 257.5
3 Pond Route Post SCM 25.73 37.45 139.0 248.1
4 NRCS Runoff Post DA1.2 20.52 30.17 57.55 102.9
5 Junction PostAP1 29.03 45.23 161.9 315.9

Hydrograph 1 -yr Summary Project Name:
Hydrology Studio v 3.0.0.29 01-02-2024
Hyd. Hydrograph Hydrograph Peak Time to Hydrograph Inflow Maximum Maximum
No. Type Name Flow Peak Volume Hyd(s) Elevation Storage
(cfs) (hrs) (cuft) (ft) (cuft)
1 NRCS Runoff Pre AP1 40.79 12.12 148,486 ----
2 NRCS Runoff Post DA1.1 72.21 12.07 213,797 ----
3 Pond Route Post SCM 25.73 12.32 207,599 2 217.13 94,917
4 NRCS Runoff Post DA1.2 20.52 12.00 47,264 ----
5 Junction Post AP1 29.03 12.30 254,863 3,4

Hydrograph 2-yr Summary Project Name:
Hydrology Studio v 3.0.0.29 01-02-2024
Hyd. Hydrograph Hydrograph Peak Time to Hydrograph Inflow Maximum Maximum
No. Type Name Flow Peak Volume Hyd(s) Elevation Storage
(cfs) (hrs) (cuft) (ft) (cuft)
1 NRCS Runoff PreAP1 64.06 12.12 221,558 ----
2 NRCS Runoff Post DA1.1 96.26 12.07 285,618 ----
3 Pond Route Post SCM 37.45 12.30 279,058 2 217.88 120,612
4 NRCS Runoff Post DA1.2 30.17 12.00 68,492 ----
5 Junction Post AP1 45.23 12.10 347,551 3,4

Hydrograph 10-yr Summary Project Name:
Hydrology Studio v 3.0.0.29 01-02-2024
Hyd. Hydrograph Hydrograph Peak Time to Hydrograph Inflow Maximum Maximum
No. Type Name Flow Peak Volume Hyd(s) Elevation Storage
(cfs) (hrs) (cuft) (ft) (cuft)
1 NRCS Runoff Pre AP1 132.5 12.10 439,404 ----
2 NRCS Runoff Post DA1.1 159.7 12.07 480,475 ----
3 Pond Route Post SCM 139.0 12.13 473,642 2 218.52 145,886
4 NRCS Runoff Post DA1.2 57.55 11.98 130,228 ----
5 Junction Post AP1 161.9 12.12 603,869 3,4

Hydrograph 100-yr Summary Project Name:
Hydrology Studio v 3.0.0.29 01-02-2024
Hyd. Hydrograph Hydrograph Peak Time to Hydrograph Inflow Maximum Maximum
No. Type Name Flow Peak Volume Hyd(s) Elevation Storage
(cfs) (hrs) (cuft) (ft) (cuft)
1 NRCS Runoff PreAP1 249.7 12.10 820,179 ----
2 NRCS Runoff Post DA1.1 257.5 12.05 792,421 ----
3 Pond Route Post SCM 248.1 12.10 785,318 2 218.94 162,964
4 NRCS Runoff Post DA1.2 102.9 11.98 235,635 ----
5 Junction Post AP1 315.9 12.05 1,020,953 3,4

APPENDIX C
WET POND SUPPORTING CALCULATIONS

APPENDIX C
STORMWATER CONTROL MEASURE SUPPORTING CALCULATIONS
FOREST HILLS
January 2, 2024
SCM #1 - Wet Pond
SPREADSHEET v1.00

SCS CN CALCULATIONS

PRE-DEVELOPMENT Drainage Area ID1
FOREST HILLS Drainage are to analysis point on southeast side of
site
January 2, 2024
Hydrologic Soil Group C
ONSITE LAND COVERS ONSITE DRAINAGE AREA
IMPERVIOUS LAND COVERS Area
SCS CN Avg%Imp [sq ft] [ac] Weighted CN
Road (pavement only) 0 0.00 0
Parking 0 0.00 0
Roof 0 0.00 0
Driveway 0 0.00 0
Sidewalk 98 0 0.00 0
Deck/patio 0 0.00 0
Other impervious 0 0.00 0
Other impervious 0 0.00 0
PERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Lawn/grass/open space 74 0 0.00 0
Grass(within R/W) 74 0 0.00 0
Woods 70 1,730,525 39.73 68
Natural wetland 70 0 0.00 0
Riparian buffer 70 0 0.00 0
Other pervious 0 0 0.00 0
Other pervious 0 0 0.00 0
COMPOSITE LAND COVERS Area
SCS CN Avg%Imp [sq ft] [ac] Weighted CN
Commercial area 94 85 0 0.00 0
Industrial area 91 72 0 0.00 0
Residential area (1 acre lots) 79 20 42,850 0.98 2
Residential (1/4 acre lot) 83 38 0 0.00 0
Residential (1/3 acre lot) 81 30 0 0.00 0 _
OTHER ONSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
SCM 0 0.00 0
Open Water 0 0.00 0
ONSITE TOTALS 1,773,375 40.71 70
TOTAL ONSITE IMPERVIOUS 0 0.00
OFFSITE LAND COVERS OFFSITE DRAINAGE AREA
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Impervious 98 0 0.00 0
Pervious 74 0 0.00 0
Woods 70 0 0 0.00 0
Riparian buffer 70 0 0 0.00 0
60' Road R/W 84 40 0 0.00 0
Roof 98 100 0 0.00 0
Driveway/sidewalk 98 100 0 0.00 0
Residential (1/2 acre lot) 80 25 564,825 12.97 80
Residential (1/3 acre lot) 84 40 0 0.00 0
OFFSITE TOTALS 564,825 12.97 80
Area
TOTAL DRAINAGE ARA [sq ft] [ac] Composite CN
2,338,200 53.68 73

POST-DEVELOPMENT TOTAL POND DA Drainage Area ID1
FOREST HILLS DA to pond
January 2, 2024
Hydrologic Soil Group C
ONSITE LAND COVERS ONSITE DRAINAGE AREA
IMPERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Road (pavement only) 113,600 2.61 10
Parking 0 0.00 0
Roof 145,425 3.34 12
Driveway 36,750 0.84 3
Sidewalk 98 21,500 0.49 2
Deck/patio 0 0.00 0
All impervious 0 0.00 0
Other impervious 0 0.00 0
PERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Lawn/grass/open space 74 759,725 17.44 49
Grass(within R/W) 74 0 0.00 0
Woods 70 0 0.00 0
Natural wetland 70 0 0.00 0
Riparian buffer 70 0 0.00 0
Other pervious 0 0 0.00 0
Other pervious 0 0 0.00 0
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Commercial area 94 85 0 0.00 0
Industrial area 91 72 0 0.00 0
Residential area (1 acre lots) 79 20 0 0.00 0
Residential (1/4 acre lot) 83 38 42,850 0.98 3
Residential (1/3 acre lot) 81 30 0 0.00 0
OTHER ONSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
SCM 25,175 0.58 0
Open Water 0 0.00 0
ONSITE TOTALS 1,145,025 26.29 79
TOTAL ONSITE IMPERVIOUS 317,275 7.28
OFFSITE LAND COVERS OFFSITE DRAINAGE AREA
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Impervious 98 0 0.00 0
Pervious 74 0 0.00 0
Woods 70 0 0 0.00 0
Riparian buffer 70 0 0 0.00 0
60' Road R/W 84 40 0 0.00 0
Roof 98 100 0 0.00 0
Driveway/sidewalk 98 100 0 0.00 0
Residential (1/2 acre lot) 98 100 564,825 12.97 98
Residential (1/3 acre lot) 84 40 0 0.00 0
OFFSITE TOTALS 564,825 12.97 98
Area
TOTAL DRAINAGE ARA [sq ft] [ac] Composite CN
1,709,850 39.25 86

POST-DEVELOPMENT TOTAL POND DA Drainage Area ID2
FOREST HILLS Pond Bypass
January 2, 2024
Hydrologic Soil Group C
ONSITE LAND COVERS ONSITE DRAINAGE AREA
IMPERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Road (pavement only) 0 0.00 0
Parking 0 0.00 0
Roof 42,050 0.97 7
Driveway 300 0.01 0
Sidewalk 98 0 0.00 0
Deck/patio 0 0.00 0
All impervious 0 0.00 0
Other impervious 0 0.00 0
PERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Lawn/grass/open space 74 586,025 13.45 69
Grass(within R/W) 74 0 0.00 0
Woods 70 0 0.00 0
Natural wetland 70 0 0.00 0
Riparian buffer 70 0 0.00 0
Other pervious 0 0 0.00 0
Other pervious 0 0 0.00 0
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Commercial area 94 85 0 0.00 0
Industrial area 91 72 0 0.00 0
Residential area (1 acre lots) 79 20 0 0.00 0
Residential (1/4 acre lot) 83 38 0 0.00 0
Residential (1/3 acre lot) 81 30 0 0.00 0 _
OTHER ONSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
SCM 0 0.00 0
Open Water 0 0.00 0
ONSITE TOTALS 628,375 14.43 76
TOTAL ONSITE IMPERVIOUS 42,350 0.97
OFFSITE LAND COVERS OFFSITE DRAINAGE AREA
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Impervious 98 0 0.00 0
Pervious 74 0 0.00 0
Woods 70 0 0 0.00 0
Riparian buffer 70 0 0 0.00 0
60' Road R/W 84 40 0 0.00 0
Roof 98 100 0 0.00 0
Driveway/sidewalk 98 100 0 0.00 0
Residential (1/2 acre lot) 98 100 0 0.00 0
Residential (1/3 acre lot) 84 40 0 0.00 0
OFFSITE TOTALS 0 0.00 0
Area
TOTAL DRAINAGE ARA [sq ft] [ac] Composite CN
628,375 14.43 76

POST-DEVELOPMENT TOTAL POND DA Totals
FOREST HILLS Total drainage areas
January 2, 2024
Hydrologic Soil Group C
ONSITE LAND COVERS ONSITE DRAINAGE AREA
IMPERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Road (pavement only) 113,600 2.61 6
Parking 0 0.00 0
Roof 187,475 4.30 10
Driveway 37,050 0.85 2
Sidewalk 98 21,500 0.49 1
Deck/patio 0 0.00 0
All impervious 0 0.00 0
Other impervious 0 0.00 0
PERVIOUS LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Lawn/grass/open space 74 1,345,750 30.89 56
Grass(within R/W) 74 0 0.00 0
Woods 70 0 0.00 0
Natural wetland 70 0 0.00 0
Riparian buffer 70 0 0.00 0
Other pervious 0 0 0.00 0
Other pervious 0 0 0.00 0
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Commercial area 94 85 0 0.00 0
Industrial area 91 72 0 0.00 0
Residential area (1 acre lots) 79 20 0 0.00 0
Residential (1/4 acre lot) 83 38 42,850 0.98 2
Residential (1/3 acre lot) 81 30 0 0.00 0
OTHER ONSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
SCM 25,175 0.58 0
Open Water 0 0.00 0
ONSITE TOTALS 1,773,400 40.71 78
TOTAL ONSITE IMPERVIOUS 359,625 8.26
OFFSITE LAND COVERS OFFSITE DRAINAGE AREA
COMPOSITE LAND COVERS Area
SCS CN Avg% Imp [sq ft] [ac] Weighted CN
Impervious 98 0 0.00 0
Pervious 74 0 0.00 0
Woods 70 0 0 0.00 0
Riparian buffer 70 0 0 0.00 0
60' Road R/W 84 40 0 0.00 0
Roof 98 100 0 0.00 0
Driveway/sidewalk 98 100 0 0.00 0
Residential (1/2 acre lot) 98 100 564,825 12.97 98
Residential (1/3 acre lot) 84 40 0 0.00 0
OFFSITE TOTALS 564,825 12.97 98
Area
TOTAL DRAINAGE ARA [sq ft] [ac] Composite CN
2,338,225 53.68 83

RUNOFF VOLUME BY SIMPLE METHOD

WATER QUALITY RUNOFF VOLUME January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
Runoff Volume by the Simple Method taken from NCDEQ Storm water Design Manual Part B
Input Data
Onsite Impervious area 7.28 acres
Offsite Impervious area 12.97 acres
Total Impervious area 20.25 acres
Drainage area A 39.25 acres
Impervious fraction la 0.516
Design storm rainfall depth Rd 1.00 inches
Output
Runoff coefficient Rv 0.51
Runoff volume V 73,282 cubic feet
R„ = 0.05 + 0.9xIA
V = 3630 x RD x RV xA

RUNOFF TREATMENT VOLUME

RUNOFF TREATMENT VOLUME January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
Runoff Volume by the Simple Method taken from NCDEQ Storm water Design Manual Part B
Input Data
Existing BUA 0.00 acres
Proposed BUA 8.26 acres
Net BUA Increase 8.26 acres
Impervious fraction la 1.000
Design storm rainfall depth Rd 1.00 inches
Output
Runoff coefficient Rv 0.95
Runoff volume V 28,470 cubic feet
R„ = 0.05 + 0.9XIA
V = 3630 x RD X RV XA

WET POND SURFACE AREA REQUIREMENT

REQUIRED WET POND SURFACE AREA January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
Drainage area 26.29 acres
Percent impervious 27.7%
Permanent pool depth 3.0 ft
Required pond surface area 0.29 acres
Required pond surface area 12,531 sq.ft.
Permanent Pool Average Depth (ft)
Impervious 3.0 4.0 5.0 6.0 7.0 8.0
10 0.51 0.43 0.37 0.30 0.27 0.25
20 0.84 0.69 0.61 0.51 0.44 0.40
30 1.17 0.94 0.84 0.72 0.61 0.56
40 1.51 1.24 1.09 0.91 0.78 0.71
50 1.79 1.51 1.31 1.13 0.95 0.87
60 2.09 1.77 1.49 1.31 1.12 1.03
70 2.51 2.09 1.80 1.56 1.34 1.17 _'
80 2.92 2.41 2.07 1.82 1.62 1.40
90 3.25 2.64 2.31 2.04 1.84 1.59 1
100 3.55 2.79 2.52 2.34 2.04 1.75
Interpolation
Average Depth
Impervious 3.0 3 3.0
20 0.84 0.84 0.84
27.709002 1.09 1.09 1.17
30 1.17 1.17 1.17
Notes:
SA/DA values taken from NCDEQ Stormwater Design Manual Section C-3, page 7 (revised 4/18/2017)
Per wet pond guidance MDC 1,the required surface area does not include the area of the forebay(s)

WATER SURFACE ELEVATION FOR DESIGN RUNOFF VOLUMES

DESIGN VOLUME WATER SURFACE ELEVATIONS January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
Elevation at WQ Volume 216.50
Calculated pond surface area at above elevation 32,692 SF
Calculated pond volume at above elevation 73,427 CF
Calculated WQ Volume from simple method 73,282 CF
Elevation at Runoff Treatment Volume 215.06
Calculated pond surface area at above elevation 28,332 SF
Calculated pond volume at above elevation 29,842 CF
Calculated Runoff Treatment Volume 28,470 CF
POND ELEVATION AND CONTOUR DATA
214.0 25,165 SF 0.0 0 0
216 31,141 SF 2.0 56,306 CF 56,306 CF
218 37,344 SF 4.0 68,485 CF 124,791 CF
220 43,773 SF 6.0 81,117 CF 205,908 CF

DEWATERING TIME CALCULATION

POND DEWATERING TIME January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
Dewatering time calculated by the falling head equation
Input Data
Pond surface area 25,165 sq.ft.
Orifice diameter 6.00 in.
Orifice coefficient 0.60
Orifice elevation 214.00
Initial elevation 216.50
Results
Orifice area 0.1963 sq.ft.
Dewatering time 4,535.7 min.
Dewatering time 75.6 hours
Dewatering time 3.1 days
Dewatering time = Areapond — (.✓Initial pond elevation x s/final pond elevation) x 12
Areaorifice x Cd g

AVERAGE DEPTH AND FOREBAY VOLUME

FOREBAY VOLUME & AVERAGE DEPTH CALCULATIONS January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
DATA FOR MAIN POOL FROM BOTTOM OF POND TO BOTTOM OF SHELF
Elev Area [SF] Incr vol [CF]
208 3,837 0
210 7,895 11,732
Enter data up to and including 212 12,240 20,135
the bottom of the shelf 213 14,521 13,381
MAIN POOL TOTAL VOLUME(UP TO BOTTOM OF SHELF) 45,248
DATA FOR MAIN POOL FROM BOTTOM OF SHELF TO TOP OF SHELF(NOT INCLDUDING FOREBAYS)
Bottom of shelf 213 1 14,521 0
Permanent pool elev 214 18,833 16,677
Top of shelf 214 18,833 0
TOTAL VOLUME OF POND SHELF(BETWEEN TOP AND BOTTOM OF SHELF) 16,677
DATA FOR FOREBAY#1 FROM BOTTOM OF FOREBAY TO TOP OF SHELF
Elev Area [SF] Incr vol [CF]
211 200 0
212 1,031 616
213 1,428 1,230
214 2,092 1,760
Total Volume of Forebay#1 3,605
DATA FOR FOREBAY#2 FROM BOTTOM OF FOREBAY TO TOP OF SHELF
Elev Area [SF] Incr vol [CF]
210 100 0
212 2,251 2,351
213 2,904 2,578
214 4,239 3,572
Total Volume of Forebay#2 8,500
DATA FOR FOREBAY#3 FROM BOTTOM OF FOREBAY TO TOP OF SHELF
Elev Area [SF] Incr vol [CF]
Total Volume of Forebay#3 0
Total Volume of Forebays 12,105 CF Average Depth by Equation 3
Main Pool Volume 61,925 CF Perimeter of perm pool 715 FT
Forebay%of main pool 19.5% Width of shelf 6.00 FT
Max depth over shelf 1.00 FT
Average Depth by Equation 2 Vol over shelf 2,145 CF
Vol at Permanent Pool 61,925 CF SA at bottom of shelf 14,521 CF
SA at Permanent Pool 18,833 SF Vol at Permanent Pool 61,925 CF
Average Depth 3.29 FT Average depth 4.12 FT

ANTI-FLOTATION BLOCK CALCULATIONS

ANTI FLOTATION BLOCK January 2, 2024
FOREST HILLS SCM #1 -Wet Pond
Constants
Unit weight of water 62.4 lb/cf
Unit weight of concrete 144.0 lb/cf
Riser Data
Length Width Area
Inside domensions 4.00 ft 4.00 ft 16.0 SF
Outside dimensions 5.00 ft 5.00 ft 25.0 SF
Cross sectional riser area 9.0 SF
Riser inside height 4.0 ft
Volume of concrete in riser 36 CF
Riser base area 25.0 SF (same as riser outside dimensions)
Riser base thickness 0.5 ft
Volume of concrete in base 13 CF
Total volume of concrete in riser+base 49 CF
Total bouyant weight of riser 3,958 lbs
Water displaced by riser 100 CF
Weight of water displaced 6,240 lbs
Anti-Flotation Block Data
Length of anti-flotation block 6.0 ft
Width of anti-flotation block 6.0 ft
Thickness of anti-flotation block 1.5 ft
Volume of anti-flotation block 54 CF
Total weight of anti-flotation block 7,776 lbs
Factor of Safety
Weight of riser+anti-flotation block 11,734 lbs
Weight of water displaced by riser 6,240 lbs
Factor of safety against flotation 1.88

APPENDIX D
ENERGY DISSIPATER/RIPRAP APRON CALCS

Forest Hills Subdivision DESIGN OF RIPRAP OUTLET PROTECTION
FES 100
August 8, 2023
OUTLET FLOWRATE 58.4 cfs
PIPE DIAMETER 42 inches
OUTLET PIPE SLOPE 1.50 %
NUMBER OF PIPES 1
PIPE SEPARATION 0 feet
ZONE FROM GRAPH 3
PIPE AREA 9.62 sq. ft.
FLOW VELOCITY 11.1 ft/sec
MATERIAL NCDOT Class I riprap
LENGTH 28.00 feet
WIDTH 10.50 feet
STONE DIAMETER 13 inches
THICKNESS 22 inches
Zone Material Diameter Thickness Length Width
1 Class A 3 9 4 x D(o) 3 x D(o)
2 Class B 6 22 6 x D(o) 3 x D(o)
3 Class I 13 22 8 x D(o) 3 x D(o)
4 Class I 13 22 8 x D(o) 3 x D(o)
5 Class II 23 27 10 x D(o) 3 x D(o)
6 Class II 23 27 10 x D(o) 3 x D(o)
7 Special 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
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DIAMETER (Ft.)

Forest Hills Subdivision DESIGN OF RIPRAP OUTLET PROTECTION
FES 200
August 8, 2023
OUTLET FLOWRATE 24.3 cfs
PIPE DIAMETER 30 inches
OUTLET PIPE SLOPE 2.80 %
NUMBER OF PIPES 1
PIPE SEPARATION 0 feet
ZONE FROM GRAPH 3
PIPE AREA 4.91 sq. ft.
FLOW VELOCITY 11.4 ft/sec
MATERIAL NCDOT Class I riprap
LENGTH 20.00 feet
WIDTH 7.50 feet
STONE DIAMETER 13 inches
THICKNESS 22 inches
Zone Material Diameter Thickness Length Width
1 Class A 3 9 4 x D(o) 3 x D(o)
2 Class B 6 22 6 x D(o) 3 x D(o)
3 Class I 13 22 8 x D(o) 3 x D(o)
4 Class I 13 22 8 x D(o) 3 x D(o)
5 Class II 23 27 10 x D(o) 3 x D(o)
6 Class II 23 27 10 x D(o) 3 x D(o)
7 Special 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
6 -
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DIAMETER (Ft.)

Forest Hills Subdivision DESIGN OF RIPRAP OUTLET PROTECTION
SCM1
January 2, 2024
OUTLET FLOWRATE 140.0 cfs
PIPE DIAMETER 24 inches
OUTLET PIPE SLOPE 1.00 %
NUMBER OF PIPES 1
PIPE SEPARATION 0 feet
ZONE FROM GRAPH 2
PIPE AREA 3.14 sq. ft.
FLOW VELOCITY 11.4 ft/sec
MATERIAL NCDOT Class B riprap
LENGTH 12.00 feet
WIDTH 6.00 feet
STONE DIAMETER 6 inches
THICKNESS 22 inches
Zone Material Diameter Thickness Length Width
1 Class A 3 9 4 x D(o) 3 x D(o)
2 Class B 6 22 6 x D(o) 3 x D(o)
3 Class I 13 22 8 x D(o) 3 x D(o)
4 Class I 13 22 8 x D(o) 3 x D(o)
5 Class II 23 27 10 x D(o) 3 x D(o)
6 Class II 23 27 10 x D(o) 3 x D(o)
7 Special 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
6 —
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1I
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DIAMETER (Ft.)

APPENDIX E
SNAP TOOL OUTPUT

Project Information Complete this sheet if required by your reviewing authority.
Contact them for any questions. Grey boxes/text are optional.
SNAPv4.2.0 LOCATION
Project Name(optional): Forest Hills Parcel ID(optional):
Submission Date(optional): date Nutrient Management Watershed: Tar Pamlico menu
Local Jurisdiction/Reviewing Agency: State/Federal menu Subwatershed: Upper_Tar_River menu
Project Latitude Coordinates(optional): N Phosphorus Delivery Zone: Tar- Upper 03020101 menu
Project Longitude Coordinates(optional): W Nitrogen Delivery Zone: Tar- Upper 03020101 menu
—
PROJECT DETAILS
Development Land Use Type: Single Family Residential menu Disturbed Area: 1,325,000 ft2
Part of Common Development Plan? no yin Project Activity: New Development menu
Designated Downtown Area? no y/n Project Drains to SA Waters? no yin
Public Linear Road/Sidewalk Project? .no yin Pre-Project Land Use: fallow/open menu
Project Owner Type: Private menu 1 Project Description(optional):
STORMWATER DETAILS
(Falls ONLY)Onsite Reduction%Req. % Project Uses LID/Runoff Volume Match? y/n
Existing BUA/Development Onsite? yin Local Gov't nutrient req's same as State? yin
Local Gov't cutoff date for Existing BUA: date Project Drains to Regional SCM? yin
Nitrogen Export Rate Target: 2.20 Ib/ac/yr Total Nitrogen Offset Credits Needed: Ib/yr
Phosphorus Export Rate Target: 0.80 Ib/ac/yr Total Phosphorus Offset Credits Needed: Ib/yr

SNAP v4.2.0
Project Area and Offsite Land Cover Characteristics Precipitation Louisburg
Station:
Copy i Paste VALUES ONLY for Best Results Click here to scroll down to error messages on this sheet.
PROJECT AREA LAND COVERS TN EMC (mg/L) TP EMC (mg/L) Pre-Project Area (ft2) Post-Project Area (ft2) Change pre-to-post(ft2)
1
Roof 1.18 0.11 187,475 187,475
Roadway 1.64 0.34 113,600 113,600
Parking/Driveway/Sidewalk 1.42 0.18 58,550 58,550
Protected Forest 0.97 0.03 0
Managed Pervious/Landscaping 2.48 1.07 1,773,375 1,388,575 -384,800
Offsite or Existing Roof 1.18 0.11 0
Offsite or Existing Roadway 1.64 0.34 0
Offsite or Existing Parking/Driveway/Sideww 1.42 0.18 0
Offsite Protected Forest 0.97 0.03 0
•
Offsite Managed Pervious 2.48 1.07 564,825 564,825 0
CUSTOM LAND COVER 1 0
CUSTOM LAND COVER 2 0
CUSTOM LAND COVER 3 0
LAND TAKEN UP BY SCM 1.18 0.11 25,175 25,175
Total (Regulated & UnReg)Area 2,338,200.00 2,338,200.00
Project (Regulated)Area 1,773,375.00 1,773,375.00

SCM Characteristics
Stormwater Control Measure (SCM) Characteristics Click here too to
Copy a Paste VALUES ONLY SCM101's Land Cover Data
SNAP v4.2.0
for Best Results
Catchment ID 1 1 1
Drains to Drains to
SCM ID 101 102 103
7 Type of SCM Wet Pond
rHydrologic soil group at SCM location C
' SCM Description Wet pond
ftDesign Storm Size(inches/24hrs) 1.00
■ Percent of Full Size 100%
Annual Effluent 72% 0% 0%
Annual Overflow 16% 0% 0%
I. %Annual ET/Infiltrated 13% 0% 0%
ElCustom%Annual Effluent I
r� Custom%Annual Overflow
rCustom%Annual ET/Infiltrated mil
l SCM Effluent TP EMC(mg/L) 0.13 0.00 0.00
III SCM Effluent TN EMC(mg/L) 0.86 0.00 0.00
■ Custom Effluent TP EMC
1.1 Custom Effluent TN EMC
r SCM Land Cover TP EMC(mg/L) 0.11 0.00 0.00
L SCM Land Cover TN EMC(mg/L) 1.18 0.00 0.00
rThis SCM Drains to Numbered SCM 0 0 0
Catchment Routing Catchments Draining to Catchments Draining to Catchments Draining to
SCM 101 SCM 102 SCM 103
Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
L. Error Check-Min/Max Size:
lim Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
Error Checks-SCM Type:
- SCM ID: 101 102 103
TArea Draining Directly to Area Draining Directly to Area Draining Directly to
SCM Drainage Area Land Covers
SCM 101 (ft2) SCM 102 (ft2) SCM 103 (ft2)
Roof 145,425
Roadway 113,600
Parking/Driveway/Sidewalk 58,250
Protected Forest
Managed Pervious/Landscaping 759,725
Offsite or Existing Roof
Offsite or Existing Roadway
Offsite or Existing Parking/Driveway/Sidewalk
Offsite Protected Forest
Offsite Managed Pervious 564,825
CUSTOM LAND COVER 1
CUSTOM LAND COVER 2
CUSTOM LAND COVER 3
LAND TAKEN UP BY SCM 25,175
TOTAL AREA DRAINING TO SCM (ft2): 1,667,000 0 0
CATCHMENT AREA(ft2): 1,667,000

SCM Characteristics
Stormwater Control Meast. Click here too to Click here too back to the
Click here to review Errors
Summary Data top
SNAP v4.2.0
I . Catchment ID 2 2 2
Drains to Drains to
SCM ID 201 > 202 > 203
r
Type of SCM
1 Hydrologic soil group at SCM location
1 SCM Description
1 Design Storm Size(inches/24hrs)
L_ Percent of Full Size [ I I I
Mom %Annual Effluent 0% 0% 0%
M %Annual Overflow 0% 0% 0%
I. %Annual ET/Infiltrated - 0% 0% 0%
. Custom %Annual Effluent
F-- Custom %Annual Overflow
ICustom %Annual ET/Infiltrated
SCM Effluent TP EMC(mg/L) 0.00 0.00 0.00
SCM Effluent TN EMC (mg/L) _J 0.00 0.00 0.00
. Custom Effluent TP EMC
. Custom Effluent TN EMC
rSCM Land Cover TP EMC(mg/L) 1 0.00 0.00 0.00
1 SCM Land Cover TN EMC(mg/L) I 0.00 0.00 0.00
1 This SCM Drains to Numbered SCM 0 0 0
L Catchment Routing Catchments Draining to Catchments Draining to Catchments Draining to
SCM 201 SCM 202 SCM 203
Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
Error Check-Min/Max Size:
Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
Error Checks-SCM Type:
SCM ID: 201 202 203
Area Draining Directly to Area Draining Directly to Area Draining Directly to
SCM Drainage Area Land Covers SCM 201 (ft2) SCM 202(ft2) SCM 203(ft2)
Roof
Roadway
Parking/Driveway/Sidewalk
Protected Forest
Managed Pervious/Landscaping
Offsite or Existing Roof
Offsite or Existing Roadway
Offsite or Existing Parking/Driveway/Sidew
Offsite Protected Forest
Offsite Managed Pervious
CUSTOM LAND COVER 1
CUSTOM LAND COVER 2
CUSTOM LAND COVER 3
LAND TAKEN UP BY SCM
TOTAL AREA DRAINING TO SCM (ft2): 0 0 0
CATCHMENT AREA(ft2): 0

SCM Characteristics
Stormwater Control Measi
SNAP v4.2.0
I . Catchment ID 3 3 3
Drains to Drains to
SCM ID 301 > 302 > 303
r
Type of SCM
hydrologic soil group at SCM location
' SCM Description
' Design Storm Size(inches/24hrs)
Percent of Full Size
%Annual Effluent 0% 0% 0%
M %Annual Overflow 0% 0% 0%
I. %Annual ET/Infiltrated ill 0% 0% 0%
Custom%Annual Effluent
ITCustom%Annual Overflow
Custom%Annual ET/Infiltrated
111.1 SCM Effluent TP EMC(mg/L) 0.00 0.00 0.00
SCM Effluent TN EMC(mg/L) J 0.00 0.00 0.00
Custom Effluent TP EMC
Custom Effluent TN EMC
r SCM Land Cover TP EMC(mg/L) 1 0.00 0.00 0.00
SCM Land Cover TN EMC(mg/L) I 0.00 0.00 0.00
This SCM Drains to Numbered SCM 0 0 0
L Catchment Routing Catchments Draining to Catchments Draining to Catchments Draining to
SCM 301 SCM 302 SCM 303
Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
Error Check-Min/Max Size:
Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
Error Checks-SCM Type:
SCM ID: 301 302 303
Area Draining Directly to Area Draining Directly to Area Draining Directly to
SCM Drainage Area Land Covers SCM 301 (ft2) SCM 302(ft2) SCM 303(ft2)
Roof
Roadway
Parking/Driveway/Sidewalk
Protected Forest
Managed Pervious/Landscaping
Offsite or Existing Roof
Offsite or Existing Roadway
Offsite or Existing Parking/Driveway/Sidew
Offsite Protected Forest
Offsite Managed Pervious
CUSTOM LAND COVER 1
CUSTOM LAND COVER 2
CUSTOM LAND COVER 3
LAND TAKEN UP BY SCM
TOTAL AREA DRAINING TO SCM (ft2): 0 0 0
CATCHMENT AREA(ft2): 0

SCM Characteristics
Stormwater Control Measi
SNAP v4.2.0
I . Catchment ID 4 4 '\ 4
Drains to Drains to
SCM ID 401 > 402 1/ 403
Type of SCM
hydrologic soil group at SCM location
' SCM Description
' Design Storm Size(inches/24hrs)
Percent of Full Size
%Annual Effluent 0% 0% 0%
M %Annual Overflow 0% 0% 0%
I. %Annual ET/Infiltrated ill 0% 0% 0%
Custom%Annual Effluent
ITCustom%Annual Overflow
Custom%Annual ET/Infiltrated
111.1 SCM Effluent TP EMC(mg/L) 0.00 0.00 0.00
SCM Effluent TN EMC(mg/L) J 0.00 0.00 0.00
Custom Effluent TP EMC
Custom Effluent TN EMC
r SCM Land Cover TP EMC(mg/L) 1 0.00 0.00 0.00
SCM Land Cover TN EMC(mg/L) I 0.00 0.00 0.00
This SCM Drains to Numbered SCM 0 0 0
L Catchment Routing Catchments Draining to Catchments Draining to Catchments Draining to
SCM 401 SCM 402 SCM 403
Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
Error Check-Min/Max Size:
Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
Error Checks-SCM Type:
SCM ID: 401 402 403
Area Draining Directly to Area Draining Directly to Area Draining Directly to
SCM Drainage Area Land Covers SCM 401 (ft2) SCM 402(ft2) SCM 403(ft2)
Roof
Roadway
Parking/Driveway/Sidewalk
Protected Forest
Managed Pervious/Landscaping
Offsite or Existing Roof
Offsite or Existing Roadway
Offsite or Existing Parking/Driveway/Sidew
Offsite Protected Forest
Offsite Managed Pervious
CUSTOM LAND COVER 1
CUSTOM LAND COVER 2
CUSTOM LAND COVER 3
LAND TAKEN UP BY SCM
TOTAL AREA DRAINING TO SCM (ft2): 0 0 0
CATCHMENT AREA(ft2): 0

SCM Characteristics
Stormwater Control Measi
SNAP v4.2.0
I . Catchment ID 5 5 5
Drains to Drains to
SCM ID 501 > 502 > 503
Type of SCM
hydrologic soil group at SCM location
' SCM Description
' Design Storm Size(inches/24hrs)
Percent of Full Size
%Annual Effluent 0% 0% 0%
M %Annual Overflow 0% 0% 0%
I. %Annual ET/Infiltrated ill 0% 0% 0%
Custom%Annual Effluent
ITCustom%Annual Overflow
Custom%Annual ET/Infiltrated
111.1 SCM Effluent TP EMC(mg/L) 0.00 0.00 0.00
SCM Effluent TN EMC(mg/L) J 0.00 0.00 0.00
Custom Effluent TP EMC
Custom Effluent TN EMC
r SCM Land Cover TP EMC(mg/L) 1 0.00 0.00 0.00
SCM Land Cover TN EMC(mg/L) I 0.00 0.00 0.00
This SCM Drains to Numbered SCM 0 0 0
L Catchment Routing Catchments Draining to Catchments Draining to Catchments Draining to
SCM 501 SCM 502 SCM 503
Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
Error Check-Min/Max Size:
Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
Error Checks-SCM Type:
SCM ID: 501 502 503
Area Draining Directly to Area Draining Directly to Area Draining Directly to
SCM Drainage Area Land Covers SCM 501 (ft2) SCM 502(ft2) SCM 503(ft2)
Roof
Roadway
Parking/Driveway/Sidewalk
Protected Forest
Managed Pervious/Landscaping
Offsite or Existing Roof
Offsite or Existing Roadway
Offsite or Existing Parking/Driveway/Sidew
Offsite Protected Forest
Offsite Managed Pervious
CUSTOM LAND COVER 1
CUSTOM LAND COVER 2
CUSTOM LAND COVER 3
LAND TAKEN UP BY SCM
TOTAL AREA DRAINING TO SCM (ft2): 0 0 0
CATCHMENT AREA(ft2): 0

SCM Characteristics
Stormwater Control Measi
SNAP v4.2.0
I . Catchment ID 6 6 6
Drains to Drains to
SCM ID 601 602 603
r
Type of SCM
hydrologic soil group at SCM location
1 SCM Description
1 Design Storm Size(inches/24hrs)
Percent of Full Size I
%Annual Effluent 0% 0% 0%
M %Annual Overflow 0% 0% 0%
I. %Annual ET/Infiltrated 0% 0% 0%
Custom%Annual Effluent
ITCustom%Annual Overflow
Custom%Annual ET/Infiltrated
111.1 SCM Effluent TP EMC(mg/L) 0.00 0.00 0.00
SCM Effluent TN EMC(mg/L) J 0.00 0.00 0.00
Custom Effluent TP EMC
Custom Effluent TN EMC
r SCM Land Cover TP EMC(mg/L) 1 0.00 0.00 0.00
SCM Land Cover TN EMC(mg/L) I 0.00 0.00 0.00
This SCM Drains to Numbered SCM 0 0 0
L Catchment Routing Catchments Draining to Catchments Draining to Catchments Draining to
SCM 601 SCM 602 SCM 603
Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
Error Check-Min/Max Size:
Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
Error Checks-SCM Type:
SCM ID: 601 602 603
Area Draining Directly to Area Draining Directly to Area Draining Directly to
SCM Drainage Area Land Covers SCM 601 (ft2) SCM 602(ft2) SCM 603(ft2)
Roof
Roadway
Parking/Driveway/Sidewalk
Protected Forest
Managed Pervious/Landscaping
Offsite or Existing Roof
Offsite or Existing Roadway
Offsite or Existing Parking/Driveway/Sidew
Offsite Protected Forest
Offsite Managed Pervious
CUSTOM LAND COVER 1
CUSTOM LAND COVER 2
CUSTOM LAND COVER 3
LAND TAKEN UP BY SCM
TOTAL AREA DRAINING TO SCM (ft2): 0 0 0
CATCHMENT AREA(ft2): 0

SCM Characteristics
Stormwater Control Measi
SNAP v4.2.0
I . Catchment ID
SCM ID
Type of SCM
hydrologic soil group at SCM location
' SCM Description
' Design Storm Size(inches/24hrs)
Percent of Full Size
%Annual Effluent
M %Annual Overflow
I. %Annual ET/Infiltrated
Custom%Annual Effluent
ITCustom%Annual Overflow
Custom%Annual ET/Infiltrated
111.1 SCM Effluent TP EMC(mg/L)
SCM Effluent TN EMC(mg/L)
Custom Effluent TP EMC
Custom Effluent TN EMC
rSCM Land Cover TP EMC(mg/L)
SCM Land Cover TN EMC(mg/L)
This SCM Drains to Numbered SCM
LCatchment Routing
I Catchment 1
Catchment 2
Catchment 3
Catchment 4
Catchment 5
Catchment 6
Error Check-Missing SCM Area:

SCM Characteristics
Error Check-Min/Max Size:
Error Check-Hydrology:
Error Check-Missing SCM Info:
Error Check-Drainage Data w/o SCM:
IIIError Checks-SCM Type:
ISCM ID:
Allowable Total Land Use
Total Land Use Area Post-Project Untreated
SCM Drainage Area Land Covers z Area to be Treated Based z
Treated By All SCMs (ft ) z Land Area (ft2)
on Post-Project Areas (ft2)
..roof 145,425 187,475 42,050
Roadway 113,600 113,600 0
Parking/Driveway/Sidewalk 58,250 58,550 300
Protected Forest 0 0 0
Managed Pervious/Landscaping 759,725 1,388,575 628,850
Offsite or Existing Roof 0 0 0
Offsite or Existing Roadway 0 0 0
Offsite or Existing Parking/Driveway/Sidew 0 0 0
Offsite Protected Forest 0 0 0
Offsite Managed Pervious 564,825 564,825 0
CUSTOM LAND COVER 1 0 0 0
CUSTOM LAND COVER 2 0 0 0
CUSTOM LAND COVER 3 0 0 0
LAND TAKEN UP BY SCM 25,175 25,175 0
TOTAL AREA DRAINING TO SCM (ft2): 1,667,000 2,338,200 671,200
CATCHMENT AREA(ft2):

Nutrient Export Summary
Landcover Et SCM Data Review
Errors/Advisories
Avg Annual precip (in) = 45.63
Total (Regulated+
2,338,200
Unregulated)Area (ft2)=
Project (Regulated)
1,773,375
Area (ft2) =
Net BUA(Project Area
359,625 Net BUA indicates new development or expansion.
BUA only ft2)=
Custom Landcovers are
present:
no
Total Nitrogen Export Target
89.56
Scaled to Project Area (Ib/yr):
Total Area Total Area Project Area
Project Area
(Onsite+ Post-Project Post-Project
Nutrient Export Summary (Onsite Only)
Offsite) before before
Pre-Project
Pre-Project Treatment Treatment
_ I
Area (All Landcover Types)(acres) 53.6777 40.7111 53.6777 40.7111
Percent Built-Upon Area(BUA) (%) 0% 0% 15% _I 15%
Built-Upon Area (BUA)(sqft) 0 0 359,625 _ 272,753
Annual Runoff Volume(ft3/yr) 400,095 303,447 1,589,597 _ 1,492,948
Annual Runoff%Change I 297% _ 392%
Total Runoff Change(cuft/yr) 1,189,501 1,189,501
Total Nitrogen EMC(mg/L) 2.48 2.48 1.59 1.53 _
Total Nitrogen Load Leaving Site(Ib/yr) 61.96 46.99 157.72 I 142.75
otal Nitrogen Loading Rate(Ib/ac/yr) 1.15 1.15 2.94 3.51
Total Nitrogen%Change Pre-to-Post 155% 204%
Total Nitrogen Change(Ib/yr)Pre-to-Post 95.76 95.76
Total Phosphorus EMC(mg/L) 1.07 1.07 0.37 t 0.33
Total Phosphorus Load Leaving Site(Ib/yr) 26.73 20.27 37.09 30.63
LTotal Phosphorus Loading Rate(Ib/ac/yr) 0.50 0.50 0.69 0.75
Total Phosphorus%Change Pre-to-Post 39% _ 51%
Total Phosphorus Change (Ib/yr)Pre-to-Post 10.36 10.36
L
SCM/Catchment Summary

SCM ID and Type Volume TN Reduction TP Reduction TN Out I TP Out
Reduction(%) (%) (%) (Ibs/ac/yr) (Ibs/ac/yr)
Catchment 1 12.66% 44.97% 57.47% 1.88 0.32
101:Wet Pond 12.66% 44.97% _ 57.47% 1.88 0.32
102:NA 0.00% 0.00% i 0.00% 0.00 0.00
103:NA 0.00% 0.00% 0.00% 0.00 0.00
Catchment 2 0.00% 0.00% 0.00% 0.00 0.00
201:NA 0.00% 0.00% 0.00% 0.00 0.00
202:NA 0.00% 0.00% 0.00% 0.00 0.00
203:NA 0.00% 0.00% 0.00% 0.00 0.00
Catchment 3 0.00% 0.00% 0.00% 0.00 0.00
301:NA 0.00% 0.00% 0.00% 0.00 0.00
302:NA 0.00% 0.00% _ 0.00% 0.00 0.00
303:NA 0.00% 0.00% 0.00% 0.00 0.00
Catchment 4 0.00% 0.00% 0.00% 0.00 0.00
401:NA 0.00% 0.00% _ 0.00% 0.00 0.00
402:NA 0.00% 0.00% 0.00% 0.00 0.00
403:NA 0.00% 0.00% 0.00% 0.00 0.00
Catchment 5 0.00% 0.00% _ 0.00% 0.00 0.00
501:NA 0.00% 0.00% 0.00% 0.00 0.00
502:NA 0.00% 0.00% 0.00% 0.00 0.00
503:NA 0.00% 0.00% 0.00% 0.00 0.00
Catchment 6 0.00% 0.00% 0.00% 0.00 0.00
601:NA 0.00% 0.00% 0.00% 0.00 0.00
602:NA 0.00% 0.00% _ 0.00% 0.00 0.00
603:NA i 0.00% 0.00% 0.00% 0.00 0.00

SNAP v4.2.0
Errors/Advisories
SCM Area (ft2) = 25,175
SCM Treated Area (ft2) = 1,667,000
Catchment Routing: No errors
Treating Runoff from Offsite Area or Existing BUA drains to an SCM. See
Existing BUA or Offsite: yes Users'Manual for instructions to determine nutrient
reduction needs and reduction provided by SCMs.
Disturbed Area (ft2) = 1,325,000
Total Phosphorus Export Target
32.57
Scaled to Project Area (Ib/yr):
Total Area 1 Project Area Total Area Project Area
Total Area Project Area
Post-Project Post-Project Post-Project Post-Project
Post-Project Post-Project
SCM-Treated SCM-Treated Untreated Untreated
after Treatment after Treatment
Area Only Area Only Areas Areas
53.6777 I 40.7111 38.2691 25.3025 15.4086 _ 15.4086
15% I 15% 19% 29% 6% 6%
359,625 _ 272,753 317,275 317,275 42,350 _ 42,350
1,419,367 ` 1,334,957 1,174,077 1,089,667 245,290 245,290
255% I 340%
1,019,272 1,031,510
1.12 1.12 0.98 0.97 1.75 + 1.75
98.85 92.81 72.03 65.98 26.82 I 26.82
1.84 2.28 1.88 2.61 1.74 1.74
60% 98%
36.89 �82
0.23 0.23 0.17 0.16 0.53 0.53
20.45 18.89 12.31 10.75 8.14 8.14
0.38 1 0.46 0.32 0.42 0.53 0.53
-23% -7%
-6.28 -1.39

Falls Lake ONLY: Onsite Reduction Compliance Check
Nitrogen Phosphorus
Onsite%Reduction Requirement
Export Target Scaled to Area(Ib/yr) 89.56 32.57
Export Load Post-Project Before Treatment 142.75 30.63
Total Reduction Need (Ib/yr)
Onsite Reduction Need (Ib/yr)
Onsite Export Target(Ib/yr)
Project Area Post-Project After Treatment 92.81 18.89

Nutrient Management Strategy Watershed - Nutrient Offset Credit Reporting Form
SNAP v4.2.0
Please complete and submit the following information to the local government permitting your development project
to characterize it and assess the need to purchase nutrient offset credits. Contact and rule implementation
information can be found online at:
http://deq.nc.gov/about/divisions/water-resources/planning/non poi nt-sou rce-ma nagement/n utrient-offset-information
PROJECT INFORMATION
Applicant Name:
Project Name: Forest Hills
Project Address:
Date: (mm/dd/yyyy) Development Land Use Type: Single Family Residential
•
County: Project Activity Type: New Development
Project Area(sqft): 1,773,375 Project Latitude: 0.000000
Post-Project Built-Upon Area%: 15.38% Project Longitude: 0.000000
WATERSHED INFORMATION
Nutrient Management Watershed: Tar Pamlico N Target Export Rate(Ib/ac/yr): 2.20
Subwatershed: Upper_Tar_River P Target Export Rate(Ib/ac/yr): 0.80
Nitrogen Delivery Zone: Tar-Upper 03020101 Nitrogen Delivery Factor: 100%
Phosphorus Delivery Zone: Tar-Upper 03020101 Phosphorus Delivery Factor: 100%
PERMANENT NUTRIENT OFFSET REQUEST
Post-Project Nitrogen Calculations-Projects with No Offsite or Built-Upon Area
(A) (B) (C) (D) (F) (G) (Where
Applicable)
TN Export TN Remaining TN Permanent Additional Total TN
TN Untreated TN Treated TN Delivery Offsets Permanent
Load(Ib/yr) Target Load Load(Ib/yr) Reduction Factor(%) Required Local Gov't Offsets to
(Ib/yr) Need(lb/yr) (Ib/yr) Offsets(Ib/yr) Buy(Ib/yr)
142.8 89.6 92.8 3.2 100.0% 3.2 3.2
Post-Project Phosphorus Calculations-Projects with No Offsite or Built-Upon Area
(A) (B) (C) (D) (F) (G) (Where
Applicable)
TP Export TP Remaining TP Permanent Additional Total TP
TP Untreated TP Treated TP Delivery Offsets Permanent
Load(Ib/yr) Target Load Load(Ib/yr) Reduction Factor(%) Required Local Gov't Offsets to
(Ib/yr) Need(Ib/yr) Offsets(Ib/yr)
(Ib/yr) i Buy(Ib/yr)
30.6 32.6 18.9 0.0 100.0% 0.0 0.0
LOCAL GOVERNMENT AUTHORIZATION
Local Government Name:I
Staff Name: Phone:
Staff Email: Date:
Local Government Authorizing Signature:

APPENDIX F
SUPPORTING DOCUMENTATION

NOAA PFDS RAINFALL DATA

8/16/2019 P recipitation Frequency Data Server
NO AA Atlas 14,Volume 2,Version 3 RALEIG H
DURHAM WSFO AP
w*OF Station ID: 31-7069
f-V Location name: M crrisville, North Carolina, USA*
• Latitude: 35.8706°, Longitude: -78.7864° i
ltillif
% �!i Elevation:
�'"`°°` Elevation(station metadata):416 ft** .,
*source:ESRI M Ts
**source:USG S
PO NT PRECIPITATIO NFREQ LENCY ESTIMATES
G M.Bonnin,D.M atin,B.Lin,T.Parzybok,M Yekta,and D.Riley
NO PA,National Weather Service,Silver Spring,M ayland
PF tabular I PF graphical I M Ts & aerials
PF tabular
PDS-based point precipitation frequency estimates w th 90% confidence intervals (in inches)1
LAverage recurrence interval(years)
Duration -
1 2 5'I I _I 0 2 5 5 0 1 00 2 00 5 00 1000
5-m in 0.394 0.461 0.530 0.587 0.647 0.689 0.727 0.758 0.792 0.819
(0.362-0.429) (0.423-0.503) (0.487-0.577) (0.539-0.639) (0.591-0.703) (0.626-0.748) (0.657-0.789) (0.682-0.825) (0.707-0.862) (0.724-0.892)
10-m it 0.630 0.738 0.848 0.939 1.03 1.10 1.16 1.20 1.25 I 1.29
(0.578-0.686) (0.677-0.804) (0.780-0.924) (0.861-1.02) (0.941-1.12) (0.998-1.19) (1.04-1.25) (1.08-1.31) (1.12-1.36) (1.14-1.41)
15-m n 0.787 0.927 1.07 1.19 1.31 1.39 1.46 1.52 1.58 1.62
(0.723-0.857) (0.850-1.01) (0.987-1.17) (1.09-1.29) (1.19-1.42) (1.26-1.51) (1.32-1.59) (1.36-1.65) (1.41-1.72) (1.43-1.76)
30-m n 1.08 1.28 1.53 1.72 1.94 2.09 2.24 2.36 2.51 2.62
(0.991-1.18) (1.18-1.40) (1.40-1.66) (1.58-1.87) (1.77-2.10) (1.90-2.27) (2.02-2.43) (2.12-2.57) (2.24-2.73) (2.32-2.86)
60-m n 1.35 1.61 1.96 2.24 2.58 2.84 3.08 I 3.31 3.60 3.83
(1.24-1.47) (1.47-1.75) (1.80-2.13) (2.06-2.44) (2.35-2.80) (2.58-3.08) (2.78-3.35) (2.98-3.60) (3.21-3.92) (3.38-4.17)
2-hr 1.56 1.86 2.29 2.65 3.08 3.43 3.76 4.09 4.51 4.84
(1.42-1.71) (1.71-2.04) (2.09-2.51) (2.40-2.90) (2.78-3.36) (3.09-3.74) (3.36-4.11) (3.64-4.46) (3.97-4.92) (4.23-5.29)
3-hr 1.65 1.98 2.44 2.84 3.33 3.75 4.15 4.56 5.11 5.55
(1.51-1.81) (1.81-2.17) (2.23-2.68) (2.59-3.10)J (3.02-3.64) I (3.38-4.09) I (3.71-4.53) (4.06-4.97) (4.49-5.57) (4.83-6.06)
6-hr 2.00 2.39 2.95 3.44 4.06 4.57 5.09 5.62 1 6.33 6.92
(1.84-2.18)_ (2.20-2.61) (2.72-3.22) (3.15-3.74) (3.70-4.40) (4.14-4.96) (4.57-5.51) (5.00-6.08) (5.56-6.85) (6.01-7.50)
12-hr 2.37 2.84 3.52 4.11 4.89 5.56 6.22 6.93 7.88 8.69
(2.19-2.58) (2.62-3.09) (3.24-3.82) (3.78-4.47) (4.46-5.30) (5.03-5.98) (5.58-6.70) I (6.13-7.45) (6.87-8.47) (7.46-9.35)
24-hr 2.83 3.42 4.27 4.94 5.84 6.55 7.27 8.01 9.01 9.79
(2.65-3.04)_ (3.20-3.67) (3.99-4.58) (4.61-5.29) (5.43-6.25) (6.08-7.01) (6.73-7.80) (7.39-8.60) (8.28-9.69) (8.97-10.6)
2-day 3.26 3.92 4.86 5.58 6.55 7.31 8.07 8.85 9.90 10.7
(3.04-3.50) (3.66-4.22) (4.53-5.23) (5.19-6.00) (6.08-7.05) (6.76-7.86) (7.45-8.70) (8.14-9.54) (9.06-10.7) (9.77-11.6)
3-day 3.45 4.14 5.10 5.86 6.87 7.67 8.47 9.29 10.4 11.2
(3.22-3.70) (3.86-4.44) (4.76-5.48) (5.46-6.29) (6.38-7.38) (7.11-8.24) (7.83-9.12) (8.55-10.0) (9.53-11.2) (10.3-12.1)
4-day 3.63 4.35 5.35 6.13 7.19 8.03 8.88 9.73 10.9 11.8
(3.40-3.90) (4.07-4.67) (5.00-5.73) (5.73-6.58) (6.69-7.72) (7.45-8.61) (8.21-9.54) (8.97-10.5) (9.99-11.7) (10.8-12.7)
7-day 4.21 5.02 6.10 6.95 8.10 9.01 9.94 10.9 12.2 13.2
(3.96-4.50) (4.72-5.36) (5.73-6.51) (6.52-7.42) (7.57-8.65) (8.40-9.63) (9.23-10.6) (10.1-11.7) (11.2-13.1) (12.1-14.2)
10-day 4.79 5.69 6.83 7.72 8.92 9.86 10.8 11.8 13.1 14.1
(4.50-5.11) (5.35-6.07) (6.41-7.29) (7.24-8.23) (8.35-9.52) (9.20-10.5) (10.1-11.6) I (10.9-12.6) (12.1-14.0) i (12.9-15.1) 1
20-day 6.39 7.54 8.90 9.98 11.5 12.6 13.8 15.0 16.6 17.8
(6.01-6.82) (7.09-8.04) (8.36-9.50) (9.36-10.7) (10.7-12.2) (11.8-13.5) (12.8-14.8) (13.9-16.0) (15.3-17.8) (16.3-19.2)
30-day 7.93 9.33 10.8 12.0 13.5 14.7 15.9 17.1 18.6 19.8
(7.48-8.44) (8.78-9.92) (10.2-11.5) (11.3-12.8) (12.7-14.4) (13.8-15.7) (14.8-17.0) (15.9-18.2) (17.3-20.0) (18.3-21.3)
45-day 10.1 11.8 13.6 I 14.9 I 16.6 1 17.9 19.2 20.5 22.1 23.4
(9.61-10.7) (11.3-12.5) (12.9-14.3) I, (14.1-15.7) (15.7-17.5) 1, (16.9-18.9) (18.1-20.4) (19.2-21.7) (20.7-23.5) 1 (21.8-24.9) 1
60-day 12.2 14.2 16.0 17.4 19.2 20.5 21.8 23.1 24.7 25.9
(11.6-12.8) (13.5-14.9) (15.2-16.8) (16.5-18.3) (18.2-20.2) (19.4-21.7) (20.6-23.1) (21.8-24.4) (23.2-26.2) (24.3-27.5)
1 Precipitation frequency(PF)estim des in this table are based on frequency analysis of partial duration series(PDS).
Num hers in parenthesis are PF estim des at lower and upper bounds of the 90%confidence interval.The probability that precipitation frequency estim axes(for a
given duration and average recurrence interval)will be greater than the upper bound(or less than the lower bound)is 5%.Estim des at upper bounds are not
checked against probable m adm un precipitation(PM P)estim des and m ay be higher than currently valid PM Pvalues.
Please refer to NO PA Atlas 14 docum art for m cre inform aion.
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8/16/2019 P recipitation Frequency Data Server
PF graphical
PDS-based depth-duration-frequency (DDF) curves
Latitude: 35.8706°, Longitude: -78.7864°
30 1 I I I I I oil I 1 1 1 1 '
Average recurrence
25 interval
(years)
t
_c 20 2
a
v I
5
c 15 1004 25
p 50
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ai
a 200
5 _ 500
0 — .
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E E E c E ry A 6 N v v v-o -0 -o v v v-o
IA Oo in M o .-I N r rV 4 r- O N o in v o
ID
Duration
30
25 -- --
'2
L Duration
lEE
EP
— 2-day— 3-da.-...... - — 4-day
— 7-day— 10-day— 20-day— 30-day— 45-day— 60-day
0 I I I I I I I - 24-hr
1 2 5 10 25 50 100 200 500 1000 —
Average recurrence interval (years)
NOAA Atlas 14,Volume 2,Version 3 Created(GMT): Fri Aug 16 18:48:37 2019
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M aps & aerials
Small scale terrain
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arch Triangle Park
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8/16/2019 P recipitation Frequency Data Server
N
Winston-Salem
• •
Greensboro Durham• Rocky Mount
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North
Carolina •
Charlotte •
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111
100km •
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US Departm Ent of Corn m ece
National 0 manic and Atm cspheric Adm histration
National Weather Service
National Water Center
1325 East West Highway
Silver Spring,M D20910
Q testions?:HDSC.Q [e-mail]
Disclaim e
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NRCS DOCUMENTATION

Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Table 2-2a Runoff curve numbers for urban areas 1i
Curve numbers for
------------------ Cover description ----------------- -----hydrologic soil group-------
Average percent
Cover type and hydrologic condition impervious area 22 A B C D
Fully developed urban areas (vegetation established)
Open space(lawns,parks,golf courses,cemeteries,etc.)a':
Poor condition(grass cover<50%) 68 79 86 89
Fair condition(grass cover 50%to 75%) 49 69 79 84
Good condition(grass cover>75%) 39 61 74 80
Impervious areas:
Paved parking lots,roofs,driveways,etc.
(excluding right-of-way) 98 98 98 98
Streets and roads:
Paved;curbs and storm sewers(excluding
right-of-way) 98 98 98 98
Paved;open ditches(including right-of-way) 83 89 92 93
Gravel(including right-of-way) 76 85 89 91
Dirt(including right-of-way) 72 82 87 89
Western desert urban areas:
Natural desert landscaping(pervious areas only) v 63 77 85 88
Artificial desert landscaping(impervious weed barrier,
desert shrub with 1-to 2-inch sand or gravel mulch
and basin borders) 96 96 96 96
Urban districts:
Commercial and business 85 89 92 94 95
Industrial 72 81 88 91 93
Residential districts by average lot size:
1/8 acre or less(town houses) 65 77 85 90 92
1/4 acre 38 61 75 83 87
1/3 acre 30 57 72 81 86
1/2 acre 25 54 70 80 85
1 acre 20 51 68 79 84
2 acres 12 46 65 77 82
Developing urban areas
Newly graded areas
(pervious areas only,no vegetation)5' 77 86 91 94
Idle lands(CN's are determined using cover types
similar to those in table 2-2c).
1 Average runoff condition,and Ia=0.2S.
2 The average percent impervious area shown was used to develop the composite CN's.Other assumptions are as follows:impervious areas are
directly connected to the drainage system,impervious areas have a CN of 98,and pervious areas are considered equivalent to open space in
good hydrologic condition.CN's for other combinations of conditions may be computed using figure 2-3 or 2-4.
3 CN's shown are equivalent to those of pasture.Composite CN's may be computed for other combinations of open space
cover type.
4 Composite CN's for natural desert landscaping should be computed using figures 2-3 or 2-4 based on the impervious area percentage
(CN=98)and the pervious area CN.The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition.
5 Composite CN's to use for the design of temporary measures during grading and construction should be computed using figure 2-3 or 2-4
based on the degree of development(impervious area percentage)and the CN's for the newly graded pervious areas.
(210-VI-TR-55,Second Ed.,June 1986) 2-5

Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Table 2-2c Runoff curve numbers for other agricultural lands 11
Curve numbers for
----------------------------- Cover description --------- hydrologic soil group---------
Hydrologic
Cover type condition A B C D
Pasture,grassland,or range—continuous Poor 68 79 86 89
forage for grazing.21 Fair 49 69 79 84
Good 39 61 74 80
Meadow—continuous grass,protected from — 30 58 71 78
grazing and generally mowed for hay.
Brush—brush-weed-grass mixture with brush Poor 48 67 77 83
the major element.3i Fair 35 56 70 77
Good 30 v 48 65 73
Woods—grass combination(orchard Poor 57 73 82 86
or tree farm).5/ Fair 43 65 76 82
Good 32 58 72 79
Woods.si Poor 45 66 77 83
Fair 36 60 73 79
Good 30 v 55 70 77
Farmsteads—buildings,lanes,driveways, — 59 74 82 86
and surrounding lots.
1 Average runoff condition,and Ia=0.2S.
2 Poor: <50%)ground cover or heavily grazed with no mulch.
Fair: 50 to 75%ground cover and not heavily grazed.
Good: >75%ground cover and lightly or only occasionally grazed.
3 Poor: <50%ground cover.
Fair: 50 to 75%ground cover.
Good: >75%ground cover.
4 Actual curve number is less than 30;use CN=30 for runoff computations.
5 CN's shown were computed for areas with 50%woods and 50%grass(pasture)cover.Other combinations of conditions may be computed
from the CN's for woods and pasture.
6 Poor: Forest litter,small trees,and brush are destroyed by heavy grazing or regular burning.
Fair: Woods are grazed but not burned,and some forest litter covers the soil.
Good: Woods are protected from grazing,and litter and brush adequately cover the soil.
(210-VI-TR-55,Second Ed.,June 1986) 2-7

Chapter 3 Time of Concentration and Travel Time Technical Release 55
Urban Hydrology for Small Watersheds
Sheet flow For sheet flow of less than 300 feet,use Manning's
kinematic solution(Overtop and Meadows 1976)to
Sheet flow is flow over plane surfaces.It usually compute Tt:
occurs in the headwater of streams.With sheet flow, 0 8
the friction value(Manning's n)is an effective rough- _ 0.007(nL)
ness coefficient that includes the effect of raindrop Tt (P 10.5 s0 4 [eq.3-3]
impact;drag over the plane surface; obstacles such as 2
litter,crop ridges,and rocks;and erosion and trans- where:
portation of sediment.These n values are for very
shallow flow depths of about 0.1 foot or so.Table 3-1 Tt = travel time(hr),
gives Manning's n values for sheet flow for various n = Manning's roughness coefficient(table 3-1)
surface conditions. L = flow length(ft)
P2 = 2-year, 24-hour rainfall(in)
s = slope of hydraulic grade line
Table 3-1 Roughness coefficients(Manning's n)for (land slope,ft/ft)
sheet flow
This simplified form of the Manning's kinematic solu-
Surface description n li tion is based on the following: (1)shallow steady
uniform flow, (2)constant intensity of rainfall excess
Smooth surfaces(concrete,asphalt, (that part of a rain available for runoff), (3)rainfall
gravel,or bare soil) 0.011 duration of 24 hours,and(4)minor effect of infiltra-
Fallow(no residue) 0.05 tion on travel time.Rainfall depth can be obtained
Cultivated soils: from appendix B.
Residue cover<_20% 0.06
Residue cover>20% 0.17 Shallow concentrated flow
Grass:
Short grass prairie 0.15 After a maximum of 300 feet,sheet flow usually be-
Dense grasses 2J 0.24 comes shallow concentrated flow.The average veloc-
Bermudagrass. 0.41 ity for this flow can be determined from figure 3-1,in
Range(natural) 0.13 which average velocity is a function of watercourse
Woods:3i slope and type of channel.For slopes less than 0.005
Light underbrush 0.40 ft/ft,use equations given in appendix F for figure 3-1.
Dense underbrush 0.80 Tillage can affect the direction of shallow concen-
trated flow.Flow may not always be directly down the
1 The n values are a composite of information compiled by Engman watershed slope if tillage runs across the slope.
(1986).
2 Includes species such as weeping lovegrass,bluegrass,buffalo
grass,blue grams grass,and native grass mixtures. After determining average velocity in figure 3-1,use
3 When selecting n,consider cover to a height of about 0.1 ft.This equation 3-1 to estimate travel time for the shallow
is the only part of the plant cover that will obstruct sheet flow. concentrated flow segment.
Open channels
Open channels are assumed to begin where surveyed
cross section information has been obtained,where
channels are visible on aerial photographs, or where
blue lines(indicating streams)appear on United States
Geological Survey(USGS)quadrangle sheets.
Manning's equation or water surface profile informa-
tion can be used to estimate average flow velocity.
Average flow velocity is usually determined for bank-
full elevation.
(210-VI-TR-55,Second Ed.,June 1986) 3-3