PERSO-048 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
ROXBORO RETENTION BASIN DAM
PERSO-048
PERSON COUNTY, NC
PREPARED BY:
WOOD E&I Solutions
2020
EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Roxboro Retention Basin Dam in Person
County, North Carolina. The purpose of the study was to analyze the existing potential of the dam
for overtopping during predetermined specified rainfall events. The analyses were completed
using the United States Army Corps of Engineers developed software Hydrologic Engineering
Center – Hydrologic Modeling System (HEC-HMS) in combination with Arc-GIS to develop
required input parameters. The rainfall events included in the analyses consist of 24-hour
durations with return periods of 25 years, 50 years, 100 years, 200 years, 500 years and 1000
years. Results of the analyses indicate that this structure is hydraulically adequate for 25 years,
50 years, 100 years, 200 years, 500 years, and 1000 years rainfall events. Data, input parameters
and final results can be found on the atta ched summary sheets and in the appendices of this
report as well as in the associated electronic file in which all program files are stored.
Drainage Area (Sq. Mi.)=
Curve Number =
Lag Time (min.)=
Rainfall Distribution Methods =
NOT TO SCALE
452
STORAGE (ACRE-FT)
77.8
80.5
83.4
86.5447
448
STORAGE (ACRE-FT)
0
14.4
29.3
44.8
60.8
444
448
ELEVATION
444
445
446
ELEVATION
449
450
451
98
3.6
SCS Type 2
SPILLWAY
Principal
Auxiliary
TYPE
Sump Pump
Conc-Lined Trapezoidal Channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 8.24 INCHES, ELEVATION 447.9 (1.1 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 7.4 INCHES, ELEVATION 447.8 (1.2 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 6.61 INCHES, ELEVATION 447.7 (1.3 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 5.84 INCHES, ELEVATION 447.6 (1.4 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.0314
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
ROXBORO RETENTION BASIN DAM, PERSO-048
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
TOP OF DAM ELEVATION = 449
24 HOUR - 1000 YEAR EVENT, 10.4 INCHES, ELEVATION 448.1 (0.9 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 9.42 INCHES, ELEVATION 448 (1 FT FREE-BOARD)
PERSO-048ROXBORO RETENTION BASIN SEPTEMBER 18, 2020
APPENDIX A
Watershed Boundary
Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P,
NRCan, Esri Japan, METI, Esri China (Hong Kong), Esri Korea,
Esri (Thailand), NGCC, (c) OpenStreetMap contributors, and the
GIS User Community
Roxboro Retention Basin Dam - Watershed Boundary
Legend
Watershed Boundary
0 0.1 0.20.05 Miles
Date: 9/14/2020
/Drainage Area: 0.0314 Square Miles
Flowpath Length: 00.0 Feet
APPENDIX B
Lag Time Calculation
Dam No: PERSO-048 Date: 9/14/2020
Dam Name: Roxboro Retention Basin Dam Engineer: E.Okorie
Maximum Potential Retention (inches):S = (1000/cn')-10
Time of Concentration (hours):Tc =l0.8*(S+1)0.7
1140*Y0.5
Basin Data:Input Values in yellow
Curve Number CN = 98.00 from NRCS CN
S =0.20408 in.
Mean Basin Slope Y = 0 % from Retention Basin Footprint, assumed minimal slope
Longest Flowpath: 0 mi. from Retention basin longitudinal Length
Flow Length l =0 ft.
Time of Concentration Tc =0.10 Hours Tc =6 mins <-- Direct runoff is from within the footprint of the Retention Basin
Lag Time TL =0.06 Hours TL =3.6 mins Assumed Minimum Tc

APPENDIX C
NRCS Curve Number Summary Sheet

Change CN according to TR-55 (see tables below)
NLCD (2016) Landuse TR-55 Landuse HSG A (sq. ft.) CN HSG B (sq. ft.) CN HSG C (sq. ft.)CN HSG D (sq. ft.) CN
Open Water Not Available 0 100 0 0 100 0 0 100 0 0 100 0
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%) 0 49 0 0 69 0 0 79 0 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 0 72 0 0 81 0 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 0 85 0 0 90 0 0 92 0
Developed, High Intensity Urban districts - Commercial and business 0 89 0 0 92 0 0 94 0 0 95 0
Barren Land Newly graded areas 0 77 0 0 86 0 0 91 0 0 94 0
Deciduous Forest Woods - Fair 0 36 0 0 60 0 0 73 0 0 79 0
Evergreen Forest Woods - Fair 0 36 0 0 60 0 0 73 0 0 79 0
Mixed Forest Woods - Fair 0 36 0 0 60 0 0 73 0 0 79 0
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 0 71 0 0 81 0 0 89 0
Hay/Pasture Pasture - Fair 0 49 0 0 69 0 0 79 0 0 84 0
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 0 80 0 0 87 0 0 90 0
Woody Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0
Emergent Herbaceuous Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0 Weighted CN= 98.00 <-- Obtained directly from the As-built Design report.
NOTE:
Direct runoff from the watershed area bounded by the perimeter of the dam on all sides. Since the top of the dams are compacted and covered with granular surfacing, it is
assumed the entire depth of rainfall on this footprint enters the basin.
Hence, Curve Number is assumed to be 98 per As-built Design Report.
APPENDIX D
Hyetograph Results Used for Analyses
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24Cumulative Depth (in)Time (hrs)
Hyetographs for 24-Hour SCS Type II Rainfall Distributions
25YR
50YR
100YR
200YR
500YR
1000YR

APPENDIX E
NOAA Atlas 14 Rainfall Table

Point precipitation frequency estimates (inches)
NOAA Atlas 14 Volume 2 Version 3
Data type: Precipitation depth
Time series type: Partial duration
Project area: Ohio River Basin
Location name (ESRI Maps): Semora, North Carolina USA
Station Name: -
Latitude: 36.4838°
Longitude: -79.0848°
Elevation (USGS): None
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.377 0.447 0.525 0.58 0.643 0.684 0.724 0.758 0.796 0.823
10-min:0.601 0.716 0.84 0.927 1.02 1.09 1.15 1.2 1.26 1.3
15-min:0.752 0.9 1.06 1.17 1.3 1.38 1.46 1.52 1.59 1.63
30-min:1.03 1.24 1.51 1.7 1.92 2.08 2.23 2.36 2.52 2.64
60-min:1.29 1.56 1.94 2.21 2.56 2.82 3.07 3.31 3.62 3.85
2-hr:1.52 1.85 2.31 2.66 3.12 3.48 3.84 4.2 4.68 5.05
3-hr:1.63 1.98 2.47 2.86 3.37 3.76 4.16 4.55 5.09 5.5
6-hr:1.99 2.4 3.01 3.49 4.15 4.68 5.23 5.8 6.59 7.21
12-hr:2.38 2.88 3.63 4.24 5.1 5.81 6.57 7.38 8.53 9.48
24-hr:2.79 3.37 4.22 4.9 5.84 6.61 7.4 8.24 9.42 10.4
2-day:3.28 3.95 4.91 5.66 6.68 7.49 8.33 9.2 10.4 11.3
3-day:3.46 4.17 5.16 5.94 7.02 7.88 8.77 9.69 11 12
4-day:3.64 4.38 5.41 6.23 7.37 8.27 9.21 10.2 11.5 12.6
7-day:4.18 5 6.1 6.97 8.18 9.15 10.1 11.2 12.6 13.7
10-day:4.74 5.65 6.82 7.76 9.03 10 11.1 12.1 13.6 14.8
20-day:6.35 7.52 8.94 10.1 11.6 12.8 14.1 15.3 17 18.4
30-day:7.88 9.28 10.8 12 13.6 14.8 16 17.1 18.7 19.9
45-day:9.98 11.7 13.5 14.8 16.6 18 19.3 20.6 22.3 23.6
60-day:12 14 15.9 17.3 19.2 20.5 21.8 23.1 24.7 25.9
Date/time (GMT): Mon Sep 14 00:32:02 2020
pyRunTime: 0.0107011795044

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NCEM Date: 9/15/2020
Project: Overtopping Study Engineer: E.Okorie
Dam: Roxboro Retention Basin Dam Dam No. PERSO-048
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
444.0 15.82 0.0 15.8
445.0 15.82 0.0 15.8
446.0 15.82 0.0 15.8
447.0 15.82 0.0 15.8
448.0 15.82 0.0 15.8
449.0 15.82 56.0 71.8
450.0 15.82 229.1 244.9
451.0 15.82 550.8 566.6
452.0 15.82 1048.0 1063.8
453.0 15.82 1744.1 1760.0
454.0 15.82 2660.1 2676.0
455.0 15.82 3815.2 3831.0
0.0
500.0
1000.0
1500.0
2000.0
2500.0
3000.0
3500.0
4000.0
4500.0
442.0 444.0 446.0 448.0 450.0 452.0 454.0 456.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)