PERSO-049 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
MAYO FGD SETTLING BASIN DAM
PERSO-049
PERSON COUNTY, NC
PREPARED BY:
WOOD E&I Solutions
2020
EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Mayo FGD Settling 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 =
Concrete-lined Trapezoidal Spillway
TOP OF DAM ELEVATION = 488.58
24 HOUR - 1000 YEAR EVENT, 10.5 INCHES, ELEVATION 485.2 (3.4 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 9.48 INCHES, ELEVATION 485.1 (3.5 FT FREE-BOARD)
NOT TO SCALE
ELEVATION
487.0
488.0
488.6
STORAGE (ACRE-FT)
11.5
15.6
18.0486.0 7.6
487.52
ELEVATION
484.0
485.0
STORAGE (ACRE-FT)
0.0
3.7
Auxiliary
98
3.6
SCS Type 2
SPILLWAY
Principal
TYPE
Sump Pump
ELEVATION
484.0
0.00656
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
MAYO FGD SETTLING BASIN DAM, PERSO-049
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
SUMMARY OF INPUT PARAMETERS
24 HOUR - 200 YEAR EVENT, 8.27 INCHES, ELEVATION 485 (3.6 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 7.41 INCHES, ELEVATION 484.9 (3.7 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 6.6 INCHES, ELEVATION 484.8 (3.8 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 5.82 INCHES, ELEVATION 484.7 (3.9 FT FREE-BOARD)
MAYO FGD SETTLING BASIN DAM PERSO-049 OCTOBER 28, 2020
APPENDIX A
Watershed Boundary
NOTE:
Dam is newly constructed and is offline structure as there is no upstream hydrologic sources flowing
into the basin. Lidar DEM does not reflect As-built and has not been updated to include the newly
constructed dam. Hence, watershed delineation could not be processed in GIS. Data for this analysis
is obtained from As-built design reports and drawings.
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:
There is no upstream hydrologic sources flowing into the settling basin. Onlystormwater runoff is direct rainfall which falls within the footprint of the settlingbasin. 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): North Carolina USA
Station Name: -
Latitude: 36.5361°
Longitude: -78.8851°
Elevation (USGS): 488.07 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.378 0.446 0.519 0.578 0.643 0.689 0.731 0.768 0.809 0.841
10-min:0.604 0.714 0.831 0.925 1.02 1.1 1.16 1.22 1.28 1.33
15-min:0.755 0.897 1.05 1.17 1.3 1.39 1.47 1.54 1.61 1.66
30-min:1.04 1.24 1.49 1.7 1.92 2.09 2.25 2.39 2.56 2.69
60-min:1.29 1.56 1.92 2.21 2.56 2.83 3.1 3.35 3.68 3.93
2-hr:1.52 1.83 2.26 2.64 3.1 3.48 3.85 4.23 4.72 5.12
3-hr:1.62 1.95 2.42 2.83 3.34 3.75 4.17 4.59 5.14 5.59
6-hr:1.97 2.37 2.94 3.44 4.1 4.66 5.22 5.81 6.61 7.29
12-hr:2.35 2.83 3.53 4.16 5.01 5.75 6.52 7.33 8.49 9.49
24-hr:2.76 3.33 4.19 4.87 5.82 6.6 7.41 8.27 9.48 10.5
2-day:3.25 3.91 4.86 5.62 6.65 7.47 8.33 9.22 10.5 11.4
3-day:3.43 4.12 5.12 5.91 6.99 7.86 8.77 9.71 11 12.1
4-day:3.61 4.33 5.37 6.2 7.34 8.26 9.21 10.2 11.6 12.7
7-day:4.14 4.95 6.05 6.93 8.15 9.13 10.1 11.2 12.6 13.8
10-day:4.7 5.6 6.78 7.72 9 10 11.1 12.2 13.6 14.8
20-day:6.3 7.46 8.88 10 11.6 12.8 14 15.3 17 18.3
30-day:7.81 9.2 10.7 11.9 13.5 14.7 15.9 17 18.6 19.7
45-day:9.89 11.6 13.4 14.7 16.5 17.9 19.2 20.5 22.2 23.4
60-day:11.9 13.9 15.8 17.2 19.1 20.4 21.7 23 24.6 25.7
Date/time (GMT): Wed Oct 28 08:04:09 2020
pyRunTime: 0.0124759674072

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway systemClient: NCEMDate: 9/15/2020Project: Overtopping Study Engineer: E.OkorieDam: Mayo FGD Settling Basin Dam Dam No. PERSO-049Reservior Water Level (RWS) Elevation (ft)Primary spillway discharge (cfs)Emergency spillway discharge (cfs)Total discharge from the spillway system (cfs)484.00 0.87 0.0 0.87484.50 0.87 0.0 0.87485.00 0.87 0.0 0.87485.50 0.87 0.0 0.87486.00 0.87 0.0 0.87486.50 0.87 0.0 0.87487.00 0.87 0.0 0.87487.50 0.87 0.0 0.87488.00 0.87 14.3 15.17488.50 0.87 53.8 54.71489.00 0.87 122.4 123.30489.50 0.87 224.3 225.15490.00 0.87 363.2 364.08‐50.000.0050.00100.00150.00200.00250.00483.00 484.00 485.00 486.00 487.00 488.00 489.00 490.00Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)The whole spillway system rating curve (cfs)