PERSO-050 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
MAYO RETENTION BASIN DAM
PERSO-050
PERSON COUNTY, NC
PREPARED BY:
WOOD E&I Solutions
2020
EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Mayo 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 attached 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 =
0.0177
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
MAYO RETENTION BASIN DAM, PERSO‐050
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
TOP OF DAM ELEVATION = 549
24 HOUR ‐ 1000 YEAR EVENT, 10.4 INCHES, ELEVATION 545.2 (3.8 FT FREE‐BOARD)
24 HOUR ‐ 500 YEAR EVENT, 9.46 INCHES, ELEVATION 545.1 (3.9 FT FREE‐BOARD)
24 HOUR ‐ 200 YEAR EVENT, 8.25 INCHES, ELEVATION 544.9 (4.1 FT FREE‐BOARD)
24 HOUR ‐ 100 YEAR EVENT, 7.4 INCHES, ELEVATION 544.8 (4.2 FT FREE‐BOARD)
24 HOUR ‐ 50 YEAR EVENT, 6.59 INCHES, ELEVATION 544.7 (4.3 FT FREE‐BOARD)
24 HOUR ‐ 25 YEAR EVENT, 5.82 INCHES, ELEVATION 544.6 (4.4 FT FREE‐BOARD)
SUMMARY OF INPUT PARAMETERS
98
3.6
SCS Type 2
SPILLWAY
Principal
TYPE
Sump Pump
ELEVATION
544
548
ELEVATION
544
545
STORAGE (ACRE‐FT)
0.0
8.3
Auxiliary
NOT TO SCALE
Concrete‐Lined Trapezoidal spillway
ELEVATION
547.0
548.0
549.0
STORAGE (ACRE‐FT)
25.8
35.1
45.0546 16.8
PERSO‐050MAYO RETENTION BASIN DAM SEPTEMBER 18, 2020

APPENDIX A
Watershed Boundary

NOTE:
Dam is newly constructed (2019) and is offline structure as there is no upstream hydrologic sources
flowing into the basin. No StreamStats data available. 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-050 Date: 9/18/2020
Dam Name: Mayo 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: 36.5290°
Longitude: -78.9008°
Elevation (USGS): 571.62 ft
PRECIPITATION FREQUENCY ESTIMATESby
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.378 0.447 0.519 0.578 0.643 0.688 0.731 0.767 0.808 0.84
10-min:0.604 0.714 0.832 0.925 1.02 1.1 1.16 1.22 1.28 1.32
15-min:0.755 0.898 1.05 1.17 1.3 1.39 1.47 1.53 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 3.67 3.93
2-hr:1.52 1.83 2.27 2.64 3.1 3.48 3.85 4.23 4.72 5.12
3-hr:1.62 1.96 2.43 2.83 3.34 3.76 4.17 4.59 5.14 5.59
6-hr:1.97 2.37 2.95 3.45 4.1 4.66 5.23 5.81 6.62 7.29
12-hr:2.36 2.84 3.54 4.17 5.02 5.75 6.52 7.34 8.5 9.5
24-hr:2.77 3.34 4.19 4.87 5.82 6.59 7.4 8.25 9.46 10.4
2-day:3.25 3.92 4.87 5.62 6.65 7.47 8.32 9.2 10.4 11.4
3-day:3.43 4.13 5.13 5.91 6.99 7.86 8.76 9.7 11 12
4-day:3.61 4.34 5.38 6.2 7.34 8.25 9.2 10.2 11.6 12.7
7-day:4.15 4.96 6.06 6.93 8.15 9.12 10.1 11.2 12.6 13.8
10-day:4.71 5.61 6.79 7.72 9 10 11.1 12.1 13.6 14.8
20-day:6.31 7.47 8.89 10 11.6 12.8 14 15.3 17 18.3
30-day:7.83 9.22 10.8 11.9 13.5 14.7 15.9 17 18.6 19.8
45-day:9.91 11.6 13.4 14.8 16.5 17.9 19.2 20.5 22.2 23.5
60-day:11.9 13.9 15.8 17.2 19.1 20.4 21.7 23 24.6 25.8
Date/time (GMT): Fri Sep 18 09:57:14 2020
pyRunTime: 0.0106239318848

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:10/28/2020
Project:Overtopping Study Engineer:E.Okorie
Dam:Mayo Retention Basin Dam Dam No.PERSO-050
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
544.00 18.05 0.0 18.05
544.50 18.05 0.0 18.05
545.00 18.05 0.0 18.05
545.50 18.05 0.0 18.05
546.00 18.05 0.0 18.05
546.50 18.05 0.0 18.05
547.00 18.05 0.0 18.05
547.50 18.05 0.0 18.05
548.00 18.05 0.0 18.05
548.50 18.05 15.4 33.43
549.00 18.05 56.0 74.05
549.50 18.05 125.8 143.89
550.00 18.05 229.1 247.15
550.50 18.05 369.6 387.64
551.00 18.05 550.8 568.84
551.50 18.05 775.9 793.98
552.00 18.05 1048.0 1066.05
0.00
200.00
400.00
600.00
800.00
1000.00
1200.00
543.00 544.00 545.00 546.00 547.00 548.00 549.00 550.00 551.00 552.00 553.00Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)