SURRY-013 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

GREEN HILL LAKE DAM

SURRY-013

SURRY COUNTY, NC

PREPARED BY:
WOOD E&I Solutions

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Green Hill Lake Dam
in Surry 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 and 50 years rainfall events. The structure is over-topped by the 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 =
208.7
212.7
NOT TO SCALE
1132.8Auxiliary24" Culvert barrel
151.6
160.7
170.1
179.5
189.1
198.81144.3
1144.8
1145
STORAGE (ACRE-FT)
99.4
107.7
116.2
124.9
133.6
142.51141.3
1141.8
1142.3
1142.8
1143.3
1143.867.8
75.5
83.3
91.3
ELEVATION
1138.8
1139.3
1139.8
1140.3
1140.824.9
31.7
38.6
45.7
52.9
60.31136.3
1136.8
1137.3
1137.8
1138.3
STORAGE (ACRE-FT)
0
5.7
11.9
18.41133.3
1133.8
1134.3
1134.8
1135.3
1135.8
ELEVATION
1131.8
1132.3
1132.8
Principal 48" Round Riser w/ 24" barrel 1131.8
68.85
38
SCS Type 2
SPILLWAY TYPE ELEVATION
24 HOUR - 100 YEAR EVENT, 8.18 INCHES, ELEVATION 1137.7 (0.2 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 1137.5
24 HOUR - 50 YEAR EVENT, 7.19 INCHES, ELEVATION 1137 (0.5 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.26 INCHES, ELEVATION 1135.9 (1.6 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.62
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
GREEN HILL LAKE DAM, SURRY-013
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
24 HOUR - 1000 YEAR EVENT, 12.1 INCHES, ELEVATION 1138.3 (0.8 FT OVER-TOPPED)
24 HOUR - 500 YEAR EVENT, 10.8 INCHES, ELEVATION 1138.1 (0.6 FT OVER-TOPPED)
24 HOUR - 200 YEAR EVENT, 9.26 INCHES, ELEVATION 1137.9 (0.4 FT OVER-TOPPED)
SURRY-013GREEN HILL LAKE DAM SEPTEMBER 11, 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
Green Hill Lake Dam - SURRY-013 - Watershed Boundary
Legend
Longest FlowPath
Watershed Boundary
0 0.2 0.40.1 Miles
Date: 8/13/2020
/Drainage Area: 0.62 Square Miles
Flowpath Length: 1.76 Miles

APPENDIX B
Lag Time Calculation

Dam No: SURRY-013 Date: 8/4/2020
Dam Name: Green Hill Lake 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 = 68.85
S =4.52433 in.
Mean Basin Slope Y = 16.53 %
Longest Flowpath: 1.7573 mi.
Flow Length l = 9278.55 ft.
Time of Concentration Tc =1.07 Hours Tc =64 mins
Lag Time TL =0.64 Hours TL =38 mins

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 397291 100 39729054 0 100 0 0 100 0
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 1879141 69 129660699 162775 79 12859192 283041 84 23775486
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 213038 72 15338714 7198 81 583032 32811 86 2821712
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 34766 86 2989909 0 91 0 13337 94 1253700
Deciduous Forest Woods - Fair 0 36 0 4772280 60 286336781 33336 73 2433552 1759084 79 138967662
Evergreen Forest Woods - Fair 0 36 0 454441 60 27266436 0 73 0 107323 79 8478523
Mixed Forest Woods - Fair 0 36 0 2446501 60 146790058 2273 73 165931 1156362 79 91352631
Shrub/Scrub Brush - Fair 0 35 0 244763 56 13706733 0 70 0 40882 77 3147925
Herbaceuous Herbaceuous - Fair 0 60 0 586665 71 41653191 5390 81 436616 569921 89 50722925
Hay/Pasture Pasture - Fair 0 49 0 1638772 69 113075259 85652 79 6766516 337163 84 28321716
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=68.85
NOTE: Curve Numbers are from SCS TR-55 with the exception of assumed HSG A (CN=60) for Herbaceous, Open Water, Woody Wetlands, and Emergent Herbaceous Wetlands.
NOTE: double-check all the land types are being calculated and texts are accurate
NOTE: double-check the exported attribute table has the same table format as the "Export_Output" table

APPENDIX D

Hyetograph Results Used for Analyses
0.02.04.06.08.010.012.014.00123456789101112131415161718192021222324Cumulative Depth (in)Time (hrs)Hyetographs for 24-Hour SCS Type II Rainfall Distributions25YR50YR100YR200YR500YR1000YR

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): Cana, Virgina USA
Station Name: ‐
Latitude: 36.5583°
Longitude: ‐80.6222°
Elevation (USGS): None None
PRECIPITATION FREQUENCY ESTIMATES
by 
duration 
for ARI 
(years):1 2 5 10 25 50 100 200 500 1000
5‐min: 0.344 0.411 0.489 0.546 0.618 0.671 0.723 0.773 0.836 0.886
10‐min: 0.549 0.657 0.783 0.873 0.985 1.07 1.15 1.23 1.32 1.4
15‐min: 0.686 0.826 0.99 1.11 1.25 1.35 1.45 1.55 1.67 1.75
30‐min: 0.941 1.14 1.41 1.6 1.85 2.04 2.22 2.41 2.65 2.84
60‐min: 1.17 1.43 1.8 2.08 2.46 2.76 3.06 3.37 3.8 4.14
2‐hr: 1.4 1.71 2.18 2.54 3.04 3.45 3.87 4.3 4.92 5.42
3‐hr: 1.52 1.86 2.36 2.75 3.3 3.74 4.2 4.69 5.37 5.93
6‐hr: 1.9 2.32 2.93 3.42 4.1 4.66 5.25 5.87 6.76 7.49
12‐hr: 2.36 2.87 3.62 4.22 5.06 5.75 6.48 7.26 8.4 9.35
24‐hr: 2.84 3.44 4.38 5.15 6.26 7.19 8.18 9.26 10.8 12.1
2‐day: 3.38 4.09 5.17 6.05 7.28 8.3 9.38 10.5 12.2 13.5
3‐day: 3.59 4.34 5.48 6.39 7.68 8.74 9.87 11.1 12.8 14.2
4‐day: 3.8 4.59 5.78 6.73 8.08 9.19 10.4 11.6 13.4 14.8
7‐day: 4.35 5.23 6.46 7.46 8.84 9.96 11.1 12.4 14.1 15.6
10‐day: 4.98 5.96 7.27 8.32 9.75 10.9 12.1 13.3 15 16.4
20‐day: 6.7 7.97 9.54 10.8 12.4 13.7 15 16.3 18.1 19.4
30‐day: 8.3 9.81 11.5 12.7 14.3 15.6 16.8 17.9 19.5 20.6
45‐day: 10.5 12.3 14.2 15.6 17.4 18.8 20 21.3 22.8 24
60‐day: 12.5 14.6 16.7 18.3 20.2 21.7 23 24.3 26 27.2
Date/time (GMT):  Mon Jun 22 17:38:52 2020
pyRunTime:  0.0157539844513

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NC DEMLR Date: 8/4/2020
Project: Overtopping Study Engineer: R. Khatri
Dam: Green Hill Lake Dam Dam No. SURRY-013
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
(from HydroCAD)
Emergency spillway
discharge (cfs)
(from HydroCAD)
Total discharge from the
spillway system (cfs)
1131.8 0.0 0.0 0.0
1132.4 25.2 0.0 25.2
1133.2 30.5 0.8 31.3
1134.0 34.7 6.7 41.4
1134.8 38.4 14.6 53.0
1135.6 41.2 19.5 60.7
1136.4 43.8 24.2 68.0
1137.2 46.3 27.8 74.0
1138.0 48.6 30.9 79.5
1138.8 50.8 33.7 84.5
1139.6 52.9 36.3 89.2
1140.4 54.9 38.8 93.7
1141.2 56.8 41.1 97.9
1142.0 58.7 43.2 101.9
1142.8 60.5 45.3 105.8
1143.6 62.3 47.3 109.5
1144.4 64.0 49.2 113.2
0.0
20.0
40.0
60.0
80.0
100.0
120.0
1130.0 1132.0 1134.0 1136.0 1138.0 1140.0 1142.0 1144.0 1146.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)