SURRY-027 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

LOW GAP WILDLIFE CLUB DAM

SURRY-027

SURRY COUNTY, NC

PREPARED BY:
WOOD E&I Solutions

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Low Gap Wildlife Club 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, 50 years, 100 years rainfall events. The structure is over-topped by the 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 =
153.1
162.5
NOT TO SCALE
72.66
32
SCS Type 2
1575.0AuxiliaryGrass-Lined Embankment
Principal Concrete Trapezoidal Channel 1570.5
99.8
108.2
116.9
125.7
134.7
143.8
1577.5
1578.0
1578.5
1579.0
1579.5
1580.0
45.5
52.7
60.1
67.6
ELEVATION
1575.0
1575.5
1576.0
1576.5
1577.0
1574.0
1574.5
STORAGE (ACRE-FT)
0.0
6.0
12.1
18.5
25.0
31.7
38.5
1571.0
1571.5
1572.0
1572.5
1573.0
1573.5
ELEVATION
1569.5
1570.0
1570.5
STORAGE (ACRE-FT)
75.4
83.3
91.4
SPILLWAY TYPE ELEVATION
TOP OF DAM ELEVATION = 1575
24 HOUR - 100 YEAR EVENT, 7.52 INCHES, ELEVATION 1574.8 (0.2 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 6.67 INCHES, ELEVATION 1574.3 (0.7 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 5.86 INCHES, ELEVATION 1573.8 (1.2 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
1.10
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LOW GAP WILDLIFE CLUB DAM, SURRY-027
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
24 HOUR - 1000 YEAR EVENT, 10.7 INCHES, ELEVATION 1576.1 (1.1 FT OVER-TOPPED)
24 HOUR - 500 YEAR EVENT, 9.69 INCHES, ELEVATION 1575.8 (0.8 FT OVER-TOPPED)
24 HOUR - 200 YEAR EVENT, 8.42 INCHES, ELEVATION 1575.3 (0.3 FT OVER-TOPPED)
SURRY-027LOW GAP WILDLIFE CLUB 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
Low Gap Wildlife Club Dam - SURRY-027 - Watershed Boundary
Legend
Longest FlowPath
Watershed Boundary
0 0.25 0.50.125 Miles
Date: 8/13/2020
/Drainage Area: 1.10 Square Miles
Flowpath Length: 2.29 Miles

APPENDIX B
Lag Time Calculation

Dam No: SURRY-027 Date: 8/3/2020
Dam Name: Low Gap Wildlife Club 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 = 72.66
S =3.76273 in.
Mean Basin Slope Y = 28.82 %
Longest Flowpath: 2.29629 mi.
Flow Length l = 12124.4 ft.
Time of Concentration Tc =0.90 Hours Tc =54 mins
Lag Time TL =0.54 Hours TL =32 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 0 100 0 364249 100 36424912 33019 100 3301865
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 69315 69 4782740 2267713 79 179149327 877200 84 73684810
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 0 72 0 90516 81 7331817 61103 86 5254849
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 0 85 0 1318 90 118590 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 230476 36 8297150 1966873 60 118012350 15603856 73 1139081464 616462 79 48700486
Evergreen Forest Woods - Fair 0 36 0 133566 60 8013946 426346 73 31123289 0 79 0
Mixed Forest Woods - Fair 0 36 0 345208 60 20712486 4412747 73 322130501 131071 79 10354582
Shrub/Scrub Brush - Fair 108472 35 3796537 655509 56 36708529 1236633 70 86564331 107149 77 8250469
Herbaceuous Herbaceuous - Fair 0 60 0 0 71 0 869922 81 70463703 19095 89 1699491
Hay/Pasture Pasture - Fair 0 49 0 0 69 0 0 79 0 48447 84 4069588
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 4102 90 369154 34656 95 3292339
Emergent Herbaceuous Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0 Weighted CN=72.66
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
0.02.04.06.08.010.012.00123456789101112131415161718192021222324Cumulative 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): Lowgap, North Carolina USA
Station Name: ‐
Latitude: 36.4855°
Longitude: ‐80.8993°
Elevation (USGS): None None
PRECIPITATION FREQUENCY ESTIMATES
by 
duration 
for ARI 
(years):1 2 5 10 25 50 100 200 500 1000
5‐min: 0.334 0.397 0.473 0.532 0.609 0.668 0.728 0.788 0.869 0.935
10‐min: 0.534 0.635 0.757 0.85 0.97 1.06 1.16 1.25 1.38 1.47
15‐min: 0.667 0.799 0.958 1.08 1.23 1.35 1.46 1.58 1.73 1.85
30‐min: 0.914 1.1 1.36 1.56 1.82 2.03 2.24 2.45 2.75 2.99
60‐min: 1.14 1.38 1.75 2.03 2.43 2.75 3.08 3.44 3.95 4.37
2‐hr: 1.35 1.64 2.1 2.46 2.98 3.41 3.87 4.36 5.08 5.68
3‐hr: 1.46 1.78 2.26 2.65 3.21 3.68 4.18 4.72 5.5 6.17
6‐hr: 1.82 2.21 2.8 3.27 3.93 4.48 5.06 5.68 6.56 7.29
12‐hr: 2.28 2.76 3.44 3.98 4.72 5.32 5.93 6.57 7.46 8.19
24‐hr: 2.71 3.29 4.17 4.87 5.86 6.67 7.52 8.42 9.69 10.7
2‐day: 3.24 3.91 4.92 5.73 6.86 7.78 8.75 9.77 11.2 12.4
3‐day: 3.45 4.16 5.21 6.04 7.19 8.12 9.1 10.1 11.5 12.7
4‐day: 3.66 4.41 5.49 6.34 7.52 8.47 9.45 10.5 11.9 13.1
7‐day: 4.21 5.04 6.16 7.03 8.24 9.19 10.2 11.2 12.5 13.6
10‐day: 4.85 5.78 6.96 7.88 9.11 10.1 11 12 13.3 14.4
20‐day: 6.53 7.73 9.16 10.3 11.8 12.9 14.1 15.3 16.8 18
30‐day: 8.11 9.54 11.1 12.2 13.7 14.8 15.9 17 18.4 19.4
45‐day: 10.3 12 13.7 14.9 16.5 17.7 18.9 20 21.5 22.5
60‐day: 12.2 14.3 16.1 17.5 19.3 20.6 21.9 23.1 24.6 25.8
Date/time (GMT):  Mon Jun 22 18:10:09 2020
pyRunTime:  0.0157248973846

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NCEM Date: 9/11/2020
Project: Overtopping Study Engineer: E.Okorie
Dam: Low Gap wildlife Club Dam Dam No. SURRY-027
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1570.5 0.0 0.0 0.0
1571.0 37.4 0.0 37.4
1571.5 110.8 0.0 110.8
1572.0 212.7 0.0 212.7
1572.5 341.7 0.0 341.7
1573.0 497.3 0.0 497.3
1573.5 679.7 0.0 679.7
1574.0 889.2 0.0 889.2
1574.5 1126.4 0.0 1126.4
1575.0 1391.8 0.0 1391.8
1575.5 1686.0 0.0 1686.0
1576.0 2009.6 0.0 2009.6
1576.5 2363.3 0.0 2363.3
1577.0 2747.6 0.0 2747.6
1577.5 3163.3 0.0 3163.3
1578.0 3610.9 0.0 3610.9
1578.5 4091.0 0.0 4091.0
1579.0 4604.4 0.0 4604.4
1579.5 5151.6 0.0 5151.6
1580.0 5733.2 0.0 5733.2
1580.5 6349.9 0.0 6349.9
1581.0 7002.1 0.0 7002.1 0.0
1000.0
2000.0
3000.0
4000.0
5000.0
6000.0
7000.0
8000.0
1568.0 1570.0 1572.0 1574.0 1576.0 1578.0 1580.0 1582.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)