WILSO-007_Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

LAKE WILSON DAM

WILSO-007

WILSON COUNTY, NC

PREPARED BY:
GEORGE ELLER, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Lake Wilson Dam in Wilson County, North
Carolina. The purpose of the study was to analyze the existing potential of the dams 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 ArcGIS 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 not hydraulically adequate for all of the above rainfall
events. The dam was overtopped during the simulations by 1.8 – 4.3 feet during these 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. The elevation-storage calculation does not include the storage below the normal pool
elevation for this study. All elevations in this report are based on North American Vertical Datum
of 1988 (NAVD88).

Drainage Area (Sq. Mi.)=
Curve Number =
Lag Time (min.)=
Rainfall Distribution Methods =
2,230.9125.0117.0 742.9
Concrete Ogee Spillway
Auxiliary N/A
STORAGE (ACRE-FT)
1,735.8
1,972.4
323.2
ELEVATION
113.9
STORAGE (ACRE-FT)
998.0
1,150.4
24 HOUR - 100 YEAR EVENT, 8.78 INCHES, MAX W.S. ELEVATION 121.6 (2.7 FT OVERTOPPING)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 7.58 INCHES, MAX W.S. ELEVATION 121.2 (2.3 FT OVERTOPPING)
24 HOUR - 25 YEAR EVENT, 6.49 INCHES, MAX W.S. ELEVATION 120.7 (1.8 FT OVERTOPPING)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
39.93
77.38
500
NOT TO SCALE
123.0
1,519.7122.0
113.0
114.0
115.0
116.0
118.0 861.8 126.0 2,491.0
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LAKE WILSON DAM, WILSO-007
640.9
511.2
1,324.5121.0
124.0
407.2
SPILLWAY
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 13.90 INCHES, MAX W.S. ELEVATION 123.2 (4.3 FT OVERTOPPING)
24 HOUR - 500 YEAR EVENT, 12.20 INCHES, MAX W.S. ELEVATION 122.7 (3.8 FT OVERTOPPING)
24 HOUR - 200 YEAR EVENT, 10.10 INCHES, MAX W.S. ELEVATION 122.1 (3.2 FT OVERTOPPING)
TOP OF DAM ELEVATION = 118.9 FT NAVD88
111.0
112.0
N/A
Principal
ELEVATION
119.0
120.0
170.0
240.8
LAKE WILSON DAM WILSO-007 June 19, 2020

APPENDIX A 
Watershed Boundary 

Sources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO,USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, OrdnanceSurvey, Esri Japan, METI, Esri China (Hong Kong), (c) OpenStreetMapcontributors, and the GIS User Community
Lake WIlson Dam - WILSO-007 Watershed Boundary
.Drainage Area: 39.93 Square MilesFlowpath Length: 94,105 Feet
Legend
Watershed Boundary
Longest Flowpath
0 2.5 51.25 Miles

APPENDIX B 
Time of Concentration Calculation 

Dam No:WILSO-007 Date:11/13/2018
Dam Name:Lake Wilson Dam Engineer:G. Eller
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 =77.38 from StreamStats
S =2.92324 in.
Mean Basin Slope Y =2.45 %from StreamStats
Longest Flowpath:17.823 mi.from StreamStats
Flow Length l =94105.4 ft.
Time of Concentration Tc =13.90 Hours Tc =834 mins
Lag Time TL =8.34 Hours TL =500 mins

APPENDIX C 
NRCS Curve Number Summary Sheet 

Change CN according to TR-55 (see tables below)
HSG
Type A CN B CN C CN D CN
Open Water 353261.0091 100 35326100.91 560212.2405 100 56021224.05 46780.6994 100 4678069.939 3061603.8975 100 3.06E+08
Developed, Open Space 27040976.0693 68 1838786373 25561163.5599 79 2019331921 4361910.9098 86 375124338.2 14042327.2389 89 1.25E+09
Developed, Low Intensity 7985105.3381 57 455151004.3 7919607.5901 72 570211746.5 1960318.7639 81 158785819.9 4563444.8745 86 3.92E+08
Developed, Medium Intensity 725131.3021 77 55835110.26 599052.9396 85 50919499.87 551896.8116 90 49670713.05 201687.2594 92 18555228
Developed, High Intensity 23986.5488 89 2134802.842 50941.2612 92 4686596.035 129993.8739 94 12219424.14 2560.1438 95 243213.7
Barren Land 0.0000 77 0 0.0000 86 0 0.0000 91 0 0.0000 94 0
Deciduous Forest 18609499.46 36 669941980.7 16892792.11 60 1013567526 2673106.17 73 195136750.4 44491411.86 79 3.51E+09
Evergreen Forest 14655487.7584 36 527597559.3 14292186.0577 60 857531163.5 2232056.7878 73 162940145.5 40749978.9626 79 3.22E+09
Mixed Forest 2836871.7892 36 102127384.4 4075810.6824 60 244548640.9 599629.5128 73 43772954.43 11014703.3099 79 8.7E+08
Shrub/Scrub 3301851.4794 35 115564801.8 3416211.4660 56 191307842.1 703884.9039 70 49271943.27 10589083.2189 77 8.15E+08
Herbaceuous 16019485.0224 50 800974251.1 15468817.9967 71 1098286078 3246438.3782 81 262961508.6 27806923.1644 89 2.47E+09
Hay/Pasture 10361415.78 49 507709373 10774759.52 69 743458407.1 2536833.757 79 200409866.8 8623258.238 84 7.24E+08
Cultivated Crops 239297594.3036 71 16990129196 216672093.5217 80 17333767482 18484835.8843 87 1608180722 81111209.8268 90 7.3E+09 designed by dan.k
Woody Wetlands 10300271.8493 80 824021747.9 8065396.2545 85 685558681.6 3112182.4593 90 280096421.3 ############95 1.23E+10
Emergent Herbaceuous Wetlands 1360366.6003 80 108829328 519193.8264 85 44131475.24 381469.7233 90 34332275.1 8021005.4891 95 7.62E+08 Weighted CN=77.39
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. Hydrologic Condition assumed as "Fair"
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 

0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0 2 4 6 8 10 12 14 16 18 20 22 24Cumulative Inches of RainHours
Hyetographs for 24-hour 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): Wilson, NC, USA
Station Name: -
Latitude: 35.7889°
Longitude: -77.9206°
Elevation (USGS): 105.72 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.41 0.473 0.535 0.611 0.686 0.753 0.814 0.872 0.941 1.01
10-min:0.654 0.756 0.856 0.977 1.09 1.2 1.29 1.38 1.49 1.59
15-min:0.818 0.951 1.08 1.24 1.39 1.52 1.64 1.75 1.87 1.99
30-min:1.12 1.31 1.54 1.79 2.05 2.29 2.5 2.72 2.98 3.23
60-min:1.4 1.65 1.97 2.33 2.74 3.1 3.45 3.81 4.28 4.71
2-hr:1.63 1.92 2.34 2.82 3.4 3.94 4.48 5.07 5.87 6.64
3-hr:1.72 2.04 2.49 3.03 3.68 4.32 4.96 5.67 6.66 7.63
6-hr:2.05 2.44 2.98 3.62 4.42 5.2 6 6.88 8.13 9.35
12-hr:2.41 2.86 3.51 4.28 5.26 6.23 7.24 8.37 9.98 11.6
24-hr:2.86 3.47 4.47 5.35 6.68 7.84 9.15 10.6 12.9 14.9
2-day:3.3 3.99 5.11 6.07 7.51 8.77 10.2 11.7 14.1 16.2
3-day:3.52 4.24 5.39 6.38 7.84 9.1 10.5 12 14.4 16.4
4-day:3.73 4.5 5.68 6.69 8.17 9.44 10.8 12.3 14.6 16.5
7-day:4.36 5.24 6.57 7.67 9.27 10.6 12 13.6 15.9 17.7
10-day:4.98 5.97 7.37 8.53 10.2 11.6 13.1 14.7 16.9 18.8
20-day:6.71 8 9.72 11.1 13.1 14.8 16.5 18.3 20.8 22.9
30-day:8.34 9.89 11.8 13.4 15.6 17.3 19 20.8 23.3 25.3
45-day:10.5 12.4 14.7 16.5 18.9 20.9 22.9 24.9 27.6 29.8
60-day:12.6 14.9 17.4 19.3 21.9 24 26 28 30.6 32.6
Date/time (GMT): Tue Jun 23 19:16:33 2020
pyRunTime: 0.0111150741577

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:Concrete Ogee Spillway
Emergency Spillway:N/A
Top of Dam :118.9 ft NAVD88
Normal Pool :113.9 ft NAVD88
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
113.9 0 0 0
114.4 85 0 85
114.9 241 0 241
115.4 440 0 440
115.9 674 0 674
116.4 938 0 938
116.9 1,227 0 1,227
117.4 1,539 0 1,539
117.9 1,763 0 1,763
118.4 1,968 0 1,968
118.9 2,146 0 2,146
119.4 2,394 0 2,394
119.9 2,702 0 2,702
120.4 3,046 0 3,046
120.9 3,419 0 3,419
121.4 3,818 0 3,818
121.9 4,239 0 4,239
122.4 4,682 0 4,682
122.9 5,144 0 5,144
123.4 5,625 0 5,625
123.9 6,123 0 6,123
SPILLWAY ASSESSMENT
Lake Wilson Dam, WILSO-007
113
114
115
116
117
118
119
120
121
122
123
124
125
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway:Emergency Spillway:Total Structure