WILSO-009 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

SILVER LAKE DAM

WILSO-009

WILSON COUNTY, NC

PREPARED BY:
TYLER MCEWEN, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Silver Lake 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 the above rainfall events.
The dam was overtopped during this event by 1.0 – 3.1 feet. 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 =
Principal Broad-crested weir 131.4
135.0 577.6
132.9
2,561.4
3,106.2
6.4
ELEVATION
7 x 42-inch pipes 124.9
STORAGE (ACRE-FT)
885.3
1,228.8
ELEVATION
136.0
137.0
0.0
4.7
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
SILVER LAKE DAM, WILSO-009
300.2
74.1
1,312.8138.0
141.0
8.5
SPILLWAY
ANALYSIS PERFORMED BY: TYLER MCEWEN, PE, DAM SAFETY
24 HOUR - 1000 YEAR EVENT, 14.6 INCHES, MAX W.S. ELEVATION 138.7 (3.1 FT OVERTOPPED)
24 HOUR - 500 YEAR EVENT, 12.7 INCHES, MAX W.S. ELEVATION 138.3 (2.7 FT OVERTOPPED)
24 HOUR - 200 YEAR EVENT, 10.5 INCHES, MAX W.S. ELEVATION 137.8 (2.2 FT OVERTOPPED)
TOP OF DAM ELEVATION = 135.6 FT NAVD88
124.0
130.0
Principal
NOT TO SCALE
140.0
2,063.4139.0
131.0
132.0
133.0
134.0
Emergency Grass-Lined Trapezoidal Channel
STORAGE (ACRE-FT)
24 HOUR - 100 YEAR EVENT, 9.05 INCHES, MAX W.S. ELEVATION 137.4 (1.8 FT OVERTOPPED)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 7.76 INCHES, MAX W.S. ELEVATION 137.0 (1.4 FT OVERTOPPED)
24 HOUR - 25 YEAR EVENT, 6.62 INCHES, MAX W.S. ELEVATION 136.6 (1.0 FT OVERTOPPED)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
35.3
77.14
447
SILVER LAKE DAM WILSO-009 May 11, 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
Silver Lake Dam - WILSO-009 - Watershed Boundary
.Drainage Area: 35.3 Square MilesFlowpath Length: 81,470 Feet
Legend
Watershed Boundary
Longest Flowpath
0 2 41 Miles

APPENDIX B 
Time of Concentration Calculation 

Dam No:WILSO-009 Date:4/23/2020
Dam Name:Silver Lake Dam Engineer:TJM
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 =77.14 from StreamStats
S =2.96344 in.
Mean Basin Slope Y =2.47 %from StreamStats
Longest Flowpath:15.43 mi.from StreamStats
Flow Length l =81470.4 ft.
Time of Concentration Tc =12.42 Hours Tc =745 mins
Lag Time TL =7.45 Hours TL =447 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 330,274 100 33,027,393 527,131 100 52,713,139 26,133 100 2,613,348 2,668,345 100 2.67E+08
Developed, Open Space 23,375,851 68 1,589,557,869 22,839,792 79 1,804,343,546 3,108,621 86 267,341,372 10,285,810 89 9.15E+08
Developed, Low Intensity 5,635,830 57 321,242,327 6,990,368 72 503,306,515 999,958 81 80,996,561 2,865,177 86 2.46E+08
Developed, Medium Intensity 370,795 77 28,551,212 515,192 85 43,791,304 506,003 90 45,540,234 74,042 92 6811832
Developed, High Intensity 23,987 89 2,134,803 33,519 92 3,083,730 129,994 94 12,219,424 0 95 0
Barren Land 0 77 0 0 86 0 0 91 0 0 94 0
Deciduous Forest 18,356,879 36 660,847,638 15,918,470 60 955,108,211 2,576,903 73 188,113,919 42,768,834 79 3.38E+09
Evergreen Forest 12,801,496 36 460,853,859 13,065,942 60 783,956,493 1,991,309 73 145,365,524 33,109,231 79 2.62E+09
Mixed Forest 2,748,685 36 98,952,670 3,917,370 60 235,042,224 586,193 73 42,792,114 10,487,601 79 8.29E+08
Shrub/Scrub 2,718,045 35 95,131,584 2,609,459 56 146,129,727 612,696 70 42,888,695 7,405,451 77 5.7E+08
Herbaceuous 14,616,108 50 730,805,424 13,494,069 71 958,078,918 3,168,285 81 256,631,111 24,830,340 89 2.21E+09
Hay/Pasture 9,162,809 49 448,977,627 6,467,369 69 446,248,437 1,572,128 79 124,198,076 6,364,581 84 5.35E+08
Cultivated Crops 221,532,685 71 15,728,820,631 199,732,833 80 15,978,626,615 16,482,842 87 1,434,007,224 69,881,009 90 6.29E+09 designed by dan.k
Woody Wetlands 9,228,958 80 738,316,677 5,648,347 85 480,109,525 1,769,427 90 159,248,461 111,961,428 95 1.06E+10
Emergent Herbaceuous Wetlands 1,293,325 80 103,466,018 215,521 85 18,319,327 381,470 90 34,332,275 5,863,303 95 5.57E+08 Weighted CN=77.14
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 Distrubutions
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: 35.8021°
Longitude: -77.9494°
Elevation (USGS): 122.45 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.407 0.469 0.528 0.604 0.679 0.747 0.807 0.865 0.931 0.999
10-min:0.651 0.75 0.845 0.967 1.08 1.19 1.28 1.37 1.47 1.57
15-min:0.813 0.942 1.07 1.22 1.37 1.51 1.62 1.73 1.85 1.98
30-min:1.12 1.3 1.52 1.77 2.03 2.27 2.48 2.69 2.95 3.2
60-min:1.39 1.63 1.95 2.31 2.71 3.07 3.42 3.78 4.23 4.67
2-hr:1.62 1.91 2.31 2.79 3.36 3.9 4.43 5.01 5.79 6.54
3-hr:1.71 2.02 2.46 2.99 3.63 4.26 4.9 5.59 6.56 7.51
6-hr:2.04 2.42 2.94 3.58 4.36 5.14 5.92 6.79 8 9.21
12-hr:2.4 2.83 3.46 4.24 5.2 6.16 7.15 8.26 9.83 11.4
24-hr:2.84 3.45 4.45 5.31 6.62 7.76 9.05 10.5 12.7 14.6
2-day:3.28 3.97 5.08 6.03 7.45 8.69 10.1 11.6 13.9 15.9
3-day:3.5 4.22 5.36 6.33 7.78 9.02 10.4 11.9 14.2 16.1
4-day:3.71 4.47 5.65 6.64 8.1 9.35 10.7 12.2 14.4 16.3
7-day:4.33 5.21 6.53 7.62 9.19 10.5 11.9 13.4 15.7 17.5
10-day:4.95 5.93 7.32 8.47 10.1 11.5 12.9 14.5 16.7 18.5
20-day:6.67 7.94 9.64 11 13 14.6 16.3 18.1 20.6 22.6
30-day:8.29 9.83 11.8 13.3 15.4 17.1 18.8 20.6 23 24.9
45-day:10.5 12.3 14.6 16.3 18.8 20.7 22.6 24.6 27.3 29.4
60-day:12.6 14.8 17.3 19.2 21.7 23.7 25.7 27.6 30.2 32.2
Date/time (GMT): Wed May 13 17:45:36 2020
pyRunTime: 0.0103650093079

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:
Emergency Spillway:60 ft width Trapazoidal Vegetated Channel
Top of Dam :135.6
Normal Pool :131.4
RWS Elevation
(ft NAVD88)
Primary Spillway Pipes
Discharge (cfs)
Primary Spillway Weir
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
124.5 0 0 0 0
125.0 13 0 0 13
125.5 83 0 0 83
126.0 163 0 0 163
126.5 245 0 0 245
127.0 326 0 0 326
127.5 403 0 0 403
128.0 491 0 0 491
128.5 615 0 0 615
129.0 656 0 0 656
129.5 695 0 0 695
130.0 732 0 0 732
130.5 767 0 0 767
131.0 801 0 0 801
131.5 833 13 0 846
132.0 864 83 0 947
132.5 894 163 0 1057
133.0 923 245 14 1182
133.5 951 326 96 1373
134.0 978 403 248 1630
134.5 1005 491 460 1955
135.0 1031 615 732 2377
135.5 1056 656 1065 2777
136.0 1080 695 1461 3237
136.5 1104 732 1923 3760
137.0 1128 767 2453 4348
137.5 1151 801 3052 5004
138.0 1174 833 3724 5730
138.5 1196 864 4469 6529
139.0 1218 894 5291 7403
139.5 1239 923 6192 8354
140.0 1260 951 7173 9384
140.5 1281 978 8237 10496
141.0 1302 1005 9385 11692
Silver Lake Dam, WILSO-009
7 x 42-inch pipes w/invert at 124.9 ft NAVD88 and broad-crested weir w/
7 support columns (2' W x 3' H) at 131.4.
ft NAVD88
ft NAVD88
SPILLWAY ASSESSMENT
124
126
128
130
132
134
136
138
140
142
0 2,000 4,000 6,000 8,000 10,000 12,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway Pipes Primary Spillway Weir Emergency Spillway Total Structure