WAYNE-012_Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

WILLS POND DAM

WAYNE-012

WAYNE COUNTY, NC

PREPARED BY:
GEORGE ELLER, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Wills Pond Dam in Wayne 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 hydraulically adequate for all of the above rainfall events.
The dam maintained a freeboard of 2.8 – 6.0 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 =
5,586.9124.0110.0 358.6
Concrete Circular Riser / Barrel
Emergency 1 Vegetated Trapazoidal Channel
STORAGE (ACRE-FT)
2,340.2
3,523.1
0.2
ELEVATION
99.2
STORAGE (ACRE-FT)
540.4
776.5
24 HOUR - 100 YEAR EVENT, 9.97 INCHES, MAX W.S. ELEVATION 114.5 (3.7 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 8.52 INCHES, MAX W.S. ELEVATION 113.6 (4.6 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 7.24 INCHES, MAX W.S. ELEVATION 112.2 (6.0 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
2.98
78.71
192
NOT TO SCALE
120.0
1,619.5118.0
102.0
104.0
106.0
108.0
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
WILLS POND DAM, WAYNE-012
215.0
100.9
114.1116.0
122.0
15.3
SPILLWAY
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 16.30 INCHES, MAX W.S. ELEVATION 115.4 (2.8 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 14.10 INCHES, MAX W.S. ELEVATION 115.1 (3.1 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 11.60 INCHES, MAX W.S. ELEVATION 114.8 (3.4 FT FREEBOARD)
TOP OF DAM ELEVATION = 118.2 FT NAVD88
98.0
100.0
ELEVATION
112.0
114.0
0.0
0.1
114.2
Principal
Emergency 2 Vegetated Trapazoidal Channel 114.2
WILLS POND DAM WAYNE-012 June 22, 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
Wills Pond Dam - WAYNE-012 Watershed Boundary
.Drainage Area: 2.98 Square MilesFlowpath Length: 19,123 Feet
Legend
Watershed Boundary
Longest Flowpath
0 0.5 10.25 Miles

APPENDIX B 
Time of Concentration Calculation 

Dam No:WAYNE-012 Date:8/22/2018
Dam Name:Wills Pond Engineer:G. Eller
Modeler:Sue White
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 =78.71 from StreamStats
S =2.70487 in.
Mean Basin Slope Y =1.19 %from StreamStats
Longest Flowpath:3.611 mi.from StreamStats
Flow Length l =19066.1 ft.
Time of Concentration Tc =5.34 Hours Tc =320 mins
Lag Time TL =3.20 Hours TL =192 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 1295918.5798 100 129591858 22616.6608 100 2261666.083 0.0000 100 0 243106.6750 100 24310667
Developed, Open Space 11445072.6017 68 778264936.9 1929620.8080 79 152440043.8 0.0000 86 0 8496960.2543 89 7.56E+08
Developed, Low Intensity 5970904.7122 57 340341568.6 825573.8263 72 59441315.49 0.0000 81 0 3552540.6051 86 3.06E+08
Developed, Medium Intensity 1263266.0198 77 97271483.53 32082.8727 85 2727044.184 0.0000 90 0 304194.0779 92 27985855
Developed, High Intensity 565848.5300 89 50360519.17 7410.3679 92 681753.8467 0.0000 94 0 0.0000 95 0
Barren Land 0.0000 77 0 31297.8841 86 2691618.032 0.0000 91 0 15141.2982 94 1423282
Deciduous Forest 0 36 0 0 60 0 0 73 0 507872.1093 79 40121897
Evergreen Forest 890591.9476 36 32061310.11 162786.5540 60 9767193.242 0.0000 73 0 4370151.5464 79 3.45E+08
Mixed Forest 38013.7467 36 1368494.883 25959.6441 60 1557578.644 0.0000 73 0 1626290.3047 79 1.28E+08
Shrub/Scrub 2373644.1110 35 83077543.88 178934.6200 56 10020338.72 0.0000 70 0 5466685.0149 77 4.21E+08
Herbaceuous 903292.0956 50 45164604.78 118602.5159 71 8420778.629 0.0000 81 0 2947170.4883 89 2.62E+08
Hay/Pasture 0 49 0 155722.0439 69 10744821.03 0 79 0 4233943.281 84 3.56E+08
Cultivated Crops 2251905.9452 71 159885322.1 676874.7326 80 54149978.61 0.0000 87 0 11068096.6313 90 9.96E+08 designed by dan.k
Woody Wetlands 1021762.2932 80 81740983.46 289861.0258 85 24638187.19 0.0000 90 0 7470628.6615 95 7.1E+08
Emergent Herbaceuous Wetlands 44290.9064 80 3543272.511 12054.1917 85 1024606.297 0.0000 90 0 150510.0311 95 14298453 Weighted CN=78.71
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
18.0
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): Goldsboro, NC, USA
Station Name: -
Latitude: 35.3880°
Longitude: -77.8930°
Elevation (USGS): 111.86 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.445 0.519 0.593 0.669 0.75 0.817 0.879 0.94 1.01 1.08
10-min:0.71 0.831 0.95 1.07 1.2 1.3 1.4 1.49 1.6 1.7
15-min:0.888 1.04 1.2 1.35 1.52 1.65 1.77 1.88 2.02 2.14
30-min:1.22 1.44 1.71 1.96 2.24 2.48 2.71 2.93 3.21 3.46
60-min:1.52 1.81 2.19 2.55 2.99 3.36 3.73 4.1 4.6 5.05
2-hr:1.79 2.14 2.65 3.14 3.76 4.33 4.89 5.51 6.36 7.14
3-hr:1.9 2.29 2.83 3.39 4.11 4.78 5.47 6.22 7.3 8.31
6-hr:2.28 2.74 3.4 4.07 4.95 5.77 6.63 7.57 8.93 10.2
12-hr:2.67 3.21 4 4.82 5.91 6.93 8.01 9.22 11 12.6
24-hr:3.08 3.74 4.83 5.79 7.24 8.52 9.97 11.6 14.1 16.3
2-day:3.57 4.32 5.53 6.59 8.18 9.57 11.1 12.9 15.5 17.8
3-day:3.8 4.59 5.84 6.92 8.53 9.92 11.5 13.2 15.8 18
4-day:4.03 4.86 6.16 7.25 8.88 10.3 11.8 13.5 16 18.2
7-day:4.73 5.7 7.16 8.37 10.1 11.6 13.2 14.9 17.4 19.5
10-day:5.41 6.48 8.02 9.3 11.1 12.7 14.3 16.1 18.6 20.8
20-day:7.33 8.74 10.6 12.2 14.4 16.2 18.1 20.1 23 25.3
30-day:9.14 10.8 13 14.8 17.1 19.1 21 23.1 25.9 28.1
45-day:11.5 13.6 16.1 18.2 20.9 23.1 25.4 27.7 30.8 33.3
60-day:13.9 16.3 19.1 21.3 24.2 26.5 28.8 31 34.1 36.4
Date/time (GMT): Tue Apr 17 17:13:40 2018
pyRunTime: 0.0397038459778

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:Concrete Circular Riser / Barrel
Emergency Spillway 1:Vegetated Trapazoidal Channel
Emergency Spillway 2:Vegetated Trapazoidal Channel
Top of Dam :118.2 ft NAVD88
Normal Pool :99.2 ft NAVD88
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
99.2 0 0 0 0
99.7 14 0 0 14
100.2 28 0 0 28
100.7 38 0 0 38
101.2 49 0 0 49
101.7 56 0 0 56
102.2 63 0 0 63
102.7 69 0 0 69
103.2 74 0 0 74
103.7 79 0 0 79
104.2 84 0 0 84
104.7 89 0 0 89
105.2 93 0 0 93
105.7 97 0 0 97
106.2 101 0 0 101
106.7 105 0 0 105
107.2 109 0 0 109
107.7 112 0 0 112
108.2 116 0 0 116
108.7 119 0 0 119
109.2 122 0 0 122
109.7 126 0 0 126
110.2 129 0 0 129
110.7 132 0 0 132
111.2 135 0 0 135
111.7 138 0 0 138
112.2 140 0 0 140
112.7 143 0 0 143
113.2 146 0 0 146
113.7 149 0 0 149
114.2 151 0 0 151
114.7 154 88 111 353
115.2 156 251 319 727
115.7 159 464 595 1,218
116.2 161 718 929 1,809
116.7 164 1,010 1,317 2,491
117.2 166 1,335 1,756 3,257
117.7 168 1,693 2,243 4,104
118.2 171 2,080 2,778 5,028
118.7 173 2,496 3,359 6,028
119.2 175 2,940 3,986 7,102
119.7 178 3,412 4,658 8,248
120.2 180 3,909 5,376 9,465
120.7 182 4,433 6,139 10,754
121.2 184 4,982 6,947 12,113
121.7 186 5,556 7,800 13,542
122.2 188 6,155 8,698 15,041
122.7 190 6,778 9,641 16,609
123.2 192 7,425 10,630 18,247
SPILLWAY ASSESSMENT
Wills Pond Dam, WAYNE-012
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 16,000 18,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway:Emergency Spillway 1:Emergency Spillway 2:Total Structure