WAKE-176_Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

WAKE FOREST WATER SUPPLY DAM

WAKE-176

WAKE COUNTY, NC

PREPARED BY:
GEORGE ELLER, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Wake Forest Water Supply Dam in Wake
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 2.6 – 5.0 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 =
N/A
Principal
ELEVATION
302.0
303.0
0.0
27.1
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
WAKE FOREST WATER SUPPLY DAM DAM, WAKE-176
271.0
204.4
579.6304.0
307.0
141.8
SPILLWAY
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 10.20 INCHES, MAX W.S. ELEVATION 300.6 (2.6 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 9.332 INCHES, MAX W.S. ELEVATION 300.1 (3.1 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 8.26 INCHES, MAX W.S. ELEVATION 299.6 (3.6 FT FREEBOARD)
TOP OF DAM ELEVATION = 303.2 FT NAVD88
295.0
296.0
NOT TO SCALE
306.0
667.5305.0
297.0
298.0
299.0
300.0
24 HOUR - 100 YEAR EVENT, 7.47 INCHES, MAX W.S. ELEVATION 299.1 (4.1 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 6.71 INCHES, MAX W.S. ELEVATION 298.7 (4.5 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 5.96 INCHES, MAX W.S. ELEVATION 298.2 (5.0 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
3.29
61.56
139
958.9308.0301.0 341.9
Concrete-lined Rectangular Channel
Auxiliary N/A
STORAGE (ACRE-FT)
759.8
856.7
82.7
ELEVATION
296.2
STORAGE (ACRE-FT)
41,730.0
496.1
WAKE FOREST WATER SUPPLY DAM WAKE-176 June 14, 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
Wake Forest Water Supply Dam - WAKE-176 Watershed Boundary
.Drainage Area: 3.29 Square MilesFlowpath Length: 22,107 Feet
Legend
Watershed Boundary
Longest Flowpath
0 0.75 1.50.375 Miles

APPENDIX B 
Time of Concentration Calculation 

Dam No:WAKE-176 Date:11/29/2018
Dam Name:Wake Forest Water Supply 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 =61.56 from StreamStats
S =6.24431 in.
Mean Basin Slope Y =7.36 %from StreamStats
Longest Flowpath:4.187 mi.from StreamStats
Flow Length l =22107.4 ft.
Time of Concentration Tc =3.87 Hours Tc =232 mins
Lag Time TL =2.32 Hours TL =139 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 82337.3974 100 8233739.736 209263.4505 100 20926345.05 0.0000 100 0 1743632.1085 100 1.74E+08
Developed, Open Space 1764497.1776 68 119985808.1 23329859.6353 79 1843058911 838459.2606 86 72107496.41 1783409.2504 89 1.59E+08
Developed, Low Intensity 615770.3841 57 35098911.9 11319193.9350 72 814981963.3 269801.6417 81 21853932.98 641463.8663 86 55165893
Developed, Medium Intensity 338142.6460 77 26036983.74 6397321.5418 85 543772331 113243.1069 90 10191879.62 410957.5537 92 37808095
Developed, High Intensity 19984.8028 89 1778647.449 927731.1433 92 85351265.18 4277.1648 94 402053.4883 84982.1218 95 8073302
Barren Land 0.0000 77 0 2467.0620 86 212167.3362 0.0000 91 0 0.0000 94 0
Deciduous Forest 2300406.649 36 82814639.36 18252404.82 60 1095144289 2073600.274 73 151372820 5863371.348 79 4.63E+08
Evergreen Forest 1697777.3234 36 61119983.64 20665765.2907 60 1239945917 2116145.4167 73 154478615.4 2794131.0237 79 2.21E+08
Mixed Forest 390413.2418 36 14054876.71 3952140.9095 60 237128454.6 406637.5402 73 29684540.44 1042707.9470 79 82373928
Shrub/Scrub 0.0000 35 0 333684.6319 56 18686339.39 14104.9731 70 987348.1193 33065.3454 77 2546032
Herbaceuous 201804.8190 50 10090240.95 1032259.2347 71 73290405.67 166035.9854 81 13448914.82 209952.2854 89 18685753
Hay/Pasture 262887.4194 49 12881483.55 8600474.764 69 593432758.7 215807.568 79 17048797.87 271527.6853 84 22808326
Cultivated Crops 0.0000 71 0 0.0000 80 0 0.0000 87 0 0.0000 90 0 designed by dan.k
Woody Wetlands 0.0000 80 0 21807.9720 85 1853677.623 0.0000 90 0 39098.0964 95 3714319
Emergent Herbaceuous Wetlands 0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0 Weighted CN=69.92
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
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): Wake Forest, NC, USA
Station Name: -
Latitude: 35.9708°
Longitude: -78.4894°
Elevation (USGS): 301.89 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.403 0.469 0.535 0.6 0.665 0.716 0.762 0.802 0.846 0.885
10-min:0.644 0.75 0.857 0.959 1.06 1.14 1.21 1.27 1.34 1.39
15-min:0.805 0.942 1.08 1.21 1.34 1.44 1.53 1.6 1.68 1.75
30-min:1.1 1.3 1.54 1.76 1.99 2.17 2.34 2.5 2.68 2.83
60-min:1.38 1.63 1.97 2.29 2.65 2.95 3.23 3.5 3.85 4.14
2-hr:1.61 1.92 2.34 2.75 3.22 3.64 4.03 4.44 4.96 5.42
3-hr:1.71 2.04 2.49 2.94 3.49 3.97 4.45 4.95 5.61 6.21
6-hr:2.05 2.44 3 3.54 4.21 4.82 5.42 6.06 6.91 7.69
12-hr:2.42 2.88 3.55 4.22 5.06 5.82 6.6 7.43 8.57 9.61
24-hr:2.86 3.46 4.33 5.02 5.96 6.71 7.47 8.26 9.33 10.2
2-day:3.32 4 4.97 5.74 6.77 7.59 8.42 9.28 10.4 11.4
3-day:3.52 4.23 5.24 6.03 7.11 7.96 8.83 9.72 10.9 11.9
4-day:3.72 4.46 5.5 6.32 7.44 8.33 9.24 10.2 11.4 12.5
7-day:4.31 5.15 6.26 7.15 8.37 9.33 10.3 11.3 12.7 13.8
10-day:4.9 5.84 7.02 7.95 9.19 10.2 11.2 12.2 13.5 14.6
20-day:6.58 7.77 9.18 10.3 11.8 13 14.2 15.4 17.1 18.4
30-day:8.17 9.61 11.2 12.4 14 15.2 16.5 17.7 19.3 20.6
45-day:10.4 12.2 14 15.3 17.1 18.4 19.8 21.1 22.8 24.1
60-day:12.5 14.6 16.5 17.9 19.8 21.3 22.6 24 25.7 27
Date/time (GMT): Tue Apr 17 12:50:24 2018
pyRunTime: 0.0406761169434

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:Concrete-Lined Rectangular Channel/Weir
Emergency Spillway:N/A
Top of Dam :303.2 ft NAVD88
Normal Pool :296.2 ft NAVD88
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
296.2 0 0 0
296.7 60 0 60
297.2 169 0 169
297.7 311 0 311
298.2 479 0 479
298.7 669 0 669
299.2 879 0 879
299.7 1,108 0 1,108
300.2 1,354 0 1,354
300.7 1,615 0 1,615
301.2 1,892 0 1,892
301.7 2,182 0 2,182
302.2 2,487 0 2,487
302.7 2,804 0 2,804
303.2 3,134 0 3,134
303.7 3,475 0 3,475
304.2 3,829 0 3,829
304.7 4,193 0 4,193
305.2 4,568 0 4,568
305.7 4,954 0 4,954
306.2 5,351 0 5,351
306.7 5,757 0 5,757
307.2 6,173 0 6,173
307.7 6,599 0 6,599
SPILLWAY ASSESSMENT
Wake Forest Water Supply Dam, WAKE-176
296
297
298
299
300
301
302
303
304
305
306
307
308
0 2,000 4,000 6,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway:Emergency Spillway:Total Structure