WAKE-072 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

PRIVETTE LAKE DAM #2

WAKE-072

WAKE COUNTY, NC

PREPARED BY:
GEORGE ELLER, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Privette Lake Dam #2 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 0.9 – 2.0 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
259.0
260.0
0.0
6.9
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
PRIVETTE LAKE DAM #2, WAKE-072
34.7
69.3261.0
24.8
SPILLWAY
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 11.90 INCHES, MAX W.S. ELEVATION 256.1 (0.9 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 10.70 INCHES, MAX W.S. ELEVATION 255.9 (1.1 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 9.17 INCHES, MAX W.S. ELEVATION 255.6 (1.4 FT FREEBOARD)
TOP OF DAM ELEVATION = 257.0 FT NAVD88
254.0
255.0
NOT TO SCALE
263.0
82.5262.0
256.0
257.0
258.0
24 HOUR - 100 YEAR EVENT, 8.13 INCHES, MAX W.S. ELEVATION 255.4 (1.6 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 7.16 INCHES, MAX W.S. ELEVATION 255.2 (1.8 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 6.25 INCHES, MAX W.S. ELEVATION 255.0 (2.0 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
0.12
69.5
27
Riprap-lined Trapazoidal Channel
Auxiliary N/A
STORAGE (ACRE-FT)
96.6
15.5
ELEVATION
257.0
STORAGE (ACRE-FT)
45.5
57.0
PRIVETTE LAKE DAM #2 WAKE-072 June 12, 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
Privette Lake Dam #2 - WAKE-072 - Watershed Boundary
.Drainage Area: 0.12 Square MilesFlowpath Length: 3,474 Feet
Legend
Watershed Boundary
Longest Flowpath
0 0.25 0.50.125 Miles

APPENDIX B 
Time of Concentration Calculation 

Dam No:WAKE-072 Date:6/11/2020
Dam Name:Privette Lake Dam #2 Engineer:T. McEwen
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 =69.5 from StreamStats
S =4.38849 in.
Mean Basin Slope Y =6.45 %from StreamStats
Longest Flowpath:0.658 mi.from StreamStats
Flow Length l =3474.24 ft.
Time of Concentration Tc =0.76 Hours Tc =46 mins
Lag Time TL =0.46 Hours TL =27 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 0.0000 100 0 44280.6241 100 4428062.413 0.0000 100 0 339307.1572 100 33930716
Developed, Open Space 8926.1118 68 606975.6043 96199.8667 79 7599789.466 0.0000 86 0 36122.5820 89 3214910
Developed, Low Intensity 13856.6749 57 789830.4714 15041.0675 72 1082956.86 0.0000 81 0 0.0000 86 0
Developed, Medium Intensity 0.0000 77 0 0.0000 85 0 0.0000 90 0 0.0000 92 0
Developed, High Intensity 0.0000 89 0 0.0000 92 0 0.0000 94 0 0.0000 95 0
Barren Land 0.0000 77 0 0.0000 86 0 0.0000 91 0 0.0000 94 0
Deciduous Forest 0 36 0 527590.2803 60 31655416.82 0 73 0 68336.8085 79 5398608
Evergreen Forest 0.0000 36 0 1265720.8840 60 75943253.04 0.0000 73 0 257163.1967 79 20315893
Mixed Forest 0.0000 36 0 0.0000 60 0 0.0000 73 0 0.0000 79 0
Shrub/Scrub 0.0000 35 0 0.0000 56 0 0.0000 70 0 0.0000 77 0
Herbaceuous 0.0000 50 0 0.0000 71 0 0.0000 81 0 0.0000 89 0
Hay/Pasture 0 49 0 576239.253 69 39760508.45 0 79 0 3614.219892 84 303594.5
Cultivated Crops 15193.2987 71 1078724.204 94855.2002 80 7588416.013 0.0000 87 0 334.7312 90 30125.81 designed by dan.k
Woody Wetlands 0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0
Emergent Herbaceuous Wetlands 0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0 Weighted CN=69.50
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
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): Zebulon, NC, USA
Station Name: -
Latitude: 35.8290°
Longitude: -78.2630°
Elevation (USGS): 254.77 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.405 0.467 0.528 0.6 0.67 0.73 0.784 0.833 0.888 0.942
10-min:0.646 0.746 0.845 0.96 1.07 1.16 1.25 1.32 1.41 1.48
15-min:0.808 0.938 1.07 1.21 1.35 1.47 1.57 1.67 1.77 1.86
30-min:1.11 1.3 1.52 1.76 2.01 2.22 2.41 2.59 2.81 3.02
60-min:1.38 1.63 1.95 2.29 2.67 3 3.32 3.64 4.04 4.4
2-hr:1.61 1.91 2.32 2.76 3.28 3.76 4.23 4.72 5.37 5.98
3-hr:1.71 2.02 2.46 2.96 3.55 4.11 4.66 5.26 6.07 6.84
6-hr:2.05 2.42 2.96 3.55 4.28 4.97 5.67 6.42 7.45 8.44
12-hr:2.41 2.85 3.49 4.22 5.12 5.99 6.87 7.84 9.19 10.5
24-hr:2.85 3.45 4.39 5.16 6.25 7.16 8.13 9.17 10.7 11.9
2-day:3.3 3.99 5.03 5.88 7.08 8.07 9.13 10.3 11.9 13.2
3-day:3.51 4.22 5.3 6.17 7.41 8.42 9.49 10.6 12.3 13.6
4-day:3.71 4.46 5.57 6.47 7.73 8.76 9.85 11 12.7 14
7-day:4.32 5.17 6.38 7.35 8.71 9.81 11 12.2 13.9 15.3
10-day:4.93 5.88 7.15 8.16 9.55 10.7 11.8 13 14.7 16
20-day:6.61 7.84 9.37 10.6 12.3 13.6 15 16.4 18.3 19.9
30-day:8.21 9.69 11.4 12.7 14.5 15.9 17.3 18.8 20.7 22.2
45-day:10.4 12.3 14.2 15.7 17.7 19.2 20.8 22.3 24.3 25.8
60-day:12.5 14.7 16.8 18.5 20.6 22.2 23.8 25.4 27.5 29
Date/time (GMT): Mon Apr 16 14:22:35 2018
pyRunTime: 0.0406501293182

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:Riprap-Lined Trapazoidal Channel/Weir
Emergency Spillway:N/A
Top of Dam :257.0 ft NAVD88
Normal Pool :254.0 ft NAVD88
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
254.0 0 0 0
254.5 20 0 20
255.0 61 0 61
255.5 121 0 121
256.0 200 0 200
256.5 298 0 298
257.0 418 0 418
257.5 584 0 584
258.0 794 0 794
258.5 1,040 0 1,040
259.0 1,322 0 1,322
259.5 1,638 0 1,638
260.0 1,988 0 1,988
260.5 2,372 0 2,372
261.0 2,791 0 2,791
261.5 3,244 0 3,244
262.0 3,733 0 3,733
262.5 4,257 0 4,257
263.0 4,817 0 4,817
SPILLWAY ASSESSMENT
Privette Lake Dam #2, WAKE-072
253
254
255
256
257
258
259
260
261
262
263
264
0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway:Emergency Spillway:Total Structure