Wake-228 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
STATE FAIR H&L DAM
WAKE-228
WAKE COUNTY, NC
PREPARED BY:
RHONDA L. BREWINGTON, PE
ALPHA & OMEGA GROUP
2019

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the State Fair H&L Dam in Wake County, North
Carolina. The purpose of the study was to analyze the existing potential of the dam 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 Arc-GIS 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 the
above rainfall events except for the 1000-year event. The dam was overtopped during this
event by 0.02 ft. 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).
STATE FAIR H DAM WAKE-228 February 3, 2020
Drainage Area (Sq. Mi.)=
Curve Number =
Lag Time (min.)=
Rainfall Distribution Methods =
24 HOUR - 100 YEAR EVENT, 7.46 INCHES, ELEVATION 439.82 (0.68 FT FREEBOARD)
ELEVATIONTYPE
24 HOUR - 50 YEAR EVENT, 6.71 INCHES, ELEVATION 439.58 (0.92 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 5.96 INCHES, ELEVATION 439.32 (1.18 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
0.22
82.65
23
NOT TO SCALE
443.50
47.70443.00
437.00
437.50
438.00
438.50
441.00 30.60
439.50
440.00
440.50
19.20
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
STATE FAIR H&L DAM, WAKE 228
12.30
15.70
9.00
43.00442.50
444.00
5.70
SPILLWAY
ANALYSIS PERFORMED BY: RHONDA L. BREWINGTON, PE (Alpha & Omega Group)
24 HOUR - 1000 YEAR EVENT, 10.10 INCHES, ELEVATION 440.52 (0.02 FT OVERTOPPED)
24 HOUR - 500 YEAR EVENT, 9.31 INCHES, ELEVATION 440.32 (0.18 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 8.24INCHES, ELEVATION 440.04 (0.46 FT FREEBOARD)
TOP OF DAM ELEVATION = 440.50
Principal Riser / Barrel 437.10
436.00
436.50
STORAGE (ACRE-FT)ELEVATION ELEVATION
441.50
442.00
0.00
0.01
STORAGE (ACRE-FT)
34.60
38.70
Auxiliary N/A
22.90
26.70
439.00
2.50
445.00
445.50
446.00
68.20
73.80
79.50
444.50
52.60
57.60
62.90

APPENDIX A 
Watershed Boundary 

Sources: Esri, HERE, Garmin, Intermap, increment P Corp.,
GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster
NL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong),
(c) OpenStreetMap contributors, and the GIS User Community
State Fair H&L Dam - WAKE-228 - Watershed Boundary
Legend
Longest Flowpath
Watershed Boundary
.
0 0.1 0.20.05 Miles
Drainage Area: 0.22 Square Miles
Flowpath Length: 3761.91 Feet

APPENDIX B 
Time of Concentration Calculation 

Dam No: Wake 228 Date: 12/17/2019
Dam Name: State Fair H&L Dam Engineer:R. Brewington
Input: Chris Stone
Check: R. Brewington
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 = 82.65 from StreamStats
S =2.09921 in.
Mean Basin Slope Y = 4.67 % from StreamStats
Longest Flowpath: 0.713 mi. from StreamStats
Flow Length l =3764.64 ft.
Time of Concentration Tc = 0.65 Hours Tc = 39 mins
Lag Time TL =0.39 Hours TL =23 mins

APPENDIX C 
NRCS Curve Number Summary Sheet 

Change CN according to TR‐55 (see tables below)HSGTypeA CN B CN C CN D CNOpen Water539634.8936 100 53963489.36 0.0000 100 0 0.0000 100 0 0.0000 100 0Developed, Open Space138848.1290 68 9441672.769 434303.3466 79 34309964.38 40982.1538 86 3524465.226 0.0000 89 0Developed, Low Intensity122795.7397 57 6999357.16 1448314.6308 72 104278653.4 692951.9840 81 56129110.7 21654.0624 86 1862249Developed, Medium Intensity72645.3317 77 5593690.539 1276191.4146 85 108476270.2 498284.3967 90 44845595.7 0.0000 92 0Developed, High Intensity0.0000 89 0 745847.7913 92 68617996.8 46687.5830 94 4388632.8 0.0000 95 0Barren Land0.0000 77 0 0.0000 86 0 0.0000 91 0 0.0000 94 0Deciduous Forest0360 06000730 0790Evergreen Forest0.0000 36 0 0.0000 60 0 0.0000 73 0 0.0000 79 0Mixed Forest0.0000 36 0 0.0000 60 0 0.0000 73 0 0.0000 79 0Shrub/Scrub0.0000 35 0 0.0000 56 0 0.0000 70 0 0.0000 77 0Herbaceuous0.0000 50 0 0.0000 71 0 0.0000 81 0 0.0000 89 0Hay/Pasture0490 06900790 0840Cultivated Crops0.0000 71 0 0.0000 80 0 0.0000 87 0 0.0000 90 0designed by dan.k Woody Wetlands0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0Emergent Herbaceuous Wetlands0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0 Weighted CN= 82.65NOTE: 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 accurateNOTE: double‐check the exported attribute table has the same table format as the "Export_Output" table

APPENDIX D 
Hyetograph Results Used for Analyses 

0.02.04.06.08.010.012.00 2 4 6 8 10 12 14 16 18 20 22 24Cumulative Inches of RainHoursHyetographs for 24‐Hour Rainfall Distributions25YR50YR100YR200YR500YR1000YR

APPENDIX E 
NOAA ATLAS 14 RAINFALL TABLE