WAKE-132 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
SPRINGDALE ESTATES LOWER DAM
WAKE-132
WAKE COUNTY, NC
PREPARED BY:
RHONDA L. BREWINGTON, PE
ALPHA & OMEGA GROUP
2019

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Springdale Estates Lower 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 inadequate for
all the above rainfall events. The dam was overtopped during these events by 0.11 ft. to 0.63
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).
SPRINDALE ESTATES LOWER DAM WAKE-132 February 4, 2020
Drainage Area (Sq. Mi.)=
Curve Number =
Lag Time (min.)=
Rainfall Distribution Methods =
419.00
193.98
218.98
245.34412.00
23.10
Riser / Barrel 407.91
406.00
407.00
STORAGE (ACRE-FT)ELEVATION ELEVATION
413.00
414.00
0.00
8.09
STORAGE (ACRE-FT)
106.39
126.43
Auxiliary N/A
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
SPRINGDALE ESTATES LOWER DAM, WAKE-132
69.88
87.53
53.28
147.69415.00
418.00
37.54
SPILLWAY
ANALYSIS PERFORMED BY: RHONDA L. BREWINGTON, PE (Alpha & Omega Group)
24 HOUR - 1000 YEAR EVENT, 9.89 INCHES, ELEVATION 410.35 (0.63 FT OVERTOPPED)
24 HOUR - 500 YEAR EVENT, 9.09 INCHES, ELEVATION 410.28 (0.56 FT OVERTOPPED)
24 HOUR - 200 YEAR EVENT, 8.07 INCHES, ELEVATION 410.18 (0.46 FT OVERTOPPED)
TOP OF DAM ELEVATION = 409.72
Principal
NOT TO SCALE
417.00
170.22416.00
408.00
409.00
410.00
411.00
24 HOUR - 100 YEAR EVENT, 7.32 INCHES, ELEVATION 410.08 (0.36 FT OVERTOPPED)
ELEVATIONTYPE
24 HOUR - 50 YEAR EVENT, 6.59 INCHES, ELEVATION 409.97 (0.25FT OVERTOPPED)
24 HOUR - 25 YEAR EVENT, 5.87 INCHES, ELEVATION 409.83 (0.11 FT OVERTOPPED)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
0.46
77.16
38

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
Springdale Estates Lower Dam - WAKE-132 - Watershed Boundary
Legend
Longest Flowpath
Watershed Boundary
.
0 0.15 0.30.075 Miles
Drainage Area: 0.4607 Square Miles
Flowpath Length: 7661.28 Feet
12/11/2019

APPENDIX B 
Time of Concentration Calculation 

Dam No: Wake 132 Date: 12/11/2019Dam Name: Springdale Estates Lower Dam Engineer: R. BrewingtonInput: Chris StoneChecked: R. BrewingtonMaximum Potential Retention (inches):S = (1000/cn')-10Time of Concentration (hours):Tc =l0.8*(S+1)0.71140*Y0.5Basin Data:Input Values in yellowCurve Number CN = 77.16 from StreamStatsS =2.96008in.Mean Basin Slope Y = 7.72 % from StreamStatsLongest Flowpath: 1.443 mi. from StreamStatsFlow Length l =7619.04ft.Time of Concentration Tc = 1.05 Hours Tc = 63 minsLag TimeTL =0.63HoursTL =38 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 Water0.0000 100 0 220580.8068 100 22058080.68 0.0000 100 0 450897.7022 100 45089770Developed, Open Space132326.1379 68 8998177.374 6416316.0742 79 506888969.9 127732.2774 86 10984975.85 714245.5702 89 63567856Developed, Low Intensity0.0000 57 0 898547.7291 72 64695436.5 12822.1687 81 1038595.668 172178.9403 86 14807389Developed, Medium Intensity0.0000 77 0 434481.7144 85 36930945.72 0.0000 90 0 133579.9104 92 12289352Developed, High Intensity0.0000 89 0 237.2666 92 21828.52754 0.0000 94 0 0.0000 95 0Barren Land0.0000 77 0 0.0000 86 0 0.0000 91 0 0.0000 94 0Deciduous Forest0 36 0 729223.6764 60 43753420.59 0 73 0 281115.1767 79 22208099Evergreen Forest7916.8856 36 285007.8816 1267173.2368 60 76030394.21 573.2757 73 41849.12757 486590.7652 79 38440670Mixed Forest28452.9301 36 1024305.482 217170.2385 60 13030214.31 2699.0656 73 197031.7868 108138.3686 79 8542931Shrub/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= 77.16NOTE: 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 

Point precipitation frequency estimates (inches)
NOAA Atlas 14 Volume 2 Version 3
Data type: Precipitation depth
Time series type: Partial duration
Project area: Neuse River Basin
Location name (ESRI Maps): Raleigh  North Carolina  USA
Station Name: ‐
Latitude: 35.8956°
Longitude: ‐78.7113°
Elevation (USGS): 410.82 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for 
ARI (years):1 2 5 10 25 50 100 200 500 1000
5‐min: 0.396 0.463 0.532 0.591 0.651 0.695 0.735 0.768 0.804 0.833
10‐min: 0.633 0.74 0.852 0.945 1.04 1.11 1.17 1.22 1.27 1.31
15‐min: 0.792 0.931 1.08 1.2 1.32 1.4 1.48 1.54 1.6 1.65
30‐min: 1.09 1.29 1.53 1.73 1.95 2.11 2.26 2.39 2.55 2.67
60‐min: 1.35 1.61 1.96 2.26 2.6 2.86 3.11 3.35 3.66 3.89
2‐hr: 1.57 1.88 2.31 2.67 3.11 3.47 3.81 4.16 4.6 4.95
3‐hr: 1.66 1.99 2.45 2.86 3.37 3.79 4.21 4.64 5.21 5.68
6‐hr: 2.01 2.4 2.97 3.46 4.09 4.63 5.16 5.71 6.45 7.07
12‐hr: 2.38 2.85 3.53 4.14 4.93 5.61 6.3 7.02 8.01 8.86
24‐hr: 2.84 3.43 4.29 4.96 5.87 6.59 7.32 8.07 9.09 9.89
2‐day: 3.28 3.95 4.9 5.63 6.62 7.4 8.19 8.99 10.1 10.9
3‐day: 3.47 4.17 5.15 5.92 6.95 7.77 8.59 9.44 10.6 11.5
4‐day: 3.67 4.39 5.41 6.2 7.28 8.13 9 9.89 11.1 12
7‐day: 4.25 5.07 6.16 7.02 8.19 9.13 10.1 11 12.4 13.4
10‐day: 4.83 5.74 6.89 7.79 9.01 9.96 10.9 11.9 13.2 14.2
20‐day: 6.46 7.62 9 10.1 11.6 12.7 13.9 15.1 16.7 18
30‐day: 8.01 9.43 11 12.1 13.7 14.9 16.1 17.3 18.9 20.1
45‐day: 10.2 12 13.7 15 16.8 18.1 19.4 20.7 22.3 23.6
60‐day: 12.3 14.3 16.2 17.6 19.4 20.8 22.1 23.4 25.1 26.3
Date/time (GMT):  Wed Dec 11 12:53:30 2019
pyRunTime:  0.0124790668488

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:
Auxiliary Spillway 1:
RWS Elevation
(ft)
Primary Spillway
Discharge (cfs)
Auxiliary Spillway 1
Discharge (cfs)
Total Structure
Discharge (cfs)
406.7 2.0 0.0 2
407.0 6.7 0.0 7
407.3 18.9 0.0 19
407.6 34.0 0.0 34
407.9 35.0 0.0 35
408.2 36.0 0.0 36
408.5 36.9 0.0 37
408.8 37.8 0.0 38
409.1 38.7 0.0 39
409.4 39.6 0.0 40
409.7 40.5 0.0 40
410.0 41.3 0.0 41
410.3 42.1 0.0 42
410.6 42.9 0.0 43
410.9 43.7 0.0 44
411.2 44.5 0.0 45
411.5 45.3 0.0 45
411.8 46.0 0.0 46
412.1 46.8 0.0 47
412.4 47.5 0.0 47
412.7 48.2 0.0 48
413.0 48.9 0.0 49
413.3 49.6 0.0 50
413.6 50.3 0.0 50
413.9 51.0 0.0 51
414.2 51.6 0.0 52
414.5 52.3 0.0 52
414.8 53.0 0.0 53
N/A
SPILLWAY ASSESSMENT
SPRINGDALE ESTATES LOWER DAM
WAKE-132
6' Diamter Corrugrated Metal Riser with 24" RCP
406.0
407.0
408.0
409.0
410.0
411.0
412.0
413.0
414.0
0.0 10.0 20.0 30.0 40.0 50.0 60.0RWS Elevation (ft) Discharge Flow Rate (cfs)
RWS Elevation vs. Discharge Flow
Primary Spillway Auxiliary Spillway 1 Total Structure