WAKE-053 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

SUNSET LAKE DAM

WAKE-053

WAKE COUNTY, NC

PREPARED BY:
GEORGE ELLER, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Sunset Lake 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 not hydraulically adequate for all of the above rainfall
events. The dam was overtopped by 1.4 – 3.7 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
311.0
312.0
0.0
3.0
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
SUNSET LAKE DAM, WAKE-053
503.0
349.4
1,256.9313.0
316.0
213.1
SPILLWAY
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 10.60 INCHES, MAX W.S. ELEVATION 312.7 (3.7 FT OVERTOPPED)
24 HOUR - 500 YEAR EVENT, 9.65 INCHES, MAX W.S. ELEVATION 312.3 (3.3 FT OVERTOPPED)
24 HOUR - 200 YEAR EVENT, 8.51 INCHES, MAX W.S. ELEVATION 311.7 (2.7 FT OVERTOPPED)
TOP OF DAM ELEVATION = 309.0 FT NAVD88
304.0
305.0
NOT TO SCALE
315.0
1,476.4314.0
306.0
307.0
308.0
309.0
24 HOUR - 100 YEAR EVENT, 7.68 INCHES, MAX W.S. ELEVATION 311.3 (2.3 FT OVERTOPPED)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 6.88 INCHES, MAX W.S. ELEVATION 310.9 (1.9 FT OVERTOPPED)
24 HOUR - 25 YEAR EVENT, 6.10 INCHES, MAX W.S. ELEVATION 310.4 (1.4 FT OVERTOPPED)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
20.5
72.34
203
2,200.6317.0310.0 670.8
Concrete-lined Broad Crested Weir with Piers
Auxiliary N/A
STORAGE (ACRE-FT)
1,706.9
1,948.4
97.0
ELEVATION
305.7
STORAGE (ACRE-FT)
852.7
1,048.6
SUNSET LAKE DAM WAKE-053 June 11, 2020

APPENDIX A 
Watershed Boundary 

Sources: Esri, HERE, Garmin, Intermap, increment P Corp.,GEBCO, USGS, FAO, NPS, NRCAN, GeoBase, IGN, KadasterNL, Ordnance Survey, Esri Japan, METI, Esri China (Hong Kong),swisstopo, © OpenStreetMap contributors, and the GIS UserCommunity
Sunset Lake Dam - WAKE-053 - Watershed Boundary
Legend
Watershed Boundary
Longest Flowpath
.
0 1 20.5 Miles
Drainage Area: 20.52 Square MilesFlowpath Length: 44,007.69 Feet
8/21/2018

APPENDIX B 
Time of Concentration Calculation 

Dam No:Wake-053 Date:8/22/2018
Dam Name:Sunset Lake Dam Engineer:G. Eller (Designer)
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 =72.34 from StreamStats
S =3.82361 in.
Mean Basin Slope Y =5.85 %from StreamStats
Longest Flowpath:8.324 mi.from StreamStats
Flow Length l =43950.7 ft.
Time of Concentration Tc =5.65 Hours Tc =339 mins
Lag Time TL =3.39 Hours TL =203 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 150118.0118 100 15011801.18 18878865.8477 100 1887886585 0.0000 100 0 455155.2394 100 45515524
Developed, Open Space 24264740.9755 68 1650002386 91387730.4445 79 7219630705 2608051.9570 86 224292468.3 19774074.2857 89 1.76E+09
Developed, Low Intensity 17872393.5109 57 1018726430 55613009.2107 72 4004136663 3044416.3705 81 246597726 9169044.0044 86 7.89E+08
Developed, Medium Intensity 4039781.4595 77 311063172.4 19046570.8217 85 1618958520 1580663.1369 90 142259682.3 4326918.0368 92 3.98E+08
Developed, High Intensity 836670.5721 89 74463680.91 5142504.1465 92 473110381.5 103769.1920 94 9754304.047 1917464.9136 95 1.82E+08
Barren Land 109478.7824 77 8429866.248 370872.4616 86 31895031.7 0.0000 91 0 56598.3467 94 5320245
Deciduous Forest 10174294.53 36 366274603.2 47142372.15 60 2828542329 1452044.806 73 105999270.8 27667195.33 79 2.19E+09
Evergreen Forest 5409841.5290 36 194754295 28086485.0062 60 1685189100 345560.5907 73 25225923.12 8705574.7729 79 6.88E+08
Mixed Forest 5531084.1581 36 199119029.7 16113842.2555 60 966830535.3 290630.4690 73 21216024.24 9906634.2408 79 7.83E+08
Shrub/Scrub 2522875.1224 35 88300629.28 8641974.7438 56 483950585.7 103603.3154 70 7252232.08 3106570.5767 77 2.39E+08
Herbaceuous 14396277.0704 50 719813853.5 21154125.3985 71 1501942903 315646.5055 81 25567366.95 7616043.8985 89 6.78E+08
Hay/Pasture 6670284.841 49 326843957.2 13222634.87 69 912361806 193661.6721 79 15299272.1 1818673.907 84 1.53E+08
Cultivated Crops 19961959.4050 71 1417299118 14023105.1647 80 1121848413 0.0000 87 0 1135147.0483 90 1.02E+08 designed by dan.k
Woody Wetlands 1117774.5628 80 89421965.03 2689553.4554 85 228612043.7 211241.1515 90 19011703.64 10456816.1755 95 9.93E+08
Emergent Herbaceuous Wetlands 471636.1861 80 37730894.88 358838.4378 85 30501267.22 0.0000 90 0 347855.9633 95 33046317 Weighted CN=72.34
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): Holly Springs, North Carolina, USA
Station Name: -
Latitude: 35.6520°
Longitude: -78.7880°
Elevation (USGS): 294.54 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.413 0.482 0.554 0.616 0.681 0.73 0.774 0.813 0.856 0.892
10-min:0.659 0.772 0.887 0.986 1.09 1.16 1.23 1.29 1.35 1.41
15-min:0.824 0.97 1.12 1.25 1.38 1.47 1.56 1.63 1.7 1.76
30-min:1.13 1.34 1.59 1.81 2.04 2.22 2.38 2.53 2.71 2.86
60-min:1.41 1.68 2.04 2.35 2.72 3.01 3.28 3.55 3.89 4.17
2-hr:1.65 1.98 2.43 2.83 3.31 3.72 4.11 4.5 5.02 5.46
3-hr:1.75 2.1 2.59 3.03 3.59 4.07 4.54 5.04 5.7 6.28
6-hr:2.09 2.51 3.11 3.64 4.33 4.92 5.52 6.14 7 7.74
12-hr:2.47 2.96 3.67 4.33 5.19 5.93 6.7 7.51 8.64 9.64
24-hr:2.9 3.5 4.41 5.12 6.1 6.88 7.68 8.51 9.65 10.6
2-day:3.36 4.04 5.05 5.85 6.93 7.79 8.67 9.59 10.8 11.8
3-day:3.55 4.27 5.31 6.13 7.25 8.14 9.05 9.99 11.3 12.3
4-day:3.75 4.5 5.57 6.41 7.57 8.49 9.43 10.4 11.7 12.8
7-day:4.34 5.18 6.32 7.23 8.48 9.47 10.5 11.5 13 14.1
10-day:4.96 5.9 7.11 8.05 9.33 10.3 11.3 12.4 13.8 14.9
20-day:6.63 7.84 9.28 10.4 12 13.2 14.4 15.6 17.3 18.6
30-day:8.24 9.7 11.3 12.5 14.2 15.4 16.7 17.9 19.6 20.8
45-day:10.5 12.3 14.1 15.5 17.3 18.7 20 21.4 23.1 24.4
60-day:12.6 14.7 16.7 18.2 20.2 21.6 23 24.4 26.2 27.5
Date/time (GMT): Mon Apr 16 12:54:25 2018
pyRunTime: 0.0400340557098

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:Concrete-Lined Broad Crested Weir with Piers
Emergency Spillway:N/A
Top of Dam :309.0 ft NAVD88
Normal Pool :306.0 ft NAVD88
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
305.5 0 0 0
306.0 59 0 59
306.5 256 0 256
307.0 523 0 523
307.5 843 0 843
308.0 1,203 0 1,203
308.5 1,596 0 1,596
309.0 2,017 0 2,017
309.5 5,123 0 5,123
310.0 6,167 0 6,167
310.5 7,273 0 7,273
311.0 8,439 0 8,439
311.5 9,660 0 9,660
312.0 10,936 0 10,936
312.5 12,264 0 12,264
313.0 13,641 0 13,641
313.5 15,066 0 15,066
314.0 16,538 0 16,538
314.5 18,054 0 18,054
315.0 19,615 0 19,615
315.5 21,217 0 21,217
316.0 22,862 0 22,862
SPILLWAY ASSESSMENT
Sunset Lake Dam, WAKE-053
305
306
307
308
309
310
311
312
313
314
315
316
317
0 5,000 10,000 15,000 20,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway:Emergency Spillway:Total Structure