WAKE-127 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

CRABTREE CREEK WS DAM #1

WAKE-127

WAKE COUNTY, NC

PREPARED BY:
GEORGE ELLER, PE
and TYLER MCEWEN, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Crabtree Creek WS Dam #1 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 4.2 – 8.0 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 =
315.2
Principal
and TYLER MCEWEN, PE, NCDEQ-DEMLR
ELEVATION
316.0
318.0
0.0
38.7
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
CRABTREE CREEK WS DAM #1, WAKE-127
378.9
272.6
961.2320.0
326.0
180.0
SPILLWAY
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 9.80 INCHES, MAX W.S. ELEVATION 316.0 (4.2 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 9.02 INCHES, MAX W.S. ELEVATION 315.7 (4.5 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 8.02 INCHES, MAX W.S. ELEVATION 315.3 (4.9 FT FREEBOARD)
TOP OF DAM ELEVATION = 320.2 FT NAVD88
302.0
304.0
NOT TO SCALE
324.0
1,152.3322.0
306.0
308.0
310.0
312.0
24 HOUR - 100 YEAR EVENT, 7.28 INCHES, MAX W.S. ELEVATION 314.4 (5.8 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 6.56 INCHES, MAX W.S. ELEVATION 313.3 (6.9 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 5.85 INCHES, MAX W.S. ELEVATION 312.2 (8.0 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
1.96
86.37
56
314.0 499.7
Rectangular Concrete Riser / Barrel
Emergency Vegetated Trapazoidal Channel / Weir
STORAGE (ACRE-FT)
1,364.9
1,599.5
102.1
ELEVATION
304.2
STORAGE (ACRE-FT)
637.2
790.8
CRABTREE CREEK WS DAM #1 WAKE-127 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
Crabtree Creek WS Dam #1 - WAKE-127 - Watershed Boundary
.Drainage Area: 1.96 Square MilesFlowpath Length: 15,037 Feet
Legend
Watershed Boundary
Longest Flowpath
0 0.5 10.25 Miles

APPENDIX B 
Time of Concentration Calculation 

Dam No:WAKE-127 Date:11/15/2018
Dam Name:Crabtree Creek WS Dam #1 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 =86.37 from StreamStats
S =1.57809 in.
Mean Basin Slope Y =5.78 %from StreamStats
Longest Flowpath:2.848 mi.from StreamStats
Flow Length l =15037.4 ft.
Time of Concentration Tc =1.56 Hours Tc =93 mins
Lag Time TL =0.93 Hours TL =56 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 31982.0946 100 3198209.463 0.0000 100 0 1928010.7956 100 1.93E+08
Developed, Open Space 0.0000 68 0 758690.8856 79 59936579.96 0.0000 86 0 8493508.8458 89 7.56E+08
Developed, Low Intensity 0.0000 57 0 954923.9484 72 68754524.29 0.0000 81 0 6850604.8388 86 5.89E+08
Developed, Medium Intensity 0.0000 77 0 2860021.1426 85 243101797.1 0.0000 90 0 9396178.4318 92 8.64E+08
Developed, High Intensity 0.0000 89 0 1872787.3719 92 172296438.2 0.0000 94 0 4222997.3027 95 4.01E+08
Barren Land 0.0000 77 0 0.0000 86 0 0.0000 91 0 0.0000 94 0
Deciduous Forest 0 36 0 897661.3382 60 53859680.29 0 73 0 4922072.079 79 3.89E+08
Evergreen Forest 0.0000 36 0 578909.2598 60 34734555.59 0.0000 73 0 5163343.4295 79 4.08E+08
Mixed Forest 0.0000 36 0 94550.6019 60 5673036.111 0.0000 73 0 830617.7339 79 65618801
Shrub/Scrub 0.0000 35 0 46434.7994 56 2600348.764 0.0000 70 0 103217.4273 77 7947742
Herbaceuous 0.0000 50 0 83770.9465 71 5947737.2 0.0000 81 0 2398197.9850 89 2.13E+08
Hay/Pasture 0 49 0 48141.66036 69 3321774.565 0 79 0 1891016.771 84 1.59E+08
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 30924.9321 85 2628619.232 0.0000 90 0 158835.5978 95 15089382
Emergent Herbaceuous Wetlands 0.0000 80 0 0.0000 85 0 0.0000 90 0 0.0000 95 0 Weighted CN=86.37
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): Morrisville, NC, USA
Station Name: -
Latitude: 35.8594°
Longitude: -78.8253°
Elevation (USGS): 317.84 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.395 0.462 0.531 0.589 0.648 0.69 0.728 0.759 0.793 0.82
10-min:0.631 0.739 0.851 0.941 1.03 1.1 1.16 1.2 1.26 1.29
15-min:0.789 0.93 1.08 1.19 1.31 1.39 1.46 1.52 1.58 1.62
30-min:1.08 1.28 1.53 1.73 1.94 2.1 2.24 2.36 2.51 2.62
60-min:1.35 1.61 1.96 2.25 2.58 2.84 3.08 3.32 3.61 3.83
2-hr:1.56 1.87 2.3 2.66 3.09 3.44 3.78 4.11 4.53 4.86
3-hr:1.66 1.99 2.45 2.85 3.35 3.76 4.17 4.58 5.12 5.57
6-hr:2.01 2.4 2.96 3.45 4.07 4.59 5.11 5.64 6.36 6.95
12-hr:2.38 2.85 3.53 4.13 4.92 5.58 6.25 6.96 7.92 8.74
24-hr:2.84 3.42 4.28 4.95 5.85 6.56 7.28 8.02 9.02 9.8
2-day:3.27 3.93 4.87 5.6 6.57 7.33 8.09 8.87 9.92 10.7
3-day:3.45 4.15 5.11 5.87 6.88 7.68 8.49 9.3 10.4 11.3
4-day:3.64 4.36 5.36 6.14 7.2 8.03 8.88 9.74 10.9 11.8
7-day:4.21 5.02 6.1 6.94 8.1 9.01 9.93 10.9 12.2 13.1
10-day:4.79 5.69 6.83 7.72 8.92 9.87 10.8 11.8 13.1 14.1
20-day:6.39 7.54 8.89 9.98 11.4 12.6 13.8 15 16.6 17.8
30-day:7.92 9.32 10.8 12 13.5 14.7 15.9 17.1 18.6 19.8
45-day:10.1 11.8 13.5 14.9 16.6 17.9 19.2 20.5 22.1 23.4
60-day:12.2 14.2 16 17.4 19.2 20.5 21.8 23.1 24.7 25.9
Date/time (GMT): Fri Nov 16 14:15:48 2018
pyRunTime: 0.0105619430542

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:Rectangular Concrete Riser / Barrel
Emergency Spillway:Grass-Lined Trapezoidal Channel / Weir
Top of Dam :320.7 ft NAVD88
Normal Pool :304.2 ft NAVD88
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
304.2 0 0 0
304.7 15 0 15
305.2 42 0 42
305.7 76 0 76
306.2 77 0 77
306.7 79 0 79
307.2 81 0 81
307.7 83 0 83
308.2 84 0 84
308.7 86 0 86
309.2 88 0 88
309.7 89 0 89
310.2 91 0 91
310.7 92 0 92
311.2 94 0 94
311.7 95 0 95
312.2 97 0 97
312.7 98 0 98
313.2 100 0 100
313.7 101 0 101
314.2 102 0 102
314.7 104 0 104
315.2 105 0 105
315.7 106 249 355
316.2 108 708 815
316.7 109 1,307 1,416
317.2 110 2,022 2,133
317.7 111 2,841 2,953
318.2 113 3,754 3,867
318.7 114 4,755 4,869
319.2 115 5,840 5,955
319.7 116 7,004 7,121
320.2 118 8,246 8,363
320.7 119 9,561 9,680
321.2 120 10,949 11,069
321.7 121 12,408 12,529
322.2 122 13,936 14,059
322.7 123 15,533 15,656
323.2 125 17,197 17,321
323.7 126 18,927 19,053
324.2 127 20,723 20,849
324.7 128 22,583 22,711
325.2 129 24,508 24,637
325.7 130 26,496 26,626
SPILLWAY ASSESSMENT
Crabtree Creek WS Dam #1 Dam, WAKE-127
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
0 5,000 10,000 15,000 20,000 25,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Primary Spillway:Emergency Spillway:Total Structure