WAKE-358_Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

E. M. JOHNSON WATER PLANT B
DAM

WAKE-358

WAKE COUNTY, NC

PREPARED BY:
TYLER MCEWEN, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the E.M. Johnson Water Plant B 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 hydraulically adequate for all of
the above rainfall events. The dam maintained a freeboard of 8.0 – 8.4 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 =
186.8450.0442.0 74.0
ASSUMED INOPERABLE
Auxiliary
STORAGE (ACRE-FT)
158.1
172.4
23.9
ELEVATION
N/A
STORAGE (ACRE-FT)
100.6
115.0
24 HOUR - 100 YEAR EVENT, 7.43 INCHES, MAX W.S. ELEVATION 436.8 (8.2 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 6.68 INCHES, MAX W.S. ELEVATION 436.7 (8.3 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 5.94 INCHES, MAX W.S. ELEVATION 436.6 (8.4 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
0.0225
99
1
NOT TO SCALE
448.0
143.7447.0
438.0
439.0
440.0
441.0
443.0 87.0
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
E.M. JOHNSON WATER PLANT B DAM, WAKE-358
61.2
48.6
129.3446.0
449.0
36.2
SPILLWAY
ANALYSIS PERFORMED BY: TYLER MCEWEN, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 10.10 INCHES, MAX W.S. ELEVATION 437.0 (8.0 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 9.27 INCHES, MAX W.S. ELEVATION 436.9 (8.1 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 8.21 INCHES, MAX W.S. ELEVATION 436.8 (8.2 FT FREEBOARD)
TOP OF DAM ELEVATION = 445.0 FT NAVD88
436.0
437.0
441.6
Principal
ELEVATION
444.0
445.0
0.0
11.8
2, 10 ft X 10 ft Riser/Barrel Structures
E.M. JOHNSON WATER PLANT B DAM WAKE-358 July 31, 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
E.M. Johnson Water Plant Dam B - WAKE-358 Watershed Boundary
.Drainage Area: 0.0216 Square Miles
Legend
Watershed Boundary
0 0.075 0.150.0375 Miles

APPENDIX B 
Time of Concentration Calculation 

The StreamStats “longest flow path” for WAKE-358 is completely contained within the Water Plant B
reservoir. The time of concentration is assumed to be 1 minute for the HEC-HMS modeling.

APPENDIX C 
NRCS Curve Number Summary Sheet 

The entire watershed is composed of the Water Plant B reservoir. The curve number is assumed to be
99.

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): Raleigh, NC, USA
Station Name: -
Latitude: 35.9111°
Longitude: -78.5964°
Elevation (USGS): 468.2 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.401 0.468 0.536 0.598 0.661 0.709 0.752 0.789 0.83 0.865
10-min:0.641 0.749 0.858 0.956 1.05 1.13 1.2 1.25 1.31 1.36
15-min:0.801 0.941 1.09 1.21 1.34 1.43 1.51 1.58 1.65 1.71
30-min:1.1 1.3 1.54 1.75 1.98 2.15 2.31 2.46 2.63 2.77
60-min:1.37 1.63 1.98 2.28 2.63 2.92 3.19 3.45 3.77 4.04
2-hr:1.6 1.91 2.34 2.72 3.19 3.58 3.95 4.33 4.82 5.22
3-hr:1.69 2.02 2.49 2.92 3.45 3.91 4.36 4.83 5.45 5.99
6-hr:2.04 2.43 2.99 3.52 4.18 4.75 5.32 5.92 6.72 7.43
12-hr:2.41 2.87 3.55 4.2 5.02 5.75 6.48 7.26 8.33 9.29
24-hr:2.85 3.44 4.32 5.01 5.94 6.68 7.43 8.21 9.27 10.1
2-day:3.31 3.98 4.95 5.72 6.74 7.55 8.37 9.22 10.4 11.3
3-day:3.51 4.21 5.22 6 7.07 7.92 8.78 9.66 10.9 11.8
4-day:3.71 4.44 5.48 6.29 7.41 8.29 9.19 10.1 11.4 12.4
7-day:4.29 5.12 6.24 7.12 8.32 9.28 10.3 11.3 12.6 13.7
10-day:4.88 5.81 6.98 7.9 9.14 10.1 11.1 12.1 13.4 14.5
20-day:6.55 7.72 9.12 10.2 11.7 12.9 14.1 15.3 16.9 18.2
30-day:8.12 9.55 11.1 12.3 13.9 15.1 16.3 17.6 19.2 20.4
45-day:10.4 12.1 13.9 15.2 17 18.3 19.6 20.9 22.6 23.8
60-day:12.4 14.5 16.4 17.8 19.7 21.1 22.5 23.8 25.5 26.8
Date/time (GMT): Fri Aug 7 19:52:25 2020
pyRunTime: 0.0104310512543

APPENDIX F 
Outlet Works Rating Curves 

WAKE-358 Water Plant B dam has a primary “Intake” structure, invert elevation approximately 411.0 ft
NAVD88, and an emergency “Inlet” spillway outlet, invert elevation approximately 446.0 ft NAVD88.
The emergency spillway invert elevation is 1.0 feet above the top of dam elevation. For this simulation,
the primary intake structure is assumed non-functioning during an flood emergency. Therefore, it is
assumed are no spillway discharges.