WAKE-362_Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

E. M. JOHNSON PLANT A DAM

WAKE-362

WAKE COUNTY, NC

PREPARED BY:
TYLER MCEWEN, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the E.M. Johnson Plant A 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 1.9 – 2.3 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 =
441.6
Principal
ELEVATION
442.0
443.0
0.0
1.4
2, 10 ft X 10 ft Riser/Barrel Structures
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
E.M. JOHNSON PLANT A DAM, WAKE-362
54.0
40.6
111.6444.0
447.0
27.4
SPILLWAY
ANALYSIS PERFORMED BY: TYLER MCEWEN, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 10.10 INCHES, MAX W.S. ELEVATION 439.7 (1.9 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 9.27 INCHES, MAX W.S. ELEVATION 439.7 (1.9 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 8.21 INCHES, MAX W.S. ELEVATION 439.6 (2.0 FT FREEBOARD)
TOP OF DAM ELEVATION = 441.6 FT NAVD88
435.0
436.0
NOT TO SCALE
446.0
126.7445.0
437.0
438.0
439.0
440.0
24 HOUR - 100 YEAR EVENT, 7.43 INCHES, MAX W.S. ELEVATION 439.5 (2.1 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 6.68 INCHES, MAX W.S. ELEVATION 439.4 (2.2 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 5.94 INCHES, MAX W.S. ELEVATION 439.3 (2.3 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
0.0235
99
1
171.9448.0441.0 67.6
ASSUMED INOPERABLE
Auxiliary
STORAGE (ACRE-FT)
141.8
156.8
14.4
ELEVATION
N/A
STORAGE (ACRE-FT)
81.6
96.6
E.M. JOHNSON PLANT A DAM WAKE-362 July 24, 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 A Dam - WAKE-362 Watershed Boundary
.Drainage Area: 0.024 Square Miles
Legend
Watershed Boundary
0 0.1 0.20.05 Miles

APPENDIX B 
Time of Concentration Calculation 

The StreamStats “longest flow path” for WAKE-362 is completely contained within the Plant A 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 Plant A 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: 35.9120°
Longitude: -78.5970°
Elevation (USGS): 430.73 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): Mon Apr 16 17:17:01 2018
pyRunTime: 0.041323184967

APPENDIX F 
Outlet Works Rating Curves 

WAKE-362 Plant A dam has riser/barrel emergency spillway outlet diversions to WAKE-118 Alum Sludge
Lagoon. The emergency spillway invert elevation is 441.6 ft NAVD88, 0.3 feet from the surveyed top of
dam elevation. For this simulation, emergency spillway outlet riser/barrel structures leading to WAKE-
118 Alum Sludge Lagoon considered an overtopping event for WAKE-362. Therefore, the top of dam
elevation for HEC-HMS modeling assumed to be 441.6 ft NAVD88 and no emergency spillway was
modeled.