WAKE-118_Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT

E. M. JOHNSON ALUM SLUDGE
LAGOON DAM

WAKE-118

WAKE COUNTY, NC

PREPARED BY:
TYLER MCEWEN, PE
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the E.M. Johnson Alum Sludge Lagoon 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 3.1 – 3.5 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
Auxiliary
STORAGE (ACRE-FT)
153.9
100.5
ELEVATION
N/A
STORAGE (ACRE-FT)
126.9
133.6
24 HOUR - 100 YEAR EVENT, 7.42 INCHES, MAX W.S. ELEVATION 375.7 (3.3 FT FREEBOARD)
ELEVATION (FT NAVD88)TYPE
24 HOUR - 50 YEAR EVENT, 6.67 INCHES, MAX W.S. ELEVATION 375.6 (3.4 FT FREEBOARD)
24 HOUR - 25 YEAR EVENT, 5.93 INCHES, MAX W.S. ELEVATION 375.5 (3.5 FT FREEBOARD)
SUMMARY OF INPUT PARAMETERS
NRCS-NOAA
0.01
99
1
NOT TO SCALE
385.0
147.1384.0
377.0
378.0
379.0
380.0
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
E.M. JOHNSON ALUM SLUDGE LAGOON DAM, WAKE-118
120.2
113.5
140.4383.0
106.8
SPILLWAY
ANALYSIS PERFORMED BY: TYLER MCEWEN, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 10.10 INCHES, MAX W.S. ELEVATION 375.9 (3.1 FT FREEBOARD)
24 HOUR - 500 YEAR EVENT, 9.24 INCHES, MAX W.S. ELEVATION 375.8 (3.2 FT FREEBOARD)
24 HOUR - 200 YEAR EVENT, 8.19 INCHES, MAX W.S. ELEVATION 375.7 (3.3 FT FREEBOARD)
TOP OF DAM ELEVATION = 379.0 FT NAVD88
375.0
376.0
378.0
Principal
ELEVATION
381.0
382.0
88.4
94.4
Auxiliary 377.0Vegetated Trapazoidal Broad-crested weir
Riprap Trapazoidal Broad-crested weir
E.M. JOHNSON ALUM SLUDGE LAGOON
DAM WAKE-118 August 7, 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 Alum Sludge Lagoon Dam - WAKE-118 Watershed Boundary
.Drainage Area: 0.01 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-118 is completely contained within the Alum Sludge
Lagoon and is therefore assumed to be 1 minute for the HEC-HMS modeling.

APPENDIX C 
NRCS Curve Number Summary Sheet 

The entire watershed is composed of the Alum Sludge Lagoon. 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: 35.9158°
Longitude: -78.6035°
Elevation (USGS): 375.14 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 0.535 0.597 0.661 0.709 0.751 0.788 0.829 0.863
10-min:0.641 0.748 0.857 0.955 1.05 1.13 1.19 1.25 1.31 1.36
15-min:0.801 0.941 1.08 1.21 1.33 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 2.45 2.63 2.76
60-min:1.37 1.63 1.98 2.28 2.63 2.92 3.18 3.44 3.77 4.03
2-hr:1.6 1.91 2.34 2.72 3.18 3.57 3.94 4.32 4.8 5.21
3-hr:1.69 2.02 2.49 2.92 3.45 3.9 4.35 4.82 5.44 5.97
6-hr:2.03 2.43 2.99 3.51 4.17 4.74 5.31 5.91 6.71 7.41
12-hr:2.4 2.87 3.55 4.19 5.01 5.74 6.47 7.25 8.32 9.27
24-hr:2.85 3.44 4.31 5 5.93 6.67 7.42 8.19 9.24 10.1
2-day:3.31 3.98 4.95 5.71 6.73 7.54 8.35 9.19 10.3 11.2
3-day:3.51 4.21 5.21 6 7.06 7.9 8.76 9.64 10.8 11.8
4-day:3.7 4.44 5.47 6.29 7.39 8.27 9.17 10.1 11.3 12.3
7-day:4.29 5.12 6.23 7.11 8.31 9.27 10.2 11.2 12.6 13.7
10-day:4.87 5.8 6.97 7.89 9.13 10.1 11.1 12.1 13.4 14.5
20-day:6.54 7.71 9.11 10.2 11.7 12.9 14.1 15.3 16.9 18.2
30-day:8.11 9.54 11.1 12.3 13.9 15.1 16.3 17.5 19.1 20.4
45-day:10.3 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 22.4 23.7 25.4 26.7
Date/time (GMT): Tue Aug 11 20:41:50 2020
pyRunTime: 0.0114300251007

APPENDIX F 
Outlet Works Rating Curves 

Primary Spillway:N/A
Emergency Spillway 1:Vegetated Trapazoidal Broad-cresed Weir
Emergency Spillway 2:Rip-Rap Lined Trapazoidal Broad-Crested Weir
Top of Dam :379.0 ft NAVD88
Normal Pool :375.0 ft NAVD88
RWS Elevation
(ft NAVD88)
Emergency Spillway 1
Discharge (cfs)
Emergency Spillway 2
Discharge (cfs)
Total Structure
Discharge (cfs)
377.0 0 0 0
377.5 31 0 31
378.0 116 0 116
378.5 265 31 296
379.0 486 116 602
379.5 791 265 1,055
380.0 1,185 486 1,671
380.5 1,676 791 2,467
381.0 2,272 1,185 3,457
381.5 2,978 1,676 4,655
382.0 3,801 2,272 6,073
382.5 4,747 2,978 7,725
383.0 5,820 3,801 9,621
383.5 7,026 4,747 11,773
384.0 8,371 5,820 14,191
384.5 9,859 7,026 16,885
385.0 11,495 8,371 19,866
SPILLWAY ASSESSMENT
Alum Sludge Lagoon, WAKE-118
376
377
378
379
380
381
382
383
384
385
386
0 5,000 10,000 15,000 20,000 25,000RWS Elevation (feet NAVD88)Spillway Discharge (cfs)
Spillway Discharge Capacity
Emergency Spillway 1:Emergency Spillway 2 Total Structure