ROWAN-055 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

HOWARD HENSLEY DAM

ROWAN-055

ROWAN COUNTY, NC

PREPARED BY:
HDR Engineering Inc. of the Carolinas

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Howard Hensley Dam in Rowan County,
North Carolina. The purpose of the study was to analyze the existing potential of the dam for
overtopping during predetermined specified rainfall events using the North Carolina Dam Safety
defined process and templates. 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 Arc-GIS 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 25 years, 50 years, 100 years, 200 years, 500 years, and
1000 years rainfall 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.
Drainage Area (Sq. Mi.)=
Curve Number =
Lag Time (min.)=
Rainfall Distribution Methods =
399.8
316.9
329.9
343.1
356.7
370.7
385.0
876.3
876.8
877.3
877.8
878.3
ELEVATION
873.8
874.3
874.8
875.3
875.8
25.8
30.1
34.5
STORAGE (ACRE-FT)
0.0
3.4
6.8
10.3
13.9
17.7
21.6
857.8
854.8
855.3
855.8
856.3
856.8
857.3
853.3
855.7
ELEVATION
853.3
853.8
854.3
STORAGE (ACRE-FT)
280.1
292.0
304.3
68.36
32
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
Grass Lined Trapezoidal Channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 8.07 INCHES, ELEVATION 856.2 (2.5 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 7.29 INCHES, ELEVATION 856 (2.7 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 6.54 INCHES, ELEVATION 855.9 (2.8 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 5.81 INCHES, ELEVATION 855.7 (3 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.17
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
HOWARD HENSLEY DAM, ROWAN-055
ANALYSIS PERFORMED BY: S. MURPHY, HDR
TOP OF DAM ELEVATION = 858.7
24 HOUR - 1000 YEAR EVENT, 9.95 INCHES, ELEVATION 856.6 (2.1 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 9.12 INCHES, ELEVATION 856.5 (2.2 FT FREE-BOARD)
ROWAN-055HOWARD HENSLEY DAM NOVEMBER 23, 2020
1085.7
NOT TO SCALE
903.4
932.2
961.6
991.7
1022.4
1053.7
746.1
770.6
795.7
821.6
848.1
875.4
612.2
633.2
654.6
676.6
699.1
722.3
497.6
515.5
533.9
552.7
572.1
591.9
414.9
430.6
446.6
463.1
480.1
891.3
891.8
892.3
892.8
893.3
888.3
888.8
889.3
889.8
890.3
890.8
885.3
885.8
886.3
886.8
887.3
887.8
882.3
882.8
883.3
883.8
884.3
884.8
879.3
879.8
880.3
880.8
881.3
881.8
878.8
235.7
246.3
257.3
268.5
178.1
187.1
196.3
205.8
215.5
225.5
129.4
137.0
144.7
152.7
160.9
169.4
88.3
94.6
101.2
107.9
114.9
122.0
53.9
59.2
64.6
70.3
76.1
82.1
39.1
43.9
48.8
872.8
873.3
869.8
870.3
870.8
871.3
871.8
872.3
866.8
867.3
867.8
868.3
868.8
869.3
863.8
864.3
864.8
865.3
865.8
866.3
860.8
861.3
861.8
862.3
862.8
863.3
858.3
858.8
859.3
859.8
860.3
ROWAN-055HOWARD HENSLEY DAM NOVEMBER 23, 2020

APPENDIX A
Watershed Boundary

Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P,NRCan, Esri Japan, METI, Esri China (Hong Kong), Esri Korea,Esri (Thailand), NGCC, (c) OpenStreetMap contributors, and theGIS User Community
Howard Hensley Dam - ROWAN-055 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.15 0.30.075 Miles
Date: 11/19/2020
/Drainage Area: 0.17 Square MilesFlowpath Length: 3805.9 feet

APPENDIX B

Lag Time Calculation
Dam No:ROWAN-055 Date:11/18/2020
Dam Name:Howard Hensley Dam Engineer:K. Trefzger
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 =68.3639 from NRCS CN Calculation
S =4.62761 in.
Mean Basin Slope Y =5.87236 %from Watershed Mean slope
Longest Flowpath:0.72 mi.from Watershed Longest flowpath
Flow Length l =3801.6 ft.
Time of Concentration Tc =0.89 Hours Tc =53 mins
Lag Time TL =0.53 Hours TL =32 mins

APPENDIX C
NRCS Curve Number Summary Sheet

Change CN according to TR-55 (see tables below)
NLCD (2016) Landuse TR-55 Landuse HSG A (sq. ft.)CN HSG B (sq. ft.)CN HSG C (sq. ft.)CN HSG D (sq. ft.)CN
Open Water Not Available 0 100 0 20621 100 2062122 0 100 0 182771 100 18277084
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 576426 69 39773364 68324 79 5397618 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 251061 72 18076413 426 81 34496 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 27543 85 2341128 0 90 0 0 92 0
Developed, High Intensity Urban districts - Commercial and business 0 89 0 0 92 0 0 94 0 0 95 0
Barren Land Newly graded areas 0 77 0 0 86 0 0 91 0 0 94 0
Deciduous Forest Woods - Fair 0 36 0 890566 60 53433941 41205 73 3007976 74840 79 5912321
Evergreen Forest Woods - Fair 0 36 0 267726 60 16063570 0 73 0 8853 79 699410
Mixed Forest Woods - Fair 0 36 0 536233 60 32174000 84070 73 6137135 9118 79 720326
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 14331 71 1017528 0 81 0 5039 89 448495
Hay/Pasture Pasture - Fair 0 49 0 1468022 69 101293549 246022 79 19435714 515 84 43274
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 0 80 0 0 87 0 0 90 0
Woody Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0
Emergent Herbaceuous Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0 Weighted CN=68.36
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.
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 Analysis

0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24Cumulative Depth (in)Time (hrs)
Hyetographs for 24-Hour SCS Type II 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): China Grove, North Carolina USA
Station Name: -
Latitude: 35.576592°
Longitude: -80.676358°
Elevation (USGS): 866.56 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.387 0.457 0.534 0.588 0.65 0.692 0.73 0.764 0.801 0.827
10-min:0.618 0.731 0.855 0.941 1.04 1.1 1.16 1.21 1.27 1.3
15-min:0.772 0.919 1.08 1.19 1.31 1.4 1.47 1.53 1.6 1.63
30-min:1.06 1.27 1.54 1.72 1.94 2.1 2.25 2.38 2.54 2.65
60-min:1.32 1.59 1.97 2.25 2.59 2.85 3.09 3.34 3.64 3.86
2-hr:1.53 1.85 2.3 2.64 3.08 3.41 3.74 4.07 4.49 4.82
3-hr:1.63 1.97 2.46 2.84 3.34 3.74 4.13 4.53 5.07 5.49
6-hr:1.99 2.39 2.99 3.46 4.09 4.59 5.09 5.61 6.31 6.86
12-hr:2.35 2.84 3.56 4.13 4.92 5.55 6.19 6.86 7.79 8.52
24-hr:2.78 3.35 4.21 4.89 5.81 6.54 7.29 8.07 9.12 9.95
2-day:3.24 3.9 4.86 5.62 6.64 7.45 8.27 9.12 10.3 11.2
3-day:3.44 4.14 5.13 5.91 6.97 7.81 8.67 9.55 10.7 11.7
4-day:3.65 4.37 5.4 6.2 7.3 8.18 9.07 9.98 11.2 12.2
7-day:4.21 5.02 6.11 6.97 8.15 9.08 10 11 12.3 13.4
10-day:4.81 5.72 6.88 7.78 8.98 9.93 10.9 11.8 13.1 14.1
20-day:6.46 7.63 9.01 10.1 11.5 12.7 13.8 15 16.5 17.7
30-day:7.99 9.39 10.9 12.1 13.6 14.8 15.9 17 18.6 19.7
45-day:10.1 11.8 13.4 14.7 16.3 17.5 18.7 19.9 21.4 22.5
60-day:12 14 15.7 17.1 18.9 20.2 21.5 22.7 24.3 25.5
Date/time (GMT): Wed Nov 18 19:29:50 2020
pyRunTime: 0.0253880023956

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:11/16/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Howard Hensley Dam Dam No.ROWAN-055
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
853.3 0.0 0.0 0.0
853.8 2.3 0.0 2.3
854.3 3.1 0.0 3.1
854.8 3.1 0.0 3.1
855.3 3.2 0.0 3.2
855.8 3.2 5.1 8.3
856.3 3.2 78.4 81.6
856.8 3.3 203.2 206.5
857.3 3.3 371.7 375.0
857.8 3.3 581.7 585.0
858.3 3.4 832.8 836.2
858.8 3.4 1125.2 1128.6
859.3 3.4 1459.3 1462.8
859.8 3.4 1836.0 1839.4
860.3 3.5 2255.8 2259.3
860.8 3.5 2719.8 2723.3
861.3 3.5 3228.9 3232.4
861.8 3.6 3783.8 3787.4
862.3 3.6 4385.6 4389.2
862.8 3.6 5035.2 5038.8
863.3 3.6 5733.4 5737.1
863.8 3.7 6481.4 6485.0
864.3 3.7 7279.8 7283.5
864.8 3.7 8129.7 8133.4
865.3 3.8 9031.9 9035.7
865.8 3.8 9987.4 9991.2
866.3 3.8 10997.0 11000.8
866.8 3.8 12061.5 12065.4
867.3 3.9 13181.9 13185.8
867.8 3.9 14359.0 14362.9
868.3 3.9 15593.6 15597.5
868.8 3.9 16886.5 16890.5
869.3 4.0 18238.6 18242.6
869.8 4.0 19650.7 19654.7
870.3 4.0 21123.6 21127.6
870.8 4.0 22658.0 22662.0
871.3 4.1 24254.8 24258.8
871.8 4.1 25914.7 25918.8
872.3 4.1 27638.4 27642.6
872.8 4.1 29426.9 29431.0
873.3 4.2 31280.7 31284.9
873.8 4.2 33200.6 33204.8
874.3 4.2 35187.4 35191.6
874.8 4.2 37241.8 37246.1
875.3 4.3 39364.5 39368.8
875.8 4.3 41556.2 41560.5
876.3 4.3 43817.7 43822.0
876.8 4.3 46149.6 46153.9
877.3 4.4 48552.5 48556.9
877.8 4.4 51027.3 51031.7
878.3 4.4 53574.5 53579.0
878.8 4.4 56194.9 56199.4
0.0
10000.0
20000.0
30000.0
40000.0
50000.0
60000.0
850.0 855.0 860.0 865.0 870.0 875.0 880.0 885.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)