SURRY-002 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

CEDAR BROOK LAKE DAM

SURRY-002

SURRY COUNTY, NC

PREPARED BY:
WOOD E&I Solutions

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Cedar Brook Lake Dam in Surry County, North
Carolina. The purpose of the study was to analyze the existing potential of the dam 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 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 =
0.46
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
CEDAR BROOK LAKE DAM, SURRY-002
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
TOP OF DAM ELEVATION = 1202
24 HOUR - 1000 YEAR EVENT, 11.3 INCHES, ELEVATION 1200.7 (1.3 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 10.3 INCHES, ELEVATION 1200.4 (1.6 FT FREE-BOARD)
24 HOUR - 200 YEAR EVENT, 8.99 INCHES, ELEVATION 1200 (2 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.05 INCHES, ELEVATION 1199.6 (2.4 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.16 INCHES, ELEVATION 1199.2 (2.8 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.3 INCHES, ELEVATION 1198.8 (3.2 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
69.83
41
SCS Type 2
SPILLWAY
Principal
TYPE
48" Riser w/ 48-in CMP barrel
ELEVATION
1196.9
STORAGE (ACRE-FT)
0.0
4.0
8.2
1195.9
1198.0
ELEVATION
1195.9
1196.4
Auxiliary Grass-line Trapezoidal channel
1197.4
1197.9
1198.4
1198.9
1199.4
36.9
1200.4
1200.9
1201.4
1201.9
1199.9
12.5
17.1
21.7
26.6
31.6
42.4
48.2
54.2
60.4
66.8
1207.9
1208.4
ELEVATION
1203.4
1203.9
1204.4
1204.9
1205.4 109.6
1205.9
1206.4
1206.9
1207.4
STORAGE (ACRE-FT)
80.2
87.2
94.5
101.9
161.1
1208.9
1209.4
1209.9
1210.0
117.6
125.8
134.2
142.9
151.9
170.6
180.4
190.4
192.5
NOT TO SCALE
73.4
1202.4
1202.9
CEDAR BROOK LAKE DAM SURRY-002 SEPTEMBER 18, 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 the
GIS User Community
Cedar Brook Lake Dam - Watershed Boundary
Legend
LongestFlowPath
Watershed Boundary
0 0.2 0.40.1 Miles
Date: 9/10/2020
/Drainage Area: 0.4607 Square Miles
Flowpath Length: 1.31 Miles

APPENDIX B
Lag Time Calculation

Dam No: SURRY-002 Date: 9/9/2020
Dam Name: Cedar Brook Lake Dam Engineer: E.Okorie
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 = 69.83
S =4.32049 in.
Mean Basin Slope Y = 8.49 %
Longest Flowpath: 1.306 mi.
Flow Length l =6895.68 ft.
Time of Concentration Tc =1.14 Hours Tc =69 mins
Lag Time TL =0.69 Hours TL =41 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 7488 100 748824 225033 100 22503274 0 100 0
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)118363 49 5799765 5750106 69 396757323 1607394 79 126984126 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 567397 72 40852582 45616 81 3694918 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 93990 85 7989173 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 67818 86 5832369 0 91 0 0 94 0
Deciduous Forest Woods - Fair 0 36 0 1307319 60 78439124 191792 73 14000808 0 79 0
Evergreen Forest Woods - Fair 0 36 0 67819 60 4069114 19377 73 1414492 0 79 0
Mixed Forest Woods - Fair 0 36 0 618145 60 37088672 50433 73 3681602 0 79 0
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 48442 71 3439364 9688 81 784750 0 89 0
Hay/Pasture Pasture - Fair 40145 49 1967127 1780079 69 122825453 191892 79 15159458 0 84 0
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 35945 80 2875597 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=69.83
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 Analyses
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): State Road, North Carolina USA
Station Name: -
Latitude: 36.3121°
Longitude: -80.8567°
Elevation (USGS): None None
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.348 0.415 0.492 0.55 0.624 0.679 0.734 0.788 0.857 0.912
10-min:0.556 0.663 0.788 0.88 0.995 1.08 1.17 1.25 1.36 1.44
15-min:0.696 0.834 0.996 1.11 1.26 1.37 1.47 1.58 1.71 1.8
30-min:0.954 1.15 1.42 1.61 1.87 2.06 2.26 2.45 2.72 2.92
60-min:1.19 1.45 1.82 2.1 2.49 2.8 3.11 3.44 3.9 4.26
2-hr:1.41 1.72 2.19 2.56 3.07 3.49 3.93 4.39 5.06 5.6
3-hr:1.54 1.87 2.38 2.78 3.34 3.81 4.29 4.81 5.56 6.18
6-hr:1.94 2.35 2.97 3.47 4.15 4.72 5.31 5.94 6.83 7.56
12-hr:2.43 2.94 3.69 4.28 5.09 5.75 6.42 7.13 8.13 8.95
24-hr:2.91 3.53 4.49 5.25 6.3 7.16 8.05 8.99 10.3 11.3
2-day:3.48 4.21 5.3 6.15 7.33 8.28 9.25 10.3 11.7 12.8
3-day:3.71 4.47 5.6 6.48 7.69 8.66 9.65 10.7 12.1 13.2
4-day:3.93 4.74 5.9 6.81 8.05 9.04 10.1 11.1 12.5 13.7
7-day:4.51 5.4 6.59 7.52 8.77 9.76 10.8 11.8 13.2 14.2
10-day:5.17 6.17 7.43 8.4 9.68 10.7 11.7 12.7 14 15.1
20-day:6.93 8.21 9.71 10.9 12.4 13.6 14.8 16.1 17.7 18.9
30-day:8.61 10.1 11.7 12.9 14.4 15.5 16.6 17.6 19 20
45-day:10.9 12.7 14.5 15.7 17.3 18.5 19.7 20.8 22.2 23.3
60-day:13 15.1 17 18.4 20.2 21.6 22.8 24.1 25.6 26.8
Date/time (GMT): Mon Jun 22 17:21:03 2020
pyRunTime: 0.0184390544891

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NCEM Date: 9/10/2020
Project: Overtopping Study Engineer: E.Okorie
Dam: Cedar Brook Lake Dam Dam No. SURRY-002
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1195.9 0.0 0.0 0.0
1196.4 13.8 0.0 13.8
1196.9 39.0 0.0 39.0
1197.4 71.6 0.0 71.6
1197.9 85.6 0.0 85.6
1198.4 95.7 36.2 131.8
1198.9 104.8 125.3 230.1
1199.4 113.2 249.3 362.5
1199.9 121.0 404.0 525.0
1200.4 128.4 587.5 715.8
1200.9 135.3 798.8 934.1
1201.4 141.9 1037.6 1179.5
1201.9 148.2 1303.5 1451.8
1202.4 154.3 1596.7 1751.0
1202.9 160.1 1917.1 2077.2
1203.4 165.7 2265.0 2430.7
1203.9 171.1 2640.5 2811.6
1204.4 176.4 3043.9 3220.3
1204.9 181.5 3475.4 3657.0
1205.4 186.5 3935.4 4121.9
1205.9 191.3 4424.2 4615.6
1206.4 196.1 4942.1 5138.2
1206.9 200.7 5489.5 5690.1
1207.4 205.2 6066.6 6271.8
1207.9 209.6 6673.8 6883.4
1208.4 213.9 7311.5 7525.4
1208.9 218.2 7980.0 8198.2
1209.4 222.3 8679.7 8902.0
1209.9 226.4 9410.9 9637.3
1210.4 230.4 10173.9 10404.3
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
1194.0 1196.0 1198.0 1200.0 1202.0 1204.0 1206.0 1208.0 1210.0 1212.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)