SURRY-026 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

RAVEN KNOB LAKE DAM

SURRY-026

SURRY COUNTY, NC

PREPARED BY:
HDR Engineering Inc. of the Carolinas

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Raven Knob 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 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, and 500 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 =
783.7
647.5
669.3
691.5
714.0
736.9
760.2
1264.9
1265.4
1265.9
1266.4
1266.9
ELEVATION
1262.4
1262.9
1263.4
1263.9
1264.4
69.3
80.1
91.2
STORAGE (ACRE-FT)
0.0
9.5
19.0
28.7
38.5
48.6
58.8
1246.4
1243.4
1243.9
1244.4
1244.9
1245.4
1245.9
1241.9
1243.1
ELEVATION
1241.9
1242.4
1242.9
STORAGE (ACRE-FT)
584.3
605.0
626.1
69.31
53
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Soil/Rock Lined TChannel
Grass Lined Trapezoidal Channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 8.49 INCHES, ELEVATION 1247.9 (0.9 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 7.58 INCHES, ELEVATION 1247.3 (1.5 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 6.72 INCHES, ELEVATION 1246.7 (2.1 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 5.9 INCHES, ELEVATION 1246.1 (2.7 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
3.12
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
RAVEN KNOB LAKE DAM, SURRY-026
ANALYSIS PERFORMED BY: S. MURPHY, HDR
24 HOUR - 1000 YEAR EVENT, 10.8 INCHES, ELEVATION 1249.2 (0.4 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 1248.8
24 HOUR - 500 YEAR EVENT, 9.79 INCHES, ELEVATION 1248.6 (0.2 FT FREE-BOARD)
SURRY-026RAVEN KNOB LAKE DAM NOVEMBER 25, 2020
1672.4
NOT TO SCALE
1462.8
1496.5
1530.7
1565.4
1600.6
1636.2
1270.3
1301.2
1332.6
1364.4
1396.8
1429.6
1094.3
1122.6
1151.3
1180.4
1209.9
1239.9
932.5
958.5
984.9
1011.7
1038.8
1066.4
807.7
831.9
856.5
881.5
906.8
1279.9
1280.4
1280.9
1281.4
1281.9
1276.9
1277.4
1277.9
1278.4
1278.9
1279.4
1273.9
1274.4
1274.9
1275.4
1275.9
1276.4
1270.9
1271.4
1271.9
1272.4
1272.9
1273.4
1267.9
1268.4
1268.9
1269.4
1269.9
1270.4
1267.4
505.2
524.5
544.0
563.9
397.3
414.4
431.9
449.7
467.9
486.4
302.5
317.6
332.9
348.5
364.3
380.6
216.7
230.5
244.5
258.7
273.1
287.7
138.4
150.8
163.5
176.5
189.7
203.1
102.6
114.2
126.2
1261.4
1261.9
1258.4
1258.9
1259.4
1259.9
1260.4
1260.9
1255.4
1255.9
1256.4
1256.9
1257.4
1257.9
1252.4
1252.9
1253.4
1253.9
1254.4
1254.9
1249.4
1249.9
1250.4
1250.9
1251.4
1251.9
1246.9
1247.4
1247.9
1248.4
1248.9
SURRY-026RAVEN KNOB LAKE DAM NOVEMBER 25, 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
Raven Knob Lake Dam - SURRY-026 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.75 1.50.375 Miles
Date: 11/19/2020
/Drainage Area: 3.12 Square MilesFlowpath Length: 3.7 Miles

APPENDIX B

Lag Time Calculation
Dam No:SURRY-026 Date:11/19/2020
Dam Name:Raven Knob Lake 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 =69.309 from NRCS CN Calculation
S =4.42814 in.
Mean Basin Slope Y =28.3897 %from Watershed Mean slope
Longest Flowpath:3.73 mi.from Watershed Longest flowpath
Flow Length l =19694.4 ft.
Time of Concentration Tc =1.47 Hours Tc =88 mins
Lag Time TL =0.88 Hours TL =53 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 40615 100 4061473 19981 100 1998065 56098 100 5609850 618306 100 61830637
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 1553961 69 107223311 1009139 79 79721992 1179318 84 99062677
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 95264 72 6859043 20613 81 1669623 396 86 34070
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 22884 85 1945152 6184 90 556584 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 2375009 36 85500336 12983999 60 779039947 38051415 73 2777753310 5404583 79 426962071
Evergreen Forest Woods - Fair 0 36 10 961104 60 57666226 1378390 73 100622488 387093 79 30580315
Mixed Forest Woods - Fair 955297 36 34390697 7360783 60 441646992 9948715 73 726256192 1660238 79 131158809
Shrub/Scrub Brush - Fair 0 35 0 1715 56 96060 25512 70 1785852 103 77 7929
Herbaceuous Herbaceuous - Fair 982 60 58915 82983 71 5891785 83970 81 6801549 83992 89 7475258
Hay/Pasture Pasture - Fair 0 49 0 314206 69 21680221 110562 79 8734425 60044 84 5043715
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=69.31
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): Mount Airy, North Carolina USA
Station Name: -
Latitude: 36.481916°
Longitude: -80.863437°
Elevation (USGS): 1347.84 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.335 0.4 0.475 0.533 0.608 0.664 0.721 0.778 0.853 0.912
10-min:0.536 0.639 0.76 0.852 0.969 1.06 1.15 1.23 1.35 1.44
15-min:0.67 0.803 0.962 1.08 1.23 1.34 1.45 1.56 1.7 1.8
30-min:0.918 1.11 1.37 1.56 1.82 2.02 2.22 2.42 2.7 2.92
60-min:1.15 1.39 1.75 2.03 2.42 2.73 3.06 3.4 3.88 4.26
2-hr:1.36 1.65 2.11 2.46 2.98 3.39 3.84 4.31 4.99 5.56
3-hr:1.47 1.79 2.27 2.66 3.21 3.67 4.15 4.67 5.42 6.04
6-hr:1.84 2.23 2.81 3.28 3.94 4.49 5.06 5.67 6.53 7.24
12-hr:2.29 2.77 3.47 4.01 4.77 5.37 6 6.65 7.57 8.32
24-hr:2.72 3.3 4.19 4.9 5.9 6.72 7.58 8.49 9.79 10.8
2-day:3.25 3.93 4.95 5.76 6.9 7.83 8.8 9.83 11.3 12.5
3-day:3.46 4.18 5.24 6.08 7.24 8.19 9.18 10.2 11.7 12.9
4-day:3.68 4.43 5.52 6.39 7.58 8.55 9.55 10.6 12.1 13.2
7-day:4.22 5.06 6.19 7.08 8.3 9.27 10.3 11.3 12.7 13.8
10-day:4.86 5.79 6.99 7.92 9.17 10.1 11.1 12.1 13.5 14.6
20-day:6.55 7.75 9.19 10.3 11.8 13 14.2 15.4 17 18.2
30-day:8.12 9.56 11.1 12.3 13.7 14.9 16 17.1 18.5 19.5
45-day:10.3 12 13.7 15 16.6 17.8 19 20.1 21.6 22.6
60-day:12.3 14.3 16.2 17.6 19.4 20.7 22 23.2 24.7 25.9
Date/time (GMT): Thu Nov 19 16:42:12 2020
pyRunTime: 0.017147064209

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:11/18/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Raven Knob Lake Dam Dam No.SURRY-026
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1241.9 0.0 0.0 0.0
1242.4 19.7 0.0 19.7
1242.9 58.3 0.0 58.3
1243.4 111.7 33.9 145.7
1243.9 179.1 149.5 328.6
1244.4 260.2 313.4 573.5
1244.9 355.0 516.6 871.6
1245.4 463.7 754.9 1218.6
1245.9 586.6 1025.7 1612.2
1246.4 723.8 1327.3 2051.1
1246.9 875.6 1658.6 2534.3
1247.4 1042.5 2018.8 3061.3
1247.9 1224.5 2407.3 3631.8
1248.4 1422.2 2823.6 4245.7
1248.9 1635.7 3267.3 4903.0
1249.4 1865.4 3738.3 5603.7
1249.9 2111.6 4236.4 6348.0
1250.4 2374.6 4761.5 7136.0
1250.9 2654.6 5313.4 7968.0
1251.4 2952.1 5892.2 8844.3
1251.9 3267.3 6497.8 9765.1
1252.4 3600.4 7130.3 10730.7
1252.9 3951.8 7789.7 11741.5
1253.4 4321.8 8476.0 12797.8
1253.9 4710.6 9189.3 13900.0
1254.4 5118.6 9929.8 15048.4
1254.9 5545.9 10697.4 16243.3
1255.4 5992.9 11492.3 17485.3
1255.9 6459.9 12314.6 18774.5
1256.4 6947.1 13164.5 20111.5
1256.9 7454.7 14041.9 21496.6
1257.4 7983.1 14947.1 22930.2
1257.9 8532.5 15880.2 24412.7
1258.4 9103.1 16841.3 25944.4
1258.9 9695.2 17830.6 27525.9
1259.4 10309.1 18848.2 29157.3
1259.9 10945.0 19894.2 30839.2
1260.4 11603.1 20968.8 32571.9
1260.9 12283.7 22072.1 34355.9
1261.4 12987.1 23204.4 36191.4
1261.9 13713.4 24365.6 38079.0
1262.4 14462.9 25556.0 40018.9
1262.9 15235.8 26775.8 42011.6
1263.4 16032.4 28025.1 44057.5
1263.9 16852.9 29304.0 46156.9
0.0
5000.0
10000.0
15000.0
20000.0
25000.0
30000.0
35000.0
40000.0
45000.0
50000.0
1240.0 1245.0 1250.0 1255.0 1260.0 1265.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)