STOKE-042 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LILLY LAKE DAM
STOKE-042
STOKES COUNTY, NC
PREPARED BY:
HDR Engineering Inc. of the
Carolinas under contract with
WOOD E&I Solutions
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Lilly Lake Dam in Stokes 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 =
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LILLY LAKE DAM, STOKE-042
ANALYSIS PERFORMED BY: S. MURPHY, HDR
24 HOUR - 200 YEAR EVENT, 9.26 INCHES, ELEVATION 896 (1.9 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.2 INCHES, ELEVATION 895.7 (2.2 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.22 INCHES, ELEVATION 895.4 (2.5 FT FREE-BOARD)
TOP OF DAM ELEVATION = 897.9
24 HOUR - 1000 YEAR EVENT, 12.1 INCHES, ELEVATION 896.5 (1.4 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 10.8 INCHES, ELEVATION 896.3 (1.6 FT FREE-BOARD)
82.04
24
SCS Type 2
SPILLWAY TYPE ELEVATION
SUMMARY OF INPUT PARAMETERS
0.25
24 HOUR - 25 YEAR EVENT, 6.31 INCHES, ELEVATION 895.2 (2.7 FT FREE-BOARD)
893.2
893.7
894.2
894.7
895.2
895.7
891.7
894.4
ELEVATION
891.7
892.2
892.7
STORAGE (ACRE-FT)
332.5
347.4
362.7
896.2
28.6
33.3
38.1
STORAGE (ACRE-FT)
0.0
3.7
7.6
11.5
15.6
19.8
24.1
ELEVATION
912.2
912.7
913.2
913.7
914.2
914.7
915.2
915.7
916.2
916.7
378.5
394.8
411.5
428.7
446.3
464.4
483.1
Primary
Auxiliary
Riser / Barrel
Grass Lined Trapezoidal channel
STOKE-042LILLY LAKE DAM OCTOBER 21, 2020
899.2
899.7
900.2
900.7
901.2
901.7
896.7
897.2
897.7
898.2
898.7
906.2
906.7
907.2
907.7
902.2
902.7
903.2
903.7
904.2
904.7
43.1
48.3
53.7
911.2
911.7
908.2
908.7
909.2
909.7
910.2
910.7
905.2
905.7
113.6
121.5
129.7
138.1
59.4
65.3
71.5
77.9
84.6
91.5
277.5
290.6
304.2
318.1
206.2
217.3
228.6
240.4
252.4
264.8
146.9
155.9
165.3
175.0
185.1
195.5
98.6
106.0
919.2
919.7
920.2
917.2
931.2
931.7
926.7
927.2
927.7
928.2
928.7
929.2
929.7
930.2
930.7
923.7
924.2
924.7
925.2
925.7
926.2
920.7
921.2
921.7
922.2
922.7
923.2
917.7
918.2
918.7
894.6
606.1
628.7
652.0
676.0
700.8
726.4
502.2
521.9
542.1
562.9
584.2
1337.1
NOT TO SCALE
1119.1
1153.7
1189.0
1224.9
1261.6
1299.0
924.8
955.6
987.0
1019.1
1051.8
1085.2
752.9
780.0
807.8
836.1
865.1
STOKE-042LILLY LAKE DAM OCTOBER 21, 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
Lilly Lake Dam - STOKE-042 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.15 0.30.075 Miles
Date: 10 /2/2020
/Drainage Area: 0.25 Square MilesFlowpath Length: 4713.3 feet

APPENDIX B

Lag Time Calculation
Dam No:STOKE-042 Date:10/21/2020
Dam Name:Lilly 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 =82.0391 from NRCS CN Calculation
S =2.18931 in.
Mean Basin Slope Y =6.47351 %from Watershed Mean slope
Longest Flowpath:0.89 mi.from Watershed Longest flowpath
Flow Length l =4699.2 ft.
Time of Concentration Tc =0.67 Hours Tc =40 mins
Lag Time TL =0.40 Hours TL =24 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 9053 100 905269 6565 100 656525 216895 100 21689487
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 65772 69 4538295 15363 79 1213701 1049484 84 88156689
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 1397 72 100614 10532 81 853084 225098 86 19358392
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 0 85 0 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 254052 60 15243137 214317 73 15645115 525069 79 41480433
Evergreen Forest Woods - Fair 0 36 0 12064 60 723854 15219 73 1111021 1214 79 95927
Mixed Forest Woods - Fair 0 36 0 3625 60 217513 23104 73 1686603 217085 79 17149677
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 49854 71 3539662 0 81 0 81926 89 7291385
Hay/Pasture Pasture - Fair 0 49 0 65290 69 4505022 493498 79 38986346 3364929 84 282654038
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 26087 80 2086938 0 87 0 9693 90 872369
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=82.04
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
14.0
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): Tobaccoville, North Carolina USA
Station Name: -
Latitude: 36.267224°
Longitude: -80.409459°
Elevation (USGS): 903.58 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.367 0.436 0.516 0.572 0.639 0.686 0.729 0.768 0.815 0.849
10-min:0.585 0.698 0.826 0.914 1.02 1.09 1.16 1.22 1.29 1.34
15-min:0.732 0.877 1.05 1.16 1.29 1.38 1.46 1.54 1.62 1.68
30-min:1 1.21 1.48 1.68 1.91 2.08 2.24 2.39 2.58 2.72
60-min:1.25 1.52 1.9 2.18 2.55 2.82 3.09 3.36 3.71 3.97
2-hr:1.49 1.81 2.29 2.64 3.13 3.51 3.89 4.28 4.8 5.21
3-hr:1.61 1.96 2.47 2.86 3.4 3.81 4.23 4.67 5.26 5.72
6-hr:1.98 2.4 3.03 3.52 4.2 4.75 5.33 5.92 6.76 7.42
12-hr:2.41 2.92 3.7 4.32 5.19 5.91 6.68 7.5 8.67 9.63
24-hr:2.89 3.49 4.44 5.2 6.31 7.22 8.2 9.26 10.8 12.1
2-day:3.4 4.11 5.18 6.03 7.23 8.21 9.24 10.3 11.9 13.2
3-day:3.61 4.35 5.47 6.36 7.62 8.65 9.73 10.9 12.5 13.8
4-day:3.81 4.6 5.76 6.7 8.01 9.09 10.2 11.4 13.1 14.5
7-day:4.34 5.21 6.43 7.4 8.76 9.86 11 12.2 13.9 15.3
10-day:4.96 5.93 7.23 8.26 9.67 10.8 12 13.2 14.9 16.2
20-day:6.69 7.94 9.5 10.7 12.4 13.7 15 16.3 18.1 19.5
30-day:8.29 9.79 11.5 12.7 14.4 15.6 16.8 18 19.6 20.7
45-day:10.5 12.3 14.2 15.6 17.4 18.8 20.1 21.4 23 24.2
60-day:12.5 14.6 16.7 18.2 20.1 21.6 23 24.3 26 27.2
Date/time (GMT): Wed Oct 21 14:36:15 2020
pyRunTime: 0.0202038288116

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:10/20/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Lilly Lake Dam Dam No.STOKE-042
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
891.7 0.0 0.0 0.0
892.2 6.9 0.0 6.9
892.7 10.3 0.0 10.3
893.2 10.4 0.0 10.4
893.7 10.5 0.0 10.5
894.2 10.6 0.0 10.6
894.7 10.7 33.3 44.0
895.2 10.8 157.6 168.4
895.7 10.9 352.3 363.3
896.2 11.1 616.3 627.4
896.7 11.2 951.2 962.4
897.2 11.3 1359.7 1370.9
897.7 11.4 1844.6 1856.0
898.2 11.5 2408.9 2420.4
898.7 11.6 3055.7 3067.3
899.2 11.7 3788.0 3799.7
899.7 11.8 4608.5 4620.3
900.2 11.9 5520.3 5532.1
900.7 12.0 6526.0 6538.0
901.2 12.1 7628.4 7640.5
901.7 12.2 8830.2 8842.4
902.2 12.3 10134.0 10146.3
902.7 12.4 11542.4 11554.7
903.2 12.5 13057.7 13070.2
903.7 12.6 14682.6 14695.2
904.2 12.7 16419.3 16432.0
904.7 12.8 18270.3 18283.1
905.2 12.9 20237.8 20250.6
905.7 13.0 22324.1 22337.0
906.2 13.1 24531.4 24544.4
906.7 13.1 26861.9 26875.0
907.2 13.2 29317.7 29331.0
907.7 13.3 31901.1 31914.4
908.2 13.4 34613.9 34627.4
908.7 13.5 37458.4 37471.9
909.2 13.6 40436.5 40450.1
909.7 13.7 43550.1 43563.8
910.2 13.8 46801.4 46815.1
910.7 13.9 50192.1 50205.9
911.2 13.9 53724.2 53738.1
911.7 14.0 57399.6 57413.6
912.2 14.1 61220.1 61234.2
912.7 14.2 65187.6 65201.8
913.2 14.3 69303.9 69318.2
913.7 14.4 73570.8 73585.2
914.2 14.5 77990.0 78004.5
914.7 14.5 82563.4 82577.9
915.2 14.6 87292.6 87307.2
915.7 14.7 92179.3 92194.0
916.2 14.8 97225.2 97240.0
916.7 14.9 102432.1 102447.0
917.2 14.9 107801.6 107816.5
0.0
20000.0
40000.0
60000.0
80000.0
100000.0
120000.0
890.0 895.0 900.0 905.0 910.0 915.0 920.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)