SURRY-066 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
HUGH CHATHAM HOSPITAL DAM
SURRY-066
SURRY 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 Hugh Chatham Hospital 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, 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 =
176.8
144.0
149.2
154.5
159.9
165.4
171.0
1020.1
1020.6
1021.1
1021.6
1022.1
ELEVATION
1017.6
1018.1
1018.6
1019.1
1019.6
13.9
16.1
18.4
STORAGE (ACRE-FT)
0.0
1.8
3.7
5.6
7.6
9.6
11.7
1001.6
998.6
999.1
999.6
1000.1
1000.6
1001.1
997.1
NA
ELEVATION
997.1
997.6
998.1
STORAGE (ACRE-FT)
129.1
134.0
138.9
73.53
16
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
NA
ELEVATION
24 HOUR - 200 YEAR EVENT, 8.93 INCHES, ELEVATION 1000.5 (20.1 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.01 INCHES, ELEVATION 999.9 (20.7 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.13 INCHES, ELEVATION 999.4 (21.2 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.28 INCHES, ELEVATION 998.9 (21.7 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.1
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
HUGH CHATHAM HOSPITAL DAM, SURRY-066
ANALYSIS PERFORMED BY: S. MURPHY, HDR
TOP OF DAM ELEVATION = 1020.6
24 HOUR - 1000 YEAR EVENT, 11.2 INCHES, ELEVATION 1001.9 (18.7 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 10.2 INCHES, ELEVATION 1001.3 (19.3 FT FREE-BOARD)
SURRY-066HUGH CHATHAM HOSPITAL DAM NOVEMBER 23, 2020
402.9
NOT TO SCALE
349.3
357.9
366.6
375.5
384.5
393.6
300.3
308.2
316.2
324.3
332.5
340.9
255.2
262.4
269.8
277.2
284.8
292.5
214.0
220.6
227.3
234.1
241.0
248.0
182.7
188.8
194.9
201.1
207.5
1035.1
1035.6
1036.1
1036.6
1037.1
1032.1
1032.6
1033.1
1033.6
1034.1
1034.6
1029.1
1029.6
1030.1
1030.6
1031.1
1031.6
1026.1
1026.6
1027.1
1027.6
1028.1
1028.6
1023.1
1023.6
1024.1
1024.6
1025.1
1025.6
1022.6
110.5
115.0
119.6
124.3
85.7
89.6
93.6
97.7
101.9
106.2
63.9
67.4
70.9
74.4
78.1
81.9
44.8
47.8
50.9
54.1
57.3
60.6
28.2
30.8
33.4
36.2
39.0
41.9
20.7
23.1
25.6
1016.6
1017.1
1013.6
1014.1
1014.6
1015.1
1015.6
1016.1
1010.6
1011.1
1011.6
1012.1
1012.6
1013.1
1007.6
1008.1
1008.6
1009.1
1009.6
1010.1
1004.6
1005.1
1005.6
1006.1
1006.6
1007.1
1002.1
1002.6
1003.1
1003.6
1004.1
SURRY-066HUGH CHATHAM HOSPITAL 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
Hugh Chatham Hospital Dam - SURRY-066 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.1 0.20.05 Miles
Date: 11/19/2020
/Drainage Area: 0.10 Square MilesFlowpath Length: 2719.2 feet

APPENDIX B

Lag Time Calculation
Dam No:SURRY-066 Date:11/19/2020
Dam Name:Hugh Chatham Hospital 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 =73.5252 from NRCS CN Calculation
S =3.60078 in.
Mean Basin Slope Y =11.0973 %from Watershed Mean slope
Longest Flowpath:0.52 mi.from Watershed Longest flowpath
Flow Length l =2745.6 ft.
Time of Concentration Tc =0.43 Hours Tc =26 mins
Lag Time TL =0.26 Hours TL =16 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 0 100 0 36055 100 3605458 70514 100 7051368
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 307070 69 21187812 171063 79 13513939 11416 84 958966
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 355543 72 25599101 23363 81 1892425 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 185662 85 15781304 9549 90 859393 0 92 0
Developed, High Intensity Urban districts - Commercial and business 0 89 0 43140 92 3968865 0 94 0 0 95 0
Barren Land Newly graded areas 0 77 0 9688 86 833172 0 91 0 0 94 0
Deciduous Forest Woods - Fair 0 36 0 279701 60 16782062 758722 73 55386672 55298 79 4368508
Evergreen Forest Woods - Fair 0 36 0 0 60 0 0 73 0 0 79 0
Mixed Forest Woods - Fair 0 36 0 88154 60 5289264 35724 73 2607836 3043 79 240378
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 34170 71 2426100 33646 81 2725310 0 89 0
Hay/Pasture Pasture - Fair 0 49 0 150511 69 10385264 48047 79 3795726 0 84 0
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=73.53
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): Elkin, North Carolina USA
Station Name: -
Latitude: 36.264004°
Longitude: -80.836198°
Elevation (USGS): 1024.59 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.35 0.416 0.493 0.551 0.623 0.675 0.727 0.777 0.84 0.89
10-min:0.56 0.666 0.79 0.881 0.992 1.08 1.16 1.23 1.33 1.4
15-min:0.7 0.837 0.999 1.11 1.26 1.36 1.46 1.55 1.67 1.76
30-min:0.959 1.16 1.42 1.61 1.86 2.05 2.24 2.42 2.66 2.85
60-min:1.2 1.45 1.82 2.1 2.48 2.78 3.08 3.39 3.82 4.16
2-hr:1.42 1.73 2.19 2.55 3.05 3.46 3.88 4.32 4.94 5.44
3-hr:1.54 1.88 2.38 2.77 3.32 3.78 4.24 4.74 5.44 6.02
6-hr:1.94 2.35 2.97 3.46 4.14 4.69 5.26 5.87 6.73 7.43
12-hr:2.42 2.94 3.68 4.27 5.09 5.74 6.41 7.12 8.12 8.92
24-hr:2.9 3.52 4.48 5.23 6.28 7.13 8.01 8.93 10.2 11.2
2-day:3.47 4.19 5.28 6.13 7.29 8.22 9.17 10.2 11.5 12.6
3-day:3.69 4.46 5.58 6.45 7.65 8.6 9.57 10.6 12 13.1
4-day:3.92 4.72 5.88 6.78 8 8.98 9.98 11 12.4 13.5
7-day:4.49 5.38 6.57 7.49 8.72 9.69 10.7 11.7 13 14.1
10-day:5.15 6.15 7.4 8.36 9.63 10.6 11.6 12.6 13.9 14.9
20-day:6.91 8.18 9.67 10.8 12.4 13.6 14.7 15.9 17.5 18.8
30-day:8.58 10.1 11.7 12.8 14.3 15.4 16.5 17.5 18.8 19.8
45-day:10.9 12.7 14.4 15.7 17.2 18.4 19.5 20.6 22 23
60-day:12.9 15.1 16.9 18.3 20.1 21.4 22.7 23.9 25.4 26.5
Date/time (GMT): Thu Nov 19 16:05:21 2020
pyRunTime: 0.0174450874329

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:Hugh Chatham Hospital Dam Dam No.SURRY-066
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
997.1 0.0 0.0
997.6 17.5 17.5
998.1 48.2 48.2
998.6 48.6 48.6
999.1 48.9 48.9
999.6 49.2 49.2
1000.1 49.5 49.5
1000.6 49.8 49.8
1001.1 50.1 50.1
1001.6 50.4 50.4
1002.1 50.7 50.7
1002.6 51.0 51.0
1003.1 51.3 51.3
1003.6 51.6 51.6
1004.1 51.8 51.8
1004.6 52.1 52.1
1005.1 52.4 52.4
1005.6 52.7 52.7
1006.1 53.0 53.0
1006.6 53.3 53.3
1007.1 53.6 53.6
1007.6 53.8 53.8
1008.1 54.1 54.1
1008.6 54.4 54.4
1009.1 54.7 54.7
1009.6 54.9 54.9
1010.1 55.2 55.2
1010.6 55.5 55.5
1011.1 55.8 55.8
1011.6 56.0 56.0
1012.1 56.3 56.3
1012.6 56.6 56.6
1013.1 56.8 56.8
1013.6 57.1 57.1
1014.1 57.4 57.4
1014.6 57.6 57.6
1015.1 57.9 57.9
1015.6 58.1 58.1
1016.1 58.4 58.4
1016.6 58.7 58.7
1017.1 58.9 58.9
1017.6 59.2 59.2
1018.1 59.4 59.4
1018.6 59.7 59.7
1019.1 59.9 59.9
1019.6 60.2 60.2
1020.1 60.4 60.4
1020.6 60.7 60.7
1021.1 60.9 60.9
1021.6 61.2 61.2
1022.1 61.4 61.4
1022.6 61.7 61.7
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
995.0 1000.0 1005.0 1010.0 1015.0 1020.0 1025.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)