RANDO-199 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
NORTHMONT ESTATES DAM
RANDO-199
RANDOLPH COUNTY, NC
PREPARED BY:
WOOD E&I Solutions
2020
EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Northmont Estates Dam in Randolph 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, and 100
years rainfall events. The structure is over-topped by the 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 =
591.6
592.1
592.6
593.1
593.6
589.1
589.6
590.1
590.6
591.1
582.1
579.1
579.6
580.1
580.6
581.1
581.6
578.6
577.6
578.1
ELEVATION ELEVATIONSTORAGE (ACRE‐FT) STORAGE (ACRE‐FT)
579.7Auxiliary Stone‐Lined Trapezoidal Channel
0.0
6.8
577.6
71
54
SCS  Type 2
SPILLWAY
Principal
TYPE
72" Round Riser w/ 36‐in DIP barrel
ELEVATION
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
TOP OF DAM ELEVATION = 582.4
24 HOUR ‐ 100 YEAR EVENT, 7.42 INCHES, ELEVATION 582.3 (0.1 FT FREE‐BOARD)
24 HOUR ‐ 50 YEAR EVENT, 6.68 INCHES, ELEVATION 581.9 (0.5 FT FREE‐BOARD)
24 HOUR ‐ 25 YEAR EVENT, 5.96 INCHES, ELEVATION 581.4 (1 FT FREE‐BOARD)
SUMMARY OF INPUT PARAMETERS
1.25
NORTHMONT ESTATES DAM, RANDO‐199
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
24 HOUR ‐ 1000 YEAR EVENT, 10 INCHES, ELEVATION 583.1 (0.7 FT OVER‐TOPPED)
24 HOUR ‐ 500 YEAR EVENT, 9.22 INCHES, ELEVATION 582.9 (0.5 FT OVER‐TOPPED)
24 HOUR ‐ 200 YEAR EVENT, 8.18 INCHES, ELEVATION 582.6 (0.2 FT OVER‐TOPPED)
582.6
583.1
583.6
584.1
328.5
343.2
358.3
373.8
594.6
595.1
595.6
594.1
389.6596.1
201.1
212.3
223.8
101.5
75.5
84.0
92.6
13.7
20.8
28.1
35.5
51.0
58.9
67.1
43.2
314.2
235.7
247.9
260.5
273.4
286.6
300.2
110.5
119.7
129.2
138.8
148.7
588.1
588.6
585.1
585.6
586.1
586.6
587.1
587.6
584.6
158.7
169.0
NOT TO SCALE
511.7
526.9
405.9
422.6
439.6
457.1
474.9
493.1
597.6
598.1
598.6
599.1
599.6
600.0
596.6
597.1
179.5
190.2
NORTHMONT ESTATES DAM RANDO‐199 AUGUST 14, 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
Northmont Estates Dam - RANDO-199 - Watershed Boundary
Legend
Longest FlowPath
Watershed Boundary
0 0.25 0.50.125 Miles
Date: 8/13/2020
/Drainage Area: 1.25 Square Miles
Flowpath Length: 2.21 Miles

APPENDIX B
Lag Time Calculation

Dam No: RANDO-199 Date: 7/16/2020
Dam Name: Northmont Estates 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 = 71.00
S =4.08451 in.
Mean Basin Slope Y = 10.603 %
Longest Flowpath: 2.209 mi. (from Streamstats)
Flow Length l =11663.5 ft.
Time of Concentration Tc =1.51 Hours Tc =90 mins
Lag Time TL =0.90 Hours TL =54 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 426172 100 42617188 0 100 0 0 100 0
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 3525159 69 243235966 1166978 79 92191241 3367027 84 282830253
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 1905545 72 137199217 33074 81 2678968 948593 86 81578961
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 447523 85 38039449 0 90 0 385434 92 35459933
Developed, High Intensity Urban districts - Commercial and business 0 89 0 109156 92 10042377 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 11568319 60 694099141 476789 73 34805561 3319639 79 262251505
Evergreen Forest Woods - Fair 0 36 0 181337 60 10880222 0 73 0 205762 79 16255184
Mixed Forest Woods - Fair 0 36 0 837765 60 50265896 10521 73 768023 826462 79 65290526
Shrub/Scrub Brush - Fair 0 35 0 79231 56 4436962 0 70 0 221026 77 17019029
Herbaceuous Herbaceuous - Fair 0 60 0 860801 71 61116876 185975 81 15063984 95932 89 8537936
Hay/Pasture Pasture - Fair 0 49 0 2701807 69 186424702 85955 79 6790459 924933 84 77694368
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 0 80 0 0 87 0 9686 90 871718
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=71.00
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
0.02.04.06.08.010.012.00123456789101112131415161718192021222324Cumulative Depth (in)Time (hrs)Hyetographs for 24-Hour SCS Type II Rainfall Distributions25YR50YR100YR200YR500YR1000YR

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): Asheboro, North Carolina USA
Station Name: ‐
Latitude: 35.7448°
Longitude: ‐79.8298°
Elevation (USGS): None None
PRECIPITATION FREQUENCY ESTIMATES
by 
duration 
for ARI 
(years):1 2 5 10 25 50 100 200 500 1000
5‐min: 0.4 0.474 0.551 0.603 0.66 0.696 0.728 0.753 0.779 0.795
10‐min: 0.639 0.758 0.883 0.965 1.05 1.11 1.16 1.19 1.23 1.25
15‐min: 0.799 0.953 1.12 1.22 1.33 1.4 1.46 1.51 1.55 1.57
30‐min: 1.1 1.32 1.59 1.77 1.97 2.11 2.24 2.35 2.47 2.55
60‐min: 1.37 1.65 2.03 2.3 2.63 2.87 3.08 3.29 3.54 3.72
2‐hr: 1.61 1.95 2.42 2.76 3.19 3.52 3.83 4.13 4.52 4.81
3‐hr: 1.72 2.09 2.6 2.97 3.46 3.83 4.19 4.56 5.03 5.38
6‐hr: 2.08 2.52 3.13 3.6 4.23 4.71 5.2 5.69 6.36 6.88
12‐hr: 2.47 2.99 3.74 4.33 5.14 5.78 6.44 7.12 8.08 8.83
24‐hr: 2.89 3.49 4.36 5.04 5.96 6.68 7.42 8.18 9.22 10
2‐day: 3.38 4.06 5.04 5.78 6.79 7.57 8.37 9.19 10.3 11.2
3‐day: 3.59 4.31 5.32 6.1 7.16 7.99 8.83 9.7 10.9 11.8
4‐day: 3.8 4.56 5.6 6.42 7.52 8.4 9.29 10.2 11.5 12.4
7‐day: 4.35 5.18 6.29 7.17 8.36 9.31 10.3 11.3 12.6 13.7
10‐day: 4.94 5.88 7.05 7.98 9.22 10.2 11.2 12.2 13.6 14.6
20‐day: 6.65 7.84 9.25 10.4 11.9 13.1 14.3 15.6 17.3 18.6
30‐day: 8.22 9.67 11.2 12.4 14 15.3 16.5 17.7 19.3 20.6
45‐day: 10.4 12.1 13.9 15.3 17 18.4 19.7 21 22.8 24.1
60‐day: 12.4 14.5 16.4 17.8 19.6 21 22.4 23.7 25.3 26.6
Date/time (GMT):  Mon Jun 22 16:34:25 2020
pyRunTime:  0.0106308460236

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NCEM Date: 7/16/2020
Project: Overtopping Study Engineer: E.Okorie
Dam: Northmont Estates Dam Dam No. RANDO-199
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
577.6 0.0 0.0 0.0
578.1 20.7 0.0 20.7
578.6 58.4 0.0 58.4
579.1 107.4 0.0 107.4
579.6 146.4 0.0 146.4
580.1 148.4 47.8 196.2
580.6 150.3 164.6 314.9
581.1 152.2 325.5 477.7
581.6 154.1 524.4 678.5
582.1 156.0 758.5 914.5
582.6 157.8 1026.0 1183.8
583.1 159.6 1326.0 1485.6
583.6 161.4 1657.8 1819.3
584.1 163.2 2021.3 2184.5
584.6 165.0 2416.1 2581.1
585.1 166.7 2842.2 3009.0
585.6 168.5 3299.7 3468.2
586.1 170.2 3788.7 3958.8
586.6 171.9 4309.2 4481.0
587.1 173.5 4861.4 5035.0
587.6 175.2 5445.6 5620.9
588.1 176.9 6062.0 6238.9
588.6 178.5 6710.8 6889.3
589.1 180.1 7392.3 7572.4
589.6 181.7 8106.7 8288.4
590.1 183.3 8854.3 9037.6
590.6 184.9 9635.4 9820.2
591.1 186.4 10450.3 10636.7
591.6 188.0 11299.2 11487.2
592.1 189.5 12182.5 12372.0
592.6 191.0 13100.5 13291.5
593.1 192.5 14053.4 14245.9
593.6 194.0 15041.6 15235.6
594.1 195.5 16065.4 16260.9
594.6 197.0 17125.0 17322.0
595.1 198.5 18220.8 18419.3
595.6 199.9 19353.1 19553.0
596.1 201.4 20522.2 20723.6
596.6 202.8 21728.4 21931.2
597.1 204.2 22971.9 23176.1
597.6 205.6 24253.2 24458.8
598.1 207.0 25572.4 25779.4
598.6 208.4 26929.9 27138.4
599.1 209.8 28326.1 28535.9
599.6 211.2 29761.1 29972.2
600.1 212.5 31235.2 31447.8
0.0
5000.0
10000.0
15000.0
20000.0
25000.0
30000.0
35000.0
575.0 580.0 585.0 590.0 595.0 600.0 605.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)