DURHA-046 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
LITTLE RIVER DAM
DURHA-046
DURHAM COUNTY, NC
PREPARED BY:
GEORGE ELLER, PE
2020
W.�=

0
EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Little River Dam in Durham County, North
Carolina. The purpose of the study was to analyze the existing potential of the dams 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 ArcGIS 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 all of the above rainfall events,
except the 1000 year return period event. The dam was overtopped during this event by 0.2 feet.
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. The elevation -storage calculation does not include the storage below the normal pool
elevation for this study. All elevations in this report are based on North American Vertical Datum
of 1988 (NAVD88).

SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE RIVER DAM, DURHA-046
ANALYSIS PERFORMED BY: GEORGE ELLER, PE, NCDEQ-DEMLR
24 HOUR - 1000 YEAR EVENT, 9.91 INCHES, MAX W.S. ELEVATION 364.3 (0.2 FT OVERTOPPED)
24 HOUR - 500 YEAR EVENT, 9.12 INCHES, MAX W.S. ELEVATION 363.4 (0.7 FT FREEBOARD)
-------------------------------------------------
24 HOUR - 200 YEAR EVENT, 8.12 INCHES, MAX W.S. ELEVATION 362.2 (1.9 FT FREEBOARD)
-------------------------------------------------
24 HOUR - 100 YEAR EVENT, 7.38 INCHES, MAX W.S. ELEVATION 361.3 (2.8 FT FREEBOARD)
-------------------------------------------------
24 HOUR - 50 YEAR EVENT, 6.66 INCHES, MAX W.S. ELEVATION 361.3 (3.6 FT FREEBOARD)
-------------------------------------------------
24 HOUR - 25 YEAR EVENT, 5.95 INCHES, MAX W.S. ELEVATION 359.6 (4.5 FT FREEBOARD)
-------------------------------------------------
SUMMARY OF INPUT PARAMETERS
Drainage Area (Sq. Mi.) = 97.1
Curve Number = 69
Lag Time (min.) = 582
Rainfall Distribution Methods = NRCS-NOAA
SPILLWAY
TYPE
ELEVATION (FT NAVD88)
Principal
Concrete Riser
354.1
Auxiliary
N/A
N/A
ELEVATION
STORAGE (ACRE -FT)
ELEVATION
STORAGE (ACRE -FT)
353.0
0.0
362.0
5,042.9
354.0
408.5
363.0
5,690.4
355.0
937.0
364.0
6,354.8
356.0
1,481.1
365.0
7,035.9
357.0
2,039.2
366.0
7,736.1
358.0
2,610.7
367.0
8,451.6
359.0
3,196.3
368.0
9,184.3
360.0
3,796.3
369.0
9,934.5
361.0
4,411.5
370.0
10,699.9
NOT TO SCALE
LITTLE RIVER DAM DURHA-048 May 20, 2020

APPENDIX A
Watershed Boundary

Little River Dam - DURHA-046 - Watershed Boundary
Drainage Area: 97.1 Square Miles
Flowpath Length: 150,664 Feet
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Legend
Watershed Boundary
Longest Flowpath
0
ly \e G
ih F/fir ripe
G
Timberlake
N
W + E
S
-- --- --- 7988
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Sources: Esri, HERE,: Garmin, Intermap, increment P Corp., GEBCO,
0 1.5 3 6 Miles USES, FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance
l i I I I I I I Survey, Esri Japan; METI, Esri China (Hong Kong), (c) OpenStreetMap
contributors, and the GIs Us'dr-Community
I lni,.',.--

APPENDIX 6
Time of Concentration Calculation

Dam No: DURHA-046
Dam Name: Little River Dam
Maximum Potential Retention (inches): S = (1000/cn')-10
Time of Concentration (hours): Tc = 10.8*(S+1)0.7
1140*Y0-5
Basin Data:
Curve Number
Mean Basin Slope
Longest Flowpath:
Flow Length
Time of Concentration
Lag Time
Input Values in
yellow
CN = 69
from StreamStats
S = 4.49275 in.
Y = 6.16 %
from StreamStats
28.535 mi.
from StreamStats
I = 150665 ft.
Date: 9/26/2018
Engineer: G. Eller
Tc = 16.16 Hours Tc = 970 mins
TL = 9.70 Hours TL = 582 mins

APPENDIX C
NRCS Curve Number Summary Sheet

Change CN according to TR-55 (see tables below)
HSG
Type
A
CN
B
CN
C
CN
D
CN
Open Water
0.0000
100
0
1838677.9880
100
183867798.8
417341.2121
100
41734121.21
1739857.7007
100
1.74E+08
Developed, Open Space
139762.7185
68
9503864.9
117325142.1513
79
9268686230
11678020.4206
86
1004309756
24998246.3656
89
2.22E+09
Developed, Low Intensity
6458.7687
57
368149.82
18741359.3541
72
1349377873
2185016.0083
81
176986296.7
4655881.9075
86
4E+08
Developed, Medium Intensity
0.0000
77
0
1468687.9740
85
124838477.8
137824.8586
90
12404237.27
233622.7823
92
21493296
Developed, High Intensity
0.0000
89
0
152499.5973
92
14029962.95
8165.1406
94
767523.2169
19374.6535
95
1840592
Barren Land
0.0000
77
0
1400381.1646
86
120432780.2
75265.7332
91
6849181.72
410918.1664
94
38626308
Deciduous Forest
9237.748121
36
332558.93
857550010.8
60
51453000645
76039741.15
73
5550901104
304826741.5
79
2.41E+10
Evergreen Forest
0.0000
36
0
156947187.8008
60
9416831268
21176992.5645
73
1545920457
44600815.6362
79
3.52E+09
Mixed Forest
0.0000
36
0
69921846.1700
60
4195310770
7928572.7884
73
578785813.6
17265873.9652
79
1.36E+09
Shrub/Scrub
0.0000
35
0
36450809.8215
56
2041245350
2779844.1894
70
194589093.3
18935331.7594
77
1.46E+09
Herbaceuous
0.0000
50
0
73235131.7668
71
5199694355
9105510.3358
81
737546337.2
24387994.0543
89
2.17E+09
Hay/Pasture
470273.7846
49
23043415
537071795.6
69
37057953896
58146367.36
79
4593563022
116012503.4
84
9.75E+09
Cultivated Crops
98235.8201
71
6974743.2
8866122.9536
80
709289836.3
1646915.1260
87
143281616
4447873.4163
90
4E+08
Woody Wetlands
0.0000
80
0
6929875.7088
85
589039435.2
1040954.4966
90
93685904.7
13604274.7824
95
1.29E+09
Emergent Herbaceuous Wetlands
0.0000
80
0
127502.3096
85
10837696.32
4435.7443
90
399216.9845
90145.8113
95
8563852
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. Hydrologic Condition assumed as "Fair"
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
desiened by dan.k
Weighted CN= 69.00

APPENDIX D
Hyetograph Results Used for Analyses

12.0
10.0
8.0
S
0
ar
s
U
ar
6.0
m
m
m
E
3
u
►11i1
2.0
0.0 1-
0
Hyetographs for 24-Hour Rainfall Distrubutions
2 4 6 8 10 12 14 16 18 20 22 24
Hours
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): Durham, NC, USA
Station Name: -
Latitude: 36.1135°
Longitude:-78.8685'
Elevation (USGS): 328.01 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):
1
2
5
10
25
50
100
200
500
1000
5-min:
0.404
0.473
0.541
0.602
0.664
0.711
0.752
0.786
0.824
0.856
10-min:
0.645
0.756
0.866
0.963
1.06
1.13
1.19
1.25
1.3
1.35
15-min:
0.806
0.95
1.1
1.22
1.34
1.43
1.51
1.57
1.64
1.69
30-min:
1.11
1.31
1.56
1.77
1.99
2.16
2.31
2.45
2.61
2.74
60-min:
1.38
1.65
2
2.3
2.65
2.92
3.19
3.43
3.75
4
2-hr:
1.65
1.97
2.41
2.8
3.27
3.66
4.04
4.42
4.91
5.33
3-hr:
1.75
2.1
2.58
3.01
3.54
3.99
4.43
4.89
5.48
6.01
6-hr:
2.11
2.53
3.1
3.64
4.3
4.88
5.46
6.06
6.87
7.59
12-hr:
2.5
2.99
3.69
4.35
5.2
5.94
6.7
7.51
8.63
9.64
24-hr:
2.92
3.52
4.38
5.051
5.95
6.66
7.38
8.12
9.12
9.91
2-day:
3.42
4.11
5.07
5.8
6.77
7.53
8.29
9.07
10.1
10.9
3-day:
3.61
4.33
5.33
6.09
7.12
7.92
8.73
9.55
10.7
11.5
4-day:
3.8
4.55
5.58
6.38
7.46
8.31
9.17
10
11.2
12.2
7-day:
4.37
5.21
6.31
7.18
8.34
9.26
10.2
11.2
12.5
13.5
10-day:
4.97
5.9
7.07
8
9.23
10.2
11.2
12.2
13.5
14.6
20-day:
6.6
7.79
9.19
10.3
11.9
13.1
14.3
15.6
17.3
18.6
30-day:
8.21
9.65
11.2
12.4
14
15.2
16.4
17.7
19.3
20.5
45-day:
10.4
12.2
14
15.3
17.2
18.6
19.9
21.3
23.1
24.5
60-day:
12.5
14.6
16.5
17.9
19.7
21.1
22.4
23.7
25.4
26.6
Date/time (GMT): Wed Apr 18 17:43:11 2018
pyRu nTi m e: 0.0402519702911

APPENDIX F
Outlet Works Rating Curves

Little River Dam, DURHA-046
Primary Spillway: Broad crested weir with support piers
Emergency Spillway: None
Top of Dam: 364.1 ft NAVD88
Normal Pool : 354.1 ft NAVD88
SPILLWAY ASSESSMENT
RWS Elevation
(ft NAVD88)
Primary Spillway
Discharge (cfs)
Emergency Spillway
Discharge (cfs)
Total Structure
Discharge (cfs)
354.1
0
0
0
355.1
820
0
820
356.1
2,285
0
2,285
357.1
4,136
0
4,136
358.1
6,273
0
6,273
359.1
8,633
0
8,633
360.1
11,174
0
11,174
361.1
13,861
0
13,861
362.1
16,666
0
16,666
363.1
19,566
0
19,566
364.1
22,541
0
22,541
365.1
25,572
0
25,572
366.1
28,643
0
28,643
367.1
31,740
0
31,740
368.1
43,562
0
43,562
369.1
48,312
0
48,312
370.1
53,222
0
53,222
371.1
58,289
0
58,289
372.1
63,507
0
63,507
373.1
68,872
0
68,872
374.1
74,381
0
74,381
375.1
80,028
0
80,028
376.1
85,812
0
85,812
377.1
91,729
0
91,729
378.1
97,776
0
97,776
379.1
103,950
0
103,950
380.1
110,249
0
110,249
381.1
116,670
0
116,670
382.1
123,212
0
123,212
383.1
129,871
0
129,871

385
380
375
370
365
360
355
350
Spillway Discharge Capacity
20,000 40,000
Primary Spillway:
60,000 80,000
Spillway Discharge (cfs)
Emergency Spillway: — —Total Structure
100,000 120,000