HomeMy WebLinkAboutWPO202200024 Assessment - Environmental 2023-01-12Rivanna Solar Site
Hydrologic & Hydraulic
Assessment
Completed for:
Adapture Renewables, Inc.
Completed By:
oa
SIERRA OVERHEAD ANALYTICS
Sierra Overhead Analytics, Inc.
PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
DARIN RAY GALLON
Lic. No. 402058736
Darin Galloway, PE
Principal Engineer
dgalloway@sierraoverhead.com
(775) 848-5540
Revision: 0
Date: 12/16/2022
Sierra Overhead Analytics, Inc.
PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
Introduction
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On behalf of Adapture Renewables, Inc., Sierra Overhead Analytics, Inc. (SOA) has prepared this
hydrology and hydraulic report (report) for the Rivanna Solar Site, located in Albemarle County,
near Charlottesville, Virginia. The approximate center point of the project is located at: 37.967' N,
-78.401' W. This report summarizes the results of the hydrology study which was performed to assess
peak flows and flood risk across the project site. All runoff flows towards the edges of the site, with
no flow entering from outside the study area. For this reason a contributing watershed model was not
necessary. A two-dimensional (2D) hydraulic model developed in HEC-RAS was used to assess on -site
depth, velocity, and scour during a 100-year, 25-year, 10-year, 2-year, and 1-year recurrence interval
storm event.
1 Site Data
1.1 Existing Topography and Drainage
SOA utilized USGS 2017 LiDAR data for the region. The site is hilly, sloping at about 0.1 ft/ft toward
the channels in the center of the site, which flow toward the south. The model domain encompasses
the contributing drainage to the site. The channels that drain the site are tributaries to Buck Island
Creek, which flows from west to east about 0.5 mile south of the site. A FEMA Zone A (100-year
floodplain) area borders the southern side of the site. The remaining site area falls in FEMA Zone X
— outside of the 100-year floodplain. Flood zones are shown in Appendix A, Figure 1.
1.2 Site Soils and Land Use
Soils data was downloaded from United States Department of Agriculture (USDA) Natural Resource
Conservation Service (NRCS) Gridded SSURGO database. Soils in the model domain are mostly silt
loam. The soils are generally poorly draining and classified as hydrologic soil group D with some small
areas of B. Hydrologic soil types are shown in Appendix A, Figure 2.
The USGS National Land Cover Database (NLCD) was used to determine land use for the model
domains. The site is mostly classified as Shrub/Scrub, Grassland/Herbaceous, and Deciduous For-
est.
1.3 Proposed Changes to Land Use and Topography
The buildable area of the site that is outside of the wetland buffer is shown in Appendix A, Figure 3.
This area will be cleared and maintained as a grassed meadow. Overall, post -construction changes to
site hydraulics will be minimal.
1.4 Precipitation
The Virginia Stormwater Management Handbook, Chapter 12, Appendix 11-B - 24-Hour Rainfall
Depth Data for Virginia was used to determine precipitation depths for the 10-year, 25-year, and 100-
year 24-hour storm event for Albermarle County (Zone 2). The precipitation depths are 9.0 inches
for the 100-year storm, 6.7 inches for the 25-year storm, 5.5 inches for the 10-year storm, 3.6 for the
2-year storm, and 3 inches for the 1-year storm. These values are slightly higher than the NOAA Atlas
Rivanna H&H 1 Friday 161h December, 2022 19:58
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PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
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14 publicly available rainfall data for the site location. These precipitation amounts were temporally
distributed through use of the Type -II, 24-hour storm.
2 Model Setup
2.1 2D Hydraulic Modeling
HEC-RAS was used to develop a 2D hydraulic model for the 100-year, 25-year, 10-year, 2-year, and
1-year 24-hour storm events to model maximum depths and velocities across the site for the pre -
construction and post -construction scenarios. Grid cells of approximately 10 feet by 10 feet were used
for the main stream channel areas and grid cells of 20 feet by 20 feet were used for the remainder of
the model area. Topography was interpolated to the grid cells based on the elevation described above.
A land use layer and soil layer were developed using the data described above, and combined to form
an infiltration layer. For the post -construction scenario, the buildable are was added as a shapefile
and incorporated into the land use data. Each land use was associated with a Manning's n value as
shown in Table 1.
Table 1: Land Cover Types and Associated Manning's n Values
Land Cover
Manning's n
Deciduous Forest
0.1
Evergreen Forest
0.15
Pasture/Hay
0.045
Mixed Forest
0.12
Developed, Low Intensity
0.08
Developed, Open Space
0.035
Shrub/Scrub
0.05
Developed, Medium Intensity
0.12
Grassland/Herbaceous
0.04
Developed, High Intensity
0.15
Woody Wetlands
0.07
Open Water
0.035
Buildable area
0.035
Hydrologic soil group data was combined with land use data to assign a CN to each land use/hydrologic
soil group combination, as shown in Table 2. These values are based on the requirements outlined in
the Virginia Stormwater Management Handbook. These CN values were used in the infiltration layer
of the 2D model. The average CN for the model domain was 75.45 for the existing conditions scenario
and 76.16 for the post -construction scenario. In the post -construction scenario, the buildable area was
also assigned an imperviousness of 4%. All other areas were assigned an imperviousness of 0%. All
cells were assigned an initial abstraction value of 0.2.
The precipitation events were simulated as spatially constant across the 2D model domain using an
internal precipitation boundary condition. Infiltration was modeled using the SCS Curve Number
method. An external boundary conditions of normal depth where friction slope = 0.01 was added to
the boundaries edge of the model domain.
Two-dimensional unsteady flow routing was performed in HEC-RAS using the Diffusion Wave Equa-
Rivanna H&H 2 Friday 16th December, 2022 19:58
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PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
Table 2: Curve Numbers
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Land Use
Curve Number
Soil Type A Soil Type B Soil Type C Soil Type D Soil Type B/D
Open Water
100
100
100
100
100
Developed, Open Space
49
69
79
84
76.5
Developed, Low Intensity
77
86
91
94
90
Developed, Medium Intensity
89
92
94
95
93.5
Developed, High Intensity
98
98
98
98
98
Deciduous Forest
30
55
70
77
66
Evergreen Forest
30
55
70
77
66
Mixed Forest
30
55
70
77
66
Shrub/Scrub
30
55
70
77
66
Grassland/Herbaceous
30
55
70
77
66
Pasture/Hay
49
69
79
84
76.5
Woody Wetlands
88
89
90
91
90
Buildable Area
39
61
74
80
70.5
tions, as described in the HEC-RAS Hydraulic Reference Manual. Model stability was maintained
through variable timestepping dictated by maximal and minimal Courant numbers where the Courant
number (C) =
C V-OT
OX
(1)
V is the flood wave velocity, AT is the computational time step, OX is the average computational
grid cell size. The maximum Courant number was set to 0.95 and the minimum was set to 0.25. The
small cell size of the computational grid dictated a small timestep, on average around 1 second.
3 Results
3.1 2D Hydraulic Model Results
3.1.1 Pre -Construction Existing Conditions
Of the total 100-year 24-hour storm precipitation depth of 9 inches, on average 2.98 inches was infil-
trated and 6.02 inches was runoff. Modeled infiltration depths across the model domain ranged from
0.24 to 5.51 inches. HEC-RAS output for the 100-year pre -construction maximum depth, velocity, and
scour is shown on Appendix A Figures 4 through 6. Of the total 25-year 24-hour storm precipitation
depth of 6.7 inches, on average 2.74 inches was infiltrated and 3.96 was runoff. Modeled infiltration
depths across the model domain ranged from 0.24 to 4.76 inches. HEC-RAS output for the 25-year
pre -construction maximum depth, velocity, and scour is shown on Appendix A Figures 10 through
12. Of the total 10-year 24-hour storm precipitation depth of 5.5 inches, on average 2.55 inches was
infiltrated and 2.95 was runoff. Modeled infiltration depths across the model domain ranged from
0.24 to 4.26 inches. HEC-RAS output for the 10-year pre -construction maximum depth, velocity, and
scour is shown on Appendix A Figures 16 through 18. Of the total 2-year 24-hour storm precipitation
depth of 3.6 inches, on average 2.11 inches was infiltrated and 1.49 was runoff. Modeled infiltration
depths across the model domain ranged from 0.23 to 3.16 inches. HEC-RAS output for the 2-year
Rivanna H&H 3 Friday 161h December, 2022 19:58
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pre -construction maximum depth, velocity, and scour is shown on Appendix A Figures 23 through
24. Of the total 1-year 24-hour storm precipitation depth of 3.0 inches, on average 1.91 inches was
infiltrated and 1.09 was runoff. Modeled infiltration depths across the model domain ranged from 0.23
to 2.73 inches. HEC-RAS output for the 1-year pre -construction maximum depth, velocity, and scour
is shown on Appendix A Figures 28 through 30.
Scour depth was calculated using the methods of Chapter 7 of the HEC 18 Scour Manual. Kl, K2,
and K3 were calculated to be 1.1, 1.3, and 1.1 respectively, and a box pile of dimensions a=1/3' and
L=1/2' were used. For simplicity, the angle of attack was assumed to be zero for all piles. The proper
excerpt pages are included in Appendix B.
During all three storm events, flow was limited to the channels on -site. During the 100-year storm
event, flow depth reached approximately 6 feet in the deepest parts of the on -site channels. Water
depth is generally between 2 and 5 feet in the channels, 0.5 to 2 feet in the flooded areas along the
channels, and less than 0.5 feet in other areas of the site. Site flow velocities follow a similar pattern
to flow depth onsite. Flow within the channels sees velocities as high as 11 feet per second, and are
generally between 3 and 7 feet per second. Overland flow is generally between 0 and 3 feet per second.
Scour depth is between 1.0 and 2.25 feet within the channels and less than 1.5 feet in other areas.
During the 25-year storm event, flow depth reached approximately 5 feet in the deepest parts of the
on -site channels. Water depth is generally between 1 and 4 feet in the channels, 0.5 to 2 feet in the
flooded areas along the channels, and less than 0.5 feet in other areas of the site. Flow within the
channels sees velocities as high as 9 feet per second, and are generally between 2 and 5 feet per second.
Overland flow is generally between 0 and 3 feet per second. Scour depth is between 1.0 and 2.0 feet
within the channels and less than 1.0 feet in other areas.
During the 10-year storm event, flow depth reached approximately 4.9 feet in the deepest parts of the
on -site channels. Water depth is generally between 1 and 4 feet in the channels, 0.5 to 1.5 feet in the
flooded areas along the channels, and less than 0.5 feet in other areas of the site. Flow within the
channels sees velocities as high as 8 feet per second, and are generally between 2 and 5 feet per second.
Overland flow is generally between 0 and 3 feet per second. Scour depth is between 0.75 and 2.0 feet
within the channels and less than 1.0 feet in other areas.
During the 2-year storm event, flow depth reached approximately 4 feet in the deepest parts of the
on -site channels. Water depth is generally between 0.5 and 3 feet in the channels, and less than 0.5
feet in other areas of the site. Flow within the channels sees velocities as high as 6 feet per second,
and are generally between 2 and 4 feet per second. Overland flow is generally between 0 and 3 feet
per second. Scour depth is between 0.5 and 2.0 feet within the channels and less than 1.0 feet in other
areas.
During the 1-year storm event, flow depth reached approximately 4 feet in the deepest parts of the
on -site channels. Water depth is generally between 0.5 and 3 feet in the channels, and less than 0.5
feet in other areas of the site. Flow within the channels sees velocities as high as 6 feet per second,
and are generally between 1 and 4 feet per second. Overland flow is generally between 0 and 2 feet
per second. Scour depth is between 0.5 and 1.5 feet within the channels and less than 1.0 feet in other
areas.
Rivanna H&H 4 Friday 16th December, 2022 19:58
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PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
3.1.2 Post -Construction
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HEC-RAS output for post -construction maximum depth, velocity, and scour is shown on Appendix
A Figures 7 through 9 for the 100-year storm, Figures 13 through 15 for the 25-year storm, Figures
19 through 21 for the 10-year storm, Figures 25 through 27 for the 2-year storm, and Figures 31
through 33 for the 1-year storm. Post -construction conditions caused minimal changes to site flow
depth, velocity and scour. Some areas of the site show a slight decrease in flow depth and a slight
increase in flow velocity and scour post -construction.
Overall runoff from the site increased in the post -construction simulation. Appendix A, Figure 34
shows locations of flow profile lines where runoff flow profiles were calculated in HEC-RAS along the
downstream boundaries of the buildable areas. The post -construction peak excess runoff for each
profile is shown in Table 3. Diversions and extended detention basins will be added to capture excess
runoff on -site.
Table 4 shows the peak flow rate at each profile under existing conditions, and Table 5 shows the total
volume of runoff at each profile under existing conditions.
4 Assumptions
1. The elevation data has been deemed appropriate for use in pre -construction 2D hydraulic mod-
eling (HEC-RAS)
2. To the greatest extent practical this model represents pending and flow conditions for excess rain-
fall occurring on the model surface. This model is an approximation of real -life flow conditions
but is limited in its accuracy by the type and accuracy of its inputs. If future calibration data
is gathered, the model can be rerun using the calibration data as inputs to check the viability
and accuracy of the model.
Rivanna H&H 5 Friday 161h December, 2022 19:58
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Table 3: Post -Construction Excess Peak Runoff Volume
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Profile Line
Volume (cubic feet)
100-yr 24-hr Flow Event 25-yr 24-hr Flow Event 10-yr 24-hr Flow Event 2-yr 24-hr Flow Event 1-yr 24-hr Flow Event
0
46
29
19
15
16
1
145
161
189
350
379
2
104
116
112
147
207
3
644
698
852
1,021
796
4
2,564
2,823
2,212
2,457
1,969
5
14
56
81
140
155
6
1,083
1,386
1,691
1,597
1,208
7
269
301
424
684
600
8
32
28
29
29
34
9
158
64
40
298
268
10
23
44
60
72
94
11
215
230
280
439
458
12
110
168
180
157
107
13
2,024
2,586
2,645
2,004
2,048
14
33
64
58
218
209
15
206
210
258
388
425
16
167
250
272
362
371
17
462
473
547
734
860
18
269
239
238
249
240
19
536
527
571
905
914
20
310
461
708
656
580
21
102
111
118
158
147
22
142
151
178
260
300
23
-1
0
0
1
1
24
323
473
709
1,013
794
Total
9,980
11,649
12,471
14,352
13,179
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Table 4: Peak Runoff Flow Rate - Existing Conditions
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Profile Line
Peak Flow Rate (cubic feet per second)
100-yr 24-hr Flow Event 25-yr 24-hr Flow Event 10-yr 24-hr Flow Event 2-yr 24-hr Flow Event 1-yr 24-hr Flow Event
0
3
2
2
1
1
1
23
15
11
5
3
2
37
24
18
8
5
3
35
22
15
6
4
4
87
50
34
10
5
5
13
8
6
2
2
6
69
43
31
13
9
7
61
40
29
13
9
8
41
27
19
9
6
9
112
41
27
10
7
10
9
6
4
2
1
11
31
21
15
7
4
12
33
21
15
7
5
13
239
154
113
52
34
14
22
16
13
6
4
15
26
17
13
6
4
16
52
34
25
11
7
17
63
42
31
14
9
18
28
19
14
6
4
19
47
31
23
10
7
20
31
19
13
6
4
21
9
6
4
2
1
22
21
14
10
5
3
23
1
1
0
0
0
24
63
41
29
13
9
Total
1,155
713
14 5
221
147
Rivanna H&H 7 Friday 16th December, 2022 19:58
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Table 5: Total Runoff Volume - Existing Conditions
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Profile Line
Volume (cubic feet)
100-yr 24-hr Flow Event 25-yr 24-hr Flow Event 10-yr 24-hr Flow Event 2-yr 24-hr Flow Event 1-yr 24-1u Flow Event
0
8,878
6,126
4,690
2,371
1,713
1
51,686
34,204
25,481
12,114
8,475
2
79,012
51,905
38,387
17,855
12,359
3
77,930
49,972
36,311
16,116
10,897
4
213,932
130,221
90,932
36,225
23,250
5
26,311
17,116
12,635
5,884
4,081
6
159,989
103,743
75,950
34,369
23,460
7
136,872
90,439
67,254
31,720
22,092
8
90,247
59,398
44,086
20,829
14,533
9
350,788
212,966
145,856
47,222
22,556
10
19,228
12,638
9,364
4,400
3,059
11
71,253
47,077
34,977
16,456
11,447
12
78,290
52,359
39,302
19,052
13,460
13
588,062
391,433
292,880
141,198
99,743
14
52,851
33,035
25,290
13,622
9,959
15
58,009
38,338
28,508
13,451
9,377
16
116,702
76,625
56,682
26,405
18,278
17
138,543
91,162
67,567
31,657
21,998
18
60,250
39,727
29,467
13,846
9,640
19
104,508
69,059
51,338
24,177
16,821
20
66,737
43,740
32,317
14,988
10,374
21
20,317
13,363
9,922
4,724
3,313
22
47,560
31,554
23,510
11,124
7,750
23
2,209
1,523
1,162
581
415
24
144,924
95,872
71,326
33,695
23,478
Total
2,765,090
1,793,592
1,315,193
594,081
402,528
Rivanna H&H 8 Friday 161h December, 2022 19:58
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PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
APPENDIX A - Figures
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Rivanna H&H 9 Friday 16th December, 2022 19:58
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Sierra Overhead Analytics, Inc.
PO Box 1716, Twain Harte, CA 95393
Phone: +1.415.413.7558
APPENDIX B - Supporting Documentation
AUJANk,
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Rivanna H&H 44 Friday 16`h December, 2022 19:58
Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Table 2-2a Runoff curve numbers for urban areas _v
Cover description
Cover type and hydrologic condition
Fully developed urban areas (vegetation established)
Open space (lawns, parks, golf courses, cemeteries, etc.) 21:
Poor condition (grass cover < 50%) ............................
Fair condition (grass cover 50%to 7596) ....................
Good condition (grass cover > 75%) ............................
Impervious areas:
Paved parldng lots, roofs, driveways, etc.
(excluding right-of-way) ...............................................
Streets and roads:
Paved; curbs and storm sewers (excluding
rightof--way)..................................................................
Paved; open ditches (including right-of-way) ............
Gravel (including right-of-way) ...................................
Dirt (including right-of-way) ........................................
Western desert urban areas:
Natural desert landscaping (pervious areas only) J .......
Artificial desert landscaping (impervious weed barrier,
desert shrub with 1- to 2-inch sand or gravel mulch
and basin borders)........................................................
Urban districts:
Commercial and business ...................................................
Industrial...............................................................................
Residential districts by average lot size:
1/8 acre or less (town houses) ............................................
1/4 acre..................................................................................
1/3 acre..................................................................................
1/2 acre..................................................................................
1 acre .....................................................................................
2 acres....................................................................................
Developing urban areas
Newly graded areas
(pervious areas only, no vegetation) ly
Idle lands (CN's are determined using cover types
similar to those in table 2-2c).
Curve numbers for
hydrologic soil group
Average percent
impervious area V A B C D
68
79
86
89
49
69
79
84
39
61
74
80
98
98
98
98
98
98
98
98
83
89
92
93
76
85
89
91
72
82
87
89
63
77
85
88
96
96
96
96
85
89
92
94
95
72
81
88
91
93
65
77
85
90
92
38
61
75
83
87
30
57
72
81
86
25
54
70
80
85
20
51
68
79
84
12
46
65
77
82
77 86 91 94
r Average runoff condition, and I, = 0.2S.
2 The average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are
directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in
good hydrologic condition CN's for other combinations of conditions may be computed using figure 23 or 2-4.
a CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space
cover type.
4 Composite CN's for natural desert landscaping should be computed using figures 23 or 2-4 based on the impervious area percentage
(CN = 98) and the pervious area CN. The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition.
5 Composite CN's to use for the design of temporary measures during grading and construction should he computed using figure 23 or 2-4
based on the degree of development (impervious area percentage) and the CN's for the newly graded pervious areas.
(210-VI-TR-55, Second Ed., June 1986) 2-5
Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Table 2-2b Runoff curve numbers for cultivated agricultural lands _v
Cover type
Cover description
Treatment
Hydrologic
condition a'
A
Curve numbers for
hydrologic soil group
B C
D
Fallow
Bare soil
—
77
86 91
94
Crop residue cover (CR)
Poor
76
85 90
93
Good
74
83 88
90
Row crops
Straight row (SR)
Poor
72
81 88
91
Good
67
78 85
89
SR + CR
Poor
71
80 87
90
Good
64
75 82
85
Contoured (C)
Poor
70
79 84
88
Good
65
75 82
86
C + CR
Poor
69
78 83
87
Good
64
74 81
85
Contoured & terraced (C&T)
Poor
66
74 80
82
Good
62
71 78
81
C&T+ CR
Poor
65
73 79
81
Good
61
70 77
80
Small grain
SR
Poor
65
76 84
88
Good
63
75 83
87
SR + CR
Poor
64
75 83
86
Good
60
72 80
84
C
Poor
63
74 82
85
Good
61
73 81
84
C + CR
Poor
62
73 81
84
Good
60
72 80
83
C&T
Poor
61
72 79
82
Good
59
70 78
81
C&T+ CR
Poor
60
71 78
81
Good
58
69 77
80
Close -seeded
SR
Poor
66
77 85
89
or broadcast
Good
58
72 81
85
legumes or
C
Poor
64
75 83
85
rotation
Good
55
69 78
83
meadow
C&T
Poor
63
73 80
83
Good
51
67 76
80
r Average runoff condition, and Ia US
2 Crop residue cover applies only if residue is on at least 5%of the surface throughout the year.
a Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas,
(b) amount of year-round cover, (c) amount of grass or close -seeded legumes, (d) percent of residue cover on the land surface (good a 2096),
and (e) degree of surface roughness.
Poor. Factors impair infiltration and tend to increase runoff.
Good: Factors encourage average and better than average infiltration and tend to decrease runoff.
2-6 (210-VI-TR-55, Second Ed., June 1986)
Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Table 2-2c Runoff curve numbers for other agricultural lands y
Curve numbers for
Cover description
hydrologic soil group
Hydrologic
Cover type
condition
A
B C
D
Pasture, grassland, or range —continuous
Poor
68
79 86
89
forage for grazing. 21
Fair
49
69 79
84
Good
39
61 74
80
Meadow —continuous grass, protected from
—
30
58 71
78
grazing and generally mowed for hay.
Brush —brush -weed -grass mixture with brush
Poor
48
67 77
83
the major element. 9'
Fair
35
56 70
77
Good
30 v
48 65
73
Woods —grass combination (orchard
Poor
57
73 82
86
or tree farm). y
Fair
43
65 76
82
Good
32
58 72
79
Woods.ly
Poor
45
66 77
83
Fair
36
60 73
79
Good
30 V
55 70
77
Farmsteads —buildings, lanes, driveways, — 59 74 82 86
and surrounding lots.
1 Average runoff condition, and Za = 0.28.
2 Poor. <50'M) ground cover or heavily grazed with no mulch.
Fair. 50 to 75%ground cover and not heavily grazed.
Good: > 75%ground cover and lightly or only occasionally grazed.
a Poor. <50% ground cover.
Fair: 50 to 7596 ground cover.
Good: >75% ground cover.
4 Actual curve number is less than 30; use CN = 30 for runoff computations.
5 CN's shown were computed for areas with 50%woods and 50%grass (pasture) cover. Other combinations of conditions may be computed
from the CN's for woods and pasture.
6 Poor Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.
Fair: Woods are grazed but not burned, and some forest litter covers the soil.
Gaod: Woods are protected from grazing, and litter and brush adequately cover the soil.
(210-VI-TR-55, Second Ed., June 1986) 2-7
Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Table 2-2d Runoff curve numbers for and and semiarid rangelands V
Cover description
Cover type
Hydrologic
condition 2/
A J'
Curve numbers for
hydrologic soil group
B C
D
Herbaceous —mixture of grass, weeds, and
Poor
80
87
93
low -growing brush, with brush the
Fair
71
81
89
minor element.
Good
62
74
85
Oak -aspen —mountain brush mixture of oak brush,
Poor
66
74
79
aspen, mountain mahogany, bitter brush, maple,
Fair
48
57
63
and other brush.
Good
30
41
48
Pinyon juniper —pinyon, juniper, or both;
Poor
75
85
89
grass understory.
Fair
58
73
80
Good
41
61
71
Sagebrush with grass understory.
Poor
67
80
85
Fair
51
63
70
Good
35
47
55
Desert shrub —major plants include saltbush,
Poor
63
77
85
88
greasewood, creosotebush, blackbrush, bursage,
Fair
55
72
81
86
palo verde, mesquite, and cactus.
Good
49
68
79
84
r Average runoff condition, and I„ = 0.25. For range in humid regions, use table 2-2c
2 Poor. <30%ground cover (litter, gmss, and brush overstory).
Fair. 30 to 7096 ground cover.
Good: > 70%ground cover.
a Curve numbers for group A have been developed only for desert shrub.
2-8 (210-VI-TR-55, Second Ed., June 1986)
Chapter 2 Estimating Runoff Technical Release 55
Urban Hydrology for Small Watersheds
Figure 2-3 Composite CN with connected impervious area
100
oonifnus CN = 90_
nil
Z
0
m 70
0
a
E
0
v 60
50
401
0
10 20 30 40 50 60 70 80 90 100
Connected impervious area (percent)
Figure 2-4 Composite CN with unconnected impervious areas and total impervious area less than 30%
MWI
NMI
90 80 70 60 50 40 0 10 20 30
Composite CN Total impervious
area (percent)
2-10 (210-VI-TR-55, Second Ed., June 1986)
V y, Downflow
Ys
Figure 7.2. Definition sketch for pier scour.
The HEC-18 equation is:
0.65
YS
Y, = 2.0 Ki Kz K3 � a Fr, .43
Yi )
(7.1)
As a Rule of Thumb, the maximum scour depth for round nose piers aligned with the flow is:
ys < 2.4 times the pier width (a) for Fr < 0.8 (7.2)
ys < 3.0 times the pier width (a) for Fr > 0.8
In terms of yja, Equation 7.1 is:
/ \ 0.35
as =2.0 Ki Kz K3 aJ Fro.43 (7.3)
where:
ys = Scour depth, ft (m)
y, = Flow depth directly upstream of the pier, ft (m)
K, = Correction factor for pier nose shape from Figure 7.3 and Table 7.1
K2 = Correction factor for angle of attack of flow from Table 7.2 or Equation 7.4
K3 = Correction factor for bed condition from Table 7.3
a = Pier width, ft (m)
L = Length of pier, ft (m)
Fr, = Froude Number directly upstream of the pier = V,/(gy,)'/2
V, = Mean velocity of flow directly upstream of the pier, ft/s (m/s)
g = Acceleration of gravity (32.2 ft/s2) (9.81 m/s2)
7.3
P `I' � L
O
�O
-
(a) Square Nose (b) Round Nose (c) Cylindrical
L L = (# of Piers) x (a)
O O
(d) Sharp Nose (e) Group of Cylinders
(see Multiple Columns)
Figure 7.3. Common pier shapes.
The correction factor, K2, for angle of attack of the flow, 2, is calculated using the following
equation:
K2 = (Cos 0 + L Sin 0)0.65
a
(7.4)
If L/a is larger than 12, use L/a = 12 as a maximum in Equation 7.4 and Table 7.2. Table 7.2
illustrates the magnitude of the effect of the angle of attack on local pier scour.
Table 7.1. Correction Factor, K,,
for Pier Nose Shape.
Shape of Pier Nose
K,
(a) Square nose
1.1
(b) Round nose
1.0
(c) Circular cylinder
1.0
(d) Group of cylinders
1.0
(e) Sharp nose
0.9
Table 7.2.
Correction Factor, K2, for Angle of
Attack, 2, of the Flow.
Angle
L/a=4
L/a=8
L/a=12
0
1.0
1.0
1.0
15
1.5
2.0
2.5
30
2.0
2.75
3.5
45
2.3
3.3
4.3
90
2.5
3.9
5.0
Angle = skew angle of flow
L = length of pier
7.4
Table 7.3. Increase in Equilibrium Pier Scour Depths, K3, for Bed Condition.
Bed Condition
Dune Height ft
K3
Clear -Water Scour
N/A
1.1
Plane bed and Antidune flow
N/A
1.1
Small Dunes
10 > H > 2
1.1
Medium Dunes
30 > H > 10
1.2 to 1.1
Large Dunes
H > 30
1.3
Notes:
The correction factor K, for pier nose shape should be determined using Table 7.1 for
angles of attack up to 5 degrees. For greater angles, K2 dominates and K, should be
considered as 1.0. If L/a is larger than 12, use the values for L/a = 12 as a maximum in
Table 7.2 and Equation 7.4.
The values of the correction factor K2 should be applied only when the field conditions are
such that the entire length of the pier is subjected to the angle of attack of the flow. Use
of this factor will result in a significant over -prediction of scour if (1) a portion of the pier is
shielded from the direct impingement of the flow by an abutment or another pier; or (2) an
abutment or another pier redirects the flow in a direction parallel to the pier. For such
cases, judgment must be exercised to reduce the value of the K2 factor by selecting the
effective length of the pier actually subjected to the angle of attack of the flow. Equation
7.4 should be used for evaluation and design. Table 7.2 is intended to illustrate the
importance of angle of attack in pier scour computations and to establish a cutoff point for
K2 (i.e., a maximum value of 5.0).
3. The correction factor K3 results from the fact that for plane -bed conditions, which is
typical of most bridge sites for the flood frequencies employed in scour design, the
maximum scour may be 10 percent greater than computed with Equation 7.1. In the
unusual situation where a dune bed configuration with large dunes exists at a site
during flood flow, the maximum pier scour may be 30 percent greater than the predicted
equation value. This may occur on very large rivers, such as the Mississippi. For smaller
streams that have a dune bed configuration at flood flow, the dunes will be smaller and
the maximum scour may be only 10 to 20 percent larger than equilibrium scour. For
antidune bed configuration the maximum scour depth may be 10 percent greater than the
computed equilibrium pier scour depth.
4. Piers set close to abutments (for example at the toe of a spill through abutment) must be
carefully evaluated for the angle of attack and velocity of the flow coming around the
abutment.
7.3 FLORIDA DOT PIER SCOUR METHODOLOGY
Equation 7.1 has been included in all previous versions of HEC-18 and has been used for
bridge scour evaluations and bridge design for countless bridges in the U.S. and worldwide.
This equation, which was developed and modified over several decades, could be improved
by including bed material size and a more detailed consideration of the bridge pier flow field
(see Section 3.6.2). An NCHRP study (NCHRP 2011a) evaluated 22 pier scour equations
and found that although the HEC-18 equation did well in comparison to the other equations,
the Sheppard and Miller (2006) equation generally performed better for both laboratory and
7.5