Culvert Scour Assessment
Little Zigzag Creek
Site Information
Site Location:
Mt Hood NF, Forest Rd 2639 MP 1.4
Year Installed:
2000
Lat/Long:
121°48’43.92”W
45°18’52.57”N
Watershed Area (mi2): 4.1
Stream Slope (ft/ft)1: 0.0421
Channel Type:
Step-pool
Bankfull Width (ft): 14
Survey Date:
April 6, 2007
1
Water surface slope extending up to 20 channel widths up and downstream of crossing.
Culvert Information
Culvert Type:
Open-bottom arch
Culvert Material: Concrete
Culvert Width:
15.5 ft
Outlet Type:
Wingwalls
Culvert Length:
133 ft
Inlet Type:
Wingwalls
Pipe Slope (structure slope): 0.045
Culvert Bed Slope: 0.039
(First hydraulic control upstream of inlet to first hydraulic control downstream of outlet.)
Culvert width as a percentage of bankfull width: 1.09
Alignment Conditions: Inline with natural channel. It is possible that this or previous installation has
changed the course of the stream and possibly altered where it joins with the mainstem of Zigzag
Creek.
Bed Conditions: Small to large boulders and embedded cobbles in culvert. One large boulder-formed
step near downstream end of culvert. One small step near culvert inlet.
Pipe Condition: Good condition.
Hydrology
Discharge (cfs) for indicated recurrence interval
25% 2-yr
2-year
Qbf2
5-year
10-year
50-year
100-year
30
118
130
168
204
292
333
2
Bankfull flow estimated by matching modeled water surface elevations to field-identified bankfull elevations.
A—298
Little Zigzag Creek
Site Evaluations
Points represent survey points
Figure 1—Plan view map.
Little Zigzag Creek
A—299
Culvert Scour Assessment
History
There is no information available for site history.
Site Description
The Little Zigzag Creek culvert is a Con/Span®
concrete bottomless arch with wingwalls and
headwalls at both the inlet and outlet. The banks
along the inlet are stabilized by riprap, bedrock,
and concrete.
Upstream of the inlet, the gradient increases and
the channel flows over a series of boulder steps.
The extreme angle of the crossing to the road
means that the channel through here flows along
the roadfill (riprap) along the right bank. There is a
small step at the inlet and then the grade flattens
out once the channel enters the culvert. A large
boulder step just prior to the outlet presumably
controls the grade through the remainder of the
culvert. The majority of the length of the culvert
consists of a relatively low gradient plane-bed
channel made up of small boulders and large
cobbles. The substrate through this riffle was
embedded. Flow in the culvert during the time of
the survey was wall to wall.
The upstream representative reach consists of a
series of steps, steep riffles/cascades, and pools.
Bedrock influences the structure and controls
grade through the reach. The channel abuts a
bedrock wall along the left bank and flows over
a bedrock cascade towards the bottom of the
reach. Fallen trees span the stream and small
pieces of wood had collected along the edges of
the channel.
Downstream of the outlet, the channel runs
through a steep riffle before joining the mainstem
of Zigzag Creek.
Survey Summary
Ten cross sections and a longitudinal profile
were surveyed along Little Zigzag Creek in April
2007 to characterize the culvert and an upstream
reference reach. No downstream reference
A—300
reach was established due to the proximity of
the culvert with the confluence of the mainstem
Zigzag Creek. In the culvert, representative cross
sections were taken through the long riffle and
at the step near the outlet. One additional cross
section was surveyed upstream to characterize
the inlet as well as the contraction of flow.
Another cross section was surveyed downstream
of the culvert to characterize the outlet and the
expansion of flow.
In the upstream reach, representative cross
sections were taken through a riffle and along a
step. An additional two sections were taken to
characterize the upstream and downstream ends
of the reach.
A cross section and pebble count were taken
through the riffle downstream of the outlet.
Profile Analysis Segment Summary
The profile analysis resulted in a total of seven
profile segments. Two segments in the upstream
channel with similar gradients were not combined
in order to separate out the portion of the
channel affected by riprap from the upstream
representative channel. The culvert consisted of
one profile segment. The culvert segment was
comparable to one representative profile segment
in the upstream channel. Two upstream transition
segments were comparable to representative
profile segments in the upstream channel. The
downstream transition segment was comparable
to two representative profile segments in the
upstream channel. See figure 2 and table 1.
Scour Conditions
Observed conditions
Footing scour – There was no observed scour
threatening structure integrity.
Culvert-bed adjustment – The channel within
the culvert has reduced slope to less than the
constructed slope (assuming the channel bed
was originally constructed at the same gradient
Little Zigzag Creek
Site Evaluations
as the structure). This may be related to the large
step at the downstream end of the culvert that
controls grade and has aggraded bed material
behind it.
Profile characteristics – The profile is mostly
uniform, with a slight concavity centered near
the culvert inlet. Natural valley transition from the
Little Zigzag valley to the mainstem Zigzag valley
may account for some of this, but it may also be
partly related to slope adjustment (flattening) of
the culvert bed (figure 2) as discussed above.
Residual depths – Culvert residual depths
are lower than the single residual depth in the
corresponding profile segment (F) (figure 21).
The single residual depth in the upper of the
upstream transition segments (D) is less than
half the residual depth of the corresponding
profile segment (E). The residual depth in the
downstream transition segment is at the upper
end of the range of depths in corresponding
profile segments (E and G).
Substrate – Culvert bed material distributions
vary considerably between the two sample cross
sections. The upstream culvert cross section
represents the bed material in the plane-bed
section that makes up 80 percent of the length of
the culvert. This bed is made up of coarse gravel
to large cobble. The step at the downstream
end of the culvert has a greater frequency of
very large cobbles to medium boulders. The
natural channel is more poorly sorted, with a
wider representation of particle sizes and a large
portion of sand at the downstream cross section.
Pebble-count data is presented at the end of this
document.
Predicted conditions
Cross-section characteristics – Flow area,
top width, and wetted perimeter are reduced
considerably by the culvert, but hydraulic radius
and maximum depth do not dramatically differ
(figures 5-9 and 12-17). Differences are most
Little Zigzag Creek
apparent at modeled flows above the 25 percent
Q2. The upstream transition segments (C and D)
also differ from the natural channel, but less so.
The downstream transition segment (A) is mostly
within the range of conditions found in the natural
channel (E and G) but generally has a greater
range of values.
Shear stress – Shear stress in the culvert is very
similar to that found in the corresponding profile
segment (F) (figure 10). Shear-stress values in
the upstream transition segments are similar
to or greater than their corresponding profile
segments. Shear in the downstream transition
segment is within the range of the corresponding
profile segments (E and G).
Excess shear – The excess shear values in the
culvert are just within or lower than the range of
values in the natural channel (figure 20). This is
a result of a combination of lower applied shears
and higher critical shears depending on the cross
section and modeled flow.
Velocity – Velocity in the culvert is greater than
the corresponding profile segment for all flows but
is most pronounced at the Qbf and above (figure
18). Velocity in the upstream transition segments
is within the range of that found in corresponding
profile segments. Velocity in the downstream
transition segment has a greater range than the
natural channel but the median values are similar.
Scour summary
There are no significant signs of scour or
structure settlement. The structure appears
adequate to transport flows up to at least the
Q100 without backwatering or significant bed load
mobilization.
The culvert has the greatest affect on flow area
and the width-related parameters top width and
wetted perimeter. Hydraulic radius and maximum
depth are affected but to a lesser degree. It
therefore follows that velocity is increased in
A—301
Culvert Scour Assessment
the culvert. Shear is not significantly affected
but excess shear is estimated to be less in
the culvert. This is due to a flatter slope at the
upstream end of the culvert and larger bed
material at the large step near the downstream
end (these are the locations where excess shear
was modeled).
The culvert appears to have been placed lower
in the profile than the original stream. This could
relate to channel adjustments from the previous
culvert or may also be a result of restrictions in
height imposed by the current road prism. The
reduced slope of the crossing and the grade
control at the downstream end reduces shear
stress and the potential for scour, but it may also
prevent the efficient transport of material through
the pipe, resulting in potential aggradation of bed
material in the pipe that could reduce capacity.
AOP Conditions
Cross-section complexity – The sum of squared
height differences in the culvert cross sections
are both within the range of those in the channel
cross sections (table 3).
Profile complexity – Vertical sinuosity in the
culvert is the same as that in the corresponding
profile segment (F) (table 4). Vertical sinuosity in
the upstream transition segments (C and D) is
lower than their corresponding channel segments
(E and G). Vertical sinuosity in the downstream
transition segment (A) is within the range of the
corresponding channel segments (E and G).
Depth distribution – The downstream cross
section in the culvert has less shallow channel
margin habitat at the 25 percent Q2 and the
upstream cross section has more when compared
to the natural channel (table 5).
Habitat units – The culvert is nearly all riffle,
whereas the natural channel is comprised of a
greater proportion of pool habitat (table 6).
A—302
Residual depths – Culvert residual depths
are lower than the single residual depth in the
corresponding profile segment (F) (figure 21).
The single residual depth in the upper of the
upstream transition segments (D) is less than
half the residual depth of the corresponding
profile segment (E). The residual depth in the
downstream transition segment is at the upper
end of the range of depths in corresponding
profile segments (E and G).
Substrate – Culvert bed material distributions
vary considerably between the two sample cross
sections. The upstream culvert cross section
represents the bed material in the plane-bed
section that makes up 80 percent of the length of
the culvert. This bed is made up of coarse gravel
to large cobble. The step at the downstream
end of the culvert has a greater frequency of
very large cobbles to medium boulders. The
natural channel is more poorly sorted, with a
wider representation of particle sizes and a large
portion of sand at the downstream cross section.
Pebble count data is presented at the end of this
document.
Large woody debris – There was a small amount
of LWD present in the culvert (table 8). The
representative channel had moderate LWD
abundance. LWD formed occasional steps and
scour pools in the channel outside the crossing.
Features in the culvert did not mimic the role of
wood in the natural channel.
AOP summary
Site observations suggest that AOP conditions
may be less desirable at this site than the metrics
suggest. Nearly 80 percent of the length of the
culvert is made up of a plane-bed riffle type unit
that has very little complexity. The substrate also
appeared embedded in fines, with few protrusions
of larger substrate that can provide important
velocity refuge for fish passage. The flows spread
out over the entire channel during low flows, and
Little Zigzag Creek
Site Evaluations
the channel through the culvert is wider than the reference channel in many places. This may create
flows that are too shallow for passage during low flow periods. There are no bank features that would
provide passage for terrestrial organisms.
Design Considerations
This is a good installation with respect to scour and structure integrity. The greatest impacts with this
site relate to AOP. This site could benefit from the placement of stable boulders as bank structures
intermittently along the structure edge in order to concentrate flows for low water fish passage and to
provide banks for terrestrial passage. Additional rock steps could also be placed across the channel to
better mimic conditions found in the channel outside the crossing.
Little Zigzag Creek
A—303
A—304
Relative elevation (ft)
500
Confluence
42
35
78
E
F
G
450
38
D
XS 1:
Pebble count
(riffle)
58
C
400
XS 2
A
97
B
XS 3
350
XS 4:
Pebble count
(step)
Table 1—Segment comparisons
XS 7
A
A
D o w n s trea m T ra n s itio n
C
C
D
U ps trea m T ra n s itio n
E
G
E
G
E
F
B
D is ta nc e a long bed (feet)
250
XS 6
C
Rip-rap along
right bank
R epres en ta tiv e
C h a n n el S eg men t
300
XS 5:
Pebble count
(riffle)
C ulvert
C u lv ert S eg men t
B
0 .0 4 6
0 .0 1 8
0 .0 5 1
0 .0 6 9
0 .0 5 1
0 .0 2 4
0 .0 5 0
S e g m e n t G r a d ie n t
Figure 2—Little Zigzag Creek longitudinal profile.
485
490
495 elevation (feet)
Relative
500
505
510
515
71
A
S egm ent
L e n g th ( f t)
S egm ent
XS 8
150
XS 9:
Pebble count
(step)
2.0%
8.5%
1.0%
9.4%
26.0%
23.4%
% D ifferen c e in
G ra d ien t
200
D
E
XS 10:
Pebble count
(pool)
100
Representative Channel
F
G
50
XS 11
0
Culvert Scour Assessment
Little Zigzag Creek
n
o
i
t
a
v
e
l
E
)
t
f
(
Little Zigzag Creek
0.0898 – 0.1167
0.0788 – 0.1006
0.0968 – 0.1465
0.1437 – 0.146
0.1063 – 0.1437
0.098 – 0.1089
0.1085 – 0.1089
A
B
C
D
E
F
G
SegmentRange of Manning's n values1
1
1
2
1
2
2
4
# of measured XSs
9
6
5
4
6
10
6
# of interpolated XSs
1
Elevation (ft)
A—305
0
3
5
7
.
3
*
3
3
3
3
1 2
. .
4 4
*
6
6
6
6
3 5 7
. .
.
4 4 4
100
*
5
8
.
4 5
1
.
5
3
.
5
4
.
5
6
.
5
7
.
5
8
.
5
*
6
6
6
6
6
1
.
.
6 7 7
200
*
5
2
3
.
7
*
5
5
.
7
*
5
7
7
.
7
Main Channel Dis tance (ft)
*
3
3
3
3
3
.
6 6
*
2 4
. .
8 8 8
7
.
8 9
*
3
3
3
3
3 5
. .
9 9
300
*
6
6
6
6
7
. 0
9 1
*
2
.
0
1
4
.
0
1
*
6
.
0
1
*
7
.
0
1
Stations with decimal values are interpolated cross sections placed along the surveyed profile.
*
5
.
1 1 2
Figure 3—HEC-RAS profile.
490
495
500
505
510
515
*
8
.
0
1
*
9
.
0
1
1
1
*
5 *
2 5
0 0
. .
1 1
1 1
400
*
5
7
0
.
1
1
1
.
1
1
Ground
W S 30 c fs
W S Qbf
W S Q10
W S Q50
W S Q100
L e ge nd
Obtained using equation from Jarrett (1984): n = 0.39S0.38R-0.16, where S=stream slope; R=hydraulic radius. Jarrett’s equation only applied
within the following ranges: S = 0.002 to 0.08, R = 0.5 ft to 7 ft. For cross sections outside these ranges, n was computed either from adjacent
sections that fell within the ranges, using the guidance of Arcement and Schneider (1987), or from the HEC-RAS recommendations for culvert
modeling.
Table 2—Summary of segments used for comparisons
Site Evaluations
)
t
f
(
A—306
Elevation (ft)
n
o
i
t
a
v
e
l
E
)
t
f
(
n
o
i
t
a
v
e
l
E
502
-10
0
.1077
10
10
Station (ft)
20
.1077
RS = 9
Station (ft)
20
RS = 11
.
1
0
7
7
30
30
.1085
40
40
50
50
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
n
o
i
t
a
v
e
l
E
)
t
f
(
n
o
i
t
a
v
e
l
E
Elevation (ft)
0
-10
502
504
506
508
510
502
504
506
508
.
512 1
0
510 3
8
0
.1437
10
.1038
10
20
Station (ft)
20
.1437
RS = 8
Station (ft)
30
30
.1437
40
.1038
RS = 10
40
50
50
60
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Figure 4—Cross-section plots. Only measured cross sections are included. Manning’s n values are included at the top of the cross section.
The stationing (RS) corresponds to the stationing on the HEC-RAS profile. Green arrows define the ineffective flow areas. Black arrows
represent points identified in the field as the bankfull channel boundary. Only those points identified in the field and supported by hydraulic and
topographic analyses are shown below.
Elevation (ft)
504
0
.1085
Elevation (ft)
506
508
510
512
504
-10
506
508
510
512
514
.
1
0
8
5
)
t
f
(
Culvert Scour Assessment
Little Zigzag Creek
n
o
i
t
a
v
e
l
E
)
t
f
(
Elevation (ft)
n
o
i
t
a
v
e
l
E
)
t
f
(
Little Zigzag Creek
494
-10
0
Station (ft)
20
30
10
.0792
Station (ft)
20
30
RS = 5
Note: n values for first profile.
10
.06
40
40
50
50
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
n
o
i
t
a
v
e
l
E
)
t
f
(
n
o
i
t
a
v
e
l
E
Elevation (ft)
492
-10
494
496
498
500
502
496
-10
498
500
502
504
506
0
0
20
Station (ft)
30
10
.0898
Station (ft)
20
30
RS = 4
Note: n values for first profile.
10
.0999
RS = 6
Note: n values for first profile.
40
40
50
50
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Figure 4—Cross-section plots. Only measured cross sections are included. Manning’s n values are included at the top of the cross section.
The stationing (RS) corresponds to the stationing on the HEC-RAS profile. Green arrows define the ineffective flow areas. Black arrows
represent points identified in the field as the bankfull channel boundary. Only those points identified in the field and supported by hydraulic and
topographic analyses are shown below. (continued)
Elevation (ft)
496
498
0
.1465
Elevation (ft)
500
502
504
-10
498
500
502
504
506
.
0
5
RS = 7
)
t
f
(
Site Evaluations
A—307
A—308
Elevation (ft)
n
o
i
t
a
v
e
l
E
)
t
f
(
n
o
i
t
a
v
e
l
E
)
t
f
(
490
10
20
0
30
.1167
10
Station (ft)
40
RS = 1
Station (ft)
20
50
.1167
30
60
.1167
40
70
50
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Bank St a
Ground
WS 30 c fs
WS Qbf
n
o
i
t
L e ge nd
a
WS Q100 v
WS Q50 e
l
WS Q10
E
Elevation (ft)
492
494
496
498
500
0
10
.1132
20
Station (ft)
30
RS = 2
40
.1132
50
.1132
60
Bank St a
Ground
WS 30 c fs
WS Qbf
WS Q10
WS Q50
WS Q100
L e ge nd
Figure 4—Cross-section plots. Only measured cross sections are included. Manning’s n values are included at the top of the cross section.
The stationing (RS) corresponds to the stationing on the HEC-RAS profile. Green arrows define the ineffective flow areas. Black arrows
represent points identified in the field as the bankfull channel boundary. Only those points identified in the field and supported by hydraulic and
topographic analyses are shown below. (continued)
Elevation (ft)
492
494
496
498
500
-10
492
494
496
498
500
.1007
RS = 3
Note: n values for first profile.
)
t
f
(
Culvert Scour Assessment
Little Zigzag Creek
w
o
l
F
a
e
r
A
q
s
(
)
t
f
Little Zigzag Creek
.
P
.
W
l
a
t
o
T
)
t
f
(
0
0
A
50
100
B
Culvert
10
0
A
50
Figure 6—Wetted perimeter.
W.P. total (ft)
20
30
40
50
60
100
B
Culvert
Figure 5—Flow area (total) profile plot.
Flow area (sq ft)
20
40
60
80
100
120
150
150
250
D
250
D
Main Channel Distance (ft)
200
C
Main Channel Distance (ft)
200
C
E
300
300
E
F
F
350
350
400
G
Flow
400
G
Flow
450
450
W.P. Total 30 cfs
W.P. Total Qbf
W.P. Total Q10
W.P. Total Q50
W.P. Total Q100
Legend
Flow Area 30 cfs
Flow Area Qbf
Flow Area Q10
Flow Area Q50
Flow Area Q100
Legend
Site Evaluations
A—309
A—310
p
o
T
h
t
d
i
W
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0
A
50
50
10
15
Figure 8—Top width.
Top width (ft)
20
25
30
100
150
200
C
250
D
E
300
300
E
F
F
350
350
Main Channel Distance (ft)
400
G
Flow
400
G
Flow
450
450
Top Width Q100
Legend
Hydr Radius 30 cfs
Hydr Radius Qbf
Hydr Radius Q10
Hydr Radius Q50
Hydr Radius Q100
Legend
Top Width 30 cfs
Top Width Qbf
Top Width Q10
0
250
Main Channel Distance (ft)
200
D
35
B
Culvert
150
C
Top Width Q50
A
100
B
Culvert
40
45
50
Figure 7—Hydraulic radius.
Hydraulic radius (ft)
)
t
f
(
r
d
y
H
s
u
i
d
a
R
)
t
f
(
Culvert Scour Assessment
Little Zigzag Creek
r
a
e
h
S
n
a
h
C
0
1
2
3
4
5
6
0
A
50
50
Little Zigzag Creek
0
Figure 10—Shear stress (channel) profile.
Shear channel (lbs/sq ft)
2
4
6
8
100
150
200
C
250
D
300
E
300
E
F
F
350
350
Main Channel Distance (ft)
400
G
Flow
400
G
Flow
450
450
Shear Chan Q100
Legend
Max Chl Dpth 30 cfs
Max Chl Dpth Qbf
Max Chl Dpth Q10
Max Chl Dpth Q50
Max Chl Dpth Q100
Legend
Shear Chan 30 cfs
Shear Chan Qbf
Shear Chan Q10
0
250
Main Channel Distance (ft)
200
D
10
B
Culvert
150
C
Shear Chan Q50
A
100
B
Culvert
12
14
16
Figure 9—Maximum depth.
Maximum channel depth (ft)
q
s
/
b
l
(
)
t
f
x
a
M
l
h
C
h
t
p
D
)
t
f
(
Site Evaluations
A—311
)
s
/
t
f
(
A—312
l
e
V
l
n
h
C
0
0
A
50
100
B
Culvert
Figure 11—Velocity (channel) profile plot.
Elevation (ft)
2
4
6
8
10
150
250
D
Main Channel Distance (ft)
200
C
E
300
F
350
G
Flow
400
450
Vel Chnl 30 cfs
Vel Chnl Qbf
Vel Chnl Q10
Vel Chnl Q50
Vel Chnl Q100
Legend
Culvert Scour Assessment
Little Zigzag Creek
80
120
B (culv)
C (US Trans)
B (Culvert)
A (DS Trans)
G
E
D (DS Trans)
C (US Trans)
E
F
Qbf
B (culv)
D (DS Trans)
25%Q2
G
E
A (ds trans)
C (usDtrans)
(us trans)
F
F
A (ds trans)
C (usDtrans)
(us trans)
E
F
Figure 12—Flow area (total).
0
A (DS Trans)
G
G
B (Culvert)
E
E
D (DS Trans)
C (US Trans)
A (DS Trans)
F
10
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
50%
of the
values
F
G
E
E
D (DS Trans)
C (US Trans)
B (Culvert)
A (DS Trans)
F
50
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
Minimum value
25th percentile
F
G
Median (aka 50th percentile)
75th percentile
Maximum value
G
Little Zigzag Creek
40
Flow Area (ft2)
Flow area (ft2)
B (Culvert)
100%
of the
values
G
Box Plot Explanation
B (culv)
F
F
E
D (DS Trans)
C (US Trans)
B (Culvert)
A (DS Trans)
100
A (ds trans)
C (usDtrans)
(us trans)
E
G
Site Evaluations
A—313
G
C (US Trans)
E
B (Culvert)
G
G
B (culv)
A (DS Trans)
G
D (DS Trans)
D (DS Trans)
B (Culvert)
A (DS Trans)
E
F
Qbf
B (Culvert)
A (ds trans)
C (usDtrans)
(us trans)
C (US Trans)
25%Q2
F
D (DS Trans)
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
E
E
Figure 14—Hydraulic radius.
0
C (US Trans)
1
Hydraulic Radius (ft)
2
3
4
A (DS Trans)
Qbf
B (Culvert)
A (ds trans)
C (usDtrans)
(us trans)
C (US Trans)
25%Q2
F
F
Figure 13—Wetted perimeter.
Hydraulic radius (ft)
A (DS Trans)
D (DS Trans)
A (ds trans)
C (usDtrans)
(us trans)
G
E
F
B (culv)
10
F
10
A (ds trans)
C (usDtrans)
(us trans)
E
A (ds trans)
C (usDtrans)
(us trans)
B (Culvert)
B (Culvert)
E
E
B (culv)
C (US Trans)
C (US Trans)
F
F
D (DS Trans)
D (DS Trans)
B (culv)
G
G
G
E
E
F
A (DS Trans)
A (DS Trans)
F
F
E
G
G
E
F
B (culv)
50
F
50
A (ds trans)
C (usDtrans)
(us trans)
E
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
B (Culvert)
B (Culvert)
B (culv)
G
C (US Trans)
C (US Trans)
F
G
G
D (DS Trans)
D (DS Trans)
E
A (DS Trans)
A (DS Trans)
E
E
0
G
G
G
G
15
Wetted Perimeter (ft)
E
F
F
B (culv)
100
F
100
A (ds trans)
C (usDtrans)
(us trans)
E
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
B (Culvert)
B (Culvert)
30
C (US Trans)
C (US Trans)
F
F
D (DS Trans)
D (DS Trans)
45
A (DS Trans)
A (DS Trans)
E
E
A—314
F
Wetted perimeter (ft)
G
G
G
G
60
Culvert Scour Assessment
Little Zigzag Creek
D (DS Trans)
C (US Trans)
F
E
G
G
G
E
F
E
F
Qbf
A (DS Trans)
25%Q2
B (culv)
B (Culvert)
A (ds trans)
C (usDtrans)
(us trans)
F
C (US Trans)
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
E
F
Figure 16—Maximum depth.
0
Maximum Depth (ft)
2
4
D (DS Trans)
Figure 15—Top width.
6
A (DS Trans)
Qbf
B (Culvert)
A (ds trans)
C (usDtrans)
(us trans)
C (US Trans)
25%Q2
D (DS Trans)
D (DS Trans)
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
E
G
G
G
E
F
B (culv)
10
F
10
A (ds trans)
C (usDtrans)
(us trans)
E
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
B (Culvert)
B (Culvert)
B (culv)
G
C (US Trans)
C (US Trans)
E
E
D (DS Trans)
D (DS Trans)
F
F
E
E
F
A (DS Trans)
A (DS Trans)
F
F
E
G
G
A (DS Trans)
B (culv)
E
F
E
F
50
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
50
A (ds trans)
C (usDtrans)
(us trans)
A (DS Trans)
G
0
B (Culvert)
B (Culvert)
20
C (US Trans)
C (US Trans)
30
G
G
G
G
40
G
G
D (DS Trans)
D (DS Trans)
A (DS Trans)
A (DS Trans)
B (culv)
E
F
F
E
F
100
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
100
A (ds trans)
C (usDtrans)
(us trans)
A (DS Trans)
A (DS Trans)
B (Culvert)
B (Culvert)
B (Culvert)
B (Culvert)
Top Width (ft)
10
Maximum depth (ft)
C (US Trans)
C (US Trans)
C (US Trans)
E
E
D (DS Trans)
D (DS Trans)
F
F
E
E
Little Zigzag Creek
F
Top width (ft)
G
G
G
G
50
Site Evaluations
A—315
B (Culvert)
F
E
C (US Trans)
B (culv)
E
D (DS Trans)
D (DS Trans)
C (US Trans)
E
F
Qbf
A (DS Trans)
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
B (Culvert)
25%Q2
F
C (US Trans)
A (ds trans)
C (usDtrans)
(us trans)
E
F
Figure 18—Velocity (channel).
0
Velocity (ft/sec)
2
4
6
8
G
G
G
Figure 17—Width-to-depth ratio.
10
A (DS Trans)
Qbf
B (Culvert)
A (ds trans)
C (usDtrans)
(us trans)
C (US Trans)
25%Q2
D (DS Trans)
D (DS Trans)
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
F
G
G
G
B (culv)
E
F
E
F
10
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
10
A (ds trans)
C (usDtrans)
(us trans)
A (DS Trans)
A (DS Trans)
E
B (Culvert)
B (Culvert)
B (culv)
G
C (US Trans)
C (US Trans)
E
E
D (DS Trans)
D (DS Trans)
F
F
E
E
F
G
G
G
B (culv)
E
F
E
F
50
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
50
A (ds trans)
C (usDtrans)
(us trans)
A (DS Trans)
A (DS Trans)
E
B (Culvert)
B (Culvert)
G
G
0
C (US Trans)
C (US Trans)
F
F
D (DS Trans)
D (DS Trans)
20
G
G
G
G
30
B (culv)
E
F
F
E
F
100
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
100
A (ds trans)
C (usDtrans)
(us trans)
A (DS Trans)
A (DS Trans)
A (DS Trans)
A (DS Trans)
B (Culvert)
B (Culvert)
10
Width-to-depth Ratio
Velocity (ft/sec)
B (Culvert)
C (US Trans)
C (US Trans)
E
E
D (DS Trans)
D (DS Trans)
F
F
E
E
A—316
F
Width-to-depth ratio
G
G
G
G
40
Culvert Scour Assessment
Little Zigzag Creek
Shear stress (lbs/ft2)
D (DS Trans)
A (DS Trans)
0
E
F
50
100
G
F
150
200
Dis charge (cfs )
250
300
G
350
G
B (culv)
E
D (DS Trans)
C (US Trans)
B (Culvert)
A (DS Trans)
F
DS Channel - riffle
Culvert (DS pebble count) - step
Culvert (US pebble count) - riffle
E
Figure 20—Excess shear stress.
G
G
US Channel (DS pebble count) - step
10
A (ds trans)
C (usDtrans)
(us trans)
E
F
B (Culvert)
Excess shear stress is the channel shear divided by the critical shear for bed entrainment of the D84
particle size. Values of excess shear greater than 1 indicate bed movement for the D84 particle size.
Excess shear (Applied/tcrit)
0
Excess Shear (Applied / tcrit)
1
2
3
4
C (US Trans)
Qbf
B (culv)
D (DS Trans)
25%Q2
G
E
A (ds trans)
C (usDtrans)
(us trans)
B (culv)
F
F
A (ds trans)
C (usDtrans)
(us trans)
E
A (DS Trans)
Figure 19—Shear stress (channel).
0
B (Culvert)
Little Zigzag Creek
C (US Trans)
4
Shear Stress (lbs/ft2)
8
12
16
B (culv)
F
F
E
D (DS Trans)
C (US Trans)
B (Culvert)
A (DS Trans)
50
A (ds trans)
C (usDtrans)
(us trans)
E
G
B (culv)
F
F
E
D (DS Trans)
C (US Trans)
B (Culvert)
A (DS Trans)
100
A (ds trans)
C (usDtrans)
(us trans)
E
G
Site Evaluations
A—317
G
G
Culvert Scour Assessment
Table 3—Sum of squared height difference
Reach
XSUnit
Location
type
Culvert
Sum of squared
height difference
Within range of
channel conditions?
US
Riffle
0.02
Yes
DS
Step
0.08
Yes
US
Pool
0.20
DS
Step
0.02
Upstream
Table 4—Vertical sinuosity
Segment
LocationVertical Sinuosity (ft/ft)
A
DS transition
1.004
B
Culvert
1.001
C
US transition
1.002
D
US transition
1.004
E
US channel
1.009
F
US channel
1.001
G
US channel
1.004
Table 5—Depth distribution
Reach
XS Location
25% Q2
Culvert
Within range of
channel conditions?
US
0
No
DS
5
No
US
2
DS
3
Upstream
Table 6—Habitat unit composition
Reach
A—318
Percent of surface area
Pool Glide
Riffle
Step
Culvert
0%
0%
97%
2%
Upstream Channel
22%
0%
63%
3%
Little Zigzag Creek
Site Evaluations
0.9
0.8


0.6

0.5

0.4

Segment G
Segment F
SEgment E
Segment A
DS Transition
0.0



Segment D
US Transition
0.1

Segment C
US Transition
0.3
Residual depth (ft)
0.2
Segment B
Culvert
Residual Depth (ft)
0.7
Culvert
Segment
B: Segment C: Segment D: Segment E Segment F Segment G
Segment
A:
US Transition
US Transition
DS Transition
Figure 21—Residual depths.
Table 7—Bed material sorting and skewness
Reach
XSUnit Sorting
Location Type
Culvert
Within range Skewness
of channel
conditions?
Within range
of channel
conditions?
US
Riffle
1.30
No
0.22
No
DS
Step
2.07
No
0.48
No
US
Pool
2.49
0.43
DS
Step
3.08
0.47
Upstream
Table 8—Large woody debris
Reach
Pieces/Channel Width
Culvert
0.21
Upstream
1.32
Terminology:
US = Upstream
DS = Downstream
RR = Reference reach
XS = Cross section
Little Zigzag Creek
A—319
Culvert Scour Assessment
View upstream of culvert outlet.
View downstream towards culvert inlet.
View upstream from inlet crown.
View downstream from outlet crown.
Upstream reference reach from upstream end.
Upstream reference reach – upstream pebble
count, pool.
A—320
Little Zigzag Creek
Site Evaluations
Upstream reference reach – downstream pebble. View of step in culvert near outlet (pebble count count, step location).
View downstream in culvert – upstream pebble Survey set up upstream of inlet.
count at flagging (riffle).
Little Zigzag Creek
A—321
Culvert Scour Assessment
Cross Section: Upstream Reference Reach – Upstream Pebble Count
Material
sand
very fine gravel
fine gravel
fine gravel
medium gravel
medium gravel
coarse gravel
coarse gravel
very coarse gravel
very coarse gravel
small cobble
medium cobble
large cobble
very large cobble
small boulder
small boulder
medium boulder
large boulder
very large boulder
bedrock
S ize R ange (mm)
C ount
Item %
C umulative %
6
5
0
5
1
3
3
6
2
7
9
4
6
15
5
1
1
0
0
22
6%
5%
0%
5%
1%
3%
3%
6%
2%
7%
9%
4%
6%
15%
5%
1%
1%
0%
0%
22%
6%
11%
11%
16%
17%
20%
23%
29%
31%
38%
47%
50%
56%
71%
76%
77%
78%
78%
78%
100%
<2
2-4
4 - 5.7
5.7 - 8
8 - 11.3
11.3 - 16
16 - 22.6
22.6 - 32
32 - 45
45 - 64
64 - 90
90 - 128
128 - 180
180 - 256
256 - 362
362 - 512
512 - 1024
1024 - 2048
2048 - 4096
> 4096
25
100%
20
80%
Frequency
70%
15
60%
50%
10
Frequency
40%
30%
5
Cumulative Frequency
90%
20%
10%
Bedrock
2048-4096
512-1024
1024-2048
362-512
256-362
180-256
128-180
64-90
90-128
45-64
32-45
22.6-32
16-22.6
8-11.3
11.3-16
5.7-8
<2
4-5.7
<2
0%
2-4
0
Cumulative Frequency
2-4
4 - 5.7
5.7 - 8
> 4096
32 -45
45-64
64- 90
8 - 11.3
90 - 128
11.316
- 16
-22.6
22.6- 32
128180
- 180
256
- 256
362
- 362
- 512
512
1024
Particle Size Category (mm)
1024
2048
- 2048
- 4096
Particle Size Category (mm)
S ize C las s
S ize perc ent finer
than (mm)
D5
D16
D50
D84
D95
D100
1
6
70
215
277
600
A—322
Material
P erc ent C ompos ition
Sand
Gravel
Cobble
Boulder
Bedrock
6%
32%
34%
7%
22%
Sorting Coefficient:
2.49
Skewness Coefficient: 0.43
Little Zigzag Creek
Site Evaluations
Cross Section: Upstream Reference Reach – Downstream Pebble Count
Material
sand
very fine gravel
fine gravel
fine gravel
medium gravel
medium gravel
coarse gravel
coarse gravel
very coarse gravel
very coarse gravel
small cobble
medium cobble
large cobble
very large cobble
small boulder
small boulder
medium boulder
large boulder
very large boulder
bedrock
S ize C las s (mm)
C ount
Item %
C umulative %
16
1
1
0
1
4
0
2
5
10
15
9
9
8
7
5
4
0
0
0
16%
1%
1%
0%
1%
4%
0%
2%
5%
10%
15%
9%
9%
8%
7%
5%
4%
0%
0%
0%
16%
18%
19%
19%
20%
24%
24%
26%
31%
41%
57%
66%
75%
84%
91%
96%
100%
100%
100%
100%
<2
2-4
4 - 5.7
5.7 - 8
8 - 11.3
11.3 - 16
16 - 22.6
22.6 - 32
32 - 45
45 - 64
64 - 90
90 - 128
128 - 180
180 - 256
256 - 362
362 - 512
512 - 1024
1024 - 2048
2048 - 4096
Bedrock
100%
16
90%
14
80%
70%
60%
10
50%
8
Frequency
6
40%
30%
Cumulative Frequency
Bedrock
2048-4096
1024-2048
362-512
512-1024
256-362
180-256
90-128
128-180
64-90
45-64
32-45
22.6-32
<2
11.3-16
0%
16-22.6
0
5.7-8
10%
8-11.3
2
4-5.7
20%
<2
4
2-4
Frequency
12
Cumulative Frequency
18
2-4
4 - 5.7 - 8
32 -45
45-64
64- 90
8 - 11.3
90 - 128
Bedrock
11.316- 16
-22.6
22.6- 32
128180
- 180
256
- 256
362
- 362
- 512
512 - 1024
Particle Size Category (mm)
1024
2048
- 2048
- 4096
Particle Size Category (mm)
S ize C las s
S ize perc ent finer
than (mm)
D5
D16
D50
D84
D95
D100
1
2
80
263
452
600
Little Zigzag Creek
Material
P erc ent C ompos ition
Sand
Gravel
Cobble
Boulder
Bedrock
16%
25%
42%
16%
0%
Sorting Coefficient:
3.08
Skewness Coefficient: 0.47
A—323
Culvert Scour Assessment
Cross Section: Culvert – Upstream Pebble Count
Material
sand
very fine gravel
fine gravel
fine gravel
medium gravel
medium gravel
coarse gravel
coarse gravel
very coarse gravel
very coarse gravel
small cobble
medium cobble
large cobble
very large cobble
small boulder
small boulder
medium boulder
large boulder
very large boulder
bedrock
S ize C las s (mm)
C ount
Item %
C umulative %
1
2
1
1
3
5
9
15
6
8
17
14
13
1
0
1
2
0
0
0
1%
2%
1%
1%
3%
5%
9%
15%
6%
8%
17%
14%
13%
1%
0%
1%
2%
0%
0%
0%
1%
3%
4%
5%
8%
13%
22%
37%
43%
52%
69%
83%
96%
97%
97%
98%
100%
100%
100%
100%
<2
2-4
4 - 5.7
5.7 - 8
8 - 11.3
11.3 - 16
16 - 22.6
22.6 - 32
32 - 45
45 - 64
64 - 90
90 - 128
128 - 180
180 - 256
256 - 362
362 - 512
512 - 1024
1024 - 2048
2048 - 4096
Bedrock
100%
16
90%
14
80%
70%
Frequency
12
60%
10
50%
8
Frequency
6
40%
30%
Cumulative Frequency
Bedrock
2048-4096
512-1024
1024-2048
362-512
256-362
180-256
128-180
64-90
90-128
45-64
32-45
22.6-32
16-22.6
<2
8-11.3
0%
11.3-16
0
5.7-8
10%
4-5.7
2
<2
20%
2-4
4
Cumulative Frequency
18
2-4
4 - 5.7 - 8
32 -45
45-64
64- 90
8 - 11.3
90 - 128
Bedrock
11.316- 16
-22.6
22.6- 32
128180
- 180
256
- 256
362
- 362
- 512
512 - 1024
Particle Size Category (mm)
1024
2048
- 2048
- 4096
Particle Size Category (mm)
S ize C las s
S ize perc ent finer
than (mm)
D5
D16
D50
D84
D95
D100
10
20
60
130
171
520
A—324
Material
P erc ent C ompos ition
Sand
Gravel
Cobble
Boulder
Bedrock
1%
51%
45%
3%
0%
Sorting Coefficient:
1.30
Skewness Coefficient: 0.22
Little Zigzag Creek
Site Evaluations
Cross Section: Culvert – Downstream Pebble Count
Material
sand
very fine gravel
fine gravel
fine gravel
medium gravel
medium gravel
coarse gravel
coarse gravel
very coarse gravel
very coarse gravel
small cobble
medium cobble
large cobble
very large cobble
small boulder
small boulder
medium boulder
large boulder
very large boulder
bedrock
S ize C las s (mm)
C ount
Item %
C umulative %
3
1
0
1
2
2
7
6
2
5
5
4
1
7
14
12
7
1
0
0
4%
1%
0%
1%
3%
3%
9%
8%
3%
6%
6%
5%
1%
9%
18%
15%
9%
1%
0%
0%
4%
5%
5%
6%
9%
11%
20%
28%
30%
36%
43%
48%
49%
58%
75%
90%
99%
100%
100%
100%
<2
2-4
4 - 5.7
5.7 - 8
8 - 11.3
11.3 - 16
16 - 22.6
22.6 - 32
32 - 45
45 - 64
64 - 90
90 - 128
128 - 180
180 - 256
256 - 362
362 - 512
512 - 1024
1024 - 2048
2048 - 4096
Bedrock
16
100%
14
90%
70%
Frequency
10
60%
50%
8
40%
Frequency
6
30%
4
20%
2
Cumulative Frequency
80%
12
Cumulative Frequency
10%
Bedrock
2048-4096
1024-2048
362-512
512-1024
256-362
180-256
90-128
128-180
64-90
45-64
32-45
22.6-32
11.3-16
16-22.6
5.7-8
8-11.3
<2
4-5.7
<2
0%
2-4
0
2-4
4 - 5.7 - 8
32 -45
45-64
64- 90
8 - 11.3
90 - 128
Bedrock
11.316- 16
-22.6
22.6- 32
128180
- 180
256
- 256
362
- 362
- 512
512 - 1024
Particle Size Category (mm)
1024
2048
- 2048
- 4096
Particle Size Category (mm)
S ize C las s
S ize perc ent finer
than (mm)
D5
D16
D50
D84
D95
D100
8
20
195
444
602
1100
Little Zigzag Creek
Material
P erc ent C ompos ition
Sand
Gravel
Cobble
Boulder
Bedrock
4%
33%
21%
43%
0%
Sorting Coefficient:
2.07
Skewness Coefficient: 0.48
A—325
Culvert Scour Assessment
Cross Section: Downstream of Culvert – Only Pebble Count
Material
sand
very fine gravel
fine gravel
fine gravel
medium gravel
medium gravel
coarse gravel
coarse gravel
very coarse gravel
very coarse gravel
small cobble
medium cobble
large cobble
very large cobble
small boulder
small boulder
medium boulder
large boulder
very large boulder
bedrock
S ize C las s (mm)
C ount
Item %
C umulative %
13
1
1
2
4
3
3
2
0
6
8
5
13
13
17
6
2
0
0
0
13%
1%
1%
2%
4%
3%
3%
2%
0%
6%
8%
5%
13%
13%
17%
6%
2%
0%
0%
0%
13%
14%
15%
17%
21%
24%
27%
29%
29%
35%
43%
48%
62%
75%
92%
98%
100%
100%
100%
100%
<2
2-4
4 - 5.7
5.7 - 8
8 - 11.3
11.3 - 16
16 - 22.6
22.6 - 32
32 - 45
45 - 64
64 - 90
90 - 128
128 - 180
180 - 256
256 - 362
362 - 512
512 - 1024
1024 - 2048
2048 - 4096
Bedrock
100%
16
90%
14
80%
70%
Frequency
12
60%
10
50%
8
Frequency
6
40%
30%
4
20%
2
10%
Cumulative Frequency
Bedrock
2048-4096
1024-2048
362-512
512-1024
256-362
180-256
128-180
64-90
90-128
45-64
32-45
22.6-32
11.3-16
16-22.6
5.7-8
8-11.3
2-4
<2
4-5.7
0%
<2
0
Cumulative Frequency
18
2-4
4 - 5.7
5.7 - 8
32 -45
45-64
64- 90
8 - 11.3
90 - 128
Bedrock
11.316- 16
-22.6
22.6- 32
128180
- 180
256
- 256
362
- 362
- 512
512 - 1024
Particle Size Category (mm)
1024
2048
- 2048
- 4096
Particle Size Category (mm)
S ize C las s
S ize perc ent finer
than (mm)
D5
D16
D50
D84
D95
D100
1
8
130
286
405
800
Material
P erc ent C ompos ition
Sand
Gravel
Cobble
Boulder
Bedrock
13%
22%
39%
25%
0%
Sorting Coefficient:
2.52
Skewness Coefficient: 0.44
*This pebble count was not used in the analysis because the downstream reach was not used as a representative reach.
A—326
Little Zigzag Creek