Assessing Stream Channel Stability At Bridges in Physiographic Regions
4. ASSESSING CHANNEL STABILITY
Based on the studies described in a previous section as well as on the observations made at bridges across numerous physiographic regions, a group of parameters that indicate
channel stability can be selected. First, however, it is necessary to redefine
stream channel stability in light of bridge engineering issues. For this
purpose, a stable channel is defined as follows, based on Knox and modified for
use at bridges: (15)
A stable channel in the vicinity of a bridge is one in
which the relationship between geomorphic process and form is stationary and
the morphology of the system remains relatively constant over the short-term
(one to two years), over a short distance upstream and downstream from bridge,
and with minimal lateral movement.
Although lateral migration of
a stream channel can be considered normal and stable within a geomorphic
definition of channel stability, it is detrimental to bridge safety and is,
therefore, considered in the stability definition used here. The distance
upstream and downstream of the bridge that should be considered in a stability
assessment depends on the problem, the channel, and the bridge. However, it is
acknowledged that a bridge inspector will not typically walk more than a few
hundred feet in either direction. That stated, it should be noted that without
walking well upstream and downstream of the bridge, channel instabilities, such
as knickpoints, that are migrating toward the bridge area may be overlooked. Remember
that the objective here is only to assess stream stability in the short term,
as inspections of bridges over water are required every 2 years. Thus, it is
not necessary to develop a complex method to examine the history or future of
channel adjustments over a long time period. It is necessary, however, for each
inspector to review previous stability analyses at the bridge of interest to
determine whether any unstable trends are developing.
A stability assessment
program for bridge inspections should be: (1) brief so that it can be completed
rapidly; (2) simple in that extensive training is not required (although some
training will be required); (3) based on sound indicators as discussed in the
literature review; and (4) based on the needs of the bridge engineering
community.
One way to insure that all
aspects of channel stability are included is to start at the watershed or
regional level and focus in on vertical and lateral aspects of the channel,
following the concepts of Thorne et al. (7) and Montgomery and
MacDonald. (20) Thus, at the broader level, watershed and flood plain
activities as well as characteristics, flow habit, channel pattern and type,
and entrenchment are selected as appropriate indicators. At the channel level,
indicators such as bed material consolidation and armoring, bar development,
and obstructions are used. Indicators of bank stability include bank material,
angle, bank and riparian vegetation, bank (fluvial) cutting, and mass wasting
(geotechnical failure). Finally, the position of the bridge relative to the
channel can be indicated by meander impact point and alignment. In the previous
method, the ratio of the average boundary shear stress to the critical shear
stress for sediment movement had been found to be important; however, average
shear conditions do not necessarily indicate processes that are occurring. (1)
Also, critical shear stress is not a reliable number. In addition, it is
difficult to measure and quantify as part of a rapid assessment. Therefore, the
shear stress ratio is not used as a stability indicator in this current
assessment method. In its place, bed material and percent of sand are used. These
results are based on the Wilcock and Kenworthy study of bed material movement
as a function of sand fractions. (86)
The 13 indicators identified
for this study are listed in table 8. For each indicator, a rating of poor,
fair, good, or excellent can be assigned based on descriptors listed in the
table. After a rating is assigned for each of the indicators, an overall rank
is obtained by summing the 13 ratings. Several assumptions are implicit in this
method of obtaining an overall rank. First, all indicators are weighted
equally. This assumption was tested by assigning weights to each of the
indicators and creating a weighted score for every bridge where observations
were made. The results showed that the weighted indicators yielded the same
results as the equally weighted indicators. Thus, there was no advantage in
using weights. Second, this method implies that each indicator is independent
of all others. While it is possible that some correlation exists between
several of the indicators, an attempt was made to select indicators that
independently describe various aspects of channel stability; thus, correlation
effects were judged to be insignificant. Third, the summing of the ratings
implies a linear scheme. The impact of this is not precisely known; however,
given that weighted ratings provided no change in the overall results, it can
be assumed that the linearity will also not affect the results significantly.
Table 9 provides the rating results
for each of the 13 stability indicators at all of the bridges where
observations were made. Using the same 13 indicators for streams in all
physiographic regions adequately described channel conditions at each of these
sites. The sums of the ratings also are given in table 9. These overall
rankings were then rated as excellent, good, fair, or poor. The division of the
overall rankings among a single set of category divisions provided limited
sensitivity to streams in some stream channel classifications and physiographic
regions. Thus, it was desirable to rank the stability based on stream type. Given
that the Montgomery-Buffington classification method is based on processes as
well as physical characteristics, this scheme was used to provide additional
sensitivity to the method. Since cascade and step-pool streams are both
transport streams and are not sensitive to changes in sediment or water
discharge, these streams were given a separate category of rankings. Plane-bed,
pool-riffle, and dune-ripple streams, along with engineered channels, were
given a second category as primarily response-type streams. Finally, braided
streams were placed in a third category, as these represent a type of stream that
is very sensitive to changes in sediment and water discharge and are primarily
located in the western and southwestern regions of the United States. These
divisions also agree loosely with the stability assessment method that Rosgen
developed. (Rosgen has divisions according to stream type, resulting in 42 divisions.
This implies a level of sensitivity for which there is no explanation given. It
also provides an unwieldy and cumbersome accounting of rankings and tables.) Tables
10-12 provide the range of rankings for excellent, good, fair, and poor ratings
of stability for each of the three divisions of stream channels. The final
rankings, in terms of excellent, good, fair, and poor, are given in table 9.
Table 8. Stability indicators, descriptions, and ratings.*
Stability Indicator |
Ratings |
Excellent (1-3) |
Good (4-6) |
Fair (7-9) |
Poor (10-12) |
1. Watershed and flood plain activity and characteristics |
Stable, forested, undisturbed watershed |
Occasional minor disturbances in the watershed,
including cattle activity (grazing and/or access to stream), construction,
logging, or other minor deforestation. Limited agricultural activities |
Frequent disturbances in the watershed, including
cattle activity, landslides, channel sand or gravel mining, logging, farming,
or construction of buildings, roads, or other infrastructure. Urbanization
over significant portion of watershed |
Continual disturbances in the watershed. Significant
cattle activity, landslides, channel sand or gravel mining, logging, farming,
or construction of buildings, roads, or other infrastructure. Highly urbanized or rapidly urbanizing watershed |
2. Flow habit |
Perennial stream with no flashy behavior |
Perennial stream or ephemeral first-order stream with
slightly increased rate of flooding |
Perennial or intermittent stream with flashy behavior |
Extremely flashy; flash floods prevalent mode of
discharge; ephemeral stream other than first-order stream |
3. Channel pattern |
Straight to meandering with low radius of curvature;
primarily suspended load |
Meandering, moderate radius of curvature; mix of
suspended and bed loads; well-maintained engineered channel |
Meandering with some braiding; tortuous meandering;
primarily bed load; poorly maintained engineered channel |
Braided; primarily bed load; engineered channel that
is not maintained |
4. Entrenchment/channel confinement |
Active flood plain exists at top of banks; no sign of
undercutting infrastructure; no levees |
Active flood plain abandoned, but is currently
rebuilding; minimal channel confinement; infrastructure not exposed; levees are low and set well back from the river |
Moderate confinement in valley or channel walls; some
exposure of infrastructure; terraces exist; flood plain abandoned; levees are
moderate in size and have minimal setback from the river |
Knickpoints visible downstream; exposed water lines or
other infrastructure; channel-width-to-top-of-banks ratio small; deeply confined; no active flood plain; levees are high and along the channel edge |
*Range of values in ratings columns provide possible rating values for each factor
H = horizontal, V = vertical, Fs = fraction of sand, S =
slope, w/y = width-to-depth ratio
Table 8. Stability indicators, descriptions, and ratings, continued.
Stability Indicator |
Ratings |
Excellent (1-3) |
Good (4-6) |
Fair (7-9) |
Poor (10-12) |
5. Bed material Fs = approximate portion of sand in the bed |
Assorted sizes tightly packed, overlapping, and
possibly imbricated. Most material > 4 mm. Fs < 20% |
Moderately packed with some overlapping. Very small
amounts of material < 4 mm. 20 < Fs < 50% |
Loose assortment with no apparent overlap. Small to
medium amounts of material < 4 mm. 50 < Fs < 70% |
Very loose assortment with no packing. Large amounts
of material < 4 mm. Fs > 70% |
6. Bar development |
For S < 0.02 and w/y > 12, bars are mature,
narrow relative to stream width at low flow, well vegetated, and composed of
coarse gravel to cobbles. For S > 0.02 and w/y < 12, no bars are
evident |
For S < 0.02 and w/y > 12, bars may have
vegetation and/or be composed of coarse gravel to cobbles, but minimal recent
growth of bar evident by lack of vegetation on portions of the bar. For S
> 0.02 and w/y < 12, no bars are evident |
For S < 0.02 and w/y > 12, bar widths tend to be
wide and composed of newly deposited coarse sand to small cobbles and/or may
be sparsely vegetated. Bars forming for S > 0.02 and w/y < 12 |
Bar widths are generally greater than 1/2 the stream
width at low flow. Bars are composed of extensive deposits of fine particles
up to coarse gravel with little to no vegetation. No bars for S < 0.02 and
w/y > 12 |
7. Obstructions, including bedrock outcrops, armor
layer, LWD jams, grade control, bridge bed paving, revetments, dikes or
vanes, riprap |
Rare or not present |
Occasional, causing cross currents and minor bank and
bottom erosion |
Moderately frequent and occasionally unstable obstructions,
cause noticeable erosion of the channel. Considerable sediment accumulation
behind obstructions |
Frequent and often unstable, causing a continual shift
of sediment and flow. Traps are easily filled, causing channel to migrate
and/or widen |
8. Bank soil texture and coherence |
Clay and silty clay; cohesive material |
Clay loam to sandy clay loam; minor amounts of
noncohesive or unconsolidated mixtures; layers may exist, but are cohesive
materials |
Sandy clay to sandy loam; unconsolidated mixtures of glacial
or other materials; small layers and lenses of noncohesive or unconsolidated
mixtures |
Loamy sand to sand; noncohesive material;
unconsolidated mixtures of glacial or other materials; layers or lenses that
include noncohesive sands and gravels |
H = horizontal, V = vertical, Fs = fraction of sand, S =
slope, w/y = width-to-depth ratio
Table 8. Stability indicators, descriptions, and ratings, continued.
Stability Indicator |
Ratings |
Excellent (1-3) |
Good (4-6) |
Fair (7-9) |
Poor (10-12) |
9. Average bank slope angle (where 90E is a
vertical bank) |
Bank slopes < 3H:1V (18E) for noncohesive or
unconsolidated materials to < 1:1 (45E) in clays on both sides |
Bank slopes up to 2H:1V (27E) in noncohesive or
unconsolidated materials to 0.8:1 (50E) in clays on one or occasionally
both banks |
Bank slopes to 1H:1V (45E) in noncohesive or unconsolidated
materials to 0.6:1 (60E) in clays common on one or both banks |
Bank slopes over 45E in noncohesive or unconsolidated
materials or over (60E) in clays common on one or both banks |
10. Vegetative or engineered bank protection |
Wide band of woody vegetation with at least 90%
density and cover. Primarily hard wood, leafy, deciduous trees with mature,
healthy, and diverse vegetation located on the bank. Woody vegetation
oriented vertically. In absence of vegetation, both banks are lined or
heavily armored |
Medium band of woody vegetation with 70-90% plant
density and cover. A majority of hard wood, leafy, deciduous trees with
maturing, diverse vegetation located on the blank. Woody vegetation oriented
80-90E
from horizontal with minimal root exposure. Partial lining or armoring of one
or both banks |
Small band of woody vegetation with 50-70% plant
density and cover. A majority of soft wood, piney, coniferous trees with
young or old vegetation lacking in diversity located on or near the top of
bank. Woody vegetation oriented at 70-80E from horizontal, often with evident
root exposure. No lining of banks, but some armoring may be in place on one
bank |
Woody vegetation band may vary depending on age and
health with less than 50% plant density and cover. Primarily soft wood,
piney, coniferous trees with very young, old and dying, and/or monostand
vegetation located off of the bank. Woody vegetation oriented at less than 70E from
horizontal with extensive root exposure. No lining or armoring of banks |
11. Bank cutting |
Little or none evident. Infrequent raw banks,
insignificant percentage of total bank |
Some intermittently along channel bends and at prominent constrictions. Raw banks comprise minor portion of bank in vertical direction |
Significant and frequent on both banks. Raw banks comprise large portion of bank in vertical direction. Root mat overhangs |
Almost continuous cuts on both banks, some extending
over most of the banks. Undercutting and sod-root overhangs |
H = horizontal, V = vertical, Fs = fraction of sand, S =
slope, w/y = width-to-depth ratio
Table 8. Stability indicators, descriptions, and ratings, continued.
Stability Indicator |
Ratings |
Excellent (1-3) |
Good (4-6) |
Fair (7-9) |
Poor (10-12) |
12. Mass wasting or bank failure |
No or little evidence of potential or very small
amounts of mass wasting. Uniform channel width over the entire reach |
Evidence of infrequent and/or minor mass wasting.
Mostly healed over with vegetation. Relatively constant channel width and
minimal scalloping of banks |
Evidence of frequent and/or significant occurrences of
mass wasting that can be aggravated by higher flows, which may cause
undercutting and mass wasting of unstable banks. Channel width quite irregular,
and scalloping of banks is evident |
Frequent and extensive mass wasting. The potential for
bank failure, as evidenced by tension cracks, massive undercuttings, and bank
slumping, is considerable. Channel width is highly irregular, and banks are
scalloped |
13. Upstream
distance to bridge from meander impact point and alignment |
More than 35 m; bridge is well-aligned
with river flow |
20-35 m; bridge is aligned with
flow |
10-20 m; bridge is skewed to flow, or flow alignment
is otherwise not centered beneath bridge |
Less than 10 m; bridge is poorly aligned with flow |
H = horizontal, V = vertical, Fs = fraction of sand, S =
slope, w/y = width-to-depth ratio
Table 9. Stability assessment ratings for each factor.
Stream |
Indicator |
Total |
Rating Based
on Tables 11-13 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
Saline R. |
6 |
9 |
7 |
3 |
9 |
6 |
4 |
9 |
3 |
4 |
2 |
2 |
3 |
67 |
Good |
SF Solomon R. |
6 |
10 |
8 |
4 |
11 |
5 |
2 |
12 |
7 |
8 |
2 |
3 |
3 |
81 |
Good |
N. Rush Cr. |
5 |
9 |
6 |
7 |
9 |
2 |
2 |
9 |
6 |
10 |
5 |
5 |
9 |
84 |
Good |
Arkansas R. |
4 |
3 |
5 |
2 |
5 |
2 |
5 |
9 |
2 |
6 |
3 |
4 |
4 |
54 |
Good |
Tomichi Cr. |
7 |
3 |
5 |
2 |
9 |
3 |
4 |
7 |
5 |
5 |
3 |
2 |
7 |
62 |
Good |
Murietta Cr. |
12 |
12 |
9 |
7 |
11 |
5 |
2 |
11 |
9 |
10 |
8 |
7 |
3 |
106 |
Fair |
Jacalitos Cr. |
9 |
12 |
10 |
7 |
11 |
8 |
3 |
11 |
8 |
10 |
6 |
7 |
8 |
110 |
Fair |
Dry Cr. |
3 |
7 |
3 |
7 |
3 |
2 |
9 |
3 |
8 |
5 |
4 |
4 |
5 |
63 |
Good |
Dutch Bill Cr. |
2 |
8 |
5 |
7 |
7 |
10 |
5 |
8 |
8 |
3 |
6 |
4 |
10 |
83 |
Good |
Buena Vista Cr. |
8 |
11 |
10 |
7 |
12 |
10 |
2 |
11 |
10 |
11 |
10 |
10 |
4 |
116 |
Fair |
Mojave R. |
10 |
12 |
12 |
6 |
12 |
12 |
5 |
12 |
7 |
12 |
12 |
11 |
8 |
131 |
Poor |
Rt. 66 Wash |
10 |
12 |
10 |
6 |
8 |
10 |
9 |
11 |
10 |
12 |
12 |
11 |
11 |
132 |
Poor |
Sacramento Wash |
9 |
12 |
10 |
6 |
10 |
10 |
9 |
11 |
12 |
10 |
12 |
9 |
4 |
124 |
Fair |
Rio San Jose |
8 |
7 |
4 |
9 |
9 |
4 |
6 |
10 |
10 |
9 |
9 |
9 |
6 |
100 |
Fair |
Rio Puerco |
8 |
7 |
6 |
10 |
11 |
10 |
9 |
12 |
12 |
11 |
12 |
12 |
10 |
130 |
Poor |
W. Elk Creek |
7 |
4 |
5 |
8 |
10 |
6 |
9 |
7 |
11 |
8 |
9 |
10 |
3 |
97 |
Fair |
Beaver Cr. |
12 |
4 |
6 |
12 |
11 |
10 |
10 |
12 |
12 |
11 |
12 |
12 |
3 |
127 |
Poor |
Brush Cr. |
10 |
6 |
6 |
10 |
8 |
4 |
7 |
11 |
10 |
9 |
7 |
8 |
3 |
99 |
Fair |
N 19 |
7 |
7 |
6 |
8 |
10 |
10 |
7 |
7 |
8 |
7 |
8 |
7 |
8 |
100 |
Fair |
R. = River, Cr. = Creek
Table 9. Stability assessment ratings for each factor, continued.
Stream |
Indicator |
Total |
Rating Based
on Tables 11-13 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
Little Skin Cr. |
6 |
5 |
6 |
5 |
5 |
6 |
9 |
7 |
9 |
9 |
6 |
10 |
8 |
91 |
Fair |
N 21 |
6 |
7 |
3 |
6 |
9 |
4 |
8 |
8 |
10 |
7 |
5 |
3 |
10 |
86 |
Fair |
Little Cypress Cr. |
8 |
6 |
6 |
11 |
10 |
8 |
7 |
6 |
12 |
7 |
12 |
9 |
8 |
110 |
Fair |
N 23 |
5 |
4 |
7 |
12 |
10 |
11 |
9 |
6 |
12 |
9 |
7 |
12 |
8 |
112 |
Fair |
N 24 |
7 |
5 |
6 |
10 |
11 |
6 |
6 |
5 |
10 |
9 |
5 |
6 |
3 |
89 |
Fair |
Honey Run |
4 |
3 |
2 |
6 |
5 |
4 |
4 |
5 |
10 |
9 |
5 |
11 |
10 |
78 |
Good |
South Fork |
3 |
3 |
3 |
7 |
7 |
2 |
4 |
4 |
8 |
8 |
6 |
6 |
3 |
64 |
Good |
East Fork |
8 |
5 |
6 |
7 |
6 |
4 |
5 |
5 |
10 |
11 |
6 |
5 |
10 |
88 |
Fair |
N 28 |
9 |
5 |
6 |
7 |
3 |
4 |
8 |
4 |
10 |
8 |
7 |
4 |
4 |
79 |
Good |
McKnown Cr. |
3 |
3 |
5 |
7 |
3 |
3 |
7 |
3 |
9 |
11 |
7 |
6 |
4 |
71 |
Good |
Wolf Run |
3 |
3 |
6 |
9 |
2 |
4 |
9 |
9 |
11 |
11 |
11 |
11 |
9 |
98 |
Fair |
Alligator Cr. |
8 |
2 |
4 |
5 |
3 |
5 |
5 |
8 |
5 |
2 |
2 |
2 |
6 |
57 |
Good |
Peace R. |
4 |
2 |
8 |
2 |
12 |
5 |
8 |
11 |
5 |
5 |
4 |
7 |
4 |
77 |
Good |
Blackrock Run |
7 |
4 |
5 |
8 |
7 |
7 |
5 |
7 |
10 |
8 |
4 |
7 |
5 |
84 |
Good |
Indian Run |
5 |
3 |
8 |
7 |
5 |
8 |
8 |
5 |
8 |
10 |
4 |
9 |
10 |
90 |
Fair |
Middle Patuxent R. |
5 |
2 |
5 |
4 |
3 |
3 |
4 |
5 |
9 |
5 |
8 |
7 |
7 |
67 |
Good |
Hammond Branch |
11 |
9 |
8 |
8 |
9 |
9 |
7 |
6 |
10 |
9 |
10 |
9 |
11 |
116 |
Fair |
Atherton Tributary |
5 |
6 |
5 |
6 |
6 |
5 |
4 |
5 |
8 |
8 |
6 |
6 |
4 |
74 |
Good |
Stocketts Run |
3 |
3 |
4 |
6 |
7 |
8 |
5 |
2 |
7 |
5 |
8 |
4 |
11 |
73 |
Good |
R. = River, Cr. = Creek
Table 9. Stability assessment ratings for each factor, continued.
Stream |
Indicator |
Total |
Rating Based
on Tables 11-13 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
Mill Stream Branch |
6 |
4 |
3 |
7 |
10 |
8 |
5 |
3 |
8 |
7 |
4 |
4 |
3 |
72 |
Good |
Kent
County Tributary |
4 |
3 |
4 |
10 |
10 |
9 |
7 |
3 |
11 |
9 |
9 |
10 |
9 |
98 |
Fair |
Morgan Cr. |
8 |
5 |
5 |
4 |
9 |
5 |
4 |
3 |
9 |
9 |
10 |
6 |
11 |
88 |
Fair |
Little Elk Cr. |
5 |
2 |
3 |
2 |
2 |
3 |
3 |
4 |
5 |
3 |
4 |
2 |
8 |
46 |
Excellent |
Big Beaver Cr. |
11 |
5 |
5 |
4 |
10 |
12 |
6 |
5 |
10 |
12 |
12 |
10 |
10 |
112 |
Fair |
Buffalo
Run |
7 |
4 |
6 |
5 |
5 |
4 |
5 |
2 |
9 |
11 |
9 |
7 |
4 |
78 |
Good |
Roaring Run |
2 |
2 |
2 |
2 |
2 |
5 |
3 |
3 |
4 |
3 |
3 |
1 |
8 |
40 |
Excellent |
Potter Run |
3 |
3 |
3 |
3 |
1 |
2 |
4 |
2 |
6 |
9 |
8 |
4 |
4 |
52 |
Good |
Bentley Cr. |
10 |
9 |
10 |
7 |
9 |
6 |
5 |
12 |
12 |
8 |
11 |
8 |
12 |
119 |
Fair |
N 48 |
2 |
3 |
3 |
5 |
1 |
2 |
4 |
1 |
5 |
1 |
4 |
2 |
8 |
41 |
Excellent |
Reids Run |
3 |
3 |
3 |
5 |
1 |
4 |
4 |
3 |
6 |
5 |
4 |
2 |
7 |
50 |
Good |
Piney Cr. |
4 |
3 |
4 |
4 |
3 |
5 |
4 |
5 |
7 |
9 |
4 |
2 |
9 |
63 |
Good |
L. Sandy Cr. |
6 |
4 |
4 |
6 |
3 |
5 |
3 |
3 |
8 |
10 |
6 |
8 |
8 |
74 |
Good |
Trout Run |
4 |
3 |
3 |
3 |
4 |
3 |
3 |
3 |
6 |
1 |
2 |
1 |
7 |
43 |
Excellent |
Pootatuck R. |
4 |
3 |
3 |
4 |
1 |
2 |
4 |
3 |
5 |
5 |
5 |
4 |
8 |
51 |
Good |
Mill R. |
3 |
2 |
5 |
3 |
6 |
2 |
4 |
8 |
6 |
6 |
6 |
2 |
12 |
65 |
Good |
Aspetuck R. |
5 |
3 |
3 |
5 |
7 |
4 |
4 |
7 |
5 |
5 |
6 |
2 |
3 |
59 |
Good |
W. Br. Saugatuck R. |
6 |
3 |
7 |
3 |
5 |
2 |
11 |
6 |
3 |
3 |
6 |
1 |
8 |
64 |
Good |
Mianus R. |
3 |
2 |
3 |
3 |
4 |
3 |
3 |
4 |
5 |
4 |
4 |
1 |
5 |
44 |
Excellent |
R. = River, Cr. = Creek
Table 10. Overall rankings for pool-riffle, plane-bed, dune-ripple, and engineered channels.
Category |
Ranking, R |
Excellent |
R < 49 |
Good |
49# R < 85 |
Fair |
85#R < 120 |
Poor |
120 # R |
Table 11. Overall rankings for cascade and step-pool channels.
Category |
Ranking, R |
Excellent |
R < 41 |
Good |
41 # R < 70 |
Fair |
70 # R < 98 |
Poor |
98 # R |
Table 12. Overall rankings for braided channels.
Category |
Ranking, R |
Excellent |
N/A |
Good |
R < 94 |
Fair |
94 # R < 129 |
Poor |
129 # R |
Table 13. Vertical versus lateral stability.
Stream |
Lateral |
Vertical |
Lateral Fraction |
Vertical Fraction |
Saline R. |
23 |
18 |
0.32 |
0.50 |
SF Solomon R. |
35 |
20 |
0.49 |
0.56 |
N. Rush Cr. |
44 |
18 |
0.61 |
0.50 |
Arkansas R. |
28 |
9 |
0.39 |
0.25 |
Tomichi Cr. |
29 |
14 |
0.40 |
0.39 |
Murietta Cr. |
48 |
23 |
0.67 |
0.64 |
Jacalitos Cr. |
50 |
26 |
0.69 |
0.72 |
Dry Cr. |
29 |
12 |
0.40 |
0.33 |
Dutch Bill Cr. |
39 |
24 |
0.54 |
0.67 |
Buena Vista Cr. |
56 |
29 |
0.78 |
0.81 |
Mojave R. |
62 |
30 |
0.86 |
0.83 |
Rt. 66 Wash |
67 |
24 |
0.93 |
0.67 |
Sacramento
Wash |
58 |
26 |
0.81 |
0.72 |
Rio San Jose |
53 |
22 |
0.74 |
0.61 |
Rio Puerco |
69 |
31 |
0.96 |
0.86 |
W. Elk Creek |
48 |
24 |
0.67 |
0.67 |
Beaver Cr. |
62 |
33 |
0.86 |
0.92 |
Brush Cr. |
48 |
22 |
0.67 |
0.61 |
N 19 |
45 |
28 |
0.63 |
0.78 |
Little Skin Cr. |
49 |
16 |
0.68 |
0.44 |
N 21 |
43 |
19 |
0.60 |
0.53 |
Little Cypress
Cr. |
54 |
29 |
0.75 |
0.81 |
N 23 |
54 |
33 |
0.75 |
0.92 |
N 24 |
38 |
27 |
0.53 |
0.75 |
Honey Run |
50 |
15 |
0.69 |
0.42 |
South Fork |
35 |
16 |
0.49 |
0.44 |
East Fork |
47 |
17 |
0.65 |
0.47 |
R. = River, Cr. = Creek
Table 13. Vertical versus lateral stability, continued.
Stream |
Lateral |
Vertical |
Lateral Fraction |
Vertical Fraction |
N 28 |
37 |
14 |
0.51 |
0.39 |
McKnown Cr. |
40 |
13 |
0.56 |
0.36 |
Wolf Run |
62 |
15 |
0.86 |
0.42 |
Alligator Cr. |
25 |
13 |
0.35 |
0.36 |
Peace R. |
36 |
19 |
0.50 |
0.53 |
Blackrock Run |
41 |
22 |
0.57 |
0.61 |
Indian Run |
46 |
20 |
0.64 |
0.56 |
Middle Patuxent R. |
41 |
10 |
0.57 |
0.28 |
Hammond
Branch |
55 |
26 |
0.76 |
0.72 |
Atherton Tributary |
37 |
17 |
0.51 |
0.47 |
Stocketts Run |
37 |
21 |
0.51 |
0.58 |
Mill Stream Branch |
29 |
25 |
0.40 |
0.69 |
Kent Co. Tributary |
51 |
29 |
0.71 |
0.81 |
Morgan
Creek |
48 |
18 |
0.67 |
0.50 |
Little Elk Cr. |
26 |
7 |
0.36 |
0.19 |
Big Beaver Cr. |
59 |
26 |
0.82 |
0.72 |
Buffalo
Run |
42 |
14 |
0.58 |
0.39 |
Roaring Run |
22 |
9 |
0.31 |
0.25 |
Potter Run |
33 |
6 |
0.46 |
0.17 |
Bentley Cr. |
63 |
22 |
0.88 |
0.61 |
N 48 |
21 |
8 |
0.29 |
0.22 |
Reids Run |
27 |
10 |
0.38 |
0.28 |
Piney Cr. |
36 |
12 |
0.50 |
0.33 |
L. Sandy Cr. |
43 |
14 |
0.60 |
0.39 |
Trout Run |
20 |
10 |
0.28 |
0.28 |
Pootatuck R. |
30 |
7 |
0.42 |
0.19 |
Mill R. |
40 |
11 |
0.56 |
0.31 |
R. = River, Cr. = Creek
Table 13. Vertical versus lateral stability, continued.
Stream |
Lateral |
Vertical |
Lateral Fraction |
Vertical Fraction |
Aspetuck R. |
28 |
16 |
0.39 |
0.44 |
W. Br. Saugatuck R. |
27 |
10 |
0.38 |
0.28 |
Mianus R. |
23 |
10 |
0.32 |
0.28 |
R. = River, Cr. = Creek
HEC-20 suggests that the
lateral and vertical stability be examined as well as the overall stability. The
indicators in table 8 can be divided into those that indicate vertical
stability and those that indicate lateral stability. Results are given in table
13 in which vertical stability is described by indicators 4-6, while lateral
stability is described by indicators 8-13. Each of the lateral and vertical stability
ratings were normalized by the total number of points possible in each category
so that they could be represented as a fraction and more readily compared. Thus,
the lateral score was divided by 72 and the vertical score by 36. If the
lateral score fraction is greater than the vertical score fraction, then it can
be expected that the channel instability is primarily in the lateral direction.
As an example, the Route 66 Wash is rated as "poor." However, the lateral score
fraction is significantly higher than the vertical score fraction (0.93 versus
0.67), indicating that lateral instability is dominant. If, on the other hand,
the vertical score fraction is greater than the lateral, then bed degradation
is the dominant source of instability. An example of this type of scores is
given by Wolf Run, for which the vertical score fraction is about double the
lateral score fraction, indicating primarily vertical instability. If both
scores are high, then the channel is unstable due to both lateral and vertical
processes. For example, Beaver Creek has lateral and vertical fractions of 0.86
and 0.92, respectively. This indicates that the channel is both degrading and
widening. The processes may be ongoing simultaneously or they may be occurring
differentially. This is frequently the
case-as a knickpoint moves upstream, the channel banks respond by collapsing
and widening, then another knickpoint moves through, and the process repeats. If
both scores are low, this indicates minimal instability in either direction. For
example, Alligator Creek has similar scores in both lateral and vertical
categories, indicating healthy adjustments in both directions.
Occasionally, rating each of
the 13 factors for a particular bridge will result in one factor which stands
out as being much higher (worse) than the others. For example, Little Elk Creek
received an excellent as the overall rating. All of the assessment factors
received scores between 2 and 5, except for the alignment factor (#13). This
factor was given a rating of 8 due to the fact that the right abutment of the
bridge was located just downstream of the outside of a gentle meander bend. The
meander bend appears to be migrating at a very slow rate; this is based on
observations that there is undercutting of tree roots on the right bank, but
all trees are oriented vertically. Although the rate of lateral migration
appears to be slow, it is worth noting and making additional observations
during future inspections.
In collecting the data and
observations for this method, the engineer or other inspector should walk some
distance upstream and downstream from the bridge, rather than just observe from
the bridge itself. The appropriate distance, however, depends on several
factors, such as uniformity of stream conditions, magnitude of disturbances
along the banks, in the flood plain, or in the watershed, time available, and
accessibility. Ideally, the observer should walk at least 10 channel widths
upstream and downstream of the bridge. Although it is possible to establish
stability conditions in less distance, the more of the stream that is observed,
the better understanding the observer will have of causes, processes, and rates
of change.
Bridges often divide property
and sometimes divide geomorphic features or regions. Thus, conditions upstream
and downstream of the bridge may be significantly different. In this case, it
may be necessary to conduct separate analyses upstream and downstream. Unless
the disturbance downstream of the bridge is traveling upstream, as in
knickpoint migration or lateral migration of an adjacent meander, then the
conditions downstream will be unlikely to affect the bridge, and more emphasis
should be placed on the upstream conditions.
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