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Publication Number: FHWA-HRT-05-072
Date: July 2006

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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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). The hydraulics and hydrology research program at the TFHRC Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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