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Assessing Stream Channel Stability At Bridges in Physiographic Regions

1. INTRODUCTION

The goal of bridge inspections is to assess the safety of bridges on a regular basis so that any deficiencies will be identified and corrected. Given the large number of bridges over water in any State, bridge inspectors must inspect the superstructure, substructure, and waterway of each bridge in a short amount of time. A typical range of time for bridge inspections is 15 minutes to 2 hours, depending on the complexity and condition of the bridge. A more detailed inspection might ensue if a deficiency is detected. In the case of waterways and erosion, a hydraulic engineer might visit the bridge to assess the situation in greater detail. For either of these levels of inspection, and given the very limited right-of-way at most bridges, the inspector or engineer typically will not walk more than a few hundred feet upstream or downstream. Most inspectors do not leave the bridge right-of-way. Thus, a method is needed for systematically assessing the stability of the stream channel with respect to the bridge. The ability to assess channel stability in the vicinity of bridges also is needed for designing road crossings, and for mitigating and predicting erosion at those structures. Bridge failures due to geomorphic or regional instability have been experienced in many locations in the United States and elsewhere. Federal Highway Administration (FHWA) guidelines for stream stability and erosion at bridges, such as Hydraulic Engineering Circular (HEC)-20 (3) and HEC-18, (4) describe examples of problems at bridges caused by regional channel degradation and lateral bank changes. These guidelines require that engineers assess channel instability in their bridge assessments. However, for most bridges, only a preliminary assessment can be conducted due to time and money constraints.

The National Highway Institute (NHI) training course for bridge inspectors and hydraulic engineers has been based on a data collection method developed by Thorne. (2) The user completes a number of data sheets by collecting primarily qualitative geomorphic data. Although the method is very complete and provides a systematic method of collecting data at every site, there are several problems in its use in bridge inspections. First, there generally is not enough time to collect such detailed data, nor are most inspectors or even hydraulic engineers adequately trained to identify all of the factors. In addition, the level of data may not be necessary for the task at hand. Finally, after the data are collected, there is no systematic method for synthesizing the data for use in determining stream stability and decisionmaking.

Johnson et al. developed a rapid channel stability assessment method based on geomorphic and hydraulic indicators for use at bridges. (1) This method has been included in the most recent revision of HEC-20. (3) It is used in HEC-20 as a method to provide a semiquantitative level 1 analysis and to determine whether it is necessary to conduct a more detailed level 2 analysis. Thirteen qualitative and quantitative stability indicators are rated, weighted, and summed to produce a stability rating for gravel bed channels. The rapid stability method provides information that can aid in decisionmaking with respect to design, repair, rehabilitation, or replacement of a bridge or culvert. Given the Federal and State requirements of inspecting bridges for local, contraction, and regional scour, it is important to have a method in place that bridge engineers and inspectors can use to make initial judgments on regional channel instability that might be detrimental to a bridge.

The rapid assessment method developed by Johnson et al. (1) was based largely on previous assessment methods. (5, 6,7)

Advantages of the method include:

• This method weights each criterion based on its impact on stream channel stability, giving lower weight to indicators, such as debris jam potential, and greater weight to indicators, such as mass wasting.
• The rapid assessment method does not have a single variable that can dominate the rating of channel stability.
• Evaluation of each indicator is categorized as excellent, good, fair, and poor with three values in each range.
• This method provides several quantitative indicators, such as bed shear stress ratio, while incorporating fewer ambiguous criteria, such as brightness and clinging aquatic vegetation, or criteria that are difficult to assess.
• The method includes bridge and culvert variables.

The assessment method was tested for selected streams in the Piedmont of Maryland and the Appalachian Plateau area of northern Pennsylvania. Since the assessment method was developed, a number of limitations have been identified, particularly when used outside of the area for which it was calibrated and tested.

One way to incorporate a large number of these complexities is to differentiate streams according to a chosen classification scheme. Montgomery and Buffington developed a stream classification scheme that is a function of processes that occur in various types of streams. (8, 9) The Montgomery-Buffington stream classification scheme is based primarily on stream channel function rather than form. They categorize streams as braided, dune-ripple, pool-riffle, plane-bed, step-pool, cascade, bedrock, and colluvial. The indicators of stream type include typical bed material, bedform pattern, reach type (transport or type), dominant roughness elements, dominant sediment sources, sediment storage elements, typical slope, typical confinement, and pool spacing. They used this classification scheme to predict the response of a channel to changes in hydrology and sediment transport.

The U.S. Army Corps of Engineers (USACE) developed a classification scheme that is based essentially on the location and function of a stream within a watershed. (10) It is the only classification scheme that also includes altered streams. This method categorizes streams as mountain torrents, alluvial fans, braided rivers, arroyos, meandering alluvial rivers, modified, regulated, deltas, underfit streams, and cohesive streams. There are no quantitative thresholds for these streams; rather, qualitative characteristics of each stream type are given.

Many other classification schemes exist, but some require relatively large amounts of data that are time-consuming to collect and that do not necessarily provide information useful to a stability analysis. Combining several classification schemes, such as the USACE and Montgomery-Buffington schemes, may provide a basis for the classification of stable channel characteristics for different stream types.

OBJECTIVE

The objective of this study was to expand and improve the Johnson et al. rapid stability assessment method to include additional factors, such as major physiographic units across the United States, range of bank materials and complexities, critical bank heights, stream type and processes, sand bed streams, and in-channel bars or lack of bars. (1) The assessment method was to be based on a similar format as Johnson et al., with improvements to be generally applicable in all types of streams across the United States. (1) The stream stability assessment method was also to be self-contained so that no additional data collection forms or methods were necessary. However, the use of forms that provide a systematic method for observations is desirable. Thus, the data collection was to be based on the reconnaissance method developed by Thorne. (2)

However, given that Thorne's method is very detailed and requires numerous data beyond that needed for bridge inspections and assessing stability, another goal of this study was to tailor Thorne's reconnaissance method for bridge inspection and stability assessment needs. The result of the project is a method to help bridge inspectors assess the stability of stream channels quickly at bridges that satisfy the following criteria:

• The method is based on the idea that only the channel stability in the short term is needed since inspectors check each waterway every 2 years.
• The method is based only on stability in the immediate vicinity of bridge (admittedly, this could overlook changes that can occur rapidly, such as knickpoint migration).
• The method must be quick and sufficiently accurate without time-consuming measurements, surveys, or calculations.

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