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4.1 4. RECOMMENDATIONS AND RESEARCH NEEDS Under this task, the research team developed draft recommendations for possible adoption of specific research results by AASHTO. This task required that the breadth of application and limitations of each recommended result be clearly documented. The research team proposed, concise scopes of work for research needed to fill gaps in stream stability analysis practice or where evaluated research results are not ready for adoption by AASHTO and use by the engineering community in general. Draft recommendations were submitted to NCHRP for review prior to beginning Task 6. Working with the project Panel, the first two topics were considered "critical" priority, the next three topics were considered "high" priority, and the remaining three topics were considered "medium" priority. The research needs topics are: ï· Critical Priority ï Impacts of River Basin Modification and Climate Change on Bridge Safety ï Prediction of Headcut Migration and Scour at Bridges ï· High Priority ï Bridge Crossings on Active Alluvial Fans ï Coupling Advanced Numerical Modeling with Sediment Transport and Bank Mechanics in Bridge Reaches â Aggradation, Degradation, Contraction Scour and Channel Widening ï Impacts of Vegetation, Restoration, Rehabilitation and Stabilization on Channel Stability in Bridge Reaches ï· Medium Priority ï Permitting and Associated Bridge Design Requirements ï Bend and Confluence Scour Near Bridges ï Advanced Mapping and Monitoring Tools for Bridges The following briefly describes the research needs that were recommended to the panel. Detailed research statements that include the research objective, tasks, any special notes, and the estimated cost and duration for each are provided in Appendix D. 4.1 Impacts of River Basin Modification and Climate Change on Bridge Safety As population densities increase and use of natural resources changes or intensifies in many basins, the impacts of agriculture, forestry, quarrying/mining, gravel extraction, dam construction and removal, river training, removal of riparian vegetation, construction and urbanization are likely to have increasingly adverse effects on bridge safety throughout affected watersheds. The effects of river basin modification and climate change relevant to bridges include channel degradation or aggradation, widening, regime change from meandering to braiding, increased rates of channel shifting, proclivity for bar formation and increased supply of debris. If watershed climate changes, this directly affects precipitation volumes and distributions leading to further direct and indirect impacts on channel stability via changes in runoff, natural
4.2 vegetation, land-use and sediment yields. Although uncertainty clouds the issue of climate change, the implications for basin-scale channel instability, with adverse impacts to bridge safety regionally and nationally, are so serious that research is now critical. Bridge engineers need tools to assess the vulnerability of bridges to potential changes in flow regime and catchment sediment supply associated with catchment modification or climate change. The aim of the proposed research is distillation of available literature and guidance on how to assess river basin sensitivity to modification and climate change, in the context of known hydrological and geomorphic processes and responses. Only those process-response mechanisms likely to adversely affect bridges would be considered. The study is primarily intended to be qualitative, but with as much quantification as the generality of the topic allows. Outcomes should include recommended strategies for identifying and responding to bridge problems likely to be induced by basin modification or climate change based on risk assessment leading to prioritized programs for basin-wide programs of bridge replacement or countermeasures to keep risks to acceptable levels. 4.2 Prediction of Headcut Migration and Scour at Bridges Headcuts, also known as nickpoints, are erosional features where an abrupt drop occurs in the stream bed elevation. Headcuts often result from base level lowering that generates one or more episodes of stream bed incision or degradation that migrates upstream through the drainage network. The drop created by a headcut can be vertical, near vertical, or steep (knickzone) in homogeneous boundary materials and overhanging when weaker layers are overlain by a more erosion resistant layer. Headcuts increase the chance of bridge failure due to scour, degradation, and channel widening and have contributed to past bridge failures such as the I-5 failure over Arroyo Pasajero in California in 1995. Thus, four interrelated stream stability and scour processes related to headcuts determine the risk to bridges along the affected stream: (1) plunge pool depth (2) overall amount of long-term bed degradation, (3) triggering of channel widening and (4) rate of upstream headcut migration. Therefore, the objective of this research is to develop practically applicable predictive equations for each of these processes. The research will include a review of the literature, laboratory studies, and other information related to each of the headcut processes listed above plus evaluation of hydraulic design, scour performance, and morphological relationships for engineered grade control and drop structures that can be used to stabilize aggressive headcuts. Data should be obtained for a variety of field conditions for development and testing of generalized, predictive relationships. These data should include: current and historical channel hydraulics, bed material properties and morphologies, and headcut geometries, scour, and rates of migration. It is anticipated that additional investigation may be required through controlled laboratory experiments coupled with numerical modeling using Computation Fluid Dynamics (CFD). 4.3 Bridge Crossings on Active Alluvial Fans Alluvial fans are fan-shaped landforms created by the distribution of significant volumes of sediment by confined and unconfined flow moving from higher to lower elevations. Alluvial fans are common throughout the western continental United States and Alaska. They are found predominantly in or along mountainous regions where flash floods, heavy precipitation, geology, and active tectonics play an important role in their development. Problems associated with active alluvial fans include flooding (sheet flow and uncertain flow paths), localized aggradation and degradation, channel shifts (avulsions), landslides and debris flows, and other hazards that have long-ranging consequences for bridge crossings. Because alluvial fans are constructed by the successive episodic and unpredictable shifting of stream flows or the successive passage of debris (colluvial) flows down different routes, alluvial fans are inherently unstable environments for bridges. Given the rapid growth of urban
4.3 development onto alluvial fans in recent years, the design of bridge crossings and roadways must consider the inherent long-term instability of such sites. Thus, the purpose of the proposed project is the development of a manual that outlines the general character of an alluvial fan, discusses active alluvial fan processes in detail, and provides guidance on incorporating alluvial fan processes and impacts in the bridge design. A brief search of the Google Scholar reveals that a wealth of relevant data and information (more than 15,000 references) has been published in the last 20 years with regard to alluvial fans and fan processes. This project will consist of an analysis and distillation of the available data and information and the preparation of a manual similar to HEC-20, but specifically tailored to bridge crossings on active alluvial fans. Given the wealth of data and information that is available on this subject, it is anticipated that no independent numerical, experimental, or field work will be needed. 4.4 Coupling Advanced Hydraulic Modeling with Sediment Transport and Bank Mechanics in Bridge Reaches â Aggradation, Degradation, Contraction Scour and Channel Widening Reach-scale channel widening is a common response to stream bed degradation or aggradation while lateral channel migration is a progressive change in the position of the stream that occurs in both vertically stable and unstable channels. Local, and in some cases extreme, channel widening can occur within the bridge opening due to contraction scour. Both channel widening and lateral channel migration can cause bridge problems such as poor flow alignment, abutment outflanking or destabilization, and scour at piers not designed to be in the main channel. Although several numerical models are available for predicting channel degradation or aggradation, only a few models have been developed for predicting channel widening and lateral channel migration. Contraction scour is primarily caused by flow acceleration and increased shear stresses and sediment transport capacity in the contracted opening at bridges. The empirical equations for calculating contraction scour are mainly based on sediment transport theory and initiation of motion, so a significant uncertainty is associated with the prediction of contraction scour depth. A numerical model that simulates changes in both bed elevation and channel width would provide better predictions of contraction scour. Bank erosion is the mechanism of channel widening and lateral channel migration, and is a complicated process affected by numerous factors such as hydraulic conditions (erosion), bank height/angle, bank materials, and vegetation (geotechnical stability). Coupling of a two- dimensional flow and sediment model with bank failure mechanisms has not received concerted attention. In order to generate a realistic distribution of boundary shear stress, better represent secondary flow within meandering channels, and properly model complex bank mechanics, a two-dimensional hydraulic model coupled with an advanced bank stability analysis is actually needed. The proposed project will improve an existing two-dimensional flow and sediment transport model by adding a module that realistically simulates potential bank failure mechanisms. It is not intended to build this model from scratch, but to extend a widely-used and well-validated model.
4.4 4.5 Impacts of Vegetation Restoration, Rehabilitation and Stabilization on Channel Stability in Bridge Reaches Research in river mechanics and fluvial geomorphology has recently established that vegetation exerts much stronger influences on channel forms and processes than was previously thought. For example; rates of bank erosion and lateral channel shifting are significantly lower along rivers flowing through mature, riparian corridors than where native vegetation has been removed from the banks, patterns of vegetation on floodplains have been shown to materially alter channel planform patterns and their evolution, and the presence of large woody debris has been found to limit degradation in incised channels. Further evidence of the profound impacts of vegetation is significant changes in channel form observed where invasive species have colonized aquatic and riparian areas. These findings come at a time when vegetation, both living and dead, is being increasingly reintroduced to channels in river restoration, rehabilitation and stabilization projects. Despite this new knowledge and growing trends for re-introduction of vegetation to managed rivers, relatively little is known concerning how vegetation of different types located in different zones of the river physically interact with bank stability and the fluvial processes of sediment scour, transport and deposition that are responsible for channel migration and change in the vicinity of bridges. This makes it difficult to assess the risks associated with vegetation succession and management (clearance, cutting or re-introduction as part of river restoration) in the channel upstream of and around bridge crossings. To address this gap in knowledge, research is required to establish causal links between vegetation and fluvial processes at the site and reach scales. The aim would be to allow bridge engineers to assess the benefits and risks associated with different types, densities and spatial distributions of vegetation upstream and around bridge crossings based on scientific and, wherever possible, quantitative relationships. The objectives of the research would be to, among other things, develop practically applicable tools to enhance existing risk assessment methods for channel scour, deposition and lateral shifting at bridges so that they can account explicitly for both the beneficial and adverse impacts of the presence, removal, or re-introduction of vegetation. Ultimately, guidelines would be formulated for assessing vegetation related risks and benefits with respect to designing bridges and implementing countermeasures. 4.6 Permitting and Associated Bridge Design Requirements State Departments of Transportation (DOT) have numerous hydraulic design standards for bridges over waterways. DOTs also have an obligation to meet regulatory requirements and obtain relevant permits and resource agency approvals for construction of bridges and countermeasures. These requirements address potential impacts on flood insurance, flood hazards, navigation, water pollution, environmental protection, and protection of fish and wildlife. Federal, State, and local agency involvement can be extensive. The permitting and approval process is often cited as a major impediment for efficient delivery of new bridges, bridge replacements, and countermeasures. Bridge hydraulic design focuses on hydraulic efficiency. However, environmental agencies have additional concerns that include aquatic, riparian, and floodplain habitat, fish passage, and wildlife passage. This project will focus on meeting the environmental concerns related to bridge design with the goal of developing model agreements that can be tailored by individual DOTs in coordination with State environmental agencies and USFWS. These agreements would establish additional performance criteria that, if met, would significantly streamline the agency approval process by directly addressing environmental concerns. The criteria may include minimum setback distances between abutments and channel banks, requirements for
4.5 clear spanning certain channels, limits on the location and number of piers in channels, constraints on exposed riprap aprons, minimum deck clearance for wildlife passage, and limits on increased velocities and shear stresses for frequent (2- to 10-year recurrence interval) flood conditions. In addition to the benefits of streamlined permitting and reduced environmental impacts, there are other, long-term benefits that DOTs can expect from this research. These include bridges with (1) fewer debris problems, (2) reduced scour, (3) fewer stream instability problems, (4) reduced long-term maintenance, (5) extended service life, and (6) fewer countermeasures. 4.7 Bend and Confluence Scour Near Bridges Bend and confluence scour are related phenomena, the first characteristic of meandering streams and the second characteristic of braided streams. Both are produced by secondary flow cells generated by streamline curvature. In meandering streams, the outside of bends tend to scour during floods and the inside fills. As a result, bed elevations on the outside of bends appear deceptively high during low flow. Bridges are commonly placed on the outside of bends, as this often allows for the anchoring of one end against a valley wall. Correct placement of pier footings and abutments is contingent upon the recognition of the amount of bend scour that might be expected during a flood. While braided streams are less common than meandering streams in the continental USA, they can be found in the western part of the country and abound in Alaska. Confluence scour occurs where two anabranches of a braided stream flow together. Confluence scour can lead to flow depths as much as five times the ambient values in anabranches. Experience in New Zealand suggests that bridges on braided streams are most likely to fail when a confluence forms at a pier. Confluence scour of essentially the same type also occurs when a large tributary enters the main stem of a meandering or wandering river with a slowly changing planform. While the effect of bend and confluence scour on bridges is well recognized in the technical literature, quantitative methods for predicting scour depths are provided in neither of the standard manuals HEC-18 and HEC-20 for bridge design. A concise design manual providing quantitative methods for evaluating bend and confluence scour at bridge crossings is needed. 4.8 Advanced Mapping and Monitoring Tools for Bridges Bridge inspection, an important step for ensuring the safety of a bridge, is conducted to identify changing conditions of a bridge structure and changing channel conditions in the vicinity of the bridge. Changing channel conditions, such as bank erosion, channel migration, and neck cutoffs, can greatly increase the threat to the foundations of piers and abutments. Existing channel conditions are compared with previous observations and data to identify potential threats to the bridge elements. Bridge inspections are performed biennially or soon after a large flood event. However, the current bridge monitoring procedures are time-consuming and labor-intensive, and the collected monitoring data is usually incomplete, qualitative, and subjective. Bridge inspections often neglect channel stability, which are a major cause of bridge failures. Even when bridge inspectors are aware of bank retreat as potential threat, visual inspection can easily miss this type of progressive change. More automated bridge monitoring could be highly valuable, especially for bridges with scour critical conditions.
4.6 Advanced mapping and monitoring technologies have recently been a research area due to an increasing demand of consistent and reliable bridge monitoring and reconnaissance data. In a digital mapping study sponsored by Iowa DOT, morphological features such as river bank positions and floodplain edges were identified on the ortho-rectified riverside images through an image processing algorithm, and a surface velocity analysis was conducted by applying Large Scale Particle Image Velocimetry (LSPIV) on image sequences of the river flow. This methodology appears to be a relatively inexpensive and practical tool for routine bridge inspections at high-risk bridges. The goal of the proposed research is to test and advance this technology, and to prepare guidelines of standard procedures for the application of advanced mapping and monitoring tools in bridge inspections. 4.9 Other Recommendations Chapter 3 provides the recommendations to AASHTO for adoption of specific research results related to geomorphic processes and predictions. The primary manual used by the U.S. transportation community on this topic is HEC-20 (Lagasse et. al 2001). An update of HEC-20 could incorporate many of the research results included in Chapter 3 without significantly reorganizing the manual because the additions could be incorporated into existing manual sections. In some cases, new sections would need to be written to cover recommended topics more thoroughly, such as a gravel bed river section. One recommendation for an update of HEC-20 is to follow the three level approach theme throughout the manual. Currently, HEC-20 presents the three level approach as a stream stability analysis procedure. When a topic, such as lateral stability, is presented in a subsequent section of the manual, the methods are not assigned to a specific level. A modest amount of reorganization regarding grouping and discussing the various approaches in relation to the three level approach may improve the utility of HEC-20. Because Level 1 methods are more qualitative and conceptual, they play an important role in identifying stream stability problems at bridge. Therefore, Level 1 methods must have in-depth coverage in HEC-20. Level 2 approaches provide quantitative tools for estimating the severity of stream stability problems and predicting future conditions. Level 2 approaches are also selected to provide reasonable results at a level of effort that is justifiable for the majority of bridges. This level should also be covered in detail in HEC-20. Level 3 approaches are reserved for highly complex problems when it is determined that Level 2 is insufficient to address a problem without excessive uncertainty or risk. Therefore, Level 3 approaches should be discussed in HEC-20, but not in the same detail as Levels 1 and 2. Level 3 methods should be discussed in concept by providing some description, guidance, and references. The strengths and limitations of all methods regardless of level should also be included in HEC-20.
