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S U M M A R Y Overview This research accomplished its basic objectives of developing guidelines for predicting the size and geometry of debris accumulations at bridge piers and methods for quantifying scour at bridge piers resulting from debris accumulations. The project produced results on two related problems: (1) predicting the accumulation characteristics of debris from potentially widely varying source areas, in rivers with different geomorphic characteristics, and on bridges with a variety of substructure geometries and (2) developing improved methods for quantify- ing the depth and extent of scour at bridge piers considering both the accumulation variables and the range of hydraulic factors involved. Waterborne debris (or drift), composed primarily of tree trunks and limbs, often accumu- lates on bridges during flood events. Debris accumulations can obstruct, constrict, or redirect flow through bridge openings resulting in flooding, damaging loads, or excessive scour at bridge foundations. The size and shape of debris accumulations vary widely, ranging from a small clus- ter of debris on a bridge pier to a near complete blockage of a bridge waterway opening. Debris accumulation geometry is dependent on the characteristics and supply of debris transported to bridges, on flow conditions, and on bridge and channel geometry. The effects of debris accu- mulation can vary from minor flow constrictions to severe flow contraction resulting in sig- nificant bridge foundation scour. Qualitatively, the impacts of debris have been well documented; however, a pressing need remains for state DOTs and other bridge owners to have improved prediction methods for the geometry (size and shape) of typical debris accumulations, the conditions under which debris can be expected to develop, and the resulting depth and extent of scour at bridge piers. Currently, only limited guidance is available on which to base critical public safety decisions during flooding on debris-prone rivers. There is a need for accurate methods of quantifying the effects of debris on scour at bridge pier foundations for use by departments of trans- portation (DOTs) and other agencies in the design, operation, and maintenance of highway bridges. Research Approach The research approach involved the following steps: 1. Completion of a literature review and evaluation of current practice with a survey of state DOTs and other bridge owners. 2. A field pilot study to evaluate instrumentation for obtaining data at debris-prone bridges and costs associated with debris-related field studies. Effects of Debris on Bridge Pier Scour 1
23. Development of a photographic database (archive) as an alternative to field work for assessing typical debris shapes and geometry relationships, nationally. 4. Development of detailed guidelines and flowcharts for estimating the potential for debris production and delivery to a bridge site, and a case study to illustrate the application of the guidelines. 5. Extensive laboratory testing of the most common debris shapes and geometries to deter- mine the relationships between debris shape and dimensions and the depth and extent of bridge pier scour. 6. Development of methods for predicting the depth, shape, and extent of scour at bridge piers resulting from debris accumulation. Application of the methodology is illustrated with example problems. 7. Discussion of approaches and limitations for incorporating debris in one- and two- dimensional hydraulic computer models. 8. Discussion of inspection, monitoring, and maintenance issues at debris-prone bridges. 9. Suggestions for implementation activities to enhance the state of practice for estimating scour at bridge piers under debris loading. Appraisal of Research Results As an extension of the original work by Diehl (1997) for the Federal Highway Administration (FHWA), guidelines and flowcharts were developed for estimating (1) the potential for debris production and delivery from the contributing watershed of a selected bridge and (2) the potential for accumulation on individual bridge elements. The application of the guidelines is illustrated by a case study of a debris-prone bridge on the South Platte River in Colorado (summarized in Chapter 3 and presented in detail in Appendix D). The case study introduces and illustrates the use of field data sheets for evaluating the potential for debris production and delivery from a given watershed. As a basis for laboratory testing, the photographic archive introduced in Chapter 2 (see also Appendix A), the field pilot study of debris sites in Kansas (see Appendix C), and the South Platte River case study (see Appendix D) were examined to develop a limited number of debris shapes that would represent the maximum number of configurations found in the field. Simplified, yet realistic, shapes that could be constructed and replicated with a reasonable range of geometric variables were needed for laboratory testing. Rectangular and triangular shapes with varying planform and profile dimensions were selected to represent prototype debris accumulations. To account for additional variables thought to be relevant to debris clus- ters in the field, a method to simulate both the porosity and roughness of the clusters was devel- oped. However, porosity and roughness were found to be, at most, second order factors in estimating scour at bridge piers under debris loading. The laboratory testing program included the use of a large indoor flume at Colorado State University and model bridge pier shapes, development of state-of-the-art instrumentation for data acquisition, and a wide range of materials to fabricate the debris clusters. Baseline tests were conducted and results were compared with several pier scour prediction equations. A series of tests under clear-water conditions with the various debris shapes were completed. The results are illustrated with tabular data, photographs, and post-test contour plots. An appraisal of testing results supported the development of an improved algorithm for predicting the scour anticipated at bridge piers from debris accumulations with rectangular and triangular planforms and varying length, width, and depth geometries. In Chapter 3, application of the methodology is presented to integrate the debris accumulation guide- lines with the equation for predicting debris scour at bridge piers using the South Platte River case study as an example.
Finally, guidelines for inspection, monitoring, and maintenance for debris-prone bridges are considered and an implementation plan for the results of this research is suggested. Conclusions from this research and recommendations for future research are presented in Chapter 4. As a result of this research, bridge owners now have documentation, guidelines, and ana- lytical procedures to quantify the effects of debris-induced scour on bridge piers: ⢠A fully documented database on debris and case studies, photographs, and data related to debris generation, movement, accumulation, and scour at bridges that can be used to inform and train design and maintenance personnel on debris-related hazards ⢠Necessary guidelines for predicting the size, location, and geometry of debris accumulations at bridge piers ⢠Methods for quantifying scour at bridge piers resulting from debris accumulations ⢠Guidance for incorporating debris effects in one- and two-dimensional hydraulic modeling ⢠Worked example problems and a case study illustrating the application of the guidelines and analytical methods The end results of this research are practical, implementable guidelines for bridge owners that enhance their ability to predict debris-related hazards at bridges and design, operate, inspect, and maintain bridges considering those hazards. 3
