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SEDIMENT AGGRADATION AT BRIDGE CROSSINGS AND AN ADAPTIVE APPROACH TO STREAM CHANNEL MAINTENANCE AND BRIDGE DESIGN
ABSTRACT
The overall objective of this study is to identify a more effective approach to maintaining stream channels at bridge crossings in the Northern Tier region of northeastern Pennsylvania. Bridge crossings in this region are experiencing extensive sediment aggradation in bridge waterways, which becomes a challenge for long-term maintenance. Current dredging mitigation practices play a role in prolonging the aggradation problems at many of the bridge crossings. The presented research identifies a more effective approach to maintaining stream channels at bridge crossings by (1) testing mitigation methods with physical models, (2) determining appropriate mathematical sediment transport analyses to aid decision-making during the design process, and (3) applying an adaptive management framework as adaptive management experiments. The mathematical sediment transport analyses; shear stress analysis, analytical stable channel design, and sediment transport modeling, all yield similar results with respect to ranking channel slope and width modifications or low chord bridge configurations according to their effects on sediment transport characteristics. Shear stress analysis is recommended as the simplest tool in terms of time, data, and expertise for including a preliminary assessment of sediment transport characteristics in the decision-making process for bridge and channel maintenance design. The physical modeling of mitigation measures is approached as four cycles of adaptive management experiments, and demonstrates that this approach results in fast and efficient learning about channel response to mitigation methods for the given conditions without posing an unacceptable level of risk that would be associated with field-scale adaptive management experiments. If physical modeling was stopped after the first cycle of adaptive management experiments that tested common instream structures (rock vanes and submerged vanes), little useful information would have been obtained other than the fact that the instream structures used in the experiment were incapable of moving sediment through the bridge opening for the flows tested. Assessment of what was learned in the adaptive management experiment cycles indicates that further testing of enhanced instream sediment traps and more focused research on bendway weir design has the potential to lead to a successful option for the management of sediment aggradation at bridge crossings in the Northern Tier region. While the general approaches that are applied in this research are focused on the sediment aggradation problems specific to the Northern Tier region, the methods can be applied to other regions and to other sediment concerns at bridges crossings.
Table of Contents
List of Tables……………………………………………………………………………………………………………….. vii
List of Figures……………………………………………………………………………………………………………….. ix
Acknowledgements………………………………………………………………………………………………………. xvi
Chapter 1. Introduction…………………………………………………………………………………………..1 1.1. Purpose and Scope………………………………………………………………………………..2
1.2. Physical Modeling Approach………………………………………………………………….4
1.3. Mathematical Modeling Approach………………………………………………………….4
1.4. Adaptive Management Approach ……………………………………………………………5 1.5. References……………………………………………………………………………………………6
Chapter 2. Site Description: Northern Tier Region…………………………………………………….7
2.1. Bridge Crossing and Channel Characteristics……………………………………………7
2.1.1. Mitchell Creek ………………………………………………………………………..8
2.1.2. Morgan Creek ……………………………………………………………………….11
2.1.3. Hills Creek……………………………………………………………………………12
2.2. Sediment Data…………………………………………………………………………………….14
2.3. Hydrology………………………………………………………………………………………….14
2.4. Hydraulics………………………………………………………………………………………….16 2.5. References………………………………………………………………………………………….17
Chapter 3. Suitability of Instream Structures in Bedload Streams at
Bridge Crossings…………………………………………………………………………………………….18
3.1. Introduction………………………………………………………………………………………..18
3.2. Objective……………………………………………………………………………………………22
3.3. Physical Modeling ………………………………………………………………………………23
3.4. Experimental Setup……………………………………………………………………………..25
3.5. Model Runs………………………………………………………………………………………..28
3.6. Experimental Results and Discussion…………………………………………………….32
3.7. Conclusions………………………………………………………………………………………..39 3.8. References………………………………………………………………………………………….40
Chapter 4. Characterization of Sediment Transport at Bridge
Sites in the Northern Tier, PA…………………………………………………………………………..43
4.1. Introduction………………………………………………………………………………………..43
4.2. Objectives ………………………………………………………………………………………….45
4.3. Methods……………………………………………………………………………………………..45
4.4. Development of a General Model………………………………………………………….49
4.5. Application of Methods ……………………………………………………………………….53
4.6. Results……………………………………………………………………………………………….55
4.7. Conclusions………………………………………………………………………………………..62 4.8. References………………………………………………………………………………………….64 Chapter 5. Approaching Physical Modeling of Stream Channel
Mitigation Methods at Bridge Crossings as an Adaptive Experiment…………………….67
5.1. Introduction………………………………………………………………………………………..67
5.2. Adaptive Management Framework………………………………………………………..69
5.3. Application of Adaptive Management……………………………………………………72
5.3.1. Identification of the Problem…………………………………………………..74
5.3.2. Sources of Uncertainty……………………………………………………………76
5.3.3. Physical Model Setup …………………………………………………………….76
5.4. Adaptive Management Experiment: Cycle 1…………………………………………..77
5.4.1. What was learned in Cycle 1?………………………………………………….79
5.5. Adaptive Management Experiment: Cycle 2…………………………………………..82
5.5.1. What was learned in Cycle 2?………………………………………………….83
5.6. Adaptive Management Experiment: Cycle 3…………………………………………..84
5.6.1. What was learned in Cycle 3?………………………………………………….85
5.7 Adaptive Management Experiment: Cycle 4……………………………………………86
5.7.1. What was learned in Cycle 4?………………………………………………….87
5.8. Discussion and Conclusion…………………………………………………………………..89 5.9. References………………………………………………………………………………………….90
Chapter 6. Conclusions…………………………………………………………………………………………94 6.1. Implications from Physical Modeling Approach……………………………………..94
6.2. Implications from Mathematical Modeling Approach ……………………………..95
6.3. Implications from Adaptive Management Approach ……………………………….96
6.4. Limitations and Additional Applications of Research………………………………97 6.5 Future Research Directions……………………………………………………………………98
Chapter 1.
Introduction
Sediment deposition at bridge crossings can cause partial or complete blockage of the waterway opening of the bridge. When the waterway area through the bridge is reduced below the minimum value needed to convey the design flood, sediment deposition becomes a hazard (Brown et al. 1981). This reduction in waterway opening can lead to more frequent overtopping of the structure and an increased potential for contraction scour, which could cause failure of the bridge structure (Johnson et al. 2001). Deposition of bed sediment in a reach of a stream channel can be the result of instability in the stream channel system. If there is an increase in sediment supply to the reach that is greater than the capacity of the stream flow to transport sediment, deposition will occur in that stream reach. If there is a decrease in the capacity of the flow to transport sediment, deposition also will occur. When the changes to sediment supply or flow transport capacity are systematic, the deposition of sediment in the reach will be long term (Johnson et al. 2001). The persistent mean changes to channel bed elevation are referred to as aggradation (Leopold et al. 1964). However, if the changes to sediment supply and flow transport capacity are brief, the channel will likely re-establish its equilibrium balance (Johnson et al. 2001). The systematic changes to the balance of hydraulic conditions and sediment conditions create the need for effective solutions to minimize the effects of sediment deposition. The presence of a rigid bridge structure and continued maintenance to a stream channel to clear the deposition help create long-term, systematic sediment aggradation conditions at a bridge crossing.
Maintaining stream channel stability at bridge crossings presents challenges. While stream channels are dynamic systems, the bridges that cross them are rigid structures. This intersection can lead to extensive, long-term management problems. For example, abutment and/or pier scour is a well-documented maintenance problem for many bridges. Scour at bridge abutments and piers has received considerable research attention (e.g., see Richardson and Davis 2001, Lagasse et al. 2001a, 2001b, Mueller and Wagner 2005). However, mitigation measures for aggradation at bridge crossings have received less attention. Solutions for dealing with sediment deposition or aggradation are summarized by Lagasse et al. (2001b). The countermeasures include channelization, sediment traps, bridge modification, and continued channel maintenance.
Channelization refers to dredging, changes in slope, or the installation of instream structures.
In order to design and maintain stable channels at bridge crossings, the channel response to changes in the stream caused by the bridge itself or by mitigation procedures must be understood. The existing models that have been developed to predict channel response have limitations in their applicability to physical conditions outside the range for which they were developed. Variability in streams often makes it necessary to focus research on a specific region or stream type. Sediment deposition is a problem particularly in the Northern Tier region of Pennsylvania, where the streams can transport large loads of gravel- to cobble-sized sediment. The research presented in this study is based on the observed characteristics of the Northern Tier region. Three detailed study sites were chosen and are described in Chapter 2 and Appendix A.
When the bridge opening is blocked by sediment, current mitigation procedures in the Northern Tier region involve dredging of the channel, which can be a factor in prolonging the sediment deposition problems at the bridge (Hey 1987, Simon 1989). Sediment deposits that block bridge openings are removed from the channel right-of-way section that includes a 15.24 m reach length upstream of the bridge and a 15.24 m reach length downstream of the bridge. Removal of sediment from this section of the channel can lead to local changes in the channel bed slope; mild bed slopes are observed near the bridges where the remaining stream reach is fairly steep. Channel widening and other modifications, backwater effects, changes in slope, along with changes in the surrounding watershed, can lead to an unstable channel in the vicinity of the bridge resulting in sediment deposition, or aggradation.
1.1. Purpose and Scope
Current bridge design procedures and sediment mitigation measures have played a role in creating or prolonging the sediment deposition problems at bridges in the Northern Tier region of Pennsylvania. The overall objective of this study is to identify a more effective approach to maintaining stream channels at bridge crossings. In order to meet this objective, three specific goals are defined related to the development of mitigation measures at existing bridge crossings and to improved design procedures for bridges and mitigation measures. The three goals involve the testing of mitigation measures for sediment deposition at bridges with physical models, determining appropriate mathematical sediment transport analyses for decision-making during the design process, and applying an adaptive management framework as adaptive laboratory experiments.
The first goal of this research is to test whether the use of instream structures could be an effective approach to maintaining bridge waterways and preventing sediment deposition at existing bridge crossings in the Northern Tier region. If bedload sediment can be transported through the bridge opening with the use of instream structures, these measures would prevent sediment deposition at bridge crossings. The premise is that a successful solution would decrease the portion of the bridge waterway that is blocked by sediment and result in a decrease in the amount of sediment deposited upstream of the bridge crossing. The resulting patterns of sediment deposition and erosion with instream structures installed should not create increased hydraulic pressures on the bridge structure.
The second goal is to determine the extent to which channel modifications near bridge sites contribute to sediment deposition problems and to develop recommendations for appropriate sediment transport analyses to be performed for bridge crossing design or design of mitigation measures at existing bridges. The result would be the identification of the simplest sediment transport analysis tool to be used in the decision-making process regarding design and maintenance of bridge crossings where sediment transport issues are a concern.
The final goal is to use an adaptive management approach for the physical modeling of mitigation measures for sediment deposition at bridges, resulting in fast and efficient learning about channel response. This part of the study focuses on the value of monitoring and the assessment of monitored data in the overall efficiency of the adaptive management approach. The application of an adaptive management framework to the physical modeling of mitigation measures is an attempt to overcome the unacceptable risk to rigid bridge infrastructure of fieldscale adaptive management experiments. The laboratory physical modeling of conditions in the Northern Tier region bridge crossings is used to decrease the level of uncertainty about channel response to mitigation methods and increase the rate of learning about the effectiveness of mitigation measures and the channel response. The knowledge gained through the adaptive experiments using physical modeling then can be applied to a field-scale adaptive management experiment.
1.2. Physical Modeling Approach
Because many of the available mathematical models for hydraulics and sediment transport are simplified or empirically based, physical modeling is widely used in the field of hydraulic engineering to investigate design and operation issues (Ettema 2000). Physical modeling is based on the use of a scaled model for replicating conditions observed in the field. If a model can reproduce events that already have occurred in the field, then the model also should reproduce events that will occur in the field in the future (French 1985). Chapter 3 of this thesis summarizes the development of a scaled model to represent the conditions observed at streams in the Northern Tier region. The effectiveness of instream structures to alleviate the sediment deposition in the bridge waterway to allow the design flood to pass through the bridge opening is tested. The instream structures that are tested in Chapter 3 are commonly used in stream restoration applications. Appendices B, C, and D use the same scaled physical model to explore the use of revised guidelines of these structures and other mitigation measures to alleviate sediment deposition at the bridge crossings in the Northern Tier region.
1.3. Mathematical Modeling Approach
Mathematical models currently are used to assess the hydraulic conditions of bridge design in Pennsylvania (PennDOT 2006). The U.S. Army Corps of Engineers’ River Analysis System model (HEC-RAS) is readily available for public use and currently is identified by PennDOT (2006) as the primary hydraulic model to be used for the design of bridge crossings. Because of the sediment aggradation problems at bridges in the Northern Tier region, sediment transport analyses also should be included in the procedures for bridge design as well as for the design of stream channel mitigation measures near bridge crossings. Chapter 4 explores the use of three sediment transport analysis methods that vary in application complexity. The three sediment transport analyses are shear stress analysis, analytical stable channel design, and onedimensional sediment transport modeling. The effects of common channel modifications and bridge characteristics in the Northern Tier region are investigated with these three methods and recommendations are made regarding the most appropriate method to use in the decision-making process regarding design and maintenance of bridge crossings where sediment transport issues are a concern.
1.4. Adaptive Management Approach
The adaptive management framework was applied to the problem of identifying mitigation measures for sediment deposition near bridge crossings in small streams in the Northern Tier region of northern Pennsylvania. The problem of identifying effective stream channel mitigation methods at bridge crossings presents a unique challenge. The integrity of the bridge structure itself has to be maintained regardless of the maintenance practices used in the stream channel near the bridge. Because of the presence of the rigid bridge infrastructure, the resilience of the stability of the stream channel system is low. Systems with low resilience are vulnerable to dramatic and surprising change (Holling and Meffe 1996). It is not feasible to apply adaptive management practices in the field at bridge crossings because of liability, cost, and time. In an effort to overcome the unacceptable risk of field-scale adaptive management experiments to rigid bridge infrastructure, an adaptive management approach for laboratory physical modeling of conditions in the Northern Tier region at bridge crossings was used to decrease the level of uncertainty about channel response to mitigation methods in this region and increase the rate of learning about the effectiveness of mitigation measures. The application of adaptive management to the physical modeling of mitigation measures for sediment deposition at the bridges in the Northern Tier region is described in Chapter 5. In this way, the more effective approach of active adaptive management can be applied to increase the rate of learning without posing unacceptable risk to the integrity of the bridge structure.
1.5. References
Brown, S.A., McQuivey, R.S., and Keefer, T.N. (1981). “Stream channel degradation and aggradation, analysis of impacts to highway crossings.” FHWA RD-80 159, Federal Highway Administration, Washington, D.C.
Ettema, R., ed. (2000). “Hydraulic modeling concepts and practice.” ASCE Manuals and Reports on Engineering Practice No. 97, American Society of Civil Engineers, Reston, VA.
French, R.H. (1985). Open-channel Hydraulics, McGraw-Hill Book Company, New York.
Hey, R.D. (1987). “River dynamics, flow regime and sediment transport.” Sediment Transport in Gravel-bed Rivers, C.R. Thorne, J.C. Bathurst, and R.D. Hey, eds., John Wiley and Sons, New York.
Holling, C.S., and Meffe, G.K. (1996). “Command and control and the pathology of natural resource management.” Conservation Biology, 10, 328-337.
Johnson, P.A., Hey, R.D., Horst, M.W., and Hess, A.J. (2001). “Aggradation at bridges.” Journal of Hydraulic Engineering, 127(2), 154-157.
Lagasse, P.F., Schall, J.D., and Richardson, E.V. (2001a). “Stream stability at highway structures, 3rd edition.” FHWA NHI 01-002 HEC-20, Federal Highway Administration, Washington, D.C.
Lagasse, P.F., Zevenbergen, L.W., Schall, J.D., and Clopper, P.E. (2001b). “Bridge scour and stream instability countermeasures – Experience, selection and design guidance, 2nd edition.” FHWA NHI 01-003 HEC-23, Federal Highway Administration, Washington D.C.
Leopold, L.B., Wolman, M.G., and Miller, J.P. (1964). Fluvial Processes in Geomorphology, Dover Publications, Inc., New York.
Mueller, D.S., and Wagner, C.R. (2005). “Field observations and evaluation of streambed scour at bridges.” FHWA RD-03-052, Office of Engineering Research and Development, McLean, VA.
Pennsylvania Department of Transportation (PennDOT). (2006). Design Manual Part 2, Highway Design(Publication 13M), Commonwealth of Pennsylvania, Harrisburg, PA.
Richardson, E.V. and Davis, S.R. (2001). “Evaluating scour at bridges.” FHWA NHI 01-001 HEC-18, Office of Bridge Technology, Washington, D.C.
Simon, A. (1989). “A model of channel response in disturbed alluvial channels.” Earth Surface Processes and Landforms, 14, 11-26.
SEDIMENT AGGRADATION AT BRIDGE CROSSINGS AND AN ADAPTIVE APPROACH TO STREAM CHANNEL MAINTENANCE AND BRIDGE DESIGN
