Impacts of River Basin Modification and Climate Change on Bridge Safety
Air Quality, Environmental Process
Research Idea Scope
This research idea was developed as part of NCHRP Project 24-27(03) “Evaluation of Bridge-Scour Research: Geomorphic Processes and Predictions,” and addresses three Focus Areas (Climate Change, Land use, Indirect Effects/Cumulative Impacts). Engineering analyses of risks associated with channel changes at bridge crossings, although imperfect, have advanced to the stage that reliable predictions can be made for years or decades in advance. Predictions are based on established relationships linking channel changes (degradation, aggradation, lateral migration, widening, and planform change) to fluvial processes (flow hydraulics, sediment transport, bank erosion), bed and bank properties (bed particle size distribution, bank materials strength, vegetation) and morphology (channel cross-section geometry, slope and planform), coupled with knowledge of current and historical behavior of the river around the bridge crossing. Predictions made using such process-form relationships and local histories are reliable provided that the river’s flow and sediment regimes, and especially the magnitudes and frequencies of flood events, are not changing.
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. If watershed climate changes, this would directly affect precipitation volumes and distributions leading to further direct and indirect impacts on channel stability via changes in runoff, natural vegetation, land-use and sediment yields. While the natural and anthropogenic causes of global warming are disputed, pronounced trends of sea level rise and unprecedented rainfall intensities recorded in recent storms suggest that climate may be changing globally and regionally. 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. The effects of river basin modification coupled 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. 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.
Urgency and Payoff
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 need for bridge replacement or remedial work to counter the effects of basin modification or climate change can be evaluated and incorporated into bridge design and planning. In some cases this information may be used to influence a proposed activity through, for example, an environmental impact statement. Since the changes considered here are basin-wide, more than a single bridge is at stake. Economies of scale result if replacement programs or countermeasures are planned and prioritized for the affected basin rather than on an individual bridge basis.
Dr. Lyle W. Zevenbergen, Ayres Associates Inc