Changing Riverine Environment

Impacts of River Restoration on Bridges

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Abstract

Many urban streams are seriously impaired due to increased flow from urban runoff and wastewater effluent. Bed degradation, bank erosion, and increased sediment often threaten instream transportation structures, such as bridge and culverts. Erosion mitigation is critical to ensure the safety of transportation structures. However, the traditional design flows based on historical gage records for achieving stream restoration goals may not be appropriate to sustain instream transportation structures. For instance, erosion control structures will result in excessive sedimentation in channels that can significantly reduce the conveyance of bridges and culverts. Overtopping flows above those structures will endanger public safety and cause failure of those structures. Design flows for stream restoration need to be determined not only based on gage records but also existing instream structures, hydrology, and hydraulic characteristics of the streams.

This paper presents a case study of the Congress Street Bridge on the Santa Cruz River, Tucson, AZ. The Congress Street Bridge was built in 1982 as the only bridge to provide traffic crossing over the Santa Cruz River in the Tucson Downtown. The bridge was built to accommodate the 100-year flood in the Santa Cruz River, which is 45,000 cfs. The design discharge is 60,000 cfs above the 100-year flood discharge. However, after 1992 flood event in the Santa Cruz River, serious erosion occurred in the lower Santa Cruz River that undermines banks and causes major incision in the main channel. To restore the erosion, a series of grade control structures and bank protections were implemented in the lower reach. These structures stopped erosion and results in sedimentation as well as vegetation growth, which also occurred near the Congress Street Bridge. The surveyed cross sectional changes at the Congress Bridge section since 1982 showed sediment deposition and vegetation growth has significantly blocked the bridge openings. The current capacity is only 27,000 cfs less than 50% of the design discharge. The reduction of flood capacity has detrimental impacts on the properties and business in downtown Tucson. Therefore, appropriate design flows need to be developed for stream restoration to avoid the negative impacts on bridge capacity.

How to Cite:

Duan, J., (2014) “Impacts of River Restoration on Bridges”, National Hydraulic Engineering Conference 2014 1(2014).

Rights: Copyright © 2014, Jennifer Duan