The bridge crosses Douglas Stream at the base of an outwash fan where the stream meets with the Waimakariri River (as part of carrying out the bridge design work). The bridge deck and old timber piers here are at the end of their design lives and need replacement. The bridge is located on SH73, some 15 km south-west of Arthur’s Pass village.
CLIENT BRIEF/PROJECT CHALLENGES
After producing a design philosophy statement, WSP provided design services for the hydraulic and hydrological assessment of the existing bridge, a site overview of the associated river structures and related issues together with a scour analysis and erosion protection design. Design flood levels and velocities were developed for the new bridge using a HEC-RAS model, allowing scour depths to be assessed and mitigation measures proposed. The bridge abutments are of variable condition, with some of the abutment gabions collapsing and the river channel bed degrading upstream. Two rock weirs located downstream are also of variable condition and are key to protecting the bridge structure into the future.
WSP developed a simple HEC-RAS computational hydraulic model to analyse the existing situation, including design (SLS and ULS) flood flow depths and velocities at the railway bridge. Together with site observations and aerial photographs, the design values were then used for scour analysis and erosion protection design, to assist in the design of the proposed bridge and especially the scour protection. Management of risks into the future were addressed, with recommendations for detailed design and considerations for ongoing maintenance.
The Client was provided with detailed design parameters for the new bridge and associated erosion protection works, to provide confidence that after reconstruction of the piers and deck the set of works carried out will allow the bridge to stand up to extreme flood events and be a long-term sustainable solution, with minimal risks to the rail line..