Bridges on Shifting Sands

Bridges are ‘masterworks’ of civil engineering, requiring structural, geotechnical, hydraulic and traffic engineering to come together. Due to their importance, and the challenge of drawing together multiple disciplines, design procedures are extensive, and often deliberately conservative. However, estimating scour remains challenging, with channel adjustment, contraction scour, response to sediment delivery, and local scour due to piers and abutments all occurring with different temporal and spatial scales.

Potential challenges in a dynamic setting are illustrated by a site in Balochistan. This stream is located adjacent to the coast and therefore can be subject to both runoff and tidal processes.

Twenty years ago, the channel was shallow and relatively narrow. Two fords provided access across. Subsequently, there have been four iterations of the stream crossing, as the channel evolved, in response to changing flow conditions and sediment supply.

Taking a step up in scale, one of the triggers for change appears to be construction of a flood levee for a secondary channel. This catchment modification was subsequently followed by a sequence of flood events, and progressive evolution of the channel mouth.

The present situation reveals a classic infrastructure issue. A new bridge has been built... but it was apparently designed from a survey when the channel was smaller, leaving a rather sad looking appendage to a causeway!

New Bridge not connected to the bank...