Which of the following methods is used for assessing scour vulnerability?

The correct answer, subsurface investigation, is essential for assessing scour vulnerability because it involves examining the soil and sediment conditions beneath and around the bridge foundation. Scour refers to the erosion of material from around bridge piers and abutments due to the flow of water, which can undermine the structural integrity of the bridge.

Subsurface investigations utilize techniques such as boring and sampling to determine the type of soil, its properties, and its stability under different water flows. This information is crucial in understanding how susceptible the foundation is to scour and designing appropriate protection measures if necessary.

In contrast, visual assessments focus on observable conditions but may not provide the detailed information needed about subsurface soil composition and behavior relating to scour. Load testing assesses how a structure performs under specific weight conditions but does not directly evaluate the effects of water flow on the soil around foundations. Water quality testing, while important for understanding environmental conditions, does not provide insights into scour risk. Hence, subsurface investigation is the most directly relevant method for assessing scour vulnerability.