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Live-bed scour is characterized by the simultaneous occurrence of local scour at a bridge's substructure combined with the transport of bed material upstream by the flow. This situation arises when the flow has enough energy and sediment load to erode the riverbed and move sediment while it scours the area around bridge piers or abutments. In this context, the water's velocity is sufficient to pick up and carry sediment particles, leading to a dynamic interaction between the sediment and the flow.

Understanding this process is crucial when assessing the stability of bridges, as live-bed scour can compromise the integrity of bridge foundations over time. It highlights the need for careful analysis in areas where sediment movement is expected to impact structural elements.

The other options do not accurately describe live-bed scour. The absence of significant water flow indicates less potential for sediment movement, while additional flow from tributaries could affect overall flow characteristics but does not define live-bed scour itself. Similarly, deep sediment layers above the streambed would not necessarily signify active sediment transport or scour processes related to a live bed scenario.