**1. Introduction**

Weirs and dikes are common hydraulic structures used in river and coastal engineering for the purposes of bank protection, bed stabilization, channel regulation, and scour protection [1–4]. Local scour downstream of weirs is an important subject in the field of hydraulic and coastal engineering. Since these weir-like structures are frequently overtopped during high flood events [5], it is essential to study local scouring under the submerged condition. When water flow approaches a weir, the flow patterns are modified by the presence of the structure and local scour holes often develop around the weir. The scour hole dimensions downstream of weirs are governed by the flow field and turbulence characteristics immediately downstream of the weir [3]. For practical purposes, the equilibrium scour depth downstream of the weir-like structures is the most important parameter to determine the depth of foundation. Therefore, many studies [6–21] have been carried out to investigate the effects of many variables, providing the publication of a series of empirical scour equations. However, limited research on the temporal development of scour hole downstream of submerged weirs [22,23] can be found in published literature. Moreover, the peak flood flow in the field generally lasts only a few hours or a number of days, which is insufficient to generate the equilibrium scour depth. Therefore, the final scour depth may be overly conservative if a design is based on the clear-water (i.e., no sediment inflow to the scour hole) equilibrium scour equation [24]. The time scale of local scour at submerged weirs under clear-water scour conditions provides the knowledge of short-term scour dimensions, and could be used for the estimation of scour dimensions during short floods [25]. Besides scour dimensions, geometrical similarity of the scour profiles is also an important issue for the investigation of scour at submerged weirs. Previous studies have shown that the geometrical similarity of the scour holes has important implications on the prediction of the eroded volume downstream of the weirs, which is useful for the design of scour countermeasures, such as the volume and extent of the riprap stones needed [11]. This paper presents an experimental study on time development of the scour hole downstream of a submerged weir. The flow intensity and overtopping ratio effects on the scour process are discussed. The temporal evolution of the scour dimension, scour profile and the relationship between the scour dimension (geometrical similarity) are analyzed based on the experimental results. All the experiments are confined to clear-water scour conditions.
