**1. Introduction**

Scour is a main design concern for hydraulic structures such as bridge or wind turbine foundations [1–4], buried pipelines [5–7], weirs and sills [8–12], etc. The scour at instream structures can be classified into three types: general scour, constriction scour and local scour [13–15]. The general scour is normally caused by the evolution of the river bed and is independent of the instream structure. The constriction scour occurs where an instream structure narrows the flow section. The local scour is caused directly by the change of flow pattern induced by the existence of the instream structure.

Bed degradation usually occurs when the bed erosion rate is greater than the upstream sediment replenishment rate. It can induce general scour at the foundations of instream structures and destabilize riverbanks, threatening the security of both the public and private properties [16]. Grade control structures (GCSs) such as submerged weirs, bed sills and check dams are common countermeasures for bed degradation [9–11,17–19]. They can raise the upstream water level and reduce the flow capacity for sediment transport, preventing the upstream river bed from being excessively degraded. Properly designed GCSs play a role as a grade control datum (GCD) for the upstream river reach [16,20]. The GCD is usually the crest elevation of the GCS which is lower than the upstream bed level.

However, the flow over GCSs can cause local scour [8–12], and the downstream general scour may continue if there are no further downstream GCSs, leading to structural damages or failures (Figure 1). The failure of a GCS induces a drop in GCD, accelerating the adverse impacts they are

initially built to prevent [16]. Thus, it is important to understand the scour process at GCDs for safe design. Also, understanding the impacts of GCD drop on the upstream bed morphology can help to assess the stability of the upstream riverbanks and instream structures if the GCS fails.

**Figure 1.** Scour at the grade control structure (GCS) downstream of the 105th Provincial Highway Bridge after the flood event on 9 July 2013 (discharge *Q* = 2710 m<sup>3</sup>/s).

The local scours at GCSs have been extensively studied and many empirical equations have been proposed [8,10–12,21–38]. Some studies also investigated the impacts of GCSs on the river bed profile [17,18]. Although very important, no studies have investigated the effects of GCS failure (i.e., GCD drop) on the upstream river reach. This paper conducted a scaled model test based on a 1.3 km-long river reach upstream of a GCS in Shiting River, China, investigating the effects of GCD drop on the upstream riverbed morphology.
