**2. Background**

Shiting River is the first tributary of Tuo River. It originates from the Longmen mountain located in Sichuan Province, China, having a length of 131.7 km and a basin area of 1501 km2. The mountainous reach (upstream reach) of the Shiting River is 61.1 km long with a gradient between 92.3–10.4‰. The gradient of the river reach downstream of the mountain decreases gradually and ranges between 12 and 2.5‰ before it joins the Tuo River. Since the Wenchuan Earthquake (Ms 8.0) in 2008, serious bed degradation occurred in the piedmont reach of Shiting River, significantly exposing and endangering the foundations of instream infrastructures [39,40]. Thus, grade control structures (GCSs) were extensively used in this river reach as countermeasures for bed degradation.

The studied reach is about 13 km downstream of the mountainous area with a gradient of around 4‰, in which the 105th Provincial Highway Bridge is located (Figure 2). The studied reach is about 1.3 km upstream of the 105th Provincial Highway bridge, and has a channel width between 300–400 m. This reach is a quasi-straight channel and has no instream structures upstream of the 105th Provincial Highway Bridge. Thus, the impacts of stream curve and other instream structures on the bed morphology are negligible. As mentioned previously, significant bed degradation occurred in the piedmont reach of Shiting River after 2008 [39,40]. Thus, an 18 m high grade control structure was built upstream of the studied reach (2 km upstream of the 105th Provincial Highway Bridge) for stabilizing the upstream riverbed. The flood events during the period of 2009–2012 degraded the river bed at the 105th provincial Hyghway Bridge to an elevation of 539 m (544 m before the Wenchuan Earthquake). In 2012, for protecting the upstream river reach and the bridge, a grade control structure (GCS) with a downstream stilling basin (for minimizing the downstream local scour) was

built downstream of the 105th Provincial Highway Bridge with a crest at *z* = 539 m (i.e., grade control datum, GCD). However, as the bed degradation downstream of the GCS continued after the flood event in July 2013 (*Q* = 2710 m<sup>3</sup>/s), the bed elevation downstream of the GCS became 527 m, exposing the GCS foundation and causing structural failure (Figure 1). In order to assess the impacts of GCD drop (or GCS failure) on the upstream bed morphology, as well as the general scour depth at the 105th Provincial Highway Bridge, a scaled model test is conducted in this study.

**Figure 2.** Plan view of the studied reach of Shiting River (**a**); location of Shiting River (**b**); a photo of the model (**c**).
