**Case Study: Model Test on the E** ff**ects of Grade Control Datum Drop on the Upstream Bed Morphology in Shiting River**

#### **Xudong Ma, Lu Wang \*, Ruihua Nie, Kejun Yang and Xingnian Liu**

State Key Laboratory of Hydraulics & Mountain River Engineering, Sichuan University, Chengdu 610065, China; maxd@scu.edu.cn (X.M.); nierh@scu.edu.cn (R.N.); yangkejun@scu.edu.cn (K.Y.); liuxingnian@126.com (X.L.) **\*** Correspondence: wanglu@scu.edu.cn

Received: 28 June 2019; Accepted: 6 September 2019; Published: 11 September 2019

**Abstract:** This paper conducted an undistorted scaled model test (geometric scale λ*L* = 1:80; the others are derived scales based on Froude similitude) of a 1.3 km-long river reach in Shiting River, China, investigating the impacts of the grade control datum (GCD, defined as the crest elevation of the grade control structure) drop on the upstream bed morphology. Three GCDs and six flood events (occurrence probability 1–50%, discharge = 600–4039 m<sup>3</sup>/s) were tested on the model. Experimental results indicate that, for a constant GCD, the increase in discharge deepens and widens the upstream river bed. For a lower GCD, the increase in channel depth and width caused by the increasing discharge is greater. For each discharge, the decrease in GCD induces a lower and steeper upstream river bed, widening the upstream main channel. For lower discharge, the GCD drop induces a head cut erosion area upstream of the grade control structure and the head cut erosion area is filled by the upstream sediment when the flow discharge is high. Experimental data also indicate that the maximum general scour depth at the 105th Provincial Highway Bridge is approximately independent of discharge for a constant GCD. For a lower GCD, the general scour depth at the 105th Provincial Highway Bridge increases slightly with discharge.

**Keywords:** bed morphology; bridge scour; general scour; grade control
