This paper conducted an undistorted scaled model test (geometric scale λL = 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 m3/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.
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