**4. Conclusions**

In this study, a simply coupled 3D bedrock erosion and sediment transport model, CCHE3D bedrock model, was developed and applied to the downstream channel of the JiJi Weir, Chushui River, where the serious channel incision and head-cut migration endanger the JiJi Dam. In this simple coupling model, a concept of a sediment mixing layer over the bedrock surface is adopted. The thickness of the sediment layer determines the bed change mechanism as follows: thick sediment layer leads to non-rock erosion calculation, while non-sediment layer indicates the dominant role of the bedrock erosion. Within the mixing layer thickness, the stream-power-induced erosion would be applied at a reduced rate, proportional to the thickness of the mixing layer. The net change rate of the mixing layer is the combined rates of rock erosion and sediment deposition.

The hydrologic data of Typhoon Matmo in 2014 and Typhoon Morakot in 2009 were used to calibrate and validate the site-specific parameters for the stream power method–based bedrock erosion model. This model was also applied to simulate the bed changes with designed channel erosion control structures installed. The objective of the study is to identify an optimal design to alleviate channel incision and stop the head-cut development.

Based on the evaluations of multiple erosion control plans proposed by WRA and the channel evolution analysis, an optimal erosion control plan was identified. Six weir structures at section JiJi-25, JiJi-26, CS-115, CS-113, CS-111, and CS-108.5 and the 150 m-wide lateral excavations from JiJi-27 to CS-108.5 along the right floodplain to widen the channel were proposed. The lateral excavation would keep flows in the thalweg, but divert the water onto the excavated area during floods. According to the longitudinal profiles along the thalweg (**Figure 1**), the CS-115, CS-113, CS-111, and CS-108.5 were selected as the locations for installing weir structures to control the channel bed slopes. The elevations of those weir structures were determined in such a way that the target weir protects a half of a segment between two neighboring weirs so that the bed slope of the pool between weirs could be reduced by half (**Figure 10**).

Using the proposed coupled bedrock erosion and sediment transport model, the optimal design and the case without any structures were evaluated and compared. According to the simulated results, the optimal design reduced the channel incision significantly, and the head-cut development was stopped by the two weir structures installed at JiJi-25 and JiJi-26. Sediment depositions were observed not only in the deep channel but also the excavated area. In general, the whole study reach from JiJi Weir to Minchu Bridge still demonstrated erosion pattern except for the first 1.2 km reach at upstream. Without any control structures, the channel would be further deepened, and the development of the head-cut would continue.
