**Abstract**

As low-head hydraulic structures, instream weirs are built across rivers to control the upstream water surface elevation and the downstream flow conditions. This chapter presents a study of erosion control using instream weirs at downstream of a reservoir; JiJi Weir was built across the longest river in Taiwan, Chuoshui Creek, a mountainous river with steep slopes. Due to the easy-to-be-eroded fine lithology layers of mud, shiver, and sandstones on channel bed, the downstream of JiJi Weir had suffered from severe channel incision and head-cut development problems, which greatly threatens the integrity of the dam. To protect the JiJi Weir and its downstream channel from serious channel erosions, the Water Resources Agency (WRA) of Taiwan proposed erosion control plans that multiple instream weir structures were to be installed along the downstream channel of JiJi Weir. A threedimensional (3D) numerical model, CCHE3D model with capabilities of simulating bedrock erosions, was used to evaluate those erosion control plans and thus explore for the optimal design.

**Keywords:** instream-weir, bedrock erosion, channel incision, erosion control, three-dimensional model
