**1. Introduction**

JiJi Weir was built across Chuoshui River, the longest river (178.6 km) in Taiwan, originating from the central mountains and flowing into the East China Sea, covering a watershed about 4323 km<sup>2</sup> . A shallow reservoir was formed due to the construction of the weir. The precipitation and geology dominate the flow and sedimentation processes in the channel and the reservoir. Particularly, during typhoon seasons, floods with high discharges will carry a large amount of sediments flowing through the river channel. Shortly after the weir construction, a pebble-boulder-sized sedimentation layer covering the channel bed was mobilized by the initial clear water releases, and the soft rocks beneath this sedimentation layer were exposed. Within the 6.5 km region downstream of the dam, the channel bed now is featured with erodible lithology layers of mud, shiver, and sandstones, resulted from severe channel incisions and a head-cut development. According to the comparisons of historical longitudinal profiles in **Figure 1**, the vertical incision of the channel thalweg from 2002 to 2014 is

**Figure 1.** *Historical longitudinal profiles along thalweg (*S *is bed slope; CS denotes "cross section").*

**Figure 2.** *Study domain with survey cross sections.*

about 10 m (please refer to **Figure 2** for the locations of those measured cross sections from CS-106 to CS-116). Looking downstream, **Figure 3** shows the incised channel near the JiJi Weir. The white dash line indicates the flat channel bed before the incision. The head-cut was still actively developing and migrating, which has threatened the integrity of the dam (**Figure 4**).

To prevent this channel from further erosion and protect the JiJi Weir Dam, the Water Resources Agency (WRA) of Taiwan has proposed several erosion control plans [1, 2]. Several weir structures and channel widening were proposed to promote sediment deposition and lower the water surface level for flood protection. This study is to provide alternative plans optimal for erosional control and flood protection by using a 3D numerical model, CCHE3D [3], with capabilities in simulating bedrock erosions with lateral erosions.

*Erosion Control at Downstream of Reservoir Using In-stream Weirs DOI: http://dx.doi.org/10.5772/intechopen.108169*

**Figure 3.** *Bedrock channel (photo by Zhang, Y., 2015.11.2; look toward downstream).*

**Figure 4.** *Head-cut development at downstream of JiJi Weir (satellite image, 2014.11.23).*

The bed rock erosion is closely related to channel lithology, tectonics, and geomorphology properties, contributed significantly by complicated weathering, plucking, abrasion (due to both bed load and suspended load), cavitation, and dissolution processes [4], which is distinguished from river sediment transport and morphologic processes on alluvial beds. Extensive research studies have been carried out for attempting to describe these complex physical mechanisms of bedrock erosions mathematically. For examples, Annandale [5] proposed a conceptual framework to correlate the flow energy to the earth mass erodibility by introducing the erodibility index, which characterizes the capability of earth materials for resisting erosions. Whipple and Tucker [6] considered the stream power to dominate the bedrock erosions. Whipple *et al*. [4] developed the conceptual models for plucking, abrasion, and cavitation in terms of the relationship between the bed shear stress and erosion rate by studying the field erosion processes. Sklar and Dietrich [7] identified the sediment supply as an important factor for bedrock erosions due to abrasion. Sklar and Dietrich [8] proposed a mechanistic bedrock erosion model by saltating bed load based on their work in 2001. Turowski *et al*. [9] extended and improved the Sklar's approach with more detailed covering effect. Stock *et al*. [10] measured rock erosion rate in several field sites including river in the United States and Taiwan. Lamp *et al*. [11] extended Sklar and Dietrich [8]'s work further by considering the impacts from not only the bed load but also the suspended load.

From the previous studies, the fluvial impacting factors for the bedrock erosions were identified as follows: bedrock erodibility (strength), stream power, shear stress, sediment supply, and grain size. Accordingly, two types of models [12], namely the hydraulic scour model (stream power-based [5, 13]) and the abrasion scour model [8] have been used in the applications with numerical simulations. Jia *et al*. [14] incorporated the abrasion-based bedrock erosion model into CCHE2D model [15] to study the channel incisions at downstream of JiJi Weir. Lai *et al*. [12] proposed a hybrid bedrock erosion model by combining the abrasionbased model and the stream-power based model. Liao *et al*. [16] implemented a stream-power-based soft bedrock erosion model into a 2D mobile-bed model EFA (Explicit Finite Analytical) to study the channel morphological process in an uplifted reach of Ta-An River by an earthquake in Taiwan. In addition to a 2D bedrock erosion model, Jia and Zhang [1, 2, 17] first developed a 3D bedrock erosion model by extending Liao *et al*. [16]'s method to CCHE3D [3] to study the channel incision problems in Taiwan.

This chapter is to present the development and applications of the CCHE3D bedrock erosion module to study the channel incisions and erosion control problems in the downstream channel of the JiJi Weir. The numerical model was calibrated and validated using field data. Based on the evaluations of the multiple alternatives, an optimal control plan, involving multiple weir constructions with the side channel excavations as well, was proposed to control the head-cut development soft rock and the channel incision.
