Influence of soft rock erosion on the channelization of a fluvial river
|關鍵字:||河川地形;軟弱岩石;深槽化;大安溪;八掌溪;沖蝕率模式;river geomorphology;soft rock;channelization;Taan River;Bachang River;erosion rate model|
Because of environmental changes (e.g., tectonic uplift, climate change, etc.) or human activities (e.g., hydraulic engineering, infrastructures construction, gravel mining, etc.), intense bedrock erosion and stream instability occurred in several major rivers in western Taiwan. Most of these rivers flow across the geological province of the western foothills which are composed of weak sedimentary rocks. Once the armor layer overlying the weak rock disappeared, the channel bedrock erosion progressed rapidly and caused enormous changes in fluvial landscape. The consequences of weak bedrock erosion have endangered the infrastructures across the river and along the river bank. The knowledge state of the erosion activities of a fluvial river is relatively matured. On the contrary, the fundamental study on the erosive behavior of a bedrock river is still in lack for evaluating the bedrock erosion processes. Therefore, the objective of this dissertation is to study the influence of soft rock erosion on the channelization of fluvial rivers. In contrast to typical bedrock erosion with a low erosion rate, the erosion rate of the bedrock river in Taiwan is exceptionally high. In several cases, the annual erosion rate could reach tens centimeters to a few meters in a weak bedrock river; the high erosion rate could result in significant changes in fluvial geomorphology. On the other hand, the rapid landscape changes provide an extraordinary opportunity to investigate the bedrock erosion processes within a manageable period of time. Two study reaches were taken in the dissertation. For the first one in the Taan River, a reach was 10-meters uplifted because of the 1999 Chi-Chi earthquake; afterwards, the reach incised 20 meters and turned into a gorge-type channel within a decade. The scale of the erosion due to knickpoint migration was 14 meters in depth and 355 meters in the horizontal direction (knickpoint retreat). For the second study reach in the Bachang River, an 11-km-long reach had been affected by human activity, such as engineering works and gravel mining. The channels transformed from an alluvial-type channel to a gorge-type channel within just a few decades. The maximum accumulative depth of the channel incision was up to 30 m approximately, with a meter-scale average annual incision rate. The maximum width of the river channel decreased to approximately one-sixth of its original width (448 m). Such an intense landscape change hence damaged various hydraulic facilities along the river bank and across the river. In this dissertation, these two reaches were chosen as the study reaches among the major rivers in western Taiwan. Chronological aerial photographs and digital terrain models were used to analyze the changes of the fluvial morphology include the channel planform, longitudinal profiles, channel incision, channel width, and channel cross sections of the study reaches. Based on the results of the channel-change analyses and field investigations, the channelization processes of geomorphology evolutions from the originally fluvial reaches were reconstructed. Moreover, three erosion-rate models for bedrock were proposed; these models include (1) an incision rate model, (2) a knickpoint migration rate model and (3) a river widening rate model. These erosion-rate models were applied along with a 2-D mobile-bed model to simulate the river channel change in the study reaches. The comparison of the field and numerical results reveals that the proposed models are capable of simulating the morphological change in a bedrock river.