Rapid channelization and incision into soft bedrock induced by human activity — Implications from the Bachang River in Taiwan

• Prior to human disturbance, the original braided river channel had remained stable.
• Unusually rapid channel evolution occurred as a result of various human activities.
• The maximum depth of channel incision has been up to 30 m approximately.
• The river width has reduced to one-sixth of its original width.
• Lessons learned from the inducing engineering problems are discussed.

Many Taiwanese river channels have been affected by human activity, such as engineering works and gravel mining. Environmental conditions such as high seasonal precipitation and young sedimentary bedrock exaggerate the narrowing and incising of river channels. The Bachang River is located in southwestern Taiwan, and the 11-km-long channel at its midstream reach has been incised to a maximum depth of approximately 30 m within several decades. Therefore, the morphologic evolution of the Bachang River in southwestern Taiwan provides an illustration of channel changes in response to human activity. The historical data used for this analysis of morphology change include one set of topographic maps and six sets of aerial photographs from the past 100 years. The channel changes are analyzed qualitatively and quantitatively and include the channel planform, longitudinal profiles, channel incision, channel width, and channel cross sections. The results indicate that remarkable channel changes have occurred since the 1980s as a result of human influence. The 11-km-long study reach transformed from an alluvial-type channel to a gorge-type channel within several decades. The maximum accumulated depth of the channel incision is approximately 30 m, with a meter-scale annual average incision rate; the maximum width has been decreased to approximately one-sixth of the original width (448 m). The process of channel evolution is divided into four stages, and we have concluded that the causes of channel change include six factors from two categories: natural factors that include geological conditions, hydrological conditions, and the process of bedrock erosion; and human factors that include gravel mining, lateral structures, and levees. The initiation of channel evolution is triggered by human factors. Finally, we discuss the potential future channel evolution and lessons learned from the case study.