Channel Planform Dynamics Monitoring and Channel Stability Assessment in Two Sediment-Rich Rivers in Taiwan

Cheng-Wei Kuo,Su-Chin Chen,Chun-Wei Chen,Tun-Chi Yang,Chi-Farn Chen

doi:10.3390/w9020084

Cheng-Wei Kuo, Su-Chin Chen + Show 3 more

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https://doi.org/10.3390/w9020084

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Abstract

Recurrent flood events induced by typhoons are powerful agents to modify channel morphology in Taiwan’s rivers. Frequent channel migrations reflect highly sensitive valley floors and increase the risk to infrastructure and residents along rivers. Therefore, monitoring channel planforms is essential for analyzing channel stability as well as improving river management. This study analyzed annual channel changes along two sediment-rich rivers, the Zhuoshui River and the Gaoping River, from 2008 to 2015 based on satellite images of FORMOSAT-2. Channel areas were digitized from mid-catchment to river mouth (~90 km). Channel stability for reaches was assessed through analyzing the changes of river indices including braid index, active channel width, and channel activity. In general, the valley width plays a key role in braided degree, active channel width, and channel activity. These indices increase as the valley width expands whereas the braid index decreases slightly close to the river mouth due to the change of river types. This downstream pattern in the Zhuoshui River was interrupted by hydraulic construction which resulted in limited changes downstream from the weir, due to the lack of water and sediment supply. A 200-year flood, Typhoon Morakot in 2009, induced significant changes in the two rivers. The highly active landscape in Taiwan results in very sensitive channels compared to other regions. An integrated Sensitivity Index was proposed for identifying unstable reaches, which could be a useful reference for river authorities when making priorities in river regulation strategy. This study shows that satellite image monitoring coupled with river indices analysis could be an effective tool to evaluate spatial and temporal changes in channel stability in highly dynamic river systems.

Highlights

Patterns of river channel planform dynamics reflect a variety of flow and sediment regimes which are controlled by climatic and geologic conditions [1]
This study focused on the channel changes from the middle to downstream area because the upstream channels are confined by steep valleys, no notable changes are observed there
The results showed that the valley width plays a key role in braided degree, active channel width, and channel activity

Summary

Introduction

Patterns of river channel planform dynamics reflect a variety of flow and sediment regimes which are controlled by climatic and geologic conditions [1]. Taiwan is located on the track of typhoons in the Western Pacific Ocean and experiences 3–4 typhoons per year on average. Recurrent flood events induced by typhoons are powerful agents to modify channel morphology in Taiwan’s rivers. Abundant sediment supply due to fragile geology and frequent landslides in Taiwan’s landscapes provides materials along the river network for reforming channels and floodplains. Frequent channel migrations reflect highly sensitive valley floors and increase the risk to infrastructure. Ideal river engineering is able to mitigate flood hazards, and this must be based on a good understanding of river characteristics in terms of spatial and temporal variations

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