The influence of landscape connectivity upon channel adjustments and sediment flux in a dynamic bedrock river system, the Liwu Basin, Taiwan

Abstract

High erosion rates in mountainous landscapes of Taiwan reflect frequent disturbances by seismic events and tropical cyclones. Patterns and rates of geomorphic processes on hillslopes and valley floors, and their interactions, alongside appraisals of magnitude–frequency relationships of disturbance events, are key considerations in analyses of landscape evolution and environmental management of these highly dynamic landscapes. This thesis incorporates analyses of sequential remotely sensed data and field investigations to examine hillslope-valley floor interactions in the Liwu Basin, Eastern Taiwan. Following a brief introduction chapter that sets up the thesis questions in relation to key literatures, thesis results are presented as three papers. The first paper examines how landscape configuration influences downstream patterns of sediment storage across the Liwu Basin. This paper shows how downstream patterns of sediment storage are controlled primarily by valley confinement and landscape configuration. This, in turn, reflects downstream patterns of river types and their longitudinal connectivity. The second paper demonstrates how landscape connectivity and landslide dynamics influence channel adjustments in the Liwu Basin. Analysis of geomorphic changes over 30 years (1980-2010) in differing parts of the catchment show that spatial variability in magnitude-frequency relations of hillslope-valley floor and upstream-downstream connectivity during the study period reflect annual reworking in source and accumulation zones, while partly-confined valleys in the mid-catchment area trap sediment behind landslide-induced dams that are formed and breached on an approximately decadal basis. The third paper applies field survey techniques to show how channels respond to flood events of low to moderate magnitudes in this bedrock-dominated river. Reach-scale monitoring by terrestrial laser scanning indicates that variability in geomorphic effectiveness reflects valley confinement and the composition/configuration of geomorphic units on the channel bed. Frequent changes of patterns of scour and fill indicate that the Liwu River is highly sensitive to flood events of low to moderate magnitude. These various threads are brought together in the discussion, which contextualizes findings on short-term (decadal) magnitude-frequency relationships in the Liwu Basin derived in this study in relation to the Taiwanese landscape as a whole. Limitations and implications of this study are outlined, alongside future research applications.