A framework for the sustainable risk assessment of in-river hydraulic structures: A case study of Taiwan's Daan River

Taiwan has frequently suffered from extreme rainfall in the past decades. A rainfall-induced flood tends to cause the failure of the in-river structure, ecological catastrophe, and even loss of human life and property. This study proposes a risk assessment framework balancing in-river structures’ design strength and ecological impact. The Daan River of Taiwan is examined as a case study to test the proposed framework. First, the study applied persistent scatterer differential Interferometric Synthetic Aperture Radar (PS-InSAR) techniques to analyze the interferometric images captured from the Sentinel-1 satellite to identify large-scale terrain deformation zones of the watershed. A numerical simulation was then applied to assess the impact of different protective in-river structures. Finally, calculations examined the Froude numbers to assess the ecological effects of landscape and fish habitat indicators. In addition, the study applied a wireless monitoring method (i.e., float-out device) to assess riverbed scouring near piers in high-risk regions. The results indicated that the majority habitat type of the stream during base flow is a glide, with an area of about 32%, with run accounting for about 10%. Riffles with high flow velocity and low water depth occupy only about 1%. The primary habitat type during the period of flooding is run with pools near the shore, providing an essential refuge habitat for fish during floods. During the constant current period, exposed sand bars occupy about 55% of the total study area of the river. When a flooding event occurs, the size of the sand bars can be reduced by 40%. Flooding hurts the quality of the habitat of the two fish species considered in this study, with the adverse ecological effects leading to degraded or scattered habitats. The creation of sufficient pool areas on the right bank of the river channel should facilitate the foraging and utilization of fish during regular flow and as a refuge during periods of flooding.