NUMERICAL AND EXPERIMENTAL STUDY ON FORMATION AND EFFECTS OF STEP-POOLS

Abstract

The demand for stream restoration utilizing step-pools sequences is increasingly due to urban developments population pressure land-use changes in recent years Fundamental formation mechanism of step-pool channels nonetheless is so far still a subject of controversy since there are two different general schools of thought on it Also insufficient understanding of artificial step-pools resulted in failure and inefficiency of many river restoration projects In order to find out actual formation mechanism of step-pool channels as well as verify capability of a three-dimensional model for modelling of step-pool morphology FLOW-3D model is used to compare calibrate and validate based on experimental results Moreover 26 physical experiments are conducted to investigate effects of artificial steps H/L/S ratio step configuration step density and upstream sediment transport on stability of steps sediment transport rate longitudinal profile energy dissipation and flow resistance Calibration and validation results showed that FLOW-3D has ability to be used in simulation of step-pool channels and both random location of keystones and mesh size play important roles in step-pool morphology formation Besides the numerical model revealed that most of existing Shields diagrams and bedload transport models are inappropriate for mountainous sediment usually characterized by big size materials Analyses of experimental results demonstrated in most of runs constructed artificial steps were stable even at the largest discharge level and artificial steps contributed to stabilize channel bed better than without steps Furthermore value 1 of H/L/S ratio and V-shape configuration provided the highest performance simultaneously the higher step density the better results of energy dissipation and flow resistance While coarse sediment feeding resulted in instability and burial of the first step and pool in some runs impacts of fine sediment feeding and flow were not strong enough to move or oscillate any component of the artificial steps Channel bed nevertheless was substantially stabilized in both coarse and fine sediment feeding Especially a great potential of artificial steps for flooding mitigation was explored since peak of sediment transport during coarse sediment feeding was much retarded

Date of Award	2017 Jul 7
Original language	English
Supervisor	Hsiao-Wen Wang (Supervisor)