Effects of entrainment on debris flow impact against single and dual barriers

Abstract

Debris flow channels are typically composed of erodible bed sediments. Debris flows can increase in flow volume and mobility by entrainment when overriding these erodible beds, exacerbating the hazardous nature. Debris resisting barriers are erected along channels to mitigate devastating effects on downstream infrastructure and settlements. The design of such barriers typically relies on flow velocities derived from mobility analysis with empirical constant erosion rates, without explicitly considering effects of terrain slope and erodible bed characteristics. The presence of multiple barriers introduces another complexity as the material overflowing the crest of barriers lands on erodible beds. This may lead to escalated erosion and affect the flow impact against downstream barriers. The primary objectives of this thesis are to investigate the effects of slope angle and flow volume on debris flow entrainment, and to study its influence on impact against barriers. A modified flow velocity equation is proposed, incorporating the net change in flow momentum and flow front angle due to entrainment. A series of flume tests with and without erodible beds are carried out using a five-metre-long and twenty-eight-metre-long flume to verify the proposed flow velocity equation and investigate flow-bed-barrier interactions. To characterise the effects of landing dynamics on erodible beds, an erosion depth equation is derived. Dual barrier tests with and without erodible beds are carried out in the twenty-eight-metre-long flume with two different flow volumes. The results are compared against single barrier tests to delineate the effects of landing-induced erosion on subsequent barrier impact. It is found that the erosion rate increases by eight times when slope angle increases from 15o to 30o. Entrainment of bed sediments into overriding flow leads to increased flow front angles up to two times compared to flows over non-erodible beds with the same flow-bed interface friction. The entrainment-induced flow momentum gain increases with slope angle, reaching values up to 50% higher than that of flows without entrainment. Consequently, increased flow velocities up to 1.2 times are exhibited when overriding saturated erodible beds. Modified flow velocity equation reasonably captures the gain in flow velocity by incorporating the entrainment-induced changes under varying slope angles and flow volumes. In the presence of upstream barriers, amplified peak erosion depths are found to occur in the overflow landing zone, where higher flow volumes lead to enhanced landing velocities and erosion. This increasing trend of erosion depths with landing velocity can be well captured by the proposed equation. The flow-bed interaction subsequently affects the impact dynamics at the terminal barrier. The peak runup height and hydrodynamic impact force increases up to 2.6 times and 1.8 times respectively for flows eroding saturated beds. Landing induced erosion in the presence of upstream barriers leads to increase in flow velocities, entrainment and flow volume, consequently increasing the second barrier impact force by up to 40% in contrast to a single barrier case. Thus, it is crucial to implement appropriate erosion protection measures within the landing zone when overflow is expected.

Date of Award	2024
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Charles Wang Wai NG (Supervisor)