Flume investigation of run-up mechanisms of granular and water surge flows impacting a rigid barrier

Abstract

Rigid barrier is a commonly adopted structural countermeasure to mitigate debris flow hazard. The height of a rigid barrier must be designed to prevent debris run-up from over-spilling downstream upon debris impacting the barrier. Despite the necessity to design barriers with suitable height based on predicted run-up, the influences of debris material properties and flow condition on run-up mechanisms are not well understood. This can lead to either over prediction or under prediction of run-up and barrier height. In this research, flume experiments were carried out to first investigate and compare flow characteristics between dry sand and water and thereafter the run-up mechanisms. Physical experiments using a five meter long rectangular flume was adopted in this research. The channel inclination was varied from 0º to 50º to study different initial conditions. Open channel flows are driven by gravitational force. The interaction with flow-impeding structures is strongly influenced by changes in momentum. Hence, the Froude number (Fr), which characterises the ratio between inertial force and gravitation force, was adopted to dynamically characterise surge flows in this research. Results revealed that the flow behaviour of granular dry sand and water surge flows is dependent on its energy dissipating mechanisms. Results also revealed that Fr can be suppressed by using larger initial volumes and shallower channel inclinations for both granular and water surge flows. However, the influence of initial volume has a more dominant effect on reaching lower Fr comparing to shallower inclinations. Debris run-up upon impacting a rigid barrier depends upon the intrinsic nature of the flow medium and the approaching Fr. Existing design recommendations underestimate run-up for watery flows. Dry granular flow exhibits a compressibility or change in density of about 40% upon impact. Obvious energy losses occur at channel inclinations greater than 20° for vertically orientated barriers. This implies that vertically orientated barriers more effectively dissipated energy. Subcritical water surge flows impacting a rigid barrier resulted in a reflective wave mechanism; whereas, supercritical water surge flows led to a formidable vertical jet run-up mechanism. Supercritical granular dry sand flows impacting a rigid barrier merely exhibited a pile-up mechanism with a granular bore propagating upstream. The conservation of mass and momentum approach proposed by Johannesson et al. (2009) can capture the pile-up mechanism when granular dry sand impacts a rigid barrier; whereas, energy principle is more appropriate for water run-up.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology