THESIS
2015
xii, 49 pages : illustrations (some color) ; 30 cm
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.
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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.
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