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Landslide and Debris Flow

posted Sep 6, 2021, 7:48 AM by jeffery jim

Landslide and debris flow is something quite normal in most tropical countries where storm is a normal occurrence. Based on actual condition and academic researches, the condition of landslide within 4m from the land surface is an outcome of two conditions which are; antecedent effects, and extreme intensity rainfall.

In most cases as reported in Siti et al. (2018) and Joe et al. (2018), rainfalls over 60mm/hr for a prolonged 180 minutes may trigger landslide and deemed as extreme rainfall. Nevertheless, for antecedent rains, it can be 72 hours to a few days of drizzling and low intensity rainfall. It is more than just deriving IDF and DDF curves.

Above all, rain is not the ultimate causal to landslide or debris flow since there are some other elements which involved indirectly such as the cohesion of cohesive soil as well as friction angle of granular soil which play important role in bonding between particles.

At microscopic level, the ionic charge from clay or even silt plays important role in generating cohesion value. On larger scale, aggregate stability of sandy to gravel size particles will also take precedence over cohesion by providing harder crust on the top layer of residual soil which reduced the penetration of water via infiltration which subsequently reduce the possibilities of hydraulic changes from at rest to active which start to nudge particles with low cohesion or friction.

For certain type of soil, this will trigger soil liquefaction and able to exert enough energy to move as non-newtonian fluid or debris flow.

Now, pedology and soil taxonomy are important consideration although these are in microscale which play important roles in triggering the major mass movement as landslide or debris movement. Antecedent effects by both journals have not consider soil taxonomy where certain soils like acrisols have tendencies to hold water until the point of saturation which is not helping with overburden on slopes exposed to runoff, rills and grills erosion, and water table or flownet.

There are a lot of precursors and also variables one have to understand before making statement on why landslide or debris flow happened. It is not as simple as it rains and it triggers landslide.

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