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Is it debris flow?

posted Sep 6, 2021, 8:07 AM by jeffery jim
A friend of mine showed me an article by Prof Emeritus Dr. Ibrahim Komoo in one English newspaper and I found the original article as attached below. In the English article, it was mentioned that the event at Mount Jerai is considered as debris flow where as in the Malay article, it was not mentioned as so.

Earlier, I told my friend that I am not in agreement with Prof Emeritus with the terminology of debris flow to describe that catastrophe. The issue here is the technicality behind the rheology and mechanics which differentiate debris flow and other terminologies. What happened at Mount Jerai is actually debris flood.

Prof Emeritus explained things in the simplest form and the mechanics and science behind the calamity where as my post is a summary of chain of events. So let me sum up in point form for ease of understanding.

1. Sulfuric acid used in mining and rock extraction may seep into the ground. Acid rock which forms most of the granite and quartz at Mount Jerai and also may be the cause to acid drain/rock drainage. Another possibility of chemical contamination is probably due to enzymes used for road and embankment stabilization.

2. This may cause depletion of aluminum ion within the soil since Ferrasols soil have good water dispersion. With depletion of aluminum ion, the cohesion binding began to reduce and subsequently may lead to instability and no longer in equilibrium state. For Ferrasols, the highly clay soil depends highly on cohesion value instead of friction angle in order to be stable against induced external forces such as hydraulics and etc. At this stage, disposition of soils in a further distance than usual is called debris flow with some water content and usually depending on the topography, mainly gully or rill for its direction. At certain steep slope, the sudden move or drop like rock fall is known as debris avalanche.

3. The erosivity of rainfall is as important as other external forces and play an important role in metamorphism. This also depends on soil's aggregate stability in order to deter flaking or breaking down due to rain drop splash. Soil erodibility is another parameter which need to be considered as it has different kind of variability in regions since highly dependent on the organic content and soil rheology to determine its classification based on HSG.

4. The final sedimentation yield based on USLE depends on runoff volume and peak discharge apart from regional topography such as slope length and so on. Imagine 283mm precipitation and runoff Qpeak and volume.

5. Clay is different than silt and sand where it requires proper wet basin and proper settlement distance in a basin. Not known to many, the occurrence of free clay in runoff and in contact with Mount Jerai lime outcrop lead to another significant issues, which is flocculation. This binds clay and settled similar to sediments.

6. Flocculated clay, when mixed with alluvial sand at water bodies will actually clog waterways and will lead to the building up of natural dammed lake. Antecedent rain of more than 20mm in a cyclic of wetting and drying manner for many days will lead to changes of pore pressure and variance of ground water in certain rocks. Changes of cross section profile also generate different level of energy or power throughout the river. This will aggravate the situation and may also increase the accumulation of more debris and increase the capacity of the dammed lake.

7. Continuous antecedent rains have increased the water volume and overtime, the dammed lake is breached and the water gushes down together with most of the larger debris during the initial sequence of the event. The immense power of the water will start to disperse and come to everyone's surprise as torrent of deadly flood. The tremendous amount of water and rapid velocity is called debris flood.

I hope this will explain the reason why I posted earlier on the possible issues from pedology perspective. I am more interested with main issue and probes like many forensic engineer rather than to identify the transportation mechanism.
Apart from that, this also verify and justify why I am not in agreement with writer(s) who transcribed Prof Emeritus' use of debris flow as terminology to explain the disaster. It supposed to be debris flood by definition.

May be an image of text that says "abtu, Rencana Aliran puing bagai 'tsunami' di daratan, hadir tanpa amaran Emeritus irkal airter Pengunjung perlu dinasihati supaya menjauhi alur sungai apabila hujan lebat berlaku kawasan pergunungan tanah diangkut ngdiT risiko jadian Hayun, Yan memberi mengurangkan asa-hadapan. epada berkaasa langkah aperk dinmhi maca)"