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Hydrological Modelling - Academician's Blunder

posted Jun 16, 2018, 7:59 AM by jeffery jim

A friend told me that a professor insist that flood is due to infiltration and not surface runoff. I am not sure whether to laugh or not about such silliness when it comes to fundamental understanding about hydrology.

Well, there are two choices or two models; rainfall-runoff models using effective rainfall and rainfall-runoff model using surface water budget. Both are appropriate at its own right. When we are discussing about flood modelling, it definitely speaks about run-off using surface water budget especially when we are talking about the tropics. If we speak about storm event, there is possibility for infiltration to take place which again depending on the percentage of permeable area with small runoff.

One have to be clear on the issue of slow infiltration rate due to high plasticity clay and the water high surface run-off which triggers flood. Continuous rain also leads to over-saturated case which indirectly swells and remove pores. This significantly further reduced the ability for infiltration or absorption. Such result shows the clear choice when selecting surface water budget for rainfall-runoff model.

Another reasonable tale-tell signs which validate the use of such model is the evidence of soil erosion based on soil taxonomy as underlined by ISRIC or FAO. The process of slope failures in high plasticity clays (at top horizon - layer stripped during surface preparation) involves formation of surface cracks, moisture infiltration through the cracks into the soil mass, a reduction in suction and hence shearing resistance of the soil, and ultimately slope failure when the driving stresses exceed the shearing resistance of the soil.

Moreover, when we relate the dominant acrisols to open catchment areas, it losses the porous surface soils and reveal its low activity clay. The A-horizon which degraded will allow the formation of hard surface crust (changes the Manning roughness coefficient value) and allow insufficient penetration of water during storm event and leads to surface erosion and slowly retaining the low activity horizon. Acrisols in low landscape are well known for showing signs of periodic water saturation. Thus, it has the lowest ability for infiltration or absorption.

When extends to the models mentioned above, it is definite that effective rain model is not the choice to even model a surface run-off during storm event. Leave flood modelling alone from this discussion. Besides, SWMM modelling commands the use of surface water budget model and limits such modelling to only the Horton or Green and Ampt infiltration equations. Never relates rainfall at Timbaktu to rainfall at Malaysia. You need to read the journal carefully and the setting of the research.

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