THE heavy rainfall the past several days caused a landslide along the Diversion Road in Davao City, causing massive traffic jams in the Ma-a and Matina areas last Thursday.
Luckily, the landslide was small and no unlucky motorist was passing through when the limestone chunks gave way. Let that be a warning though.
As we have seen, the areas within the poblacion, including Shrine Hills are soil on limestone, they're not solid rocks. Meaning, they are not as stable as rockbeds will be.
Expect rainwater to not just soften soil but also cause these to be waterlogged, and where the vegetation doesn't provide much anchor for the soil to hold on, then landslide should be expected.
There have been warnings raised for several years now, and yet very few anti-erosion measures are in place, and this is interesting considering that there are numerous ways to prevent soil erosion.
A flank of a hill or cliff can be vegetated. This is the simplest and most natural way. But when we consider that what's beneath the soil is limestone, then maybe we need plants that can pierce through the limestone to anchor the soil. Otherwise, a waterlogged soil with the vegetation on top might just slide down after several days of rain.
The most common now that we see along the highways are the use of geotextiles that are laid down all through the slopes and then vegetated. This has been proven effective, several studies have shown.
The most unnatural and drastic is putting up retaining walls to prevent water runoff that lead to further erosion and landslide.
We would assume that the Department of Public Works and Highways has the Environment and Management Bureau's geological maps that show where the unstable areas are. With the type of weather we are getting these days -- long hot and dry days that cause the soil to crack, and incessant rains that can cause these cracked soil to fall -- DPWH should be thinking of reinforcing what needs to be reinforced. They can start with using geotextiles along with vegetation as retaining walls are not just eyesores but area also not friendly to the environment.
Otherwise, expect more inconveniences (at the very least) like that one caused by the Diversion Road landslide.
Published in the SunStar Davao newspaper on July 15, 2017.
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