Gabion dams

The videos posted on socmed over the weekend showing the ferocious floods that hit parts of the metropolis prompted many to ask and search for answers on how to mitigate these hazards in the medium and long term. We have to admit and realize the problem can’t be erased immediately and all that can be done is to put up an effective warning system and set up measures to lessen the inundation. But the real solution has to be installed.

It’s sometimes funny that we call government intervention projects flood control projects. To control means to regulate but almost all projects done tend to increase water flow. Maybe we need to fall back and reassess the intention. Bringing the water to flow faster will mean bringing more volume to low-lying areas which inevitably cause flooding. There might be an increase in rainwater over the decades because of climate change but this is not much to cause the kind of flooding we have now. These raging floods are caused by water flowing faster.

Precipitation is measured in volume of water per unit of time, and since this is constant, it’s the same measure as the surface water flow after evaporation, transpiration, infiltration, and percolation are subtracted. Building a drainage structure (canal, pipe, etc.) will make water flow faster, thus bringing heavier volume downstream. It’s always like a series of rungs in a ladder – the next drainage structure will receive the water and bring it to the next. At any time, a structure’s capacity can’t take the volume, it overflows. One thing is often overlooked – solving a flooding problem in an area (by making the water flow faster) almost always brings a bigger problem to the next lower area. It would take an immense gigantic amount of money to build something that can automatically drain a city in an instant without causing flooding buildups along the way.

That’s why the other viable concept is not to drain faster but to regulate water flow. Drainage facilities are underused 95% of the time because these are designed for maximum capacity. If a certain thunderstorm will produce a certain amount of water, it’s not necessary to bring these down to the sea immediately, as one would think. If the water can be made to flow towards the sea over a longer period of time, then the water flow is much, much less, we need less drainage capacity, and flooding will not, or seldom, occur.

This is the concept behind the proposed gabion dams of a time long ago. We have hundreds of smaller waterways in the hinterland where these can be built easier and cheaper – just earth mounds with some rock support, maybe held by chicken wire. We have a much larger hinterland area, than we have built-up lowlands. These will need regular maintenance, of course, but not difficult or sophisticated. In the lowlands, low-lying flat surfaces, or even underground storage structures, can retard water flow and make it manageable. Parking areas are excellent places to start, with innovative designs. Also natural sinkholes. But gabion dams will still be the best, easy, and available measure to grab. Unfortunately, these are very unsophisticated, and most government leaders brush them aside.