Poor land use is worsening floods
25 February 2020
KERRY KIMIAFA
GOROKA - It would be absurd to blame effects of climate change as the sole reason for sudden and unprecedented flooding in certain parts of Papua New Guinea, such as the recent case here in the Highlands.
But the truth is that we humans have abetted and induced flooding through land use changes, especially massive vegetation clearance.
This is because trees, bushes, grass, vines and creepers act as buffers to assist hold back surface water flow, or runoff, and diminish the accelerated flow of water into streams during rainfall events.
Trees and vegetation also help percolate rainfall into groundwater that otherwise would have made its way straight to rivers had there been no vegetation.
However, in recent times vegetation clearance has happened at a much faster rate than before due to population pressures impacting everywhere in the country.
This is creating a dangerous scenario because rainwater now flow as if on a freeway with high speed and increasing volume because there is nothing to reduce its strength and volume.
So we have to be careful and consider this, especially when removing trees and vegetation from mountain slopes, in water catchment sites or at high altitudes because this will lead to unprecedented flooding events and destruction along the waterway.
During heavy and continuous downpours, the infiltration capacity of soils is quickly exceeded, leading to surface run off referred to as Hortonian flows, where rivers quickly become flooded, carrying away debris (logs, fallen trees, branches) and the bedload (silt, sand, mud, soil, rocks).
What happens now is the sediment fills the river channel leading to over-flooding - incisions, undercutting, collapsing banks and overland flow - causing so much destruction.
The peak wet season adds greatly to the problems, and authorities need to consider this when planning civil works such as roads, bridges, culvert or drains.
People living within the headwaters and water catchments need to be warned of the dangers, as do people downstream.
The Kyoto Protocol identified land use change from deforestation as being the main cause of CO2 increases globally and the cause of habitat destruction, extinction of flora and fauna and dysfunction of the natural cycles which govern rainfall events.
Hence, when we clear vegetation, we are inviting unforeseen events that bring devastating consequences, including loss of life.
Kerry Kimiafa is an environmental scientist specialising in surface and groundwater investigations. He is also a volunteer with the Travel4Green (T4G) PNG project (www.t4gpng.org) which is advocating for climate change remediation, forest conserving and carbon sequestration. Email [email protected]
In Australia where water is precious a few farmers are building fords and low dam walls to slow the water down that passes through their land in creeks and streams.
This is having a remarkable effect on the surrounding land and its ability to retain moisture longer. Where once the land was dry and pastures poor the method has produced greenery.
Mother Nature seems to know best and when we interfere it usually turns out poorly in the long run.
Posted by: Philip Fitzpatrick | 27 February 2020 at 09:03 AM
I enjoyed your analysis Kerry. Here in the UK where stronger, more destructive, floods have raised new demands for river control, the debate has developed on similar lines to those outlined by yourself.
We are having to face up to much heavier rain at the same time as farmers have drained hill ground to make their fields more productive. So no surprise that rain water on high land is being rushed into the major rivers instead of flowing into them more slowly.
As a result the downstream sections of these rivers cannot be contained and more towns and cities on their lower stretches are being flooded - some of them several times a year.
Reaction to this is interesting. Some farmers/landowners insist the solution is to deepen the river beds through dredging.
They say river courses have silted up and become shallow, cannot hold the volumes they did in the past, and flood water spills from them much more easily.
The counter argument is that dredging increases the speed with which water moves through the river system which makes downstream flooding more likely because system capacity is reached earlier.
The slow-flow response is favoured by government which is encouraging hill farmers to make an effort to ease river flow in high reaches and hold more water up there immediately after violent storms.
Methods include decelerating water flow itself by re-introducing bends and squiggles that only recently were straightened out.
And then removing embankments that were installed to protect farm land. This allows storm water to spread out in the upstream sections and some of this land being submerged temporarily.
This, says government, is infinitely preferable to that water being responsible for dramatic flood damage to roads, bridges and urban structures themselves because it has been allowed to rush downstream and add to the volume pressure.
Posted by: Robert Forster | 26 February 2020 at 08:43 PM