Sabo dams
Mountainous areas of the world have major threats towering over them; sometimes during heavy rains the mountains will come down.
A debris flow is a fast-flowing mixture of rock, water, mud, and sand that flows rapidly downhill under the force of gravity. Debris flows can engulf villages and cities rapidly, sometimes with little to no warning. The energy of the flowing water is dangerous enough, but the large chunks of rock can be particularly hazardous as they’ll plow right through a person, car, or house.
Sabo dams (also called check dams) rely on technology that has been in use around the world for thousands of years.
Today dams like these are most commonly deployed in Japan, a country that has to deal with the combined threats of debris flows, tropical storms/typhoons that can trigger flooding, and volcanic lahars, although other Sabo dams have been deployed in a variety of locations around the world.
The idea behind a Sabo dam is pretty simple. If a stream is flowing normally, the stream should be allowed to continue flowing. It should be able to carry its normal sediment load downhill, eroding and depositing normally, without having sediment pile up behind the dam where it could be a threat to its integrity. A Sabo dam therefore is a structure built around a stream that lets the stream flow through during normal times. It can be something like this – a metal grate across the stream – or a concrete structure with a channel in the center that allows for normal river flow.
In the event of a flood, debris flow, or lahar, suddenly the dam’s presence becomes known. If the water level on the stream rapidly increases, suddenly the vertical bars are in the way. Friction with the vertical bars will actually slow the speed of the water rushing in the flooding river and as the water is slowed, some of the sediment will drop out behind the dam. Furthermore, if the river begins carrying large objects such as boulders or felled trees, those objects will be fully trapped behind the dam and will actually interrupt the flow even more.
These structures can be built at many scales; even small, temporary structures placed in the path of a river likely to flood can serve as obstacles that protect people downstream (based on my reading, small/temporary structures generally are described as check dams, although the terms can be interchangeable).
These structures can be particularly useful in areas at risk of Lahars. Lahars are volcanic debris flows and mudflows that can occur catastrophically. Volcanoes tend to be high peaks and therefore can generate snow and glaciers, they tend to have layers of poorly consolidated rocks and ash that are easily eroded away, and they also have heat supplies that can suddenly cause glaciers and snow to melt. Lahar threats from the volcanoes in Japan are another major reason why that country has deployed these structures in a number of areas.
-JBB
Image credit: machmud junus
https://machmudjunus.wordpress.com/2009/05/11/sabo-2/ (broken link at this page but originally hosted here)
Read more:
http://www.sabo-int.org/dott/
http://staff.civil.uq.edu.au/h.chanson/sabo.html
http://bit.ly/1SWC7Vn
http://www.dpri.kyoto-u.ac.jp/
