One single such structure has a limited effect. Rather, series of sabo dams are built
along a river for incremental capacity. Around Mount Merapi, the upstream structures
are designed to withhold boulders only, letting sand and gravel pass, to be captured by
structures further downstream.
Figure 4: A sabo dam (with an outlet for irrigation)
It is important to maintain a certain basic level of sediment transport past the structures.
Otherwise, there will be increased erosion in the downstream parts of the river system.