Legowo, D. Volcanic debris control applied in Indonesia
Most rivers in Indonesia originate in volcanic regions. Volcanic eruptions cause enormous Tows of ash and lava into these rivers. This debris is easily eroded and often turns into lahars after rain. Lahars erode the sediment of river beds and banks and achieve considerable size and enormous energy on the way down the mountain, overflowing and causing considerable damage. The large amount of sediment brought down by Iahars raises river beds and gives rise to flooding. Sediment carried further down to main rivers also causes aggradation, instability of river courses, obstruction of irrigation facilities, and flooding.
Control of excessive volcanic sediment flow has been achieved to a great extent using checkdams built in steep-gradient river beds, but the optimum requirements of these checkdams are still indeterminate. To localise the spread of volcanic material and to safeguard the menaced areas at the foot of a volcano, lahar pockets are built across the streams: As there are no data available regarding the actual nature and mechanism of movement of the volcanic debris, the design of lahar pockets follows experience gained in the past. River courses will be improved at places where the danger cannot be entirely handled solely by such structures for coping with sediment. River course improvement works such as bed excavation, levees, embankments, bank protection works and groynes are therefore provided so that there will be minor river bed fluctuation or meandering or deviation of flow, no flooding or bank collapse, and smooth conveyance of flood waters and sediment downstream.