'Last week I spent 4 days mapping a stretch of the Putih River, or 'White River' as it is translated into English. This involved marking in changes in channel dimensions and defences, to produce a mini hazard map. With review and comparison to previous maps I can create an updated report of the area. The exercise has been more than useful in helping towards my 4th year project, being able to see first hand how the river interacts with local communities and the impact the lahars have. I was accompanied on my visit by a student from Gadjah Mada Uinversity, who was able to help me communicate with the locals. They are able to provide information on lahar duration and magnitude for the past two to three years at specific locations, as many villagers do not move out of the area despite the hazards. We also spoke to an engineer who was assessing the quality of the defences that are currently being constructed and he gave an in depth explanation of a new type of dam they have installed that will hopefully minimise the risk of damage further downstream (see the first photo below, the engineer entitled this a floating dam). A destroyed dam on the Putih river is shown in the second image below).
What has struck me more than anything is how resilient the local population are. Where bridges have been destroyed they merely construct new ones, (quite often out of bamboo which looks marginally dangerous when I see motorbikes using them - see the photo below), and where houses have been knocked down they simply rebuild. The river is also a major source of income to them and their excavation of material generally helps conditions during the rainy season, removing material that would otherwise be reworked. Issues do arise if they dig too close to the dams, which can risk the structure being undermined.
Overall the government seems to have the river well maintained; it just requires constant maintenance. With Merapi erupting so frequently there is a constant supply of material to be washed down every year; it's not like the problem will disappear after a few years. This means that hazard maps need to be constantly assessed - if the volcano erupts in a different direction then a different drainage basin will suffer from an influx of material. On a wider scale it is important to note that fluctuations in the rainy season have been increasing, possibly due to climate change, and so it is hard to predict when the lahars will occur. This could be an interesting topic to investigate.
From Wednesday this week I have been tasked with mapping the Krasak River and hopefully I will be able to compare the two rivers after this exercise.'