Since this is not my area of expertise, I searched Google under the key words "lahars volcano" to get these possible sources:
There are many more articles.
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps. Thanks for asking.
Volcanologists can use this data to predict if current eruptions are likely to occur and have a rough idea as to when