Wataru SAKURAI, Yoshio TOKUNAGA, Takeo MITSUNAGA, Michael M Alpassan,Jerry E Fano, Ryosuke TSUNAKI, Hidehiko MANZEN and Takao YAMAKOSHI
Abstract
The area around Mt. Mayon in Albay Province Philippines was affected seriously
by large mudflow caused by heavy rainfall of the typhoon 21 in Nov. 2006. The
loss of human lives amounted to 1,300 deaths and many houses, infrastructures
were damaged. After the disaster, an urgent survey conducted by a research team
dispatched by Ministry of Land Infrastructure and Transportation Japan revealed
that all of the riverbeds in the mountain foot area where mudflow passed were
scoured deeply and widely in spite of the gentle slope, which was around 3]4
degree. Maximum depth of the scouring reached to 10m, and maximum width reached
to 30]50m. It was remarkable that the gradient of the sections where we found
the deep scouring was too gentle to result in such a deep scouring. We tried
to clarify this mechanism of the deep scouring by a numerical riverbed fluctuation
model to refer to further counter measures. Due to lack of data in detail such
as rainfall data in upper stream and the latest map to know the condition of
catchment area, it was impossible to know an exact peak discharge and a hydrograph,
so it didn't allow us to say anything quantitative. However, our result infers
that the very low sediment concentration in the subsequent flow and the fine
and loose material of the sediment in riverbed might cause the deep scouring.
Key wordsFmudflow, sediment concentration, riverbed material