In the year following the Hōei eruption, a secondary disaster occurred when the Sakawa flooded due to sediment build-up resulting from the ash fall.
Volcanic sands fell and widely covered the cultivated fields east of Mount Fuji. To recover the fields farmers cast volcanic products out to dumping-grounds and made sand piles. The rain washed sand piles from the dumping grounds away to the rivers again and again and made some of the rivers shallower, especially into the Sakawa, into which huge volumes of ash fell, resulting in temporary dams. Heavy rainfall on August 7 and 8, 1708, the year following the Hōei eruption, caused an avalanche of volcanic ash and mud and broke the dams, flooding the Ashigara plain.
Read more about this topic: Hōei Eruption Of Mount Fuji