When it rains, raindrops impact to forest floor send fallen leaves, fallen branches and soil particles flying (raindrop erosion). Together with it, ¹³⁷Cs outflows from the forest floor. (Figure 1)

Investgation in Kawamata Town in Fukushima Pref. reveals that ¹³⁷Cs outflow rate of this mechanism is 0.5-2.1 % in about a month (Oct 4-24, 2017). The amount of outflow of ¹³⁷Cs from forest floor is tend to decrease together with an increase of the coverage rate of forest floor by fallen leaves and branches.(Table 1)

• 

  Fig.1　Sending soil particle flying by impact of a raindrop

• 

  Fig.2　Morgan-type splash cup（stainless steel, outer diameter 30cm，inner diameter 10cm，height 10cm）

  A hole is center of the splash cup in order to exposure the forest floor. When a raindrop hit this part, soil particles are send flying and gathered in a collection tray.

Table 1　Coverage rate of the forest floor and outflow rate of ¹³⁷Cs

Coverage rate by fallen leaves, branches, etc.	Amount of the soil move into downstream of the slope	Amount of outflow of 137Cs	Amount of 137Cs in the soil around investigation point	Outflow rate of 137Cs
5.1 ％	21.2 g/m2	710 Bq/m2	33,929 Bq/m2	2.1 ％
48.2 ％	11.3 g/m2	295 Bq/m2		0.9 ％
95.4 ％	8.5 g/m2	523 Bq/m2	101,595 Bq/m2	0.5 ％

Related articles

• When it rains, does radioactive cesium flow out of forests with soil? 【¹³⁷Cs flow from river basins and mountain slope】
• When it rains, does radioactive cesium flow out of forests with soil? 【Tendency of the change of the ¹³⁷Cs concentration in soil】
• How much does additional internal dose exposure would be if we assume highest concentration during the fire event was exposed? How harmful for the health?

Reference

1. Anderson, R.S. et al., Geomorphology: The Mechanics and Chemistry of Landscapes, Cambridge University Press, 2010, 651p.
2. 恩田裕一編, 人工林荒廃と水・土砂流出の実態, 岩波書店, 2008, 245p.