Growth of Vegetation in Fluvial-Fan Segment Due to Decrease of Destruction of Vegetation by Floods

Abstract

Recently in many rivers, vegetation covers wide parts of floodplain of fluvial-fan segment of particularly with a dam on the upstream. Rich vegetation is disadvantageous from the viewpoint of safety against flood but also it is not necessary good for ecosystem of fluvial-fan rivers. When the changes of river landscape of the Tedori river (with dam) and the Abe river (without dam) are compared with each other by aerial-photo analysis, the growth of vegetation after dam construction is remarkable in the Tedori river. When the records of floods of the two rivers are compared with each other, it is clarified how the major floods were controlled by a dam in the Tedori river. The previous studies by us show that repetition of medium-size floods and low-stage waters promotes vegetated area to expand. On the other hand, major floods sometimes destroy the vegetation. Usually the growth and destruction are in balance. Thus the obvious growth of vegetation in the Tedori river probably implies the decrease of chances for vegetation to be destroyed by major floods. Then, models to simulate destruction of vegetation by flood and growth of individual trees have been proposed to explain the difference of landscape changes in the Tedori and the Abe river.

Effects of Tree Destruction and Debris Dam Formation at the Gravel-Bed Bars in Kumagaya in the Arakawa River on the Maximum Water Depth and Flood Hydrograph During the Flood Event Caused by Typhoon Hagibis

“…WOI is defined as τ*90/τ*c90, which expresses the ratio of the shear stress acting on d90 and the critical value. Similar methods using the critical shear stress were also proposed by Tsujimoto et al 15) and Egger et al 16) For judging the washout of a tree, the critical value of WOI was set as 1 using the data in Tanaka et al 5) . When a tree trapped debris in the scoured region by horseshoe vortices around a tree trunk, the scoured region just around a tree trunk was observed to decrease, although the scour slightly occurred around the TDC (Fig.…”

“…Then the tree washout itself can be decreased and the WOI judgment for washing out trees was not used further. For large perennial grasses, referring to Yagisawa and Tanaka 15) , the critical values of Miscanthus sacchariflorus, Phragmites japonica and Eragrostis curvula, were set as 2.0, 2.0, and 1.55 respectively. When the perennial grasses were judged to be washed out, the Manning roughness coefficient was changed to 0.035, the same as the value in the low channel.…”

“…Fig 15. Discharge hydrograph at the downstream location of fine grid area (Fig.6), (a) whole flood event, (b) around a peak, (c) after the peak.…”