Coarse sediment movement around U-shaped vegetation zone on gravel bar with two meandering channels

Introduction

Numerical analysis
Numerical method and condition

In mid-upper regime of Kinugawa River, for example, the channel morphology was such as braided or
two lane meandering more than seventy years ago. In these days, however, channel morphology ischanging into single lane meandering, forming water colliding fronts which cause river disasters.
Furthermore, habitats of alien species such as Eragrostis curvula is being enlarged and those of endemic
species for gravel bed, such as Aster kantoensis Kitam, becomes almost extinct.
As a countermeasure against these issues, from about ten years ago, gravel bar restoration project
started in Kinugawa River. This project intends to increase submergence frequency by cut down of theheight of gravel bars, and to control flood flow by installing cobble mount at the upstream end of a gravelbar. However, partially because effective maintenance has not been performed after the restoration,
gravel bars were covered with fine sand, and grassland of alien species and forest area were enlarged.
In the present study, vegetation transition on a gravel bar with two meandering channels wasinvestigated over three years after implementation of gravel bar restoration. Subsequently, numericalanalyses were performed to examine the influences of vegetation distribution pattern and its partial
removal on the flow structure and coarse sediment behavior on gravel bars with two meanderingchannels.

Table 1. Numerical analysis conditions
Case
Case A1
Case A2
Case B1
Case B2
Case C1
Case C2
Case D1
Case D2(a) Year 1947(b) Year 2002

Transition of channel morphology in mid-upper
regime of Kinugawa River

Field investigation
210A
BC
DEFG

3rd channel

Elevation (m)E

205

200Cobble
mountMain channel

Coarse material

None

Casted (5mm f)

Whole U-shape

Aerial photograph of the field investigationsite and traverse lines for topographic
survey (A-G).

100200300
Lateral distance (m)Bed roughness

None
None
Casted (5mm f)
None

The bottom partremoved

Casted (5mm f)

The main channel side
partially removed

Casted (5mm f)

330x10-6

1/30

0-0.15

9.25

10.00
Longitudinal distance (m)Case B1 and B2 : whole U-shape

10.75

0.028

None

Nays-2DH solver of iRIC system (iRIC-UC, 2020) was used for numericalanalyses of the two-dimensional flow and movable bed. In numerical analyses, 10
sets of the gravel bar topography obtained from laboratory experiment was installed
periodically in a rectangular straight channel of 30cm .
Case A1 and A2 were with no vegetation. Case B1 and B2 were with U-shaped
vegetation area. The U-shaped has longer side arm on the main channel side. Case
C1 and C2 were cases for removal of the bottom part of the U-shape. Case D1 andD2 were cases for partial removal of the main channel side of the U-shape.

Case C1 and C2 : removal of the bottom part of the U-shape

Case D1 and D2 : partial removal of the main channel side of the U-shape

Vegetation pattern in numerical analyses

Year 2012

Case A1.

Case A2.

400

Elevation along the traverse lines (A-G)

The field investigation site is a gravel bar near the Uwadaira-Bashi Bridge in mid-upper regime ofKinugawa River, located at about 99km point from the confluence to Tonegawa River in Kanto Plain,
Japan. This gravel bar was about 700m long and 400m wide, and the average bed slope was 1/200.
This gravel bar was formed between the two meandering. The restoration project of this gravel bar
was implemented in the year 2011, which cut down the bar height, covered the bar surface with cobble
and installed cobble mount at the upstream end of this bar.

Case B1.

Communities of endemic (and mixed endemic) species for gravel bed were rare but recognizedaround outer edge of rather upstream part of the U-shaped zone, where gravel was deposited morethan the other area. Alien species, which adapts to sandy bed, were distributed in very large area on
the gravel bar, especially in the internal area of the U-shape, where fine sand was deposited. Thedifference of sediment size between the outer edge and the internal area is supposed to be due tovelocity difference in flood flow, affected by the flow resistance of the U-shaped zone. Therefore, theU-shaped vegetation zone, established in initial stage immediately after the restoration, plays a very
important role in vegetation transition on gravel bars.

Laboratory experiment for Gravel bar topography

Case C1.

Case D1.

Velocity vector and water depth contour
Year 2014
Endemic

Alien

PoaceaeMixed endemic

Mixed alien

Woody plants

Transition of vegetation distribution from the year
2012 to 2014 (Brown dotted line indicates U-shaped
vegetation zone of Poaceae)

A laboratory experiment was performed on stable gravel bar topography with two meandering channels for numerical analyses. In
the experiment, a straight flume of 8m length, 30cm width and 24cm height was used. In order to obtain stable bar topography,
groynes were installed every 1m from the upstream end so that the channel width was narrowed periodically. The channel slope was
set to 1/30, the bed material diameter was 0.9mm, and the flow rate was 430 cm 3/s. Then, 2 hours after supplying water, bar
topography was stably formed with two meandering channels where the difference between the main channel and the second
channel was clear.
0.30
4m

0.15
01cm
05m
Two meandering channels with groyne

Case B2.

Year 2013

Vegetation distribution survey was executed every year from the year 2012 to 2014. During thesethree years, we had two floods, which made almost no change of gravel bars in mid-upper reach of
Kinugawa River. As a whole, most of vegetation grew thickly in/around a U-shaped zone, open toward
the upstream direction. This U-shaped zone is mainly composed of Poaceae, and formed within acertain range of relative height to water surface of the main channel and the second channel.

Dimensions of agroyne.

Bed slope

195
0Installation of agroyne

Flow rate (m3/s)

0.15

Numerical results

Main channel

2nd channel

Vegetation pattern

Lateral distance (m)Hirokazu Ikeda and Kosuke Iimura (Utsunomiya University), Yuka Soeta (Saitama Prefectural Government),
Chie Kawashima (Tokyo Metropolitan government), and Satoshi Komura (Niigata Prefectural Government)

0.75
Longitudinal Distance(m)1.5Smoothed topography of a bar with two meandering channels.

Case A1: Meandering flow is observed along the main channel, and a flow
along the second channel, divided from the main flow, is suggested.
Case A2: A large deposition area can be seen along the right edge of themain meandering flow.
Case B1: The flow is decelerated in/around the vegetation area itself andalso the behind of the vegetation.
Case B2: Additional noticeable deposition area is spread over a wide area
in/around the U-shape, which encourage the alien species.

Case C2.

Case D2.

Velocity vector and bed elevation change contour
Case C1 & C2: The effect of removal of the bottom part of the U-shape
almost eliminates the low velocity region in the internal and the behindregion, therefore, the deposition area around the vegetation is reduced.
The influence of the main channel side of the vegetation still remains.
Case D1 & D2: The deposition area along the right bank side is a little
reduced. However, the most serious deposition areas are not at all
reduced because the low velocity areas in the internal and behindregion of the U-shaped vegetation still remain.

Conclusion
On a large-gravel bar with two meandering channels, a U-shaped
vegetation zone, open to the upstream side, was formed along the main
channel and the second channel immediately after the gravel bar
restoration. This U-shaped zone was mainly composed of Poaceae, andformed within a certain range of relative height to water surface of themain channel and the second channel.
Endemic species for gravel bed were mainly recognized around outeredge of the U-shaped zone, and alien species were distributed especiallyin the internal area of the U-shape where fine sand was deposited. It issuggested that flow resistance of the U-shaped zone affects the flood flow
structure and sediment distribution, which influences vegetation
segregation on the gravel bar.

In the present study, stable gravel bar topography with two meanderingchannels was successfully reproduced in a laboratory experiment withgroynes. This topography was used in the present numerical analyses.
Numerical results show that low velocity regions are formed in the internaland behind area of the U-shape vegetation, where coarse sediment isdeposited more than the other areas, and the deposition area in the main
channel side arm of the U-shape is merged with that along the main
channel.
Numerical results also show that removal of the bottom part of U-shapeeliminates the low velocity regions, which is most effective on the wholearea. And if the main channel side arm can be removed in addition,
significant improvement is expected.