Flume experiments on high-water channel erosion andrevetment failure during flood
BACKGROUND To protect banks and levees from river flow’s lateral erosion, a revetment isinstalled on levee and bank slope, especially at an outer bend of a meandering channel.
In a flood event of eastern Hokkaido in August 2016, one of noticing points was that many ofrevetment failures were with high-water channel erosion behind revetments and without damages
to revetment’s foundation, as in Otohuke River, eastern Hokkaido.

PURPOSE As there are various factors in revetment failures, a detailed explanation about

revetment failures remains unclear. In this study we focused on high-water channel erosion behind arevetment during flood. By means of flume experiments and a numerical simulation, we investigate
the mechanism of the high-water channel erosion behind revetments.

FLUME EXPERIMENTS First, to replicate the erosion and revetment failure, we conducted a flume

experiment which had a meandering channel with a revetment model. The flume experiment showedcbcharacteristic high-water channel erosion along revetment’s top.
a4.6m

meander angle= 40 ° 7.6m
flow

0.4m

fixed
bed

1.2m

revetment

1.5m
1.7m

4.4m

a

overtopping

right bank

40cm

left bank

4.23cm3.23cmhigh-water
channelmosaic tile sheet as revetment model
was inserted into flume’s riverbed
low-water
channelcmain flow
2 minutes after the start

150cm

d

confluencebovertopping
confluencedmain flow
15 minutes after the start

overtopping & confluence over revetmenta : overtopping happened at left outer bend
conditionsof meandering.
3
bed slope: 1/120, bed material: silica sand (d=0.765mm), discharge: 0.005m /s,b : the flow joined at downstream area
sediment supply: 2.5*10-6m3/s, experimental period: 73 minutesc, d : some tiles of revetment model came off
bed elevation change
revetment
A: deepest scoring point
main flowB:
confluence
point
flow
2 minutes after the start• there was a remarkable
A B
erosion along back of
0.07
0.07
revetment around 2mbeforeexperiment
beforeexperiment
0.06
0.06
main flow
0.05
0.05
from downstream end
after experiment
30 minutes after the start
after experiment
0.040.04
revetment’s top
revetment’s top
0.03
0.03
• the erosion on high-water
0.02
0.02
0.01
0.01
channel was 40mm at
00-0.01 0-0.01 0
0.5
1
1.5
0.5
1
1.5
most, almost same depth
-0.02
-0.02
main flow
60 minutes after the start
-0.03
-0.03
as low-water channel
distance
fromrightbank[m]distance from right bank [m]
-0.04
-0.04
AB
channel transformation
26 minutes after the startflow velocity
the meandering channel 25 minutes after the startparticle on
revetment
revetment
overtopping flow
measurement by Particle
gradually migrated
particle on lowImage Velocimetry
downstream andwater channel
we used this measured
meandering angle
velocity [m/s]
decreased.
0.05
0.2
0.35 velocity data to check its
right-bank outline
reproducibility
bed elevation [m]bed elevation [m]8cm

CONCLUSION• Overtopping, confluence across revetment and large erosion that would cause revetment failure wereobserved in the flume experiment.
• From the result of numerical simulation, it was found that in a meandering channel there is a tendencyof fluctuation of bedload and local bedload imbalance on high-water channel.
• This tendency would lead to high-water channel erosion along revetment as observed in Otohuke River.

The 22nd IAHR-APD CONGRESS 2020

in Sapporo, Japan

levee

Graduate School of Engineering,
Hokkaido University, Sapporo, Japanflow

high-water channel erosion

revetment

revetment failure and channel erosion
in Otohuke River, Hokkaido, Japan

Jun OKUDA
Tomoko KYUKA
Toshiki IWASAKI
Yasuyuki SHIMIZU

NUMERICAL SIMULATION Next, we conducted reproduction computation by iRIC-Software’sNays2DH and checked its reproducibility with experiment’s data; bed elevation change and flow
velocity. From the computation, we investigated factors in high-water channel erosion.
experimentcomputational conditions
model: Nays2DH (model for unsteady horizontal
2D-flow, sediment transport and bedform changes)

cell size: 15mm × 12.5mm
Manning’s roughness: 0.022 for riverbed
0.014 for revetment

numerical simulation

other conditions were same as in the experiment
revetment model in this computation
cells for revetment in the flume were regarded as fixed bed

checking reproducibility
Comparing elevation change and velocitydistribution in computation with experiment’s
result, they had enough consistency to examine
details of high-water channel erosion.
600s

revetment

t= 1500 sec… area where PIV results indicatedinvestigating high-water channel erosion
•

1200s

process of the erosion development

t=600~1200sec: Overtopping flow spread on
Line-C high-water channel and local scouring happened
t=1800~2400sec: The scouring propagatedtoward upstream
t=3600sec: By the meandering channel’s
migration, main flow concentrated into thescouring area, resulting in a large erosion
along revetment’s back

1800s
2400s
3600selevation and amount of bedload-flux along Line-C
high-water channel

low-water channel

low-water channel

revetment
revetment

t=1800sec

T=3600s

distance

rapid flow returning to low-water channel
behind the revetment

As shown in a graph, bedload flux increased toward
downstream and this condition had continued until thecomputation ended.
 We considered that this tendency was a factor oflarge-scale high-water channel erosion
From flow velocity distribution in left figure, on high-waterchannel,• the overtopping’s velocity was relatively low, though thelarge-scale erosion was developing there• the maximum velocity was observed where flow
returned to low-water channel behind the revetment
 This would cause the revetment failure as in the photoof Otohuke River above.