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EFFECT OF CLIMATE CHANGE ON EROSION PROCESS OF COHESIVE RIVERBANKS
ࠐAhmed ALY EL-DIEN, Hiroshi TAKEBAYASHI, Masaharu FUJITADPRI, Kyoto University, Ujigawa Open Laboratory, (Higashinoguchi, Shimomisu, Yokoouji Fushimi, Kyoto,612-8235, Japan)
1. Introduction
The mean annual temperature has increased by
1.0°C in Japan over the last century while it isexpected to rise by 2 to 3°C within the next 100 years1).

days with no precipitation have increased1). Based on
this fact, two patterns of rainfall are assumed as shownin Fig.2. Rainfall pattern for the current situation

As a result, precipitation and the frequency of intense
precipitation events are also projected to increase
throughout East Asia2). Summer precipitation in Japanhours. While the rainfall pattern for the future situation(Case 2) has a rainfall intensity (i) = 10 mm/hr for 120
hours. Both cases have the same volume of rainfall.

is expected to increase 17 to 19%1). There has been an
increase in the maximum amount of rainfall during theperiod 1961 to 20003).

Rainfall is assumed as uniform and continuous during
the simulation time. The initial ground water condition

Heavy rainfall is commonly known to cause failure
to slopes and the like riverbanks. Infiltration of rainwater results in reduction in soil matric suction and(Case 1) has a rainfall intensity (i) = 5 mm/hr for 240

was set up by conducting a steady state seepage
analysis under the specified hydraulic boundary headcondition.

shear strength, which eventually leads to reduction inslope stability. This reduction in slope stability has
been identified to be affected by both rainfall intensityand duration.
There are three factors controlling the riverbankfailure; 1) Undercutting by the effect of river flow. 2)
the bank geometry, and 3) soil characteristics whichvary with the saturation degree which in turn dependon rainfall intensities. Therefore, for a given river flowhydrograph, rainfall event, and bank geometry theseproperties control the evolution of factor of safety.
The objective of current paper is to investigate theFig.1 The simulated riverbank and boundary conditions

influence of current and future rainfall events(intensity and duration) on the stability of a typicalsynthetic riverbank by using numerical modeling. Thefactor of safety and the plane of failure are calculated
under different rainfall events. Progress of the porewater pressure is also calculated.
2. Problem Geometry and Hydraulic Conditions.
Hypothetical riverbank with shape, soil properties andboundary conditions as shown in Fig.1 is consideredin this study. The slope above the water level and thetop of the riverbank are assumed to be rainfall
boundaries. As a result of climate change, the numberof days with heavy precipitation and the number ofFig.2 Rainfall record measured for month of August, 2014
3. Simulation Procedure
Undercutting process by the flowing river isconsidered. The river bed is assumed rigid so thehydraulic erosion occurs at the river bank only, while$erosion GRHVQ¶W RFFXU DW WKH ULYHU EHG. Uniform flow(2) is higher than that of Case (1).

3rate is also assumed as 300 m /sec. Unsteadysaturated-unsaturated seepage flow model5) based on
the Richard equation is coupled with a slope stabilitymodel6) which is based on the strength reduction
technique. Both models
finite-element method.

arebasedonthe

4. Results and discussion
The trend of factor of safety (FOS) through time isshown by Fig.3. The factor of safety reduces
progressively in both cases because the removed
material and the eroded toe of the bank reduce thegravity forces resisting failure.

Fig.4 Seepage flow field at the end of simulation time
5. Conclusions
The four factors of 1) Rainfall intensity andduration, 2) hydraulic conditions of the river, 3) theriverbank geometry, and 4) soil characteristics arethought to control the riverbank failure.
Riverbank failure may occur due to reduction inFig.3 Variation of factor of safety through time
For Case (2), the increase in rainfall amountproduces an increase in the height of the ground watertable to levels higher than those produced from Case
(1). The riverbank in Case (2) becomes more saturated.
Saturation reduces the negative pressure (apparentcohesionሺ‫ܥ‬ሻ) in the unsaturated zone. On the otherhand, the positive pore water pressure is developed
and reduces the effective stress. In addition, saturation
reduces friction ሺ߮ሻ between soil particles andnegative pressure (suction) which in turn decreased thesoil shear strength even without any change in positive
pore water pressure in the soil.
The change of rainfall pattern as a result of climatechange has a significant effect on the stability ofriverbanks.
References1)

increases the unit weight of soil (become heavy). In
Case (2), failure occurs earlier (after 150 hours) thanthat of Case (1) (after 188 hours). It should be noticed
that the FOS decreases rapidly at the start of thecalculation because the GWT inside the bank isinitially assumed below the WSL in the river, and withthe passing time the GWT rises while the WSL
remains almost constant. The high rainfall intensity isthought to be the cause of the early failure in that case.
Seepage flow field at the end of simulation time forboth cases is shown in Fig. 4. Level of GWT in Case

2)

3)

4)

5)
6)

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