D17

Channel Migration Characteristics in Alluvial Meanders before and after Implementing Bank
Protection Structures: A Case Study of Lohandra River in Nepal〇Saroj KARKI・Hajime NAKAGAWA・Kenji KAWAIKE・Kazuki YAMANOI・Ashok GAUTAM

1. INTRODUCTION

2. METHODOLOGY

Every year hundreds of hectares of agricultural areas

The study area is a meandering reach of an alluvial

are lost to river bank erosion in the alluvial plains ofriver in Nepal. The approximate length of the study

Nepal which is the major food production region of thereach is 1kilometer. The methodology is divided intocountry. Therefore, it is vital to understand the channel

two steps. First, the analysis of channel geometricmigration behavior in order to appropriately plan andcharacteristics based on the field observations and theimplement the mitigation measures against river bankgoogle earth images of the period 2010-2014 (beforeerosion. In this study, an attempt has been made to

the implementation of groynes) and 2015-2019 (afteranalyze the channel migration characteristics of athe implementation of groynes).

a) 2010-3-24b) 2011-2-26c) 2013-11-24d) 2014-12-8e) 2016-10-15f) 2019-1-3

Fig 1: Chronology of the channel planform evolution.

After Groynes

Before Groynes(2014-12-8)(2013-11-24)

Fig 2: Velocity distribution before and after the groynes installation obtained from 2D Simulation.
Secondly, we perform 2D numerical simulation with

bed topography. Since the study channel don’t have any

bank erosion to understand the channel evolution

discharge measurement, the bankfull discharge waspattern and predict future evolution.

calculated based on the average channel width, depthand the channel slope.

3. PRELIMINARY RESULTS

Fig. 2 shows the depth averaged velocity distribution

Figure 1 depicts the chronology of the change inin the channel for two different period-one before andchannel planform. It can be observed that in the absence

the other after the installation of groynes. It can be seenof bank protection structures, the channel dominantly

that due to the presence of groynes the velocity on thehad a tendency to expand laterally increasing both thedownstream of the groynes increased significantly.

amplitude and the sinuosity (Fig. 1 a-d). However, after

This may be the reason for the downstream translation

the groynes were installed at the outer bank, upstreamof the channel after the groynes installation in the realof the apex for deterring bank erosion, the lateralfield.

migration ceased and the channel began to translate in
the downstream direction by eroding the banks

4. PRELIMINARY CONCLUSION AND FUTURE

downstream of the apex which can be observed fromWORKS

Fig 1 (e-f). Meanwhile, the radius of curvature of theThe preliminary analysis indicated that even if thebend which had been consistently decreasing in thebank protection structures control bank erosion at theabsence of bank protection works, increased after theinstalled location, it might shift the erosion to anotherintroduction of groynes.

locations. Therefore, in meandering channels theNext the 2D numerical simulation was conducted tocountermeasures on one bend should be complemented

investigate the hydro-dynamic characteristics of theby additional protection works on the opposite bend.

channels before and after the implementation ofgroynes. Currently we considered fixed bed with flat-In the next step, the simulation will be conductedcoupling the sediment transport and bank erosion.