River Encroachment Induced Flood Risk Assessment: A Case of Bagmati River
Student: Sandeep Shrestha
Supervisor: Dr. Khadananda Dulal
Submitted Date:
September, 2022
Abstract
River encroachment has been increasing in recent times especially in urban and
suburban regions. Encroachment of river corridors tends to change the Land cover type
of the flood plain and thus increases the magnitude of the flood peak discharge and
flood risk. Flood risk, climate change and social change have increasingly become a
global concern, and also, vulnerabilities related to Land cover change and climate
change have, potentially, a very strong effect on catchment hydrology, floods and
damages. In most of the urban cities of Nepal, flooding’s are due to the combination of
extreme rainfall and urbanization. Flood is the most common destructive natural
hazards that occur regularly across the world. The patterns of Land cover, urban sprawl,
population growth, and other factors have an increasing impact on the environment and
thus put more elements at risk and consequently increase vulnerability of people and
their environment.
The major significance of the study is to assess the trend of encroachment of Bagmati
River section by settlements and analyze variation in flooding pattern accordingly. The
study is intended to determine the change in Land cover of Bagmati River and its flood
plains, estimate the river discharge during flooding and prepare floodplain mapping so
as to assess the impact of flood on the land that surface that has been encroached. HEC-
RAS is software that allows the modeler to model hydraulic problems using one
dimensional steady flow analysis, one and two dimensional unsteady flow analysis,
quasi – unsteady and full unsteady flow sediment transport – mobile bed modeling,
water temperature analysis, and generalized water quality modeling.
The Land cover change has been assessed for three different years of 2003, 2011 and
2022. Flood estimate for 2, 5, 10, 20, 50 and 100 years return periods are calculated by
using Gumbel's, Log Pearson and Log Normal methods among which the result obtain
from Gumbel's method has been used for the flood inundation mapping. The flow
parameters depth, velocity and inundation area increases when the return period
increases due to the increase in discharge for higher return period. The inundation area
for 20, 50 and 100 years return period are 43.24 ha, 46.52 ha and 49.55 ha.
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The assessment of hazard level is categorized with depth and the vulnerability due to
the flooding was made with regard to the Land cover type in the flood areas. The
maximum extreme hazard for 100 years return period was obtained to be 4.98 m. The
assessment of the flood area indicates that a large percentage of vulnerable area is Built-
up area. For 20, 50 and 100 years return period the inundation of build-up area are 19.75
ha, 22.59 ha and 25.21 ha respectively. The flood risk assessment was made by
combining the results of vulnerability and hazard assessment. The flood risk is
classified as low, medium and high risk zones. The section from confluence of
Bishnumati River and Balkhu is at high risk zone. The relation between changes in risk
with respect to change in Land Cover is best satisfied by a polynomial equation. The
unit increase in Land cover will increase the risk by 0.093.
Strong policy for urban planning near the Bagmati River corridor should be
implemented to reduce the effect of change in Land cover of river bank and flood risk.
It is recommended to setup flood defense in risk zones and early warning system and
also increase the resilience of the river plains to future inundations by installation of
protective structures to decrease vulnerability in the studied area.
Keywords
River encroachment, Flood, Hazard, Vulnerability, Risk