Flood Mapping and Mitigation Measures for Upper Awash River
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Date
2020-09
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Addis Ababa University
Abstract
This study focuses mainly on the flood mapping and proposes mitigation measures by
using the application of arc-Gis based hydrologic model (HEC-HMS) for estimation of
peak discharge and hydraulic modeling (HEC-RAS) for estimation of water surface profile
for different return periods. The DEM (30m*30m) of the study area was used to extract
the physical characteristic of a watershed using Geo-Spatial Hydrologic modeling
extension HEC-GeoHMS. The satellite data analysis was used in the calibration and
validation of the Awash Belo flooded area. The six indices were selected in water area
extraction from Landsat-7 Geotiff like NDWI, AWEI, NDVI, NDMI, WRI, and NDMI.
Among them, NDWI was the best fit with simulated flow and used for both calibration
and validation for events of 9/3/2001 and 8/25/2001 respectively.
Finally, different mitigation alternatives were proposed to take corrective measures for
Awash Belo flood-damaged area. The appropriate mitigation measure for the Belo flood
plain was selected based on three main criteria after their design like flood risk,
Environmental impact, and balance of the two criteria. Even if, it was difficult to decide
the efficient alternative because some alternative contradicts each other, the multi-criteria
decision analyses were used. The two-stage channel modification was best in the riskoriented
decision
but
least
in
environmentally
oriented
decisions.
The
one-stage
channel
modification
was
good
in
environmental
impact-oriented
whereas
least
preferred
in
riskoriented.
The
levee in
combination with
channel
modification
was
good in
environmental
oriented
but,
less
in
risk-oriented.
The
balance
of
all
environmental
and
flood
risk
gives
higher
priority
for
levee
construction
and less
priority
for
channel
modification.
The
peak
discharge
developed
at
upstream
flood
plain
was
82.8
and
100
m
/s for 50 and
100 years respectively. The area inundated by this peak was 500.6 ha and 505 ha for 50
and 100 years return period respectively without any protection measure. The area falls to
52.27 and 57.28 hectares when the levee construction mitigation measure was applied for
50 and 100 years return period. Scenario analysis confirms the superiority of levee
construction to other proposed alternatives with the highest rated value of 0.733 as well as
strong weakness of two-stage channel modification with the least preferred rated value of
0. Although increasing the size of the channel or decreasing its roughness can lead to a
reduction in flood level because of additional channel capacity, channel modification could
also have a negative effect such as increasing inflow velocity and stream power. Therefore,
from the different alternatives, the construction of a levee has been proposed as the best
alternative remedial measure to protect the flood from the damaged area.
Description
Keywords
HEC-HMS, HEC-RAS, HEC-GeoHMS, HEC, NDWI, AWEI, WRI