Abstract:
Floods are most frequent and the most widely experienced catastrophic geologic hazards and the
cause the many damage of all natural disasters. It perhaps takes the greatest toll in terms of
human lives and property damage throughout the world. The river channel is directly related to
the quantity of water that it usually carries, if they overflow its banks, it causes the flood.
Ethiopia experiences two types of floods: flash floods and river floods. This study was carried
out in Ribb River watershed in South Gondar, Amhara Region, Ethiopia, where flood occurs on
regular basis. The main aim of this study is to assess the effect of flood risk related to Ribb River
watershed. The study used different types of data such as toposheet, ALOS PALSA DEM
(12.5m), Sentinel-2L-A sensor data, soil, and precipitation data, Gross population data; all these
data were processed with the help of GIS tools. To prepare the flood risk mapping, remote
sensing and GIS tools were used to integrate eleven parameters such as slope, elevation, soil,
rainfall intensity, distance to the main river, topographic wetness index, normalized difference
vegetation index, drainage density, lithology, land use/land cover and population density. The
analytic hierarchy process (AHP) was applied to derive factor weights using Excel with
reasonable consistency ratio (CR=0.04). In addition to analytic hierarchy process analysis and
GIS, flood frequency analysis was calculated by using Gumbel’s method for 5, 10, 50, 100 and
150 years return period and the flood magnitude is estimated about 7.50 m, 7.92m, 8.85m, 9.25m
and 9.49m respectively. The produced risk map indicates that most of the study area is highly
susceptible to flood risk. Finally from generated risk map, 20 % (300.36 Km2) in very high flood
risk level, 36 % (559.27km2) in high risk level, 38 % (588.46 km2) in moderate flood risk level,
5.5 % (71.23 km2) in low flood risk level and 0.5 % (0.24 Km2). The resultant map can aid as
baseline data for decision makers in disaster management system, better land use planning and
future preventive measures
