2D Hydrodynamic Modeling of Rib River Flooding
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Date
2011-10
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Addis Ababa
Abstract
Flood occurs repeatedly in Ethiopia and cause tremendous losses in terms of property and
life, particularly in the lowland areas. The majority of flood disasters’ victims are poor people
living in nearby stretch of floodplain. Therefore, the study was carried out to perform
floodplain analysis and risk assessment of Fogera and nearby areas. This research involves
the integration of 2D Structured Mesh Generator (CCHE-MESH) and hydrodynamic
numerical model (CCHE2D model) with Geographic Information Systems (GIS) to develop a
regional model for floodplain determination and representation.
The study describes the flood extent and depth in the area for flow condition derived from the
historical flow data of the Rib River. Triangulated Irregular Network (TIN) was prepared
from shape file generated from spot elevations of the floodplain through field survey data and
the DEM of the study area in ArcGIS. Triangulated Irregular Network (TIN) is converted to
an ASC II format, which is supported by the CCHE-MESH and CCHE2D model.
Boundary condition for the inlet is specified as total discharge and discharge hydrograph for
steady and unsteady simulation respectively and for the outlet a constant water level of Lake
Tana (1788m) is specified as a downstream BC which allows for simulation of the free flow
condition at the river outlet. Initial conditions are defined based on information on gauges or
through introducing a so called warm-up that precedes the actual simulation period and
boundary conditions are generally specified from hydrometric data such as time series data of
discharge and stage or simulated discharge hydrograph.
Maps of flood Extent, Level, Depth and Velocity are developed using 100 years return period
flood. The Flood has the inundation area of 47.13km2 while the simulated flood extent has
flood depths less than 5.9m. The depth between 5.9m and 2m is located along the river
channel and on the pond which is located south of the rib river. However, the depths below
2m exist largely on the flood plain which extend 4 km and 3 km outside right bank and the
left bank of the main river respectively. The simulated water level ranges between 1798.4m
ASL at the upstream and 1788m ASL at the downstream end of the main river. The
simulation result indicates much of the flooding occur in the middle part of the river where
the flow behaviour is mainly dominated by the inflow from the Rib River. Most of the areas
around the flood plain are cultivated land with less proportion of Agro-pastoral, Marsh and
Pastoral. 85-90% of the flood inundated areas are covered by agricultural land. The
remaining 10-15% is covered by Agro-pastoral near inlet to Lake Tana.
Description
Keywords
Hydrodynamic Modeling