Flood Mapping and Modeling on Fogera Flood Plain A Case Study of Ribb River
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Date
2011-06
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Addis Ababa University
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 HEC-Hydrologic Modeling System (HECHMS)
and HEC-River Analysis System (HEC-RAS) 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 different flow conditions
derived from the historical flow data of the Ribb River. The hydrologic model is
calibrated using HEC-HMS for hourly time series data for return periods of 2, 10, 50 and
100 years. The value derived by the hourly data of the HEC-HMS is compared with
different frequency analysis methods. One dimensional hydraulic model HEC-RAS with
HEC-GeoRAS interface in co-ordination with ArcGIS was applied for the analysis.
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. Required data sets as stream centerline, banks, flow paths and cross sections
were prepared in HEC-GeoRAS thus, creating import file and imported in HEC-RAS. In
HEC-RAS, boundary condition for downstream were defined. Similarly, flood
discharges for different return periods were also inputted and steady flow analysis was
done for the results.
The result of hydrologic model by HEC-HMS shows a flow value of 91.8m3/s, 202.4
m3/s, 273.1m3/s, and 308.4m3/s for return periods of 2, 10, 50 and 100 respectively. The
result was compared with the frequency analysis using event flow values of the Ribb
River.
According to the food map, the flooded area for the return periods 2, 10, 50 and 100
years are 12.63km2, 18.63km2, 21.31km2 and 22.5km2 respectively. The classification of
flood depth area showed most of the flooding area had water depth less than 1.5m. On
the other hand 88% of agricultural land, 11.6% of agro-pastoral land and 1.36% River is
inundated by the flood.Key words: HEC-HMS, HEC-RAS, TIN, DEM, flood map
Description
Keywords
Hec-Hms;Hec-Ras; Tin; Dem; Flood Map