Nyarko, Benjamin Kofi
The Geographic Information System Model (Figure 1.3), adapted and modified for analysis is
the Modeling Flow based on the Relational Rule used by (Meijerink et al., 1994). The model
identifies four (4) main stages that could be used for flood risk zoning or assessment.
The first stage involves the generation of various thematic maps of the area of study, using aerial
photographs, satellite images, topographic maps and field observation and measurements to
check the accuracy of these data.
The second stage involves the incorporation of the thematic data into the Geographic
Information System Model (GISM) through digitizing and creation of attribute tables of each
theme.
Thirdly, it involves the use of arithmetic overlay operation (addition and division to help
integrate the hydrological model into the geographic information system model.
The fourth stage deals with the generation of flood risk hazards maps for the Accra area under
investigation.
Figure 1.3 MODIFIED MODELING FLOW DIAGRAM FOR RELATIONAL-RULE-BASED FLOOD
ASSESSMENT
OTHER DATA SOURCES
FIELD OBSERVATION
LABORATORY STUDIES
INTERPRETATION OF
AERIAL PHOTOS AND
SATELLITE IMAGES
THEMATIC DATA
LAND USE
SOIL
CONTOUR MAP
DRAINAGE MAP
INPUTTING DATA
INTO THE GIS
PLATFORM
THROUGH
DIGITIZING AND
IMPORTING
GENERATE
DIGITAL MAPS
FLOW DIRECTION,
DEM ETC.
GENERATION OF
ATTRIBUTE
TABLES
APPLICATION OF RULES USING
ARITHMETIC OVERLAY MODEL
- ADDITION
- DIVISON
FLOOD RISK
ZONE MAP
Source: Meijerink et al. (1994)
4 FLOOD RISK ZONES DETERMINATION
Bonell & Balek (1993) noted that in traditional hydrologic methods, estimating surface runoff
does not always use elevation/height (DEM), because derivation of elevation/height (DEM)
information involves laborious operation. Despite that, the possible combination of DEM and
discharge maps using an overlay operation method with the geographic information system
platform should lead to derivation and the understanding of spatial association between the two
which could be used to predict runoff rates and flood risk zones.
4.1 Runoff discharge of the study area
The modified rational model Viessman & Lewis, 1996; and Mannaerts, 1996 (equation 1)
presented a general step to help calculate individual discharge for each section in the entire
catchment areas (Table 1.1).
1042 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.
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