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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