Nyarko, Benjamin Kofi 
Li (m? /Sec/m) is the runoff concentration at various elevations 
ZrnA is the value for flood risk areas. 
Both equations 2 and 3 were used separately to calculate the discharge that each pixel within the 
segment generated at a given elevation and a weighted pixel indicating areas that fall within the 
zone that experience flood or not. 
4.3.1 Assumptions for using the overlay operation 
The application of the arithmetic overlay method (equations 2 and 3) for the study was based on 
the following assumptions: 
1. A steady state condition exists within the variables to be used. 
2. The discharge parameter [X] only varies if total rainfall changes. 
4.4 Runoff Concentration on Elevation 
The modified rational method used to calculate discharge gave a general view of the rate at 
which sections within each catchment area produced runoff with a given rainfall event. To show 
variability of runoff over the topography, equation (2) was used for the map integration. 
Figure 1.5 Runoff Concentration on elevation 
  
  
  
  
  
  
  
ood | 
Runoff concentration at various elevation 
1 to Runoff Con. (m3/sec/m) 
23202 
| 2200.0 
Y a 2000.0 
hic 1800.0 
fa 1600.0 
an 1400.0 
phy 1200.0 
sion 1000.0 
d is 800.0 
by | 600.0 
of IS 400.0 
1193 
the D M poase u Hat 
1: 1512284. 
The resultant map (Figure 1.5) showed discharge rate over the topography at varied elevations 
within Accra and its environs. With a given discharge rate, the catchment runoff concentration 
are value would only vary with elevation. For instance, a discharge value of 155 m”/sec for Upper 
Sakumo when combined with an elevation of 1550 meters resulted in a calculated discharge of 
1705 m’/sec/m. Also an elevation of 250 meters will experience a discharge of 405 m?/Sec/m. 
Hence, any point within the 155 m?/sec discharge zone will experience the same runoff rate if 
only they are of the same elevation. Therefore, the discharge concentration values determined 
from the arithmetic map overlay decreases as elevation decreases, indicating a slow runoff rate 
that has the potential of creating a backwater effect and generating flooding. 
However, it is noted that no matter the elevation if the area contributing to discharge is large as 
in the case of the Middle Sakumo, a high runoff concentration is produced as compared with the 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 1045