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The lithological properties of an area influence porosity and permeability. Such factors/properties of a rock can influence flood intensity. The geology affects the catchment runoff by formation of different permeable and impermeable rocks. The Bilate catchment is separated into four primary divisions geologically such as paleozoic sedimentary rocks, tertiary volcanic rocks, quaternary volcanic rocks, and quaternary sedimentary rocks (Figure 3 (m)). The area coverage of the surficial geology of the Bilate catchment is presented in Table 15.

Table 15

The surficial geology factor of the Bilate catchment

Surficial geologyArea (km2)Area (%)
Paleozoic sedimentary rocks 1,900.7 33.8 
Tertiary volcanic rocks 455.9 8.1 
Quaternary volcanic rocks 401.3 7.1 
Quaternary sedimentary rocks 2,867.1 51 
Total 5,625 100 
Surficial geologyArea (km2)Area (%)
Paleozoic sedimentary rocks 1,900.7 33.8 
Tertiary volcanic rocks 455.9 8.1 
Quaternary volcanic rocks 401.3 7.1 
Quaternary sedimentary rocks 2,867.1 51 
Total 5,625 100 
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Table 16

The geomorphology factor of the Bilate catchment

GeomorphologyArea (km2)Area (%)
Volcanic landforms 2,644.33 47.1 
Structural landforms 1,245 22.1 
Residual landforms 997.0 17.7 
Alluvial landforms 738.7 13.1 
Total 5,625 100 
GeomorphologyArea (km2)Area (%)
Volcanic landforms 2,644.33 47.1 
Structural landforms 1,245 22.1 
Residual landforms 997.0 17.7 
Alluvial landforms 738.7 13.1 
Total 5,625 100 
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Table 17

The population density factor of the Bilate catchment

Population density (person/km2)Area (km2)Area (%)
<52 137.1 2.4 
53–67 3,213.7 57.1 
68–115 1,210.5 21.5 
116–203 906.5 16.1 
>203 157 2.8 
Total 5,625 100 
Population density (person/km2)Area (km2)Area (%)
<52 137.1 2.4 
53–67 3,213.7 57.1 
68–115 1,210.5 21.5 
116–203 906.5 16.1 
>203 157 2.8 
Total 5,625 100 
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Table 18

The DD factor of the Bilate catchment

DDArea (km2)Area (%)
0–0.00021 804.6 14.3 
0.00022–0.0006 2,470.7 43.9 
0.00061–0.0012 2,200.7 39.1 
0.0013–0.002 100.1 1.8 
0.0021–0.0037 49.1 0.9 
Total 5,625 100 
DDArea (km2)Area (%)
0–0.00021 804.6 14.3 
0.00022–0.0006 2,470.7 43.9 
0.00061–0.0012 2,200.7 39.1 
0.0013–0.002 100.1 1.8 
0.0021–0.0037 49.1 0.9 
Total 5,625 100 
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Table 19

The TRI factor of the Bilate catchment

TRIArea (km2)Area (%)
0–0.37 891.8 15.9 
0.38–0.45 742.1 13.2 
0.46–0.52 1,790.9 31.8 
0.53–0.61 1,836.6 32.7 
0.62–0.89 362.6 6.5 
Total 5,625 100 
TRIArea (km2)Area (%)
0–0.37 891.8 15.9 
0.38–0.45 742.1 13.2 
0.46–0.52 1,790.9 31.8 
0.53–0.61 1,836.6 32.7 
0.62–0.89 362.6 6.5 
Total 5,625 100 
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Table 20

Distance from the river factor of the Bilate catchment

Distance from the river (m)Area (km2)Area (%)
0–1,520 834.7 14.8 
1,521–2,550 2,270.7 40.4 
2,551–5,010 2,219.7 39.5 
5,011–9,000 205.3 3.6 
9,001–20,000 94.8 1.7 
Total 5,625 100 
Distance from the river (m)Area (km2)Area (%)
0–1,520 834.7 14.8 
1,521–2,550 2,270.7 40.4 
2,551–5,010 2,219.7 39.5 
5,011–9,000 205.3 3.6 
9,001–20,000 94.8 1.7 
Total 5,625 100 
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Table 21

Standardization matrix

FactorsSlopeDEMSTRainfallTWIDDLULCDRSPISTIWeight
Slope 0.123 0.117 0.101 0.147 0.199 0.290 0.126 0.171 0.102 0.102 0.111 
DEM 0.123 0.117 0.101 0.147 0.133 0.174 0.126 0.171 0.102 0.102 0.112 
ST 0.123 0.117 0.101 0.147 0.199 0.058 0.063 0.057 0.102 0.102 0.093 
Rainfall 0.123 0.117 0.101 0.147 0.199 0.116 0.252 0.171 0.102 0.102 0.13 
TWI 0.041 0.059 0.034 0.049 0.066 0.058 0.042 0.171 0.102 0.102 0.098 
DD 0.025 0.039 0.101 0.073 0.066 0.058 0.025 0.057 0.102 0.102 0.08 
LULC 0.123 0.117 0.202 0.073 0.199 0.290 0.126 0.171 0.102 0.102 0.168 
DR 0.041 0.039 0.101 0.049 0.022 0.058 0.042 0.057 0.102 0.102 0.052 
SPI 0.123 0.117 0.101 0.147 0.066 0.058 0.126 0.057 0.102 0.102 0.064 
STI 0.123 0.117 0.101 0.147 0.066 0.058 0.126 0.057 0.102 0.102 0.09 
FactorsSlopeDEMSTRainfallTWIDDLULCDRSPISTIWeight
Slope 0.123 0.117 0.101 0.147 0.199 0.290 0.126 0.171 0.102 0.102 0.111 
DEM 0.123 0.117 0.101 0.147 0.133 0.174 0.126 0.171 0.102 0.102 0.112 
ST 0.123 0.117 0.101 0.147 0.199 0.058 0.063 0.057 0.102 0.102 0.093 
Rainfall 0.123 0.117 0.101 0.147 0.199 0.116 0.252 0.171 0.102 0.102 0.13 
TWI 0.041 0.059 0.034 0.049 0.066 0.058 0.042 0.171 0.102 0.102 0.098 
DD 0.025 0.039 0.101 0.073 0.066 0.058 0.025 0.057 0.102 0.102 0.08 
LULC 0.123 0.117 0.202 0.073 0.199 0.290 0.126 0.171 0.102 0.102 0.168 
DR 0.041 0.039 0.101 0.049 0.022 0.058 0.042 0.057 0.102 0.102 0.052 
SPI 0.123 0.117 0.101 0.147 0.066 0.058 0.126 0.057 0.102 0.102 0.064 
STI 0.123 0.117 0.101 0.147 0.066 0.058 0.126 0.057 0.102 0.102 0.09 
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