Geography Reference
In-Depth Information
However, quantifying and estimating population exposed to risk has
become necessary and this has further raised another line of study-how to esti-
mate vulnerable population to hazard with minimal error. The most popular
method is the used of density data, which basically involves sharing the popu-
lation equally across the specified areal territory.
A fundamental weakness of this procedure is that population is distributed
evenly across the various land uses of the territory. Areas with no human
settlement (such as forest, swamps and water bodies) are likewise assigned
population value. Since population is generally known to reside within an
urban structure, this chapter presented a better procedure that distribute
population according to the urban structure using information from remotely
sensed data. The maximum likelihood supervised classification data
previously generated (Figure 7b) was disaggregated into various land uses and
the urban footprint (areas covered with human settlements and other building
structures) was extracted for further analysis.
To estimate the population potentially endangered by Local Government
Area (LGA), the vectorized local administrative element represented by 20
polygons (corresponding with the LGAs of Lagos State) in ArcMap was
loaded with population data for each of the polygons. To ensure a substantial
level of accuracy in the estimation, the 2006 population census data for the 20
LGAs of the state was projected to 2013 parallel to Landsat data of 2013 from
which the urban footprint was extracted. The growth rate of 3.2 prescribed by
the national population commission was adopted for the projection.
A topological overlay of the GIS layer carrying administrative/population
data and the urban footprint layer was performed based on attributes and
features spatial join (Figure 13a). The urbanized area land use classes was
merged and then superimposed on the flood risk magnitude map using spatial
intersect. The new map so-generated captured and clipped out the urbanized
areas within the potentially exposed LGA by vulnerability class (Figure 13b).
Figure 14 reveals that though Ikorodu is the fourth largest LGA after Epe,
Badagry and Ibeju/Lekki, it has the largest settled area (over 139 km
2
) in the
state. In addition, 6 LGAs are at the verge of achieving 100 percent urban land
coverage, these are Surulere, Agege, Ajeromi/Ifelodu, Ifako/Ijaye, Mainland
and Mushin. The result shows that 15 LGAs are prone to flooding disaster in
the state and a total of 14.587 km
2
, 13.318 km
2
, 11.559 km
2
, 16.899 km
2
and
26.348 km
2
for very high, high, moderate, low and very low vulnerability
classes respectively are exposed to flood hazard (Table 5). Among the flood
prone 15 LGAs with an overall population of 8,423,846, an estimated total of
172,444 are vulnerable to the hazard.
