Abstract:
Land-use changes due to urbanization influence the surface runoff. Therefore, surface
runoff assessment has become crucial in urban areas, since the increment of surface
runoff leads to the occurrence of floods, soil erosion, and pollutants transportation. Rapid
urbanization is mostly being observed in cities of developing countries. Matara city is
one of such cities subjected to rapid expansion which is located in southern Sri Lanka.
However, a proper surface runoff assessment for Matara city has not been reported in the
literature. Making hypothesis that Matara city has been subjected to surface runoffrelated
issues, we evaluated the relationship between urbanization and surface runoff in
Matara city, Sri Lanka for the period from 1990 to 2020. The Geographic Information
System (GIS) based SCN-CN method was used as the tool for the study. Rainfall data,
source maps, and satellite images were obtained from primary and secondary sources. It
was observed that the urban land area has been increased from 1990 to 2020 by 31.79%,
while the land area used for forest, home garden, and water bodies have been declined
by -9.52%, -22.35%, and -2.35%, respectively. The results of the SCN-CN method
demonstrated a positive change of conversion of runoff area (33.01%) by runoff depths
(41-60 mm). However, all the other runoff intervals (0-20 mm, 21-30 mm, 31-40 mm,
and 61-70 mm) show a negative change of conversion. The transformation of runoff
volume in high runoff potential areas shows a positive change (18.61%) while low and
moderate runoff potential areas show a negative change of conversion of volume (-1.60%
and -17.01%). Therefore, it can be concluded that due to urbanization, the imperviousness
of the land increases leads to high runoff potential area. This trend will not achieve a
positive impact because a high runoff potential area leads to destructive phenomena,
such as floods and slidings. The results revealed that, change of land usage has led to
higher surface runoff over the studied period in the study area. Therefore, assessing the
surface runoff which rises with the urbanization is an essential task in the urban planning
and development.
