Abstract:
The study intended to determine the effect of hydrological behavior of the Nilwala
upper catchment corresponding to the flood, to prepare maps for the flood inundation
and the effect of damages to agriculture with reference to the major historical floods
and, to identify flood management and flood-disaster mitigation strategies.
Long-term historical time series data on stream flow at Bopagoda, and rainfall at
Mawarala, Panilkanda, Aninkanda and Arpthop were analyzed to discern changes in
hydrologic effects at Akuressa and Aturaliya divisional secretariat divisions, which are
frequently and considerably affected by floods. GIS package “ArcviewGIS 3.2a” was
used for calculations of flooded area and mapping. Damages and flood level data were
collected by a field survey. Flood frequency analysis and regression model were used to
predict the flood events. Changes in rainfall and runoff pattern and flow regime during
30 years were studied.
The rainfall varies from 2500mm to 4000mm with an average annual rainfall of 3386.8
mm. An increasing trend of annual rainfall is shown while runoff and runoff to rainfall
ratio are decreasing due to decreasing the base flow runoff. However, in short term,
there is a linear relationship between rainfall and runoff. Frequency analysis resulted
249.3, 332.1, 441.5, 524.3 discharges in cusec for 5 year, 10 year, 25 year and 50 year
return periods respectively. The return period of flash flood may occur by 25 to 30
years. The flood is mainly affected to paddy cultivation in the area. Future mitigation
measures should be focused on the routing high intensity storm runoff and adjust the
cultivation practices according to rainfall pattern.
