Climate Change and Flood Assesment : A Future towards NWP
Case Study: Sri Lanka
 Image of flood in Sri Lanka (Source:- Google images) |  Image of flood in Sri Lanka (Source:- Google images) |
|---|
Country description
Sri Lanka is in the Indian Ocean southwest of the Bay of Bengal, between latitudes 5° and 10°N, and longitudes 79° and 82°E.The Climate experienced during 12 months period in Sri Lanka can be characterized in to four climate seasons as follows.
 Figure 01. Four rain seasons in Sri Lanka (Source: - Department of Meteorology, Sri Lanka) |
|---|
Flood in Kelani River
​
•Floods are the most frequent natural disasters in Sri Lanka. Riverine Floods and Flash floods are most common in Sri Lanka.
​
• The Kelani River Basin is highly vulnerable for flooding due to its geographical setting where upper catchment is, mountainous and the lower catchment is plain with the entire catchment being situated in the wet zone receiving high high amount of annual rainfall.
 Figure 02. Topography of the Kelani river basin (Source:- Google images) |  Figure 03. Study area (Source:- Google images) |
|---|
•The Kelani River is a 145 km (90 miles) long river with a catchment area of 2,292km2. It is ranked as the fourth longest river in Sri Lanka.
​
•The River flow variation during rainy seasons (monsoons) is from 800m3/s to 1,500 m3/s generally . Because the river catchment is completely confined to the wet zone of the country, it gets high amount of annual rainfall close to around 3000mm on average.
•Colombo is the commercial capital of Sri Lanka. Major part of Colombo is located in the flood plains of Kelani River that undergoes frequent floods.
​
•Kelani river is extremely important as this is a region with a high population density and economic activities.
​
​
Reasons for flood in Kelani river
​
-
The inrease in population density in these vulnerable areas .
-
Heavy rainfall occurring over a short period of time.
-
Deforestation and improper land use.
-
The absence of scientific soil conservation practices.
-
Urbanization with the insufficient infrastructure facilities such as drainage system.
Discussion on the Analysed Data
The observed rainfall data was collected from the Department of Meteorology, Colombo.
​
Monthly average rainfall data from 2005 to 2015 are used for the following graphs (MS Excel and R were used for plotting)
 Figure 04. The graph of monthly average Rainfall in Colombo |  Figure 05. The graph of monthly average Rainfall in Colombo |
|---|
Figure 05.
The graphs of figure 04 and figure05 were plotted using monthly average rainfall data (from 2005 to 2015) which were collected from the Department of Meteorology, Sri Lanka. In 2010, Sri Lanka experienced the worst floods. These graphs clearly show that high amount of rainfall occurred in 2010 November. The reason for that is, a low-pressure area developed over the Colombo and high amount of rainfall received during a short period of time. Also in 2010 May, Colombo experienced food caused by exceptionally high pre-monsoon rainfall over a short period of time.
​
​
 Figure 06. The graph of precipitation average |
|---|
The graph of figure 06 was plotted using monthly average rainfall data from 2005 to 2015. According to this graph highest amount of rainfall can be observed in November and October. The reason for that is weather systems such as depressions and tropical cyclones are formed in Bay of Bengal during this period. Due to the influence of these weather systems the rainfall of Sri Lanka increases. Considerably high amount of rainfall can be seen during the months of April and May. During the month of April Sri Lanka experiences first inter monsoon. Due to atmospheric disturbances heavy thundershowers can be experienced during the month of April and it leads to the flash flooding in Colombo. Normally South-west monsoon of Sri Lanka begins in May. The highest amount of annual rainfall receives in this South-west monsoon period. At the beginning of South-west monsoon heavy precipitation can be experienced and it also causes food in south-west quarter of Sri Lanka especially in Colombo area.
Research methods used in flood analysis in Kelani River Basin, Sri Lanka
(source :- Report on Climate Change Adaptation Research by Mitsui Corporation)
​
The SDSM was used for long term forecasting and weather model, WRF was used for short term forecasting (2-3 days) were used to forecast rainfall. Flood discharge and inundation along the Kelani River reach below Hanwella was analyzed by the application of two-dimensional flood simulation model (FLO-2D). Inflow to the model at Hanwella, is estimated by the HECHMS model under future extreme rainfall events. The methodological framework is as following Figures.
 Figure 07. SDSM modelling process (source :- Report on Climate Change Adaptation Research) |
|---|
 Figure 08. WRF Modelling Process (source :- Report on Climate Change Adaptation Research) |
|---|
 Figure 09. HEC-HMS modelling process (source :- Report on Climate Change Adaptation Research) |
|---|
Methodological Framework for Assessment of Extreme Rainfall Events along the Kelani River Basin
 Figure 10. Methodological Framework for Assessment of Extreme Rainfall Events along the Kelani River Basin (source :- Report on Climate Change Adaptation Research) |
|---|