Abstract:
Flash floods are becoming more and more common and frequent in many urban areas all
over the world. Unplanned urbanization, inadequate and poorly maintained macro and
micro drainage systems, shortage of wetlands and lack of awareness can be highlighted
as major reasons for flash floods. Nevertheless, according to climatologists there is a
significant change in precipitation patterns which cause serious distortion of rainfall
extremes due to climate variability. Today, Sri Lanka has no exception with regards to
flash floods in urban areas. Mode canal basin which is geographically located at the
heart of the city of Galle and frequently confront with flash flood has been selected as
the study area of this research. Mode canal basin consists of gently undulating plains and
low-laying flatlands with a high density of drainage paths. A rapid urbanization and
thereby alteration of catchment characteristics have been taken place within the Mode
canal basin. Recently, some research attempts had been made to investigate flash floods
in Mode canal basin through ID hydrological modelling approach. Tidal fluctuation is
also a significant parameter within the lower reaches of the mode canal. This paper
discusses the development of a 2D hydrodynamic model incorporating the tidal effect.
Precipitation data, tidal data and cross section data were collected from various sources
as the main required data sets for the development of the model. The SOBEK software
had been used in this study. Whole catchment was sub divided into twenty two sub
catchments according to the land use pattern variations and contour variations. Eighteen
number of SCS nodes were used in upstream and four number of paved nodes were used
within the downstream in the rainfall runoff model development. Seventy seven number
of cross section nodes and three boundary nodes were also incorporated within the flow
model. Initially, ID model had been developed then 2D model wasdeveloped by
coupling DEM with ID model. Subsequently, applicable flood mitigation techniques
were investigated through model output.According to the model outputs it can be
identified that canal capacity is not sufficient from Julgaha to Minuwangoda Junction.
But in front of the Sangamiththa College it can be recognised as a bottleneck within the
canal which results frequent inundation. Inadequate drainage capacity is main factor for
frequent flash floods within the whole basin. Sedimentation is also a major problem
throughout the Mode Canal. Therefore, the expected estimated canal capacity cannot be
achieved throughout the canal. In order to minimize the increment of salinity level
within Mode canal, it is highly recommended further canal deepening appropriately