River and Floodplain Modelling for the Development of Flood Risk Map:
A Case Study of Sg. Selangor
Flood is one of the natural disasters in this country. However human activities in river basin changes flood behaviour such as increase in the flood magnitude which affects human life and properties. Structural measures to mitigate flood such as river widening and straightening, must be well understood since this kind of work may transfer the flood problem from upstream to the downstream part of the river system. Therefore an understanding of the flood behaviour is an important step in reducing the damage cause by flood. River hydrodynamic modeling using computer is able to carry out flood simulation along a river which will produce flood level along the river and floodplain.
Sg. Selangor basin was selected as the study area. This basin is located besides Klang Valley which is highly developed. It is expected that there is a potential development in the near future within this basin. The main river of Sg. Selangor is about 106 km in length with the catchment area of 1960 km2. The basin has been identified having enough and suitable information to carry out the intended research.
Among input required to make the present research successful is the river cross section, floodplain information, activities in the catchment, contour, soil types and landuse. Other information on flood record such as flood level and discharge at hidrological station within the floodplain is useful for the purpose of the calibration process.
The approach use in this research is to develop a river hydrodynamic model covering the main river and its flood plain. The river tributaries are modelled as hydrologic input from the respective subcatchments. Calibrations were carried out at several locations along the river namely downstream, middle reach of the river and in the floodplain. Flood behaviour from the model is compared with 1971 flood which is the largest flood that had occured in the basin.
The impact from land reclamation to the flood behaviour was further analysed for and area of 400 hectares along Kg Asahan. The simulation shows that water level increases from 38 mm to 128 mm at surrounding areas. Analysis between cross-section km 53 to km 67 shows that the areas flooded are 736 hectares and 889 hectares for 50 and 100 year return period respectively. The development of the flood risk map will assist the Lembaga Urus Air Selangor (LUAS), river engineer and planner in carrying out effective measures for planning and implementing of projects within Sg. Selangor river basin.