Rainfall and Flood Frequency Analysis in Pahang River Basin, Malaysia
Sammanfattning: In the past years different parts of the Pahang River basin have been affected by problems related to flooding. Many people died or were dislocated from their place, and government and private properties have been damaged causing huge impact on the country’s economy. The main reason for this catastrophe is the lack of appropriate knowledge about the river ba-sin’s hydrology. The rapid industrialization and urbanization has led to deforestation and un-planned land use altering the rainfall-runoff relationship.
In this study, three main causes of flooding in the basin are analyzed: heavy local rainfall, extreme increase in river discharge and sea wave from South China Sea. The Royal city of Pekan is located at a place where these three causes have a high probability to happen, so the analysis will mainly focus on the area around Pekan (Lower Pahang basin).
The basin is exposed to two different monsoon winds drawing moist air either from the Indian ocean, or South China Sea. It also receives local convectional rainfall during the inter-monsoon period due to high temperatures in the lowlands. The spatial variability of rainfall in the basin is caused by the complex nature of topography integrated with the monsoon sea-sons. The basin’s average annual rainfall is 2170mm.
Geographical Information System (GIS) was a used to delineate the watershed and extract terrain and physical feature of basin. Daily rainfall data of 39 year (1970-2008) from 12 me-teorological stations located predominantly at the Lower Pahang basin used to analyze the spatial and temporal variability of rainfall. Different data analysis techniques has been used including Principal Component Analysis (PCA). The first principal Component (PC1) which describes 69% of the total variance is related to the northeast monsoon season rainfall, and the Second principal Component (PC2) which describes 29% of the variance is related to the convective rainfall occurring during the inter-monsoon season due to high temperature at the lowlands. Homogeneous rainfall sectors are then determined over the area with the annual rainfall pattern and grouped in three homogenous sectors. The result shoed that mountainous areas surrounding the basin and the southeast coast of the basin which is exposed to northeast monsoon wind receive a larger amount of rainfall compared to the lowland.
The increase in the river discharge is mainly dependant on the amount of precipitation that falls on the mountainous areas where the intensity of rainfall is high and rainfall is quickly converted into runoff due to the steep slopes. River discharge increases in the monsoon sea-sons and mainly during northeast monsoon season. The flood frequency was performed using 37 years (1972-2008) of gauged records using Log-Pearson type ΙΙΙ method. It showed that there is a 20% probability for the areas downstream of Lubuk-Paku to be flooded since the river discharge exceeds the maximum safe water level once in every 5 years.
Flood routing has been performed to estimate the peak discharge at any distance downstream of Lubuk-Paku because the river passes through many physical factors that can alter the flow hydrograph. There are lateral flow from the tributary river joining the river downstream of the last gauging station (Lubuk-Paku) and runoff generated from the sub-watersheds. The result showed that the hydrograph of the river flow is attenuated by approximately 200m3/s, and 24 hours lag in time.
Flood resulted from the effect of sea waves from the South China Sea is also important to analyze particularly at the southeast coast of the basin. The waves convey energy and mo-mentum to the beach where they break. Upon breaking, the momentum is transferred to the water column resulting in longshore and onshore forces exerted on the water column and in-creases the water level in the river column which is said to be a wave setup (Robert, 2001). This helps to estimate the area of land susceptible to be flood due to the wave action. Gradu-ally varied flow equation is used to estimate the effect of wave setup and showed that the water level at Pekan will increase by 0.5m above the dangerous water level.
Depending on the radius of curvature of the river channel and the velocity of the water, when the river passes through curved channels, the water level rises towards the outer bank than the inner bank due to centrifugal force on the water. The flow velocity is also faster at the outer bank of the river than the inner bank. This leads to a greater increase in water level towards the outer bank. According to the analysis the water level at the outer bank will increase ap-proximately by an additional 0.5m in Pekan.
The results of this work are useful in flood control projects and assessment of flood character-istics of basins for best management practices such as flood protection and early warning process.
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