LOSS OF STORAGE AREAS DUE TO FUTURE URBANIZATION AT UPPER RAMBAI RIVER AND ITS HYDROLOGICAL IMPACT ON RAMBAI VALLEY, PENANG, PENINSULAR MALAYSIA

Rambai Valley is a coastal floodplain located in Penang northwest coast of Peninsular Malaysia. It is undergoing substantial urbanization at present. This valley is drained by two main channels, Rambai River and Canal 4. The paddy fields of the upper section of Rambai River and Canal 4 (Permatang Rotan) are flood storage areas. They attenuate part of the peak flows that enter the flood prone central region of this valley which is extensively urbanized. This paper through statistical analyses examines the change in potential peak stages resulting from the present and future conversion of upper Rambai River paddy land to urban surfaces. The changes in potential peak stages are simulated using XP-Storm with the purpose of studying the impact of the loss of these storage areas on the downstream floodplain. Channel roughness and surface runoff flow time data were used for model calibration. Simulation results indicated that extensive loss of the paddy fields could lead to higher flood peaks to the immediate downstream sections, i.e. between 9% to 22% for 50% and 100% losses of storage area. The results also indicated that for the same percentage of storage area losses, flood peak stage increases 2.5 to 3.25 times higher for stream point located immediately downstream of the target area (i.e. 500 m away) compared to further downstream points (i.e. 3 to 6 km away) that showed no significant changes. As a whole, the results implied that the increase and propagation of peak stages downstream is not proportional (rational) to the percentage of urbanization and loss of storage areas. The impact of urbanization on peak stage is declines with increasing distance from the target areas.