Abstract: 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 highe...