Flood catastrophe continuously hit Malaysia through monsoonal flood and flash flood as well posing threat towards people, infrastructure and environment. Batu Pahat is among the district in Johor affected by flood disaster for almost every year. This study conducted to determine the hydraulic and hydrological characteristic and the ability of drainage system in Parit Besar and Jalan Pontian-Batu Pahat catchment area at Batu Pahat using HEC-RAS software. Through HEC-RAS, floodplain map was also produced to access the overview of potential floodplain areas. Survey data, geometric data and flow data were computed in HEC-RAS to produce drainage flow and cross-sectional area along the catchment area. Four (4) flow rates of 0.05284, 0.05894, 0.06910 and 0.07113 (m3/s) were utilized to identify the drainage ability to accommodate additional water. Several stations in Parit Besar catchment cannot accommodate the runoff with the minimum flow rate of 0.05284 (m3/s). Meanwhile, Jalan Pontian-Batu Pahat catchment cannot accommodate flow rates of 0.06910 and 0.07113 (m3/s). This is due to the catchment area has low elevation and insufficient drainage system. From the results, floodplain areas were identified. Modification of drainage cross-section was proposed to reduce the floodplain areas. This study is vital for better understanding of flooding and help the authorities to produce better flood management plan.

Flood is associated with the climatic change where the climate variable has detrimental impact on hydrologic cycle. Johor is among the affected states by flood catastrophe for almost every year. Over the year, Johor has experienced increasing amount of rainfall. However, studies on future rainfall pattern in Johor are still quite limited. Thus, this study was carried out to identify climate change impacts by projecting rainfall data using Statistical Downscaling Model (SDSM) and predicting flood events utilizing the Standardised Precipitation Index (SPI). Through SDSM, the rainfall data was projected in corresponding with General Circulation Models (GCM) predictor under three different representative concentration pathway (RCP) of RCP2.6, RCP4.5 and RCP8.5 that consider different emission level in future. Future observation in the upcoming 30 years for both studied stations at Kota Tinggi, Johor recorded increasing trend in the projected mean annual rainfall under three scenarios where the highest change in projection was +12.2% under RCP8.5 at Ladang Telok Sengat station. From SPI analysis, it is predicted that there will be frequent occurrence of flooding in the upcoming 30 years due to more positive SPI values (> 1) indicated by the studied stations. From the study, the authorities can come up with better mitigation plan to supervise the flood event in the future.