Comparison between one-dimensional and two-dimensional flood modelling at Batang Lawas, Sarawak

Abstract

Flood occurrence in Malaysia can be categorized into two namely flash flood and monsoon flood. Lately, almost every year flood events were recorded within Batang Lawas catchment near Lawas town. Most of the flood events were caused due to heavy downpour within Batang Lawas catchment. Water levels from 0.15m to 1.2m from ground level were recorded and resulting in many of the local resident needs to be evacuated. The aim of this study is to investigate the flood inundation area along Batang Lawas near Lawas town and to compare between 1-D and 2-D hydraulics model. In achieving this aims, hydrologic model using HEC-HMS was developed. 1-D and 2-D HEC-RAS hydraulics model also has been used to study the river hydraulics behaviour at Lawas town. Next, flood peak design simulation was performed within Batang Lawas catchment for 2, 5, 10, 20, 50 and 100-year ARI using three different rainfall temporal patterns namely, normalized, actual storm profile and 25% intensity position. Based on the simulated design hydrograph, the river hydraulics analysis and comparison has been carried out. Last but not least, the flood inundation map for Batang Lawas near Lawas town was produced. The collected information and raw data obtained from various resources was processed to serve as an input for HEC-HMS and HEC-RAS hydrodynamics model. From the overall analysis, it was found that the simulated Tc for Batang Lawas near Lawas town and river mouth is 18 hours and 33 hours respectively. From this, two storm duration have been used which is 12-hour and 24-hour for determination of peak hydrograph under different ARI of 2, 5, 10, 20, 50 and 100-year. Three rainfall temporal patterns were used to determine the highest peak discharge. The normalized rainfall pattern with 12-hours storm duration generates the highest values of 1304.3 m3/s, 1534.3 m3/s, 1747.9 m3/s, 1958.4 m3/s, 2218.8 m3/s and 2616.2 m3/s for 2, 5, 10, 20, 50 and 100 year ARI respectively. From 1-D and 2-D HEC-RAS model simulation results, it is founded that the 1-D model gives a smaller WSE values and steeper recession slope compared to 2-D model. In comparison, 1-D HEC-RAS model only require small amount of data as compared to 2-D which require more substantial and complex data collection. Setting up the geometry also differs for both models. 1-D model is more preferable for project reduction cost as it only require cross sectional data rather than 2-D model that require terrain data for whole domain. The main difference identified during this study between pure 1-D and 2-D model is the illustration of generated flood inundation map. 2-D model produce more realistic presentation than 1-D model in terms of water particles flow path over a floodplain area. As conclusion, this study shows the comparison between 1-D and 2-D hydraulics model and generating a flood inundation map for Batang Lawas near Lawas Town area.