The unprecedented flood event in the central Chao Phraya basin in 2011 was a catastrophic natural disaster affecting Thai society and economic sectors. During this event massive amounts of overland flow played a major role in damaging properties and environmental systems, with typical drainage systems rather ineffective in low-lying flat lands. The capability to effectively handle flood problem under these conditions is indeed a great challenge. To achieve such a goal, hydrological management schemes need to be improved based on insightful knowledge of flood hydrodynamics. In this study, we aim to gain insight into the aforementioned issue by performing and analyzing simulation of overland flow in low-lying flat lands. The flood plain of Sam-Khok and Klong Luang districts was selected as the study area and the simulation performed with an unstructured grid shallow water model together with a high-fidelity topographic data (LIDAR). The simulation results have been validated with field data (water trace height). The major hydrodynamic mechanisms such as flow patterns, flow magnitudes are characterized and the effective discharge quantified.