Time of concentration for large catchment based on hydrodynamic modelling

Department of Hydraulics and Hydrology, Faculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

^* Corresponding author: abdjalil.hassan@gmail.com

Abstract

Flood mitigation design requires accurate computation of discharge at any interest location to sustain the protection level. The design flood hydrograph generates from rainfall runoff model which used unit hydrograph method depends on the time of concentration (T_c) of the catchment. Common factors which influence T_c are the catchment properties including length, slope, soil properties and surface cover. However, when dealing with large catchment, more complex factors which also requires attention are the rainfall intensity, catchment wetness and initial water in the channel due to rain prior to the storm event. For large catchment, the travelling time which govern the T_c is more dominant in the channel rather than on the soil surface. Since water flowing in the river channel is unsteady and nonuniform, the use of Manning formula is inappropriate. This paper explains the application of hydrodynamic modelling approach to determine T_c for large catchment with long river channel. A hydrodynamic river model for Sg Relai, Kelantan with area of 460 km² and covering 90 km distance was developed using InfoWorks ICM. Results shown that as the rain intensity increased, the travelling time will be shortened. The traveling time also reduce when initial water level in the channel increase which indicate that T_c will reduce if the catchment already received some rainfall prior to the storm event. Based on this analysis and results, the use of hydrodynamic model as part of the rainfall runoff model is significant for large catchment to handle complex factor such as wide range of rainfall intensity, spatial effect and catchment wetness.