Time of concentration for large catchment based on hydrodynamic modelling

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 (Tc) of the catchment. Common factors which influence Tc 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 Tc 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 Tc 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 Tc 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.