Flood Runoff Estimation of Ungauged River Catchments Using Soil Conservation Service (SCS) Method
The research in this project is to estimate the flood runoff from an ungauged catchment in eninsular Malaysia using the Soil Conservation Service (SCS) Method. The objective for the selection of the SCS Method is to have an alternative method to estimate the flood runoff from rural, and or mixed development for ungauged catchments in Penisular Malaysia. The SCS Method is able to provide the volume and the peak discharge of a flood hydrograph based on the rainfall events. Before the SCS Method can be applied to an ungauged catchment, the parameters of the SCS Method need to be calibrated from the gauged catchment(s).
The gauged catchment(s) should have similar land use and physical characteristics of the ungauged catchment. The physical characteristics of a catchment are the catchment slope, and hydrologic soil conditions. The unit hydrograph theory theory is used to calibrate the SCS Method. The calibrated parameters also known as the corrected coefficients will then be applied to SCS Method to estimate the flood runoff of the ungauged catchment. The flood hydrographs derived by using the SCS Method will then be compared with the flood hydrographs derived by using the Jabatan Pengairan dan Saliran (JPS) Hydrological Procedures (HP) No. 5, 11, and 16. The HP16 is not forurbanized catchments, HP5 and HP11 are for rural catchments condition in peninsular Malaysia.
The use of Convolution Method to generate the design direct runoff hydrographs is used in the comparison study. In the calibration process, it is difficult to obtain a good rainfall-runoff relationship for the gauged catchments. The rain-runoff relationship of a catchment is determined by the ability of rainfall stations in or within the catchment boundary to capture the rainfall events. There are many hydrological records (rainfall and runoff records) in the sample catchments that are not used in the calibration process. This is due to the reason that the poor rainfall records with respect to the runoff records where high amount of rainfall recorded in one or two rainfall stations but no records of high runoff in the streamflow station of the catchment.
Other reasons are to the missing of either rainfall or runoff data for the same period of records and difficulty of isolating single storm hydrograph event where usually the storms occurred in multiple storm type. There are limitations to the usage of the SCS Method. These limitations are due to the lack of soil maps throughout the Peninsular Malaysia for soils textural classifications, soils infiltration rates and land use of the catchment. The land use of the catchment for agricultural usage for SCS Method is classified as fallow, row crops, pasture and others. However, how is the land use of the catchment relates or equivalent to Malaysian context? For urban catchment, how is the land use been equivalent to the terrace houses? With the calibration of the correction coefficients, it is able to overcome some of the limitations as stated above.