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Flood Runoff Estimation of Ungauged River Catchments Using Soil Conservation Service (SCS) Method

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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.