The comparative study of peak discharge at Ngotok watershed by using the method of SCS, Snyder and Nakayasu for flood control needs

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Flood problems often hit the Ngotok River watershed every year. The problem of flooding is generally caused by changes in land use and a decrease in river functions in the Ngotok River watershed area as well as frequent back water from rivers which empties into Brantas River when experiencing maximum discharge. For that reason, a comparative study of peak flood discharge needs to be done because there is no research on flood control. So that research is conducted on the analysis of synthetic unit hydrograph models. The synthetic unit hydrograph method used is SCS, Snyder, and Nakayasu. Rainfall data used is data from 1998-2016 from 14 rain stations in the Ngotok River watershed. The Thiessen polygon method is used to determine the amount of rain scattered in the Ngotok River watershed. The average rainfall in the Ngotok River watershed in the period 1998-2016 was 97.05 mm. In the modeling phase, the sub-catchment division of the watershed is carried out by dividing it into 5 sub-catchments. The modeling results using the SCS, Snyder, and Nakayasu methods show the amount of discharge for the return period of 2 years, 5 years, 10 years, 20 years, 25 years, 50 years, 100 years, and 200 years which varies. AWLR data approaching the modeling results are 2014 data. The hydrograph calibration results for the SCS method with a 25 year return period are 0.88, for the Snyder method with a 25 year return period of 0.74, and the Nakayasu method with a 25 year return period of 0.43. Thus the SCS hydrograph approaches the existing hydrograph model based on the AWLR data that exists and is following the observations during the population survey.