Hydrological Study of the Nakayasu Hydrograph Method for Design of Water Retention in the JIIPE Gresik Industrial Estate

The development area of the JIIPE industrial estate could change the land use around the development site. The research area mostly has clay lithology. This development area must be protected from flood. This study aims to plan a reservoir that can process the drainage system. The data needed are geographic data, rainfall data, meteorological data, area data, and land function data. This study used the Nakayasu Hydrograph method. The Nakayasu hydrograph method parameters are time interval from the rain surface to the top of the hydrograph, period time from the center of gravity of the rain to the center of gravity of the hydrograph, period time for the hydrograph, the area of the river basin, and the length of the longest main river channel. The calculation result from this method is the peak discharge value and this calculated based on the existing and planned conditions. In this study, the calculation of the probability of discharge in the return period of 25 years is carried out. From this calculation, a hydrograph graph of the flood discharge from the existing and planned conditions is generated. The calculation for the difference between the maximum debit between of existing and planned conditions is to determine the debit that must be held every second in the storage. The generated discharge value is 18.7 m3/s. The calculation of storage volume the reservoir to control the volume need is 168,300 m3. The calculation of the storage volume based on the area of the reservoir is used as the volume of availability, which is 169,956 m3, so that the recommended water retention dimensions are obtained with an area value of 36,947 m2 and depth of water retention of 4,5 m.