Surface irrigation networks in Indonesia are damaged by several factors, and sedimentation is among the most severe challenges. Sand traps play a substantial role in improving irrigation system efficiency by reducing sedimentation. There are two periods in sand trap operation: the operational and maintenance periods. Pengasih is one of the irrigation schemes implemented in the Progo Opak Serang (POS) River Basin, which has a high level of erosion. This study aimed to propose an appropriate management strategy for the Pengasih sand trap as the first barrier in irrigation network sedimentation based on mathematical modeling. The HEC-RAS simulation software was used to simulate the sand trap hydraulic behaviour. The results show that the validated Manning’s coefficient was 0.025. The optimal transport parameters were Laursen for the potential function, Exner 5 for the sorting method, and Rubey for the fall velocity method. The recommended flushing timeframe is 315 min, with a discharge of 2 m3/s. We suggest that the sand trap flushing frequency be performed twice a year, and it can be performed at the end of March and October. This coincides with the end of the first and third planting seasons of the irrigation scheme.

Rainfall-runoff transformation is carried out when the series of discharge data is limited or unavailable. One of the components of rainfall-runoff transformation is unit hydrograph, which can be derived synthetically. The selection of the representative synthetic unit hydrograph is fundamental related to the results of the further calculation. This study compared three types of synthetic unit hydrograph, that were Gama I, Nakayasu and SCS. The study was conducted in Juana Watershed, which is located in Central Java Province and composed of 52 sub-watersheds. The calculation was carried out in the control point of Sentul Weir by using HEC-HMS version 4.2.1, in the case of January 2014 flood events. The results showed that the peak discharge from Gama I, Nakayasu, and SCS synthetic unit hydrograph were 80.78 m3/s, 85.32 m3/s, and 78.89 m3/s respectively. Those results then compared with the flood mark in Sentul Weir, which was estimated 76.53 m3/s. Therefore, the SCS method was determined as the representative synthetic unit hydrograph in Juana Watershed, refers to the minimum error value of 3.08%. Then the analysis of design flood hydrograph for the 52 sub-watersheds in Juana Watershed can be approached by using the SCS synthetic unit hydrograph method.

The construction of a new CFPP in Indonesia, which was located next to three existing power plants and utilized an existing discharge channel, faced the problem of insufficient capacity of the existing discharge channel to deliver water to four power plants. The problem occurred not only because of the overcapacity of the cooling water flow proposed by the new CFPP but also because of the small size of the culvert located in the discharge channel. This paper discusses several methods to overcome this problem by enlarging the culvert area or by removing the culvert from the channel and replacing it with a bridge. A hydraulic study was investigated using the HEC-RAS software by utilizing inputs obtained from the existing channel geometry and flow measurement data. It was found that additional culverts on both sides with a size of 2 m x 4 m and 3 m x 1 m could reduce the water level by 1.12 m and 0.39 m, respectively. Meanwhile, removing the culvert provided a significant water level reduction of 1.39 m. Enlarging the culvert was chosen as the solution to the discharge channel capacity issue since removing the culvert would require temporarily closing the channel during construction and stopping the operation of the existing power plant.