Dadahup lowland irrigation area, known in Bahasa as Daerah Irigasi Rawa (DIR) Dadahup, is one of the Ex-PLG areas that has been now included in the food estate program. It has been experiencing low productivity, producing only 1.7 -2.9 ton/ha/year of paddy. Farming on lowland areas is often hindered by inundation during rainy season. Such problems were observed at Blok A5 of DIR Dadahup where inundation might last for a long time. Drainage was hindered since the water level in the channel, being at +1.4 to 1.6 m, was higher than the ground level whose elevations were +1.30 m for the right area and +1.00 m for the left area. Flow simulation by employing HEC-RAS is proposed to obtain a drainage system that can overcome this inundation problem. The use of gates in the secondary and tertiary channels was analyzed to improve drainage performance. The results show that by using gates and pumps can reduce the inundation area and maintain the water level in the channel according to cultivation needs. Such as in tertiary channel, the water level in the right tertiary channel can be maintained at +1.30 m ~ +1.46 m, and that in the left tertiary channel can be maintained at +1.12 m.

Environmental issue has been considered more significant in many aspects of engineering decision-making process particularly in river management. There is an increasing effort to conserve functioning of rivers for human use as well as nature, therefore environmental flow assessment has been widely developed. This paper discusses on environmental flow assessment of the Sekampung River, particularly on its middle reach. A new analytical approach based on water-sediment equations was introduced in order to determine a minimum environmental flow at the certain cross section of a river. The result of the new method was then compared with a minimum environmental flow provided by using two hydrological based methods, namely, Tennant and Flow Duration Curve Analysis (FDCA) method. The result shows that the concerned discharge provided by the water-sediment method (3.5 m 3 /s) is the smallest compare with a minimum environmental flow that is provided by both Tennant (5.7 m 3 /s) and FDCA method (4.5 m 3 /s). It is promising that the water-sediment method can be used as a simple approach on preliminary state of environmental flow assessment. The method involves not only water discharge but also its related sediment flow of the river in order to mitigate further ecological and morphological risks.

The Wonogiri Reservoir with 1.343 km2 catchment area has a major problem of sedimentation. To overcome this issue, a new spillway has been built and closure dike is being constructed to localize sediment inflow from the Keduang watershed. Study on the effect of the closure and overflow dike on the reservoir operation in flood period is necessary to evaluate the performance of flood control related to the flood risk in the downstream area of the reservoir. For this purpose, the reservoir routing simulation model was developed under two condition, namely old condition and new condition with the new spillway and closure dike. The reservoir routing simulation was conducted for three inflow hydrographs of 60 and 500 years return period, and probable maximum flood (PMF). The results show that the presence of closure dike causes the peak outflow discharge increases to 1.45%, 75.18% and 56.28% for inflow hydrograph of 60 years, 500 years return period and PMF, respectively. Furthermore, the maximum water level also increases by 0.3 m, 1.9 m and 0.9 m for those three new design floods respectively. In order to reduce the dam overtopping failure chance of the 500 years return period flood, it is recommended to operate full opening of the new spillway gate when the water level reaches elevation +135.6 m MSL.

The Indonesian government in 2020 initiated a food estate program to revitalize Ex-PLG areas in order to strengthen national food security. Dadahup lowland irrigation area (DIR Dadahup) was part of the food estate program. Water insufficiency during the dry season has been Dadahup’s problem to date. The channel’s water level during the dry season drops to +0.48 m, which is lower than the average ground level of +1.00 m and +1.30 m at the left and right banks, respectively. This causes stunted growth of the cultivated paddy. The use of gates in secondary and tertiary channels serves to maintain the water level of the channels so that it is possible to irrigate paddy fields whose elevation is +1.00 to +1.20 m. This can be achieved by placing and regulating the opening and closing of the gates with the assistance of flow simulation using the HEC-RAS program.

Dam breaks can result in flash floods which have enormous destructive power. This destructive force becomes even more significant when the dam break occurs in a dam with a large capacity. An example is the Jatigede Dam, which has a capacity of 1,060 million m3. To determine the flash flood characteristics and potential impact of the collapse of the Jatigede Dam, an analysis of the dam break was carried out using HEC-RAS software. The dam break scenario uses a Probable Maximum Flood (PMF) inflow with the partial opening of two spillway gates in the middle, which causes the dam overtopping. The Froehlich and Von Thun, and Gillette regression methods were used to defining the breach parameters. Based on the dam break analysis, the simulated flash flood of the Von Thun-Gillette method resulted in a higher velocity and lower water surface elevation than the Froehlich method. The difference in the velocity, dimension of the breach shape, water surface elevation, and discharge is caused by the breaching shape and breach formation time.

Flows that occur in rivers are usually accompanied by scouring/erosion and sedimentation/deposition processes. The scouring process that occurs can be caused by river morphological conditions and the existence of river structures that obstruct the flow. Structures such as bridge piers can change flow patterns so that in general can cause local scouring. One method for reducing the scouring that occurs around a bridge pier is to consider the shape of the pier bridge design. One of the most common pier bridge designs is the oblong shape. This research aims to find out the effect of streamlining the oblong pier shape on the change in flow characteristic and scour depth reduction using FLOW-3D numerical modeling.

The Palingkau Swamp Irrigation Scheme SP1 SP2 SP3 extends tidal irrigation between Kapuas Murung River and Kapuas River, which local peoples developed. The development of the new agricultural land behind the old tidal irrigation system creates oxidation of potential acid sulphate (FeS2) soil. The result of the leaching process polluted local’s agricultural area causes the decrease land productivity. The field observation recorded the water pH ranges between 3.9 - 5.3; 3.7 - 4.31 at the handil; 2.5 - 4 at the collector channels; and 2.8 - 3.6 at the primary channels. Good water management is needed to overcome these problems, both at the new irrigation system and handil. The HECRAS mathematical model is used to evaluate water circulation in the system and its reliability. The Efforts to normalize the handil impact increasing the amount of water entering the handil although it is not significant. Moreover, it still cannot meet the water demand for the leaching process in the network system by 2.06 million m3/day. It is necessary to increase the capacity of the channel to allow freshwater flow from the river for the leaching process in the network system.

Wonogiri Reservoir, one of the large reservoirs in Indonesia, regulates water for flood control, irrigation, domestic use, and hydropower. It is located at the upper part of Bengawan Solo River Basin, Central Java. A closure dike has recently been constructed as a countermeasure against severe sedimentation that separates the reservoir into two parts, i.e., Sediment Storage Reservoir (SSR) and Main Reservoir (MR). A two-gate spillway was constructed in the SSR to facilitate sediment flushing and sluicing. During flood events, it controls reservoir water level jointly with the MR spillway. This brings the need for an update on the reservoir operation rules, especially on the gates opening of the new spillways to maintain the flood control performance. An assessment of reservoir routing simulation results was carried out by considering the overflow from SSR to MR, or vice versa with two options of Control Water Level (CWL), i.e. +135.8 m and +136.3 m. The flood control performance of the new reservoir operation rules was evaluated by considering the reservoir water level recovery, the maximum total outflow discharge, and the dam safety due to the risk of overtopping. These assessments give a foundation for the selection of CWL and the spillway operation to ascertain an optimal flood control of the reservoir.