Flood forecasting at Wonogiri Reservoir is restricted on the availability of hydrologic data due to limited monitoring gauges. This issue triggers study of unit hydrograph modeling using Geomorphological Instantaneous Unit Hydrograph (GIUH) which is based on Geographic Information System (GIS). Analysis of physical watershed parameters was conducted on Digital Elevation Model (DEM) data using software Watershed Modeling System (WMS) 10.1 and ArcGIS. Nash model and S-curve method were used to process triangular GIUH into hourly Instantaneous Unit Hydrograph (IUH) and Unit Hydrograph (UH) and then was compared with the observed UH of Collins method. A sensitivity analysis was conducted on parameter of RL and Nash-model k. Evaluation of accuracy of the simulated GIUH runoff hydrograph was also conducted. The GIUH model generated UH with smaller peak discharge Qp, also slower and longer of tp and tb values than the observed UH. Accuracy test of the simulated GIUH runoff hydrograph using Nash-Sutcliffe Efficiency (NSE) shows that Keduang watershed gives a satisfying result, while Wiroko watershed gives less satisfactory result. The inaccuracies occur due to limited flood events used to derive the observed UH and stream tributaries that were not properly modeled based on Strahler method.
Flash flood is defined as “a flood of short duration with a relatively high peak discharge,” which leaves little time to take action to reduce property damage and the risk to life. Flash floods occur not only because of heavy rainfall but some co-factors that can trigger it. This study aims to determine the co-factors that trigger the flash flood. Observations are carried out using a descriptive-qualitative approach of five small catchments in Indonesia, namely Bahorok Catchment (Langkat, North Sumatra), Kalijompo, and Kalipakis Catchment (Jember, East Java), Nasiri Catchment (Western Seram, Maluku), Wasior Catchment (Wondama Bay, West Papua). The dominant co-factors are related to rainfall IDF, morphological characteristics (slope, channel properties, flow pattern), geological conditions (rock, soil, structure, geohydrology), catchment conditions (vegetation, land use). Flash floods generally occur due to landslides in the upstream part of the river. Debris consisting of water, rock, and tree trunks can stem the river’s flow and form natural dams. In five flash flood cases under investigation, the causes of a flash flood triggered by heavy rainfall and the morphological characteristics are 60% and 40%, respectively. The quantitative measure of each co-factor that triggers flash floods is essential for further research to identify flash flood symptoms.
Riverbeds in the Mt. Merapi area are dominated by gravel originating from Mt. Merapi. The 2010 Mt. Merapi eruption has formed a riverbed consisting of sediments of various size. Since then, the morphology of the rivers has changed dynamically, depending upon the natural and the anthropogenic factors. Most of the rivers drain into the Progo River, whereas the main river system of Progo River drains into the Indian Ocean. The dynamics of the tributaries of volcanic rivers along the Progo River are the major concern to identify the capacity of the tributaries against potential lahar flow disasters that may occur. This paper presents the results of a series of observations of several tributaries of the volcanic rivers, taking into account the natural flow phenomena as well as the sediment mining activity along the river under investigation. The investigation included the hydraulic simulation, which employed a numerical model. The results of the simulation showed that the downstream part of the Progo River is influenced by sediment from Mt. Merapi through its tributaries. These also suggest cost-effective river tributary maintenance to mitigate the risks of lahar flow disasters.
Nasiri lays in the Luhu village, Huamual district, West Seram Regency, Maluku province. Nasiri experienced in flash flood on August 1 st , 2012 which had never happened before. There was no rainfall station and water level recorder at that time. It is rather difficult to find out the cause and yet Nasiri River was only 8 meters wide. The research started with identifying base flow, soil characteristics, learning flood video record, routing the river reach, finding the nearest rainfall station, and also interviewing some peoples there. Field data area was complemented with satellite radars. AutoCAD 2007, IFAS 2.0.1.2, Geostudio 2004, ArcGIS 10.2, HEC-HMS 4.2.1, and HEC-RAS 5.0.3 were used to perform simulations of the natural river with and without precipitation calibration, and also with and without landslide dam in the river. HEC-RAS was subject to perform 2 (two) dimensional flood routing. The result was fairly satisfying. Nasiri watershed was experiencing in flash flood caused by 2 (two) landslide dams which collapsed in 2 (two) different times. The first landslide dam was 7.55 meters high which collapsed at 09:52 (UTC+9) with 83.58 m3/s of peak discharge. The second landslide dam was 8.91 meters high which collapsed at 14:24 (UTC+9) with 54.16 m 3 /s of peak discharge.
Sei Teras fishpond area was developed by the Banjar community in 2006. The conditions of the ponds in Sei Teras are still very simple, where the availability of seeds and feed is completely dependent on nature. The evaluation is based on the level of control and the production rate, while efforts for improvement are planned through controlling water quality. In the rainy season, the shrimp productivity ranges from 180 Kg/ha, while it can reach 450 kg/ha in the dry season. From the measurements results in November 2020-Mei 2021, it was noted that the salinity of the water in fishponds and canals ranged from 2 to 24.3 ppt while the water pH ranged from 6.11. to 7.87. Water quality parameters such as DO and pH have met the criteria for applying higher aquaculture technology, while other parameters such as Nitrate, Nitrite, Ammonia, TDS and Salinity require improvement and/or control. An alternative design is in the form of rearranging the fishpond system such as individual fishpond size. In addition, it is necessary to manage the fishpond irrigation network, namely by planning the main control structures separating fresh water and saline water, and rehabilitation of channels that experience sedimentation.
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