Hydrology and Hydraulic Analysis of Nasiri Flash Flood Disaster Event on the 1st August 2012

Abstract

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.

Review on Co-factors Triggering Flash Flood Occurrences in Indonesian Small Catchments

“…The results showed that the flash flood in the Nasiri was caused by two natural dams that collapsed at two different times. The first natural dam has a height of 7.55 meters collapsed at 09:52 AM with a peak flow of 83.58 m 3 /s, while the second natural dam has a height of 8.91 meters collapsed at 02:24 PM with a peak flow of 54.16 m 3 /s [5].…”

“…Flash floods caused by the natural dam-break occurred in the Nasiri Catchment in 2012. It caused 19 people to die and disappeared, 37 people were injured, 163 houses were severely damaged, 4,438 houses were slightly damaged, and 105,768 people were suffered and displaced [5]. In 2010, a flash flood was like a tsunami that occurred in the Wasior Catchment, in which the mud was mixed with water due to the river's overflow.…”

Contributed Indicators to Fluvial Flood Along River Basin in Urban Area of Indonesia

“…Landslides are influenced by physical factors such as slope, vegetation density, soil texture, soil permeability, soil solum thickness, and weathering of rock (selvena). Specific Indicators of Flash Floods are the following: (a) occur suddenly, in a short time, (b) large volume of water (peak discharge) (Kurniawan 2013), (c) can be associated with river dam damage due to erosion (Hidayatulloh et al 2018), (d) travel time from trigger to flood is less than 1 hour (Hidayatulloh et al 2018), (e) high rainfall (Azmeri et al 2015, Tewal et al 2018). (f ) the size of the catchment area, which affects the peak discharge, (g) vegetation density, (h) watershed slope, (i) the shape of the watershed (oval, round), a round watershed will be more prone to flooding because more water is accommodated (Savitri dan Pramono 2017), (j) soil permeability.…”

“…An example of a Flash Flood incident in Indonesia was observed on 1 Jan 2006, when a disaster occurred in the Kaliputih River, Jember, causing 80 deaths and hundreds of injuries. It was caused by extreme rain damaging a natural dam, which triggered a landslide with a distance of 12.8 km, flood arrival time was 1.7 hours, the speed varied depending on elevation, and the peak discharge was estimated at 891.53 m3/s (Kurniawan 2013, Hidayatulloh et al 2018. In 2003, a flash flood in the Bahorok Catchment killed 200 people and destroyed nearby buildings.…”