VIBRATION EFFECT FROM CHECK DAM IN THE BATANG KURANJI RIVER, PADANG CITY

Up to the end of year 2012, along Batang Kuranji River - Padang had occurred several times a flash flood. In addition to the loss of material and psychological trauma to the people of city of Padang, this flood also caused a decline in the riverbed and eroded the river bed and the river embankment. The Batang Kuranji River, which has a length of more than 27.4 km, has 202 km2 of watershed in Bukit Barisan, which is suffer from damaged along its flow from upstream to downstream. The disaster in Batang Kuranji is paid the serious attention from the government, it can be seen through the construction of several check dams and 2 groundsill with costs of over 238 billion rupiah in a few years. One of the groundsill built on a river bed close to the residential area in the Pilla Tarok area was completed in 2016. However, in line with the completion of Groundsill, the community around the upper reaches experienced a disturbance in the form of vibrations felt in their homes which was reported has caused cracks in house buildings and psychological disturbance. This study was motivated by a vibration phenomenon that caused difficulty to rest and discomfort for residents who felt the vibration directly and linked it to the existence of Groundsill buildings in the Batang Kuranji River. Groundsill (some time also called check dam) is a along the river structure which is made with the aim of keeping the riverbed away from being eroded and excessive, holding sediments and river rocks not washed away by flooding. The goal is then expected by make the slope of the riverbed in a certain value so that the speed of the water flow decreases and the water level rises, so that the scouring on the river bed can be prevented. In general as in engineering plan of a groundsill, in Batang Kuranji River, the scour control structure is also considered the main things related to the stability of the structure such as piping effect, quicksand, seepage velocity / ground water flow, up-lift pressure and dynamic stability of the threshold. Unlike dams, hydraulic design of the groundsill is not overlooked and specifically planned. To follow up on the vibration phenomena after the construction of groundsill in the Batang Kuranji, a numerical and field investigation was conducted to clarify the problem. Vibration testing in the field was carried out at three points, namely at the Groundsill body, at the top of the cliff of Batang Kuranji and inside the house of residents who felt the vibrations. In the field there was also an observation of the flow of water on the surface of the upstream of the groundsill. From this field observation it was found the emergence of harmonic water waves that occurred above the Groundsill. The wave then creates harmonic vibrations due to the release of energy in the terms of waterfall in the groundsill downstream. This vibration is very likely to change and will have an amplitude and frequency approaching the natural frequency of the surrounding geological layer, it the create resonance vibrations that propagate to the homes of surrounding are. Then a numerical analysis was carried out to get an idea of the effect of vibration propagation that occurred due to water fall in the spillway in Groundsill to nearby points, namely at the crest of the river and in the vibrating houses. Based on field testing and numerical simulations, the problem can be explained in the form of a vibration phenomenon that arises after the existence of this Groundsill. This study is very useful to understand and explain the occurrence of vibration phenomena due to the existence of Groundsill such as those in the Batang Kuranji. In the future, improvements can be made to the basic design in order to eliminate the previous weaknesses so that the vibration phenomena do not arise which may results in discomfort and traumatic effect among the people who live around the of Groundsill and / or other hydraulic construction. Furthermore, a series of laboratory tests will be carried out to investigate the effect of the shape of a water constructions that involves energy release, so that it does not cause negative effects to society in general.