Modeling the Slippage of Mount Merapi Cold Lahar Flow Due to the Placement of Non-Permanent Broad-Crested Weir Structures Downstream of the Pabelan Bridge

The flow pattern that occurs in rivers due to the descent of cold lava often causes damage in various areas. This phenomenon is inseparable from the volume of water mixed with volcanic material flowing through the open channel, which results from input water supply or water storage passed by cold lava. Hydrology researchers study the water mixed with volcanic eruption material that remains on higher ground. River flow is influenced by the riverbed surface profile and varying river widths. When given a certain volume with specific viscosity, a dense flow may form, which can be observed on the water surface, while the denser viscosity underneath the surface remains invisible to the naked eye.

If the runoff of cold lava material in these channels is not measured, it can lead to uncontrolled jumps. The structured form of such uncontrolled and dense jumps can be observed through the resulting flow patterns. To prevent undetected flow patterns, hydraulic measuring structures are needed, one of which includes a weir with a bottom opening—known as flow over a weir—along with surrounding bridge structures. This sluice gate is easy to operate, even manually. However, estimating water operation needs under excess water availability is not straightforward due to the effects of depth and jumps occurring around the bridge. Hydraulic jump patterns often overlook measurable aspects, which can be a constraint when there is an increase in cold lava supply mixed with rainwater during peak discharge flow events.

To address this, observations must be validated through model testing using an apparatus known as a weir, which produces jumps of water along with sediment material—an area that still requires in-depth research in modeled channels. The weir modeling and connecting bridge include instruments and various weir treatments, with adjustable pressurized water using a pump at specific discharges. To analyze structural damage height, flow pattern determination, and flow velocity, observations are made using valve openings on certain pumps. The researcher tests and measures the deposition positions of cold lava sediments, one by one, based on slope positions of specific river sections. The testing begins with valve openings of 0.5 and 0.75, reading instruments, moving the weir position from a 20 cm to 240 cm distance from the bridge. A graph of the weir test results is then generated based on bottom opening movements below the bridge, with three test repetitions for each pump opening, recording depth values, jump lengths, and travel time measured using test instruments.

The results of the weir and bridge test analysis are recommended as reference information for bridge construction planners. The results show that varying weir positions under certain conditions indicate a collapse value for the Glidik II bridge, correlating with jump length and cold lava flow speed. This research is a laboratory study using weir and bridge models. The target output is a journal article based on simulation results of flow pattern determination in a channel, by shifting the weir position relative to the bridge structure. The experiment uses various model material diameters, presented in tabular and graphical formats, particularly focusing on flow with sediment and cold lava materials in hydraulic laboratory models.

The findings are observed up to the point of bridge collapse at a depth of 4.5 cm with a ½-second valve opening at the 37th second (in model scale), equivalent to 45 meters (in empirical scale). The results will then be written into a journal article for publication in an international journal.