The emergence of a new lake after being hit by a tropical cyclone is a natural phenomenon that is quite rare to witness, especially in Indonesia. This is very interesting, especially when viewed in the context of Hydrogeology. Hydrogeology is a branch of applied geology where the focus of study is on the movement of groundwater and the interaction of groundwater with the rocks it passes through.
Recently, a similar natural phenomenon occurred in Kupang City, East Nusa Tenggara (NTT) Province, precisely in the Sikumana Village, Maulafa District after the Seroja tropical cyclone which occurred between April 3-5, 2021.
According to BMKG, tropical cyclone Seroja is a development of cyclone seed 99S which was detected appearing on April 2, 2021. The cyclone seed grew and became tropical cyclone Seroja on April 5, 2021 at 01.00 WIB.
Researcher and Geologist of FMIPA UI Mochammad Prahastomi M.S, S.T., M.Sc. explained, scientifically the emergence of the lake in Kupang, NTT is inseparable from the complex hydrogeological system in the limestone area. The Kupang area and its surroundings, said Tomi, are regionally composed of Pleistocene and Neogene rocks.
“Pleistocene rocks dominate the Kupang area and are composed of coral limestone. This coral limestone is what usually forms the karst landscape in the NTT area,” explained Tomi.
Furthermore, he said that the Seroja cyclone seemed to have an important role in forming the lake. In geology, this lake is often referred to as a karst lake. The recent cyclone was accompanied by an increase in the intensity of rainwater that would seep into the ground.
“This high volume of rainwater increases the erosion and dissolution process of limestone. This groundwater can appear on the surface as a spring. One of the reasons why this spring has only just appeared now is most likely related to the formation of new underground river channels that eventually connect to the surface, triggered by heavy storms and high rainfall in the area. This is reinforced by observations of a fairly high water discharge which can indicate the presence of an underground river flow that appears on the surface as a spring,” said Tomi to the FMIPA UI Public Relations team.
The lakes that are formed are usually temporary. This is controlled by the flow of groundwater that appears on the surface, so that if the subsurface water supply recedes, the water in the lake also recedes. Of course, this requires a deeper study related to the management and evaluation of groundwater potential in very complex karst areas.
Karst areas themselves have previously been found in Indonesia, for example in Padalarang and Sawarna, West Java. According to him, this karst area is very unique, one of its characteristics is the formation of caves and rivers below the ground surface.
Underground river in the Sawarna karst area (Personal documentation)
“These caves are the result of the dissolution of meteoric water that enters through cracks in the limestone. This meteoric water can dissolve limestone due to the low levels of lime saturation (CaCO3) in the meteoric water. The cracks in the limestone can enlarge over time. The long-term dissolution process is what forms caves and underground rivers in the karst environment,” said Tomi explaining to the FMIPA UI Public Relations Team.
According to him, the emergence of this phenomenon is a very appropriate momentum to conduct comprehensive research related to the emergence of these springs so that the groundwater potential in this area can be mapped properly. Cooperation and involvement between universities, ministries and local governments are the keys to success in managing groundwater in areas that are very difficult to water. In addition, it is very likely that if the New Lake in Kupang, NTT is managed and developed properly, it will become a new potential tourist attraction that presents the charm of exotic natural beauty.
limestone dissolution processes in karst areas. The source of karst lake water is indicated to come from a new path connecting the underground river and topography (orange box).
