An insight into subterranean flow proposition around Alleppey mudbank coastal sector, Kerala, India: inferences from the subsurface profiles of Ground Penetrating Radar

The mudbank formation along south-western Indian coast has been a scintillating phenomenon during monsoon season, and the source of mud is a debatable issue till now. In the midst of many hypotheses, the subterranean passage is considered a possible one but needs to have supportive scientific evidence. This present study endeavours to demonstrate the potential use of the Ground Penetrating Radar (GPR) in generating subsurface information to locate buried paleochannels in coastal zone which could act as the conduit for transport of mud from terrestrial sources to sea. The historical maps of the late 1600 s show that the present-day Vembanad Lake was a part of the nearshore area and the coastline was nearly 60 km inside, drained by the rivers from the Western Ghats. Due to shoreline migration towards the seaside since late Holocene, the present-day landforms came to existence while the earlier channels got buried under recent sand. Nearly 500-m-long GPR subsurface (6-m depth penetration) profile was generated, and various paleochannels were traced. These buried channels were categorised based on the pattern of their occurrences. Two erosional surfaces at around 1 and 2-m were mapped, and their controls on paleochannels were derived. During phase-I, many paleochannels were in active condition and due to heavy deposition during phase-II, though these channels managed to maintain themselves, but the width of their courses got narrowed and in recent times they got filled up and buried. The pre-monsoon and monsoon GPR profiles signify the high degree of wetness characteristics of these buried channels during monsoon season and their possibility of being passages for possible transport of water charged with fine clay materials through the unconsolidated in-filled material in these buried channels. This demonstrative and first-hand subsurface information through GPR studies would further support to facilitate in consolidating the subsurface evidence for strengthening the subterranean passage concept in the mudbank area of the south-western coast of India.