[H-QR04] Deep time perspective on the geological response to climate change

[HQR04-03] A 5000-yr Record of Storm Surge, Fluvial Flooding, and Tsunami Inundation in Dapengwan Lagoon in SW Taiwan

*Neng-Ti Yu¹, Jiun-Yee Yen², Shyh-Jeng Chyi³, Jia-Hong Chen², I-Chin Yen⁴, Liang-Chi Wang⁵ (1.National Tsing Hua University, 2.National Dong Hwa University, 3.National Kaohsiung Normal University, 4.National Central University, 5.National Chung Cheng University)

Keywords:Coastal Hazards, Storm Surge, Fluvial Flood, Tsunami Inundation, SW Taiwan, Late Holocene

Coastal hazards have been frequently documented with short recurrence in southwestern Taiwan from the last four centuries and are very likely to be boosted by ongoing global warming. New borehole coring concentrated in the largest lagoon, Dapengwan, of Taiwan in the region has been undertaken to unveil the event deposits from the Common Era and further beyond for insights into the future hazard scenario of great concern. The upper 20-m-thick lagoon fill is dominated by massive carbonaceous shelly mud with flaser lamination, mottled coloring, and rootlet structure indicative of sedimentary processes from subtidal fall-out to intertidal ebb-flood cycle and supratidal pedogenesis. The mud forms a shallowing-upward progradational succession alongside with a shifting-seaward and upward accumulation of sharp-based, massive-bedded, shelly, well-stratified sands reflecting constant beach-foreshore wave reworking. In the landward core transect, meter-thick lenses of sharp-based, fining-upward, poorly sorted, muddy gravelly sand are encountered recording fluvial flooding and/or channel avulsion. The stratigraphic scheme shows that (1) the lagoon area was a beach–shoreface nearshore that was represented by the widespread massive-bedded shelly sand from 5 to 3 kya. (2) The early-stage lagoon was established by the formation of a longshore sand shoal in the shoreface area and constantly received storm surge and fluvial flood sand depositions between the 3 and 2 kya. (3) During the latest prehistorical period prior to the 17th century, the lagoon expanded and continued to be episodically intruded by extensive sand deposition of storm washover and fluvial flood. (4) From the age of discovery, the lagoon has been gradually filled up and protected by the well-developed natural barrier island and possibly also the manmade irrigation and drainage system. As a result, the storm washover and fluvial avulsion have diminished in the shallow intertidal–supratidal waterbody. Nevertheless, a decimeter-thick, sharp-based shelly sand layer dating the late 17th–early 18th centuries is found extending landward for nearly 4 km and all over the backbarrier lagoon shores. This marine inundation deposit was attributed to tsunami process and rare preservation in the recent era when the storm surge and fluvial reworking was less active in the lagoon. As shown by the recent sea level rise and coastal failure due to current global warming in the SW Taiwan coast, it is very likely that the sand-starved Dapengwan Lagoon may soon return to its previous sand-rich form with episodic and extensive hazards of storm surges and fluvial floods.