Presentation Information
[G-O-11]Low-Gradient Erosional Valleys in the Ultra-Shallow Zone (< 3 m depth) of a Wave-Dominated Lacustrine Coast: A Case Study from Shimosakahama, Lake Biwa, Japan
*Shintaro YAMASAKI1, Qian YU2 (1. Disaster Prevention Research Institute, Kyoto University, 2. Graduate School of Science, Kyoto University)
Keywords:
ultra-shallow zone,erosional valley,lateral spreading,underwater archaeology,coastal hazard
Owing to the risks of vessel grounding and severe accessibility constraints, precise geomorphological investigation of ultra-shallow coastal zones with water depths shallower than 3 m has rarely been conducted. To elucidate the formation mechanisms of underwater archaeological sites along the coast of Lake Biwa—several of which have been inferred to be earthquake-related—we performed a high-resolution bathymetric and geophysical survey along the shore off Shimosakahama, Nagahama City, Shiga Prefecture, central Japan. The survey revealed the presence of multiple shallow erosional valleys oriented perpendicular to, or slightly oblique to, the shoreline.
These valleys are characterized by widths of 100 to 500 m, lengths of approximately 300 m, and remarkably gentle longitudinal gradients of about 1°, with floors mostly covered by sand and gravel. The occurrence of such broad, low-gradient erosional features is difficult to reconcile with conventional depositional models for wave-dominated coasts characterized by arcuate shorelines, beach ridges, and buried lagoonal deposits.
Within a single valley, two separate archaeological sites interpreted as former cultivated fields were identified at different locations, indicating that the present morphology results from multiple episodes of lateral spreading rather than a single catastrophic event. Surface-wave surveys across the valley floors detected significant reductions in shear-wave velocity compared to the adjacent lacustrine shelf. On the adjacent onshore area, screw-weight sounding (SWS) tests revealed a low-N-value layer below approximately 3 m depth, interpreted as a buried lagoonal deposit. The spatial correspondence between this weak layer and the valley features strongly suggests that it has served as a shear plane for lateral spreading.
Integrating these observations, we propose that the low-gradient valleys were formed through a feedback process involving repeated liquefaction of the buried weak layer under recurrent seismic loading, cumulative lateral spreading, progressive strength degradation, and subsequent selective erosion. These low-gradient erosional valleys represent a hitherto unrecognized geomorphic element indicative of latent instability in coastal ground and carry substantial implications for coastal hazard assessment and the evaluation of ultra-shallow-water development potential.
These valleys are characterized by widths of 100 to 500 m, lengths of approximately 300 m, and remarkably gentle longitudinal gradients of about 1°, with floors mostly covered by sand and gravel. The occurrence of such broad, low-gradient erosional features is difficult to reconcile with conventional depositional models for wave-dominated coasts characterized by arcuate shorelines, beach ridges, and buried lagoonal deposits.
Within a single valley, two separate archaeological sites interpreted as former cultivated fields were identified at different locations, indicating that the present morphology results from multiple episodes of lateral spreading rather than a single catastrophic event. Surface-wave surveys across the valley floors detected significant reductions in shear-wave velocity compared to the adjacent lacustrine shelf. On the adjacent onshore area, screw-weight sounding (SWS) tests revealed a low-N-value layer below approximately 3 m depth, interpreted as a buried lagoonal deposit. The spatial correspondence between this weak layer and the valley features strongly suggests that it has served as a shear plane for lateral spreading.
Integrating these observations, we propose that the low-gradient valleys were formed through a feedback process involving repeated liquefaction of the buried weak layer under recurrent seismic loading, cumulative lateral spreading, progressive strength degradation, and subsequent selective erosion. These low-gradient erosional valleys represent a hitherto unrecognized geomorphic element indicative of latent instability in coastal ground and carry substantial implications for coastal hazard assessment and the evaluation of ultra-shallow-water development potential.
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