Boulder transport and wave height of a seventeenth century South China Sea tsunami on Penghu Islands, Taiwan
Abstract. The widespread tsunami risks in the South China Sea have diverse origins from trench megathrust to intraplate earthquake or landslide and remain poorly understood due to the scarce historical and geological records. The cliff-top paleotsunami gravels and basalt boulders on Penghu Islands in the Taiwan Strait present facies constraints on sediment transport, wave estimates from incipient motion formulas, and stratigraphic links to the probable sources. The boulders are supported by a pumice-bearing mud matrix that reflects a suspension-rich turbulent flow process and the typical rolling–saltation transport that results from bore-like waves. Calibrating for ancient sea level height and 100 year surge indicates that the storm waves that are likely to form in the shallow interisland bathymetry only enable boulder sliding–rolling and are incapable of the 2.5 m high cliff-top deposition. The estimated minimum height of tsunami waves is also insufficient and needs to add to 3.0 m high for a minimum cliff-top overflow of 0.5 m depth for terminal rolling before deposition. Coeval gravels in two other outcrops also record the time and extent of tsunami deposition, and are characterized by beach-derived bioclasts and stranded pumices, sharp base, matrix support, poor sorting, and elevation reaching above the 100 year surge. The gravels mark the local minimum wave run-ups and reach 2.4–4.0 m above sea level. The 1575–1706 radiocarbon age of the studied boulder suggests a probable tie to the disastrous 1661 earthquake in the SW Taiwan Orogen and the megathrust source in the northern Manila Trench.
Neng-Ti Yu et al.
Status: final response (author comments only)
RC1: 'Comment on nhess-2022-293', Anonymous Referee #1, 16 Feb 2023
AC1: 'Reply on RC1', Neng-Ti Yu, 22 Feb 2023
RC2: 'Reply on AC1', Anonymous Referee #1, 23 Feb 2023
- AC2: 'Reply on RC2', Neng-Ti Yu, 24 Feb 2023
RC3: 'Reply on AC1', Anonymous Referee #1, 23 Feb 2023
AC3: 'Reply on RC3', Neng-Ti Yu, 24 Feb 2023
RC4: 'Reply on AC3', Anonymous Referee #1, 24 Feb 2023
- AC5: 'Reply on RC4', Neng-Ti Yu, 19 Mar 2023
- RC4: 'Reply on AC3', Anonymous Referee #1, 24 Feb 2023
- AC3: 'Reply on RC3', Neng-Ti Yu, 24 Feb 2023
- RC2: 'Reply on AC1', Anonymous Referee #1, 23 Feb 2023
- AC1: 'Reply on RC1', Neng-Ti Yu, 22 Feb 2023
RC5: 'Comment on nhess-2022-293', Anonymous Referee #2, 10 Mar 2023
- AC4: 'Reply on RC5', Neng-Ti Yu, 13 Mar 2023
Neng-Ti Yu et al.
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Review on Boulder transport and wave height of a seventeenth-century South China Sea tsunami on Penghu Islands, Taiwan
The authors used fundamental hydrodynamic formulas to identify the wave source of cliff-top boulders in Taiwan. I found some issues with the methodology (application of the formulas) to this study and they are outlined below. This study is worth for earth-science researchers/readers and the revised manuscript can be published in the journal.
Lines 94-95: Why did you assume the wave height of the 100-year return period is 1.8 m? Have you done any probabilistic study for this assumption? Because Table 1 shows 100-year significant wave height is greater than 10 m.
Table 1 – what is “observation”? Is it the number of waves?
Nandasena et al. (2022) formulas do not calculate the minimum wave height but the minimum flow velocity to initiate boulder transport. Therefore, the first four formulas given in the manuscript cannot be referred from Nandasena et al. (2022). The given formulas have a significant difference (perhaps typos) compared to the formulas in Nandasena et al. (2022). Therefore, the authors must include a section to explain how they derived their equations based on Nandasena et al. (2022).
Line 135: Hudson formula is used for the design of armor-breakwaters against gravity waves (sea and swells). The formula was not validated for tsunamis and storms. However, Esteban et al. (2014) applied the Hudson formula to assess the damage to breakwaters by tsunamis. The authors may cite their paper to support the application of the Hudson formula in this study.
Lines 136-137: the assumption of Fr =1 and 2 for storms and tsunamis, respectively, is outdated. Because, both the high-energy events can have similar Froude numbers varying from as small as 0.5 to as high as 2.5 or more. It is difficult to predict the exact Froude number at the pre-transport location of the boulders without knowing flow characteristics (flow depth and flow velocity). Therefore, the results based on this assumption have a low scientific value. Alternatively, I suggest the authors to conclude based on flow velocities if permitted.
Table 3: Some tsunami periods are highly unrealistic. For example, 3.4 S, and 3.6 S. Tsunamis are considered long-period waves. The calculated numbers fall in short-period waves. The authors need to declare which formulas used to calculate wave period (Lorang or Barbano) and describe their results carefully following the established scientific definitions.