Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on a Complex Rocky Shoreline of Malta
Abstract
:1. Introduction
2. Study Site and Context
2.1. Geographical and Geological Setting
2.2. Geomorphology of the Coastal Slope
3. Materials and Methods
3.1. Field Observations and Time-Sequential Imagery
3.2. Boulder Transport Modelling
3.3. Wave Velocity Decay Model
4. Results
4.1. Boulders of Interest: Morphometric Considerations
4.2. Boulders of Interest: Field Observations and Time-Sequential Imagery
4.2.1. Boulders Emplaced by Storms 1957–2013 (Recent Movers, RM)
4.2.2. Boulders Interpreted as Potentially Tsunamigenic in Origin (Ancient Movers, AM)
4.2.3. Boulders of Indeterminate History (Indeterminate Movers, IM)
4.3. Boulders of Interest: Hydrodynamic Considerations
5. Discussion
6. Conclusions
- The study adopted a mixed methods approach for detecting and contrasting boulder histories, bringing together evidence from field survey, analysis of time-sequential imagery and hydrodynamic modelling. Each method proved valuable in generating unique insights but in complement they have enabled a degree of confidence in revealing the impacts of both contemporary storm waves and ancient extreme waves on the complex shore landscape at Żonqor.
- Tracking of boulder movements between 1957 and 2013 focused on the role of contemporary storm waves and enabled an irrefutable group of recent movers to be identified and their transport pathways to be reconstructed and compared. Out of six intervals between successive images, four reveal notable movements. Surprisingly, storm waves are rearranging boulders along the Żonqor coast at frequent intervals and with varying magnitude, with some storms merely performing tinkering work while others are quite capable of pivoting and lifting megaclasts over the Globigerina scarp, thus freeing them from their lower shoreline trap and initiating their subsequent and intermittent shunting up the platform ramp.
- The field survey provided corroborating evidence which added detail to recent mover pathways and enabled speculation of the nature of often complex movements but, inevitably, attention was drawn to the more ancient boulders, classified as such by a range of diagnostic criteria. This led to establishing the notable boulder berm as the boundary separating the shoreward contemporary platform surface, impacted by regular storm waves, from the landward ancient surface with numerous boulders exhibiting significant weathering. Close inspection of a number of the ancient movers suggests that they must have remained static for centuries in order to develop their delicate weathering features. A unique finding for this site is the landward-facing (reverse) imbrication of individuals in the boulder fields at c. 10–12 m asl. The implication is that a wave (or waves) of sufficient magnitude must have travelled further inland to higher elevations in order to create powerful enough backwash necessary to create the reverse imbrication attitudes. Therefore, there is strong evidence that the majority of the ancient movers inspected have been positioned on the landscape by either historical extreme storm waves, of greater magnitude than recent events, or they are tsunamigenic in origin.
- The hydrodynamic modelling results are offered as auxiliary evidence, acknowledging the large uncertainty associated with estimating velocities required for boulder transport/emplacement and whether waves are of sufficient power to generate them. Despite this caution, the results provide the wave run-up context both to define the realm of the recent movers and also to suggest a limited range of storm wave activity, leaving many of the ancient movers clearly immovable by events of comparable magnitude to those of the last half century or so. In addition, the modelling exercise explored the likely velocities required to overturn two ancient movers, closer to the shoreline, with somewhat inconclusive results for one of the boulders suggesting that its pivot over the Globigerina scarp theoretically could have been performed by a storm wave; the other of sufficient elevation to cautiously suggest a tsunami-driven movement.
- The result is a complex assemblage of boulders at Żonqor, with clearly defined groupings, attributable in different ways to both contemporary storm waves and ancient extreme events—a palimpsest where regular storms of varying magnitude appear to rework the distribution of boulders close to the shoreline at surprisingly frequent intervals but over long time periods the landscape becomes reset by tsunami. In light of this, a fully informed understanding of the nature and implications of impacts of such events on coastal environments needs to be developed by agencies in Malta responsible for coastal safety and management.
- Finally, the study provided a test-bed for performing a novel revised method for defining the velocity decay profile associated with wave run-up, which can be compared with existing boulder transport models for ascertaining those boulders likely to have been moved under different design wave conditions. The theoretical formulation is provided together with equations for practical application. The method requires calibration against a known boulder movement and overcomes some of the criticisms associated with converting wave velocity to wave height in the Nott approach, thus providing an alternative means of discriminating between the impact of extreme waves of both storm and tsunami origin.
Supplementary Materials
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Appendix A
Appendix A.1. Wave Velocity Decay Model
Appendix A.1.1. Theoretical Derivation
Appendix A.1.2. Equations for Practical Application
Appendix B
Appendix B.1. Characteristics of Boulders of Interest
Group | Variable | Median | Maximum | Minimum | Range |
---|---|---|---|---|---|
Recent Movers (RM) | a-axis (m) | 3.8 | 8.5 | 1.5 | 7.0 |
Box volume (m3) | 6.99 | 55.25 | 1.80 | 53.45 | |
Box mass (1000 kg) | 13.29 | 104.98 | 3.42 | 101.56 | |
Elevation (m asl) | 3.54 | 5.88 | 2.60 | 3.28 | |
Distance to shoreline (m) | 22.0 | 31.0 | 10.0 | 21.0 | |
Indeterminate Movers (IM) | a-axis (m) | 3.6 | 6.7 | 2.0 | 4.7 |
Box volume (m3) | 8.30 | 62.31 | 1.65 | 60.66 | |
Box mass (1000 kg) | 15.77 | 118.39 | 3.14 | 115.25 | |
Elevation (m asl) | 6.10 | 7.74 | 2.34 | 5.40 | |
Distance to shoreline (m) | 39 | 55 | 11 | 44 | |
Ancient Movers (AM) | a-axis (m) | 1.6 | 6.2 | 1.0 | 5.2 |
Box volume (m3) | 0.58 | 31.25 | 0.25 | 31.00 | |
Box mass (1000 kg) | 1.09 | 59.37 | 0.48 | 58.89 | |
Elevation (m asl) | 10.06 | 11.97 | 4.70 | 7.27 | |
Distance to shoreline (m) | 65 | 92 | 11 | 81 |
Appendix C
Appendix C.1. Kruskal–Wallis Test between Boulder Groups
Variable | H-Factor | p-Value | |
---|---|---|---|
Boulder Characteristics | a-axis (m) | 39.30 | <0.01 |
Box volume (m3) 1 | 15.30 | <0.01 | |
Contextual Factors | Elevation (m asl) | 28.37 | <0.01 |
Distance to shoreline (m) | 27.51 | <0.01 |
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Factor | Parameters | Derivation |
---|---|---|
Location | Distance from shoreline | Field measurement, Google Earth |
Elevation | Elevation above shoreline | Field survey, LiDAR contours |
Magnitude | Principal axes | Field measurement |
Volume | Field axial measurement | |
Mass | Field axial measurements, density observation in laboratory, supported by literature | |
Attitude | Normal/transverse to shoreline, inverted, stacked, imbricate | Direct field observation |
Age in situ | Weathering state | Direct field observation |
Hydrodynamic implications | Flow velocity and wave height at deposition | Estimated via boulder transport modelling |
Image Date | Type 2 | Recent Mover Boulder (RM) ID 1 | Moves per Interval 3 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
RM 1 | RM 2 | RM 3 | RM 4 | RM 6 | RM 7 | RM 8 | RM 9 | RM 11 | RM 12 | RM 13 | |||
17/08/1957 | AP | ⊗ | ◯ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | - |
10/11/1967 | AP | ❶ | ◯ | ⊗ | ❶ | ⊗ | ❶ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | 3 |
10/12/1978 | AP | ❷ | ❶ | ⊗ | ❷ | ⊗ | ◯ | ⊗ | ⊗ | ⊗ | ⊗ | ⊗ | 3 |
31/08/1988 | AP | ❸ | ◯ | ❶ | ◯ | ❶ | ◯ | ❶ | ❶ | ❶ | ❶ | ❶ | 8 |
04/05/1994 | AP | ◯ | ❷ | ◯ | ❸ | ❷ | ❷ | ◯ | ◯ | ◯ | ◯ | ◯ | 4 |
18/02/2002 | GE | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | 0 |
15/04/2013 | GE | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | ◯ | 0 |
Total moves: | 3 | 2 | 1 | 3 | 2 | 2 | 1 | 1 | 1 | 1 | 1 | 18 |
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Mottershead, D.N.; Soar, P.J.; Bray, M.J.; Hastewell, L.J. Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on a Complex Rocky Shoreline of Malta. Geosciences 2020, 10, 400. https://doi.org/10.3390/geosciences10100400
Mottershead DN, Soar PJ, Bray MJ, Hastewell LJ. Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on a Complex Rocky Shoreline of Malta. Geosciences. 2020; 10(10):400. https://doi.org/10.3390/geosciences10100400
Chicago/Turabian StyleMottershead, Derek. N., Philip J. Soar, Malcolm J. Bray, and Linley J. Hastewell. 2020. "Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on a Complex Rocky Shoreline of Malta" Geosciences 10, no. 10: 400. https://doi.org/10.3390/geosciences10100400