Analysis of multi-scale beach variability

Harshinie Karunarathna, Hajime Mase, Yasuyuki Baba

Thursday 2 july 2015

16:30 - 16:45h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Morphodynamics of estuaries and coastal areas

Parallel session: 13A. Sediment - Coast

Beach change occurs as a result of complex interactions between beach morphology and a number of dynamic sediment processes acting at a wide range of time and space scales. At short term, beaches change as a result of storms. Medium term change occurs as a result of variability of the incident wave climate resulting from local weather patterns. Long term beach change may take place as a result of global climate variability. Monthly cross shore beach profiles measured at the Ogata Wave Observation pier located in Joetsu-Ogata Coast, Japan, was analysed to investigate multi-scale beach change. The beach faces the Sea of Japan and is subjected to high energy wave conditions with strong winter/summer seasonal signature. The shoreline position seems to follow the seasonal variability of incident wave climate. The measured profiles show very significant variability where shoreward movement of the shoreline position exceeds 20 m and lowering of the beach exceeds 4 m at some cross-shore locations. During summer months, the beach profile variability mostly occurs in the sub-tidal part of the profile, however, a significant amount of upper beach change can be seen during winter months. The beach profile shape rotates between three different beach states in time; (i) concave reflective profile, (ii) profile with sub-tidal berm and (iii) gentle, dissipative profile. Empirical Orthogonal Function (EOF) analysis of the profiles show that the variability of beach profile shape is dominated by (a) upper shoreface steepening (b) sub- tidal berm development and dissipation and (c) variability of the overall profile slope, which have some longer than seasonal cyclic signatures. Comparison of temporal EOFs with climate indices such as Southern Oscillation Index and Pacific Decadal Oscillation index shows significant correlations between profile change and inter-annual to inter-decadal climate variability in the region. The impact of the results found through this analysis on coastal management practise should be noted. Understanding the timescales at which beach changes take place and the scale of variability is essential to develop sustainable coastal and flood defence management practise. Therefore, the insights to multi-scale beach dynamics provided by this study will be a useful asset to coastal engineers and managers.