Indicators of a GIS-based Aerial Images Analysis of the Portuguese Northwest Coastal Dynamics

J. Pais-Barbosa, F. Veloso-Gomes, F. Taveira-Pinto

Faculty of Engineering of Porto University - FEUP, Hydraulics and Water Resources Institute - IHRH, Porto (PT)

The Portuguese northwest coast has been changing. The sea action regime is the most energetic and dynamic force acting in this area and, is therefore, its main modeller agent. This coastal zone, in general, has significant coastal erosion problems, which are clearly visible in some areas. These problems are due to both natural causes and anthropic reasons. The coastal morphodynamics is changing, over small time scales (days, weeks) to several hundred years.
The main objective of this work, in the stretch Maceda - Furadouro, near Porto, based on an aerial survey analysis and GIS tools and techniques, is to locally typify, classify and physically analyse the evolution of the coastal dynamic, in a medium and long term, as well as the hydromorphologies and hydroforms due to the wave action.
With this work, it's expected to improve the identification and typification of hydromorphologic patterns of breaking waves, sea bottom, and relation with local wave and tide regime, as well as, to improve the identification of coastal morphology patterns related with wave direction, sea bottom forms, wave heights and periods and beach profile.

Figure 1, 2 and 3 shows, for the same area in different years, the sediment drift, the limit of the sediment dispersion and some coastal forms in a small area of the Portuguese northwest coast.
Besides, in figure 1 and 3 it is possible to observe asymmetric mega cusp horns that are related with the generated rip and drift currents and oblique wave direction. In figure 1 it can be also seen small high tide cusps.
In figure 2 it is verified that mega cusp horns are smoother and smaller than in figure 1 and 3. That could be related to the reduction of rip and drift currents strength, to the wave obliquity and specially with the tide level, since that photographs have different tide levels.

Figure 1 - Aerial photograph of 29th September 1996, 12.35 hours (source: Water Institute)

This way and according to the six beach transition states recognized by the Australian School of beach morphodynamics, (Wright and Short, 1983) referred by Carter, 1995, this beach area could be classified according with figure 1 and 3 as an intermediate rhythmic bar beach and according with figure 2 an intermediate longshore bar-trough.

Figure 2 - Aerial photograph of 17th September 2001 (source: Water Institute) Figure 3 - Aerial oblique photograph of 13th August 2002, 12.00 hours (source: F. Piqueiro)

References:

Carter, R. W. G., 1995 - "Coastal Environments - An Introduction to the Physical, Ecological and Cultural Systems of Coastlines". Academic Press Limited. London, UK.