Resumo

RILO, Ana et al. Methodological framework for the definition and demarcation of the highest astronomical tide line in estuaries: the case of Tagus Estuary (Portugal). RGCI [online]. 2014, vol.14, n.1, pp.95-107. ISSN 1646-8872.  https://doi.org/10.5894/rgci450.

Important coastal management instruments, such as Estuary and Watershed Planning and Management Plans, require the cartographic demarcation of the boundary between the estuarine bed and fringe. This boundary has important implications on the jurisdictional areas of public entities that manage and regulate public water bodies, as well as on some aspects of the Water Framework Directive application, particularly the ecological assessment and the evaluation of human disturbance factors. Although this boundary is clearly defined by the Portuguese law as the water line at the highest astronomical tide, its practical determination is not straightforward. First, different hydromorphological conditions and anthropogenic occupations of estuarine margins can affect the position of the line. Also, maximum tidal elevations vary along the estuary. Finally, tidal effects are sometimes difficult to distinguish from other factors, from riverine and atmospheric origin, that affect the water levels. Therefore, the development of a methodology to determine this line, based on objective criteria, is both a challenge and a requirement to estuarine management. A new approach based on different criteria is proposed and illustrated in the Tagus estuary (Portugal). Four types of criteria are proposed: a) a biophysical criterion, based on the presence or absence of halophyte vegetation; b) the type of structure/elevation criterion, based on the height and permeability characteristics of existing marginal structures; c) a land use criterion; d) a legal criterion based on the Law 54/2005 of 15 November. The application of the new approach requires the collection and assessment of diverse information on the study area, including legal documents, cartographic material, aerial photographs and orthophotos (0.50 m spatial resolution, RGB and near-infrared). Other imagery sources were assessed, such as Bing Maps aerial (bird’s eye option) and Google Earth (using the historical imagery tool). Previous reports regarding the definition of this boundary in other Portuguese systems were also analyzed and taken into consideration. The criteria associated to the type of structure/elevation and to land use were supported by a recent (2011) topographic survey of the Tagus estuary marginal areas. Finally, the LANDSAT 7 ETM+ image corresponding to the highest tide level in the Tagus registered by this satellite (from 22 February 2002 at 11:03:23) was processed using the Image Analysis tool from ArcMap. The 752 composition bands were selected in order to highlight the extension of the submerge area for those tide conditions, and allowed the validation of biophysical criteria. These criteria definition was further validated by topographic field data and biophysical field observation acquired during January, August and September 2011. The biophysical criterion is based on the presence, distribution and behavior of various species of salt marsh (salt tolerant). The underlying principle is that the transition from this type of vegetation to another, with terrestrial features and little tolerance for salt water, is a reliable indicator of the boundary between the bed and the margin. This criterion was applied in estuarine beaches, cliffs, limits of salt marsh areas and waterways. When the margin is occupied by anthropogenic structures (e.g., defense adherent structures, jetties, piers, dikes, salt pans and tidal mills) the type of structure/altimeter criterion was applied. A previous analysis of extreme water levels in this estuary, considering tides and storm surges, indicates maximum water levels ranging between 2.3m above mean sea level (MSL) at the estuary mouth and 2.8 m MSL in the upper estuarine area, for a return period of 100 years. Considering that wave run-up was neglected in that study, the 3.0m (MSL) height was taken as representative of altimetry positions that are not subject to frequent flooding. Thus, impermeable structures lower than 3.0 m (MSL) are included in the bed and the structures higher than 3.0m (MSL) are in the margin. Permeable structures are always included in the estuarine bed. The land use criterion is based on the observation that there can be agriculture lands in the estuary margins that are below high tide level, due to historical human efforts to preserve those areas from estuarine flooding. Thus, these areas are included in the margin. Finally, the legal criterion is based on the paragraph 1 of article 10 of Law 54/2005 dated from 15 November, which indicates that the “mouchões” (local term that designates islands and sandbanks formed by alluvial deposition) are included in the estuarine bed.

Palavras-chave : coastal management; estuarine bed; estuarine fringe; integrated methodology.

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