Geomorphological evidence from the MAPPA-Web-GIS: explanatory notes

Detecting landforms in floodplains is perhaps the most challenging activity for a geomorphologist (Castiglioni 2001). In fact the natural evolution of a floodplain tends to cancel landforms soon after they are no longer active. The Pisa Plain, in particular, was formed in sea-level rise conditions, and thus its evolution was accompanied by a constant rise of base-level. Aggradation was then combined with progradation, resulting in a progressive burial of landforms. Nevertheless those landforms that were buried by more recent alluvium, such as stream channels or marshes, leave a fingerprint which may be visible on modern topography in the form of weak undulations in the ground floor. These are normally undetectable in the field and need an extremely detailed micro-relief representation to be highlighted. Such “inherited” landforms, even if recognized, are then difficult to classify and constrain chronologically. Finally their mutual relationships are hard to assess. Long-aged settlement contributes to modify past fluvial landforms in floodplains, creating an artificial drainage network and enhancing natural topographic highs building artificial ground levels. Specific great-scale surveys are necessary to investigate floodplains geomorphological setting, and a cross-disciplinary approach is in most cases indispensable (Piovan et alii 2006). A Digital Terrain Model reproducing the topography of investigated area with a very high spatial resolution becomes fundamental for studying some landscapes of difficult interpretation as the floodplains where the original morphologies can be lost or modified by the natural environmental changes or by the human activity (Ninfo et alii 2011). Mapping landforms is the first step to perform landscape interpretation. The representation code used by Italian scholars (Servizio Geologico Nazionale, 1994) is a powerful tool that provides all the necessary information to genetically constrain landforms and to assess their mutual relationship in time and space. In this work, though, we preferred not to use this type of representation. In fact in the MAPPA Project all data (archaeological, geological and geomorphological) are included in a digital mapping instrument (the MAPPA web-GIS) which provides access to all the project results for a wide community of end-users, such as researchers, professionals, operators of local public institutions, dealing with archaeological heritage protection, environmental management, natural hazard mitigation. This tool must be simple to consult and must enable real-time queries of data. For this reason a specific legend has been worked out for the MAPPA Web GIS geomorphological map. The milestone of geomorphological maps of the Pisa Plain (Mazzanti 1994) was actually based on cross-checking information on surface lithology with evidence from aerial photography and hystoricalarchaeological data. More recent documents (e.g. Provincia di Pisa http://sit.provincia.pisa.it), improved the resolution of the data but with limited accuracy due to the lack of a suitable topographic base. In the framework of the MAPPA Project new geomorphological evidence was collected thanks to the availability of a Lidar survey and new detailed remote sensing analyses (Bini et alii 2012). The Airborne Lidar Scanning (ALS), acquiring spatially dense altimetry data set over short periods of time, allows the production of very detailed Digital Terrain Models (DTM) even in areas strongly urbanized or covered by dense vegetation. Remote sensing enables to map those features that are hardly detectable in the field due to their scarce relief energy. The nature of surface fingerprints of buried landforms could be verified thanks to the project data base. These new data were represented according to a special code worked out in order to incorporate our data in the MAPPA-Web-GIS; this code will be illustrated in the following, together with the criteria used for landforms detection