Anthropogenic landscape changes cause significant disturbances to fluvial system dynamics and such is the case of the watersheds studied near the Spanish Mediterranean coast (Cartagena). Economic growth resulted in the addition of external water resources from the Tajo River (1979) as part of the National Water Plan (1933). Irrigation water has caused the water table to rise since 1979. Furthermore, water resources have boosted urban touristic expansion, industrial estates, and road infrastructures. This study presents a diagnosis of the official flood hazard maps by applying remote sensing techniques that enable the identification of (i) areas flooded during recent events; and (ii) the possible effects of anthropogenic actions on fluvial processes affecting flooding (land use and land cover change—LULCC). The flooded areas were identified from a multispectral satellite image taken by a sensor on Sentinel-2. A multi-temporal analysis of aerial photographs (1929, 1956, 1981, 2009, and 2017) showing the fluvial and anthropic environment at a detailed scale (1:25,000) was used to define the fluvial geomorphology and the main anthropic alterations on the Rebollos ephemeral stream. Official inputs from geographical information repositories about land use were also gathered (LULC). The result was compared to the official flood hazard maps (SNCZI) and this revealed floodable areas that had not been previously mapped because official maps rely only on the hydraulic method. Finally, all the recent changes that will have increased the disastrous consequences of flooding have been detected, analyzed, and mapped for the study area.

The behaviour of floods depends on two main factors: Manning's roughness coefficient and the threshold runoff, P0. In both parameters, land cover plays a vital role in the characterisation of small streams, which traditionally are altered in the Mediterranean basin. However, in the absence of an assessment protocol that optimises the geographical information stored in the official repositories, studies in these areas tend to be subjective, depending on the personal criteria of the technicians in charge of the study. In this paper a new method is proposed for the determination of these parameters, based on the integration of the Spanish national system for the mapping of flood-prone areas (SNCZI), which was designed by the Spanish Geological and Mining Institute (IGME), and the spatial data of the Spanish land cover and use information system (SIOSE database), designed by the Spanish National Geographic Institute (IGN). The methodology generates roughness data based on objective criteria and on a threshold runoff map, which can be reviewed by the technician in charge of the study, but which is based on updated, regulated and open official data. The result is a thorough hydrological and hydraulic characterisation, which has been tested in a western Mediterranean area characterised by the complexity of land use: the foothills between Sierra Helada and Sierra de la Cortina in the Municipality of Benidorm, located in the province of Alicante, Spain.