Identification and mapping of beach ridges in the Acheron River delta (Western Greece)

A beach ridge is a coastal depositional landform defined as a relict shore ridge that is more or less parallel with the coastline and with other landward-adjacent ridges. It is built by wave swash that may be surmounted by wind-deposited sediments. Once such a ridge becomes isolated from daily active beach processes by coastal progradation, it becomes a beach ridge. These landforms can serve as indicators of the positions of past shorelines and relative sea-levels. The older beach ridges are located further inland, and new ones are built progressively seaward. The main aim of this study is to identify and map in detail a series of beach ridges developed and preserved at the delta of the Acheron River, in Western Greece. The part of the deltaic plain north of the main river channel is characterized by a ridge and swale topography consisting of several beach ridges, which are almost parallel to the present-day shoreline of Phanari Bay. These beach ridges have accreted rapidly and gradually filled the Phanari Bay over the last 900 years. To recognize and map the beach ridges two different methodologies have been applied. The first method is based on the application of indicators on Sentinel 2 satellite images for inverting the swales since sometimes it is difficult to discriminate the ridges themselves. The Normalized Difference Vegetation Index (NDVI), a normalized difference index that describes the difference between visible and near-infrared reflectance of vegetation cover, was used to discriminate swales based on their vegetation. In addition, the Normalized Difference Water Index (NDWI), the most suitable index for mapping water bodies, was used to separate the swales from the ridges. NDWI allowed us to detect swales using satellite images taken in the winter months since topographic depressions are covered by water during this period of the year. The second methodological approach has been performed by using a 2 m cell size Digital Elevation Model (DEM) obtained from the Hellenic Cadastre. The identification of the beach ridges was mainly based on the analysis of the DEM and the application of Red Relief Image Map, a ground surface visualization methods based on the combination of three landform element layers i.e. topographic slope, positive openness and negative openness. The two crossing methods led to the construction of a preliminary map of the beach ridge system, which was verified in the field to produce the final map. The identification and mapping of the beach ridges was the first necessary step to trace the palaeogeographic evolution as well as to reconstruct the accretion history of this part of the Acheron River delta over the last 900 years.