Defining beaches and their evolutionary states in estuaries

Abstract

Projected rates of global sea level rise and human pressures have increased attention to the potential for landform change in estuaries. This paper assesses the status of the fetch-limited beaches in the Tagus estuary, one of the largest estuaries in Europe, with a focus on distinguishing active beaches from inactive vegetated banks and identifying conditions under which they change state. A total of 26 beaches were identified in the inner estuary and 49 in the tributary basins on 2007 aerial photographs and compared with conditions on older photographs (1944-1958). Lengths, widths and maximum fetch distances for beaches were measured and site visits were made to determine their origins and present conditions. Beaches occur at eroding uplands or marshes or on spits extending from eroding uplands. Human-created beaches occur on spoil areas, within niches formed by structures and where vegetation is eliminated or prevented from colonizing (e.g. boat launches and recreational surfaces). Basin infilling, with increase in the elevation of low tide terraces and the formation of bars, is reducing wave energies, and some beaches are reverting to vegetated banks. Beaches that become vegetated banks because of human actions occur where use for boating or recreation is abandoned and where spits that form off spoil deposits reduce fetch distances upwind.