Paraglacial coasts responses to glacier retreat and associated shifts in river floodplains over decadal timescales (1966-2016), Kongsfjorden, Svalbard

The aim of this paper is to quantify and map the impact of the post-LIA climate change on the coastal evolution on three glacier catchments in the Kongsfjorden area in Svalbard. Climatic data of the meteorological station of Ny-Ålesund indicate an increase in the annual mean air temperature of +4°C from 1969 to 2016 and an increase in precipitation. On the northern coast of the Brøgger Peninsula, the Austre Lovénbreen, Midtre Lovénbreen and Vestre Lovénbreen glaciers have experienced a net retreat in response to changing meteorological conditions. As a consequence of this retreat, the glaciers have disclosed a large area of 7 km² composed of terrigenous sediments which is reworked by runoff and forms coastal sandur deltas. Channel network behavior has been studied using the computation of the active floodplain width by photo-interpretation, which decreased in average from 1966 to 2010. This demonstrated a contraction of the active braided belt and a decrease in the amount of braided channels. A photo-interpretation analysis combined with acquisition of dGPS data during field work shows a mean shoreline progradation of + 0.16 m/y from 1966 to 2016, with a maximal advance of + 82 m seaward. Since 1966 coastal progradation has decreased in time with higher mean values at the beginning of the studied period and an erosional trend from 1990. The sublittoral area was studied using analog side scan sonar in 2009, 2011 and 2012. Three pro-deltas were identified and underwent an extension of 30,000 m² from 2009 to 2012. In the light of this knowledge, our main conclusion is that, by retreating, glaciers have an impact on the sediment availability and on the capacity of the fluvial system to effectively transport sediment to the shoreline. These two factors control the overall coastal evolution by regulating the sediment supply to the coastal area. The coastal zones that were fed with sediments by runoff have experienced a coastal progradation and those that lost this supply have undergone a coastal recession. Due to the contraction of proglacial floodplains, current progradation concerns restricted coastal areas.