Abstract

The 75-km reach of the Skeidararsandur coast of southern Iceland is a mesotidal area (spring tidal range 2 m) dominated by an accreting series of barrier spits oriented east-west and backed by wide wind-tidal flats and active braided streams. The spits average 5-10 km in length and increase from 200 m to 750 m in width in a westerly (downdrift) direction. Eolian beach ridges are absent, but the uppermost spit surfaces are covered with transverse dunes migrating in response to strong easterly and westerly winds. Sediment is provided by two major glacial-outwash stream systems that exit through a series of narrow river distributaries cut through the barriers. Major flow occurs during the summer melt season when up to 5.1 g/l of sediment is carried in suspension by currents that attain surface velocities of 3.3 m/sec. Sand carried seaward by upper-regime flow (particularly at low tide) builds river distributary deltas. Maximum recorded longshore current velocities in the surf zone of 130 cm/sec are generated both by storm waves from the SE and longer period swell from the SW. The net sediment transport direction is to the west. The beach and nearshore zones feature well developed rhythmic topography and large offshore bars. Offshore bottom profiles (PDR) indicate a large subtidal sediment bulge downdrift of the major river systems and seaward of the widest barriers. Beach profiles at 40 localities, detailed morphological mapping, together with natural cuts through the spits, show beach accretion by multiple berm growth. A generalized model based upon sedimentary structures shows that these berms, consisting of ridge (swash bar), berm-ridge, berm-top, and beach-face stratification compose a thin veneer (1-2 m) on top of fluvial sediment derived from river distributary migration and from glacier-burst floods (Jokulhlaups). These floods fragment and inundate certain portions of the coast, thereby significantly affecting the coastal geomorphology. Where floods have not occurred recently, inactive outwash plains (sandurs) and wind-tidal flats exist behind the spits and are covered by mound-like vegetated eolian dunes.

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