Role of backwash in sedimentation from tsunami on confined coastal plain on western Greenland - insights from textural properties of 2000 AD landslide-generated tsunami deposits

Abstract:

Sedimentation from tsunami on coastal plains is usually interpreted to be mainly an effect of flooding wave (runup). The role of backwash or water drainage phase in sedimentation is often local, particularly on flat wide coastal plains. However, in settings where coastal plain is more steep or the relatively narrow coastal plain is confined by steep slopes the significance of backwash phase in formation of tsunami deposits may be higher. Here we present data on sediment texture (grain size and roundness) from tsunami deposits and coastal sediments (beach, nearshore, marshes, slopes) collected in Vaigat Strait, western Greenland. This area was affected by landslide-triggered tsunami that occurred on 21st November 2000 and reached maximum reported runup height of 50 m a.s.l.. The studied sediments were collected from twelve cross-perpendicular transects. The tsunami deposits covered most of the inundation distance, which ranged from several tens of meters to over 300 m. The deposits thickness is over 30 cm thick close to the coast and in front of inland scarps. They were composed mainly of coarse sand to gravel. The analyses revealed that the major sediment source was beach and beach ridge. The grain size trends were fining landward in case of the widest coastal plain. However in narrower setting confined by steep slopes on the landward site, the deposits revealed coarsening landward, although they originated from beach and beach ridge. It may suggest that the sediments eroded by the tsunami from the beach were transported landward during the runup but not deposited. The deposition occurred during the backwash resulting in sorting of fractions.

The study was funded by Polish National Science Centre grant No. 2011/01/B/ST10/01553. The help of participants of the field expeditions is highly appreciated.