Reconciling Storegga tsunami sedimentation patterns with modelled wave heights : a discussion from the Shetland Isles field laboratory.

The Shetland Isles represent an ideal field laboratory for tsunami geoscience research. This is due to the widespread preservation of Holocene tsunami sediments in coastal peat deposits. This study uses published accounts of the Holocene Storegga Slide tsunami to illustrate how two different approaches – mapping of tsunami sediments and numerical modelling – produce radically different run‐up heights. The Storegga Slide is one of the world largest submarine slides and took place ca 8150 cal yr bp on the continental slope west of Norway. The tsunami generated by the landslide deposited locally extensive sheets of marine sand and gravel, as well as redeposited clasts of peat across the contemporary land surface. These sediment accumulations have subsequently been buried by peat growth during the Holocene while exposures of the deposits are locally visible in coastal cliff sections. In several areas, the tsunami sediments can be traced upslope and inland within the peat as tapering sediment wedges up to maximum altitudes of between ca 8·1 m and 11·8 m above present sea level. Since reconstructions of palaeo‐sea level for Shetland for ca 8150 cal yr bp suggest an altitude of 20 m below high tide on the day that the tsunami struck, it has been inferred that the minimum tsunami run‐up was locally between 28·1 m (8·1 + 20 m) and 31·8 m (11·8 + 20 m). However, numerical models of the tsunami for Shetland suggest that the wave height may only have reached a highest altitude in the order of +13 m above sea level on the day the tsunami took place. In this paper a description is given of the sedimentary evidence for tsunami run‐up in the Shetland Isles. This is followed by an evaluation of where the palaeo‐sea level was located when the tsunami occurred. Significant differences are highlighted in tsunami inundation estimates between those based on the observed (geological) data and the theoretically‐modelled calculations. This example from the Shetland Isles may have global significance since it exemplifies how two different approaches to the reconstruction of tsunami inundation at the coast can produce radically different results with modelled wave height at the coast being considerably less than the geological estimates of tsunami run‐up

Skredfarevurdering for ny fjøs på Øvre Ljøsne, Lærdal kommune

Den nye fjøsen til Jens Reidar Ljøsne ligg utanfor faresona for skred med årleg sannsyn på 1/1000. Flaum- og jordskred har lengst rekkevidde i området. I august i 2003 gjekk eit flaumskred utover terrassen på Øvre Ljøsne, og det kom vatn og slam ned terrasseskråninga mot tomta. Me finn at utlaupslengda for flaum- og jordskred ikkje vil kunna nå ned til fjøstomta. Skredmassane vil verta avsette øvst oppe på terrassen. Men vatn og slam frå flaumskreda vil kunna renna til terrassekanten og vidare ned i ravina og ned til vegen (E16), og vatn kan teoretisk nå ned til fjøstomta. Terrenget der fjøsen vert bygt bør difor hevast slik at det vert fall ned mot elva. På den måten vil flaumskredvatn frå terrassen over ikkje kunna renna inn i fjøsen, men drenera ned til elva

Propagation of the Storegga tsunami into ice-free lakes along the southern shores of the Barents Sea

There is clear evidence that the Storegga tsunami, triggered by the giant Storegga slide offshore western Norway 8100-8200 years ago, propagated into the Barents Sea. Cores from five coastal lakes along the coast of Finnmark in northern Norway reveal major erosion and deposition from the inundation of the tsunami. The deposits rest on a distinct erosional unconformity and consist of graded sand layers and re-deposited organic remains. Some of the organic remains are rip-up clasts of lake mud, peat and soil and suggest strong erosion of the lake floor and neighbouring land. In this part of the Arctic coastal lakes are usually covered by > 1 m of solid lake ice in the winter season. The significant erosion and deposition of rip-up clasts indicate that the lakes were ice free and that the ground was probably not frozen. We suggest that the Storegga slide and tsunami event happened sometime in the summer season; between April and October. Minimum run-up has been reconstructed to 3-4 m

Groundwater fluctuations during a debris flow event in western Norway - triggered by rain and snowmelt

Pore pressure is crucial in triggering debris slides and flows. Here we present measurements of groundwater pore pressure and temperature recorded by a piezometer 1.6 m below the surface on a slope susceptible to debris flows in western Norway. One of the largest oscillations in data collected over 4 years coincided with a debris flow event on the slope that occurred during storm Hilde on 15–16 November 2013. More than 100 landslides were registered during the storm. Precipitation totaled about 80–100 mm in 24 h, locally up to 129 mm, and an additional trigger factor for the landslides was a rapid rise in air temperature that caused snowmelt. In the studied slope a fraction of the precipitation first fell as snow. On 15 November, the groundwater level in the hillslope rose by 10 cm/h and reached 44 cm below the surface. At the same time, air temperature rose from 0 ∘C to over 8 ∘C, and the groundwater temperature dropped by 1.5 ∘C. The debris flow probably occurred late in the evening of 15 November, when the groundwater level reached its peak. Measurements of the groundwater in the hillslope in the period 2010–2013 show that the event in 2013 was not exceptional. Storm Dagmar on 25–26 December 2011 caused a similar rise in groundwater level but did not trigger any failures. The data suggest that during heavy rainstorms the slope is in a critical state for a landslide to be triggered for a short time – about 4–5 h

Calendar year age estimates of Allerød - Younger Dryas sea-level oscillations at Os, western Norway

A detailed shoreline displacement curve documents the Younger Dryas transgression in western Norway. The relative sea-level rise was more than 9m in an area which subsequently experienced an emergence of almost 60 m. The sea-level curve is based on the stratigraphy of six isolation basins with bedrock thresholds. Effort has been made to establish an accurate chronology using a calendar year time-scale by 14C wiggle matching and the use of time synchronic markers (the Vedde Ash Bed and the post-glacial rise in Betula (birch) pollen). The sea-level curve demonstrates that the Younger Dryas transgression started close to the Allerød–Younger Dryas transition and that the high stand was reached only 200 yr before the Younger Dryas–Holocene boundary. The sea level remained at the high stand for about 300 yr and 100 yr into Holocene it started to fall rapidly. The peak of the Younger Dryas transgression occurred simultaneously with the maximum extent of the ice-sheet readvance in the area. Our results support earlier geophysical modelling concluding a causal relationship between the Younger Dryas glacier advance and Younger Dryas transgression in western Norway. We argue that the sea-level curve indicates that the Younger Dryas glacial advance started in the late Allerød or close to the Allerød–Younger Dryas transition

The start of a major sea-level rise indicates that icesheet expansion in western Norway commenced before the Younger Dryas

The relative sea level rose 10 m on Sotra, western Norway, during the Younger Dryas (YD). Based on dating of isolation basins, the transgression started in late Allerød at ~13 110 and ended in late YD at ~11 780 cal yr BP, with an average sea-level rise of ~7 mm yr-1. Between ~11 780 and ~11 560 cal yr BP the relative sea level was stable. Shortly after the YD/Holocene boundary the sea level rapidly fell 37 m at ~23 mm yr-1. The shorelines for the regression minimum in Allerød and the transgression maximum in YD are almost parallel with tilt of 1.2-1.4 m km-1, indicating that no isostatic tilting, and thus neither uplift or depression occurred during the sea-level rise. We conclude that the transgression was caused by a YD ice-sheet re-advance mapped in the same area. This stopped the isostatic uplift and increased the gravitational attraction on the sea elevating the geoid in this area. There may also been a contribution from rising glacio-eustatic sea level. Our results show that the transgression started around 13 110 cal yr BP. Thus, we conclude that the YD ice-sheet advance in western Norway started before the onset of the YD

Skredfarevurdering - kva kan kommunane sjølve gjere

Notatnummer 8/14Formålet er å gje eit oversyn over kva som bør vera med i ei skredfarevurdering, og vurdere kor stor del kommunane sjølve kan gjere av dette arbeidet. Ei slik skredfarevurdering kan delast i: 1. Innsamling av grunnlagsmateriale som ulike kart og data. 2. Feltarbeid, og 3. Samanstilling og vurdering. Kommunane kan sjølve gjere mykje av del 1, og spare tid og utgifter. Pkt. 2 og 3 må gjerast av fagfolk som fyller krava i NVE sine retningsliner. Aktsemdkarta for skred syner oftast for store areal som er skredutsette, og feltarbeid vil erfaringsmessig avgrense desse skredutsette areala. Lokalhistorie er eit viktig grunnlag for skredfarevurdering, då databasen skrednett ikkje har med alle hendingar, og heller ikkje areal med rekkevidde for skreda, men berre punktregistrering. Rapporten gir også eksempel, mest frå bratt terreng på Vestlandet, på skredtypar som går inn i ei skredfarevurdering