Katja Laute

Fluvial transport during thermally and pluvially induced peak runoff events in a glacier‐fed mountain catchment in western norway

Abstract. Results compiled after three years of process monitoring and analysis in the glacier‐fed Erdalen catchment, situated in the steep fjord‐landscape of western Norway, indicate that there are significant intra‐ and inter‐annual variations with respect to fluvial sediment transport rates and sediment yields. Three different periods with a high frequency of major discharge events can be identified over the year, with these three periods showing a significant inter‐annual variability. High runoff in spring (April–June) is mainly caused by snowmelt whereas major discharge events in summer (July–August) are due to thermally caused glacier melt. In autumn (September–November), major discharge events are associated with heavy rainfall events. Autumn is the most important period with respect to fluvial sediment transport and fluvial sediment yields. Fluvial sediment transport and fluvial sediment yields in Erdalen are altogether supply‐limited. The intensity of fluvial transport in autumn and over the entire year depends strongly on the number of heavy rainfall events that trigger transfers of sediments from slopes into channels via saturation overland flow with connected slope wash and debris flow events. Annual suspended sediment yields are about two times greater than annual solute yields corrected by atmospheric inputs. Suspended sediment concentrations in glacier melt water during summer and annual sediment yields in Erdalen are both lower than in many other glacierized catchments worldwide. Because of the quantitative importance of single meteorological events for fluvial sediment transport and yields, and the high intra‐ and inter‐annual variability of sediment yields in Erdalen, process monitoring and analysis will be continued to be able to calculate more reliable annual sediment yields and denudation rates.

Characteristics of floodplain deposits within a braided sandur system in upper Erdalen (Nordfjord, western Norway).

Abstract. This study aims to characterize the morphology and contemporary floodplain deposit dynamics within a braided sandur system in upper Erdalen, a steep U‐shaped valley in western Norway. The braided sandur system is fed by two tributary valleys connected to the Jostedalsbreen Ice Field. Based on field observations, two different sub‐systems can be identified within the braided sandur system: fine‐grained to sandy flood sediments down‐valley, and coarser, gravelly deposits in the upper part. A combination of different methods is applied for studying sediment dynamic processes within the braided sandur system (sedimentological and stratigraphic analyses, 14C dating, dendrochronology and lichenometry). The coarser deposits are interpreted as originating from the Little Ice Age advance in Erdalen. Preliminary results indicate that the floodplain sediments were deposited after the Little Ice Age advance. Different sedimentary facies types are present and different phases of flooding can be identified. Within the entire braided sandur system, multiple sub‐systems characterized by different levels of process intensities exist.

Subrecent sediment dynamics and sediment budget of the braided sandur system at Sandane, Erdalen (Nordfjord, Western Norway)

A study was carried out of the subrecent sediment budget of a braided sandur system in a U-shaped valley linked to the Jostedalsbreen ice field in Nordfjord, in Western Norway. Special focus was on (i) the detection of different zones with negative, positive or balanced subrecent (following the Little Ice Age advance) sediment budget within the braided sandur system, (ii) the identification of sediment sources upstream of Sandane and from the slope systems to both sides of the braided sandur system, and (iii) the analysis of the subrecent sediment budget of the entire Sandane system. A combination of methods was applied in the analysis. The upstream part of Sandane was found to be characterised by a negative subrecent sediment balance, with erosion of coarse sediments from the Little Ice Age advance. In comparison, the downstream parts of Sandane have a balanced to slightly positive subrecent sediment budget, with formation of younger flood sediments and more stable channels. Thus, the subrecent sediment budget of Sandane appears to be slightly negative. Present-day coupling of slope and fluvial systems is limited and only a rather small amount of sediments is directly transported from the slopes into the braided sandur. In sum, fluvial sediment transport following the Little Ice Age period appears to be supply-limited.

Environmental Controls and Geomorphic Importance of a High‐Magnitude/Low‐Frequency Snow Avalanche Event in Bødalen, Nordfjord, Western Norway

Abstract Snow avalanches are common phenomena in western orway. During the winter–spring period 2011/2012 an extreme snow avalanche occurred within the upper valley part of a steep and glacier‐connected mountain catchment (ødalen) in western orway. Compared with annually occurring regular snow avalanches, so‐called ‘extreme snow avalanche events’ are more difficult to monitor and to study as they are characterized by recurrence intervals often longer than a decade. Morphometric and meteorological controls of a high‐magnitude/low‐frequency snow avalanche event, its geomorphic effects as well as its related relative role in mass transport compared with the annually monitored snow avalanche activity within the ødalen catchment were explored. Maximum values of snow height, velocity and pressure were predicted by applying a numerical run‐out simulation. The formation of this snow avalanche resulted from the combination of extraordinary meteorological conditions and a favourable morphometric setting of the source area. The snow avalanche path covered a total distance of 2900 m, including a stretch of 850 m where the snow avalanche slid downwards on top of the Bødalsbreen outlet glacier. Within the run‐out zone, directly located in front of the ødalen outlet glacier, 2032 stones with b‐axes >5 cm were remobilized which corresponds to a total transferred debris mass of 460 t. Compared with annually occurring snow avalanches within the ødalen drainage basin the relative importance of extreme‐sized snow avalanches is comparably low with respect to direct erosion and sediment transfer along rockwalls at higher slope areas. However, extreme‐sized snow avalanches play a significant role with respect to the remobilization of debris/sediment at lower slope areas as well as to down‐valley transport of sediment, including recognizable transfers of debris into the main stream channels of the drainage basin system whenever extreme snow avalanches reach the main channel.