1,721,592 research outputs found
Susceptibility and Triggers for Debris Flows: Emergence, Loading, Release and Entrainment
No full textDating past geomorphic processes with tangential rows of traumatic resin ducts
No full textPast activity of geomorphic processes can be reconstructed on tree-ring series using the presence of injuries, reaction wood or abrupt changes in the annual increment. The analysis of these features provides valuable data on years with process activity. In contrast, an intra-annual dating has so far normally only been possible through the analysis of injuries. In this technical note, it is shown that, in tree-ring studies realized with conifers, resin ducts may have the potential for providing information on the intra-seasonal timing of past geomorphic processes as well. However, because ducts may occur as a result of influences other than geomorphic, detailed field investigations and the identification of processes present at the study site imperatively need to precede dendrogeomorphological investigations. Data obtained from 1298 cross-sections indicate that the presence of resin ducts in Picea abies (L.) Karst. and Larix decidua Mill. can be considered to be the result of geomorphic activity if they form tangentially oriented rows with a compact and continuous arrangement of traumatic ducts. The presence of resin ducts may also help to improve the quality of reconstructions in studies using Abies alba Mill. as vertical resin ducts occur exclusively at or next to injuries. In contrast, resin ducts apparently cannot be used for dendrogeomorphological analyses of Pinus ssp
Magnitude-frequency relationships of debris flows - A case study based on field surveys and tree-ring records
No full textDebris-flow activity in a watershed is usually defined in terms of magnitude and frequency. While magnitude–frequency (M–F) relations have long formed the basis for risk assessment and engineering design in hydrology and fluvial hydraulics, only fragmentary and insufficiently specified data for debris flows exists. This paper reconstructs M–F relationships of 62 debris flows for an aggradational cone of a small (< 5 km2), high elevation watershed in the Swiss Alps since A.D. 1863. The frequency of debris flows is obtained from tree-ring records. The magnitude of individual events is given as S, M, L, XL, and derived from volumetric data of deposits, grain size distributions of boulders, and a series of surrogates (snout elevations, tree survival, lateral spread of surges). Class S and M debris flows (< 5 × 103 m3) encompass a typical size of events and have mean recurrence intervals of 5.4 (SD: 3.2) and 7.4 years (SD: 6.7), respectively. Class XL events (104–5 ×104 m3) are, in contrast, only identified three times over the past 150 years, and major erosional activity on the cone was restricted to two of these events in 1948 and 1993. A comparison of results with hydrometeorological records shows that class L and XL events are typically triggered by advective storms (rainfall > 50 mm) in August and September, when the active layer of the rock glacier in the source area of debris flows is largest. Over the past ∼ 150 years, climate has exerted control on material released from the source area and prevented triggering of class XL events before 1922. With the projected climatic change, permafrost degradation and the potential increase in storm intensity are likely to produce “class XXL” events in the future with volumes surpassing 5 × 104 m3 at the level of the debris-flow cone
- …
