The significance of channel recharge rates for estimating debris‐flow magnitude and frequency

116

Abstract. Steep mountain catchments typically experience large sediment pulses from hillslopes which are stored in headwater channels and remobilized by debris-flows or bedload transport. Event-based sediment budget monitoring in the active Manival debris-flow torrent in the French Alps during a two-year period gave insights into the catchment-scale sediment routing during moderate rainfall intensities which occur several times each year. The monitoring was based on intensive topographic resurveys of low-and high-order channels using different techniques (cross-section surveys with total station and high-resolution channel surveys with terrestrial and airborne laser scanning). Data on sediment output volumes from the main channel were obtained by a sediment trap. Two debris-flows were observed, as well as several bedload transport flow events. Sediment budget analysis of the two debris-flows revealed that most of the debris-flow volumes were supplied by channel scouring (more than 92 %). Bedload transport during autumn contributed to the sediment recharge of high-order channels by the deposition of large gravel wedges. This process is recognized as being fundamental for debris-flow occurrence during the subsequent spring and summer. A time shift of scour-and-fill sequences was observed between low-and high-order channels, revealing the discontinuous sediment transfer in the catchment during common flow events. A conceptual model of sediment routing for different event magnitude is proposed.

Section: Introductionmentioning

confidence: 99%

Sediment budget monitoring of debris-flow and bedload transport in the Manival Torrent, SE France

Theule¹,

Liébault²,

Loye³

et al. 2012

116

“…The supply of loose debris plays an important role in the occurrence of future debris flows during subsequent rainfall events (Jakob et al, 2005). Both the critical rainfall threshold for debris flow initiation and the number of debris flows caused by a rainfall event may differ before and after an extreme rainfall event or a major earthquake.…”

Section: Introductionmentioning

confidence: 99%

Characteristics of rainfall triggering of debris flows in the Chenyulan watershed, Taiwan

Chen

Jan

Huang

2013

This paper reports the variation in rainfall characteristics associated with debris flows in the Chenyulan watershed, central Taiwan, between 1963 and 2009. The maximum hourly rainfall I_m, the maximum 24 h rainfall R_d, and the rainfall index RI (defined as the product R_dI_m) were analysed for each rainfall event that triggered a debris flow within the watershed. The corresponding number of debris flows initiated by each rainfall event (N) was also investigated via image analysis and/or field investigation. The relationship between N and RI was analysed. Higher RI of a rainfall event would trigger a larger number of debris flows. This paper also discusses the effects of the Chi-Chi earthquake (CCE) on this relationship and on debris flow initiation. The results showed that the critical RI for debris flow initiation had significant variations and was significantly lower in the years immediately following the CCE of 1999, but appeared to revert to the pre-earthquake condition about five years later. Under the same extreme rainfall event of RI = 365 cm² h⁻¹, the value of N in the CCE-affected period could be six times larger than that in the non-CCE-affected periods

“…Individual surges and presumably smaller events may have caused GD to trees but did not eliminate entire sectors in the past. Jakob et al (2005) highlighted the importance of channel recharge rates in the triggering of debris-flow events. At Torrent de la Fouly, small rocks are readily available and debris supply cannot be considered a limiting factor here.…”

Section: Discussionmentioning

confidence: 99%

Debris flows on forested cones – reconstruction and comparison of frequencies in two catchments in Val Ferret, Switzerland

Bollschweiler

Stoffel

2007

Abstract. Debris flows represent a major threat to infrastructure in many regions of the Alps. Since systematic acquisition of data on debris-flow events in Switzerland only started after the events of 1987, there is a lack of historical knowledge on earlier debris-flow events for most torrents. It is therefore the aim of this study to reconstruct the debrisflow activity for the Reuse de Saleinaz and the La Fouly torrents in Val Ferret (Valais, Switzerland). In total, 556 increment cores from 278 heavily affected Larix decidua Mill., Picea abies (L.) Karst. and Pinus sylvestris L. trees were sampled. Trees on the cone of Reuse de Saleinaz show an average age of 123 years at sampling height, with the oldest tree aged 325 years. Two periods of intense colonization (the 1850s-1880s and the 1930s-1950s) are observed, probably following high-magnitude events that would have eliminated the former forest stand. Trees on the cone of Torrent de la Fouly indicate an average age of 119 years.As a whole, tree-ring analyses allowed assessment of 333 growth disturbances belonging to 69 debris-flow events. While the frequency for the Reuse de Saleinaz study site comprises 39 events between AD 1743 and 2003, 30 events could be reconstructed at the Torrent de la Fouly for the period 1862-2003. Even though the two study sites evince considerably different characteristics in geology, debris-flow material and catchment morphology, they apparently produce debris flows at similar recurrence intervals. We suppose that, in the study region, the triggering and occurrence of events is transport-limited rather than weathering-limited.