Ritigraben torrent (46°11’N, 7°49’E)

Reconstructed Ritigraben event years and flooding data in the selected rivers show significant similarities. Comparisons indicate that 68% of reconstructed debris flow event years at Ritigraben correspond with flooding in at least one of the other catchments. This correspondence is especially true for the late 19th and the 20th century, during which only one debris flow appears nowhere else. The indices of similarity between the different data sets indicate that Ritigraben data best match with data on flooding in the Vispa (indsi=0.33) and the Saltina Rivers (indsi=0.31). In contrast, similarities of archival data for the Mattervispa and the Saaservispa Rivers appear to be less evident (largely insufficient flooding data rather than low similarities).
The probability of debris flow occurrence in the Ritigraben is highest when there is simultaneous flooding in the Saltina or the Vispa Rivers. In the opposite way, debris flow occurrence in the Ritigraben is most frequent when flooding simultaneously occurs in the Alpine Rhone River.

In fact, reconstructed Ritigraben data show that phases with accentuated activity and shorter recurrence intervals than today existed in the past, namely after 1827 and until the late 19th century. This period of short recurrence intervals of debris flows in the Ritigraben coincides with a phase of high flooding activity in major Swiss rivers. After 1835 (and until the mid-1890s), considerable above-average precipitation sums in summer and fall are noted for the Swiss Alps. Furthermore, increased debris flow activity in the Ritigraben goes together with the last “Little Ice Age” advance of the Gorner Glacier, which reached its maximum extent about 1859. Similarly, the period of less frequent debris flow activity after 1922 is also replicated in the major Swiss rivers, where low flooding frequencies were recorded between 1927 and 1975.

The analysis of the spatial distribution of sites affected by flooding shows that the 3 types of events that have been observed during the past 25 years also commonly occurred in the past.
• Ritigraben debris flows in June, July, and (exceptionally) August appear to have been initiated by type I event.
• Similar to the timing of the recent type II events, data indicate that throughout the past 4 centuries this meteorological situation exclusively occurred in August and September.
• Type III events occurred mostly in October or May. These results confirm the seasonal timing of debris flow activity, indicating that events are restricted to the period between June and September.

Long-term fluctuations of late summer and fall precipitation sums not only influence the flooding frequency in the Swiss and Ticino Alps, but they apparently drive the debris flow activity of the Ritigraben torrent. Correlations between global warming and modifications in the number or the size of debris flow events, as hypothesized by Haeberli and Beniston (1998), cannot, so far, be confirmed in the study area.

Owing to the elevated position of the source area (2600-3200 m a.s.l.), Ritigraben debris flows only occur between June and September. Over the past 4 centuries, events were most frequent in August (43%) and September (25%). This concentration of events is especially true for the 1948-2002 period, where all but one event occurred in August or September. The exception was not initiated by heavy rain but by rapidly melting snow. Between the early 19th century and 1947, however, all events occurred in June, July (both 26%), or August (48%), and not a single event can be found in September. This subtle shift at the middle of the 20th century from June, July, and August debris flow activity to August and September activity can also be observed in the 17th and 18th centuries, when all attributable events took place in September.

(4) - Remarques générales

Reconstructed debris-flow data for the Ritigraben fit very well with flooding data in neighboring catchments. The importance of the late-twentieth-century events has to be thoroughly revised. Events comparable to that in 1993 have previously existed in the region and do not represent a new phenomenon. Even the occurrence of several events within a short period of time can be observed in the mid–nineteenth century (i.e., six substantial events between 1834 and 1868).