By Rupert Bainbridge, Northumbria University
So I was recently in the European Alps helping out on some fieldwork for a colleague, Mark Allan. Whilst we were there to look at landslides onto glaciers the weather quite often had other plans! On the 26th August 2014 in Val Veny, Italy we had some heavy, but as I’m told for the summer of 2014 not unusually heavy rainfall, which caused extensive damage to infrastructure and rapid channel change. So, unable to reach our landslide fieldsites, fuelled on delicious European pastries and with a flooding river we were unable to resist the urge to document some of what happened that day. Below is a brief overview of some observations and impacts from the flooding.
Val Veny is the main trunk valley for ~7 alpine glaciers. However the Brenva and Miage glaciers are the only ones that currently flow into the valley bottom. The river is heavily managed using gabions, boulder riprap and engineered channelisation. Figure 1 shows a number of sites visited during and after the 26th September flood to monitor changes during and look at the impacts after the event.
Figure 1: Satellite image of Val Veny showing the major glaciers and the locations of the sites discussed below. Click the picture fore a higher resolution version!
Site 1: Campsite access road, Pertud
The first site we visited, conveniently, was the access road and bridge to our campsite (Site 1). At this site the river is channelised by permanent walls along its southern bank above and below the bridge; boulder riprap is used to protect the bank upstream of the bridge whilst permanent wall structures are used to protect the banks downstream (Figure 2A). Unfortunately we missed the flood peak, but caught it on the waning stage.
The bridge in Figure 2 was restricting flow causing localised overbank flooding. Some minor infrastructure damage occurred here with debris on the roads, some large woody debris within the bridge structure and localised scour around boulder riprap which had eroded sections of bank, damaging the road (Figure 3). Interestingly ice inclusions were found in some of the debris washed up onto the bank (Figure 2B), which indicated to us at the time a possible glacial source for some of the flood water.
Figure 2: (A) Main road and Perthud access bridge showing the channelised river in waning stage. The red dashed line shows debris indicating the peak flood height; (B) Example of ice inclusion in flood debris.
Figure 3: Localised scour on the campsite access road. Boulder riprap upstream of the road cut show evidence of scour suggesting this may have been the initiation mechanism for the road erosion.
Site 2: Miage Glacier
Access to the Miage Glacier was possible on the 27th September, the day after the floods. During this visit that there were a number of small freshly drained supraglacial lakes along the eastern margin of the glacier which are thought to have contributed to the flood waters. However on this trip we did miss the big smoking gun! A colleague Tom Shaw ventured to the Miage glacier in late September to collect some monitoring equipment and aside from some of the equipment disappearing in the flood, the glacial Lake Miage had drained as well (Figure 4). Combined with other possible supra-glacial lake drainages there could have been a significant glacial contribution to the August 26th flood. Some anecdotal evidence suggests that there may also have been lake drainages on the Freyney Glacier.
Figure 4: The basin of the former Lake Miage. The red line approximates the previous lake surface. (Photo credit: Thomas Shaw)
Site 3: Freney track wash-out
One of the more impressive impacts of this relatively small flood was the Freney farm track wash-out. Having been running on this track in blissful sunshine just the day before it was impressive to see very little evidence of the road having existed in the first place! A number of avulsion channels were present through the forested area where the road had been, one following the general path of the road. The channel headcut seen in Figure 5 appears to have continued migrating upstream between our visits on the 26th (A) and 27th (B).
Figure 5: Channel avulsion on the Freney farm track. (A) During the August 26th flood, note the channel headcut which is not present the following day; (B) August 27th return visit, the dashed line approximates the line of the road and the circle indicated the same boulder in each photo.
Fresh boulder and debris run-ups behind trees in Figure 6 highlight the competence of the flow to transport large material through over a large area (not just in channelised flows) and the extensive impact at this site.
Figure 6: Large scale boulder deposition through the forested area near Freney
Site 4: Road and bridge wash-out, Brenva Glacier/Mont Blanc Tunnel
One of the last sites we visited on the 27th (due to road closures) was near the bottom of the Brenva Glacier. There were extensive ice-cored moraine complexes being undercut by the floodwaters (Figure 7). As a result of this, large quantities of superficial sediment was collapsing into the flood flows and being reworked downstream.
Figure 7: Superficial sediment collapse into flood waters
A little further downstream one of the access roads to the Mont Blanc Tunnel crosses the river. The bridge surface itself had survived but much of the surrounding area had been severely eroded. Head-cuts on either side of the bridge surface had cut it off completely, whilst it is clearly evident from the boulders over the bridge surface that flows had been much higher and transporting large material (Figure 8).
Figure 8: Bridge and road damage on one of the Mont Blanc access roads. The red dashed line indicates the old road surface. Black dashed lines indicate erosion scars, some of the lower scars to the right of the bridge may be abandoned head-cuts. Black arrows indicate the active river channels, each of which is an active channel head-cut.
Hopefully this has been an interesting overview of the flash flooding we saw on August 26th 2014, they were certainly impressive on the day! There are some videos taken by Mark on the new BSG Postgraduate YouTube page: