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ICELAND_DEBRISFLOWS: A Study of the Hazard and Geomorphic Change Caused by Debris Flows in Iceland

Start date: 15-06-2012 - End date: 15-08-2012

Aircraft:

Aircraft name: DO228 - NERC - ARSF

Airport: Súgandafjörður and Skutulsfjörður in the Westfjords of Iceland- this region was covered by the ARSF LiDAR instrument in 2007 - hence change detection can be performed. Western Skagafjörður, this area has large and frequent debris flows, but the geological setting is different to that in the Westfjords. This makes it an ideal site to test the wider applicability of the debris flow dynamics models developed on the basis of the data from the Westfjords. Armelle Decaulne makes yearly visits to both these areas, and has many complimentary datasets that can be used to support the project. The other collaborators have visited one or both of these sites in the past and so are also familiar with the area.

Project description

Project theme: Earth Science: Geomorphology and Risk

Science context: One of the main risks to population in Iceland, particularly in the fjords, is the action of debris flows. These are mixtures of boulders, sediment and water that surge downslope after prolonged/intense rainfall or sudden snowmelt (Decaulne & Sæmundsson, 2007). Metre-size boulders can be carried in debris flows, and cause damage to property and, in some cases, loss of life. To better understand the risk posed by these flows, both the physics of their motion and the role of the geological setting in initiating slope instability need to be understood. We propose to collect high resolution topographic data in Súgandafjörður and Skutulsfjörður, which were previously surveyed in 2007, and from the slopes of western Skagafjörður. By studying previously surveyed sites, we will measure accurate quantification of sediment movement rates and volumes. By studying the slopes in Skagafjörður, we will be able to compare different geological settings. These data will help us develop a new numerical model of debris flow activity, and help us to understand the long-term slip and creep processes that contribute to slope instability. We will create difference maps (showing deposition and erosion of sediment) for Súgandafjörður and Skutulsfjörður and for Skagafjörður we will accurately characterise the morphology for comparison with data from Conway et al (2010) for Súgandafjörður and Skutulsfjörður.

Measurements to be made by aircraft: Objectives ------------- The objectives are to better understand the dynamics and behaviour of debris flows through the study of their geomorphometry. This will help to assess the potential hazard posed by these flows and also contribute to a better understanding of their contribution to post-glacial hillslope degradation. Proposed Work -------------------- We plan to collect elevation data and air photography from slopes above the towns of Ísafjörður and Suðureyri in the Westfjords of Iceland and the slopes north of Sauðárkrókur in north-central Iceland (see attached map). These locations have very frequent medium- to large-scale debris flows (Decaulne et al., 2009; Decaulne et al. 2007; Decaulne & Sæmundsson 2006) and are thus ideal for studying the morphology of fresh hillslope debris flows. This new survey would cover the same region as the NERC-ARSF survey performed in 2007 (Conway, et al. 2010) and also cover a new site, Sauðárkrókur, in north-central Iceland. The significance of resurveying the same sites as in 2007 is that we can produce an elevation difference map, allowing accurate quantification of material moved by any process across the whole area. From this we aim to, 1) create “before and after” maps for selected individual debris flows and 2) accurately quantify the medium-term rate of slope degradation. The “before” maps will provide input into a numerical code describing debris flow motion (Mangeney, et al. 2010) and the code tested and calibrated by comparison to the “after” maps. We will use this calibrated code to predict the path and final deposition area of potential future flows – a vital tool in improving geohazard prediction and mitigation. Determining the rate of slope movement is particularly important for the Gleiðarhjalli bench (located above Ísafjörður town), where gradual sediment creep is an important additional factor in triggering debris flows (Decaulne, et al. 2005). The general sediment transport map produced will be used to assess slope stability over the whole area, an important outcome in itself. We also propose to survey a second area, the western slopes of Skagafjörður, which is even more active (Decaulne et al. 2009), but less populated. The geological setting of the debris flows here is different from that in the Westfjords. These flows are confined in their upper reaches and deposit over colluvial fans. We can therefore test whether the numerical model calibrated in the Westfjords can be used in this new setting. We will use this dataset to test whether model parameters can be easily adapted across regions. The high resolution data will enable us to make estimates of how erosion and deposition change along the flow and see if they obey the same trends and thresholds as found by Conway et al. (2010) near Ísafjörður. In addition a new survey here will form a baseline for future GPS or LiDAR surveys in the area. In addition to risk analysis, this project contribute to quaternary science in general. For example, the difference maps will help us to understand the contribution of different processes to the post-glacial sediment budget. For example are debris flows more important in slope degradation (despite their relatively short duration) than slope creep or rockfall? Is the rate in the westfjords comparable to elsewhere and if not why not? Anticipated Outputs ------------------------- We anticipate refining the debris flow hazard map for the towns of Ísafjörður and Suðureyri by use of our numerical model. We intend to publish articles in high-impact peer-reviewed journals such as Geomorphology, Natural Hazards, and Earth Surface Processes and Landforms.

Season: Summer 2012, or 2013. Summer 2012 would be preferable to allow clustering with a scheduled NERC-ARSF deployment to Greenland (requiring a stop-off in Iceland).

Weather constraints: Clear skies, or very slight cloud.

Time constraints: Summer season.

Flights (number and patterns): Two flights and a total of 5 flight hours for data collection. Proposed flight patterns are shown on the attached map. We have not recalculated the flying times needed for the Westfjords as we assume the same flying time as in 2007 (4hrs), this is likely an over-estimate, because the present Leica LiDAR instrument onboard the aircraft can obtain a greater sampling rate over a larger swath than the Optech instrument operated in 2007. Estimated transit time from the Westfjords to Sauðárkrókur is 1hr 45mins. The calculations for Sauðárkrókur are given below: LiDAR system Laser Freq (Hz) 60000 ~max rate for flying height Scan Freq (Hz) 45 (max for this scan angle is ~45) Scan Angle (deg) 7 Scan Factor 315 Swath width (m) 491 Resolution (m) Cross-track 0.74 Resolution (m) Along Track 0.74 Aircraft Altitude (m) 2000 Ground Speed (knots) 130 Overlap (%) 25 Overlap (m) 123 Turn time (min) 5 Site Width (m) 2000 Length (m) 8200 No. of Passes 6 Duration (hh:mm:ss) 00:37:15 We anticipate that the aircraft will need to be on-site for one week to ensure reasonable probability of suitable weather conditions for the duration of the data collection.

Instruments: LiDAR (Leica ALS50 II), air photography (Leica RCD105). N/A

Scientific contact

Name: Armelle DECAULNE

PI email: armelle.decaulne@univ-bpclermont.fr