Magnitude-frequency of sea cliff instabilities

Abstract

Abstract. The magnitude-frequency relationship of sea cliff failures in strong, low retreat rate cliffs, was studied using systematic historical inventories carried out in the coasts of Portugal and Morocco, in different geological and geomorphological settings, covering a wide size scale, from small to comparatively large rockslides, topples and rockfalls, at different time and spatial scales. The magnitude-frequency expressed in terms of volume displaced and of horizontal area lost at the cliff top showed good fit by inverse power laws of the type p=a.x −b , with a values from 0.2 to 0.3, and exponents b close to 1.0, similar to those proposed for rockfall inventories. The proposed power laws address the magnitudefrequency for sea cliff failures, which is an important component of hazard assessment, to be completed with adequate models for space and time hazard components. Maximum local retreat at the cliff top provided acceptable fitting to inverse power laws only for failures wider than 2m, with a=4.0, and exponent b=2.3, which may be useful to assess the cliff retreat hazard for the use of areas located near the cliff top.

Influence of Wave Action and Lithology on Sea Cliff Mass Movements in Central Algarve Coast, Portugal

“…With respect to the Miocene carbonate rocks, the vast majority of mass movements are rock falls and topples, which is in agreement with the results obtained by Teixeira (2006). In addition, although some authors have argued that notching has an insignificant effect on cliff stability (e.g., Wolters and Muller, 2008), the deepening of notches is considered to be responsible for the toppling failures that represent the more frequent mass movements in the calcarenites along the south Portuguese coast (Marques, 1997(Marques, , 2008Teixeira, 2006). As sectors II and III are exposed to different wave conditions (Figures 2 and 3), the common factor contributing to the mobilization of large volumes of hard rock (rock falls and topples) is the vertical facies variation.…”

“…Wolters and Muller (2008) used finite element model predictions that pointed to an insignificant effect of cliff undercutting on cliff stability. However, in contrast, field measurements show that the deepening of such notches is responsible for the toppling failures that represent 42% of the mass movements in calcarenite rocks along the south Portuguese coast (Marques, 2008).…”

Short-term retreat statistics of a slowly eroding coastal cliff

“…They found a linear trend in the non-cumulative distribution of log volumes, including the small changes that were resolved with their uniquely highresolution observations. These results suggest undersampling of small cliff failures caused the deviation of the magnitude-frequency relationships found previously in coastal (Dong and Guzzetti, 2005;Teixeira, 2006;Marques, 2008) and inland (e.g. Hovius et al, 1997;Stark and Hovius 2001;Dussauge-Peisser et al, 2002;Malamud et al, 2004) observations.…”

“…Teixeira (2006) also found power-law relationships between volume, horizontal area, and retreat. Marques (2008) further investigated the magnitude-frequency statistics of strong, slowly retreating coastal cliffs (< 10 cm yr −1 ) in terms of volume, horizontal area, and cliff retreat. The database included about 600 events from regional scale observations obtained from aerial photographs spanning up to 53 years in Portugal and Morocco, and smaller scale insitu field observations (from Teixeira, 2006) in Portugal.…”

“…For example Teixeira (2006) and Marques (2008) report horizontal (planimetric) area while Rosser et al (2007) and Lim et al (2010) consider area parallel to the cliff face. Some studies focus on magnitude-frequency relationships of cliff retreat (Dong and Guzzetti, 2005;Teixeira, 2006) while others on failure volume (Rosser et al, 2007;Lim et al, 2010).…”