Alan P. Dykes

Weathering-limited rainfall-triggered shallow mass movements in undisturbed steepland tropical rainforest

Rainfall-triggered landslides in undisturbed tropical rainforests may have been underestimated as contributors to slope development and denudation in the past. Theoretically, ideal conditions for such geomorphic processes, i.e. steep slopes and frequent high magnitude and intensity rainfall events, occur in a number of tropical regions, particularly within Southeast Asia. Therefore, a high frequency of occurrence of shallow slope failures was expected in the undisturbed steeplands of southeast Brunei. Stability conditions of the steep planar slopes were examined using a deterministic modelling approach in order to examine the possibility that most slopes could not fail in response to rainfall because they did not possess a sufficiently thick mantle of residual soil. A simple hillslope hydrology model based on a soil moisture balance approach was used to simulate hillslope responses to measured and simulated rainfall events. The stability of saturated slopes could then be analysed using the infinite slope model, the input shear strength parameters for which were obtained from direct shear tests and then calibrated by back-analysis of a failure which occurred in late 1991. The findings suggest that any slope of 40° and steeper should fail several times every year in response to storm events, but that in reality most of the slopes have failed previously and have not yet regained a critical depth of residual soil. Some approximate values for rates of weathering and slope development suggest that any given slope will not fail at intervals of less than 10,000 years. Therefore, the occurrence of shallow failures will be infrequent but nevertheless significant in terms of regional denudation and ecological diversity.

The use of salt dilution gauging techniques: ecological considerations and insights

The salt dilution (gulp injection) technique is a well established and widely used technique to measure stream discharge, flow velocity and water residence characteristics in small headwater streams. However, the impact of the technique on water quality and instream ecology has been largely ignored in field investigations. A series of experiments were undertaken in a regulated and groundwater-dominated river to examine the effects on aquatic invertebrate drift at high, medium and low discharges. In the groundwater-dominated river, drift significantly increased as a result of the introduction of the saline solution under all flows. Drift increased at the regulated site under low and intermediate flow but not during high flows, probably due to a natural increase in drift associated with spate conditions. Following the application of the saline solution several taxa absent or infrequently occurring in background samples, such as the cased caddisfly, Agapetus fuscipes, were recorded. The wider implications of the technique are discussed in relation to short-term pulsed pollution episodes and the management of riverine ecosystems.

Geomorphological maps of Irish peat landslides created using hand-held GPS

Abstract Please click here to download the map associated with this article. Peat mass movements are relatively common geomorphological phenomena in the uplands of Ireland and parts of northern Britain. Geomorphological mapping of many peat landslides has led to the identification of different types of failures based on repeatedly occurring morphological and geometric characteristics. Furthermore, much of the recently enhanced understanding of mass failures in peat deposits, published in several recent reviews, has been obtained through detailed analysis of the geomorphological maps produced from the new field surveys that are herein presented together for the first time. The map that is the focus of this paper is a composite figure that shows geomorphological maps of the majority of extant bogow and bog slide types of peat mass movements in Ireland, together with two peat ows and a large peat slide that has not previously been described. The paper examines the methodology used to create the maps, and highlights the value of the maps in presenting potentially critical evidence that may not be discernible by field inspection alone. Creating geomorphological maps of peat landslides from field sketches provides highly satisfactory results if selected features are geo-referenced using a hand-held GPS at an average frequency of around once for every 30 m of source area perimeter or other linear component of a landslide. In the case of very large failures and very old and degraded failures, lower frequencies of GPS points can be tolerated because critical morphological evidence appears to scale-up with failure size and smaller-scale morphological details are lost with increasing failure age due to natural degradation of the disrupted peat.

Peat slope failures and other mass movements in western Ireland, August 2008

Abstract Peat mass movements are relatively common in Ireland and potentially highly damaging. They are usually triggered by heavy rainfall but occasionally wind farm construction or other construction activities also cause significant peat slope failures. In August 2008, heavy rainfall in northwestern Ireland triggered several landslides near Geevagh, Co. Sligo, mostly involving peat-covered hillslopes, and a large peat flow occurred a few days later at the site of a new wind farm near Tralee, Co. Kerry. All were inspected very soon after they occurred to obtain visual evidence of site characteristics and conditions as little changed as possible from the time of failure. This paper reports the nature and implications of the landslides and highlights some of the difficulties of assessing potential landslide hazards from Irish blanket bogs.

The Vaiont landslide : re-assessment of the evidence leads to rejection of the consensus

There appears to be a clear general consensus in the literature regarding four critical issues that define the problem of the October 1963 Vaiont landslide and its behaviour that are central to the disaster: (1) the 1963 failure was a reactivation of an ancient landslide; (2) failure took place along thin clay seams (already at residual strength); (3) the sliding surface had a ‘chair’ shape with a (sub)horizontal base; and (4) failure was triggered by inundation of the toe of the slide mass by rising reservoir levels. The key to understanding the Vaiont landslide is the failure surface geometry, which was controlled by the structural geology. It now appears that the so-called chair structure (that was assumed to define the shape of the failure surface) does not exist, and without it, the first consensual point is untenable, and the fourth may not contain the whole truth. We have systematically re-examined the published evidence and undertaken our own new research in order to test the logical and geotechnical validity of the four elements of the consensus. Glacial processes can account for the pre-failure morphology of the landslide site; the clay seams must therefore have been at peak shear strength as there was no ancient landslide. Tectonic processes can account for the failure surface geometry, which does not have a ‘chair’ shape, as well as small-scale structures; and rainfall appears to have been an essential element in the initiation and development of the landslide. Our findings largely contradict the consensus position and thus form the basis of a new overarching hypothesis for the landslide that should account for all of the observed and known features, events and data.

New, simplified and improved interpretation of the Vaiont landslide mechanics

Both the occurrence and behaviour of the Vaiont landslide have not been satisfactorily explained previously because of difficulties arising from the assumption that the failure surface was ‘chair’ shaped. It is now known that there was no ‘chair’, which means that the 1963 landslide could not have been a reactivated ancient landslide because the residual strength of the clay interbeds would have been insufficient for stability prior to 1963. Furthermore, the moderately translational geometry reduces the influence of reservoir-induced groundwater and hence of submergence. Standard stability analyses now show that prior to 1960, the average shear strength must have significantly exceeded the peak shear strength of the clay interbeds known to have formed the majority of the failure surface. Three-dimensional stability analyses confirm these results and show that at the time of the first significant movements in 1960, the rising reservoir level had a negligible effect on the Factor of Safety. According to these results, the Vaiont landslide was most likely initiated by pore water pressures associated with transient rainfall-induced ‘perched’ groundwater above the clay layers, in combination with a smaller than hitherto assumed effect of reservoir impounding, then developed by brittle crack propagation within the clay beds, thus displaying progressive failure. Further, very heavy rainfall accelerated the process, possibly due to reservoir-induced groundwater impeding drainage of the rainwater, until the limestone beds at the northeast margin failed. With the shear strength suddenly reduced to residual throughout, the entire mass was released and was able to accelerate as observed.

Geomorphological maps of Irish peat landslides created using hand-held GPS - second edition.

Abstract Please click here to download the map associated with this article.

Discussion of ‘A5 Llyn Ogwen peatslide, Capel Curig, North Wales’ by D. Nichol, G.K. Doherty & M.J. Scott Quarterly Journal of Engineering Geology and Hydrogeology , 40, 293–299

A.P. Dykes & J. Warburton write: Nichol et al. (2007) present an interesting case study of a small shallow landslide that occurred in North Wales in November 2005. They describe the landslide and its local context in sufficient detail to conclude that it was caused by the combination of high rainfall and subsurface hydrological controls. They also conclude that a bio-engineering approach to remediation of the affected slope is necessary to protect the highway and will provide the most appropriate solution given the environmental sensitivity of the site. Although we agree fully with the general conclusions as inferred from the information presented, we feel that it would be beneficial to provide a broader context for this study in terms of recent research into peat instability on hillslopes. The purpose of this discussion is therefore to comment on some of the details provided in the study and, in doing so, to highlight some of the problems and misconceptions often associated with the study of peat landslides. Nichol et al. (2007) begin by stating that ‘Peatslides are relatively uncommon in the UK’ and that ‘as far as can be determined, this is the first example of a natural peatslide event to be documented for Wales’. The latter comment is probably true, as we have not yet verified the reportedly similar event in January 2005 that affected the A470 road 45–50 km south of Llyn Ogwen (Mason 2005). However, peat landslides are perhaps more common in the UK than is often realized. Of all the recorded global peat mass movements, around 20% have occurred in mainland Britain (i.e. excluding Northern Ireland) and 60% in Ireland (Northern Ireland and the Republic of Ireland combined). Over 50 separate landslides involving blanket bog are known from Northern Ireland and at least 35 in northern England (excluding bog bursts involving raised bogs). In Scotland, Acreman (1991) reported a number of peat landslides among the many that occurred in 1983 near the English border, Bowes (1960) described one on the Isle of Lewis, and in Shetland five peat slides occurred on 19 September 1990 (Veyret & Coque-Delhuille 1993) followed by more than 20 peat slides on 19 September 2003 (Dykes & Warburton 2008a). More recently, two large peat landslides occurred in the North York Moors, northern England, following severe local flooding on 19 June 2005 (Fig. 1). Our comments here are broadly divided into three topics: classification and causal factors; shear strength and stability analysis; post-failure remediation. Classification and causal factors. Nichol et al. (2007) state that the blanket peat lies ‘directly on bedrock or on gravel subsoil’ (p. 293), and their figures 2 and 6 clearly show a thin layer of peat overlying a thinner layer of mineral material, with both layers having been removed to reveal ‘smooth rockhead’. As part of our work to understand the causes and mechanisms of peat landslides, we set out a definition of what constitutes a ‘peat failure’ and then defined specific types of peat failures (Dykes & Warburton 2007a). According to this classification scheme, the landslide at Llyn Ogwen appears be a ‘peaty-debris slide’ assuming the average depth of peat is at least 0.4 m; that is, the defining characteristic is failure of the mineral substrate below the base of the peat. (If there is less than 0.4 m of peat on average, the landslide should simply be identified as a ‘debris slide’ sensu Corominas (1996).) In this type of failure, the properties of the peat material are largely irrelevant, with the exception of the unit weight, as this contributes to the shear stress acting on a potential failure plane within the substrate material. However, the hydrological characteristics of the peat mass (compare rock mass characteristics), such as pipes, desiccation cracks, tension cracks, etc., will be critical, as they can transmit rainwater directly to the base of the peat (see review by Warburton et al. 2004). At Llyn Ogwen the water would enter the ‘gravel subsoil’ where high and potentially artesian water pressures could rapidly develop beneath the effectively impermeable peat cover. This was

Prediction of climate change effects on the runoff regime of a forested catchment in northern Iran

ABSTRACT The southern coast of the Caspian Sea in northern Iran is bordered by a mountain range with forested catchments which are susceptible to droughts and floods. This paper examines possible changes to runoff patterns from one of these catchments in response to climate change scenarios. The HEC-HMS rainfall–runoff model was used with downscaled future rainfall and temperature data from 13 global circulation models, and meteorological and hydrometrical data from the Casilian (or “Kassilian”) Catchment. Annual and seasonal predictions of runoff change for three future emissions scenarios were obtained, which suggest significantly higher spring rainfall with increased risk of flooding and significantly lower summer rainfall leading to a higher probability of drought. Flash floods arising from extreme rainfall may become more frequent, occurring at any time of year. These findings indicate a need for strategic planning of water resource management and mitigation measures for increasing flood hazards. EDITOR M.C. Acreman ASSOCIATE EDITOR not assigned

Correction to: The Vaiont landslide: re-assessment of the evidence leads to rejection of the consensus

The published version of this article, unfortunately, contained error. Figure 10 correction was not carried out. Given in this article is the correct figure.