The role of groundwater in cliff instability: an example at Cape Blanc-Nez (Pas-de-Calais, France)

The role of groundwater in cliff instability 31

Copyright © 2005 John Wiley & Sons, Ltd. Earth Surf. Process. Landforms 31, 31–45 (2006)

Earth Surface Processes and LandformsEarth Surf. Process. Landforms 31, 31–45 (2006)Published online 21 September 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1229

The role of groundwater in cliff instability: anexample at Cape Blanc-Nez (Pas-de-Calais, France)Guillaume Pierre* and Philippe LahousseLaboratoire de Géomorphologie et Gestion des Milieux Naturels, Université des Sciences et Technologies de Lille, France

AbstractCliff retreat in northern Boulonnais is described. The distribution of the amount of retreat isrelated to structural and topographic factors. Both control the runoff–infiltration balanceand therefore the mechanical behaviour of the rocks, which determines the modes of failure.The spatial variability of the retreat rate is explained, but predicting the temporal variabilityof the retreat rate, which is central to risk management, is much more difficult. Rainfall andpiezometric surface data enhanced a ‘piston flow’ mechanism during November 2000. Theresult of its occurrence on the stability of the cliff and conditions of its recurrence areexamined with a view to better understanding the rate of recession of coastal cliffs and wisermanagement of risk. Copyright © 2005 John Wiley & Sons, Ltd.

Keywords: cliff retreat; groundwater; ‘piston flow’ mechanism; Boulonnais

*Correspondence to:G. Pierre, Laboratoire deGéomorphologie et Gestion desMilieux Naturels EA 3339, UFRde Géographie et Aménagement,Université des Sciences etTechnologies de Lille, Av. PaulLangevin 59, 655 Villeneuved’Ascq cedex, France. E-mail:[email protected]

Received 11 February 2004;Revised 24 January 2005;Accepted 17 February 2005

Introduction

The evolution of soft rock or hard rock coastal cliffs is generally examined using the amount of retreat (Catalaoet al., 2002; Pierre and Lahousse, 2003; Zviely and Klein, 2004), the retreat rate (Rudberg, 1967; May, 1971; May andHeeps, 1985; Allison, 1989; Hénaff et al., 2002), the modes – marine processes or subaerian processes (Robinson,1977; Sunamura, 1982; Nott, 1990; Alveirinho Dias and Neal, 1992; Kirkgöz, 1995; Allison and Kimber, 1998;Benumof et al., 2000; Duperret et al., 2002a; Lahousse and Pierre, 2003a, Brossard and Duperret, 2004), or the causesof the retreat (static causes, Bonte, 1959; Jones and Williams, 1991; Davies et al., 1998; Duperret et al., 2002b; Costaet al., 2003; or dynamic causes, Caudron et al., 2001). However, these various parameters are often investigatedseparately, or, in the case of Blanc-Nez, over a reduced and structurally homogeneous section of the coastline (Lahousseand Pierre, 2003b).

There are structurally differentiated cliffs over a distance of 7 km, between Strouanne and Sangatte (Figures 1 and2). The Gault-chalk cliff of Strouanne is similar to the cliffs of southern England, which are in the same sedimentarybeds (Osman, 1917; Wood, 1955; Brunsden and Jones, 1976), and on which stability analyses and investigations onstructural parameters have been carried out (Hutchinson, 1969; Hutchinson et al., 1980; Hutchinson, 1983). The cliffof Sangatte, cut in silty Pleistocene slope deposits, can be compared with other cliffs consisting of non-consolidatedclastic rocks, such as the Algarve cliffs cut into friable Miocene and Quaternary sediments (Alveirinho Dias and Neal,1992), or the cliffs of Northern Ireland (McGreal, 1984) and west Wales (Jones and Williams, 1991), which are cutinto till and boulder clay. Except for the west Wales coast, the studies quoted above do not provide precise measure-ment of cliff retreat. The chalk cliff between Petit Blanc-Nez and Fond du Guet is similar to those of the Pays de Cauxand south-east England, for which the amount (Hénaff et al., 2002; Costa et al., 2004, May, 1971; May and Heeps,1985) and the modes of retreat (Watremez, 2001; Hénaff et al., 2002; Duperret et al., 2004; Mortimore et al., 2004a,2004b) are well known.

Along the section between Strouanne and Sangatte, cliff retreat over the last 60 years (1939 or 1944–2003) has beendetermined, and the modes of cliff evolution have been analysed by direct observation since 1999. Investigations on alarge scale have made it possible to analyse the spatial variability of the amount and rate of retreat, and have allowedus to point out the significant influence of structural and topographical parameters on cliff retreat. With regard to thecauses of the retreat, this paper attempts to determine the influence of groundwater on cliff instability. Hutchinson

32 G. Pierre and P. Lahousse


Figure 1. Location map of the study area (A–C, photographs of Figure 2; water-table contours after Bracq and Brunin, 1999).

(1969) described the role of groundwater in the coastal landslides of Folkestone Warren, involving Gault clay; Backet al. (1984) discussed the dissolution of limestones by fresh, brackish and saline water and Norris (1990) emphasizedthe role of emerging groundwater on the cliff face in weakly cemented sediments. Caudron et al. (2001) and Duperretet al. (2002a) mention the role of groundwater on chalk cliff instability, and the excess load caused by the rise of thewater level. However, the relationship between the behaviour of the aquifer and the instability of chalk cliffs has notbeen investigated either in northern France, where piezometric surface data are lacking, or in southern England, wherethe characteristics of the chalk aquifer are well known (Jones and Robins, 1999). Data from the borehole of Crand’Escalles make it possible to examine this relationship without using geophysical methods (Miehé et al., 2001).

Methods

The amount of cliff retreat was measured by superimposing topographic plans, derived photogrammetrically, whichprovide results of high precision (Overton et al., 1996; Lahousse et al., 2000; Moore, 2000; Catalao et al., 2000, 2002;



Costa et al., 2001; Pierre and Lahousse, 2003). A first photogrammetric restitution (scale 1/2000) was made from theaerial photographs of the IGN (Institut Géographique National), taken on 20 April 1939 (scale of the prints 1/11 000).This made it possible to chart the position of the top of the cliff 64 years ago, with a margin of error of ±1 m. Theamount of cliff retreat was then obtained by the readjustment, in the IGN Lambert 1 system, of the 1939 cliff position

Figure 2. Views of the three types of cliff encountered in the investigated area. Sangatte cliff, cut into Pleistocene deposits (a),chalk cliff at Cape Blanc-Nez, with a remnant of the 1993 rockfall (b), Gault-Chalk cliff at Cran Saint-Pô outlet and Saint-Pô Head(c). See Figure 1 for locations.



on a topographic map that was also derived photogrammetrically (margin of error ±14 cm) from a specificaerial mission made on 28 March 2002 (scale of the prints 1/7500). A tacheometric survey of the top of the cliff,made on 15 September 2003, does not show any significant retreat since March 2002. The same method was appliedto the cliff of Sangatte (and to a short segment north of the Cran d’Escalles, see Figure 5 below) from prints datedSeptember 20, 1944 (scale 1/7000, Royal Air Force), making it possible to cover a 58 year period (1939 aerialphotographs do not cover these two sections). The retreat of the top of the cliff was then measured every five metres,along the coastline.

The retreat rate during the period from 1939 to 2003 has been derived using three different types of photographicdocuments:

• vertical aerial photographs, at scales ranging between 1/7000 and 1/30 000 IGN, 1939, 1949, 1957, 1963, 1969,1970, 1971, 1975, 1976, 1977, 1979, 1981, 1982, 1983, 1985, 1986, 1988, 1989, 1990, 1992, 1993, 1995, 1997,2000; RAF, 1944; ‘NAI photo aérienne’, 1999, 2002, 2003);

• archives of the French newspaper La Voix du Nord (since 1960) and of the Direction Régionale de l’Environnementdu Nord–Pas-de-Calais (reports and oblique aerial photographs);

• photographs taken from the shore (1981, Colbeaux, unpublished; 1990, in Wissocq, 1992; 1999, 2000, 2001, 2002,2003).

The meteorological data (daily and monthly rainfalls) are provided by the weather stations of Boulogne-sur-Mer(1950–2000, averages calculated from 1961 to 1990, after Météo France) and of Guines (1970–2003, after Eaux deCalais). Piezometric surface data from the observation well of Escalles (after Eaux de Calais) have been recordedmonthly between 1989 and 1999, and daily since 2000 (a measurement every 20 minutes; automatic recording).

Structural and Hydrogeological Background

The Blanc-Nez cliffs (Figure 1) constitute the northern limit of the Boulonnais, corresponding to the northern edge ofthe Artois block, which is raised above the Flanders plain and the Straits of Dover (Colbeaux et al., 1980). Theirelevation drops to some 20 metres towards Strouanne and Sangatte, as at Cran Saint-Pô and Cran d’Escalles, andreaches 40 m at Saint-Pô Head, 50 m at Petit Blanc-Nez and about 100 m at Cape Blanc-Nez. The cliffs are cut in theAlbo-Cenomano-Turonian series, which outcrops between Strouanne to the south and the Pleistocene formations atSangatte to the north. Overlying the Gault, the different facies of the chalk (Robaszynski and Amédro, 1993; Amédroand Robaszynski, 2001) outcrop in various combinations, depending on the height of the cliff and the SW–NE dip ofthe series. The succession from bottom to top comprises (Figure 3) the following:

• at Saint-Pô, between Strouanne and Petit Blanc-Nez, the whole or part of the Gault (Albian), and the lowerCenomanian blue chalk;

• at Petit Blanc-Nez the whole of the lower Cenomanian blue chalk and the middle Cenomanian grey chalk;

• at the Crupes the upper blue chalk and the lower grey chalk of lower to middle Cenomanian age respectively;

• south of Cape Blanc-Nez the whole of the middle Cenomanian grey chalk, the upper Cenomanian white chalk andthe lower nodular Turonian chalk;

• north of Cape Blanc-Nez the white chalk of the upper Cenomanian and the Turonian chalk.

Although the effective porosity of the chalk matrix is low (Price, 1997), post-sedimentary features increase thehydraulic conductivity of these materials. The chalk of the Blanc-Nez site is very faulted and the fracturation (whosemajor orientations are N 30° and N 120°) contributes to a slow fissural drainage, on which a faster flow of karsticorigin is superimposed (Bracq et al., 1992; Bracq and Brunin, 1999). Because of these discontinuities, and in spite ofthe presence of relatively impermeable marly seams in the Cenomanian and Turonian series, the aquifer can bedescribed as being single layer (Mania, 1978). The phreatic zone corresponds to the middle Cenomanian grey chalk,and the lower Cenomanian blue chalk acts as a boundary to the aquifer.

The dip of the series is approximately 7° NE, and two synclinal axis of the same orientation are located some 500 mbehind the coastline (Figure 1). Along the syncline of Escalles, water in the zone of saturation of the aquifer flowstowards the NE. A high groundwater level results in an outflow of the water along the springs lining the contactbetween the impermeable blue chalk and the grey chalk from the Cran d’Escalles to Petit Blanc-Nez. North of theCran d’Escalles, the higher level of the groundwater never rises above the altitude of the foot of cliff (i.e. 4–5 m NGF,Nivellement Général de la France).



Between Petit Blanc-Nez and Strouanne, despite the presence of a synclinal undulation parallel to the coast(Destombes and Destombes, 1963), the phreatic water disappears at the same time as the middle Cenomanian greychalk (Figure 1). The low permeability of the lower Cenomanian blue chalk causes an overland flow, which iscanalized down to the sea by the Cran Saint-Pô. At Strouanne, where the thickness of blue chalk is reduced, slightinfiltration determines the presence of seepage at the Gault–blue chalk contact towards the top of the cliff.

North of Cape Blanc-Nez (Figure 1), between the old German cemetery and the Sangatte spit, the cliff is cutinto Pleistocene deposits which fossilized the former chalk cliff (whose orientation becomes WE). These depositsstart with marine sediments (ancient shingle bars and sandy beach), covered at the foot of the fossil cliff by chalkyscrees, and elsewhere by argillaceous, muddy or sandy slope deposits whose facies are described in Figure 4 (Somméet al., 1999).

The formation of Sangatte is entirely in the zone of aeration overhanging the water-table (Bracq et al., 1992; Bracqand Brunin, 1999), and the water that percolates through it runs to the NNE, in accordance with the stratification andthe dip of the deposits.

Marine Influences

On the French Channel coast, the main tidal wave travels from the SW towards the NE. Along the Boulonnaiscoast, the mean spring tidal range is 7 m, and the foreshore extends for 520 m from the coast, a little less near CapeBlanc-Nez (370 m). Spring tide reaches an elevation of 4 m NGF, a level at which the cliff base is not exposed to

Figure 3. Lithostratigraphy of the Cretaceous of Blanc-Nez cliffs (after Robaszynski and Amédro, 1993).



Figure 4. Lithostratigraphy of the Pleistocene deposits in the Channel-tunnel pit (after Sommé et al., 1999).

waves during fair weather conditions, except along the Crupes, the Blanc-Nez and the Fond du Guet (Figure 5). FromPetit Blanc-Nez to Strouanne, the high water spring tide (HWMST) shoreline is between 1 and 30 m from the cliffbase. Near the Cran d’Escalles and east of the Fond du Guet, depending on the width of the shingle bar that fronts thecliff, the HWMST is about 10 m from the cliff base.

The coast between Strouanne and Sangatte is oriented N 45°, parallel to the direction of the most violent storms(with gusts > 28 m/s) blowing from SW (N 240°). Due to the confines of the Channel, the incidental swells have areduce height exceeding 5 m only once every ten years on average (Clique and Lepetit, 1986). During the storm of25 January 1990, wave height off the Cape Gris-Nez reached 8 m (Wissocq, 1992). Nevertheless, even in this case, thewide, flat and largely bare shore platform that fronts the cliffs reduces wave attack and marine abrasion at the cliffbase. At Cape Blanc-Nez, Wissocq (1992) indicated that, during winter 1989–1990, violent storms without rainfallonly resulted in the removal of scree at the cliff base.

Cliff Erosion over some 60 Years

There has been a great spatial variability in the amount of cliff retreat, but three cases can be distinguished (Figure 5):

• regular retreat (sections I, III and V);

• irregular retreat ( juxtaposition of stable and unstable sectors over some 60 years, sections II and IV);

• high values of retreat over a short section (Cape Blanc-Nez and Cran Saint-Pô).

The occurrence of these cases is not only related to lithologic factors. For example, the Pleistocene of Sangatteundergoes regular retreat as well as the Turonian chalk to the west of the old German cemetery (sector I), the chalk ofthe lower and middle Cenomanian at the Crupes (sector III) and the Albian clay and the chalk of the lower Cenomanianat Saint-Pô (sector V). In the same way, sections undergoing irregular retreat occur in various stratigraphic situations(Cenomanian and Turonian chalk at Cape Blanc-Nez (II) and Cenomanian chalk at Petit Blanc-Nez (IV). Last, veryhigh values of retreat (between 30 and 40 m) occur over short sections both at the high chalk cliff of Cape Blanc-Nez



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(80–100 m) and at the low Gault–chalk cliff of Cran Saint-Pô (23 m; Figure 2, B and C). To understand this distribution,it is necessary, in addition to the structure, to take into account the elevation of the cliff and the topography at the backof the cliff, which has a direct influence on the drainage of rain water, and therefore on the mechanical behaviour ofthe rocks.

Along the chalky sections, and where the rock is permeable, hollows concentrate the water which infiltrates andalters the chalk. This occurs at the Crupes (middle Cenomanian, section III, Figure 5) and west of the old Germancemetery (middle Turonian, section I, Figure 5) where the surface runoff, channelled by the Fond du Guet, is directed.The retreat (0·21 m/year in both places) is then regular and achieved by soilfalls and small rockfalls. BetweenStrouanne and Petit Blanc-Nez (Figure 5), the regular cliff retreat is related to the outcropping of the Gault at thebottom of the cliff. However, the average amount of retreat (0·20 m/year) first increases towards the Saint-Pô Head atthe same time as the cliff rises, and then at the outlet of the Cran Saint-Pô (0·33 m/year), which concentrates surfacerunoff (chalk of lower Cenomanian has a low permeability) and canalizes it towards the sea. Consequently, the watercontent of the clay which outcrops at the foot of the cliff face varies. This variation induces collapse structures wherethe creep is slow, or mud-flows that promote rockfalls in the overlying chalk where the creep is fast (Figure 2C).

The irregular retreat (sectors II and IV) always corresponds, behind the cliff, to steep and well drained slopes,where the runoff exceeds the infiltration. The rock thus remains sound, and the spatial occurrence of the failuresbecomes uncertain. In this case, the average amount of retreat (0·14 m/year) increases with the height of the cliff(Cape Blanc-Nez – 0·25 m/year, Petit Blanc-Nez – 0·16 m/year), except for the upstream part of the Fond du Guet(80 m), where hardgrounds of upper Turonian outcrop at the top of the cliff, forming a ledge (Figure 5). Between theCrupes and Petit Blanc-Nez, irregular retreat is involved with the outcrop of a resistant nodular chalk level at the footof the cliff. With regard to the high values of retreat (up to 40 m since 1939), they are first of all correlated to theheight of the cliff at Cape Blanc-Nez, and to the topography of the back slopes at the Cran Saint-Pô.

Relevant comparisons between mean rates of cliff retreat should take into account disparities in coastal environ-ments, and above all the period of observation and methods of measurement (comparison of land registers or largescale topographic maps, photogrammetric analyses). Thus, the mean value of retreat along the chalk section (0·14 m/year) at Cape Blanc-Nez is in accordance with those obtained on the Cenomanian–Turonian section of the chalk cliffsof Pays de Caux (from 0·14 m/year to 0·17 m/year; Costa et al., 2003). Cliff top changes in the English Channel rangefrom 0·10 m/year to 0·50 m/year (May and Heeps, 1985). Cliffs cut into clastics recede at an average rate ranging from0·07 to 0·25 m/year (Jones and Williams, 1991; Zviely and Klein, 2004) which is a little less than the Sangatte cliff(0·26 m/year). Cliff retreat ranges between 0·10 and 0·30 m/year at Folkestone Warren (May and Heeps, 1985) andalso at Strouanne. Nevertheless, for risk management purposes the temporal variability in cliff recession events ismore important than the mean rates of recession.

Hydroclimatology and Rate of Retreat

The exceptionally high rate of retreat over a short section at Cape Blanc-Nez results from huge, isolated rockfalls,which are not related to climatic events (20 m recorded in 1963 and 40 m in 1993, Figures 2(B) and 5). Theoccurrence of such cliff failures is unforeseeable, and their return period is even longer since the volumes concernedexceed 100 000 m3 and the stability of the cliff is accentuated by scree at its foot, which is only slowly removed by thesea. Other events, more frequent but of lower magnitude (100 m3 to 5000 m3), are ubiquitous and occur essentiallycontinuously, as a result of rock weathering related to climatic conditions. These events induce constant and more orless regular cliff retreat.

However, in 1939, 1970, 1988 and 2000, the cliff underwent generalized retreat (the whole length of the clifflineretreats in a single erosional episode – involving several failures; Figure 6). Daily observations during November 2000enabled us to investigate the role of overland flow on cliff retreat (Lahousse and Pierre, 2003a, 2003b), and analysis ofwater-table fluctuations made it possible to isolate the effect of this factor on cliff stability.

Rainfall and retreat rateFigure 6 shows the years of major retreat, and the coastline sections concerned. Four episodes had a wide extent: thoseof 1939, 1970, 1988 and 2000. It is probable that the cliff at Sangatte in Pleistocene deposits and the one that lines theCrupes was affected in 1939, 1970 and 1988, but the quick removal of small and fine grain size screes makes theiridentification uncertain. Over these two sections, retreat was achieved by high frequency and low magnitude events.Four other episodes had a smaller extent: those of 1966, 1981, 1986, and 1995. Thus, the entire stretch of the coastlinedoes not evolve in a synchronous way.



Figure 6. Retreat rate of the Blanc-Nez cliffs (shading: sections with cliff failures, and dating of failure occurrence).

Since 1950, the occurrence of years with rainfall above normal (674 mm in Boulogne, 843·2 mm in Guines, Figure 7)did not necessarily result in episodes of generalized (such as those of 1939, 1970, 1988 and 2000) or partial retreat(1966, 1981, 1986 and 1995). It is not even possible to affirm that the cliff is unstable during particularly wet years (asin 1981, 1986, 1988), since in 1974 (total annual rainfall 1137·1 mm in Guines) the cliff remained stable. Conversely,the erosive episode of 1970 took place during a multiannual phase of weak precipitation. It is not obvious that theregular increase in annual rainfall since 1950 has multiplied the major erosive episodes: their occurrence remainsirregular. Finally, the considerable decrease in total rainfall during the last three years (2001, 2002, 2003) cannot beinterpreted as the direct cause of the steady state of the cliff since December 2000, for this diminution is effective onlyfrom fall 2001 (rainfall totalled 170 mm in September 2001, compared with a normal monthly average of 65·5 mm).

Between Petit Blanc-Nez and Cape Blanc-Nez, it has been demonstrated that, other things being equal, the episodesof significant retreat occur only if the cliff is not in steady state (Lahousse and Pierre, 2002). The return period of anunstable state thus varies from 4 years (1966–1970) to 16 years (1970–1988), independently of the annual variation ofrainfall. Hence, it appears logical that the cliff remained stable during the abundant rainfall of 1974, which followedclosely the erosive episode of 1970. However an analysis of the rainfall indicates that the annual totals can hide quitedifferent monthly distributions, which determine the dynamic of the aquifer (s.l.). This seems to play a significant rolein the retreat rate and in the way events unfold.

Hydrogeological behaviour of the aquiferExcept for the Crupes where the groundwater, lodged in a synclinal structure, can come out of springs which follow thecontact line between the blue chalk and the grey chalk, the higher level of the water-table is always at a lower altitude

Figure 7. Mean annual rainfall, weather stations of Boulogne-sur-mer (1950–2000, after Météo France) and Guines (1970–2003,after Eaux de Calais). Dark shading, generalized erosional episode; pale shading, partial erosional episode.



Figure 8. Depth of the water-table ( January 1989–December 2003) in the borehole of Escalles (after Eaux de Calais).

than that of the foot of the cliff, which is between 3 and 5 metres (Figure 1). Under these conditions, the piezometricsurface has no influence in itself on the stability of the cliff (Lahousse and Pierre, 2003b). However, fluctuation of thewater-table testifies to the changing state of the unsaturated zone, as illustrated by the comparison between twoperiods, from January 1997 to December 1998 and from January 2000 to December 2001. As the piezometric surfacedata available at the borehole of Escalles were a little irregular between 1989 and 2000 (monthly recording, Figure 8),we kept the pumping water level for 1997 to 1998. Consequently, the rest water level stands some 2 m above thevalues of the graphic (Figure 9(D)), that is to say the height of the cone of exhaustion during pumping.

Figure 9. Monthly rainfall and depth of the water-table (2000–2001 and 1997–1998); weather station of Boulogne-sur-mer (afterMétéo France), weather station of Guines and borehole of Escalles (after Eaux de Calais).



These two sequences have a similar pluviometric profile: more abundant rainfall in autumn, with in both cases apeak in November of the first year (237·6 mm in Guines and 302·2 mm in Boulogne in November 2000; 208·8 mmand 221 mm in November 1997), which shifts to September the second year (Figure 9(A) and (C)). These values areabove the monthly average over 30 years (i.e. 102·9 mm in Guines, and 81·7 mm in Boulogne). Meanwhile, thepiezometric curves are not superposable. The precipitation of November 1997 had no effect on the water-table beforeJanuary 1998 (rest water level then reached −13 m), while those of November 2000 had an immediate impact: thewater-table rose abruptly to −11 m (Figure 9(B) and (D)). This quick response is the result of a ‘piston flow’.

The hydraulic conductivity in Cenomanian chalks ranges around 10−4 m/day (Price, 1997), and the height of theunsaturated zone of the chalk aquifer varies from about 10 m (Cran d’Escalles) up to 90 m (Cape Blanc-Nez, Figure1). So, as the water moves in the unsaturated zone of the aquifer by successive waves of infiltration (Horton andHawkings, 1965), the water which reaches the water-table during a rainfall event is not the rain-water but theantecedent water content of the soil. Hence, even if a heavy rainfall event in a context of soil moisture deficit mayexceptionally induce drainage up to 30% (Jones and Cooper, 1997), recharge to the aquifer will not exceed a fewmetres, and no elevation of the water-table will occur.

Heavy rainfall during the first 10 months of 2000 (795·8 mm – the mean for these months is 525·7 mm) consider-ably increased the water content of the soil sensu lato, so that the precipitation of October 30 to November 2 prompteda fast rise of the piezometric surface (from −15·5 m to −12·5 m), and those of November 20 (40 mm) had a nearlyimmediate effect on the water-table, which rose from −12·5 m to −11 m in a few hours (Figure 10(A) and (B)). Duringthis period, the unsaturated zone of the aquifer experienced considerable hydrostatic pressure and an excess load(chalk porosity ranges around 40%), both acting against the equilibrium of the cliff.

Data for the year 1997 illustrate the percolation rate in a soil moisture deficit situation. As the beginning of the yearwas less humid than the average (453·1 mm for the first 10 months, instead of 525·7 mm), and in spite of a very wetNovember (221 mm and 208·8 mm in Boulogne and in Guines respectively) and the occurrence of a heavy rainfallevent on 10 November (67 mm, Figure 10(C)), the water-table, from the relatively low level of −17 m (rest waterlevel), rose gradually until January 1998 (Figure 9(D)). In February 1998, total rainfall was 12·6 mm, and the water-table fell from −13 to −14 m (rest water level), while a single and medium-sized rockfall (5000 m3) occurred north ofthe Cran d’Escalles on 25 February 1998.

The hydroclimatic conditions leading to ‘piston flow’ were not fulfilled in autumn 1998: the rise of the water-tableoccurred one month after the rainy months, because the beginning of the year was not particularly wet (total rainfall ofthe first 8 months was 386 mm, instead of 381·5 mm in a normal year). Neither were they in 2001, although the yearwas wet (632·9 mm during the first 8 months). In this case, the rainy month (September, 175·6 mm) followed tooclosely the summer period during which the evapotranspiration induced moisture deficit in the higher fringe of thesoil, reducing downward flow.

As piezometric surface data are not available before 1989 it is difficult to be sure that, where the aquifer develops,the ‘piston flow’ mechanism occurred during the preceding episodes of retreat (more especially as the exact date ofthe events is seldom known). However, the pluviometric rate of winter 87–88 makes the case possible (Figure 11).Indeed, rainfall from October 1987 to January 1988 totalled 524·7 mm (instead a normal 285·8 mm), involving aconsiderable recharge. There was a heavy shower (21·3 mm) on 5 February 1988, and on 11 February the newspaperLa Voix du Nord reported a series of collapses. The other episodes of more or less generalized retreat, even if they arenot included in a low rainfall period like the one of 1970, are not related to this hydrogeological mechanism. Theseepisodes are probably related to factors such as high precipitation and surface runoff, eventually combined with stormsduring spring tides. Isolated and small or medium failures can occur at any time.

Conclusion

The hydrodynamic behaviour of the aquifer in November 2000 contributed to the occurrence of an exceptionalmorphodynamic episode distinguished by its extent and by the amount of cliff retreat. In some places, almost allthe retreat since 1939 occurred between 2 and 22 November 2000 (Pierre and Lahousse, 2003). Such a succession ofhydroclimatic events is rather infrequent, as it is rare that the matric pore space remains completely water filled inchalk, so that the flow occurs through cracks prompting an instantaneous rise of the water-table (Price, 1997). In termsof management, it would be imprudent to assert that this scenario should necessarily result in destabilization of thecliff, as the triggering factors would have no effect if the cliff had not reached a disequilibrium point. This is why adaily survey of rainfall and groundwater level is not sufficient to predict cliff behaviour, more especially as we haveno accurate data on the rate of evolution over 60 years. Thus, except for small and frequent isolated events, three casesmay occur:



Figure 10. Daily rainfall, November 1997 (Guines, after Eaux de Calais) and November 2000 (Boulogne, after Météo France);depth of the water-table, November 2000 (borehole of Escalles, after Eaux de Calais).

• hydroclimatic conditions are favourable for cliff failure, but the cliff is in steady state; small to medium isolatedfalls (up to 5000 m3) may occur as well as no falls at all;

• hydroclimatic conditions are favourable for cliff failure, and the cliff is unstable; a generalized or partial retreat,with falls of variable sizes (100 m3 to 8000 m3), leads to a more or less sustainable equilibrium;

• huge and hazardous falls (100 000 m3) of the highest section of the cliff lead to a sustainable equilibrium (40 yearsat Cape Blanc-Nez!).

Nevertheless, it is obvious that in a single layered chalk aquifer the rise of the water-table is not responsible for highhydrostatic pressures in matric pores and an excess load leading to cliff failure (Caudron et al., 2001), since the depthof the unsaturated zone generally exceeds the height of the cliffs (Bracq and Brunin, 1999; Duperret et al., 2002a). Onthe contrary, the observation of a ‘piston flow’ during November 2000 highlights the role of water content in theunsaturated zone on cliff instability. Such a mechanism should not occur as it stands in a multilayered aquifer, such asthose developed in Turonian and Coniacian chalk (cliffs of the Pays de Caux or East Sussex; Duperret et al., 2002a),unless the fractures contribute significantly to water movement.

The Gault–chalk cliff of Saint-Pô and the Pleistocene cliff of Sangatte, which have experienced an almost continuouschange over 60 years, are not affected by this particular mechanism. The rate of their retreat may increase during wet



Figure 11. Rainfall data, 1988, Boulogne-sur-mer (after Météo France).

periods, but it is not related to fluctuations of the water-table in the chalk. At Folkestone Warren (UK), Hutchinson(1969) demonstrated that there is no relationship between the majority of the groundwater in the Gault slipped massesand that of the inland chalk aquifer. Actually, variation of the liquid limit in the Gault and mass movementsat Strouanne (Figure 2(C)) are the result of the infiltration of rain water on the cliff face where the Gault outcrops(Pierre and Lahousse, 2004). Groundwater action on weakly cemented clastics (Norris, 1990) is well illustrated by theprocesses observed in the bedded deposits of the Sangatte cliff. Runoff water on the cliff backslope percolates throughopen cracks and flows out on the cliff face, creating tapping holes and gullying, which finally reaches the top ofthe cliff face. The gullying eventually causes debris falls, in addition to desquamation and small liquid soilfalls(Figure 2(A)).

The single, medium-sized rockfall of 25 February 1998, was the first significant event to occur since a 1995 lawinvolving mayors’ responsibility for public safety was enforced. This event led to restrictions on access to thecliff, near the sector thought to be unstable. As a result of the episode of generalized retreat in November 2000, riskmanagement for this highly visited site (up to 1 million people each year) now focuses on public information ratherthan on restriction. The advisory medium is signposting, and the warning is ‘Danger, rock fall hazard’, although itshould be ‘The cliff, in constant evolution, retreats at an average rate of 0·14 m/year, but the probability of aninstantaneous retreat of 40 m is to be considered!’.

AcknowledgementsThis work was funded by the French Direction Régionale de l’Environnement du Nord–Pas de Calais (grant No. 8459). The authorswish to thank Professor A. S. Trenhaile (University of Windsor, Ontario) for his critical and helpful comments.

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The role of groundwater in cliff instability: an example at Cape Blanc-Nez (Pas-de-Calais, France)

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