1The French Riviera (Fig. 1) forms the maritime rim of the French Alps, and is the steepest continental margin in France. The attraction exerted by this part of the Mediterranean coast over the last decades has generated not only a significant development of coastal tourist and recreational facilities and infrastructure (Anthony, 1994), but also rapid urban and peri-urban growth. This attraction is hinged not only on the climatic qualities of this sunny Mediterranean coast but also on the bold landscapes offered by a steep coastal margin. The rapid socio-economic development of the French Riviera has been matched by the emergence of numerous environmental problems paradoxically related to the climatic and landscape qualities of this coast. The steep coast and shelf gradients, the Mediterranean climate and river discharge pattern, and high population pressures jointly generate a number of natural risks ranging from landslides to chronic beach erosion, the diversity and concentration of which render this Riviera coast exceptional in terms of coastal hazards. These hazards are particularly marked in the lower river valleys indenting the Riviera coast because of the space they offer for development.
2Within this coastal complex, five major hazards may be singled out (Fig. 2): earthquakes, landslide activity on the steep mountain slopes and the hills surrounding Nice, flooding and flood-generated sedimentation in the lower river valleys, beach erosion, and submarine landslides on the steep Var delta front. Seismic activity, an outgrowth of the regional tectonic setting, constitutes a potential hazard on this coast, as earthquake vibration effects may be amplified by the hilly topography surrounding Nice (Julian and Anthony, 1996). The Riviera is part of a passive margin reactivated during the Quaternary by compressive movements between the African and European plates (Mascle and Rehault, 1991). Although there are no modern examples of damaging earthquake activity, earthquakes may also have a destabilising influence on both terrestrial and marine terrain by generating landslides that could be potentially hazardous in the coastal zone. Following heavy precipitation events, the hilly terrain also generates numerous minor landslides that are not only a common hazard and nuisance to road users and residences, but also necessitate onerous clean-up operations. The review will focus on the last two types of hazards: river flooding and sediment surcharge in the coastal zone and submarine landslides on the steep shelf slope.
3These two types of hazards are well illustrated by the Var river floodplain and delta (Fig. 1), by far the most important river on the French Riviera, and the main water and sediment purveyor to the Riviera coast. Processes liable to be involved in the generation and durability of geomorphic and hydrological hazards are briefly examined. The review shows that a fine comprehension of the sediment dynamics and geomorphic processes affecting the lower Var valley, and of the hazards they generate, is hampered by a lack of data on these aspects, notably morphology and sediment supply interconnections, and their spatial and temporal scales.
Fig. 1 -The French Riviera, and a Spot image of the lower Var river valley and the airport constructed on the reclaimed delta.
La Côte d’Azur, et une image SPOT de la basse vallée du Var et de l’aéroport construit sur le delta.
White bars across the Var depict the last four downstream gravel-retention dams. Two other dams further downstream were destroyed by the exceptional November 1996 flood.
Les barres blanches sur le Var montrent quatre des barrages de rétention de graviers. Les deux derniers barrages en aval ont été détruits par la crue exceptionnelle de novembre 2006.
Fig. 2 - Diagram summarising the five potential, and inter-connected, coastal hazards affecting the French Riviera, generated jointly by the geologic, geomorphic and climatic characteristics of this Mediterranean margin.
Diagramme résumant les cinq risques côtiers liés auxquels est potentiellement exposée la Côte d’Azur, et engendrés conjointement par le contexte géologique et géomorphologique et les caractéristiques climatiques de cette marge méditerranéenne.
Grey box comprises a set of complex, poorly apprehended inter-connected hazards that include exceptional flooding of the lower Var valley associated with hazardous sediment surcharge, and cut-off of gravel supply to the neighbouring beaches in the Baie des Anges, thus exacerbating the beach erosion hazard (see Cohen and Anthony, this issue).
La boîte grise représente une série complexe de risques liés et mal connus, y compris des crues exceptionnelles dans la basse vallée du Var responsables d’une surcharge en graviers, et l’arrêt de fourniture de graviers aux plages de la Baie des Anges, ce qui exacerbe le risque d’érosion de ces plages (voir Cohen et Anthony, ce volume).
4The various rivers debouching in the Baie des Anges have relatively narrow valleys that are all prone to flooding. Because of the size of its lower valley and the presence of a delta, the Var is the most important of the hazard-prone rivers emptying into the Baie des Anges. Sustained development pressures on the Var have considerably affected shoreline stability by modifying sediment inputs to both the delta front and the gravel beaches bounding the bay. In this regard, the Var is a fine example of interconnected hazards associated notably with river discharge and sediment cascades from the mountains to the sea (Anthony and Julian, 1999).
5The Var is a 135 km-long river with an extremely varied catchment lithology composed of various types of limestones, marls, sandstones and conglomerates (Julian, 1980). The valley morphology is narrow throughout. The lower-valley gradient is moderate, at 4.3 m km-1, in the last 16 km down to the sea. Several gravel-retention dams were built in the 1970s just upstream of the coastal reach (Fig. 3a). Much of the narrow lower valley downstream of these dams is occupied by a rectilinear braided gravely channel (Fig. 3b) that shows progressive downstream incision of up to 10 m at the river-mouth. The Var river mouth forms a Gilbert-type deltaic protuberance built up during the Pleistocene and the Holocene (Dubar and Anthony, 1995). The flow regime of the Var may be considered as seasonal, but characterised by torrential discharge, typical of the Mediterranean Alpine margin. Discharge is relatively high in autumn and spring, and low and variable in summer. Measurements of liquid discharge concerning the lower Var valley are rather irregular over time. The estimated mean discharge is about 70m3 s-1, and the mean maximum flood discharge about 830m3 s-1. Discharge can increase dramatically in a matter of hours following sustained rainfalls. As shown by the data for two critical years, 1994 and 1996 (Fig. 4), the significance of which will be discussed later, discharge is so variable, however, that such mean values tend to become meaningless, so that attention is rather given to peak 100-yr and 1000-yr flood return intervals. As indicated later, such peak return intervals have changed with the recurrence of several ‘exceptional’ floods over the last three decades. The lower Var valley is the most important water reservoir for the Nice conurbation, while the floodplain is also an ecologically important zone.
Fig. 3 - Ground photographs of a gravel-retention dam damaged by the November 1994 flood (A), and braided gravely channel in the coastal reaches downstream of the dams during low discharge (B).
Photographies d’un barrage de rétention de graviers endommagé par la crue de novembre 1994 (A), et du lit anastomosé du Var près de la côte, en aval des barrages, durant une période d’étiage (B).
Source: Edward Anthony, August, 1997.
6As with liquid discharge, reliable data are wanting on the sediment budgets in the lower Var valley. Estimates of the suspension load attaining the lower Var valley vary widely, from 5 to 20 million tonnes a year (Sage, 1976), to 0.6 to 1million tonnes a year (Mulderet al., 1998). Although useful in modelling sediment transport rates, the relevance of an annual estimate of suspended loads may be questionable since such loads can significantly fluctuate from one year to the next. The bedload (dominantly gravel) has been estimated at 100,000 to 200,000m3 a year (Sage, 1976). Much of the solid load of the Var is supplied during high-discharge episodes from poorly consolidated deposits and from loose material released by landslides (Fig. 2) in the upper and middle catchment areas following intense autumn rainfalls and spring melt water flows and rainfalls (Anthony and Julian, 1999). Such sediment loads undergo source-to-sink sorting, with rapid downstream evacuation of increasingly finer-grained material, compared to the gravel bedload, which works its way downstream much more slowly, under the influence of episodic high-energy floods. Hence the importance of the suspension load attaining the lower Var valley and delta.
7The exposure of the lower Var valley to flooding has somewhat moderated residential development in this valley. However, the strong development pressures have spilled onto the valley, essentially in the form of space-consuming but less ‘risky’ agricultural, industrial and administrative estates, and transport infrastructure and installations. The lower floodplain and delta of the Var form an important multi-mode transport hub. The most important development included the complete reclamation of the delta itself for the progressive expansion of Nice Riviera international airport (Fig. 1), France’s second busiest airport, with a potential of 13 million passengers. The reclamation went far beyond the original surface area of the subaerial delta itself, as up to 3.5 km² of additional space was gained by reclamation spillover onto the steep delta front through an impressive series of groynes, embankments and infill structures (Fig. 5). This armouring of the artificial shores of the delta has deprived the beaches bounding the Baie des Anges of gravel, exacerbating the beach erosion hazard (see Cohen and Anthony, this volume). Space consumption by development activities at the expense of the floodable lower valley and delta has been accompanied by the construction of flood-protection embankments. Rapid development in the 1960s and 1970s also created an acute need for gravel for construction, resulting in bed extractions. Both the narrowing of the river channel and aggregate extraction led to a lowering of the main channel bed by up to 10 m in the last few kilometres upstream of the delta (Guglielmi, 1993). Aggregate extraction and massive pumping of the alluvial aquifer also resulted, in the 1970s, in a significant lowering of the aquifer and increasing saltwater intrusion (Guglielmi, 1993), a difficult situation in the face of increasing socio-economic development with important water consumption needs. Remedial measures aimed at restoring aquifer performance included regulation of aggregate extraction and the construction of eleven gravel-retention dams across the lower channel (Fig. 1).
8Not unsurprisingly, these various forms of intrusion on the floodable lower valley could only result in increasingly greater risks of flood damage during phases of exceptional river discharge. This happened in 1994, and then in 1996 (Fig. 4). The November 1994 flooding event (peak discharge of 3770m3 s-1 on November 5, compared to a proclaimed 100-yr return interval discharge of 2000m3 s-1) was caused by highly localised but heavy rainfalls that affected the upstream Var catchment from November 2-5, with peak falls over a 12-h period on November 5 (Carrega, 1994) that were rapidly evacuated downstream in a water-saturated channel. This event was particularly devastating, as it led to considerable damage to the industrial and administrative estates in the lower Var valley, transport (rail and road links) infrastructure and airport following prolonged flooding lasting over two weeks. Embankments and the last two dams downstream were damaged (Fig. 3a), and millions of tons of fine-grained sediments that had been trapped over the years behind the gravel-retention dams were reworked and deposited throughout the flooded lower valley and reclaimed delta, necessitating expensive clean-up operations. Moreover, flood sediment supply from the upper catchment and important seaward evacuation of fine sediment from the lower river valley by this flood following dam bursting led to a significant gravity flow on the submarine delta slopes (Mulder et al., 1998, 2001b). The river velocities were so high as to entrain several blocks of 1m3 composing the damaged dams over 100 m downstream. Indeed, an important lesson to be gleaned from geophysical studies carried out on the Var submarine slopes (see following section) concerns the fact that the abundant gravel found along the Var canyon and submarine fan (Klauckeet al., 2000) clearly reflects the transport capacity of high-magnitude flooding events that periodically affect the lower Var valley and delta. Following the 1994 flood, the 100-yr return flood peak was upgraded from 2000m3 s-1 to 3500m3 s-1. The January 1996 flood attained a peak discharge of 1310m3 s-1, but caused less damage as a result of both the lower peak discharge values and the valley incision caused by the November 1994 flood.
9Since the 1996 flood, the lower valley has been spared by the flooding hazard, largely because there have been no significant floods since. Moreover, the important seaward evacuation of fine sediment from the lower river valley by the November 1994 flood must have created accommodation space for flood waters. Under these circumstances, it is not unreasonable to assume that the last decade may have been one of channel bed accretion through trapping of new incoming sediment in the accommodation space created by the erosion-dominated floods of 1994 and 1996. There are, unfortunately, no data on bed elevation changes, but expected gradual accretion of the lower valley bed may pave the way for flooding in the coming years.
Fig. 4 - The 1994 and 1996 liquid discharge curves for the Var river near the delta.
Les courbes de débits liquides du Var près de son delta en 1994 et 1996.
Note the exceptional November 1994 flood peak. Source: http://www.hydro.eaufrance.fr/.
A noter le pic de la crue exceptionnelle de novembre 1994.
Source : http://www.hydro.eaufrance.fr/
The landfill zone for the accommodation of Nice international airport adjacent to the extremely narrow shelf and submarine canyon morphology fronting the Var delta, and the October, 1979 submarine landslide zone that included the complete destruction of a harbour under construction (inset).
La zone de remblai sur le delta du Var aménagée pour l’aéroport international de Nice en bordure du plateau continental très étroit et le canyon sous-marin qui prolonge le Var. Encart : zone affectée par le glissement de terrain sous-marin qui a complètement détruit un port en construction.
10The steep coastal morphology of the French Riviera is mimicked by the extremely narrow continental shelf and submarine slopes (Fig. 5), the morphology of which reflects the effects of the Messinian salinity crisis and significant thermal and water load-induced flexural subsidence during the Quaternary (Savoye and Piper, 1991). The combination of sudden high-discharge floods, the relatively steep lower valley gradient, the important fine-grained discharge, and the deep continental shelf contribute in generating suspension plumes (Fig. 6) from which settling feeds the Var delta front and prodelta in mud. Turbulent mixing favourable to flocculation and settling is assured by wind forcing and waves, and probably by submarine freshwater discharge on the delta front. However, under very high solid discharge conditions, density flows may develop, generating mass movements of various intensities (Mulderet al., 1994, 1997, 1998; Klauckeet al., 2000; Migeonet al., 2002, 2006). The unconsolidated muds of the delta front and slope are strongly ravined by these movements (Leuridanet al., 1988; Klauckeet al., 2000), and the ravines and canyons serve as active mud transport pathways from the delta to the abyssal plains (Bellaiche, 1993).
11While landslides on the delta front are common, they are generally low-volume events (Mulderet al., 1994), and therefore do not constitute a direct hazard to the coast. Apparently, although irregular, river discharge and sediment supply to the delta front are sufficiently high over a timescale of a few years as to regularly feed the delta front in sediments that are evacuated every four to five years by gravity flows (Mulderet al., 1997). This should result in slope regulation, thus precluding large-sized events on this small Var delta.
12However, in contrast to the foregoing observation, the delta front experienced a singular massive landslide on October 16, 1979 (Fig. 5) that had all the makings of an exceptional event (Anthony and Julian, 1997). Although this event was taken for a natural landslide linked to a turbidity current, the true causes are more likely related to a combination of exceptional loading of the unstable delta front slopes by the reclamation operations and strong, but not unexceptional, river discharge (Anthony and Julian, 1997). Nearly 11million tonnes of fill (Fig. 5) were emplaced in a space of six months prior to the landslide, at depths of up to –25 m and distances of up to 300 m offshore from the delta shoreline (de la Tullaye, 1989). The hazardous character of this major submarine landslide was undoubtedly due to the landfill at the rim of the delta. This landslide caused several casualties, destroyed part of the reclamation works on the upper delta front, including a harbour under construction at the time (Fig. 5), and generated a small tsunami that underwent resonance in the Baie des Anges, leading to further damage of sea-front property. The trigger mechanism was probably sustained flood discharge with a peak liquid flow that attained 1200m3 s-1 on the day of the event. This gravity flow likely represents the re-establishment of an equilibrium delta front slope following the massive artificial reclamation loading of the upper delta slope over a very short period (less than one year).
Fig. 6 - Oblique aerial photograph of the Var delta during a high-discharge phase (January 1996) characterised by a suspension-sediment plume.
Photographie aérienne oblique du delta du Var pendant une crue (janvier 1996) et son panache turbide au large.
Such plumes are the common fine-grained sediment purveyors to the shelf, and are generally diverted by wind-induced currents to the southwest. Courtesy of Pierre Carrega, January, 1996.
Ces panaches turbides constituent le mécanisme d’approvisionnement normal des sédiments fins d’origine continentale au plateau continental, et sont généralement déviés vers le sud-ouest par des courants induits par le vent. Photo de Pierre Carrega, janvier 1996.
13The Nice area of the French Riviera is a fine example of a risk-prone coast that has undergone significant urban growth in the last few decades, thus increasing pressures on a coast with steep geomorphology and rivers subject to variable but potentially high discharge. This has had the effect of exacerbating hazards due to flooding and direct structural damage to infrastructure from sediment loading, especially in the lower valley and delta of the Var river. Landslides provide significant quantities of both coarse and fine sediments that are transported downstream by episodic high-energy floods.
14However, while a significant amount of work has been carried out on the submarine slopes of the Baie des Anges, and especially on the Var submarine system, leading to a much better understanding of the sediment transport processes operating in this zone, and therefore their potential hazards (Leuridanet al., 1988; Savoye and Piper, 1991; Bellaiche, 1993; Mulderet al., 1994, 1998, 2001b; Klauckeet al., 2000; Migeonet al., 2002, 2006), the same level of understanding has not been gained on the more critical subaerial coastal landscapes that have been largely urbanised. Although delta-front processes associated with river discharge may pose a threat to submarine cables, the sediment movements they involve are not a hazard to the coast because they essentially concern geomorphic regrading and equilibrium mud cascades downslope under high river discharge conditions. The damaging 1979 turbidity current must be considered as a totally exceptional event since the gravity flow involved in this event appeared to have been generated by failure of landfill deposits on the upper rim of the delta that caused further downslope erosion and release of consolidated and under-consolidated delta-slope sediments. Similarly, the density flow deduced by Mulderet al. (1998, 2001b) following the 1994 flooding event was undoubtedly generated by massive downstream sediment transport and sediment release from behind the gravel-retention dams in the lower Var.
15The geophysical ‘unity’ of the submarine realm is lacking in the subaerial portions of the lower Var system, where the diversity of actors and stake-holders, the variety of geomorphic and hydrological processes, and rapid development pressures have tended to blur the image of the interconnected hazards that lie await. Anthony and Julian (1999) argued that planners and environmental engineers have had a rather poor grasp of the geomorphic and hydrological relationships operating within the Var system, while the pressures of rapid urban development have clearly outpaced foresight in valley and coastal management. Although there has been a considerable amount of scientific work, much of it is grey literature (readily available through the internet), replete with recommendations that still need to be implemented, and there appears to be a lack of durably coordinated action indispensable in addressing issues related to complex environmental systems. There are, for instance, no complete liquid discharge data for the Var since 2000 (see http://www.hydro.eaufrance.fr/), and no systematic sediment budget surveys and cartography of geomorphic and bed level changes.
16There has been a rather poor appreciation of the related processes of sediment sourcing, temporary or durable storage in channels, sediment textural segregation, timescales of segregated sediment transport and storage downstream, sediment budgets, and lower river channel, floodplain, delta floodplain, delta front, and beach morphodynamics. The role of dams in modulating the sediment budgets and in inducing sediment segregation of fine-grained sediment and bedload, and the potential effects of sudden dam bursting as happened during the course of the exceptional November 1996 flood, have not received coordinated attention. Fine-grained sedimentation, for instance, has been suspected to progressively form an impermeable surface aquifer limit that tended to ‘seal off’ the free river discharge from the groundwater aquifer, thus impeding the recharge of the latter, and affecting the flow characteristics of the Var (Guglielmi, 1993), but the way this may have exacerbated the flood hazard is not clear.
17The problem is compounded by the marked spatial and temporal variability of high-magnitude rainfall and flooding events, not to mention unpredictable but potentially hazardous earthquake activity, in the face of the necessity of balancing strong socio-economic development pressures. The diversity and concentration of coastal and coast-related hazards in the Baie des Anges does not simply require a recalculation of the 100 and 1000-yr flood peaks but clearly call for a full systems approach involving the coordinated input of earth scientists from various disciplines.