Chronic offshore loss of nourishment on Nice beach, French Riviera: A case of over-nourishment of a steep beach?

Résumé En

The 4.5 km-long gravel beach fronting the exclusive resort of the city of Nice, on the French Riviera, insoutheastern France, was artificially nourished from 1976 to 2005 to the tune of 558,000 m3, making thislong-term operation one of the most significant for gravel beaches in the world. Nourishment has ranged fromnil in certain years (1979, 1980, 1983–85, and 2001–2002) to a peak of over 97,000 m3 in 2000. Analyses of 50transects covering the beach highlight no significant change in net beach width over this 30-year period ofmassive gravel nourishment. A Principal Components Analysis and a Cluster Analysis used to detect patternsin the 87 beach-width measurement dataset show no clear spatial trends in transect groups that can beinterpreted in terms of the morphology of the beach and the steep inner shoreface. Significant wave height offNice shows no change over the period 1979–2005. Since there is no possibility for alongshore gravel leakageon the strongly embayed Nice beach, the relative stability in beach width clearly implies loss of rechargedgravel offshore. Gravel loss following nourishment is favoured by: (1) the steep inner shoreface inheritedfrom the geological context of Nice beach at the flanks of the southern Alps, and (2) the practise of artificialbeach widening through flattening, in summer, of a narrow (5–15 m-wide) mobile zone of the profile in orderto enhance the ‘carrying’ capacity of this highly touristic beach. Beach widening and flattening followingnourishment bring close to the very steep inner shoreface zone several cubic metres of gravel for each metreof beach that may be permanently lost downslope during autumn and winter storms. Recharged gravel isredistributed alongshore and offshore leakage is probably enhanced where small narrow submarine canyonheads impinge on the beach, resulting in a very narrow shoreface. Mean beach width shows an oscillatingalongshore pattern that may be due to the influence of these canyons as pathways of gravel loss offshore.However, there is no correlation between mean beach width and distance to the 10-m isobath, used as asurrogate for inner shoreface width. Storms are associated with plunging waves that are particularly effectiveand concentrated, on this almost tideless shore, over the narrow mobile zone of the beach profile where aseries of steep reflective berms are built during storms. The high dynamic pressures associated with thisnarrow zone of concentrated wave breaking, and energy reflection from the steepened profile, are deemed tocontribute to the permanent downslope loss of gravel. This situation of long-term gravel loss is probablyaccepted by the beach management authority because of the low cost of obtaining nourishment material andthe advantages derived from a temporarily wider beach in terms of recreational space.