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Wetland Vertical Accretion and Soil Elevation Change in the Rhône River Delta, France: The Importance of Riverine Flooding

Philippe E. Hensel, John W. Day Jr. and Didier Pont
Journal of Coastal Research
Vol. 15, No. 3 (Summer, 1999), pp. 668-681
Stable URL: http://www.jstor.org/stable/4298983
Page Count: 14
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Wetland Vertical Accretion and Soil Elevation Change in the Rhône River Delta, France: The Importance of Riverine Flooding
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Abstract

Accretion and surface elevation change were measured in riverine, marine and impounded wetland habitats of the Rhone Delta from 1992 to 1996 using a sedimentation-erosion table (SET) and marker horizons. Riverine habitat accreted at a significantly greater rate than the other habitats throughout the period of study, averaging $13.4 \pm 7.0 mm yr^{-1}$ compared to $1.1 \pm 0.1$ and $1.2 \pm 0.5 mm yr^{-1}$ for impounded and marine habitats, respectively. Elevation change was similar to accretion in the riverine habitat ($11.3 \pm 6.1 mm yr^{-1}$), reflecting an average 16% compaction and consolidation of recent, primarily mineral deposits. Over time, elevation change and accretion became more linearly correlated, showing that variation between these two processes decreases with time. Accretion and elevation change in impounded and marine habitats were less than current rates of relative sea-level rise, a result of isolation from riverine flooding and the lack of marine storms during the study period. There was more than 30 mm of accretion in the riverine habitat deposited during the 50 and 90-year floods in the Rhone in 1993 and 1994. Impounded and marine habitats gave no record of these events. Wetlands connected to the Rhone River can therefore accrete rapidly from sediments deposited during floods. Impoundments, the most common "natural areas" left in the delta, are not keeping pace with relative sea-level rise and may become vulnerable to increased sea-level rise if current management practices are continued. /// Une $\acute{e}tude$ d'accretion et des changements de niveaux du sol a $\acute{e}ffectu\acute{e}$ dans les marais caractéristiques du Delta du Rhone entre 1993 et 1996, utilisant une "Table a Sedimentation et Erosion" (S.E.T.) et des horizons de kaolin. $L'accr\acute{e}tion$ dans les marrais en connection avec le fleuve etait significativement $sup\acute{e}rieure\grave{a}$ celle des autres milieux (moyenne: $13,4 \pm 6,0 mm an^{-1}$ contre $1,1 \pm 0,1$ et $1,2 \pm 0,5 mm an^{-1}$ pour les milieux marins et endigues, respectivement) Les changements du niveau du sol étaient semblables à $l'accr\acute{e}tion$ dans les marrais en connection avec le fleuve ($11,3 \pm 6,1 mm an^{-1}$), soulignant le faible compactage et consolidation des $d\acute{e}pots$ $r\acute{e}cents$, forts en teneur minerale. Globalement, la $corr\acute{e}lation$ entre $l'accr\acute{e}tion$ et les changements de niveaux devenait plus forte a travers le temps, montrant que la $variabilit\acute{e}$ entre ces deux proces s'affaiblit à la longue. $L'accr\acute{e}tion$ etait $inf\acute{e}rieure$ à l'augmentation du niveau marin dans les sites marins et $endigu\acute{e}s$ $d\hat{u}$ au manque d'apports fluviales ou marins. l y avait plus de 30 mm $d'accr\acute{e}tion$ $d\acute{e}pos\acute{e}$ suite aux crues de 50 et 90 ans en 1993 et 1994. Les milieux marins et $endigu\acute{e}s$ n'ont $enr\acute{e}gistr\acute{e}$ auqu'un effect de ses crues. Les résultats montrent la $capacit\acute{e}e$ du fleuve comme source de $s\acute{e}diments$ pour les milieux inondables, $malgr\acute{e}$ les $am\acute{e}nagements$ dans le bassin versant. Les milieux $endigu\acute{e}s,$ qui comprennent la $majorit\acute{e}$ des terrains $delta\ddot{i}ques$, sont $d\acute{e}fficients$ en $s\acute{e}dimentation$ et $pr\acute{e}sentent$ un risque $vis-\grave{a}-vis$ à l'accroissement du niveau marin si la gestion du delta reste insensible à l'importance des apports fluviaux.

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