Deltas on the move; Making deltas cope with the effects of climate c

KvR report 001/2006The ranking of deltas for our selection of specific indicators for ‘soft’ system-basedapproaches (Table 5.4) is discussed in the next sections.Best deltas for restoring sediment dynamicsThree indicators for the potential of restoring sediment dynamics are shown in Figure6.10. These are delta area, uplift or subsidence, and offshore slope. Other specificindicators mentioned in Table 5.4 are area coastal plain, accommodation space anddike height. Their relative importance is related to the exact measure (e.g.nourishment, removal of dams) considered. Delta area and lowland coastal plain(area 0-2 m above mean sea level) are not correlated (Annex 1, Section 3), probablybecause an active delta does not need to be formed in the vicinity of existing coastallowlands (compare 6.9 and 6.10). Still, a large area of the delta proper should storevast quantities of sediment that could be released in a controlled fashion.Table 6.1:Delta rankings for the potential of restoring sediment dynamics usingvarious indicators.Delta area Uplift Offshore slope1. Ganges-Brahmaputra 1. Mississippi * 1. Parana2. Mekong 2. Mackenzie 2. Chao Praya3. Yangtze-Kiang 3. Nile 3. Yangtze-Kiang4. Lena 4. Danube 4. Ganges-Brahmaputra5. Huang He (Yellow) 5. Po 5. Lena* In contrast to what you would expect from this ranking, the Mississippi delta is prone to unrelentingsubsidence, which occurs even beyond the limits of the deltaic plain (Dokka, 2005). In the DIVAdatabase uplift or subsidence (mm.yr -1 ) was calculated from estimates of glacio-isostatic adjustment(inverse distance weighting interpolation of a point dataset of Peltier, 2000) minus an assumed 2mm.yr -1 for deltaic areas. The erroneous value for the Mississippi could simply be de to aninterpolation error.The Ganges-Brahmaputra, Yangtze-Kiang, and Lena rank high for delta area andhave low offshore slopes (Table 6.1). These indicators are negatively correlated (seeAnnex 1). Total delta area varies greatly, but at least half of the deltas studied haveand area of over 10.000 km 2 in size. Here it should be feasible to find opportunitiesfor restoring the sediment dynamics, but equally so, to find space for water retentionand controlled flooding measures (Table 6.1). Most deltas have a rather gradualoffshore slope, but some have a steeper slope. From Niger (0,075%) to SenegalRiver (0,48%), offshore slopes may be too steep to allow much scope for substantialsediment entrainment along the coast. Conversely, deltas with shallow slopes andextensive submerged deltas should offer opportunities for nourishment.Uplift could be an interesting indicator because particular measures such as beachnourishment might be more effective when it is placed in an uplifting area. Uplift is aDIVA indicator that results from an interpolated point data set of estimates of glacioisostaticadjustment plus an assumed inherent subsidence of deltaic areas at a rateof 2 mm/yr (Vafeidis et al., 2005).<strong>Deltas</strong> on the moveFigure 6.10: Ranking of deltas according to several indicators for restoring sedimentdynamics (which is one category of soft system-based measures):delta area, rate of uplift or subsidence, and offshore slope. Delta areaand offshore slope originate from WDD, uplift/subsidence is from DIVA.Ganges-BrahmaputraMekongYangtze-KiangLenaHuang He (Yellow)IndusMississippiVolgaOrinocoIrrawaddyAmu DaryaNigerShatt el ArabGodavariDneiperPoNileRedChao PhrayaMahanadiPechoraMacKenzieKrishnaFlyParanaYukonDanubeSenegalOrdGrijalvaZambeziMahakamBurdekinKlangSao FranciscoMagdalenaColvilleMangokyEbroPungoeBaramMississippiMacKenzieNileDanubePoShatt el ArabNigerEbroOrinocoMagdalenaMahakamRedMangokySao FranciscoZambeziYukonKrishnaIndusBurdekinMekongHuang He (Yellow)FrazerIrrawaddyGodavariChao PhrayaParanaMahanadiDneiperPechoraYangtze-KiangSenegalFlyLenaGanges-Brahmaputra0 20000 40000 60000 80000 100000 120000total delta area (km2)-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5 3uplift/subsidence (mm/y)6465