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Fluvial Processes In River Engineering Downloads Torrent


Fluvial Processes In River Engineering Downloads Torrent

Abstract:Fluvial avulsion is an important process in the dynamics of the riverscapes and plays a key role in the drainage network evolution in lowland areas, also influencing past and present social processes and economic activities. Crevasse splays represent significant geomorphological features for understanding the fluvial morphodynamics in lowland areas dominated by avulsion processes. Within wide floodplains characterized by very low elevation ranges, the detection and accurate mapping of crevasse splay morphology and features, such as crevasse channels, levees, and deposit, can be very challenging considering floodplain extension, anthropic impact on the natural channels network, logistic difficulties, and in some cases, climate conditions that prevent field work. This research aims at improving the detection and mapping of crevasse splays in lowland areas through the combination of different remote sensing techniques based on optical multispectral imagery and topographic data derived from satellite earth observation missions. The Lower Mesopotamia Plain (LMP) offers a unique opportunity to study the avulsion processes because it presents numerous examples of crevasse splays, characterized by different sizes and states of activity. Furthermore, in this area, a strong correlation exists between the formation and development of crevasse splays and the expansion of agriculture and early societies since the Early Holocene. Different supervised classification (SC) methods of Landsat 8 satellite images have been tested together with topographic analysis of the microrelief, carried out based on two different 1-arcsec DEMs (AW3D30 and GDEM2). The results of this study demonstrate that the combination of multispectral imagery analysis and topographic analysis of the microrelief is useful for discerning different crevasse elements, distinguishing between active and relict landforms. The methodological approach proved helpful for improving the mapping of erosional and depositional landforms generated by the avulsion process and, in the study area, provided the best results for the active landforms.Keywords: Multispectral analysis; relief analysis; crevasse splays; avulsion processes; Mesopotamian plain

Relation between average fan slope and Melton number for the five test sites in comparison with results from the literature. Threshold line A separates fluvial sediment transport processes from mixed processes, and threshold line B separates mixed processes from debris flows. The following abbreviations were used: DF = debris flow, DFL = debris flood, FST = fluvial sediment transport, ME = Melton number, and Sf = average fan slope (modified from Scheidl and Rickenmann, 2010).

Results of the statistical tests for the comparison between different processes (fluvial sediment transport FST and debris flow DF) based on deposition depths and relative process intensities. The applied significance level (p-value) was equal to 0.05.

Mediterranean deltas range from a few km2 in area, associated with small catchments (tens to hundreds of km2), to major protuberances at the mouths of the larger rivers, the most important of which are the Danube, the Po, the Nile, the Ebro and the Rhône (Figure 1). Several of these large open-coast deltas, notably the Danube and the Rhône, started their development as bay-head deltas in embayments and rias. Numerous smaller deltas, especially in the Central and Western Mediterranean, also developed in these embayed settings under conditions of high fluvial sediment supply, notably rich in sand and gravel, and locally impeded longshore drift between bedrock headlands (Anthony et al., 2014).

The vulnerability of modern deltas strongly depends on fluvial sediment supply that is increasingly impacted by human activities, leading notably to accelerated subsidence and erosion. Many deltas in the Mediterranean are linked to river catchments that have been dammed over the last few decades. Whereas accelerated subsidence has received a lot of attention in recent years, notably synthesized in the benchmark papers by Ericson et al. (2006) and Syvitski et al. (2009), delta shoreline change, especially erosion, also recognized as an important corollary of the diminution of sediment supply to deltas, has been treated in numerous case studies rather than in a synthetic approach.

Schematic continuum of delta morphology and potential net long-term trajectory of evolution. The morphology ranges from symmetric to strongly longshore-deflected and asymmetric, as a function of river influence relative to wave-induced longshore transport. Delta destruction occurs as river influence becomes weakened by a variety of natural (changes in catchment climate and vegetation linked to the Little Ice Age, for instance, avulsion) and human-induced changes (catchment land-use and reforestation, catchment engineering, dams). From Anthony, 2015. DOI:

In complement to the overall delta shoreline change, the area change affecting the vicinity of the mouth(s) of each delta was also calculated using arbitrary limits depicted in Figure 1b. The reason for this operation is that the mouth zone is the primary receptacle of bedload exiting from the river, such that significant changes in this zone may be a good indicator of changes in fluvial bedload supply and/or in the intensity of reworking by waves and currents.

The impacts of human activities on Mediterranean river catchments and sediment flux have been documented in several case studies and in basin-scale syntheses (e.g. Bravard, 2002; Poulos and Collins, 2002; Hooke, 2006; Milliman and Farnsworth, 2011). Engineering works aimed at torrent management and channel embanking to assure flood control and navigation, as well as in-channel gravel and sand extractions, have significantly affected fluvial sediment supply to the coast. Over the last fifty years, dams intercepting and storing much of the fluvial sediment flux appear to stand out, however, as the dominant cause of reduction of river sediment to coastal sinks (e.g. Surian and Rinaldi, 2003). Hundreds of dams constructed across rivers draining into the Mediterranean Sea are deemed to have generated significant reductions in fluvial sediment loads, and none of the river catchments feeding the ten deltas selected for this study has been spared. Eight of the ten deltas are associated with catchments that have lost more than 60% of their sediment flux (Figure 12a, b, c). The Ombrone, Rhône, Ebro, Moulouya and Nile deltas have lost more than 80% of their fluvial loads following the construction of dams, this figure attaining 98% in the iconic case of the Nile. The sediment load of the Danube is still relatively high at 19.9 Mt/yr, despite a 70% drop after the construction of dams. The Ceyhan catchment, the latest to be affected by dams, has also lost a significant amount of its fluvial sediment flux, although this loss has been much less severe than in several other catchments such as the Arno, Ombrone, Ebro, Moulouya and Nile, that now supply less sediment to their deltas than the Ceyhan. Although the relationship between dams and river sediment flux reduction appears, as expected, to be the overarching element of river catchment management in the Mediterranean and Black Seas in recent decades, there is a spatial and temporal variability in this relationship that implies that other factors need to be taken into account (Anthony et al., 2014). Land-use changes in Mediterranean catchments, especially the abandonment of farmland in the mountainous hinterlands, have led to reforestation, and concomitant reductions in fluvial sediment yields.

Although eight out of the ten deltas show both an erosional tendency and important decrease in fluvial sediment load, the statistical relationship between these two variables is not significant (Figure 11). This relationship is, in fact, only strongly expressed for the Moulouya and Medjerda deltas, which have significantly retreated over the study period. All the other deltas show very little loss or relatively mild gain (cases of the Danube and Po), notwithstanding significant decreases in fluvial sediment flux, notably in the Nile, Ebro, Arno and Rhône deltas (Figure 12d). The poor relationship could suggest that: (1) the negative effect of dams on sediment supply decrease to deltas in the Mediterranean is presently over-estimated, (2) the relative shoreline stability reflects a lag in the downstream propagation of the effect of dams on the reduction of bedload transfer from river channels to delta shorelines. Liquete et al. (2005) noted that sediment load reduction effects have, to date, had, little effect on many of the deltas of the small, steep rivers and torrents of the coast of Andalusia. Elsewhere, deltas have actually accreted as a result of land-use changes, a fine example being the Meric in Turkey (Ekercin, 2007). Although the Ceyhan delta shows a net loss of area, much of this loss is attributed to significant retreat of its twin Seyhan lobe following dam construction (Alphan, 2005). The first major dam on the Seyhan was constructed only in 1984 (Ataol, 2015), and since then, nine other dams have been constructed between 1989 and 2013. The less-dammed Ceyhan has shown net delta area gain following deforestation (Kuleli, 2010).

Graphs depicting changing river sediment loads and delta-plain subsidence confronted with changes in delta protrusion area for the ten deltas. (a) Pre- and post-dam sediment loads in Mt/year for the Arno River (Billi and Rinaldi, 1997), the Medjerda River (Sliti, 1990; Rand McNally Encyclopedia of World Rivers, 1980; Meybeck and Ragu, 1996; Tiveront, 1960; Milliman and Farnsworth, 2011), the Ebro River (Palanque et al., 1990; Vericat and Batalla, 2006), the Po River (Idroser, 1994 cited in Simeoni and Corbau, 2009; Syvitski and Kettner, 2007), the Ceyhan River (EIE, 1993, cited by Cetin et al., 1999), the Rhône River (Milliman and Meade, 1983; Ollivier et al., 2010; Dumas et al., 2015; OSR, 2016), the Moulouya River (Snoussi et al., 2002), the Ombrone River (Milliman and Farnsworth, 2011), the Nile River (Syvitski and Saito, 2007; Milliman and Farnsworth, 2011), and the Danube River (Milliman and Farnsworth, 2011; Preoteasa et al., 2016); (b) Post-dam sediment loads in Mt/year (note difference in scale between (a) and (b); (c) Percentage change in fluvial sediment loads following dam construction; (d) Percentage change in surface protrusion area over 30 years; (e) Mean delta-plain subsidence rates in mm/year: Arno (CNR, 1986), Danube (Vespremeanu et al., 2004), Moulouya (Church et al., 2004), Nile (Becker and Sultan 2009; Marriner et al., 2012; Stanley and Clemente, 2017), Po (Bondesan et al., 1995), Rhône (Vella and Provansal, 2000), Ebro (Ibáñez et al., 1997), Ombrone (Pranzini, 1994), Medjerda (World Bank, 2011; Louati et al., 2014). Note that sediment loads in the European rivers have also undergone reductions related to catchment reforestation during both the pre-dam and the recent to present post-dam periods, as shown by the budget calculations of Provansal et al. (2014) for the Rhône. DOI: 153554b96e

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