Fluvial processes and avulsion

New Kingdom royal cult temples in Thebes (Luxor, Egypt) are all located on the lower desert edge. Kom el-Hettân (Amenhotep III: reign 1391-1353 BCE, 18th Dynasty) is an exception, as it is located in the present Nile floodplain. Its anomalous position has puzzled Egyptologists, as has the termination of its use, which traditionally has been attributed to natural hazards such as flooding or earthquakes. Geoarchaeological analyses of the subsurface shows that Amenhotep III's temple was initially founded on a wadi fan that stood several metres above the contemporary surrounding floodplain landscape. The temple was fronted by a minor branch of the Nile, which connected the temple to the wider region, but the temple itself was relatively safe from the annual flood of the Nile. This geoarchaeological study comprised a coring programme to determine the c. 4000-yr landscape history of the local area. Chronological control was provided by the analysis of ceramic fragments recovered from within the sediments. This study shows that the New Kingdom period was, at least locally, characterised by extremely high sedimentation rates that caused a rapid rise of the floodplain and gradual submergence of the pre-existing high temple grounds. This is, however, not a plausible reason for the destruction of the temple, as frequent inundation did not begin until the temple was already out of use and largely dismantled .

Large river channel confluences are recognized as critical fluvial features because both intensive
and extensive hydrophysical and geoecological processes take place at this interface. However,
identifications of suspended sediment routing patterns through channel junctions and the roles
of tributaries on downstream sediment transport in large rivers are still poorly explored. In this
paper, we propose a remote sensing-based approach to characterize the spatiotemporal
patterns of the post-confluence suspended sediment transport by mapping the surface water
distribution in the ultimate example of large river confluence on Earth where distinct water
types meet: The Solimões-Amazon (white water) and Negro (black water) rivers. The surface
water types distribution was modeled for three different years: average hydrological condition
(2007) and two years when extreme events occurred (drought-2005 and flood-2009).
Amazonian surface water domination along the main channel is highest during the water
discharge rising season. Surface water mixing along the main channel depends on the
hydrological seasons with the highest mixed-homogenized area observed during water
discharge peak season and the lowest during discharge rising season. Water mixture also
depends on the yearly hydrological regime with the highest rates of water mixing in 2009,
followed by 2005 and 2007. We conclude that the dominant mixing patterns observed in this
study have been persistent over a decadal scale and the anabranching patterns contribute to
avoid a faster mixing in a shorter distance. Our proposed approach can be applied to a variety
of morphodynamic and environmental analyses in confluences of large rivers around the world.

Fluvial lowlands have become attractive human settling areas all around the world over the last few millennia. Because rivers kept changing their course and networks due to avulsion, the sedimentary sequences in these areas are archives of both fluvial geomorphological and archaeological development. We integrated geological and archaeological datasets to demonstrate the concurrence of the gradual abandonment of a major Rhine channel (Utrecht, The Netherlands), the development of human habitation in the area, and the interactions between them.

The Utrecht case study highlights the stage-wise abandonment of a natural river channel, due to avulsion, coincident with intensifying human occupation in Roman and Early Medieval times (1st millennium AD). The analyses make maximum use of very rich data sets available for the study area and the tight age control that the geo-archaeological dataset facilitates, offering extra means of time-control to document the pacing of the abandonment process. This allows us to quantify change in river dimensions and meander style and to provide discharge estimates for successive stages of the abandonment phase over a 1000-year period of abandonment succession, from mature river to eventual Late Medieval overbuilt canal when the Rhine branch had lost even more discharge.

Continued geomorphic development during this period - which includes the 'Dark Ages' (450-1000 AD) - appears to have been crucial in the development of Utrecht from Roman army fortress to Medieval ecclesial centre. The settlement dynamics in and around the city of Utrecht changed during the various phases of abandonment. In the bifurcating network of river branches forming the Rhine-Meuse delta, the main Rhine branch hosted the Roman limes military border and transport route. The Rhine- Vecht bifurcation at Utrecht provided an excellent location to raise a Roman fort. Continued geomorphic activity during abandonment in Early Medieval times was characterised by enhanced overbank sedimentation and shifts in the position of bifurcations. River flooding became more incidental in this stage, and alluvial-ridge occupancy became sensitive to flooding events for several centuries. We conclude by demonstrating that similar human-river interactions during Roman times occurred in several other deltas within the former Roman empire, with differences depending on the position of a settlement within the delta, the overall hydrological situation, and the ability of societies to control the changing environment.

Deltaic land inundated by storm surges may reform by sedimentation from natural or human-induced river diversions. This is a well-known trigger mechanism for creation of new channels in coastal plains and deltas, which may develop into main channels and lead to abandonment of older (avulsion), particularly in the downstream parts of deltas that host tidal rivers. These new channels develop as part of deltaic splay complexes that heal initial diversion scars and fill up flooded basins at a certain pace.

We study a case with excellent historical and geological data of a diversion of the river Rhine following catastrophic inundations (1421–1424 AD) into medieval reclaimed land. Numerical modelling of deltaic splay and channel development is combined with reconstructions from historical maps and geological data. This yields detailed insight in pacing of splay sedimentation and changing hydrodynamics in the channel upstream of the diversion in the two centuries following the inundation.

The equivalent of the full sand budget of the river Rhine was effectively trapped in the developing splay. The tidal-avulsion splay evolution on aspects is similar to that of fluvial crevassing into flood basins documented for settings lacking ‘downstream’ tidal control. The typical small-scale delta-lobe avulsion cycles: mouth bar formation, backward sedimentation, upstream avulsion, channel progradation and mouth bar formation are reproduced in the splay-modelling. The pacing of splay development, however, is relatively fast due to the presence of tides and the water depth in the receiving basin. The diversion had a strong upstream impact, in particular on water levels in the feeding river channel at stages of peak flow. For two centuries levels were significantly raised, because bifurcation-imposed reduced transport capacity and associated sedimentation at the diversion site increased hydraulic roughness and hampered flow.

These findings have implications regarding flood mitigation for presently-planned lower-delta engineered diversions. Furthermore, they elucidate the differences between upstream fluvial avulsion and downstream tidal-river avulsion that are important to recognise if we want to understand how deltaic distributary-networks are maintained.

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Link to the full paper is found here: http://dx.doi.org/10.1016/j.geomorph.2009.12.009

The academia download is my new geological cross-sections of the Biesbosch area in mini-poster format; supplementary material to the paper.

In the Holocene Rhine-Meuse delta, the geography, architecture, and chronology of the channel belts and their
flood basins is known in exceptional high detail. This is due
to a long history of intensive geological, geomorphological,
and archeological research by various universities and knowledge institutes and archaeological consultancy companies.