Quaternary lowland gravel-bed rivers: climate forcing at different time-scales

In recent decades, research on fluvial systems at mid-latitudes has identified strong links between stepped terrace system formations and Quaternary climatic cycles during the last 1 My. During both glacial and interglacial periods, geomorphological and sedimentological processes within terrace sequences are closely controlled by climatic forcing. This takes the form of the production of coarse gravel to sand in large braided rivers during glacial periods, and the deposition of overbank silts by flood events in meandering channel systems or tidal sands in downstream areas during interglacial periods.  Recent investigations have also demonstrated that calcareous tufa accumulations at the top of fluvial series provide evidence (including molluscs, leaf-prints, mammal remains and stable isotopes) of past interglacial environmental dynamics. In areas where other continental archives, such as lacustrine deposits or speleothems are lacking, these data are particularly valuable for the reconstruction of both climates and associated biocenoses. Studying how river systems have evolved over time provides a basis for evaluating their capacity to adapt to present-day global changes. This session is open to communications focusing on multidisciplinary research on Pleistocene river systems and their connexion with slope processes and landscapes (e.g. morphology, sedimentology, palaeontology, geochemistry). Investigations including comparisons with data from other continental records or developing diachronous approaches with present-day analogues are encouraged, especially if they aim to better understand environmental responses to recent climate change.

Conveners: P. Antoine (CNRS), J. Dabkowski (CNRS), JL. Grimaud (Mines Paris, PSL).

 

Modes of alluvial plain construction and estimation of texture and volume of alluvial deposits

The nature, texture and volume of alluvial infilling is a major control of bed and river course mobility. Though this aspect is fundamental in estimating the potential for restoration of rivers,  because of its “hidden” nature, it remains poorly understood. Furthermore, we still largely ignore how indirect (land-use changes) and direct anthropogenic (fluvial engineering works and maintenance practices) have come to overlap with climate variability and pre-Neolithic legacies in the construction of alluvial plains. This session welcomes contributions focusing on the modes and temporalities of alluvial plain construction before and after the Neolithic, with a particular interest in the identification of tipping points and in how the growing influence of direct anthropogenic interventions in or close to the riverbed, such as drainage systems, leats associated with water mills, weirs, ripraps, embankments or gravel mining have modified the stratigraphy and texture of alluvial deposits. Furthermore, the session is interested in original methodological approaches which enable the evolution of overflow frequencies and associated sediment deposits to be determined, as well as approaches that can quantify the stock of coarse (> 2mm) and fine (< 2mm) sediments in alluvium and estimate their spatial variability.

Conveners: L. Lespez (UPEC), E. Wohl (Utah State University).

 

Hydrological and sedimentary connectivity between riverbed and floodplain

Multidimensional (vertical, lateral and longitudinal) exchanges of sediment and water between riverbed and floodplain are the main drivers of fluvial landscape shaping and of the biodiversity in riverine ecosystems. Sediment transfer and exchange between the channel and the floodplain are controlled by closely interdependent factors: the hydrologic functioning interacts with the available sediment stock from longitudinal and lateral inputs, as well as the fluvial pattern, which determines the mobility of the channels. Yet, such exchanges have been deeply modified over recent centuries in response to the construction of fluvial engineering works such as weirs, riprap and dykes…This session aims at exploring the consequences of these anthropogenic interventions on recent hydro-sedimentary connectivity with a special focus on the modalities and rhythms of transfer and storage of water and sediment between floodplain and riverbed across temporal scales. We especially welcome a variety of contributions that cover novel data acquisition methods and innovative approaches.

Conveners: E. Gautier (Université Paris 1 Panthéon-Sorbonne), H. Piégay (CNRS).

 

Bedload: fluxes, mobility, processes and metrology

Due to the relatively low energy of lowland gravel-bed rivers and the coarse size of the particles that make up their beds, they are generally considered to have bed material that is rarely mobile, with low bedload velocities and amounts. Yet, depending on the size of and volume of coarse particles composing the alluvium, this assumption is often challenged and bed-material on many of these rivers turns out to actually be very mobile. At the same time, bed-material size and structure as well as bedload mobility could have been strongly altered by the long-standing presence of fluvial engineering works that have inhibited the longitudinal and transversal coarse connectivities, thus reducing the coarse sediment supply to riverbeds. Bedload behaviour is therefore under the control of an inherited and strongly intertwined complex combination of natural and anthropogenic factors that has to be disentangled to improve the management of lowland gravel-bed rivers. Furthermore, bedload velocity and fluxes, the thickness of the active layer, the contribution of lateral erosion to the amount of bedload or even how particle displacements interact with bed morphology are still poorly understood in such rivers. Lastly, such issues are directly related to our ability to reliably determine and map the variability of bed-material grain-size. The use of modern methods based on UAV in braided or ephemeral rivers where part or the totality of the bed is often exposed, cannot be applied in lowland rivers because of the immersion of most of their bed all year round. For this reason, there is a strong need to develop methods and tools to extract such information. This session aims at exploring the various issues described above as well as the range of methods and devices that can be deployed.

Conveners: T. Dépret (CNRS), F. Arnaud (CNRS).

 

Restoration/Rehabilitation

Most of the operations of geomorphological restoration/rehabilitation of lowland rivers are still very often based on restoration of forms. These generally involve recreating meanders and sequences of pools and riffles. It is not uncommon to supplement these efforts with sediment reinjections and/or the creation of structures (such as large woody debris). With the exception of operations to remove weirs and small dams, process-based restoration is less common, perhaps due to the slowness of processes in lowland gravel-bed rivers. The time needed to reach a new equilibrium following restoration, and thus to determine the efficiency of such operations is thought to be relatively long. This session is interested in contributions focusing on 1) how to estimate the potential for hydro-geomorphological restoration/rehabilitation of lowland gravel-bed rivers, paying particular attention to defining conditions for which passive and active actions are relevant, 2) identifying the spatial and temporal scales at which restoration/rehabilitation operations should be defined and determining their efficiency at these scales, 3) developing original methodological frameworks and metrics allowing the effects of restoration/rehabilitation actions implemented in such rivers to be monitored.

Conveners: F. Gob (Université Paris 1 Panthéon-Sorbonne), B. Belletti (CNRS), V. Chardon (Université de Strasbourg).

 

Urban and suburban rivers

Urban and suburban rivers are, by definition, among the hydrosystem most modified by human activities. Because of the severe changes that have affected their geometry as well as their fluxes of water and sediments and because of the increasing urban sprawl worldwide, these rivers are the subject to ever-increasing concern. As urban environments experience an increased demand for nature, one of the main issues concerns the preservation or restoration of the river’s functionalities in a context of high levels of persistent pressure and a lack of vertical and/or lateral adjustment possibilities. This session welcomes contributions documenting 1) the diversity of the ways in which forms, structures and processes have been altered in lowland gravel-bed rivers, and the importance and velocity of these alterations, 2) the range of solutions that can be implemented to manage and/or restore these rivers.

Conveners: N. Thommeret (UPEC), O. Navratil (Université Lyon 2).