While most of the world's large rivers are heavily engineered, channel response to engineering measures on decadal to century and several 100 km scales is scarcely documented. We investigate the ...response of the Lower Rhine River (Germany‐Netherlands) to engineering measures, in terms of channel slope and bed surface grain size. Field data show domain‐wide incision, primarily associated with extensive channel narrowing. Remarkably, the channel slope has increased in the upstream end, which is uncommon under degradational conditions. We attribute the observed response to two competing mechanisms: bedrock at the upstream boundary increases the channel slope over the upstream part of the alluvial reach to compensate for the reduction of net annual sediment mobility, and extensive channel narrowing reduces the equilibrium slope. Another striking feature is the advance and flattening of the gravel‐sand transition, suggesting its gradual fading due to an increasingly reduced slope difference between the gravel and sand reaches.
Plain Language Summary
Over two thirds of the world's large rivers are heavily engineered. Human intervention has important consequences for river channels, which erode and aggrade in response to measures like dam construction, channelization, and diversion. Such bed level change can directly (and severely) affect flood safety, navigation, and ecology. River channels respond over decades to centuries, and over hundreds of kilometers, and we usually do not have large enough datasets to investigate this response. Here, we study the response of the highly engineered Lower Rhine River (Germany‐Netherlands), which has been monitored over the past century in terms of bed elevation and grain size of the bed surface sediment. Channel narrowing in the past has caused significant channel bed incision. Such narrowing is expected to reduce the channel slope domain‐wide, but instead, the slope has become steeper in the upstream part of the domain. We attribute this behavior to the presence of bedrock in the upper segment of the river. In addition, advance and flattening of the Rhine River's gravel front have transformed a sand‐bed reach into a gravel‐bed reach. This knowledge can help us better understand other eroding river systems.
Key Points
Channel narrowing has unexpectedly led to a slope increase rather than a slope decrease in the upper part of the incising Lower Rhine River
This slope increase is associated with the presence of bedrock in the upper part of the considered domain
The Rhine River's gravel front has advanced and flattened, suggesting its gradual fading
Gravel augmentation is a river restoration technique applied to channels downstream of dams where size‐selective transport and lack of gravel resupply have created armored, relatively immobile ...channel beds. Augmentation sediment pulses rely on flow releases to move the material downstream and create conditions conducive to salmon spawning and rearing. Yet how sediment pulses respond to flow releases is often unknown. Here we explore how three types of dam releases (constant flow, small hydrograph, and large hydrograph) impact sediment transport and pulse behavior (translation and dispersion) in a channel with forced bar‐pool morphology. We use the term sediment “pulse” generically to refer to the sediment introduced to the channel, the zone of pronounced bed material transport that it causes, and the sediment wave that may form in the channel from the additional sediment supply, which can include input sediment and bed material. In our experiments, we held the volume of water released constant, which is equivalent to holding the cost of purchasing a water volume constant in a stream restoration project. The sediment pulses had the same grain size as the bed material in the channel. We found that a constant flow 60% greater than the discharge required to initiate sediment motion caused a mixture of translation and dispersion of the sediment pulse. A broad crested hydrograph with a peak flow 2.5 times the discharge required for entrainment caused pulse dispersion, while a more peaked hydrograph >3 times the entrainment threshold discharge caused pulse dispersion with some translation. The hydrographs produced a well‐defined clockwise hysteresis effecting sediment transport, as is often observed for fine‐sediment transport and transport‐limited gravel bed rivers. The results imply a rational basis for design of water releases associated with gravel augmentation that is directly linked to the desired sediment behavior.
Key Points
Sediment pulse movement is controlled by hydrograph type
Short peaked hydrographs lead to translation
Broad‐crested hydrographs lead to dispersion
The origin of the well‐known ‘grain size gap’ within the size range 1–10 mm in fluvial gravels is explored in a laboratory experiment to ascertain the effect of hydraulic sorting. A widely graded ...sand/gravel mixture was fed into a flume containing a bed composed of the same sediment mixture. The rate of sediment feed was varied but the water flow remained constant throughout the experiment at a rate that sustained size‐selective bedload transport with negligible suspension. We observed persistent deposition of the coarsest sediments (+16 mm) due to lack of competence to mobilize these sizes, partial entrapment of the finest sizes (−1 mm) in interstices of the gravel bed, and preferred transport of the intermediate sizes, including uptake of grains from the bed, establishing the conditions for development of the grain size gap.
An experiment tests the hydraulics‐based theory of preferred transport of granule gravel for the development of the well‐known ‘grain size gap’ in fluvial gravel deposits. In the condition of size‐selective sediment transport – common in gravel‐bed rivers – the sediment transport process leading to creation of the gap is successfully created.
A looming tragedy of the sand commons Torres, Aurora; Brandt, Jodi; Lear, Kristen ...
Science (American Association for the Advancement of Science),
09/2017, Letnik:
357, Številka:
6355
Journal Article
Recenzirano
Increasing sand extraction, trade, and consumption pose global sustainability challenges
Between 1900 and 2010, the global volume of natural resources used in buildings and transport infrastructure ...increased 23-fold (
1
). Sand and gravel are the largest portion of these primary material inputs (79% or 28.6 gigatons per year in 2010) and are the most extracted group of materials worldwide, exceeding fossil fuels and biomass (
2
). In most regions, sand is a common-pool resource, i.e., a resource that is open to all because access can be limited only at high cost. Because of the difficulty in regulating their consumption, common-pool resources are prone to tragedies of the commons as people may selfishly extract them without considering long-term consequences, eventually leading to overexploitation or degradation. Even when sand mining is regulated, it is often subject to rampant illegal extraction and trade (
3
). As a result, sand scarcity (
4
) is an emerging issue with major sociopolitical, economic, and environmental implications.
Downstream fining of bed sediment in alluvial rivers is usually gradual, but often an abrupt decrease in characteristic grain size occurs from about 10 to 1 mm, i.e., a gravel‐sand transition (GST) ...or gravel front. Here we present an analytical model of GST migration that explicitly accounts for gravel and sand transport and deposition in the gravel reach, sea level change, subsidence, and delta progradation. The model shows that even a limited gravel supply to a sand bed reach induces progradation of a gravel wedge and predicts the circumstances required for the gravel front to advance, retreat, and halt. Predicted modern GST migration rates agree well with measured data at Allt Dubhaig and the Fraser River, and the model qualitatively captures the behavior of other documented gravel fronts. The analysis shows that sea level change, subsidence, and delta progradation have a significant impact on the GST position in lowland rivers.
Key Points
Even a limited gravel supply to a river sand bed reach results in the formation of a gravel‐sand transition (GST)
A GST migrates more slowly as the gravel reach lengthens. It can halt under base level rise, subsidence, or delta progradation
We propose analytical formulations for the GST migration celerity and the stable GST position
This paper explores the potential of unmanned aerial system (UAS) optical aerial imagery to characterize grain roughness and size distribution in a braided, gravel-bed river (Vénéon River, French ...Alps). With this aim in view, a Wolman field campaign (19 samples) and five UAS surveys were conducted over the Vénéon braided channel during summer 2015. The UAS consisted of a small quadcopter carrying a GoPro camera. Structure-from-Motion (SfM) photogrammetry was used to extract dense and accurate three-dimensional point clouds. Roughness descriptors (roughness heights, standard deviation of elevation) were computed from the SfM point clouds and were correlated with the median grain size of the Wolman samples. A strong relationship was found between UAS-SfM-derived grain roughness and Wolman grain size. The procedure employed has potential for the rapid and continuous characterization of grain size distribution in exposed bars of gravel-bed rivers. The workflow described in this paper has been successfully used to produce spatially continuous grain size information on exposed gravel bars and to explore textural changes following flow events.
Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel‐bed rivers. However, the functional aspects of river systems are often left out ...of rehabilitation monitoring programmes. Despite temperature being a fundamental parameter determining the general health of rivers, a limited number of studies have tested whether gravel augmentation can aid restoring thermal functions. Using airborne thermal infrared (TIR) imagery, this paper explores potential positive outcomes through the monitoring of gravel augmentation actions on three rivers in France using hydro‐morphological indicators within a trajectory‐based before–after control–impact (BACI) framework. This design, combining long‐term geomorphic evolution with TIR‐based CI strategy, indicated that restoring forms was not sufficient to restore thermal functions in their spatial dimension. Despite forms showing various degrees of recovery, the thermal regime of rehabilitated reaches (i.e. temperature gradient, cold‐water patches density, etc.) failed to recover. Nonetheless, hydro‐morphological indices can be used to estimate long‐term evolution of groundwater‐surface water interactions. We emphasise the benefits of trajectory‐based BACI assessment to identify current conditions, understand the past evolution (trajectory) of the system to define the framework within which rehabilitation can be objectively assessed, especially when assessing hydrological (here thermal) processes. From a hydrological perspective, the gap between restored forms and restored thermal processes stems from the alteration of connectivity pathways, which potentially require more time (or efforts) to be recovered than forms do. With an increasing number of rehabilitation schemes and increasing pressure of global changes on rivers, we suggest that monitoring of water temperature becomes a routine part of rehabilitation projects.
A specific trajectory‐based before–after control–impact (BACI) framework, combining long‐trend evolution in hydro‐morphological indices with a thermal infrared‐based CI strategy, is proposed to assess the effects of river rehabilitation on thermal function. Results following gravel augmentation on three gravel‐bed rivers indicate that restoring forms is not sufficient to restore thermal functions. The rising number of rehabilitation schemes and increasing pressure of global change call for routine monitoring of water temperature in such projects.
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