128 research outputs found
Reconstruction of an extreme flood hydrograph and morphodynamics of a meander bend in a high-peak discharge variability river (Powder River, USA)
AbstractUnderstanding of morphodynamic processes associated with largeâscale floods has recently improved following significant advances of modern technologies. Nevertheless, a clear link between flood discharge and inâchannel sedimentation processes remains to be resolved. The hydrological and geomorphological data available for the meandering Powder River (Montana, USA) since 1977 makes it a perfect laboratory to investigate connections between flood discharge and pointâbar sedimentation processes. This study focuses on a pointâbar that accreted laterally ca 70âm during a 50âyear recurrence flood, which lasted about 14âdays in May 1978. In September 2018, a trench ca 2âm deep and 70âm long was excavated through the axial pointâbar deposits, and the 1978 flood deposits were delineated based on georeferenced preâflood and postâflood crossâsection surveys. Sedimentological data show that pointâbar deposits accumulated at the early and late flood stages, when the flow was confined to the channel, and have similarities with classical facies models in terms of palaeocurrent patterns and vertical grainâsize trend. However, during highâstage flood conditions, when the flow overtopped the bar, crossâcutting of the bar and armouring were typical processes. Integration of sedimentological and palaeoâhydrological data highlight that the relation between channel crossâsectional area and flood discharge play a key role in preserving bar deposits. The integrated approach adopted here provides a basis for advancing palaeoflood hydrology beyond the stage of estimating peak discharges to the next stage of estimating palaeoflood hydrographs
Field migration rates of tidal meanders recapitulate fluvial morphodynamics
The majority of tidal channels display marked meandering features.
Despite their importance in oil-reservoir formation and
tidal landscape morphology, questions remain on whether tidalmeander
dynamics could be understood in terms of fluvial processes
and theory. Key differences suggest otherwise, like the
periodic reversal of landscape-forming tidal flows and the widely
accepted empirical notion that tidal meanders are stable landscape
features, in stark contrast with their migrating fluvial counterparts.
On the contrary, here we show that, once properly
normalized, observed migration rates of tidal and fluvial meanders
are remarkably similar. Key to normalization is the role of
tidal channel width that responds to the strong spatial gradients
of landscape-forming flow rates and tidal prisms. We find that
migration dynamics of tidal meanders agree with nonlinear theories
for river meander evolution. Our results challenge the conventional
view of tidal channels as stable landscape features and
suggest that meandering tidal channels recapitulate many fluvial
counterparts owing to large gradients of tidal prisms across meander
wavelengths
Seaward expansion of salt marshes maintains morphological self-similarity of tidal channel networks
International audienceTidal channel networks (TCNs) dissect ecologically and economically valuable salt marsh ecosystems. These networks evolve in response to complex interactions between hydrological, sedimentological, and ecological processes that act in tidal landscapes. Thus, improving current knowledge of the evolution of salt-marsh TCNs is critical to providing a better understanding of bio-morphodynamic processes in coastal environments. Existing studies of coastal TCNs have typically focussed on marshes with either laterally stable or eroding edges, and suggested that TCN morphology evolves primarily through the progressive landward erosion of channel tips, that is, via channel headward growth. In this study, we analyze for the first time the morphological evolution of TCNs found within salt marshes that are characterized by active lateral expansion along their seaward edges and anthropogenically-fixed landward boundaries. We use remote-sensing and numerical-modeling analyses to show that marsh seaward expansion effectively limits headward channel growth and prompts the evolution of TCNs that maintain self-similar morphological structures. In particular, we demonstrate that the overall TCN length increases proportionally to the rate at which marshes expand laterally and that these morphological changes do not significantly alter the drainage properties of the coupled marsh-TCN system. Such behavior is not observed in marshes that are not expanding laterally. Our results allow for elucidating the mechanisms of TCN formation and evolution in tidal wetlands, and are therefore critical to improving our current understanding of coastal-landscape ecomorphodynamics, as well as to developing sustainable strategies for the conservation and restoration of these environments
The effects of differential subsidence and coastal topography on high-order transgressive-regressive cycles: Pliocene nearshore deposits of the Val d\ue2\u20ac\u2122Orcia Basin, Northern Apennines, Italy.
This study focuses on the lowstand and early transgressive systems tracts of a basin-fill sequence of lower Pliocene nearshore deposits in the Val d\u2019Orcia Basin of the Northern Apennines, Italy. The basin at that time was a semi-enclosed marine embayment, and, in the study area, its margin was subject to highly variable subsidence along the depositional strike, attributed to a decrease in tectonic displacement. The nearshore succession in the more rapidly-subsiding segment of the basin is around 20 m thick, comprising three storeys of laterally-stacked Gilbert-type delta lobes overlain by a shoal-water delta, whereas the nearshore succession in the adjacent, more slowly-subsiding segment, is up to 9 m thick. This succession is characterised by alternation of shoreface and offshore deposits, moderately wave-worked and covered by shoal-water deltaic facies. These coeval nearshore successions consist of several transgressive-regressive cyclothems. The development and lateral variation of the cyclothems was controlled by the local subsidence rate and coastal topographic gradient. Some of the cyclothems are considered to be higher-order sequences and others to be parasequences, with the former passing laterally into the latter in the area where the sea-level fall was countered by fast local subsidence. Some of the bounding surfaces are of limited lateral extent, with two parasequences passing laterally into a single one. Coastal topography controlled particularly the thickness of transgressive deposits. In the low-gradient setting of a delta plain, the relative sea-level rises caused major landward shifts of the shoreline and reduced fluvial sediment supply, with the formation of a transgressive lag in sediment-starved conditions. In the high-gradient coastal setting of the non-deltaic zone, the shoreline shift was minimal and had relatively little impact on local sediment supply, which promoted an accretionary transgression. At the end of the lowstand stage, the rate of sediment accumulation in the non-deltaic nearshore zone was lower, allowing the onset of subsequent transgression to be recorded considerably earlier than in the deltaic nearshore zone. This diachroneity suggests that facies criteria alone may not necessarily be a reliable basis for the recognition of systems tract boundaries
Reconstruction of an extreme flood hydrograph and morphodynamics of a meander bend in a highâpeak discharge variability river (Powder River, USA)
Tectonic driven evolution of a Pliocene Gilbert type delta (lower Val d\ue2\u20ac\u2122Orcia Basin, southern Tuscany, Italy).
Pliocene alluvial to marine deposits of the Val d\ue2\u20ac\u2122Orcia Basin (Northern Apennines, Italy): sequence stratigraphy and basin analysis
Since the Middle-Late Miocene, the Apennine chain was characterised by the development of several basins. These basins have been alternatively explained as extensional post-orogenic basins or syn-orogenic compressional basins. In this framework, the Pliocene depositional history of the Val d'Orcia Basin (Southern Tuscany) is described, based on a sequence stratigraphic approach. Three depositional sequences (Vd3a-c) have been recognized. The Vd3a sequence records marine ingression and subsequent relative sea-level highstand in a narrow W-E trending depression. Subsequent relative sea-level fluctuations led to a progressive basin widening and deposition of Vd3b and Vd3c sequences. Basin scale sedimentation was mainly controlled by relative sea-level changes, although accumulation of bioclastic deposits of the Vd3c sequence could have been influenced by climatic factors. Based on its wide bowl-shaped geometry, the Pliocene Val d'Orcia Basin has been interpreted as an inherited depression. This depression is thought to be originated by erosion at the Late Miocene-Early Pliocene transition
PLIOCENE ALLUVIAL TO MARINE DEPOSITS OF THE VAL DâORCIA BASIN (NORTHERN APENNINES, ITALY): SEQUENCE STRATIGRAPHY AND BASIN ANALYSIS
Since the Middle-Late Miocene, the Apennine chain was characterised by the development of several basins. These basins have been alternatively explained as extensional post-orogenic basins or syn-orogenic compressional basins. In this framework, the Pliocene depositional history of the Val d'Orcia Basin (Southern Tuscany) is described, based on a sequence stratigraphic approach. Three depositional sequences (Vd3a-c) have been recognized. The Vd3a sequence records marine ingression and subsequent relative sea-level highstand in a narrow W-E trending depression. Subsequent relative sea-level fluctuations led to a progressive basin widening and deposition of Vd3b and Vd3c sequences. Basin scale sedimentation was mainly controlled by relative sea-level changes, although accumulation of bioclastic deposits of the Vd3c sequence could have been influenced by climatic factors. Based on its wide bowl-shaped geometry, the Pliocene Val d'Orcia Basin has been interpreted as an inherited depression. This depression is thought to be originated by erosion at the Late Miocene-Early Pliocene transition.
- âŠ