Reduced sediment supply and rising sea levels are driving land submergence on deltas worldwide, motivating engineering practices that divert water and sediment to sustain coastal landforms. However, ...lobe response following channel abandonment by diversions has not been constrained by field‐scale studies. Herein, avulsion and engineered diversion scenarios are explored for the Huanghe delta (China), where three lobes were abandoned in the last 40 yr. Two lobes were completely cut off by diversions, and one naturally by an avulsion. Shoreline retreat rates are strikingly different: ∼400 m/yr for diverted lobes and ∼90 m/yr for avulsed lobe. We hypothesize that this variability is linked to vegetal cover across lobes, and therefore the capacity to buffer hydrodynamic reworking of shoreface sediment. Furthermore, the vegetal cover is related to lobe salinity and elevation, which vary by abandonment style. We offer this as a case study to inform about the efficacy of future delta diversions.
Plain Language Summary
Sediment and water diversions are an important tool to combat land loss of deltaic coastlines. However, nourishment of drowned coastal land typically requires diverting water and sediment away from another region of the coast, potentially resulting in additional land loss. This study finds that the stability of shorelines facing reduced sediment supply is dependent on elevation before sediment loss, freshwater supply, and vegetation coverage. These findings have implications for future sediment and water diversions, by informing strategies that optimize land preservation.
Key Points
Coastline retreat following channel abandonment is assessed for engineered diversions and natural avulsions on the Huanghe delta (China)
Antecedent channel topography, riverine water input, and vegetation vary depending on abandonment style, and impact delta lobe stability
By mimicking conditions for natural channel avulsions, lobes abandoned by engineering practices may be made resilient to shoreline retreat
A surface model for aeolian bedform topography is adapted from a surface model of subaqueous bedform topography. The aeolian bedform surface model is developed using a uniform grid with a ...cell-centered finite volume approximation of the sediment continuity equation. The resulting modeling framework approximates the dynamic motions of aeolian bedform topography driven by bedform field boundary conditions. The numerical model is applied to simulate bedforms growing from unimodal and bimodal transport regimes from both a fixed elevation (sediment source area) and within a domain with fully periodic boundary conditions. The rates at which modeled aeolian bedforms grow and morphologically mature are sensitive to the chosen boundary conditions. Video files of model simulations and source code for the presented aeolian bedform surface modeling framework are available in supplemental materials. The aeolian bedform surface model code is malleable and readily modified for exploratory study of dynamic bedform topography that inherits morphological traits from aeolian bedform field boundary conditions.
Many published interpretations of ancient fluvial systems have relied on observations of extensive outcrops of thick successions. This paper, in contrast, demonstrates that a regional understanding ...of palaeoriver kinematics, depositional setting and sedimentation rates can be interpreted from local sedimentological measurements of bedform and barform strata. Dune and bar strata, channel planform geometry and bed topography are measured within exhumed fluvial strata exposed as ridges in the Ruby Ranch Member of the Cretaceous Cedar Mountain Formation, Utah, USA. The ridges are composed of lithified stacked channel belts, representing at least five or six re‐occupations of a single‐strand channel. Lateral sections reveal well‐preserved barforms constructed of subaqueous dune cross‐sets. The topography of palaeobarforms is preserved along the top surface of the outcrops. Comparisons of the channel‐belt centreline to local palaeotransport directions indicate that channel planform geometry was preserved through the re‐occupations, rather than being obscured by lateral migration. Rapid avulsions preserved the state of the active channel bed and its individual bars at the time of abandonment. Inferred minimum sedimentation durations for the preserved elements, inferred from cross‐set thickness distributions and assumed bedform migration rates, vary within a belt from one to ten days. Using only these local sedimentological measurements, the depositional setting is interpreted as a fluvial megafan, given the similarity in river kinematics. This paper provides a systematic methodology for the future synthesis of vertical and planview data, including the drone‐equipped 2020 Mars Rover mission, to exhumed fluvial and deltaic strata.
The formative conditions for bedform spurs and their roles in bedform dynamics and associated sediment transport are described herein. Bedform spurs are formed by helical vortices that trail from the ...lee surface of oblique segments of bedform crest lines. Trailing helical vortices quickly route sediment away from the lee surface of their parent bedform, scouring troughs and placing this bed material into the body of the spur. The geometric configuration of bedform spurs to their parent bedform crests is predicted by a cross‐stream Strouhal number. When present, spur‐bearing bedforms and their associated trailing helical wakes exert tremendous control on bedform morphology by routing enhanced sediment transport between adjacent bedforms. Field measurements collected at the North Loup River, Nebraska, and flume experiments described in previous studies demonstrate that this trailing helical vortex‐mediated sediment transport is a mechanism for bedform deformation, interactions and transitions between two‐dimensional and three‐dimensional bedforms.
Abstract Stormwater is a vital resource and dynamic driver of terrestrial ecosystem processes. However, processes controlling interactions during and shortly after storms are often poorly seen and ...poorly sensed when direct observations are substituted with technological ones. We discuss how human observations complement technological ones and the benefits of scientists spending more time in the storm. Human observation can reveal ephemeral storm-related phenomena such as biogeochemical hot moments, organismal responses, and sedimentary processes that can then be explored in greater resolution using sensors and virtual experiments. Storm-related phenomena trigger lasting, oversized impacts on hydrologic and biogeochemical processes, organismal traits or functions, and ecosystem services at all scales. We provide examples of phenomena in forests, across disciplines and scales, that have been overlooked in past research to inspire mindful, holistic observation of ecosystems during storms. We conclude that technological observations alone are insufficient to trace the process complexity and unpredictability of fleeting biogeochemical or ecological events without the shower thoughts produced by scientists’ human sensory and cognitive systems during storms.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Stormwater is a vital resource and dynamic driver of terrestrial ecosystem processes. However, processes controlling interactions during and shortly after storms are often poorly seen and ...poorly sensed when direct observations are substituted with technological ones. We discuss how human observations complement technological ones and the benefits of scientists spending more time in the storm. Human observation can reveal ephemeral storm-related phenomena such as biogeochemical hot moments, organismal responses, and sedimentary processes that can then be explored in greater resolution using sensors and virtual experiments. Storm-related phenomena trigger lasting, oversized impacts on hydrologic and biogeochemical processes, organismal traits or functions, and ecosystem services at all scales. We provide examples of phenomena in forests, across disciplines and scales, that have been overlooked in past research to inspire mindful, holistic observation of ecosystems during storms. We conclude that technological observations alone are insufficient to trace the process complexity and unpredictability of fleeting biogeochemical or ecological events without the shower thoughts produced by scientists’ human sensory and cognitive systems during storms.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hurricanes can cause immediate catastrophic destruction of marsh vegetation and erosion of soils; however, they also have long-lasting ecological impacts. Those impacts include the deposition of ...tremendous amounts of saltmarsh litter ('wrack') onto upland ecosystems, the hydrologic effects of which have not previously been investigated. When Hurricane Irma battered the southeastern US coastline, widespread wrack deposition was reported (often exceeding 0.5 m depth), especially in vulnerable coastal hammock ecosystems: locally-elevated forests within the saltmarshes that rely on freshwater inputs from rain. We report the impacts of this deposited wrack, which has persisted for 2 years, on effective precipitation inputs to coastal hammock soils. At a coastal hammock site, wrack deposits of 22-38 cm depth were estimated to store 10.2-19.9 mm of rain, reducing net rainfall to the surface by 66% over the study period (Oct 2018-Jun 2019). Three months of calibration data collected from wrack lysimeters in the field supported this interception estimate, as only 49 mm of the total 170 mm (29%) of rain that fell on the wrack was transmitted through to the soil surface. These litter interception effects on precipitation inputs far exceed those that have been described in other ecosystems and we hypothesized that they alter the growing conditions of these precipitation-dependent trees. The marshgrass (Spartina alterniflora), from which the wrack that was studied originates, is a globally abundant native and often invasive plant; thus, understanding the duration and extent of those effects on ecohydrological processes may be crucial to managing and conserving these ecosystems, especially given rising sea levels and changing hurricane regimes.
The distribution of cross‐set thicknesses is important data for reconstructing ancient aeolian dune fields from the strata they accumulated, but most aeolian strata on Mars must be observed from ...satellite. We hypothesize that remote sensing resolution limits will affect cross‐set thickness measurements and the dune‐field reconstructions that follow. Here we test this hypothesis using a numerical experiment mimicking the effects of satellite image resolution limits performed on a distribution of aeolian cross‐set thicknesses measured in the field from the Jurassic Page Sandstone, Arizona, USA. Page set thicknesses are exponentially distributed, representing the accumulations of dry dune fields (no water table interactions with the dunes) in a state of net‐sediment bypass. When observed from satellite, set‐thickness measurements increase as adjacent sets become indistinguishable, based on the map‐view distance between their upper and lower bounding surfaces. This is termed the exposure distance of a cross set and is a function of (1) the set thickness, (2) the dip of the outcrop surface, and (3) the number of satellite image pixels required to detect a set (detection limit). By running experiments using outcrop dips from 1° to 60° and detection limits from 0.75 to 2.50 m (3 to 10 High‐Resolution Imaging Science Experiment pixels), we find that gently sloping surfaces (< 13°) at all detection limits are associated with the least blending of adjacent sets, conserving the net‐bypass interpretation made from the true set thicknesses. Although these results are specific to the Page, they can be used as a guide for future Mars work.
Plain Language Summary
Cross sets are sedimentary deposits left by wind‐blown sand dunes. Cross sets can be preserved for long amounts of time as sedimentary rocks, where variability in their thickness can be analyzed to understand the motion of ancient dunes and the processes that helped the dunes move, including ancient winds, the presence or lack of near‐surface groundwater, ancient topography, and tectonics. On Earth, we can accurately measure cross‐set thicknesses in the field, but for Mars we are mostly limited to satellite images. Here we mimic the uncertainties present in a satellite image of Mars by altering field‐measured thicknesses from Earth. We perform this numerical experiment on field‐measured cross‐set thicknesses from the Page Sandstone, Arizona. The altered thicknesses are checked for how they might affect our understanding of the ancient Page dune fields. Based on these experiments, satellite images offer a risk of misinterpretation, but good measurements can be made at High Resolution Imaging Science Experiment image resolution if the slope of the rock outcrop is shallow, 13° from horizontal or less. At these shallow slopes, the thinnest cross sets are exposed over long distances, making them detectable from orbit. Though these results are specific to the Page, they can help guide future Mars work.
Key Points
The perceived thicknesses of aeolian cross sets are increased (and their distributions altered) due to remote sensing resolution limits
Image resolution limits can severely alter interpretations of aeolian sandstones, even at HiRISE resolution (best available at 25 cm/pixel)
Accurate measurements from HiRISE images require outcrop slopes less than 13°, such that thin sets are exposed over detectable distances
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