Search

Riverbeds often fine downstream, with a gravel‐bedded reach, a relatively abrupt gravel‐sand transition (GST), and a sand‐bedded reach. Underlying this behavior, bed grain size distributions are often bimodal, with a relative paucity (gap) around the range 1–5 mm. There is no general morphodynamic model capable of producing the grain size gap and gravel‐sand transition autogenically from a unimodal sediment supply. Here we use a one‐dimensional morphodynamic model including size‐specific bedload and suspended load transport, to show that bimodality readily evolves autogenically even under unimodal sediment feed. A GST forms when we include a floodplain width that abruptly increases at some point. Upstream of the transition, non‐gap gravel ceases to move and gap sediment is preferentially transported. At the transition, non‐gap sand rapidly deposits from suspension, enhancing gap sediment mobility and diluting its presence on the bed. Plain Language Summary: The bed surface layer of many rivers is a mixture of sand and gravel. This mixture is described by the probability distribution of grain sizes, and in particular by the median size. Consider the long profile of such a river. Surface median size commonly becomes finer downstream, but often changes abruptly from a value above 5 mm to a value below 1 mm over a short reach. The range 1–5 mm is termed "gap sediment." Here we explain how this abrupt change evolves, even when there is no deficit of gap sediment supplied to the reach, and even though particle abrasion is not included. The grain size distribution autogenically develops two peaks, one in the sand range and one in the gravel range above 5 mm. When abrupt floodplain widening is included, the gravel peak is stronger in the upper reach and the sand peak is stronger in the lower reach, leading to a relatively abrupt gravel‐sand transition. Gap sediment can be diluted both upstream and downstream by a combination of effects due to bedload and suspended load, so that it dominates nowhere in the grain size distribution. Key Points: Numerical simulations show that bed bimodality evolves autogenically, even with unimodal sediment feed and no abrasionAbrupt floodplain widening can lead to the formation of a distinct gravel‐sand transitionThe behavior can be explained by preferential mobility of 1–5 mm sediment over a sand bed combined with fallout of sand from suspension [ABSTRACT FROM AUTHOR]

River channels are maintained by coordination between flow hydraulics, sediment supply, riparian vegetation, and sediment transport. This coordination is challenging to understand in natural flow regimes, where climatic and environmental drivers produce episodic flood and sediment supply events. To better understand the response of channels to flood sequences, we have undertaken laboratory flume experiments on sediment storage and export across a sequence of alternating hydrographs. Our experiments indicate that accumulated sediment storage before floods predicts sediment transport during floods, with sediment storage depletion during floods causing a nonlinear variation of sediment‐transport rates through time. Likewise, sediment storage between floods follows a growth‐to‐saturation pattern, whereby the sediment transport gradually increases toward the sediment feed rate depending on the occupation of available sediment storage zones. To describe these non‐linear variations, we developed a mathematical model which represents sediment transport and storage as a coupled dynamical system. This work highlights the crucial role that within‐channel sediment storage and its history play in determining sediment export in rivers. Plain Language Summary: The amount of sediment moving in a river channel largely determines its stability during floods; therefore, numerous ecological and engineering applications utilize predictions of sediment‐transport rates in rivers. Most prediction models assume that the force of the water and the size of sediment are the main variables needed to predict sediment‐transport rates. This implies that no information about the historical patterns of sedimentation in the channel is required to predict the amount of moving sediment. To test this implication, we conducted laboratory experiments on sediment transport across a sequence of floods. In contrast to existing prediction methods, we determined that the amount of sediment accumulated in a channel is a leading predictor of sediment transport both between and during floods. We characterized this dependence of sediment transport on the history of sediment storage and described it with a mathematical model. This work helps us better understand how floods affect the stability of river channels. Key Points: Sediment‐transport rates across flood sequences are largely predicted by the sediment storage accumulated in periods between floodsCoupling between sediment transport and storage generates non‐linear temporal variations of mean sediment‐transport ratesHistorical sediment transport and supply trends therefore control channel stability in conjunction with present‐time flow and sediment characteristics [ABSTRACT FROM AUTHOR]

Key Clinical Message: Contraceptive implant migration is a rare complication associated with contraceptive implants: migration to the ulnar nerve, emphasizing the importance of accurate diagnosis, imaging, and a multidisciplinary approach to mitigate neurovascular risks during insertion and removal procedures. The case report demonstrates the necessity for careful removal techniques and thorough patient follow‐up to ensure positive outcomes and prevent long‐term nerve damage. There are some potential risks and complications associated with contraceptive implants, including neurovascular injury. The aim of this case report is to report a rare complication associated with contraceptive implants. A 32‐year‐old female, right‐hand dominant, presented to the orthopedic clinic for the extraction of a contraceptive implant (Implanon) from her left arm. She reported intermittent numbness in the ring and little fingers. Upon examination, the Implanon was not palpable. Both Phalen's test and Tinel signs were negative. An x‐ray of the arm revealed the implant's position. Under local anesthesia through a longitudinal incision, the Implanon was found within the perineurium of the ulnar nerve. Two weeks after the operation, the patient returned to the clinic. Upon examination, there were no indications of ulnar nerve neuropathy. If a patient undergoes subdermal implant‐associated pain or is at risk of neurovascular damage during removal, it is advisable to refer the patient to a family planning specialist experienced in handling challenging implant removals, and subsequently to a peripheral nerve surgeon, to optimize outcomes. The migration of a contraceptive implant to the ulnar nerve is an exceedingly rare but possible complication. [ABSTRACT FROM AUTHOR]

Follicular thyroid carcinoma (FTC) is a well-differentiated malignancy, while insular thyroid carcinoma (ITC) is a very rare and poorly differentiated tumor. The present study reports a case of ITC arising within the background of FTC. A 52-year-old housewife presented with an anterior neck swelling for a duration of 6 months. An ultrasound examination revealed a well-defined solid nodule measuring ~58x37x28 mm in the mid-lower third of the right thyroid lobe, and two small nodules of <3 mm in size in the left lobe. Fine needle aspiration cytology of the lesion led to the suspicion of follicular neoplasm. A right thyroid lobectomy was performed, and the histopathological examination revealed poorly differentiated insular carcinoma arising in the background of FTC. As a result of this diagnosis, a completion thyroidectomy was performed. ITC is a rare and highly aggressive thyroid malignancy that is morphologically and biologically regarded as an intermediate between fully differentiated and undifferentiated/anaplastic thyroid carcinomas. In comparison to patients solely with follicular carcinoma, patients with insular carcinoma are older, more often male, have larger tumor sizes, are at greater risk of metastasis, and have a lower survival rate. Poorly differentiated ITC with follicular carcinoma is a very rare and aggressive condition that could be managed by thyroidectomy followed by radioiodine therapy. [ABSTRACT FROM AUTHOR]

This study examines the spatial pattern of fractional sediment mobility and assesses the influence of morphologic units on bedload transport in a small pool‐riffle reach with limited supply. Using a 2D hydraulic model and a subsurface‐based sediment transport model, shear stresses, fractional sediment mobility, and bedload transport were examined for flow events ranging in magnitude between 0.2Qbf and 1.5Qbf. Results reveal that while spatial variations in shear stress decrease as discharge increases, only a small proportion of the bed experiences high transport rates. At the reach scale, riffles are the primary morphological unit contributing to fully mobile sediment for all size fractions. However, at a subunit scale, there is evidence of sediment transport reversal for grains >32 mm at flows near or exceeding bankfull discharge in association with shear stress reversal. These transport reversals are important for maintaining pools despite their infrequent occurrence in the study reach. Sediment transport maps indicate that bed morphology considerably influences sediment transport at low to moderate flows. During these events, the shear stress is sensitive to local bed topography and partial mobility is the dominant transport process. In contrast, variations in bedload transport rates decrease during high flows when the flow is less sensitive to variations in bed topography and the bed becomes fully mobile. Key Points: A scaled version of a subsurface‐based model was utilized to evaluate the impact of morphologic units on fractional sediment mobilityRiffles were found to be the main source of fully mobile sediment of all sizes, with local transport reversal observed for grains >32 mmThe impact of morphologic units on bedload transport is more pronounced during low and moderate flows and diminishes during high flows [ABSTRACT FROM AUTHOR]