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Semi-theoretical model for mean sediment resting time of spherical particles: the role of hydrodynamic impulses and sphere size nonuniformity

Wyssmann Micah A., Papanicolaou Athanasios N., Kyriakopoulos Theodoros

Acta Geophysica

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2023

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Vol. 71, no. 6

2883--2904

EN

Prediction of bedload rates in gravel-bed rivers at low-to-moderate flow conditions, where bedload movement is intermittent, remains a challenging problem. While the virtual velocity concept provides a useful approach to bedload rate estimation in the intermittent movement regime, virtual velocity estimation remains hindered by a lack of tools for predicting mean sediment resting time. As a first step toward sediment resting time estimation in gravel beds, the present study develops a semi-theoretical resting time model applicable to nonuniform gravel-sized spherical particles. The model is based on the consideration that interactions of near-bed flow with bed material lead to mobilization of individual resting particles during hydrodynamic momentum transfer events (i.e., impulses). Thus, resting time is affected by impulse magnitude and timing. The primary premise underpinning model development is that an instantaneous velocity time-series generation approach based on the velocity spectrum can be used to mimic hydrodynamic impulses and simulate resting times. Based on past findings, two model parameters are considered important to advancing resting time predictions in gravel beds. First, the relative particle size allows size-fractional resting time predictions for a nonuniform sediment mixture. Second, the hindrance coefficient accounts for hiding effects and enables resting time predictions for different bed structure types. To provide calibration and verification data, laboratory experiments documenting impulse statistics and mean resting times for a range of flow and relative particle size conditions were also performed. The verified model exhibits mean resting times with similar magnitude and trends with increasing stress compared with experimental verification data.

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Preliminary evidence on laboratory experiments to detect the impact of transient flow on bedload transport

Przyborowski Łukasz, Nones Michael, Mrokowska Magdalena, Książek Leszek, Phan Cong Ngoc, Strużyński Andrzej, Wyrębek Maciej, Mitka Bartosz, Wojak Szymon

Acta Geophysica

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2022

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Vol. 70, no 5

2311--2324

EN

Trapezoidal-shaped hydrographs are typical of anthropized rivers, as this form is generally associated with the release of water from hydropower dams. To investigate how such unnatural waves can affect bedload rate, preliminary fume experiments were performed in Krakow, Poland, looking at bedload transport rate, bed shear stress and bed morphology. In addition, close-range bed surface photogrammetry was used to investigate bed changes due to the passage of the food wave. Three scenarios, having the same water volume but different wave magnitudes, were tested. The lowest wave showed almost no sediment transport and no visible changes in bed morphology, while higher waves changed the bed morphology, creating erosion and accumulation zones. The highest wave was characterized by an 8-shaped hysteresis of the bedload rate with a peak during the wave maximum. The lag time between the maximum bedload rate and the wave plateau was longer than expected due to zero-slope conditions.

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Experimental investigation of flow pattern over a fully developed bed at a 60° river confluence in large foods

Balouchi Behnam, Shafai Bejestan Mahmood, Ruther Nils, Rahmanshahi Mostafa

Acta Geophysica

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2022

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Vol. 70, no 5

2283--2296

EN

River confluences have a complex flow and sedimentation pattern that have vital influences on the hydraulic and bed morphology of river reach and the surrounding area. Confluences can be observed in waterways with various situations such as live bed conditions. This condition is a hydro-morphological situation with a high densiometric Froude number, i.e., bed load transport is supplied from upstream. According to the literature review, most of the experimental studies investigate the flow pattern on the flatbed and not on the developed riverbed, or/and in the low densiometric Froude number, or/and without supplying the sediment from upstream. Therefore, in the present study for the quantification of the flow pattern under these conditions, each developed bed was fixed with the cement blanket method in the laboratory. Then, the 3D velocity was measured at specific points at the confluence. The current study was designed to understand the flow pattern corresponding to the river bed behavior in the case of large foods. It is expected that the morphological features downstream of the confluence have a different pattern than the ones in the condition described in other literature. Thus, this paper describes briefly what are the different bed features and investigates the corresponding flow pattern. The results of the flow pattern on the developed bed show that all zones at the river confluence can be observed except the point bar due to the approximate equality of the mean longitudinal velocity of the separation zone and the main channel upstream of the confluence. Moreover, results show that by increasing the bedload ratio (sediment discharge to water discharge of the main channel of upstream of the confluence) from 0 to 3× 10−4, for defection to the outer bank of the channel decreased down to 45%, the stagnation equivalent area decreased down to 2.5 times, and bed shear stress decreased down to 40%. Hence, the momentum of lateral flow decreased with increasing bedload. Besides, the recovery zone occurred at a longer distance after the confluence compared to the case without bedload. Hence, the location of the maximum velocity zone, vortices, and secondary flows changed downstream of the confluence, by changing the bed load value.

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Transport rumowiska wleczonego ze zlewni źródliskowej (przykład źródlisk Łyny)

Afelt A

Inżynieria Ekologiczna

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2012

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Nr 31

5--16

PL

Wstępne wyniki badań źródliska Łyny wskazują na proces intensywnego odprowadzania substancji mineralnej w postaci stałej – jako składowej erozji wodnej zachodzącej w krajobrazie młodoglacjalnym na styku wód podziemnych i powierzchniowych. Pomiary polowe eksportu substancji mineralnej w formie rumowiska wleczonego wykazały, iż sumarycznie w roku erozyjnie ze zlewni odprowadzane jest w przybliżeniu 840 kg materiału (okres badań lata 2008-2009). Oznacza to średnio ubytek masy rzędu 580 kg z 1 km2 powierzchni jednostkowej zlewni rocznie. Woda źródlana pełni bardzo ważną rolę w odprowadzaniu substancji nierozpuszczonej ze zlewni topograficznej. Erozja wodna – sterując intensywnością transportu rumowiska, stymuluje w źródlisku Łyny proces odgrzebywania lokalnej rynny polodowcowej.

EN

The spring area of the Łyna river is a geological reserve, where naturally occurring retrogressive erosion is protected. Intensity of removal of solid and dissolved mineral material from the spring area of the river is the study aim. The study area is exceptional for Polish lowlands due to its complexity, as it includes 9 spring niches and 4 effluents. Geological setting plays here a decisive role, because all springs are situated in a head of a deep, buried tunnel valley which is developed in older sediments. The tunnel valley is filled with sands and gravels deposited during the last phase of the Vistulian glaciation. Removal of solid and dissolved material by retrogressive erosion leads to enlargement of the spring area and to uncovering of the buried tunnel valley. Intensive increase of the flow rate occurs downstream as tributary streams join the main river. Data from 3 gouge profiles covering 850 m downstream are as follows; 8,5; 22,8 and 48,8 dm3/s on average (based on a period 2004-2010). Intensity of the dragged material increases from 0,01 up to 0,08 g/s (measured in 2007-2008), 0,027 g/s on average. Mineralization of river water ranges between 150 and 234 mg/dm3 (178 mg/dm3 on average). It is assessed that the dissolved material removed from the area equals to 630 kg per day (230 tons per annum). On the other hand, the solid dragged material equals to 2,3 kg per day (0,84 tons per annum) for 34,8 dm3/s discharge measured at the lowermost gouge profile and for average mineralization of spring waters. Springs of the Łyna river have a high and stabilized outflow leading to the dynamic removal of mineral material. Removal of the dissolved material by the spring waters represents denudational potential of the regional aquifers which feed the springs. On the other hand, removal of the dragged material represents only erosion occurring within the uppermost part of the topographic catchment adjacent to the spring area which supplies the river channel with the solid material.

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Charakterystyka ilościowa i jakościowa transportu rumowiska w eksperymentalnej zlewni Wilczego Potoku w Karkonoszach w latach 1994-1999

Katrycz M.

Przegląd Naukowy Inżynieria i Kształtowanie Środowiska

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2002

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Vol. 11, No. 2

80--90

EN

In 1989 a m easuring overflow was built on a small water-course in the East Karkonosze Mountains in order to determine the dynamics of flow and bedload transport. The data collected in a 0.99 square kilometers basin reveal that the bedload m ass in valley river-beds is considerably bigger than in slope streams. It has been shown that the bedload transport process is different for the naturally afforested areas and in conditions of anthropopressure, in which the material transport increases drastically. The observed minor retention of bedload in river-beds and its fast renewal indicate that big precipitation and ablation stimulate quick bedload transport and renewal of its resources during the settling of the flood wave. Since the measurements commenced, an upward trend of bedload transport has been noticed, which results from the increasing influx of water into the basin as well as from the progressing anthropogenisation of the whole area. It has been noted that the rnain bedload mass is carried out of the basin during intensive summer rainfall, w hen floods are the biggest although they take place rarely: once in a few years. In all, the flowing water has carried from the basin area over 50 tons of river bedload, there of 8.8 tons by the slope stream system.