Hysteretic Implications for Graded Bed Load Sediment Transport in Symmetrical Hydrograph Flows

Differential parametric values associated with bed load sediment transport, that result at the same discharge levels on the rising and falling limbs of a flood hydrograph, are usually defined as bed load hysteresis. This hysteresis in bed load sediment transport rates is of considerable interest in the field of fluvial hydraulics. Within this study, a series of well-defined, symmetrical hydrograph flows are generated over a graded, mobile sediment bed to fully examine the hysteresis of the resulting bed load sediment transport in terms of the threshold of motion, and differential bed load transport rates and bed load yields during the hydrographs. The experiments are conducted in a titling flume without sediment supply specified at the upstream inlet, thereby representing typical river reach conditions immediately downstream of a dam that are exclusively subject to net in-channel bed degradation from sediment transport initiated during flood events. Our results show that the fractional bed load transport of defined fine, medium and coarse size classes within the graded sediment bed generally display clockwise, no/mixed and counter-clockwise hysteresis patterns, respectively, with clockwise hysteresis most commonly found for the coarse size class mobilised by hydrographs with long durations. By contrast, counter-clockwise hysteresis is usually observed for fine size class transported by hydrographs with short durations. Accordingly, the corresponding reference stresses for each size class vary between different hydrographs and are primarily controlled by the hydrograph flashiness (i.e. unsteadiness) and magnitude (i.e. total water work). Moreover, it is shown that the hysteresis effect, particularly for those size classes and hydrograph combinations that result in clockwise and counter-clockwise behaviour, should be fully accounted for when reproducing bed load transport rates using separate-limb based method. Finally, we investigate the relative fractions of the overall bed load yields generated during the rising and falling limbs of all symmetrical hydrographs (i.e. the bed load yield ratio), which are found to be primarily dependent on bed load transport hysteresis. Finally, the relationship between the bed load yield ratio and the ratio of reference stresses for the fractional sediment motion of each size class on both limbs is found to follow a power law.

Grain shape effects in bed load sediment transport

Bed load sediment transport, in which wind or water flowing over a bed of sediment causes grains to roll or hop along the bed, is a critically important mechanism in contexts ranging from river restoration to planetary exploration. Despite its widespread occurrence, predictions of bed load sediment flux are notoriously imprecise. Many studies have focused on grain size variability as a source of uncertainty, but few have investigated the role of grain shape, even though shape has long been suspected to influence transport rates. Here we show that grain shape can modify bed load transport rates by an amount comparable to the scatter in many sediment transport data sets. We develop a theory that accounts for grain shape effects on fluid drag and granular friction and predicts that the onset and efficiency of bed load transport depend on the mean drag coefficient and bulk friction coefficient of the transported grains. Laboratory flume experiments using a variety of grain shapes confirm these predictions. We propose a shape-independent sediment transport law that collapses our experimental measurements onto a single trend, allowing for more accurate predictions of sediment transport and helping reconcile theory developed for spherical particle transport with the behavior of natural sediment grains.

Collisional Langevin approach to bed load sediment velocity distributions

<p>Bed load experiments reveal a range of possibilities for the downstream velocity distributions of moving particles, including normal, exponential, and gamma distributions. Although bed load velocities are key for understanding fluctuations in transport rates, existing models have not accounted for the full range of observations. Here, we present a generalized Langevin model of particle transport that includes turbulent drag and episodic particle-bed collisions. By means of analytical calculations, we demonstrate that momentum dissipation by particle-bed collisions controls the form of the bed load velocity distribution. As collisions vary between elastic and inelastic, the velocity distribution interpolates between normal and exponential. These results add context to conflicting experiments on bed load velocities and suggest that granular interactions regulate sediment dynamics and transport rate fluctuations.</p>

Enhancing shoreline advance by ploughing the intertidal beach: Physical simulation

<p>Understanding shoreline behaviour and developing tools to deal with erosion has increasing interest nowadays. Coastal erosion and accretion produce changes on the beach width. These changes condition the uses given to dry beach and coastal areas. As the beach becomes narrower, the hazard of coastal areas increases. Additionally, due to the tourism, the demand and interest for wider beaches in early spring have risen.</p><p>Natural and human factors determine shoreline evolution. Storms erode beaches during winter, and calm weather conditions produce accretion. Assisted recovery techniques aim to propose new soft engineering methods that enhance accretion during calm periods. These human interventions need to be thoroughly analysed to ensure their effectiveness. In this study, we propose the ploughing of the intertidal beach area to accelerate the natural recovery process of the beach.</p><p>The effect of ploughing the intertidal area of a beach has been analysed through real scale physical simulations in the wave-current-tsunami flume (COCoTsu) of IHCantabria. The effect of the ploughing was monitored by measuring the sand transported shoreward with cell pressures beneath sediment trap boxes. The channel was longitudinally split into two equal channels (1 m wide each), one of them with plane sloping sand and the other including five crests and holes emulating a real plough made by a tractor. The comparison of both sides derives the effect of the ploughing.</p><p>Simulated geometry includes wave generator, 11 m of flat bottom, 17 m of concrete variable sloping fixed bed, 10 m of sand with D<sub>50</sub> = 0.318 mm movable bed, 2 m of trap box for continuous capturing and weighting shoreward transported sand and 10 m of wave dissipators. Concrete and sand slopes were designed to mimic the real geometry of a sandy beach intertidal accreting bar.</p><p>Sixteen experiments were conducted with fixed wave dynamics and bottom geometry and varying water level. Wave conditions were irregular waves with Hs = 0.3 m and Tp = 7 s, which produce dimensionless fall velocity Ω ≤ 1.5 ensuring accretion over the sandy bottom. Water level ranged from the level of the top of the sand to 50 cm above it. Additionally, one test was conducted with rising water level from -20 cm to 50 cm (from the top level of the sandy area), emulating a rising tidal cycle.</p><p>Hydrodynamics and morphodynamics were measured continuously during each experiment by means of 16 free surface elevation sensors, 4 ADV, 2 OBS, 8 pressure cells and 6 video cameras. Bottom load sediment transport was calculated as the difference of the measured total load (pressure cells beneath the aforementioned sand trap boxes) and suspended load sediment concentration measured by the OBS. Additionally, the laser scanner accurately determined the initial and final 3D geometry of the movable bed area.</p><p>All this data allows the analysis of the suitability of ploughing technique for accelerating natural accretion processes. Preliminary results show that ploughing affects the roughness of sandy bottom, increasing the wave dissipation and with a variable effect on sediment transport depending on the water level.</p>