Mechanism of Tsunami-Induced Erosion of Bridge-Abutment Backfill and Its Countermeasures

Tsunamis can destroy bridges in coastal areas. Studies have attempted to unravel the mechanism of tsunami-induced damage and develop effective countermeasures against future tsunamis. However, the mechanisms of tsunami-induced erosion of bridge-abutment backfill and its countermeasures have not been studied adequately. This study investigates this topic using numerical analysis. The results show that the tsunami flowing down along the downstream wing of the abutment induces bedload sediment transport on the ogive section of the backfill on the downstream side of the abutment, resulting in the onset of backfill erosion. Sediment suspension and bedload sediment transportation occur when the backfill inside the abutment starts to flow out from below the downstream wing. This leads to subsidence of the backfill at the upstream side of the downstream wing. The subsequent backfill erosion is mainly caused by bedload sediment transport. Numerical experiments on countermeasures show that extending the wings downward can prevent the acceleration of backfill erosion in the presence of the abutment. A combination of multiple countermeasures, including extended wings, would be more effective in maintaining the stability of the abutment after a tsunami. This suggests the application of such countermeasures to actual bridges as an effective countermeasure against backfill erosion.

Bed Load Distribution Analysis of Pondo Poboya River to The Hydraulics Flow Parameters

The aim is to know the effect of bedload sediment to river hydraulics parameters. The research was conducted by taking samples in three-part of the river. Each location is taken ten cross-sections with left, middle, and right parts of the river. Bedload sediment is calculated by the grain analysis method. There are several approaches in determining the roughness: Manning, Raudkivi, Subramanya, Meyer, and Muller roughness. From the analysis results, sediment in the form of fine sediment (d ≤ 0.15 mm) located in the middle of the cross-section does not settle due to high velocity. In contrast, in the wet area, the velocity is smaller so that grain material is deposited. The relationship of bedload sediment to the morphological form of the river shows that the middle part has the most material content in grain sediment (d ≤ 0.15 mm). The wet edge has the most content in gravel and sand (d < 6 mm). The Manning equation obtained a minimum roughness of 0.0257 and a maximum coefficient of 0.0365 with an average value of 0.0311. This value is matched with the coefficient of roughness on the Manning table does not differ much, i.e., natural, straight, and meandering channels.

Sediment displacement evolution after dam removal in a mountain river (Oioki dam, Leitzaran River)

&lt;p&gt;Bedload sediment transport was monitored from 2016 to 2020 in the Leitzaran River, in a reach affected by the removal of 7-meters high dam (Oioki dam). The removal was accomplished in two phases, the 3 first meters were removed in September 2018 and the second phase (September 2019) involved the removal of the remaining 4 meters. The study area was divided into three subreaches: control (unaffected by the dam), upstream and downstream of the dam. A sample of 300 RFID-tagged stones were seeded every year (100 at each reach).. Prior to this, the grain-size distribution of the surface sediment was characterized using the Wolman method. Then, the grain-size chosen for the tracer stones was distributed according to three Wentworth intervals: that corresponding to the surface d&lt;sub&gt;50&lt;/sub&gt;, d&lt;sub&gt;50&lt;/sub&gt;+1 (immediate upper interval), and d&lt;sub&gt;50&lt;/sub&gt;-1 (immediate lower interval). It was not possible to follow completely, and the lower interval had to be dismissed as the sediment was very small or narrow to insert the tracer.&lt;/p&gt;&lt;p&gt;We conducted an extensive surveying field campaign every summer.&lt;/p&gt;&lt;p&gt;The number of retrieved tracers was relatively high, around 40-70% (considering all field campaigns), although with differences amongst the different sub-reaches. The obtained results were organized by displacements and volumes of sediment moved. The maximum (3,500 meters) and higher mean displacement (~1,550 meters) were registered in the hydrologic year 2019/20. These values are from the upstream reach of the dam and match simultaneously with (i) the whole removal of the dam, and (ii) the period showing a lower discharge (note the critical discharge for the movement of our particles is ~25-30 m&lt;sup&gt;3&lt;/sup&gt;&amp;#183;s&lt;sup&gt;-1&lt;/sup&gt; (d&lt;sub&gt;50&lt;/sub&gt; = 64.0&amp;#8805;&amp;#216;&lt;90.5 mm); mean discharge and peak flow from 2013 to 2020 were ~5.3 m&lt;sup&gt;3&lt;/sup&gt;&amp;#183;s&lt;sup&gt;-1&lt;/sup&gt; and ~125.0 m&lt;sup&gt;3&lt;/sup&gt;&amp;#183;s&lt;sup&gt;-1&lt;/sup&gt;, respectively and at the end of the watershed).&lt;/p&gt;&lt;p&gt;We also estimated the bulk bedload volumes during the time spanned by this research and we report how the hydrologic year 2019/20 was the more active in terms of displaced volumes, moving up to 27,500 tons in the upstream reach. In fact, this year also presents the maximum for the downstream reach.&lt;/p&gt;&lt;p&gt;At this moment, besides the raw data of displacements and volumes, our observations highlight how the fact that a copious load of sediment was made available with the dam removal seemed to be more determinant than the magnitude of the flow to get larger tracer displacements.&lt;/p&gt;

Analytical approach unifying the two-regime scaling for bedload particle motions

&lt;p&gt;Bedload particle hops are defined as successive motions of a particle from start to stop, characterizing one of the most fundamental processes describing bedload sediment transport in rivers. Although two transport regimes have been recently identified for short- and long-hops, respectively &lt;strong&gt;(Wu et al., &lt;em&gt;Water Resour Res&lt;/em&gt;, 2020)&lt;/strong&gt;, there still lacks a theory explaining how the mean hop distance-travel time scaling may extend to cover the phenomenology of bedload particle motions. Here we propose a velocity-variation based formulation, and for the first time, we obtain analytical solution for the mean hop distance-travel time relation valid for the entire range of travel times, which agrees well with the measured data &lt;strong&gt;(Wu et al., &lt;em&gt;J Fluid Mech&lt;/em&gt;, 2021)&lt;/strong&gt;. Regarding travel times, we identify three distinct regimes in terms of different scaling exponents: respectively as ~1.5 for an initial regime and ~5/3 for a transition regime, which define the short-hops; and 1 for the so-called Taylor dispersion regime defining long-hops. The corresponding probability density function of the hop distance is also analytically obtained and experimentally verified.&amp;#160;&lt;/p&gt;

Validation of an Experimental Procedure to Determine Bedload Transport Rates in Steep Channels with Coarse Sediment

The current study presents an experimental procedure used to determine bedload sediment transport rates in channels with high gradients and coarse sediment. With the aim to validate the procedure for further investigations, laboratory experiments were performed to calculate bedload transport rates. The experiments were performed in a laboratory tilting flume with slopes ranging from 3% to 5%. The sediment particles were uniform in shape (spheres). The experiments were divided into four cases based on sediment size. Three cases of uniform sizes of 10 mm, 15 mm and 25 mm and a case with a grain size distribution formed with the uniform particle sizes were considered. From the experimental results a mathematical bedload transport model was obtained through multiple linear regression. The experimental model was compared with equations presented in the literature obtained for gravel bed rivers. The experimental results agree with some of the models presented in the literature. The closest agreement was seen with models developed for steep slopes especially for the highest slopes considered in the present study. Therefore, it can be concluded that the methodology used can be replicated for the study of bedload transport rates of channels with high gradients and coarse sediment particles to study more general cases of this process such as sediments with non-uniform shapes and sizes. However, a simplified model is proposed to estimate bedload transport rates for slopes up to 5%.

Rapid River Bed Recovery after the In-Channel Mining: The Case of Vistula River, Poland

The effects of in-channel wet-pit mining is nowadays widely discussed in terms of negative influence of the created pits on the river ecosystem and fluvial processes. The pits induce an alteration of natural flow or sediment transport. This paper describes the post-mining channel recovery observed in a relatively short time in a gravelly sand bed lowland river. The study was based on repeated bathymetry of the channel and grain size analyses of bed material taken from the mining area and its surrounding upstream and downstream pit. We also use calculations of possible bedload sediment movement in the studied river reach. We noticed that the excavation pit exceeded the maximum depth of 8.8 m in 2014 and, immediately after the end of mining, the bedload started to infill the pit. The bathymetric measurements in 2019 indicated that the process of pit infill was completed after five years, though the former pit is refilled with material finer than the natural bedload observed in the discussed river reach, and consists mainly of sand. The studied process of pit infilling runs continuously, even during the annual average water stages.