Computing the sediment and ensuing its erosive activities using HEC-RAS to surmise the flooding in Kulfo River in Southern Ethiopia

Purpose The change in sediment transport phenomenon and morphological characteristics of Kulfo River in the southern part of Ethiopia is estimated using one-dimensional hydraulic modelling. The purpose of this study is to predict erosion and sedimentation using hydrological engineering center-river analysis system (HEC-RAS) model. Design/methodology/approach Geometrical survey data of 2005 and 2019 were used to assess the impact of flood depth with 100 years of return per period on the morphology of the river. The bed and bank materials at selected sites of the river were sampled to estimate the grain size using manning roughness coefficient. Discharge and suspended sediment concentration were sampled thrice per day for a stretch of three months to develop a rating curve. Findings HEC-RAS model indicates that flood depth with 100 years return period had a significant inundated area during 2019 in comparison to 2005 demonstrating a temporary change in the morphology of the river. Acker and White method in HEC-RAS was used extensively to calculate the sediment load and subsequently calibrated. In the upper reach of the study area, there was aggradation and degradation, whereas the only degradation was noted in the middle to the downstream reach of the channel. Seasonal flood during peak flow due to a rise in bed level is most prevalent during the aggradation period. Originality/value Comparison of a flood depth inundating the catchment and sediment deposition has been intricately analyzed by using HEC-RAS model.

Mathematical model and algorithmic process of solving the problem of the intra-day regulation of discharge of water through the Nizhny Novgorod low-pressure hydroelectric complex

Introduction of measures for intra-day of regulating runoff incoming water flow rates from the Nizhny Novgorod HPP and their discharge through the spillway of the Nizhny Novgorod low-pressure hydroelectric complex requires a specific algorithm for dispatcher actions of the created hydraulic system. At the same time, there are serious difficulties in predicting the water regime over time. As previous studies have shown there is a large unevenness and irregularity of water discharges not only during the same day, but also in the same periods of each day, as well as weeks, months, and years. Тhis article analyzes the boundary conditions when introducing measures of regulating runoff, a mathematical model and an algorithm for solving the problem of intraday regulation are developed, describing the sequence of actions for solving the problem of "smoothing" the flow rates supplied to the lower stream of the Nizhny Novgorod low-pressure hydroelectric complex. The implementation of the proposed measures is carried out according to a three-stage (or two-stage) schedule for regulating the flow rate and water level. The proposed measures will improve the hydraulic and hydrological conditions of the downstream reach of the Nizhny Novgorod low-pressure hydroelectric complex by which the necessary depths for navigation will be reached. Conditions have also been created to mitigate erosion processes.

Quantifying spatial and seasonal variations of groundwater- surface water interaction for the prediction of hydrological turnover on the catchment scale

<p>Targeting hypohreic exchange as well as gains and losses as the means of interaction between ground- and surface water in a stream leads forward to the consideration of both influencing the apparent hydrological turnover at the catchment scale i.e. the cumulative effect of gains and losses on physical water composition along a stream. The variability in hydrological turnover across a catchment is governed by the spatially varying connectivity between groundwater and the streambed. Especially under low flow conditions, expansion of turnover relative to stream flow is prominent and its spatial variability is intensified.</p><p>Studying the scaling behaviour of hydrological turnover processes, we measured hydrological turnover along two representative stream segments of about 500-600m length at a second order tributary to the river Mosel in Trier, western Germany by applying differential sault dilution gauging (after Payn et al., 2009) over 10 campaigns in summer and 7 in winter. Each stream reach represents a typical geomorphological setting in the catchment. The upstream reach is characterized by steep sloping terrain towards the stream with pastures and forest at higher elevations as the dominant land use. At the downstream reach the terrain is flatter with the stream meandering. The land use is diverse with meadow, pastures and forest as well as settlements. Each respective reach was split into two equidistant parts, resulting in three measurements of hydrological turnover, first and second section as well as the whole reach. Thus, acquiring data accounting for the spatial variability in each reach as well as between reaches. The measurements were carried out weekly, at the two stream reaches from August to September with stream flow ranging from ca. 2 l/s to 94 l/s and at the downstream reach from November to February with stream flow ranging from 200 l/s to over 1000 l/s.</p><p>The results show clearly the positive relationship between discharge and the relative volume of water exchanged between stream, hypohreic zone and groundwater as gains and losses at the reach scale. In addition to that, exchange processes vary independently at both investigated reaches. However, the dataset suggests a distinctive relationship between turnovers of an entire reach compared to the sum of the two sub-reach sections. The slope of this relationship may be a first step for the upscaling of observed exchange and turnover processes from the reach to the network scale.</p>

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

<p>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<sub>50</sub>, d<sub>50</sub>+1 (immediate upper interval), and d<sub>50</sub>-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.</p><p>We conducted an extensive surveying field campaign every summer.</p><p>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<sup>3</sup>·s<sup>-1</sup> (d<sub>50</sub> = 64.0≥Ø<90.5 mm); mean discharge and peak flow from 2013 to 2020 were ~5.3 m<sup>3</sup>·s<sup>-1</sup> and ~125.0 m<sup>3</sup>·s<sup>-1</sup>, respectively and at the end of the watershed).</p><p>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.</p><p>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.</p>

Flood season segmentation and scheme optimization

River flood season segmentation is a significant measure for flood prevention. The objective of this study is to carry out theoretical analysis on flood season segmentation methods and put forward a framework for reasonable flood season segmentation. The proposed framework consists of a Fisher optimal partition method for determining the optimum numbers of the sub-seasons, an ensemble approach for segmenting a defined flood season, and a Non-parametric bootstrap combined with fuzzy optimum selection method (NB-FOS) for testing the rationality of the flood season staging schemes. The present research findings show that different methods could result in different staging schemes. It is proved through rational analysis that the staging scheme obtained by Probability change-point (PCP) is superior to others. The flood season of the downstream reach of Yellow River can be segmented into 3 sub-seasons, i.e. early flood season(Jun.01-July 20), main flood season (July 21- Sept.28), and late flood season (Sept.29- Nov.08). The segmentation results of the flood season should play active role in flood prevention.

Rapid assessment of floating macroplastic transport in the Rhine

<p>Most marine litter pollution is assumed to originate from land-based sources, entering the marine environment through rivers. To better understand and quantify the risk that plastic pollution poses on aquatic ecosystems, and to develop effective prevention and mitigation methods, a better understanding of riverine plastic transport is needed. To achieve this, quantification of riverine plastic transport is crucial. Here, we demonstrate how established methods can be combined to provide a rapid and cost-effective characterization and quantification of floating macroplastic transport in the River Rhine We combine visual observations with passive sampling to arrive at a first-order estimate of macroplastic transport, both in number (10 - 75 items per hour) and mass per unit of time (1.3 – 9.7 kg per day). Additionally, our assessment gives insight in the most abundant macroplastic polymer types the downstream reach of the River Rhine. Furthermore, we explore the spatial and temporal variation of plastic transport within the river, and discuss the benefits and drawbacks of current sampling methods. Finally, we present an outlook for future monitoring of major rivers, including several suggestions on how to expand the rapid assessment presented in this paper.</p>

Investigating the impact of low-head dams on sediment transport dynamics in gravel-cobble streams

<p>Sediment connectivity, though typically viewed as subsidiary to concerns surrounding fish passage, serves an important role in a functioning riverine ecosystem, with both substrate stability and particle size distribution acting as key determinants of benthic community structure and spawning habitat. However, despite more than a decade of pressure to restore stream continuity under the Water Framework Directive (WFD), there have been very few empirical studies on the impact that low-head dams (i.e. weirs) have on bed and suspended sediment conveyance, and little progress in the development of replicable quantitative methodologies for doing so. In this study we explore these knowledge gaps through field investigations of three gravel-cobble streams in southeast Ireland using RFID technology to investigate bedload connectivity, and integrated high-resolution monitoring of turbidity and discrete suspended sediment sampling to establish above dam vs, below dam patterns of suspended sediment conveyance.</p><p>Suspended sediment inputs and outputs over a range of flow conditions (above baseflow) reveal elevated sediment flux at the downstream station (below dam) compared to that coming into the reach (above dam). These observations are indicative of a local source of sediment between monitor­ing stations. Here we suggest that as sediment inputs became exhausted before peak discharge, the structure’s impounded zone (typically considered a depositional area) becomes the dominant source of sediment to the downstream reach. We argue that if sediment trapped behind the structure is available for transportation during high flow events, the system must be trapping sediment under lower flows, which is consistent with field observations.</p><p>Results for bedload connectivity and tracer transport over low-head dams demonstrate that particles exceeding the reach D<sub>90</sub> can be carried through and over these structures, which is consistent with what has been reported from the US. This observation suggests that both structures may have reached a state of ‘transient storage’ as hypothesized by other authors. However, RFID tracer data when reinterpreted as fractional transport rates using a workflow based on existing empirical relations, indicate patterns consistent with supply-limited conditions downstream, demonstrating conflicting lines of evidence between the event-scale tracer movement and long-term sediment regime. Utilizing our empirical data and additional observations collected from a stationary RFID antenna mounted on a weir crest, we expand on existing models and mechanisms to show how a system may continue to exhibit supply-limited conditions downstream without the need for a net attenuation of sediment to occur indefinitely.</p><p>These results indicate that low-head dams may continue to alter the hydrosedimentary processes of fluvial systems long after dam construction and any hypothetical storage capacity has been reached. Though the impact low-head dams have on sediment disconnectivity to the downstream reach is likely to be variable and relatively localized, we hypothesize that the magnitude of any supply-limitation experienced downstream is predominantly a function of both dam height and the structure’s propensity to become drowned out under high flows.</p>

Geomorphic Changes Related to Anthropogenic Interference Along the Ganga River From Rishikesh to Haridwar, Uttarakhand, India

The authors, in this study, have quantified the changes in the geomorphic activity of the Ganga River between Rishikesh and Haridwar by using Survey of India (SOI) 1:5000 scale topographic sheets, satellite data, digital elevation model (DEM), and hydrological data from published resources. They have also correlated the change in settlement area in Haridwar and Rishikesh and connected it to the changing geomorphic features in the downstream reach of the Tehri dam site. The study finds that the change in geomorphology of the river in the studied reach of the Ganga River is attributable to reduced water and sediment supply. The study area being in the active tectonic zone, bound between MCT in the north and HFT in the south, may have experienced offsetting of the reduction in sediment supply by dam in the post dam era (after 2006) due to massive earthquakes delivering sediment directly delivered to streams through enhanced landslide and other mass wasting processes.

Temporal Effects of Groundwater on Physical and Biotic Components of a Karst Stream

Although most lotic ecosystems are groundwater dependent, our knowledge on the relatively long-term ecological effects of groundwater discharge on downstream reaches remains limited. We surveyed four connected reaches of a Chinese karst stream network for 72 consecutive months, with one reach, named Hong Shi Zi (HSZ), evidently affected by groundwater. We tested whether, compared with other reaches, HSZ had (1) milder water temperature and flow regimes, and (2) weaker influences of water temperature and flow on benthic algal biomass represented by chlorophyll a (Chl. a) concentrations. We found that the maximum monthly mean water temperature in HSZ was 0.6 °C lower than of the adjacent upstream reach, and the minimum monthly mean water temperature was 1.0 °C higher than of the adjacent downstream reach. HSZ had the smallest coefficient of variation (CV) for water temperature but the largest CV for discharge. Water temperature and discharge displayed a significant 12-month periodicity in all reaches not directly groundwater influenced. Only water temperature displayed such periodicity in HSZ. Water temperature was an important predictor of temporal variation in Chl. a in all reaches, but its influence was weakest in HSZ. Our findings demonstrate that longer survey data can provide insight into groundwater–surface water interactions.

Three-decadal dynamics of mid-channel bars in downstream of the Three Gorges Dam, China

The downstream of the world’s largest Three Gorges Dam (TGD) along Yangtze River (1560 km) hosts numerous mid-channel bars (MCBs). The morphodynamics of these MCBs are crucial to the river’s hydrological processing, local ecological functioning, and socioeconomic development. However, a systematic understanding of such dynamics and their linkages to the TGD across the entire downstream reach remain largely unknown. Using Landsat archive images and a thematic extracting scheme, the work presents comprehensive monitoring of MCB dynamics in terms of number, area, and shape, in the downstream of the TGD during 1985–2018. Total 140 MCBs were extracted and grouped into four types to represent small size (< 2 km2), middle size (2 km2–7 km2), large size (7 km2–33 km2) and extra-large size (> 33 km2) MCBs, respectively. Most of the MCBs in terms of number (70 %) and total area (91 %) were concentrated in the lower reach (Hankou to Estuary). The number of small size MCBs decreased after TGD closure and most of such decreasing events happened in the lower reach. Although all four types of MCBs showed overall increasing trends in area, small MCBs had smaller rate and received more influence by the TGD operation than the large MCBs. Small size MCBs tended to become relatively shorter and wider whereas the others became slimmer after TGD operation. Impacts from the TGD operation could decrease along with the increasing distance from TGD to Hankou (for shape dynamics) or to Jiujiang (for area dynamics). The quantified longitudinal and temporal dynamics of MCBs across the whole downstream of the TGD provides a crucial monitoring basis for continuous investigations of changing mechanisms in the morphology of the Yangtze River system.