Tetsuya Sumi

Sustainable sediment management in reservoirs and regulated rivers: Experiences from five continents

By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam construction globally, these impacts are increasingly widespread. There are proven techniques to pass sediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream impacts, but they are not applied in many situations where they would be effective. This paper summarizes collective experience from five continents in managing reservoir sediments and mitigating downstream sediment starvation. Where geometry is favorable it is often possible to bypass sediment around the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reaches with rates and timing similar to pre-dam conditions. Sluicing (or drawdown routing) permits sediment to be transported through the reservoir rapidly to avoid sedimentation during high flows; it requires relatively large capacity outlets. Drawdown flushing involves scouring and re-suspending sediment deposited in the reservoir and transporting it downstream through low-level gates in the dam; it works best in narrow reservoirs with steep longitudinal gradients and with flow velocities maintained above the threshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend that these sediment management approaches be utilized where possible to sustain reservoir capacity and minimize environmental impacts of dams.

A physically based distributed hydrological model of wadi system to simulate flash floods in arid regions

Many hydrological approaches have been developed for humid areas; however, we still have many challenges regarding flash floods simulation in arid regions. Thus, the main purpose of this study is to develop a physically based hydrological model for flash floods simulation, to understand the hydrological processes, and to overcome the water relating problems, as water scarcity, data deficiency. In this study, Hydrological River Basin Environmental Assessment Model incorporating wadi system (Hydro-BEAM-WaS) integrated with both Geographic Information System (GIS) and remote sensing data is proposed. To overcome the lack of observational data, Global Satellite Mapping of Precipitation (GSMaP) data has been compared with the Global Precipitation Climatology Center (GPCC) data. We found that the GSMaP has an overestimated or underestimated systematic seasonal bias. Wadi El-Khoud in Oman was chosen for model calibration, while the River Nile basin in Egypt was chosen for flash flood simulations. The simulation has been successfully carried out where it exhibits a reasonable fit between both the simulated and observed results, in spite of the deficiency of high quality observations. This approach can be used to evaluate and simulate the wadi runoff behaviors, such as discontinuous surface flow. It also can help in estimating the initial and transmission losses. Flash flood water has been assessed and evaluated as new water resources, which would be properly utilized to overcome the problem of water shortage in such regions. The proposed model has been proven to provide reliable simulations of flash floods referring to ungauged wadi systems.

Effects of sediment bypass tunnels on grain size distribution and benthic habitats in regulated rivers

ABSTRACT Four dams in Japan and Switzerland with Sediment Bypass Tunnels (SBT) as a measure against reservoir sedimentation were monitored to analyse the effects of sediment supply on the downstream environment based on up- to downstream differences in geomorphological and biological characteristics. SBT operation times ranged from 93 years at Pfaffensprung and 17 at Asahi to only three years at Solis and no operation at Koshibu. Sediment grain size distribution was monitored, and microhabitats and invertebrates were analysed in terms of richness and composition. Results showed that grain sizes were coarser down- than upstream at dams with newly established SBTs, while they were similar or finer for dams with long SBT operation. Analysis of biotic data revealed that microhabitat and invertebrate richness was low directly below the dam but increased further downstream the longer the SBT operation. Sedentary species dominated at locations where bed conditions were stable, e.g. directly downstream of the dam at Koshibu. Recovery of downstream environment with increasing SBT operation time was disclosed by the Bray–Curtis similarity index, which evaluated an overlap between up- and downstream reaches for both microhabitat composition and invertebrate communities. With increasing operation time, both indices increased, revealing the positive effects of long-term SBT operation.

Sediment transport in high-speed flows over a fixed bed: 2. Particle impacts and abrasion prediction

Abstract Single bed load particle impacts were experimentally investigated in supercritical open channel flow over a fixed planar bed of low relative roughness height simulating high‐gradient non‐alluvial mountain streams as well as hydraulic structures. Particle impact characteristics (impact velocity, impact angle, Stokes number, restitution and dynamic friction coefficients) were determined for a wide range of hydraulic parameters and particle properties. Particle impact velocity scaled with the particle velocity, and the vertical particle impact velocity increased with excess transport stage. Particle impact and rebound angles were low and decreased with transport stage. Analysis of the particle impacts with the bed revealed almost no viscous damping effects with high normal restitution coefficients exceeding unity. The normal and resultant Stokes numbers were high and above critical thresholds for viscous damping. These results are attributed to the coherent turbulent structures near the wall region, i.e. bursting motion with ejection and sweep events responsible for turbulence generation and particle transport. The tangential restitution coefficients were slightly below unity and the dynamic friction coefficients were lower than for alluvial bed data, revealing that only a small amount of horizontal energy was transferred to the bed. The abrasion prediction model formed by Sklar and Dietrich in 2004 was revised based on the new equations on vertical impact velocity and hop length covering various bed configurations. The abrasion coefficient kv was found to be vary around kv ˜ 105 for hard materials (tensile strength ft > 1 MPa), one order of magnitude lower than the value assumed so far for Sklar and Dietrichs model. Copyright

Abrasion prediction at Asahi sediment bypass tunnel based on Ishibashi's formula

Abrasion in a concrete-lined sediment bypass tunnel is estimated using a Japanese state-of-the-art prediction model and validated by measured invert abrasion data at Asahi Reservoir, Japan. The model is described in detail, certain shortcomings are disclosed, and a revised version is proposed. The model consists of a kinetic energy term accounting for the impact by saltating particles, and a friction work term accounting for the grinding stress. It is found that the latter term yields concrete abrasion values being consistently a multiple compared to its kinetic term contradicting other research. Based on that, and a possible particle impact angle inconsistency, it is proposed to omit the friction work term. It is shown that the calculated abrasion is overestimated by 138% on average compared with that measured, if both terms are accounted for. However, promising results are obtained with only 30% overestimation by neglecting the friction work term.

Sediment transport in high-speed flows over a fixed bed: 1. Particle dynamics: Sediment transport in high-speed flows: 1. Particle dynamics

Abstract Particle dynamics are investigated experimentally in supercritical high‐speed open channel flow over a fixed planar bed of low relative roughness height simulating flows in high‐gradient non‐alluvial mountain streams and hydraulic structures. Non‐dimensional equations were developed for transport mode, particle velocity, hop length and hop height accounting for a wide range of literature data encompassing sub‐ and supercritical flow conditions as well as planar and alluvial bed configurations. Particles were dominantly transported in saltation and particle trajectories on planar beds were rather flat and long compared with alluvial bed data due to (1) increased lift forces by spinning motion, (2) strongly downward directed secondary currents, and (3) a planar flume bed where variation in particle reflection and damping effects were minor. The analysis of particle saltation trajectories revealed that the rising and falling limbs were almost symmetrical contradicting alluvial bed data. Furthermore, no or negligible effect of particle size and shape on particle dynamics were found. Implications of experimental findings for mechanistic saltation‐abrasion models are briefly discussed. Copyright

Development of the Suspended-Sediment Concentration Measuring System with Differential Pressure Transmitter in Rivers and Reservoirs

It is important to know quantity of wash load for the integrated sediment management from upstream through downstream river basins. Generally, both continuous turbidity measuring and bottle sampling are used for suspended-sediment concentration measurement in rivers or reservoirs. Here, we have developed a new suspended sediment concentration measuring system with differential pressure transmitter (hereafter, we call SMDP). The SMDP has advantages in long-term and high turbidity measurement because differential pressure transmitter is measuring density of water directly. Based on measuring conditions, a submersible type and a water circulating type SMDP are available, and both of them have shown good performances through laboratory tests and field experiments at the Miwa dam, the Tenryu river and the Kurobe river.

Advances and approaches in river sediment research

Rivers remain nowadays and will be in the future one of the main resources for drinking and irrigation water but also for the production of hydroelectricity. Human interventions, such as the construction of dams and reservoirs, are thus essential to ensure a sustainable water supply. However, each physical modification of the river system has an impact on the river network, including water discharge, sediment quantity and quality and affects as consequence the river morphology and ecology. As a result of an increasing water demand, rivers in their natural states are nowadays very seldom. A deeper understanding of the basics in sediment transport, morphodynamics as well as of the effects of modifications on the river network is therefore required. In particular, it is important to implement appropriate and sustainable management strategies in order to minimize negative impacts of anthropogenic works on the aquatic environment. The International Symposium on River Sedimentation (ISRS), held in Stuttgart, Germany, 19–22 September 2016, served as a platform to give international researchers from engineering and natural science, stakeholders and decisionmakers the possibility to exchange knowledge, discuss advances in river sediment research and to present new and transdisciplinary approaches with respect to sediment dynamics, sustainable water resources management as well as ecological functioning of river systems. High-quality submissions were selected for special issues to further distribute new achievements and improved knowledge to the community. The present special issue in the International Journal of River and Basin Management contains four of these peer-reviewed articles, focusing mainly on research related to water management for ensuring a secure water supply. The herein presented state-of-the-art research articles include an analysis of the hydro-sedimentological characteristics of intermittent rivers in Afghanistan, modelling of cohesive sediments in the Upper Rhine, measurements of sediment fluxes in periglacial reservoirs and a study of effects of sediment bypass tunnels on sediment grain size distribution and benthic habitats. Around 30% of the global river network consists of intermittent river systems. These systems are characterized by a high seasonal variability in water, sediment transport and a recurrent habitat discontinuity. Sadid et al. (2017) analysed the hydro-sedimentological characteristics of the ShakarDara and Maidan Rivers in Afghanistan to make an attempt to outline major characteristics of these two intermittent river systems. First, the interaction between groundwater and surface water was investigated, where the importance of precipitation is shown to recharge the groundwater. Also, an increase of suspended sediments over the last decades was observed from the analysed data. The Maidan River showed in general higher suspended sediment concentrations due to less vegetated areas and finer available sediment sizes. The prediction of cohesive sediment behaviour represents a special challenge due to the presence of physicochemical and biological interactions. Further understanding of settling, consolidation and remobilization of cohesive sediments is therefore essential to determine the risk of erosion of e.g. contaminated cohesive sediments in reservoirs. Hoffmann et al. (2017) focused on an evaluation of uncertainties related to erodibility measurements and their implications for the modelling of erosion risk. Data on the erodibility of cohesive sediments from the impounded Upper Rhine River and a conceptual framework, including 10 reservoirs along this river stretch, were used. Even if the statistically analysed data showed large uncertainties in the estimation of critical shear stresses and erosion rates, good agreement were obtained for the modelled results of the evolution of the bed level changes of the most downstream reservoir at Iffezheim, compared with measurements. Reservoir sedimentation is a serious problem in many reservoirs worldwide. Shrinking glaciers, as result of climate change, will potentially increase this problem also for periglacial reservoirs. Ehrbar et al. (2017) presented particle size and suspended sediment concentration measurements in three periglacial reservoirs located in the Swiss Alps. The combination of water sample analysis, Laser In-Situ Scattering Transmissometry and Acoustic Current Doppler Profiler measurements was proven to be a suitable method to investigate even the relatively low occurring suspended sediment concentrations, whereby sediments are mainly composed of clay and silt. The combination of the chosen instruments made it possible to study the mixing of inflowing river water and the evolution and decay of turbidity currents. Application experiences as well as limitations of the methods were also presented and discussed within the article. Sediment bypass tunnels are increasingly used as a measure against reservoir sedimentation. The advantage of these structures is that sediment particles, including coarse particles, will be guided directly to the river stretch downstream of the dam and, as a consequence, the transport will not be interrupted by reservoirs. Auel et al. (2017) investigated four dams in Japan and Switzerland to monitor and analyse the effects of sediment supply by sediment bypass tunnels on the downstream environment. The analysis is based on a monitoring of both upstream and downstream reaches. Due to the investigation of structures, which are newly established and a comparison to tunnels, which are in operation since a longer period, conclusions were drawn regarding the refinement of sediments, microhabitat composition and invertebrate communities as well as the recovery of the downstream environment. Within these four articles, the broad spectrum of ongoing research in the field of river sediment research can be seen. Not only the temporal and spatial scales differ in the papers, but also the sediment characteristics and research methodologies. However, all selected articles show the importance of future research in order to understand the processes behind sediment movement and

Heavy flooding generating a precipitous decrease of fine particles along a riverbed: the case of a downstream reach of the futase dam

Dams and their reservoirs are operated for multiple purposes, including flood protection.1 However, all dams trap sediment and inevitably lead to downstream physical and ecological changes.1 In the downstream reaches, flows are more stable and sediment transport is more limited than in those upstream.2 This can cause several problems in downstream reaches including sediment deficiency and riverbed degradation.3 To compensate the sediment deficit downstream and reduce ecological damage, sediment replenishment is sometimes performed.3 Specifically, a certain amount of sediment is excavated from an upstream reservoir, transported to downstream channels, and then flushed via channel flooding.4 However, the total volume of sediment transported downstream is influenced not only by the annual volume of replenished sediment, but also by the flood discharge. The combination of these factors complicates prediction of sediment flow patterns. Analysis of a range of flood discharge rates and sediment flow data over many years at a single site is required in order to better predict these sediment flow patterns. However, existing data of this type spans only relatively short time periods (a few years at most). In this study, we examine the impacts of flood discharge on fine material on a riverbed by investigating the effects of sediment replenishment in the Futase Dam in Saitama Prefecture, Japan.

STUDY ON BEDLOAD MEASUREMENT BY PLATE-TYPE VIBRATION SENSOR

Measuring the bedload transport rate is utmost interest to understand and quantify all sediment transport related phenomena. Three existing bedload measuring systems are 1) the hydrophone, 2) the geophone, and 3) the plate microphone as their modified system. All systems estimate transport rate based on sediment impact, whereas each has respective disadvantages. In this paper, as the other improved system, plate-type vibration sensor is discussed. Calibration experiment is conducted and the system is compared with the other plate hydrophone. In the experiments, impact data is perceived as both vibration pressure and vibration pulse, and analyzed the same as plate microphone. Although it has been clarified that fine sediments lower than 10 mm are not detectable with plate hydrophone, plate-type vibration sensor detected vibration pulses of them clearly. Moreover, detection rate can be linked to the sediment bulk density and the gravel jump length.