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This review provides a critical, multi-faceted assessment of the practical contribution tidal stream energy can make to the UK and British Channel Islands future energy mix. Evidence is presented that broadly supports the latest national-scale practical resource estimate, of 34 TWh/year, equivalent to 11% of the UK’s current annual electricity demand. The size of the practical resource depends in part on the economic competitiveness of projects. In the UK, 124 MW of prospective tidal stream capacity is currently eligible to bid for subsidy support (MeyGen 1C, 80 MW; PTEC, 30 MW; and Morlais, 14 MW). It is estimated that the installation of this 124 MW would serve to drive down the levelized cost of energy (LCoE), through learning, from its current level of around 240 £/MWh to below 150 £/MWh, based on a mid-range technology learning rate of 17%. Doing so would make tidal stream cost competitive with technologies such as combined cycle gas turbines, biomass and anaerobic digestion. Installing this 124 MW by 2031 would put tidal stream on a trajectory to install the estimated 11.5 GW needed to generate 34 TWh/year by 2050. The cyclic, predictable nature of tidal stream power shows potential to provide additional,whole-system cost benefits. These include reductions in balancing expenditure that are not considered in conventional LCoE estimates. The practical resource is also dependent on environmental constraints. To date, no collisions between animals and turbines have been detected, and only small changes in habitat have been measured. The impacts of large arrays on stratification and predator–prey interaction are projected to be an order of magnitude less than those from climate change, highlighting opportunities for risk retirement. Ongoing field measurements will be important as arrays scale up, given the uncertainty in some environmental and ecological impact models. Based on the findings presented in this review, we recommend that an updated national-scale practical resource study is undertaken that implements high-fidelity, site-specific modelling, with improved model validation from the wide range of field measurements that are now available from the major sites. Quantifying the sensitivity of the practical resource to constraints will be important to establish opportunities for constraint retirement. Quantification of whole-system benefits is necessary to fully understand the value of tidal stream in the energy system.

This paper presents numerical modeling methods to predict long term reservoir sedimentation. The Generalized Stream Tube computer models for Alluvial River Simulation version 3.0 (GSTARS3) model which has capability of semi-two dimensional hydraulic computation and sediment routing by using stream tube concept combined with the theory of minimum stream power was implemented. In GSTARS3, non-equilibrium sediment transport equation is incorporated in accounting for the reservoir sedimentation processes. Capability of numerical simulation with and without applying stream tube concept, theory of minimum stream power, and non-equilibrium sediment transport equation was evaluated with nine years of sedimentation in the Kardeh Reservoir, Iran, and the best combinations of theories were investigated in this study. Simulated thalweg profile and channel geometry were compared to the measured results. From the comparisons, non-equilibrium sediment transport equation is preferably recommended method to predict reservoir sedimentation. The application of theory of minimum stream power improved the performance of model predictions significantly. In addition, simulations with three stream tubes were slightly better than the one with five stream tubes. Numerical simulations indicated that a semi-two dimensional modeling with non-equilibrium sediment transport equation, three stream tubes, and theory of minimum stream power is applicable to a long term prediction of reservoir sedimentation.

On 2 September 2018, an intense rainstorm swept Mengdong Town in Yunnan Province of China, inducing a serious landslide and debris flow disaster with 10 deaths and 11 missing. Image interpretation, field survey, and slope stability analysis were used to examine the characteristics and initiation mechanism of this hazard. A total of 1774 landslide scars were identified, the area of which occupied 8.26% of the study region. Due to the spatial inhomogeneity of precipitation, these scars mainly concentrated along valleys in the lower part of the study area. Besides, well-forested hillslopes were more prone to landslides, indicating the limited role of trees in stabilizing slopes in extreme rainfall events. The initiation of landslides is mainly attributed to the week cohesion of the saturated clayey sand beneath the root zone, where the tensile resistance of roots was absent, and the increase of positive pore water pressure. Additionally, 288 landslide scars were situated adjacent to roads, demonstrating that the road construction activity had intensified the landslide disaster. Owing to the relatively low mobility of landslides, a considerable portion of landslide debris deposited on the valley floor in smaller watersheds (< 6.03 hm2), while the remaining portion entered high-order channels. In these channels, where stream power was relatively large, the woody debris and soil carried by landslides were entrained by streamflow, and debris flows were formed. Moreover, the magnitude of debris flow was amplified by the vertical and lateral erosion of the stream channel.

This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s−1. Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12-13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f−2/3 inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f>0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11-14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.

The Mahi—one of the major rivers in Western India—is subject to frequent major flooding, which severely affects the local economy and infrastructure. Little has been done, however, to assess the flood patterns and severity along its course. Here, the Mann–Kendall and Pettitt tests are used to identify long-term trends of precipitation and peak streamflow at multiple locations in the catchment. Then, flood susceptibility mapping is performed by the analytical hierarchy process, accounting for 14 geomorphic, hydraulic, and geologic factors. The analyses suggest a decline in total precipitation and peak flow discharges at most locations, consistently with the general climatic trend of the area, featuring a weakening summer monsoon. Nonetheless, a significant portion of the catchment area remains highly susceptible to flooding, with stream powers capable of mobilizing boulders up to 1 m in size in extraordinary floods. These results can support the work of engineers and policymakers dealing with floods in the study area, but the proposed methodology can also be applied to other fluvial catchments to evaluate the role of climate trends in modulating flood susceptibility.

River beds are traditionally assumed to become mobile at a fixed value of nondimensional shear stress, but several flume and field studies have found that the critical value is higher in steep shallow flows. Explanations for this have been proposed in terms of the force balance on individual grains. The trend can also be understood in bulk‐flow terms if total flow resistance has “base” and “additional” components, the latter due to protruding immobile grains as well as any bedforms, and the stress corresponding to “additional” resistance is not available for grain movement in threshold conditions. A quantitative model based on these assumptions predicts that critical Shields stress increases with slope, critical stream power is near‐invariant with slope, and each has a secondary dependence on bed sorting. The proposed slope dependence is similar to what force‐balance models predict and consistent with flume data and most field data. Possible explanations are considered for the inability of this and other models to match the very low critical values of width‐averaged stress and power reported for some low‐gradient gravel bed rivers. Key Points New conceptual explanation for increase in critical Shields stress with slope Model predicts secondary dependence on bed sorting New model for critical stream power as spinoff

PurposeThe degree of the river channel change, and particularly the amount of transported sediment, defines the geomorphic effectiveness of a flood event. This study aims to improve the level of understanding of the control exerted by the streamflow and the flow energy on the sediment flux regimes during flood events, which are ranked depending on their geomorphic effectiveness.Materials and methodsThe study focused on the Trotuș River, Eastern Carpathians, Romania. Two data sources were used: gauging stations (discharge and suspended sediment load) and field observations (grain size, channel bed slope and channel width corresponding to bankfull discharge). The bedload sediment transport was estimated using the Bedload Assessment for Gravel-bed Streams (BAGS) program. The parameters and variables taken into consideration (magnitude, duration, stream power, total energy expenditure, total sediment load) were statistically analysed for each flood event.Results and discussionJust 15% of the total 76 investigated flood events ranked in the class which includes the flood events with the highest geomorphic effectiveness. Of the 41 × 106 t of sediment carried through the four gauging stations during flood events, approximately 74% was transported at stream power values above 300 W m−2. In the years with type B flood events, the sediment flux was much higher than the multiannual average value. Changes of the sediment rating curves were documented at all gauging stations subsequent to major flood events. Clockwise loops accounted for the highest percentages in all types of flood events. Major flood events generate significant changes in river channels, which are subsequently reflected in the sediment regime.ConclusionsThe main cause of the changes occurring in the sediment regime likely lies in the channel adjustments subsequent to flood events. This investigation introduces a methodology for ranking flood events depending on their magnitude, duration, stream power and transported sediment.

Soil erosion by water is a significant problem in arid and semi-arid areas of large parts of Iran. Water erosion is one of the most effective phenomena that leads to decreasing soil productivity and pollution of water resources; especially, in the Mazayjan watershed in the southwest of Fars Province gully erosion contributes to the sediment dynamics in a significant way. Consequently, the intention of this research is to identify the different types of soil erosion processes acting in the area and to assess the process dynamics in an integrative way. Therefore, we applied GIS and satellite image analysis techniques to derive input information for the numeric models. For sheet and rill erosion the Unit Stream Power-based Erosion Deposition Model (USPED) was utilized. The spatial distribution of gully erosion was assessed using a statistical approach, which used three variables (stream power index, slope, and flow accumulation) to predict the spatial distribution of gullies in the study area. The eroded gully volumes were estimated for a 7-year period by fieldwork and Google Earth high-resolution images. Finally the gully retreat rates were integrated into the USPED model. The results show that the integration of the SPI approach to quantify gully erosion with the USPED model is a suitable method to qualitatively and quantitatively assess water erosion processes. The application of GIS and stochastic model approaches to spatialize the USPED model input yields valuable results for the prediction of soil erosion in the Mazayjan catchment. The results of this research help to develop an appropriate management of soil and water resources in the southwestern parts of Iran.

Meandering rivers are among the most dynamic Earth-surface systems, which generally appear in fertile valleys, the most valuable lands for agriculture and human settlement. Landsat time series and morphological parameters are complementary tools for exploring river dynamics. Karun River is the most effluent and largest meandering river in Iran, which keeps the Karun’s basin economy, agriculture, and industrial sections alive; hence, investigating morphological changes in this river is essential. The morphological characteristics of Karun have undergone considerable changes over time due to several tectonic, hydrological, hydraulic, and anthropogenic factors. This study has identified and analyzed morphological changes in Karun River using a time series of Landsat imagery from 1985–2015. On that basis, morphological dynamics, including the river’s active channel width, meander’s neck length, water flow length, sinuosity index, and Cornice central angle, were quantitatively investigated. Additionally, the correlation between the stream power and morphological factors was explored using the data adopted from the hydrometric stations. The results show that the dominant pattern of the Karun River, due to the sinuosity coefficient, is meandering, and the majority of the river falls in the category of developed meander rivers. Moreover, the number of arteries reduced in an anabranch pattern, and the river has been migrating towards the downstream and eastern sides since 1985. This phenomenon disposes a change in the future that can be hazardous to the croplands and demands specific considerations for catchment management.

In recent decades, the number of rainfall-induced landslides has increased significantly in many parts of Malaysia, especially in the urbanized and hilly areas. The disturbance of hilly morphology as a result of human activities has increased the potential for erosion on man-made slopes, especially during extreme rainfall during rain events. Most hilly areas in Malaysia are covered by a thick layer of soil, which is known to have a significant impact on soil erosion. However, little is known about how soil erosion and rainfall could be the driving force behind landslide initiation, especially on stabilized slopes. Therefore, this study focuses on the soil detachment rate of landslides triggered by rainfall at different rainfall intensities. A sandbox model is used to represent real slope conditions. The relationship between the soil detachment capacity, soil properties (water content, slope, clay layers and soil compaction), hydraulic parameters (flow shear stress and stream power) and rainfall intensities (low, medium and high) was investigated. The results showed that the hydraulic parameters and the rainfall intensity are directly proportional to the detachment rate of the soil. Water content and slope show a higher soil detachment rate and a lower critical flow shear stress than other soil properties. It can be concluded that high saturation and steep slope increase the risk of soil erosion because the cohesion and friction of the soil are significantly reduced, leading to a weakening of the soil structure at the surface. The results of this study can feed into the existing analysis of slope stability and formulate the onset of a landslide triggered by rainfall, especially in eroded soils.