No abstract

Please cite this article as: Bejarano, M.D., Sordo-Ward, A., Alonso, C., Nilsson, C., Characterizing effects of hydropower plants on sub-daily flow regimes, Journal of Hydrology (2017), doi: http://dx. Abstract 9A characterization of short-term changes in river flow is essential for understanding the 10 ecological effects of hydropower plants, which operate by turning the turbines on or off to 11 generate electricity following variations in the market demand (i.e., hydropeaking). The goal 12 of our study was to develop an approach for characterizing the effects of hydropower plant 13 operations on within-day flow regimes across multiple dams and rivers. For this aim we first 14 defined ecologically meaningful metrics that provide a full representation of the flow regime 15 at short time scales from free-flowing rivers and rivers exposed to hydropeaking. We then 16 defined metrics that enable quantification of the deviation of the altered short-term flow 17 regime variables from those of the unaltered state. The approach was successfully tested in 18 two rivers in northern Sweden, one free-flowing and another regulated by cascades of 19 hydropower plants, which were additionally classified based on their impact on short-term 20 flows in sites of similar management. The largest differences between study sites 21 corresponded to metrics describing sub-daily flow magnitudes such as amplitude (i.e., 22 difference between the highest and the lowest hourly flows) and rates (i.e., rise and fall rates 23 33 34 Key words: hydrological alterations; hydrological characterization; hydropeaking; impact 35 assessment; short-term; sub-daily flows 36 37 1. Introduction 38 Critical components of the flow regime such as magnitude, frequency, duration, timing 39 and rate of change control ecological processes in river ecosystems (Poff et al., 1997), and 40 modification of flow regimes constrains the distribution of species, their adaptive capacity, 41 survival, dispersal and reproduction (Lytle and Poff, 2004). Each of these five flow 42 components describes the variability over a wide range of spatial and temporal scales (Ward, 43 1989). Flow variability may be considered at long time scales, which are commonly 44 controlled by inter-and intra-annual variations in climate. Year-to-year variation in flows 45 associated to the Interdecadal Pacific Oscillation index and shifts in the El Niño Southern 46 Oscillation phenomenon (Biggs et al., 2005), and month-to-month variation in flows 47 3 associated to seasons (Bejarano et al., 2010) are examples of large time-scale flow variability. 48 Additionally, topography and geology are usually superimposed on climate and shape intra-49 annual flow variation in, for example, snowmelt-fed or groundwater-fed rivers (Bejarano et 50 al., 2010). Furthermore, flow variability may also be considered at shorter time scales, from 51 months to hours (or smaller). Day-to-day and within-day water gains or losses are ultimately 52 caused by varying rates of precipitation, evapotranspiration, infiltration, ...

No abstract

Flash floods are of major relevance in natural disaster management in the Mediterranean region. In many cases, the damaging effects of flash floods can be mitigated by adequate management of flood control reservoirs. This requires the development of suitable models for optimal operation of reservoirs. A probabilistic methodology for calibrating the parameters of a reservoir flood control model (RFCM) that takes into account the stochastic variability of flood events is presented. This study addresses the crucial problem of operating reservoirs during flood events, considering downstream river damages and dam failure risk as conflicting operation criteria. These two criteria are aggregated into a single objective of total expected damages from both the maximum released flows and stored volumes (overall risk index). For each selected parameter set the RFCM is run under a wide range of hydrologic loads (determined through Monte Carlo simulation). The optimal parameter set is obtained through the overall risk index (balanced solution) and then compared with other solutions of the Pareto front. The proposed methodology is implemented at three different reservoirs in the southeast of Spain. The results obtained show that the balanced solution offers a good compromise between the two main objectives of reservoir flood control management

Questions: Do Mediterranean riparian guilds show distinct responses to stream water declines? If observed, which are the most sensitive and resilient guilds and their most affected attributes?Location: Tiétar river below the Rosarito dam, central-western Spain. Methods:We identified riparian guilds based on key woody species features and species distribution within this Mediterranean river corridor, and evaluated similarity of their responses to long-term flow alteration (i.e. stream water declines since dam construction in 1959). Hierarchical cluster analysis was used to group surveyed vegetation bands according to species composition. The groups were designated as riparian guilds where each vegetation group comprising a guild: (1) contains species sharing similar features (using PCA); and (2) shares a similar environment (using DCA). Changes in several guild attributes (i.e. dominance and species composition, diversity and establishment patterns) during the regulated period were compared statistically. We used pre-and post-dam established vegetation bands identified based on old (1956) and modern (2006) aerial photographs and field measurements of woody species diameter. Results:Responses to flow alterations varied between guilds according to ecological requirements of their species. The ability to survive water stress (i.e. 'Xeric' guilds) and drag forces caused by floods ('Torrential' guilds) allowed certain pioneer shrub-dominated guilds (e.g. Flueggea tinctoria and Salix salviifolia) to spread on newly emerged surfaces downward to the main channel after flow alterations, although new shrubland had less species diversity than pre-dam shrubland. In contrast, new hydromorphological conditions following damming limited recruitment of native late-successional tree guilds sensitive to floods (to drag forces, inundation and anoxia; i.e. 'Slow-water' and 'Flood-sensitive', respectively) and those with greater water requirements (i.e. 'Hydric') (e.g. Alnus glutinosa and Celtis australis), although species diversity increased in this mature forest through co-existence of remaining riparian species and new arrival of upland species.Conclusions: Changes in several riparian attributes after flow alterations differed between guilds. Stream water declines after damming caused shifts in species-poor pioneer shrubland downwards to the watered channel, resulting in severe declines of mature native forest. Understanding vegetation guild responses provides information about general trends in plant populations and assemblage structures expected to occur during river development and flow regulation, increasing our capacity to detect and synthesize complex flow alteration-riparian ecosystem response relationships, and anticipate irreversible impacts.

SUMMARYThis study characterises the abatement effect of large dams with fixed-crest spillways under extreme design flood conditions. In contrast to previous studies using specific hydrographs for flow into the reservoir and simplifications to obtain analytical solutions, an automated tool was designed for calculations based on a Monte Carlo simulation environment, which integrates models that represent the different physical processes in watersheds with areas of 150-2000 km 2 .The tool was applied to 21 sites that were uniformly distributed throughout continental Spain, with 105 fixed-crest dam configurations. This tool allowed a set of hydrographs to be obtained as an approximation for the hydrological forcing of a dam and the characterisation of the response of the dam to this forcing. For all cases studied, we obtained a strong linear correlation between the peak flow entering the reservoir and the peak flow discharged by the dam, and a simple general procedure was proposed to characterise the peak-flow attenuation behaviour of the reservoir. Additionally, two dimensionless coefficients were defined to relate the variables governing both the generation of the flood and its abatement in the reservoir. Using these coefficients, a model was defined to allow for the estimation of the flood abatement effect of a reservoir based on the available information. This model should be useful in the hydrological design of spillways and the evaluation of the hydrological safety of dams. Finally, the proposed procedure and model were evaluated and representative applications were presented.

No abstract

No abstract