Ecohydraulics: linkages between hydraulics, morphodynamics and ecological processes in rivers
Correspondence to: Xavier-François Garcia, Department of Limnology of Shallow Lakes and Lowland Rivers, Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany.
The articles in this issue are a selection of the 15 main presentations made at the EUROMECH Colloquium 523 ‘Ecohydraulics: linkages between hydraulics, morphodynamics and ecological processes in rivers’ that was organized in Clermont-Ferrand, France, from 15 to 17 June 2011. The Colloquium was attended by 51 participants from 15 countries. Copyright © 2013 John Wiley & Sons, Ltd.
Many river systems have been heavily regulated and channelized in the past, with the aim to reclaim fertile land in the floodplain, to protect cities and constructions against natural hazards, to improve navigation, or to use water for hydropower and irrigation. River regularization and channelization often lead to modified hydrological cycles with reduced flooding frequencies, high linearity, high spatial homogeneity in flow conditions and bed morphology and reduced longitudinal (main channels and tributaries) and lateral (main channel and riparian corridor) ecological connectivity. The renaturation projects aim at re-establishing the major ecological functions of the river system and enhancing the connectivity, without jeopardizing the economical functions of the river (e.g. navigation and hydropower production) or the protection against natural hazards (stabilization of structures such as bridge piers and abutments or channel banks). River renaturation, therefore, essentially aims at recreating some natural river dynamics in a controlled way.
The effective design and implementation of river renaturation projects requires good knowledge of hydrodynamic, morphodynamic and ecological processes, as well as their mutual interactions. At present, knowledge and expertise are still largely divided along monothematic lines. To bring together scientists, practitioners and engineers with different background and expertise, the EUROMECH Colloquium 523 ‘Ecohydraulics: linkages between hydraulics, morphodynamics and ecological processes in rivers’ was organized in Clermont-Ferrand, France, from 15 to 17 June 2011. The Colloquium was attended by 51 participants from 15 countries. This Special Issue of the Journal Ecohydrology reports a selection of 15 main contributions to the EUROMECH Colloquium 523.
Obviously, linkages between hydrodynamic, morphodynamic and ecological processes cover an extremely wide field of topics and research methodologies. Tables 1 and 2 classifies the contributions in the present special issue according to their topic and methodology, respectively. This preface aims to briefly introduce the contents of the current special issue and to distill some lessons learned from the Colloquium that can guide future research.
Table 1. Topics covered in the present special issue.
|Casado et al.||Henning and Hentschel||Blanckaert et al.||Branco et al.||García-Arias et al.|
|Hondzo et al.||Jamieson et al.||Bruno et al.||Han et al.||Garófano-Gómez et al.|
| ||Gostner et al.|| ||Parasiewicz et al.||Luce et al.|
| || || || ||Ncube et al.|
| || || ||Ye et al.|
Table 2. Research methodologies covered in the present special issue.
|Blanckaert et al.||Henning and Hentschel||Branco et al.||Han et al.||García-Arias et al.|
|Branco et al.||Jamieson et al.||Garófano-Gómez et al.||Ye et al.||Ye et al.|
|Bruno et al.|| ||Hondzo et al.|| || |
|Han et al.|| ||Luce et al.|| || |
| || ||Ncube et al.|| || |
|Ye et al.|
CONTENT OF THIS SPECIAL ISSUE
It is obvious that river regularization and channelization impairs the ecological functions of a river. A good understanding of the eco-hydro-morphological response of a river system to anthropogenic modifications is a first requirement for the successful design of mitigation measures and river renaturation schemes.
Dams have been built on numerous rivers in the past, to increase flood retention capacity and to exploit water for hydropower and irrigation. Dams affect all components of the river ecosystem on a large spatial scale. In the present special issue, Bruno et al. (this issue) and Casado et al. (this issue) investigated the effect of dams on the thermal regime of the river, Garófano-Gómez et al. (this issue), Ncube et al. (this issue) and Ye et al. (this issue) investigated the impact on vegetation and Han et al. (this issue) investigated their effect on the fish dynamics.
Other human interventions, such as groynes that protect riverbanks and improve navigation, only have an effect in the immediate vicinity of the structure. In the present special issue, Henning and Hentschel (this issue) and Jamieson et al. (this issue) optimized the design of groynes to improve their ecological value and hydraulic function. River channelization often causes bottlenecks in the ecological connectivity of the river system. Branco et al. (this issue) optimized the design of boulders as a local mitigation measure that improves the migration of fish.
Monitoring is essential to assess the evolution of the ecological status of a river in response to anthropogenic modifications. The monitoring typically has to be performed over long temporal scales. Dam construction, for example, modifies the hydrological characteristics and typically reduces the inundation frequency of riparian zones and floodplains. Riparian vegetation will only slowly adapt to new hydrological conditions on a timescale that may range from years to decennia. In a similar way, the river ecosystem also responds to renaturation measures. Garófano-Gómez et al. (this issue) and Ncube et al. (this issue) reported long-term monitoring of changes in riparian vegetation and floodplains following dam construction, whereas García-Arias et al. (this issue) monitored changes in plant assemblages and distribution following river renaturation.
Changes in the river ecosystem are relatively easy to observe and monitor, but it is extremely difficult to assess quantitatively the eco-hydro-morphological status of a river and its evolution over time. Water managers, policymakers and decision makers do, however, need a quantitative assessment by means of simple metrics in order to evaluate the cost–benefit ratio, the feasibility and the success of renaturation projects. Gostner et al. (this issue) and Parasiewicz et al. (this issue) proposed metrics for quantitative assessment. These metrics are essentially based on the hypothesis that physical heterogeneity, mainly in flow depth and velocity, is an indicator of ecological integrity. These metrics quantify the status of a river on the temporal scale of a river reach. These metrics are not process based but merely represent a descriptive statistical quantification of the global ecological state of a river.
The most important but also the most difficult component in the optimal design of a human intervention on a river is the prediction of the ecological response of the river system. Ecological models can be divided into two categories: statistical models and process-based models.
Statistical models relate in a statistical way the main characteristics of a biological variable (such as biomass of vegetation) to the main hydro-morphological drivers (such as characteristics of the velocity and the flow depth). García-Arias et al. (this issue) and Ye et al. (this issue) used statistical models to investigate the evolution of riparian vegetation. A limitation of statistical models is that the statistical relation between biological and hydro-morphological parameters is case dependent. Therefore, these models cannot easily be commuted between different rivers and require a reference state for calibration.
Process-based models represent the main processes in the life cycle of the investigated species. Ye et al. (this issue) developed a process-based model for riparian vegetation and compared its capabilities to a statistical model. For the same reach on the River Lijaing in China, Han et al. (this issue) developed a process-based model for fish dynamics. Process-based models are less case dependent and have a more general validity range than statistical models. But the modelling of all main processes involves a multitude of empirical parameters that inherently leads to a relatively large uncertainty in the model predictions.
Enhanced understanding of the main ecological processes is a requisite to improve ecological models. This special issue reports experimental research on real rivers, in mesocosms and in laboratory flumes.
Experimental research on real rivers often aims at monitoring and documenting processes as well as gathering data for the validation of ecological models. Garófano-Gómez et al. (this issue), Ncube et al. (this issue) and Ye et al. (this issue) reported field experiments on riparian vegetation, and Branco et al. (this issue) reported on the use of boulders to improve fish migration. Hondzo et al.(this issue ) analysed and modelled the dynamics of dissolved oxygen, and Luce et al. (this issue) investigated the biomass loss of periphyton due to abrasion by sediment transport.
To prevent the complexity of the real river environment, experiments are often performed in dedicated mesocosms and laboratory flumes. Branco et al. (this issue) complemented their field research on the use of boulders to improve fish migration with experiments in a full-scale experimental fishway. Bruno et al. (this issue) investigated the response of benthic invertebrates to interacting hydropeaking and thermopeaking in a set of open air flumes directly fed by an Alpine stream. Han et al. (this issue) investigated the movement rules of a fish under volitional swimming conditions in dedicated laboratory experiments. Blanckaert et al. (this issue) focussed on the role of turbulence on the drift of benthic invertebrates in laboratory experiments.
The large spectrum of topics and research methodologies covered in this special issue demonstrate the relevance and importance of the relatively recent field of ecohydraulics. The discussions during the EUROMECH 523 colloquium and the selected papers in this special issue provide some tentative conclusions and guidelines for future research:
- Most contributions investigate and model separately water quality, thermal regime, river morphology, benthic invertebrates, fish and vegetation. It is logical to focus in a first step separately on each of these topics, but future development should strive to a more integrated approach that includes the multiple linkages between the different components and processes.
- Some contributions investigate and quantify the river ecosystem on a large spatial scale, and some contributions investigate ecological processes on a small spatial scale. River systems are characterized by a broad range of relevant spatial and temporal scales that interact. We believe that the interaction between the different spatial and temporal scales is a topic that merits further attention in the form of a multi-scale approach. As an example, patterns of flow, turbulence and morphology that are ecologically important do exist on small spatial scales, such as behind obstacles in the flow field, or in the vicinity of irregular banks. These small spatial scales cannot be resolved by the hydro–morphological models that are typically used for the modelling of river reaches. Hence, research could focus on a parameterization of these small-scale processes that can be incorporated in large-scale models. Moreover, efforts have to be made to understand how hydraulic processes occurring at large spatial scale affect and drive processes at smaller spatial scales.
- This special issue includes contributions that adopt a statistical approach and contributions that adopt a process-based approach. Obviously, both approaches have advantages and limitations regarding generalization, prediction skills, complexity and costs. The best predictions of the response of river ecosystems to human interventions can probably be obtained by a combination of both approaches and by combining the expertise of mathematicians, statisticians, hydrologists, hydraulicians, geomorphologists and ecologists. Incorporating uncertainty in deterministic models provide much more information and allows to value the results. In all cases, well-documented field surveys that include ecology and hydromorphology as well as laboratory studies of the physical and biological processes remain of key importance.
We hope that the present special issues will provide inspiration and ideas and contribute to the further progress in the field of Ecohydraulics.