Christoph Hauer

Morphodynamic Effects on the Habitat of Juvenile Cyprinids (Chondrostoma nasus) in a Restored Austrian Lowland River

At the Sulm River, an Austrian lowland river, an ecologically orientated flood protection project was carried out from 1998–2000. Habitat modeling over a subsequent 3-year monitoring program (2001–2003) helped assess the effects of river bed embankment and of initiating a new meander by constructing a side channel and allowing self-developing side erosion. Hydrodynamic and physical habitat models were combined with fish-ecological methods. The results show a strong influence of riverbed dynamics on the habitat quality and quantity for the juvenile age classes (0+, 1+, 2+) of nase (Chondrostoma nasus), a key fish species of the Sulm River. The morphological conditions modified by floods changed significantly and decreased the amount of weighted usable areas. The primary factor was river bed aggradation, especially along the inner bend of the meander. This was a consequence of the reduced sediment transport capacity due to channel widening in the modeling area. The higher flow velocities and shallower depths, combined with the steeper bank angle, reduced the Weighted Useable Areas (WUAs) of habitats for juvenile nase. The modeling results were evaluated by combining results of mesohabitat-fishing surveys and habitat quality assessments. Both, the modeling and the fishing results demonstrated a reduced suitability of the habitats after the morphological modifications, but the situation was still improved compared to the pre-restoration conditions at the Sulm River.

Challenges of river basin management: Current status of, and prospects for, the River Danube from a river engineering perspective.

In the Danube River Basin multiple pressures affect the river system as a consequence of river engineering works, altering both the river hydrodynamics and morphodynamics. The main objective of this paper is to identify the effects of hydropower development, flood protection and engineering works for navigation on the Danube and to examine specific impacts of these developments on sediment transport and river morphology. Whereas impoundments are characterised by deposition and an excess of sediment with remobilisation of fine sediments during severe floods, the remaining five free flowing sections of the Danube are experiencing river bed erosion of the order of several centimetres per year. Besides the effect of interruption of the sediment continuum, river bed degradation is caused by an increase in the sediment transport capacity following an increase in slope, a reduction of river bed width due to canalisation, prohibition of bank erosion by riprap or regressive erosion following base level lowering by flood protection measures and sediment dredging. As a consequence, the groundwater table is lowered, side-arms are disconnected, instream structures are lost and habitat quality deteriorates affecting the ecological status of valuable floodplains. The lack of sediments, together with cutting off meanders, leads also to erosion of the bed of main arms in the Danube Delta and coastal erosion. This paper details the causes and effects of river engineering measures and hydromorphological changes for the Danube. It highlights the importance of adopting a basin-wide holistic approach to river management and demonstrates that past management in the basin has been characterised by a lack of integration. To-date insufficient attention has been paid to the wide-ranging impacts of river engineering works throughout the basin: from the basin headwaters to the Danube Delta, on the Black Sea coast. This highlights the importance of new initiatives that seek to advance knowledge exchange and knowledge transfer within the basin to reach the goal of integrated basin management.

The importance of morphodynamic processes at riffles used as spawning grounds during the incubation time of nase (Chondrostoma nasus)

An ecologically orientated flood protection project was implemented at the Austrian lowland Sulm River. Habitat modelling was conducted during a 3-year monitoring program to assess the effects of river bed embankment and the initiation of a new meander by constructing an initial side channel, and allowing self-developing side erosion. Hydrodynamic and physical habitat models were combined with statistical methods. This study focused on the necessity for including stability analysis when modelling spawning grounds. The critical erosion parameter of the cobbles at spawning grounds was analysed using numerical modelling combined with substrate maps of potential spawning grounds of nase (Chondrostoma nasus). Due to the specific characteristics of this reach of the Sulm River, instability of the riffles can hamper successful reproduction in nase. Spawning grounds of Chondrostoma nasus and their stability must be guaranteed at least for the duration of incubation. This morphodynamic necessity should be incorporated in future restoration projects and when artificially restoring spawning habitats.

The status of the Danube River sediment regime and morphology as a basis for future basin management

Hydromorphological alterations of large rivers are evident and have to be related to multiple anthropogenic pressures. Here we present the results of an integrated study concerning the actual status of the hydromorphology of the Danube River Basin that shows the sediment regime features a heavily disturbed system at various scales. Combined impacts of flood protection, navigation, and hydropower measures applied over a long period of time have been identified based on the river scaling concept as being responsible for these specific alterations (lack of bed load and suspended load in the remaining free-flowing sections). Moreover, long sections of the Danube River have been narrowed, channelized, disconnected from floodplains, and morphologically degraded over the last 200 years. This has caused increased bottom shear stresses, increased sediment transport capacities, and in addition a lack of lateral self-forming processes and corresponding reduced morphodynamics in the non-impounded sections. As a consequence of both, longitudinal and lateral disturbances of the sediment supply and additional impacts of the channelization, in opposition to a surplus of sediments in impoundments the remaining free-flowing sections are subject to various forms of river bed degradation. Such degradation or river bed incision leads to a loss of instream structures in general, with a disappearance of gravel bars at the Upper Danube and changes to sand bars in the Lower Danube. Hence, for river systems and large river basins, it has to be stated that the preservation and restoration of sediment continuity and morphodynamics is one of the most relevant issues for river engineering and ecology. This has to be considered especially for the implementation of legal directives and/or future river basin management plans.

Floodplain evaluation matrix (FEM): An interdisciplinary method for evaluating river floodplains in the context of integrated flood risk management

Abstract Strengthening of non-structural measures for flood protection by enforcing natural retention processes on river floodplains not only is required by the EU Floods Directive but also implicates a variety of socio-economic conflicts related to the multipurpose use of floodplains. In order to ensure the achievement of multiple benefits of such non-structural measures in the most effective way, it is crucial to determine which floodplains are highly relevant for preservation and/or restoration not only concerning flood protection (hydrology/hydraulics) but also for ecological and sociological reasons. Therefore, the main objectives of this paper are to identify on different spatial scales those hydrological/hydraulic, ecological and sociological parameters that are important for an integrated evaluation of floodplain effectiveness. The results are finally assembled in the novel multidisciplinary floodplain evaluation matrix (FEM) serving as decision support for the relevant stakeholders and indicating where efforts of floodplain preservation/restoration should be spent first within an integrated flood risk management. At the end of this paper, an integrative evaluation on macro-scale level for the Austrian Kamp River is exemplarily presented. Detailed sectoral investigations of the hydrological and hydraulic parameters are further described in a case study of the Austrian Danube (see paper “Assessing the role of floodplains along the Austrian Danube for flood hazard reduction using the hydrological and hydraulic parameters of the FEM-method”).

Dynamic disturbance regime approach in river restoration: concept development and application

In recent years, numerous restoration measures have been initiated to ecologically and morphologically improve rivers based on self-dynamic development. A wealth of monitoring studies has been implemented to evaluate these restoration measures. Such restored river systems, however, must develop for years to decades to achieve a (dynamic) equilibrium, with an equally lengthy period before serious evaluation is possible. Thus, modelling approaches that accurately quantify the underlying processes and their manifold interactions are useful tools for river management. This paper presents a new conceptual approach for analyzing the interrelationship among plant succession, morphology, and hydrological impacts. Based on the dynamic disturbance regime approach, the model concept addresses interdisciplinary processes in river morphodynamics. The development of the concept is outlined, and the approach is applied considering three different process types: (1) metastable, (2) oscillation, and (3) acyclic. All three describe the relationship between vegetation succession/retrogression and the impact of disturbances. We show that addressing these three different process types helps the prediction of intermediate and long-term river system development by going beyond steady-state monitoring results to consider future dynamic developments. Moreover, classifying these process types and comparing them with reference (natural) conditions helps evaluation of the river system and subsequently definition of management strategies.

Evaluation of hydropeaking impacts on the food web in alpine streams based on modelling of fish- and macroinvertebrate habitats

Hydropeaking as a result of peak-load electricity production has been identified as one of the most significant pressures in alpine streams. Scouring of macroinvertebrates leads to downstream transport of aquatic organisms (catastrophic drift). Additionally, invertebrates are affected by periodic drying of wetted area during the dewatering of gravel bars and exposed areas along the banks. Even though fish are physiologically better adapted to switch to suitable habitats, artificial flow fluctuations may be followed by lethal stranding and quick alteration in habitat quantity and quality. Nevertheless, the interactions between pressures on fish and macroinvertebrates in terms of hydropeaking have not been investigated so far. The aim of this paper is to evaluate effects of flow fluctuations on potential epibenthic feeding grounds. Therefore, we evaluated changes in habitat distribution resulting from rapid flow fluctuations in river reaches with different river morphological characteristics, for five different macroinvertebrate taxa. Additionally, microhabitats for brown trout at two different life stages were calculated using representative peaking events (seasonal analysis) based on mid- to long term times series. Moreover, GIS-analysis allowed the evaluation of hydropeaking impacts (interaction) on both, macroinvertebrates and fish. In this study, it could be documented that feeding from the benthos for juvenile and subadult brown trout is inhibited during peak flow and is therefore reduced to times of base flow. Moreover, potential benthic feeding areas occurring at base flow have been found to increase with the level of morphological heterogeneity within analyzed river reaches. Likewise, hydrological sensitivity testing in terms of reducing ∆Q at different levels was performed and revealed that possible positive effects required heterogeneous river morphology as a precondition. However, this approach might be applied for estimating the impacts of hydrological mitigation measures in hydropeaked rivers concerning physical condition and/or growth rate of salmonids considering the river morphology of the investigated stream.

Habitat use and tolerance levels of macroinvertebrates concerning hydraulic stress in hydropeaking rivers – A case study at the Ziller River in Austria

Artificial flow fluctuations due to the operation of hydropower plants, frequently described as hydropeaking, result in a constant decrease of biomass of specific macrozoobenthos (MZB) taxa. For the presented case study, we assessed three reaches in the Ziller River catchment. At each sampling reach we performed the Multi-Habitat-Sampling (MHS) method with a Water Framework Directive (WFD) compliant AQEM/MHS net according to the Austrian guideline. Additionally, a hydraulic-specific sampling was conducted with a modified Box (Surber) sampler. As a basis for predictive habitat modelling of the MZB fauna, we measured abiotic parameters like mean (v40) and bottom-near (vbottom) flow rate or water depth respectively, for each box sample. In addition, the choriotope type, representing grain size classes, was determined. One of the main results is, that the national status assessment was not capable to reflect the impact of pulse release at the investigated river stretches on the basis of status classes. Moreover, we figured out that 1) habitats of stagnophilic macroinvertebrate taxa are minimized in channelized stretches affected by hydropeaking, leading to heavy quantitative losses for populations, becoming apparent in significant decreases in total individual numbers and biomass for many taxa. 2) The minor respond of the ecological status class in affected stretches by applying the WFD compliant national assessment method for macroinvertebrates owes to the tolerance of rheobiont or rheophilic taxa commonly classified as indicators for good conditions regarding saprobity or degradation score. 3) A development of a stressor-specific sampling design is required as the MHS method largely ignores vulnerable habitats. 4) The habitat suitability of selected species provides efficient expertise for impact assessment and mitigation measure design in terms of predictive habitat modelling.

Ecological degradation of a meandering river by local channelization effects: a case study in an Austrian lowland river

Anthropogenically induced siltation has serious effects on micro-habitat diversity and thus on aquatic organisms in lotic systems. The present study deals with the impact of siltation on the macroinvertebrate community in the River Lafnitz, south-eastern Austria. Our aim was to examine various ecological parameters in a meandering river stretch. The results significantly show (1) a generally lower taxa diversity, (2) a dominance of tolerant taxa such as Chironomidae and a clear reduction of sensitive taxa such as Ephemeroptera, Plecoptera, Trichoptera (EPT-taxa) and (3) overall lower abundances and biomass in sandy fractions. The present case study highlights various processes of fine sediment sources and sinks. While in most cases, fine sediment input is locally caused by erosion of adjacent terrestrial areas, even small-scale channelization of meandering rivers leads to subsequent trapping of huge sandy fractions especially in morphologically natural river sections. This inconspicuous but steady process is a risk to biodiversity and masks serious ecological degradation. Suitable management and restoration measures for anthropogenically silted rivers are therefore strongly recommendable. In summary, the study underlines the sensitivity of meandering systems and its dependence on catchment scale degradation and questions the reversibility of human impacts.

Assessing the impact of climate change on brown trout ( Salmo trutta fario ) recruitment

Climate change influences air temperature and precipitation, and as a direct consequence, the annual discharge pattern in rivers will change as climate warming continues. This has an impact on bedload transport and consequently on aquatic life, because coarse sediments in streams provide important habitat for many species. Salmonids, for example, spawn in gravel, and during their early life stages live in or on top of the substrate. We used a multiple model approach to assess how predicted discharge changes affect bedload transport and the vulnerable early life stages of brown trout (Salmo trutta fario) in a prealpine catchment in Switzerland. In the study area, future discharge scenarios predict an increased frequency of flood occurrence in winter and long-lasting low-flow periods in summer. As a result, bed erosion will become more frequent during winter, leading to less stable spawning grounds and deeper scouring, but during summer, an improvement in habitat diversity can be expected, which is advantageous for young-of-the-year fish. To face the future challenges of climate change, we recommend widening of riverbeds and improvements in longitudinal connectivity.