REFORM - D4.2 Evaluation of hydromorphological restoration from existing data

Large river regulation and rehabilitation in Europe – six selected case studies

tom.buijse@deltares.nl — Fri, 04 Dec 2015 12:08:11 +0000

Large rivers have been selected as one of the satellite topics both within WP3 and WP4, because of their particular features which could not be analysed in the case study catchments framework. Large rivers are considered rivers with a catchment larger than 10,000 km² and > 100 m³/s. This encompasses rivers such as the Danube, Rhine, Rhône, Ebro, Vistula but also major tributaries such as the Sava, Narew, and Main rivers. Most fulfil major socio-economic functions, which will remain strongly modified and thus direct the options for rehabilitation. Because of their multifunctional use, large rivers can often only be partially rehabilitated or mitigated to achieve Good Ecological Potential according to the Water Framework Directive. This report addresses both hydrological modifications and restoration (rehabilitation, mitigation) following a DPSIR approach for six case studies that are spread across Europe

The historical trajectory of driving forces, river regulation (100 – 200 years) and rehabilitation (20 years) is used to underpin and illustrate the state-of–the-art regarding the effectiveness and potential of large river rehabilitation. For this, experiences and case studies from various large rivers in Europe are presented. For each case study the following information is given:

General characteristics of the river (stretch);
Description of historical state or reference condition(s) used in the rehabilitation project;
Functions of the river (stretch): for which socio-economic functions is the river used, and what are the resulting pressures for its ecological functioning?
The effects of identified pressures on hydromorphology and ecology;
Mitigation and rehabilitation measures; what measures have been taken or planned to improve the hydromorphological and ecological status of these rivers?
Ecological effects of measures.

The six case studies are representatieve of various European conditions with regard to climate, hydromorphological characteristics and catchment size. The case studies are situated in three biogeographical regions and six countries, viz. Atlantic region: River Trent (UK) and Delta Rhine (Netherlands), Continental region: Middle Vistula (Poland), Lower Danube and Po River (Italy) and Mediterranean region: Ebro (Spain). All these rivers can be characterized as large rivers (viz. catchment area larger than 10,000 km²), although they differed strongly in climatic zone, river length, catchment size, discharge, slope and river style. Large rivers can be considered as unique ecosystems and results are difficult to generalize. Still these case studies together give a good impression on the present regulation and rehabilitation of large rivers in Europe.

The case studies share but also differ substantially in drivers and associated pressures. Both flood protection and navigation are important drivers for the occurrence of many pressures. The rivers Trent, Po, Ebro and Delta Rhine have a large number of drivers and associated pressures, while the Danube Delta and middle Vistula are less impacted. For the majority no information was available regarding the extent of drivers and pressures.

There was a general pattern in the chronological sequence of the impact of drivers and associated pressures. The primal drivers for early regulation of all rivers were flood protection (embankments) and agriculture (deforestation). For most, these forms of river regulation started already centuries ago. Navigation became an important driver during the 19^th century requiring further channelisations. As a result, the occurrence of highly dynamic habitats strongly declined caused by stabilisation of the river bed (by groynes, bank protection) as well as by deepening of the main channel. Of our case studies, only the river Vistula in Poland is currently not regulated for navigation purposes, and – hence – large parts of the main channel of the river have not been channelised. More recently, especially after the Second World War, many dams were constructed in the rivers, which resulted in a decreased longitudinal connectivity, thereby impeding conditions for migratory fish and other species. Additionally, the hydrological regime of rivers was strongly altered and sediment supply to downstream sections was strongly reduced. Especially the rivers Trent, Po, Ebro and Lower Danube have been severly impacted by the construction of dams.

For the majority of the case studies, only limited information was available regarding the impacts of pressures on hydromorphology and ecology. Large rivers are impacted by multiple stressors which complicate to identify the primal causes for degradation. It seems that the sequence of drivers (and associated pressures, see above) have initiated major transition points for ecological processes and biota along large rivers. We discuss the effects briefly in respect to the time line of occurrence of these drivers and pressures.

There are some striking differences in the restoration measures taken. Along the lowland stretches of large rivers, such as the Lower Danube and the Delta Rhine, measures focus on restoring lateral connectivity gradients between main channel and floodplains. Because of constraints imposed by navigation, only a limited number of measures are taken that improve conditions for lateral migration to rejuvenate riparian zones and bar and island formation, because these will affect navigational depth in the main channel. Along the river Trent and Po (and to some extent, the Delta Rhine), measures are taken that increase variation in width and depth of the main channel, which variation is an important variable for the occurrence of several hydromorphological processes. Restoring conditions for island and shoal formation will only be carried out along the river Vistula where navigation is not an important driver.

In summary, along relatively intact river stretches, such as the Vistula and Danube delta only a limited amount of measures can already improve ecological conditions. In highly regulated rivers such as the river Trent and Delta Rhine having extensive and diverse pressures a large number of measures are required and have been taken or planned. By contrast, the Mediterranean Rivers Ebro and Po are also highly regulated, but along these rivers only a small number of measures are planned at present.

3.5_Satellite topic Large Rivers 09 Nov 2015-def.pdf

Work packages:

Deliverables:

Document type:

Report

File status:

Final

Policy Brief:

Evaluation of hydromorphological restoration from existing data

tom.buijse@deltares.nl — Fri, 15 Aug 2014 08:15:59 +0000

An increasing number of rivers have been restored over the past few decades but only a small number of these projects have been monitored, and hence, the knowledge on the effect of river restoration on biota is limited. Nevertheless, monitoring results of several projects are available in peer-reviewed scientific literature and have been compiled in recent research projects. The objective was to evaluate the effect of hydromorphological restoration on biota based on these existing data. Specific objectives were to quantify restoration success, to identify catchment, river reach, and project characteristics which influence (either constrain or enhance) the effect of restoration, and to derive recommendations for river management. The study is complemented by a satellite topic on urban river restoration.

Some narrative reviews have already been published, but a comprehensive quantitative meta-analysis which summarizes the findings of these existing studies was lacking. In the meta-analysis, quantitative research findings from a large number of studies were compiled. A meta-analysis is restricted to one single type of research finding and qualitative information cannot be used, but it is less subjective than narrative reviews and allows to investigate the effect of “moderator variables”, i.e. to identify variables which influence restoration success and hence, to identify effective measures and to describe favourable conditions for river restoration. The meta-analysis was complemented by a satellite topic on urban restoration to identify differences between the characteristics of urban and non-urban restoration projects.

Based on the results of the meta-analysis, the satellite topic, and other comprehensive reviews on river restoration already published in literature, the following conclusions were drawn. It is important to note that - as for all statistical analysis - it is not possible to infer causal relationships and hence, results should be interpreted with caution. Furthermore, for most results of this study, restoration success refers to an increase in the number of individuals and taxa simply because these metrics were reported in literature, but other metrics might be better suited to quantify success (e.g. stream-type specific conditions, functional approaches). It is strongly recommended to read the results and discussion sections before applying the results to avoid oversimplified interpretations.

In summary, it was possible to draw some first conclusion for river management from the evaluation of hydromorphological restoration based on existing monitoring data. However, monitoring data are still scarce and more robust, practical relevant, and quantitative results (e.g. thresholds) could be derived and river management would benefit from (i) original monitoring data, which would allow to use functional metrics to investigate the underlying processes and to infer causal relationships, (ii) full before-after-control-impact monitoring designs, which most probably would substantially decrease scatter in the datasets and analyses, (iii) a larger number of monitored projects, which easily could be accomplished since a large number of hydromorphological restoration measures will be implemented in the upcoming years, (iv) the availability of long-time monitoring data sets to investigate the effect of project age,which was identified as the most important variable affecting restoration success. A more intensive exchange and collaboration between river science and river management in planning monitoring programs is strongly recommended. This would offer a great opportunity to make fundamental advances in our understanding of how river restoration affects river hydromorphology and biota and to identify (cost)-effective restoration measures.

Overall, the effect of hydromorphological restoration on biota is positive but variability is high: Restoration in general has a positive effect on floodplain vegetation, ground beetles, macrophytes, fish, and invertebrates. Since variability is high, adaptive management approaches are recommended.

Restoration effect differs between organism groups:It cannot be expected that all organism groups benefit from restoration to the same extent. Results indicate that, in general, restoration effect on diversity is highest for terrestrial and semi-aquatic groups like floodplain vegetation and ground beetles, intermediate for macrophytes, lower for fish, and lowest for macroinvertebrates.

Restoration has a higher effect on the number of individuals than on the number of taxa:The effect of restoration is more pronounced on the number of fish and invertebrate individuals than on the number of taxa.

Restoration effect does only slightly differ between measures, i.e. there is no single “best” measure:There are no large differences in the overall effect of different measures but there is a tendency that terrestrial and semi-aquatic organism groups like floodplain vegetation and ground beetles as well as macrophytes benefit most from channel-planform measures and aquatic groups like fish and invertebrates from instream measures.

Urban restoration projects do not substantially differ in respect to the pressures occurring and the measures applied: Urban restoration projects were mainly applied in small rivers and length of the restored reaches was shorter compared to non-urban restoration projects. However, approaches towards (sub)urban and non-urban river rehabilitation practices were similar.

Urban restoration projects are rated less successful compared to non-urban projects by scientists and river managers: …which is probably due to the low absolute effect, which is rated as a failure, but which often is a high relative effect since urban river start from a low base, and hence, should not be assessed too negative.

Conditions which favour restoration success can be identified but restoration outcome cannot be predicted: Restoration success is especially high for the different organism groups under specific conditions (see section 2.2.5) but the variance explained by the models is too low and low sample size restricted the use of rigorous statistical tests to really predict the restoration outcome.

Overall, restoration success most strongly depends on project age, river width, and is affected by agricultural land use:Success is generally lower but restoration still has a positive effect in catchments dominated by agricultural land use. Since land use is a proxy for e.g. water quality, there is an urgent need to identify the underlying causal relationships.Project age is the most important predictor affecting restoration success, but the direction of the effect of project age on restoration success differs between organism groups (no simple increase of effect with time). There is an urgent need for long-time monitoring to investigate the restoration effect over time, to better understand the trajectories of change induced by restoration measures, and to identify sustainable measures which enhance biota in the long-term.

4.2 Evaluating HyMo restoration from existing data FINAL.pdf

Work packages:

Deliverables:

D4.2 Evaluation of hydromorphological restoration from existing data

Document type:

Report

File status:

Final