Table of contents
- General comments
- 0. Guidance to the reader
- 1. Introduction
- 2. Heavily modified and artificial waters
- 3. Ecological status and potential
- 4. Hydromorphological pressures and impacts
- 5. Barriers – transversal structures
- 6. Abstraction and flow regulation and water level regulation
- 7. Morphological alterations
- 8. Hydromorphological issues in transitional and coastal waters
- 9. Sector and activity chapters
- 10.Hydropower and WFD
- 11. Navigation and WFD
- 12.Towards sustainable flood risk management
5. Barriers – transversal structures
5. Barriers – transversal structures
5.1. Reservoirs, dams, weirs
A river that has been left in its natural state is generally freely passable to migrating aquatic organisms in an upstream and downstream direction. Structures such as dams, weirs, barrages and other in-stream structures alter the natural hydromorphological form of a channel, with impoundment of water and reduced velocity profile on the upstream side and may act as a sediment trap. Since dams interrupt the natural continuity of rivers and reservoirs and change the hydrological cycle, their ecological consequences can be manifold. For example, access to spawning sites for migratory fish may be prevented. This is a particular problem for fish such as salmon, trout, eel and sturgeon, but also invertebrate species may be affected. However, even small dams cause problems, as they are impassable to most species of fish. Bypass rivers, fish ladders or fish passes at such structures may maintain or improve the continuity. Dams and weirs have an effect on the natural transport of sediment, resulting in its retention upstream of dams and loss downstream, so that material may have to be imported to stabilise the river bed and prevent incision.
The WFD anticipates that restoration of “high” and “good” water quality would include ensuring that the continuity of the river is not disturbed by anthropogenic activities and that undisturbed migration of aquatic organisms and sediment transport can occur.
Figure 5.1: Examples of barriers and transverse structures in European rivers.
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Photos not included |
Photos not included |
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5.2. European overview
Comments: The data reported on significant pressures and stored in WISE-WFD database are not fully suited for illustrating water bodies having barriers and transversal structures as a significant pressure. In many of the individual RBMPs there are sections and maps and graphs describing water bodies being affected by transversal structures. It is an aim to provide an overview of this information in selected of river basin districts, see the examples on barriers in the table below.
There are several hundred thousands of barriersand transverse structures in European rivers. Some of them are large dams for hydropower production or irrigation storage reservoirs, but the majority are smaller. In Germany, for example, it has been estimated that there are some 200 000 transverse structures. In relation to the overall length of Germany’s network of rivers of around 400,000 km, therefore, the continuity of the rivers is interrupted every second kilometre by a technical structure. Several of the RBMPs have identified a large number of barriers in the river basin district. Figure 5.2 illustrates the large number of barriers the Thames RBS and the Seine-Normandy RBD. For both river basin districts large part of the river system is affected by barriers.
During the last two centuries there has been a marked increase in both size and number of large dams and the storage capacity reservoirs, especially with the development of hydropower and irrigation reservoirs. There are currently about 7000 large dams in Europe. In addition, there are thousands of smaller dams.
Table 5.1: Barriers in river basins
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1 688 barriers are located in the large Danube rivers with catchment areas greater than 4 000 km2. 600 of the 1 688 continuity interruptions are dams/weirs, 729 are ramps/sills and 359 are classed as other types of interruptions. 756 are currently indicated to be equipped with functional fish migration aids. Therefore, 932 continuity interruptions (55%) remain a hindrance for fish. Source: International River Basin Management Plan of the Danube |
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There are currently thought to be some 200 000 transverse structures in Germany. In relation to the overall length of Germany’s network of watercourses of around 400,000 km, therefore, the continuity of the rivers is interrupted every second kilometre by a technical structure. Source: UBA 2010: Water resource management in Germany, part 2 Water quality (p. 49) The German Elbe RBD notes that 91% of river length is failing GES due to hydromorphological pressures. 276 transversal structures out of 11.000, such as dams and weirs, are found to significantly disrupt fish migration in rivers that were identified as basin-wide priority for fish migration. Source: EEB 2010: 10 years of the Water Framework Directive: A Toothless Tiger? http://www.eeb.org/?LinkServID=B1E256EB-DBC1-AA1C-DBA46F91C9118E7D&showMeta=0 |
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The Loire-Bretagne RBMP (SDAGE 2009) identifies over 10,000 infrastructures which reduce longitudinal river continuity and have negative impacts on the ecological status. Around 90% of this infrastructure is obsolete. 1430 infrastructures are listed for priority action. Source: EEB 2010: 10 years of the Water Framework Directive: A Toothless Tiger? http://www.eeb.org/?LinkServID=B1E256EB-DBC1-AA1C-DBA46F91C9118E7D&showMeta=0 |
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There are over 2,500 weirs and impoundments, and 5,000 culverts on Scottish rivers. Source: SEPA, Significant Water Management Issues (SWMI) www |
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The Czech article 5 report identified around 6000 barriers above 1 m with 2153 in the Danube RBD; 2805 in the Elbe RBD and 1065 in the Odra RBD. Source WFD Article 5 Characterization chapter http://heis.vuv.cz/_english/data/spusteni/projektydat/vodniutvary/dokumenty/cz/Part_2.pdf |
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In the Slovak RBMP 779 barriers inpassable to fish have have been identified, only 84 of these have functional fish passes. Source: SK RBMP p. 45 http://www.vuvh.sk/rsv2/download/VPS/VPS.pdf |
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The Dutch Rhine RBMP identified over 9 000 dams, including over 700 in flowing waters and the Duthch Meuse RBMP identified more than 2 000 dams, half of them in flowing waters. The weirs and dams are needed for flood protection and for regulation water levels to different functions (such as urban, agriculture and nature). Only a small part is made passable for fish. Source Rhine RBMP and Meuse RBMP (5.1.5 Regulation of water movement and morphological changes). |
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In Belgium 779 barriers have been identified on a 3000 km long priority network of rivers (Rivers in contact with the sea and those considered to have greatest value and strategic importance). In addition many barriers are found on other rivers. Source : Biodiversity Indicators, 2011. Fish migration barriers. |
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In Sweden 3875 out of 15 598 river water bodies (25 %) and 1372 of 7252 lake water bodies (19 %) are affected by continuity interruptions. In Northern Baltic Sea RBD, for example, there are identified 945 dams, 2825 other barriers and nearly 5000 road crossings that may act as barriers. Source: Swedish RBMPs |
5.3. Case studies
We hope that Member States and relevant stakeholders will comment on the current case studies and contribute with updates and new case studies. The current case studies are partly copy and paste from RBMPs or other relevant documents
5.3.1. River fragmentation, Loire River, France and recovery of fish species in the River Rhine
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River fragmentation in the Loire River, France by obstacles in 1700 and 2005, affecting adult salmon migrating upstream.
Source: EEA, 2007 |
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Almost all fish species have returned to the Rhine, but access to habitats for salmon should be improved. Measures targeted at improving water quality in the Rhine have enabled many fish species to return to the river. However, some specific measures are required to enable salmon to really re-colonize the Rhine basin, in particular to improve access from the sea past the sluices of the Haringvliet, Netherlands. The target year for this improvement and for improved access to the salmon habitats in the tributaries of the Upper Rhine is 2015. (ICPR, 2009) |
5.3.2. Ireland barriers
Source: Gargan et al. 2011
In Ireland, regulation of rivers via dams and weirs took place extensively in the 18th and 19th centuries. These structures generally consisted of a full-width barrier across the channel with a “mill race” leading water to a mill wheel installation and a tail race returning the water downstream to the channel. In other cases, a series of lock gates were involved in facilitating navigation. For mills, the barrier height was designed to obtain maximum value from the available stream power, trapping an optimal length of backwatered channel without forming an excessively high weir.
In Ireland, the Water Framework Directive (WFD) Freshwater Morphology Programme of Measures and Standards (POMS) identified barriers to fish migration as one of the principal issues placing channels “at risk” in terms of failing to achieve good hydromorphology status.
5.3.3. Sediment retention in reservoirs
Source: UBA 2010: Water resource management in Germany, part 2 Water quality
Hydrological changes, interruptions to the river continuum and intervention into the watercourse structure all disrupt the sediment regime. Large dams and weirs have an effect on the natural sediment transportation, resulting in the retention of sediment upstream of dams and the loss of sediment downstream of dams. The river is only able to compensate for this deficit of sediments by gathering material from the bottom, causing it to “dig into” the landscape more extensively along certain sections. As a result of such intervention, for example, the Rhine has become up to 7 m deeper, the Isar up to 8 m, and the Elbe up to 1.7 m deeper. This trend towards further deepening is continuing. It can be assumed that the majority of rivers in Germany exhibit an unnaturally high level of depth erosion.
Source WWF based on Schwarz U. 2007: Assessment of the balance and management of sediments of the
Text box: Reduced sediment transport and Ebro delta
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The delta of the Ebro River is a site of high economic and environmental importance. Almost 50,000 people live on the delta, and several economic activities (fisheries, aquiculture, agriculture (rice farms) and tourism) are associated with the ecosystems of the delta. Existing dams in the Ebro River currently trap approximately 95% of the suspended sediment load as compared to measurements from the beginning of the 1900s. In the last forty years, low flows past the town of Tortosa, 40 km upstream of the mouth of the river, have decreased approximately 40 %. The decrease in river discharge at its mouth also leads to salt water intrusion within the river system and because the sedimentation rate has been reduced from 3-15 mm/year to 0.1-4 mm/year, the lack of accretion is leading to coastal retreat and land subsidence. A situation that may be aggrevated with increasing sea level rise due to climate change. |
5.4.1. Case study: Measures to remove barriers in the Danube River Basin district
The current diagram and table illustrate how measures related to remove barriers will be implemented in the RBMP planning period up to 2015 in the international Danube RBD.
5.4.2. Case study: Indicator: Defragmentation of rivers
Source: Biodiversity Indicators, 2011.
The text box below illustrate progress in removing fish migration barriers along a 3000 km long priority network of rivers in Belgium.
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This indicator shows the number of fish migration barriers that must be removed on a 3000 km long priority network of rivers in Belgium. Analysis and Policy goal In 1996, Belgium, The Netherlands and Luxemburg agreed to take all measures necessary to allow free fish migration through all hydrographic basins by 2010. A postponement to 2015 (90%) and 2021 (100%) is under preparation, synchronising the target with the European Water Framework Directive. Trends and distance to target By the end of 2009, 171 of the 789 fish migration barriers (22%) had been removed. Others are being processed at different administrative levels. Nevertheless, progress is still far behind schedule Expectations At the current speed of the mitigation of river fragmentation (fish migration barriers) the target will not be reached. With the current policy only 20% of the fish barriers will be solved till 2010. |
5.4.3. Fish conservation plans
Plans for getting fish species (e.g. Salmon (Rhine, Thames, Meuse etc); sturgeon (Danube); eel (French rivers) and lampreeys) back into the river systems to be included here.
5.4.4. References
Biodiversity Indicators, 2011. Fish migration barriers: Defragmentation of rivers . Research Institute for Nature and Forest, Brussels. www.natuurindicatoren.be (updated 05-09-2011). http://www.natuurindicatoren.be/indicatorenportal.cgi?detail=567&id_structuur=54&id_categorie=&lang=en&jump=yes
Gargan et al. P. G,2011: “Comparison of field‐ and GIS‐based assessments of barriers to Atlantic salmon migration: a case study in the Nore Catchment, Republic of Ireland,” Journal of Applied Ichthyology 27 (December 1, 2011): 66-72.
ICPR, 2009
UBA 2010: Water resource management in Germany, part 2 Water quality