3.2 Hydromorphological pressures

3.2 Hydromorphological pressures

For decades, humans have altered the shape of water bodies and the flow of river courses to farm the land, facilitate navigation, construct hydropower plants and protect settlements and agricultural land against flooding. For these purposes, rivers have been straightened, channelised and disconnected from their floodplains; land has been reclaimed, dams and weirs have been built, embankments have been reinforced, and groundwater levels have changed. These activities have resulted in altered habitats, changed flows, interruptions in river continuity, loss of floodplain connectivity and severe impacts on the status of the aquatic environment. These changes have caused damage to the morphology and hydrology of the water bodies, i.e. to their hydromorphology (EEA, 2018; EEA, 2019).

Hydromorphology plays a key role for aquatic ecosystems. For example, water flow and substrate provide physical habitat for plants and animals, such as fish and benthic invertebrates. Good hydromorphological functioning is an essential element of ecosystem health and underpins the delivery of many ecosystem services and benefits for society (EPA Catchments Unit, 2016; Houlden, 2018).[1] 

[1] https://www.catchments.ie/hydromorphology-what-is-it/ and http://www.hrwallingford.com/news/Hydromorphology-the_forgotten_facet_of_the_WFD

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In the second RBMPs, hydromorphological pressures are the most commonly occurring pressure on surface waters, affecting 34 % of all such water bodies (EEA, 2018). The most reported hydromorphological pressures are physical alterations related to flood protection, urbanisation, agricultural development and navigation as well as barriers including dams and weirs built for different purposes (hydropower, flood protection, irrigation, or navigation). In addition, several thousands of water bodies are affected by hydrological alterations driven by water abstractions (for public water supply, agriculture, or industry) and reservoirs used mainly for hydropower and irrigation. However, in the second RBMPs of most Member States, the identified hydro-morphological pressures are not clearly apportioned to specific drivers (EC, 2019).

Further, 16 % of European water bodies have been designated as heavily modified (13 %) or artificial (3 %) water bodies.

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Key hydromorphological pressures are described in the following sections of this report elaborating on the role of hydropower, navigation, flood protection and agricultural drainage as major drivers of impacts on hydromorphology in Europe. Also, separate sections address the role barriers to illustrate their very dense distribution and far-reaching impacts on the European river network and key issues related to the loss of lateral connectivity to floodplains.

Certain aspects of hydromorphological pressures and impacts are less well-known so far in terms of their extent and implications on European scale. One of these aspects is the issue of changed sediment dynamics due to hydromorphological pressures which is gaining more and more attention and will require targeted management interventions in the near future (see Box 2). In the meantime, the issue should remain in focus of further data collection and research to identify the main underlying processes, impacts on water bodies and appropriate management approaches.

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 Box 2                  Sediment quantity and hydromorphology

Sediment and sediment transport are essential and integral natural elements of the hydromorphology of rivers, lakes, estuarine and coastal systems. Sediment is also vital to the ecology of these systems, providing and supporting habitats as well as nutrients for aquatic plants, invertebrates, fish, and other organisms. Although the WFD does not explicitly take account of sediment, ecological status is clearly dependent on habitat (including sediment quantity) and clearer understanding is needed on the role of sediments in the WFD and related legislation such as the Floods Directive and Marine Strategy Framework Directive.

The management of most European rivers by humans has resulted in substantial modifications to natural sediment transport processes, sometimes with dramatic consequences for the stability of rivers and coastlines (SedNet, 2014). Dams act as a barrier in the hydrological system as they interrupt the continuity of sediment transport through rivers systems. Sediments trapped in reservoirs cause a deficit of sediments downstream reservoirs leading to erosion, morphological and ecological consequences in the downstream rivers (Kondolf et al, 2014). Also, the dredging of sediment, which is necessary to maintain and develop ports or navigable waterways, can increase tidal floods and damage ecology by directly affecting physical habitats, disrupting riverine processes and reducing connectivity with the floodplain (England & Burgess-Gamble, 2013).

The relevance of sediments for achieving fundamental management goals in river basins is obvious. However, the perceived complexity often hinders the full integration of sediment issues into river basin management (SedNet, 2017). The WFD takes a river basin scale approach to water management which is well aligned with the need to manage sediments at this scale, through the development of sediment management plans, rather than locally as has been the case traditionally. To date, most European countries though do not have sediment management plans in place (Dworak and Kampa, 2019).

Some major European river basin commissions have taken up the challenge to work towards transboundary sediment management plans as part of river basin management planning, such as the Rhine and Danube commissions (Brils, 2008). Also, in the Elbe, a comprehensive sediment management concept has been developed in support of management planning in a large international river basin, serving as an inspiring example on how to integrate sediment in river basin management (SedNet, 2017). A transboundary dimension to sediment management plans beyond national borders is important so that national plans are coordinated and have similar levels of ambition within transboundary catchments.

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The WFD explicitly requires Member States to manage the effects on the ecological status of water which result from changes to physical characteristics of water bodies. It requires action in those cases where hydro-morphological modifications are having an impact on the ecological status interfering with the ability to achieve the WFD objectives and to avoid deterioration due to new modifications. The restoration of hydromorphological conditions can take place using a wide range of measures such as removing river obstacles to restore river continuity, setting ecological flow requirements, improving physical habitats in rivers and on their floodplains or implementing natural water retention measures.

At the same time, WFD measures for hydromorphological pressures should not be taken in a silo approach but it is beneficial, both in terms of the effects to be achieved and funding opportunities, to coordinate the planning of WFD measures with the planning process for other sectors (e.g. planning for the energy, transport and agricultural sectors) (EC, 2019).

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