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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.

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).