European Geologist Journal 45

Riverbed aggregates dredging

by Tamás Hámor1, Gábor Kovács2

1 Directorate General Joint Research Centre, European Commission

Hungarian Government Veszprém County Office Mining Department



Dredging of aggregates, mainly sand and gravel, is a common practice in rivers and coastal marine settings, and less frequently also in lakes. The driver for this activity is usually two-fold: the extraction of raw materials for construction and the maintenance of waterways to ensure safe shipping, avoid bank erosion and prevent local flood risks. The major criticism usually made is that dredging disturbs aquatic ecosystems, destroys natural filter layers of potable water reserves, and may emit polluting substances. As a “flow-type” mineral commodity, the permitting and legislation issues can be complicated. In addition, sophisticated and high-resolution sediment re-charge supply models that would allow precise forecasts on the sustainability of this activity are scarce on both a local and river basin scale. This article addresses the above aspects of river dredging by providing a short review of the literature and legislation, and an assessment of its sustainability, using dredging in Hungary as an example of activity at the Member State level.


Aggregates are granular materials used in construction. Primary aggregates are obtained by extraction from natural sources and include sand, gravel and crushed rock (Blengini et al., 2012). Secondary aggregates are by-products from other industrial processes, like blast or electric furnace slags or china clay residues, mineral processing residues, or recycled aggregates derived from construction and demolition wastes. Aggregates are extracted from quarries, pits, and marine and fluvial dredging. In locations on land with a high groundwater table, aggregate extraction technology can also be achieved using vessel-based dredging, resulting in the formation of artificial lakes. An insignificant amount of aggregates may also originate from the removal and reworking of overburden sediments and rocks targeted at the extraction of other mineral commodities.

The aggregates sector is the largest amongst the non-energy extractive industries in Europe. The European aggregates demand is 2.7 billion t/year, 5 t/capita, representing a turnover of more than €15 billion. Eighty seven per cent of all aggregates produced are from quarries and pits. The aggregates industry comprises 15,000 companies (mostly SMEs) with 200,000 employees, operating in 26,000 sites (UEPG, 2018).

The share among the different aggregates is: 47 % crushed rock, 41 % sand and gravel, 8 % recycled aggregates from construction and demolition waste, 2 % from marine dredging, and 2 % manufactured aggregates (from slag and ash). Although no data are available on aggregates derived from fluvial dredging, this source is estimated to represent less than 0.1 % of the overall output.

The correlation between economic performance (GDP) and the demand for aggregates is well documented, and attempts to decouple this, i.e. via resource efficiency and recycling, show no obvious results yet. In this respect, it is surprising that the aggregates production of the EU-EFTA countries shows a limited and delayed effect of the 2008 crisis, and slow recovery after 2013 (Figure 1).

Figure 1Aggregates production in the European Union and EFTA countries (in billions of tonnes) (UEPG, 2018). 

Dredging is the removal of sediments from the bottom of water bodies (rivers, lakes, coastal marine areas). It is a routine necessity in waterways to offset the effects of silting – the natural sedimentation of sand, silt, clay and gravel.  Environmental dredging is also applied for removing polluted sediments or non-polluted clay fractions that inhibit oxygenation and the transparency of water (Manap & Voulvoulis, 2016). Extraction involves a variety of technologies from cutter suction to bucket dredging, each with different efficiencies and impacts.

EU Community legislation

An analysis of the European Union Community legislation was presented in detail for the extractive industry in general (Hámor, 2004) and more specifically on aggregates sector (Hámor et al., 2011). Recently, the Raw Materials Information System of the European Commission, developed by the Joint Research Centre, provided an updated review of the acquis communautaire (the accumulated legislation, legal acts, and court decisions which constitute the body of European Union law)[1] (Manfredi et al., 2017). Based on a simple text search for dredging, the following provisions are the most relevant and specific in the acquis.

Dredging, as an economic activity, is listed in both the Directive on concession contracts (2014/23/EU) and the Directive on public procurement (2014/24/EU) with NACE code F 45.24 among “Construction of water projects”. Directive 2017/2397/EU on recognition of professional qualifications in inland navigation considers dredging and its manoeuvres as a navigational operation, and acknowledges that floating equipment means a floating installation carrying working gear such as dredging equipment. In addition, Directive 2006/87/EC on laying down technical requirements for inland waterway vessels sets detailed provisions, e.g. on the stability of dredging vessels.

Regulation 2017/352/EU establishing a framework for the provision of port services and common rules on the financial transparency of ports provides a definition on dredging in ports: “the removal of sand, sediment or other substances from the bottom of the waterway access to the port, or within the port area that falls within the competence of the managing body of the port, including the disposal of the removed materials, in order to allow waterborne vessels to have access to the port; it comprises both the initial removal (capital dredging) and the maintenance dredging carried out in order to keep the waterway accessible, whilst not being a port service offered to the user”.

The Environmental Impact Assessment Directive (2011/92/EU) classifies “extraction of minerals by marine and fluvial dredging” in Annex II 2. (b) as an activity for which Member States shall determine whether the project shall be made subject to an assessment. Member States shall make that determination through a case-by-case examination or thresholds (or criteria), or both procedures.

The most relevant part of the acquis is in relation to the Water Framework Directive (2000/60/EC), which sets a number of environmental objectives both for surface waters and groundwaters. While normally obliged to prevent inputs of pollutants, Member States can grant exemptions when, for instance, pollutants occur when intervening in surface waters to mitigate the effects of floods and droughts or for managing waters and waterways. These activities, which can include cutting, dredging, relocation and deposition of sediments in surface water, must be conducted in accordance with the regulations of the Member State. The exemptions may be used where the competent authorities have established an efficient monitoring system.

The Marine Strategy Framework Directive (2008/56/EC) and Commission Decision 2010/477/EU on criteria and methodological standards on good environmental status of marine waters are worth mentioning in the context of marine dredging. The EU Natura 2000 network is based on the Habitats Directive (92/43/EEC) and the Birds Directive (2009/147/EC). Local protected sites may represent a conflict to both marine and fluvial dredging.

Dredging generates hardly any waste and therefore the waste acquis is less relevant; nevertheless, the Extractive Waste Directive (2006/21/EC) and the European Waste Catalogue Chapter on Mining (Comm. Dec. 2001/118) are deemed applicable. This interpretation is backed by the fact that “spreading of … sludges from dredging”, and “deposit of non-hazardous dredging sludges alongside small waterways from where they have been dredged out and of non-hazardous sludges in surface water including bed and subsoil” are excluded from the scope of the Landfill Directive (1999/31/EC).

Case law with regard to dredging, under the relevant jurisdictions of the European Court of Justice, mostly deals with state aid issues and concerns conflicts with the Natura 2000 sites (e.g. Cases C-226/08, C-461/13) and with interpretation of the Environmental Impact Assessment Directive.

Community policies and funded R&I projects

Since the Raw Materials Initiative was published in 2008[2], the safe supply of raw materials has been among the political priorities of the EU. Its second pillar on safeguarding domestic resources and supporting enhanced extraction is relevant to aggregates. However, fluvial dredging does not receive any specific attention.

Assessments of critical raw materials for the EU economy[3] included aggregates in the scope of the assessment. Although the calculated supply risk exceeded the threshold for criticality, the economic importance was below the threshold, and therefore aggregates were not categorised as critical raw materials.

In many locations it is a challenge to ensure compatibility of dredging with Natura 2000 and the corresponding environmental management. Across Member States there is a difference in interpretation of directives that relate to Natura 2000. This is reported to thwart the EU single market, as reflected in numerous European Court of Justice judgments. In 2011 the European Commission published a Guidance Document on non-energy mineral extraction and Natura 2000[4], where marine dredging is presented in detail. The lack of references on riverbed dredging either indicate the minor weight of this activity in the supply mix or the insignificant number of cases conflicting with nature conservation. River dredging is only mentioned in one case, the Cliffe Pools case (UK) where sand and gravel dredged from a nearby riverbed were used to backfill former clay pit impoundments later flooded by saline water.

Dredging is almost completely missing as a theme in any of the EU FP7 and H2020 calls and projects, as determined from a review of the list of raw materials related research projects recently published by the ORAMA consortium[5]. The ERDF Interreg project “Danube Sediment”[6] represents one of the very few exceptions. It calls for the sustainable sediment balance management of the river, also with a view to dredging (Maier & Skiba, 2017). The EC Joint Research Centre recently published a study also dealing with these issues (Vigiak et al., 2017).

Aggregates and dredging in Hungary

Hungary is used here as an example of a land-locked Member State where river dredging is a minor but existing aggregate production activity.

The framework conditions

In compliance with the provisions of the Water Framework Directive, the River Basin Management Plan of Hungary was published and approved by the European Commission in 2012[7]. Hungary is a landlocked country which covers a significant part of the downstream section of the Danube River Basin. The Plan has very few references to dredging: It presents dredging as an environmental pressure due to the physical modification on river and lake water bodies that are reduced or modified because of the hydromorphological impacts of dredging. This statement is based on a detailed survey on hydromorphological alterations of all surface waters between 2006 and 2008. In engineering activities, including dredging, it is unclear what tools were applied to define their level of significance, as only the number of water bodies affected was provided.

The national legislation in Hungary on dredging in relation to environmental management and water quality is similar to the Community law (European Commission, 2017).  The thematic scope of the national legislation covers rules on fiscal instruments, pollution thresholds, and technical details on water works.

The national Mining Act is also applicable for dredging (Szabados & Hámor, 2010). Where the primary objective of dredging is aggregate extraction, an exploration permit, a mineral reserves report, the establishment of a mining plot and an extraction technical operation plan are the major requirements. This permitting process may take 2-3 years. When aggregates dredging is a collateral activity to water works primarily directed at improving navigation ways, flood prevention, etc., the permit is issued under the scope of the Water Act[8].

Both primary mining by dredging and collateral production as part of water works activity are incorporated into the sphere of authority of the so-called County Government Offices, which are typical one-stop-shop permitting entities. Either ways, the dredging company must pay a mining royalty based on the volume of aggregate extracted; this feeds into the central state budget. Therefore, local municipalities, which also have a role in permitting as an invited co-authority, have no particular interest in supporting this activity.

Production, environmental pressures

Hungary covers the major, central part of the Pannonian Basin (the Carpathian Basin in geographical terms). As a depositional center of sediments, it has significant resources and reserves of fossil (Quarternary and Holocene) river terrace gravel and sand. The location of these reserves are along the current and paleo riverbed lines of the Danube, Tisza, Dráva, Maros, Hernád and Sajó rivers, and along the forelands of the Mátra and Bükk mountains in the Northeast.

The production of the Hungarian mining industry was more than 52.9 million tonnes in 2011, of which 75% was non-metalliferous mineral raw material. The growing significance of non-metalliferous mineral raw materials is reflected by the fact that their proportion grew (84% in 2014). According to the national inventory (MBFSZ 2018) as of 2017, there were 513 registered aggregates mining plots covering ca. 218 km2, representing 0.2% of the territory of Hungary. Over time, aggregates extraction in Hungary (Figure 2) does not match the EU curve. For example, the effect of the 2008 crisis was delayed by one year, and the 2014 positive peak was most likely driven by the infrastructure developments financed by the EU funds. Companies are active in exploration, too; there are 154 licensed exploration plots for aggregates, covering 308 km2.

Figure 2: Extraction of aggregates in Hungary, 1999-2016 (in million m3) (MBFSZ, 2018).

Many of the side effects of aggregates extraction in open pits are present on the Csepel Plain (Figure 3), located south of Budapest, which provides the majority of supply demanded by construction in the capital city. Situated on the Danube terrace, most of the extraction is performed below the groundwater level via dredging; as a result, these sites are principally lakes with dredging technologies. The evaporation of open water has led to a remarkable fall in groundwater levels in the wider region. The change in land use is also significant: the total area of the involved settlements is 615 km2, of which 45 km2 is covered by the mining plots, representing 7 % of the area. It is unlikely that many former pits can be returned to previous types of land use. Conflict with locally protected Natura 2000 sites does occur.

Figure 3: Current and past aggregates extraction on the Csepel Plain, 10 km south of Budapest.

Riverbed dredging

Riverbed dredging has long traditions in Hungary, being in use since the mid-19th century. At that time gravel production originated mostly from the Danube and the gravel quarry-lakes. The heydays of industrial dredging were in the 1970-80s, when the production was in the scale of millions of cubic metres annually. During the last 20 years dredging output dropped significantly due to several factors, including a more stringent legal regime, expanding nature conservation, dams built upstream, the transition to a market economy and the enhanced competition from other supply sources. At present, sand and gravel extraction by dredging is on the order of a few thousand m3 annually (Figure 4). The ratio of the extracted bulk volumes with the water authority permit (shown in blue in Figure 4) versus gravel sold and delivered indicates that bi-product production arising from waterworks activity is far greater than primary production. This raises the question of how the missing material volume is managed in an environmentally responsible manner.

Figure 4: Aggregates production water rights and sand and gravel volumes of riverbed dredging in Hungary.

Concerning technology, some dredging sites are more reminiscent of artisanal and small-scale mining (ASM) sites outside Europe than of an industrial extraction site (Figure 5). Good practice sand dredging is being carried out on the Maros (Mureș) River, where 4–5 dredging river sections operate 5 to 10 km apart. As a result of this dredging, high quality plaster sand is obtained. Despite dredging for many decades along the same stretch of the river, the water authority has not reported any negative impact on the river bed or aquatic wildlife.


Figure 5: Dredging technology on the Maros River.

Sustainability issues

Dredging has numerous advantages and positive environmental effects, such as (a) the extraction of raw materials for construction, (b) maintenance and order of waterways to ensure safe shipping and to avoid river bank erosion, (c) preventing and mitigating local flood risks, (d) removal of polluted anthropogenic sediments, and (e) improving water quality and living conditions for benthic fauna by removing fine sediments to help oxygenation and avoid eutrophication. Wet extracted aggregate material has a higher quality; it is already washed, thus has better granulometric grading. The most frequently cited criticism of dredging is that it may disturb aquatic ecosystems. However, as an example to the contrary, very rapid recovery of the fauna was observed after the Baia Mare cyanide spill (BMTF Report 2000). Dredging may also destroy natural filtering layers for potable water reserves, and may emit polluting substances (engine fuels, lubricants, exhaust gases). These impacts need to be assessed and evaluated in a balanced way against the advantages of dredging at both the local and regional scale in a river basin.

Furthermore, if comparing extraction and dredging based on their effects on the environment (Table 1) river dredging is advantageous. There is no negligible difference in the fact that the extracted mineral raw material in the riverbeds re-charge from time to time, which does not occur in quarries or pits.

Table 1: Environmental impact matrix of the mining activities (- Low damage, – – Moderate damage, – – – High damage, 0 Neutral impact,  + Positive impact). 


Responsible fluvial dredging in landlocked continental settings may contribute to a more sustainable mix of aggregate supply, better ecological status of water ecosystems and water quality, and maintaining waterways. The proposal of minimum conditions as a list of baseline measures for responsible dredging are: (a) good governance in a broad sense that involves (aa)  clear national legislation, (ab)  streamlined permitting, (ac) favourable financial framework, and (ad) a better co-operation of the competent authorities; (b) smart spatial development and land use planning at regional and local scale which is supported by (c) modelling the sedimentary balance of rivers in the river basin management plans that can also feed in to (d) complex sustainability assessments in addition to conventional environmental impact statements. It is understood that dredging cannot replace aggregate quarrying but it can be a good example that is sustainable and contributes to developing circular economy. Systematic data collection and river basin scale assessment models are needed for better established conclusions which may aid decision-makers engaged in considering sustainability issues in this specific sub-sector of the extractive industry.


The authors thank the Mining and Geological Survey of Hungary for their data service, Gabriella Jelinek for her expert support, Károly Fiala (Lower Tisza Water Directorate) for providing dredging data on the Maros River, and Oliver Bonham and Robin Nagano for their careful review and constructive suggestions.










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This article has been published in European Geologist Journal 45 – Environmentally sustainable mining in Europe

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