- 1 Remove sediments
- 1.1 General description
- 1.2 Applicability
- 1.3 Expected effect of measure on (including literature citations):
- 1.4 Temporal and spatial response
- 1.5 Pressures that can be addressed by this measure
- 1.6 Cost-efficiency
- 1.7 Case studies where this measure has been applied
- 1.8 Useful references
- 1.9 Other relevant information
Category 06. In-channel structure and substrate improvement
Fine sediments fill the interstitial spaces in gravel-bed rivers and affect spawning habitat for fish and interstitial habitat for invertebrates. Moreover, fine sediment deposits (e.g. in groyne fields or reservoirs) often are contaminated with pollutants or nutrients (Schwartz and Kozerski 2003). Contaminated sediments especially affect sediment dwelling (benthic) species and decrease reproduction or health of animals consuming benthic species. Therefore, from an ecological point of view, it is desirable that fine or contaminated sediments are removed from the river. This fact-sheet deals with “environmental dredging” (a special case of sediment management to remove contaminated sediment mainly for restoration of impacted aquatic ecosystems) (Salomons & Brils 2004), and not with other cases like dredging to ensure the navigability of waterways.
Removal or dredging of fine or polluted sediments is one out of several remedial options besides capping, in situ treatment, or monitored natural recovery. Suitability and applicability depend on site-specific characteristics (Reible et al. 2003, Bridges et al. 2006):
Dredging causes resuspension of some sediment. Contaminants can bio-accumulate during long-term sediment disturbance practices to levels exceeding regulatory ecological criteria (Su et al. 2002). Therefore, prior to consideration of extensive dredging as a remedial alternative for any river system, the potential significant and long-term impacts on the food web must be evaluated. Furthermore, it must be considered that maximum dissolved concentrations of the contaminants can occur well away from the point of dredging (Birdwell et al. 2007). Moreover, some residual contamination is always left, which can be minimized by overdredging (removing an additional layer of less contaminated underlying sediment). Finally, contaminated material must be transported to special disposal sites.
In situ capping of sediments has proven to be a useful alternative to removal via dredging. In some cases, even with a large share of the cap failing, the exposure and risk associated with capping is approximately equal to or below dredging (Reible et al. 2003). However, there is limited information on the long-term performance of caps and capping leaves the area unsuitable for colonization by biota.
In situ treatment technologies offer some promise, although they are presently in the early stages of technology development; however, it is unrealistic to expect such technologies to resolve all the technical and logistical challenges posed by contaminated sediments (Bridges et al. 2006).
Removing fine or contaminated sediments is a mitigation or “end-of-pipe” measure. From an ecological and economical point of view, it would be more useful to prevent the input of fines, nutrients, and pollutants (e.g. by developing riparian buffers strips) or to prevent sedimentation of fines (e.g. by modifying hydraulic structures like groynes). However, there are many existing sediment deposits, i.e. historical contamination, for which treatment and disposal are necessary as “end-of-pipe” solutions (Salomons & Brils 2004).
Expected effect of measure on (including literature citations):
There are several publications on the effect of fine sediments on spawning habitat conditions of fish and interstitial habitat for invertebrates (see fact sheet “Recreate gravel bars and riffles”), some on the effect of contaminated sediments (e.g. Clements et al. 2010), but virtually no information on the physico-chemical or biological effect of removal of fine or contaminated sediments. Therefore, the assessment of the expected effect of environmental dredging is mainly based on expert judgement.
HYMO (general and specified per HYMO element)
- Increase in clean gravel sediment.
- Enhancement of physico-chemical conditions if contaminated or organic sediment (decreasing oxygen depletion) is removed.
Biota (general and specified per Biological quality elements)
- Fast recovery of sensitive invertebrate species can be expected if source populations are present (in metal-polluted streams) (Clements et al. 2010).
- Especially non stream type specific sediment-dwelling invertebrate species will probably decrease after sediment removal.
- Especially juvenile fish will probably increase due to the availability of clean sediment deposits in shallow low-velocity areas.
- Fish rapidly re-colonize reaches after dredging of heavily contaminated sediment (Unruh et al. 2009).
- Probably favours stream type specific, less tolerant species.
- Potentially has a medium effect on phytoplankton since availability of nutrients and excessive, non stream type specific algal growth is decreased.
Temporal and spatial response
Pressures that can be addressed by this measure
- Sediment discharge from dredging
- Loss of vertical connectivity
- Sedimentation and sediment input
Low cost efficiency due to the high cost (for depositing contaminated sediments) compared to the low to medium ecological effect.
Case studies where this measure has been applied
- Anzur. Intervención de mejora ambiental de un tramo del Río Anzur
- Chícamo Life project. Conservation of Aphanius iberus´ genetics stocks ( Murcia ).
- Pisuerga. Improvement of ecological state of the river between the dam Pisuerga Aguilar de Campo and Alar del Rey (Palencia) 1st Stage.
- Spree - Restoration and remeandering of the Müggelspree - downstream Mönchwinkel
- Olivenza. Hydrological and Forestry Restoration of the Olivenza riverside.
- Vreugderijkerwaard - Side channel
- Odra. Actions for environmental restoration and flood control in the lower basin of the Odra River (Burgos)
- Narew river restoration project
- Tajo. Improvement of ecological state of the Tajo and tributaries riverside affected by the spill of kaolin, at Poveda de la Sierra and Taravilla (Guadalajara)
- Eggenstein-Leopoldshafen - Living Rhine floodplain near Karlsruhe (LIFE04 NAT/DE/000025
Birdwell, J. E., Thibodeaux, L.J. (2007) A kinetic model of short-term dissolved contaminant release during dredge-generated bed sediment resuspension. Environmental Engineering Science, 24, 1431-1442.
Bridges, T. S., Apitz, S. E., Evison, L., Keckler, K., Logan, M., Nadeau, S. & Wenning, R. J. (2006) Risk-based decision making to manage contaminated sediments. Integrated Environmental Assessment and Management, 2, 51-58.
Clements, W. H., Vieira, N. K. M. & Church, S. E. (2010) Quantifying restoration success and recovery in a metal-polluted stream: a 17-year assessment of physicochemical and biological responses. Journal of Applied Ecology, 47, 899-910.
Reible, D., Hayes, D., Lue-Hing, C., Patterson, J., Bhowmik, N., Johnson, M. & Teal, J. (2003) Comparison of the long-term risks of removal and in situ management of contaminated sediments in the Fox River. Soil and Sediment Contamination, 12, 325-344.
Salomons, W. & Brils, J. (2004) Contaminated sediments in European river basins. European Sediment Research Network SedNet, unpublished report, http://www.sednet.org/.
Schwartz, R. & Kozerski, H.-P. (2003) Entry and deposits of suspended particulate matter in groyne fields of the Middle Elbe and ist ecological relevance. Acta hydochim. hydrobiol., 31, 391-399.
Su, S. H., Pearlman, L. C., Rothrock, J. A., Iannuzzi, T. J., Finley, B. L. (2002) Potential long-term ecological impacts caused by disturbance of contaminated sediments: a case study. Environmental Management, 29, 234-249.
Unruh, D. M., Church, S. E., Nimick, D. A. & Fey, D. L. (2009) Metal contamination and post-remediation recovery in the Boulder River watershed, Jefferson County, Montana. Geochemistry-Exploration Environment Analysis, 9, 179-199.