Reduce impact of dredging

Reduce impact of dredging

Reduce impact of dredging

06. In-channel structure and substrate improvement

General description

In-stream sediment mining (referred to as dredging in the following) directly alters the channel geometry and bed elevation at the mining site. Moreover, it causes a sediment deficit and typically induces upstream- and downstream-progressing river incision (Fig. 1).

This incision, in turn, induces higher average shear stresses on the bed under the same flows and, therefore, may aggravate the process of degradation (Martin-Vide et al. 2010). The resultant incision alters the frequency of floodplain inundation along the river courses, lowers valley- floor water tables and frequently leads to destruction of bridges and other infrastructure works. Moreover, mining causes lateral channel instability, bed armouring or even changes of the channel planform and type (from braiding to single-channel or from gravel-bed to bedrock river types).

As a consequence, mining results in the loss or impoverishment of aquatic and riparian habitats.

Fig. 1. Downstream sediment deficit and erosion, and upstream nick point migration caused by instream gravel-mining (modified from Kondolf 1997).

The type and magnitude of channel response to sediment mining depend mainly on the ratio between extraction and sediment replenishment rates. The effects of mining will be especially severe and difficult to reverse:

(i) where material is extracted at a rate greatly exceeding the replenishment rate;

(ii) in single-thread rivers, that are generally associated with relatively low rates of catchment sediment supply;

(iii) in channelized reaches;

(iv) where rivers are underlain by a thin cover of alluvium over bedrock; and

(v) where mining coincides with other human activities that reduce upstream sediment delivery (see review on sediment mining in Rinaldi et al. 2005).

Due to these detrimental effects and based on the above considerations, sediment mining must not be allowed in incised, incising or vertically stable rivers.

Applicability

In aggrading rivers, sediment mining can be considered at locations where it may have beneficial effects for flood-control purposes, channel stability and restoration, though an accurate geomorphic study on the following aspects is mandatory: trends of channel adjustments, sources of sediments in the basin and along the river, modes and times of sediment transfer through the fluvial system, and the effect of natural (e.g. lakes) or artificial features (e.g. dams, weirs) altering sediment fluxes through the system. Based on these information, it can be assessed if the river is aggrading and sediment production in the drainage basin is high and sediments are frequently delivered to the river (Rinaldi et al. 2005).

Field experiments indicate that infrequent mining of small amounts of gravel in near-natural rivers with high sediment supply probably do not cause significant adverse effects on fish and invertebrate communities on the reach scale, obviously since substrate composition and bar topography recover quickly and due to the high re-colonization potential (one-time removal of 69,000 m3 of gravel in a large river, 232,000 km2, i.e. about 30% of the annual sediment load, Rempel and Church 2009). However, there are no further empirical data to derive recommendations for the amount of gravel which could be dredged and frequency or timing of dredging without causing a significant ecological impact, especially in less natural rivers.

Expected effect of measure on (including literature citations):

There are several publications on the effect of dredging on physical habitat characteristics (see review in Rinaldi et al. 2005), only few studies on the biological effect (e.g. Brown et al. 1998), and virtually no information on the physical or biological effect of reducing dredging activities as a restoration measure in peer-reviewed literature (except Rempel and Church 2009). Therefore, the assessment of the expected effect of reducing dredging activities is mainly based on expert judgement.

HYMO (general and specified per HYMO element)

• Reducing dredging activities will probably decrease channel depth, increase riffle area, and re-connect the river to its floodplain due to the higher sediment load and deposition.

• Where rivers are underlain by a thin cover of alluvium over bedrock, channel incision is limited and the rivers erode laterally. In these rivers, gravel mining increases channel width, decreases stream competence, which results in filling of pools. As a result, the channel-bed becomes uniform and flat. Decreasing dredging activities will probably result in sedimentation along the banks and narrowing of the channel. However, such bedrock streams will not fully recover since bank and floodplain material has been eroded and riparian vegetation will markedly differ from the pre-disturbance state (Brown et al. 1998).

Physico-chemical parameters

• No information available.

Biota (general and specified per Biological quality elements)

BQE	Macroinvertebrates	Fish	Macrophytes	Phytoplankton
Effect	medium	medium	no effect	no effect

Macroinvertebrates:

• Infrequent mining of small amounts of gravel in rivers with high sediment supply probably do not cause significant adverse effects on invertebrate communities on the reach scale, obviously due to the high re-colonization potential in near-natural rivers and the small scale mining (see field experiment in Rempel and Church 2009).

• Increase in number and biomass of invertebrates, especially collector-filterer and scraper, which are most severely affected by gravel-mining (Brown et al. 1998).

Fish:

• Infrequent mining of small amounts of gravel in rivers with high sediment supply probably do not cause significant adverse effects on fish communities on the reach scale, obviously due to the high re-colonization potential in near-natural rivers and the small scale mining (see field experiment in Rempel and Church 2009).

• Increase in number of fish, which is lower especially at large scale mining sites (Brown et al. 1998).

Macrophytes:

• Probably no or minor effect on macrophytes

Phytoplankton:

• Probably no effect on phytoplankton

Pressures that can be addressed by this measure

• Sediment discharge from dredging
• Sand and gravel extraction

Cost-efficiency

Due to missing information on the economic value of gravel mining and the ecological effect, cost-efficiency can not be assessed.

Case studies where this measure has been applied

• Vääräjoki - Niskakoski
• Kuivajoki - Hirvaskoski

Useful references

Brown, A. V., Lyttle, M. M. & Brown, K. B. (1998) Impacts of gravel mining on gravel bed streams. Transactions of the American Fisheries Society, 127, 979-994.

Kondolf (1997) Hungry water: Effects of dams and gravel mining on river channels. Environmental Management, 21, 533-551.

Martin-Vide, J. P., Ferrer-Boix, C. & Ollero, A. (2010) Incision due to gravel mining : Modeling a case study from the Gallego River, Spain. Geomorphology, 117, 261-271.

Rempel, L. L. & Church, M. (2009) Physical and ecological response to disturbance by gravel mining in a large alluvial river. Canadian Journal of Fisheries and Aquatic Science, 66, 52-71.

Rinaldi, M., Wyzga, B. & Surian, N. (2005) Sediment mining in alluvial channels: physical effects and management perspectives. River Research and Applications, 21, 805-828.

Other relevant information

Instream mining is prohibited in several European countries (among others): United Kingdom, Germany, France, The Netherlands, and Switzerland. It has been reduced or prohibited in: Italy, Portugal. (Kondolf 1997).