Remove or modify in-channel hydraulic structures

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Remove or modify in-channel hydraulic structures

Remove or modify in-channel hydraulic structures06. In-channel structure and substrate improvement

General description

Figure 1: Regular groynes and alternative V-shaped and notched groyne forms (modified from Kleinwächter et al. 2005).

In-channel hydraulic structures like groynes and deflectors are used in river engineering to stabilize banks and to concentrate the flow in the middle of the channel to achieve a sufficient fairway depth at medium and low discharges, resulting in a fixed river planform, a navigation channel that is relatively deep over a large part of its cross-section, and sedimentation between the groynes (groyne fields). In waterways, groyne fields are often the only shallow, low-velocity areas with some flow- and substrate diversity (although often impacted by ship-induced waves).

By removing or modifying in-channel hydraulic structures like groynes, flow- and substrate diversity can be further increased and aquatic and semi-terrestrial habitats can be improved. Different types of groynes have been used: V-shaped and notched groynes (see Figure 1) and L-shaped groynes.

Regular inclined groynes deflect the water away from the river bank when submerged. V-shaped groynes also deflect the water away from the river bank, but in addition direct the current to the middle of the downstream groyne field, resuspending fine material which has deposited during low-flow periods. In contrast to one large eddy in regular groyne fields, flow-diversity is higher during low- and mean-flow periods. Notched groynes result in a second flow profile near the river bank at high flows and a more diverse flow-pattern during low- and mean-flows (Kleinwächter et al. 2005). Another groyne type are L-shaped groynes, where an additional parallel structure is build at the head of the inclined groyne parallel to the flow to protect the groyne field from ship-induced waves.

Applicability

The fine sediment deposition in the groyne fields often are contaminated with pollutants or nutrients (Schwartz and Kozerski 2003). Removing or modifying groynes may remobilize these sediments and affect downstream river reaches. Therefore, from an ecological point of view, it is desirable that the organic mud should be removed from the river and deposited in a controlled landfill prior to removing or modifying groynes (see fact-sheet “Remove Sediments”).

Expected effect of measure on (including literature citations):

HYMO (general and specified per HYMO element)

  • Increase in flow- and sediment diversity (higher flow velocities, coarser sediment), especially in groyne fields of notched groynes (Kleinwächter et al. 2005, Rödiger et al. 2010).
  • Increase of low-velocity zones protected from ship-induced waves and increase in substrate diversity due to sedimentation of fines in groyne fields of L-shaped groynes (Gies and Lorenz 2009).


Physico-chemical parameters

  • No information available


Biota (general and specified per Biological quality elements)

BQE Macroinvertebrates Fish Macrophytes Phytoplankton
Effect medium medium medium no effect

Macroinvertebrates:

  • Increase in invertebrate abundance and diversity in groyne fields of L-shaped groynes (Gies and Lorenz 2009).
  • Increase of abundance of invertebrates inhabiting shallow low-velocity areas (Kleinwächter et al. 2005) and probably also invertebrates inhabiting stable sandy substrate.
  • Habitat quality for macroinvertebrates is especially enhanced by the notched groynes (Rödiger et al. 2010).


Fish:

  • Increase in fish diversity in modified groyne fields (Fladung et al. 2003).
  • Increase of high-flow velocity areas, leading to coarser grain sizes, potentially favour juveniles of rheophilic fish species (Kleinwächter et al. 2005).


Macrophytes:

  • Possibly influences macrophyte composition due to changes in flow-diversity.


Phytoplankton:

  • Probably no effect on phytoplankton.

Temporal and spatial response

Pressures that can be addressed by this measure

Cost-efficiency

High cost-efficiency if existing groynes are modified especially within the common maintenance work, medium cost-efficiency if new groynes are built (due to low to medium cost and medium ecological effect).

Case studies where this measure has been applied

Useful references

Bischoff, A. & Wolter, C. (2001) Groyne-heads as potential summer habitats for juvenile rheophilic fishes in the Lower Oder, Germany. Limnologica 31: 17-26.

Gies, M. & Lorenz, A. (2009) Revitalisierung eines Uferabschnitts am Rhein - Auswirkungen auf die Biozönose. Deutsche Gesellschaft für Limnologie, Proceedings 2008, 466-470.

Kleinwächter, M., Eggers, T. O., Henning, M., Anlauf, A., Hentschel, B. & Larink, O. (2005) Distribution patterns of terrestrial and aquatic invertebrates influenced by different groyne forms along the River Elbe (Germany). Archiv für Hydrobiologie Supplement 155/1-4, 319-338. Large Rivers Vol. 15, No. 1-4.

Rödiger, S., Schröder, U., Anlauf, A. & Kleinwächter, M. (2010) Ökologische Optimierung von Buhnen in der Elbe. Limnologie Aktuell, 14, in press.

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.

Sukhodolov, A., Uijttewaal, W. S. J. & Engehardt, C. (2002) On the correspondence between morphological and hydrodynamicsl patterns in groyne fields. Earth Surface Processes and Landforms, 27, 289-305.

Other relevant information

The number and shape of gyres developing in groyne fields mainly depends on the ratio between the groyne length (Lg) and the length of the groyne field (Lf). A two-gyre circulation pattern develops when aspect ratio is less than a critical value (Lg/Lf < 0.5), and a one-gyre circulation for the groyne fields with greater than critical aspect ratios (Lg/Lf > 0.5). This is of ecological interest since the sedimentation pattern in the groyne fields corresponds to the patterns of recirculating flow. The low velocity area in the centre of gyres promotes accumulation of relatively fine sediments. The thickness of the layer of deposited fines decreases toward the gyre margins (Sukhodolov et al. 2002).

Another modification of groynes concerns the material of which the groyne has been constructed. Especially in lowland rivers groynes were found to substitute former gravel bars to a certain degree and to provide spawning and nursing habitats for lithophilic fish (Bischoff & Wolter 2001). However, it is quite important to cover the groyne heads with differently sized gravel instead of concrete or similarly impermeable embankments.

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