Difference between revisions of "Remove or modify in-channel hydraulic structures"

From REFORM wiki
Jump to: navigation, search
(→‎General description)
(→‎General description)
Line 4: Line 4:
 
==General description ==
 
==General description ==
  
−
'''In few words''': Remove in-stream hard structures to allow natural processes to re-establish
+
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).
  
−
Groynes or wing deflectors are commonly employed at rehabilitation projects as structures for modifying the dynamics and directing the flow, increase the complexity of the substrate and the water level. There are many kinds of wing deflectors, but rock and rock-filled log crib deflector structures are the most common. When they are placed in sand bed streams it might be needed to put a filter layer or geotextile underneath the deflector for a correct settlement.  
+
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 below) and L-shaped groynes.
  
−
Since they redirect the flow and narrow the width of the channel, microturbulences are created even at low flows. Sometimes these microturbulences alternate the sediments size distribution, diversifying the substrate structure and enhancing habitat diversity.  
+
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.
  
−
On the other hand, these structures might be degradation source, since the turbulences created and a higher flow velocity can cause streambanks and channel bed erosion (figure 1). The backwater effects would damage riffle areas if they are placed too near to upstream riffles. Flooding frequency and periods of inundation might be intensified by increasing channel roughness.
 
  
−
[[Image:Wing_deflectors_.jpg|400px|thumb|center|Figure 1: Effects of wing deflectors on streambanks (modified from Malavois et al., 2007).]]
+
[[Image:GroyneForms.jpg|400px|thumb|center|Figure 1: Regular groynes and alternative V-shaped and notched groyne forms (modified from Kleinwächter et al. 2005).]]
  
 
==Applicability ==
 
==Applicability ==

Revision as of 11:39, 23 November 2010

Remove or modify in-channel hydraulic structures

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

General description

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 below) 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.


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

Applicability

Expected effect of measure on (including literature citations):

  • HYMO (general and specified per HYMO element)
  • physico � chemical parameters
  • Biota (general and specified per Biological quality elements)

Temporal and spatial response

Pressures that can be addressed by this measure

Cost-efficiency

Case studies where this measure has been applied

Useful references

Adam, P, N. Debiais and J. R. Malavois. 2007. Manuel de restauration hydromorphologique des cours d´eau. Fiche 3. Épis. Direction de l´eau, des milieux aquatiques et de l´agriculture (DEMAA) Service Eaux de Surface. L´Agence de l´eau Seine-Normandie.

Stream corridor Restoration: Principles, Processes and Practices. 2001. Federal Interagency Stream Restoration Working Group. USDA- Natural Resources Conservation Service

Other relevant information

Retrieved from "https://wiki.reformrivers.eu/index.php?title=Remove_or_modify_in-channel_hydraulic_structures&oldid=4549"