Initiate natural channel dynamics to promote natural regeneration

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Initiate natural channel dynamics to promote natural regeneration

General description

Naturally, river flow causes erosion and deposition of the river banks and bed. These natural channel dynamics result in a specific channel planform and channel features like steep cutbanks, bars, and pools, which can only persist in the long-term if they are rejuvenated during high flow events (otherwise e.g. pools fill with fine sediment, cutbanks flatten). In degraded rivers, channel dynamics are limited by bed and bank fixation, hydrological alterations like the reduction of morphogenic flows due to damming or water abstraction, changes in sediment transport (e.g. sediment deficit downstream of dams), and the lack of natural flow obstructions like large wood and boulders.

Instead of actively building a more natural channel planform and channel features using heavy machinery, natural channel dynamics can be initiated to “let the river do the work” (passive restoration). This is usually done by simply abandoning maintenance as well as removing bed and bank fixation and placing natural flow obstructions in the river to accelerate the morphological changes (e.g. large wood or boulders re-directing the flow towards the river banks or bed and causing bank and bed erosion).

Natural channel dynamics in a mid-sized gravel-bed river

Restoring a meandering planform by initiating lateral channel dynamics is described in more detail in the fact-sheet on re-meandering: Allow/increase lateral channel migration or river mobility

Applicability

Natural channel dynamics can be initiated if the respective changes in channel-bed level and bank erosion do not compromise flood protection or damage infrastructure works on the floodplain. Therefore, it can especially be applied in rural areas and applicability is limited in urban areas. Natural channel dynamics are limited in low gradient rivers with highly cohesive bank material (e.g. lowland rivers in loamy alluvial plains), and significant changes in channel morphology can only be expected in the long-term. Alternatively, the cohesive bank material can be excavated. However, this increases the risk of initiating non-natural channel dynamics and planform. Besides a low channel slope and cohesive bank material, natural channel dynamics might be limited due to hydrological changes or a sediment deficit. Therefore, it is crucial to ensure natural sediment transport and that morphogenic flows occur which cause natural erosional and depositional processes.

Expected effect of measure on (including literature citations):

There are only few studies that explicitly investigated the effect of passive restoration (initiating natural channel dynamics). The results reported in Jähnig et al. (2013) indicate that passive restoration can have similar positive effects on habitat diversity compared to active restoration.

References

Jähnig, S. C., Lorenz, A. W., Lorenz, R. R. C., Kail, J. (2013). A comparison of habitat diversity and interannual habitat dynamics in actively and passively restored mountain rivers of Germany. Hydrobiologia, 712, 89-104.


Pressures that can be addressed by this measure

Case studies where this measure has been applied

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