Revision as of 12:24, 14 October 2010

Allow/increase lateral channel migration or river mobility

Allow/increase lateral channel migration or river mobility05. River bed depth and width variation improvement

General description

To increase lateral channel dynamics and in the long term re-meander channelized and straightened rivers is one of the most visually striking restoration measures which is also perceived as aesthetically pleasing by the public (Kondolf 2006). In principle, there are three possible approaches for re-meandering: Creating a new channel and initiating lateral channel migration (which are covered by this fact-sheet) as well as re-connecting meanders and oxbows (see fact-sheet “Reconnect backwaters (oxbows, side channels) and wetlands”). Creating a new channel: A new channel with natural channel-dimensions is created using heavy machinery.

However, the “stable” channel-dimensions (mean cross-section width, depth, sinuousity) of a river in its dynamic equilibrium state can only be roughly assessed based on nearby reference sites, empirical equations or regime models. Historical data must be used with care, since discharge and sediment regime as well as bank vegetation, which have a strong influence on channel planform and dimensions, may have been altered (e.g. downstream from reservoirs, peak-flows from impervious areas, grazing of riparian areas). Several projects failed since meandering channels were created in places where a braiding planform naturally occurs (Kondolf and Railsback 2001, Kondolf 2006). Therefore, it is crucial to adequately assess the channel planform and dimensions which can be expected given the catchment characteristics (e.g. grain size, discharge, sediment load, bank material and riparian vegetation).

Initiating lateral channel migration: Instead of creating a new channel using heavy machinery, lateral channel dynamics and migration can be initiated by using flow deflectors (“let the river do the work”). However, this “passive restoration” potentially causes high sediment loads in the beginning (which may have negative effects downstream like filling pools) and takes several tenth of years until a meandering planform is reached, especially in streams with cohesive banks or banks reinforced by reeds and dense vegetation. Moreover, a meandering planform will not be reached within an engineering time scale in streams with limited stream power (small streams or rivers with reduced peak flows like reaches downstream of dams). Besides re-meandering, the increase of lateral channel dynamics can substantially enhance habitat diversity even if a meandering planform is not reached.

The happy medium? Given the problems and constraints of the two approaches mentioned above, a third, intermediate approach would be to create a new channel with a width, depth and sinuousity well below the values assessed based on the catchment characteristics. Since the capacity of the channel is lower than in its dynamic equilibrium state, the following high flows will most probably result in bank erosion and reshaping of the cross-sections. However, sediment output from the restored reach will be considerably less than with the passive restoration approach and a meandering planform can be reached in an engineering time scale.

The role of riparian forests: Several studies indicate that planting and developing riparian forests may be crucial for the success of re-meandering projects: Flow velocity and depth are typically lower in re-meandered streams which can significantly increase water temperature if riparian trees and shade is missing (Buckaveckas 2007). Moreover, riparian vegetation would increase bank stability and appeared to be the key to long-term improvements of fish habitat (Klein et al. 2007.)

Applicability

See problems and constraints of the different approaches mentioned above. Restricted to reaches where lateral channel migration can be admitted (up to whole meander belt width) as well as frequent flooding. As with every reach-scale measure, catchment scale pressures like urbanization or fine-sediment input may constrain the effect of re-meandering (Moerke et al. 2004, Tullos et al. 2009). Length of the restored reach should at least be several meanders in length to achieve all ecological benefits.

Expected effect of measure on (including literature citations):

HYMO (general and specified per HYMO element)

• Increase in travel-time of discharge (Bukaveckas 2007)

• Short-term increase of sediment load and export downstream, long-term decrease due to sedimentation on floodplain (Sear et al. 1998, partly based on conceptual model)

• Increase of depth variability (pool / riffle sequence, shallows) (Pedersen et al. 2007, Passy and Blanchet 2007, Klein et al. 2007, Jungwirth et al. 1993)

• Increase in flow variability (Pedersen et al. 2007, Jungwirth et al. 1993)

• Increase in substrate diversity (sediment sorting) (Pedersen et al. 2007, Passy and Blanchet 2007, Klein et al. 2007, Jungwirth et al. 1993)

• Bank features (e.g. undercut-banks).

Physico-chemical parameters

• Increase in temperature if riparian forest is missing (Bukaveckas 2007)

• As a consequence of shallower cross-sections, which are usually built when re-meandering a river:

• Increase in groundwater recharge and summer low-flow (Tague et al. 2008)

• Nutrient retention due to increase in travel-time and storage (retention rates <10% reported in literature) as well as more frequent inundation of floodplain area (Bukaveckas 2007, Pedersen et al. 2007b, Hoffmann et al. 1998, Krovang et al. 1998)

Biota (general and specified per Biological quality elements)

Macroinvertebrates:

• Short-term increase of species which are indicators for disturbance (Tullos et al. 2009).

• Pre-restoration number and abundance of taxa were typically found 1-2 years after restoration (Friberg 1988, Biggs et al. 1998, Pedersen 2007).

• Increase in invertebrate diversity (Jungwirth et al. 1993) and density if source populations are present (Friberg et al. 1994), which might also have positive effects on carrying capacity of fish.

• A more even distribution of taxa (eveness) (Pedersen et al. 2007).

• Similar to macrophytes, colonization in larger streams is probably faster (if source populations are present) and slower in headwater streams, where upstream source populations are missing (since it is the most upstream part of the system). This may especially affect invertebrate species without terrestrial life stages (hololimnic species).

Fish:

• Increase in fish diversity, density and biomass (Jungwirth et al. 1993)

Macrophytes:

• Pre-restoration number and abundance of taxa were typically found 1-2 years after restoration (Friberg 1988, Biggs et al. 1998, Pedersen 2007).

• More natural species composition and growth patterns in the edge habitats, if source populations are present (Pedersen et al. 2007).

• Colonization in larger streams was found to be faster (if source populations are present) and slower in headwater streams, where upstream source populations are missing (since it is the most upstream part of the system) (Baattrup-Pedersen et al. 2000).

Phytoplankton:

• Longer retention time possibly favours phytoplankton.

Temporal and spatial response

Pressures that can be addressed by this measure

• Channelisation / cross section alteration
• Embankments, levees or dikes
• Alteration of instream habitat

Cost-efficiency

Case studies where this measure has been applied

• Renaturierung Untere Havel
• Asseltse Plassen - Bank erosion
• current deflector Eichenfelde
• Lahn Cölbe
• Vreugderijkerwaard - Side channel
• Rijkelse Bemden - River bed widening
• Thur
• Narew river restoration project
• Low reach of River Cinca
• Töss
• Skjern - LIFE project
• River Quaggy, Chinbrook Meadows
• Becva
• Lower Traun
• Ruhr Binnerfeld

Useful references

Other relevant information