Remeander water courses

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Remeander water courses

Remeander water courses

05. River bed depth and width variation improvement

General description

Re-meandering 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)

BQE Macroinvertebrates Fish Macrophytes Phytoplankton
Effect high high medium low

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

Cost-efficiency

Largely depends on the land purchase cost and on the approach used (creating new channel vs. passive restoration).

Very cost-effective if land is already owned due to the high and sustainable ecological effect.

Case studies where this measure has been applied

Useful references

Baattrup-Pedersen, A., Riis, T., Hansen, H. O. & Friberg, N. (2000) Restoration of a Danish headwater stream : short-term changes in plant species abundance and composition. Aquatic Conservation: Marine and Freshwater Ecosystems, 10, 13-23.

Biggs, J., Corfield, A., Gron, P., Hansen, H. O., Walker, D., Whitfield, M. & Williams, P. (1998) Restoration of the rivers Brede, Cole and Skerne: a joint Danish and British EU-LIFE demonstration project, V - Short-term impacts on the conservation value of aquatic macroinvertebrate and macrophyte assemblages. Aquatic Conservation: Marine and Freshwater Ecosystems, 8, 241 - 255.

Bukaveckas, P. A. (2007) Effects of Channel Restoration on Water Velocity, Transient Storage, and Nutrient Uptake in a Channelized Stream . Environmental Science und Technology, 41, 1570 - 1576.

Friberg, N., Kronvang, B., Hansen, H. O. & Svendsen, L. M. (1998) Long-term, habitat-specific response of a macroinvertebrate community to river restoration. Aquatic Conservation: Marine and Freshwater Ecosystems, 8, 87 - 99.

Friberg, N., Kronvang, B., Svendsen, L. M. & Hansen, H. O. (1994) Restoration of a channelized reach of the River Gelsa, Denmakr: effects on the macroinvertebrate community. Aquatic Conservation: Marine and Freshwater Ecoystems, 4, 289 - 296.

Hoffmann, C. C., Pedersen, M. L., Kronvang, B. & Ovig, L. (1998) Restoration of the Rivers Brede, Cole and Skerne: a joint Danish and British EU-LIFE demonstration project, IV - implications for nitrate and iron transformation. Aquatic Conservation: Marine and Freshwater Ecosystems, 8, 223 - 240.

Jungwirth, M., Moog, O. & Muhar, S. (1993) Effects of river bed restructuring on fish and benthos of a fifth order stream, Melk, Austria. Regulated Rivers: Research and Management, 8, 195-204.

Klein, L. R., Clayton, S. R., Alldredge, J. R. & Goodwin, P. (2007) Long-Term Monitoring and Evaluation of the Lower Red River Meadow Restoration Project, Idaho, U.S.A. Restoration Ecology, 15, 223 - 239.

Kondolf, G. M. & Railsback, S. F. (2001) Design and performance of a channel reconstruction project in a coastal California gravel-bed stream. Environmental Management, 28, 761-776.

Kondolf, G. M. (2006) River Restoration and Meanders. Ecology And Society, 11, 42.

Kronvang, B., Svendsen, L. M., Brookes, a., Fisher, K., Moller, B., Ottosen, O., Newson, M. & Sear, D. (1998) Restoration of the rivers Brede, Cole and Skerne: a joint Danish and British EU-LIFE demonstration project, III - Channel morphology, hydrodynamics and transport of sediment and nutrients. Aquatic Conservation: Marine and Freshwater Ecosystems, 8, 209 - 222.

Moerke, A. H., Gerard, K. J., Latimore, J. A., Hellenthal, R. A. & Lamberti, G. A. (2004) Restoration of an Indiana, USA, stream: bridging the gap between basic and applied lotic ecology. Journal of the North American Benthological Society, 23, 647 - 660.

Nakano, D. & Nakamura, F. (2006) Responses of macroinvertebrate communities to river restoration in a channelized segment of the Shibetsu River, Northern Japan. River Research and Applications, 22, 681 - 689.

Passy. S. I. & Blanchet, F. G. (2007) Algal communities in human-inmpacted stream ecosystems suffer beta-diversity decline. Diversity and Distributions, 13, 670-679.

Pedersen, M., Andersen, J., Nielsen, K. & Linnemann, M. (2007b) Restoration of Skjern River and its valley: Project description and general ecological changes in the project area. Ecological Engineering, 30, 131 - 144.

Pedersen, M., Friberg, N., Skriver, J., Baattruppedersen, A. & Larsen, S. (2007) Restoration of Skjern River and its valley - Short-term effects on river habitats, macrophytes and macroinvertebrates. Ecological Engineering, 30, 145 - 156.

Sear, D., Briggs, a. & Brookes, a. (1998) A preliminary analysis of the morphological adjustment within and downstream of a lowland river subject to river restoration. Aquatic Conservation: Marine and Freshwater Ecosystems, 8, 167 - 183.

Tague, C., Valentine, S. & Kotchen, M. (2008) Effect of geomorphic channel restoration on streamflow and groundwater in a snowmelt-dominated watershed. Water Resources Research, 44, W10415.

Tullos, D. D., Penrose, D. L., Jennings, G. D. & Cope, W. G. ( 2009) Analysis of functional traits in reconfigured channels: implications for the bioassessment and disturbance of river restoration. Journal of the North American Benthological Society, 28, 80-92.

Wolter, C. (2010) Functional vs scenic restoration - challenges to improve fish and fisheries in urban waters. Fisheries Management and Ecology, 17, 176 - 185.

Other relevant information

A short note on bank fixation: In the beginning, river banks were fixed after creating a new, more sinuous channel in many restoration projects in Central Europe and in many so called “Natural Channel Design” projects in the US, where bank erosion and sediment input is considered detrimental to spawning habitat for fish. However, most habitats in meandering channels like undercut-banks and pools can only be maintained in the long-run by natural channel dynamics (natural bank erosion and sedimentation). Therefore, creating sinuous, fixed channels is clearly no sustainable restoration measure! If it is necessary to fix river banks due to constraints like housings in urban restoration projects or other infrastructure work, it should be carefully checked if the target species or biota are really limited by the newly created habitats and if there are other, more cost-effective measures to create these habitats besides re-meandering (see Nakano 2006, Friberg 1988, Wolter 2010 for examples).

Assessing “stable” channel form: As mentioned above, it is crucial to adequately assess the “stable” channel planform (meandering or braiding) and dimensions (mean cross-section width, depth, sinuousity) of a river in its dynamic equilibrium state, especially if the new meandering channel is build.

There are three different approaches, which have several pros and cons:

  • Empirical equations for channel planform (e.g. Leopold and Wolman 1957, Ferguson 1987, Van den Berg 1995, Bledsoe and Watson 2001) and channel dimensions (e.g. Hey and Thorne 1986, Parker et al. 2007): 
  Pros: Easy to use

  Cons: Strictly can only be applied for streams in the same region
  • Regime models for channel planform (e.g. Millar 2000, Eaton et al. 2010) and dimensions (e.g. Millar and Quick 1993, 1998, Millar 2005, Eaton et al. 2004, Eaton 2006):

Pros: Not restricted to a specific region 

  Pros:Explicitly consider riparian vegetation and bank stability, which strongly influence channel planform and dimensions.

  Cons: Not fully physically based since all regime models include one kind of “extremal hypothesis” (e.g. assuming that stream power is at its minimum in stable rivers, which are “in regime”, i.e. local erosion and deposition but no net erosion)
  • Physically based approaches for channel planform (e.g. Crosato and Mosselman 2009): 
  Pros: Not restricted to a specific region

  Pros: Fully physically based approach

  Cons: Mean channel width of stable channel has to be known in advance.


Literature on stable channel form:

Bledsoe, B. P. & Watson, C. C. (2001) Logistic analysis of channel pattern thresholds: meandering, braiding, and incising. Geomorphology, 38, 281-300.

Crosato, A. & Mosselman, E. (2009) Simple physics-based predictor for the number of river bars and the transition between meandering and braiding. Water Resources Research, 45, W03424.

Eaton, B. C. (2006) Bank stability analysis for regime models of vegetated gravel bed rivers. Earth Surface Processes and Landforms, 31, 1438-1444.

Eaton, B. C., Church, M. & Millar, R. G. (2004) Rational regime model of alluvial channel morphology and response. Earth Surface Processes and Landforms, 29, 511-529.

Eaton, B. C., Millar, R. G. & Davidson, S. (2010) Channel patterns: Braided, anabranching, and single-thread. Geomorphology, 120, 353-364).

Ferguson, R. I. (1987) Hydraulic and sedimentary controls of channel pattern. in: K. S. Richards. River channels: environment and process. Blackwell Science, Oxford, 129-158.

Hey, R. D. & Thorne, C. R. (1986) Stable channels with mobile gravel beds. Journal of Hydraulic Engineering, 112 (8), 671-689.

Leopold, L. B. & Wolman, M. G. (1957) River channel patterns: braided meandering and straight. U.S. Geological Survey Professional Paper, 282-b, 39-85.

Millar, G. & Quick, C. (1998) Stable width and depth of gravel-bed rivers with cohesive banks. Journal of Hydraulic Engineering, 124, 1005-1013.

Millar, R. (2000) Influence of bank vegetation on alluvial channel patterns. Water Resources Research, 36, 1109-1118.

Millar, R. G. & Quick, M. C. (1993) Effect of bank stability on geometry of gravel rivers. Journal of Hydraulic Engineering, 119, 1343-1363.

Millar, R. G. (2005) Theoretical regime equations for mobile gravel-bed rivers with stable banks. Geomorphology, 64, 207-220.

Parker G., Wilcock P.R., Paola, C., Dietrich W.E. & Pitlick J. (2007) Physical basis for quasiuniversal relations describing bankfull hydraulic geometry of single-thread gravel bed rivers. Journal of Geophysical Research 112: F04005. Van den Berg, J. H. (1995) Prediction of alluvial channel pattern of perennial rivers. Geomorphology, 12, 259-279.

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