Revision as of 14:31, 14 October 2010

Widen water courses

Widen water courses

05. River bed depth and width variation improvement

General description

In the Alpine and lower-mountain regions, many rivers have been widened or re-braided in the last years to restore a more natural planform (wandering or braiding). Channel migration and braiding are enabled within the widened reach, leading to greater structural, hydraulic, and habitat heterogeneity. As with re-meandering, it should carefully be checked if a multiple-channel planform would naturally develop given the present discharge and sediment conditions. 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). 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). In principle, there are two possible approaches for widening or re-braiding: Creating a wider, shallower planform or initiating natural lateral channel dynamics. Creating a “braiding” planform: Actively widening or re-braiding usually includes removal of bank fixation, the setback of the flood levees. Although widening intends to increase sedimentation of gravel and the formation of gravel bars, over-widening may result in too low flow depth and velocities and in turn sedimentation of fines and clogging of interstitial spaces. This is especially true for catchments with high suspended sediment loads due to agricultural land-use in the upstream catchment. Some guidelines for dimensioning the widening of rivers are available (e.g. assessing maximum pool depth in the widened reach and at the downstream end) (Hunzinger 1998, 2004).

Initiating natural lateral channel dynamics: Instead of creating a new channel using heavy machinery, lateral channel dynamics can be initiated by removing bank fixation and placement of 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 wider or braiding planform is reached. Moreover, significant changes in channel planform and a natural braided will not be reached within an engineering time scale in streams with reduced stream power (rivers with reduced peak flows like reaches downstream of dams) or sediment deficits. 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 well below the value 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 braiding planform can be reached in an engineering time scale.

Applicability

See problems and constraints of the different approaches mentioned above. Restricted to reaches where lateral channel migration can be admitted (up to whole braiding channel 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 widening or re-braiding. River widenings are especially successful if the reaches are long, if bedload is high, and if upstream near-natural reaches with source populations exist (Rohde 2005). Widenings are particularly appropriate for the rehabilitation of formerly braided rivers with intact or little impaired bed load.

Widening in eroding rivers: In eroding rivers (e.g. sediment deficits), a local active widening potentially increases channel-bed erosion downstream due to sedimentation in the restored reach and downstream increase of sediment deficit, as well as upstream due to the decrease of the water level in the restored reach. To decrease the upstream erosional effect, stepwise active widening or initiating natural lateral channel erosion is recommended (Requena et al. 2005). Moreover, it is recommended to add gravel material from the banks and floodplain to the restored reach to decrease downstream sediment deficits and erosion (Hunzinger 2004).

Expected effect of measure on (including literature citations):

HYMO (general and specified per HYMO element)

• Increase in mean overall width shore length variation for depth, current velocity, number of substrates and higher substrate diversity (Jähnig and Lorenz 2008, Jähnig et al. 2009).

• Increase in mesohabitat diversity (Jähnig et al. 2009b, Weber et al. 2009). Increase in microhabitat composition (more even distribution of different grain-sizes) but no increase in the number of aquatic microhabitats (no completely new microhabitats) (Jähnig et al. 2009b).

• Especially increases pioneer habitats like gravel bars (Rhode 2005.

Physico-chemical parameters

• River widenings potentially influence many physico-chemical processes. However, there is no empirical study on the effect of this restoration measure on physico-chemical parameters in the peer-reviewed scientific literature.

Biota (general and specified per Biological quality elements)

Macroinvertebrates:

• No significant increase in taxa number, abundances, evenness, several biological metrics, traits, and alpha-diversity (Jähnig and Lorenz 2008, Jähnig et al. 2009, 2009b).

• The low effect on invertebrates might be due to several reasons: (i) New high quality habitats have not been created by river re-braiding (e.g. large wood), which is supported by the fact that microhabitat diversity did not increase in several projects (Jähnig et al. 2009b). (ii) Other anthropogenic stressors like catchment land-use or the lack of source populations might limit the effect of local re-braiding restoration projects, which is supported by the fact that other organism groups (ground beetles) and floodplain vegetation - with a higher dispersal ability - significantly improved after widening or re-braiding (Jähnig et al. 2009), especially if near-natural reaches with seed banks are located nearby (less than 10 km upstream, Rohde et al. 2005).

• It is difficult to assess if the restoration measure really has a low effect on invertebrates or if the effect in the restoration projects investigated was constraint by other anthropogenic stressors, the lack of source populations, and low dispersal abilities. Therefore, it is recommended to focus on the creation of new, high quality habitats in river widenings or re-braiding projects and to ensure that nearby source populations are present if the measure intends to enhance invertebrate assemblages. If these prerequisites are met, river widenings probably have a medium to high effect on invertebrates.

Fish:

• Highest winter abundances in deep, well-structured backwaters of river-widenings. However, no significant higher number of species or overall abundance compared to un-restored reaches (Weber et al. 2009).

• The low effect on fish might be due to several reasons: (i) River widenings did not create habitats which are presently limiting fish (e.g. thermal refugia missing due to the reduced thermal heterogeneity). (ii) Other anthropogenic stressors like fragmentation or residual flow might limit the effect of local widenings. (iii) Widenings are too short compared to the surrounding reaches which display degraded morphological conditions: Therefore, the ecological effect of local river widenings (< 1km in length) in degraded rivers probably only have small positive effects on fish (conservative assessment), respectively the local improvement of juvenile fish recruitment will not translate into measurable stock improvements later on.

Macrophytes:

• River widenings potentially favour macrophytes since shallow, slow flowing areas and backwaters are created, but there is no empirical evidence for this.

Phytoplankton:

• River widenings could favour phytoplankton since water depth is decreased and travel-times are increased. However, the length of widenings (usually < 1km) is too short to significantly affect the abiotic parameters that influence 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

Largely depends on the land purchase cost and on the approach used (creating new channel vs. passive restoration). Medium cost-effective if land is already owned due to small to medium ecological effect.

Case studies where this measure has been applied

• Anzur. Intervención de mejora ambiental de un tramo del Río Anzur
• Charlottenburg artificial bay
• Morava
• Lahn Cölbe
• Rijkelse Bemden - River bed widening
• Thur
• Deva River. Bank protection on the right bank of the Deva River in Molleda
• Narew river restoration project
• Low reach of River Cinca
• Töss
• Skjern - LIFE project
• Drava - Kleblach
• Enns - Aich
• Ahlen-Dolberg - Optimisation of the pSCI “Lippe floodplain between Hamm and Hangfort” (LIFE05/NAT/D/000057)
• Soest - Optimisation of the pSCI “Lippe floodplain between Hamm and Hangfort” (LIFE05/NAT/D/000057)
• Becva
• Lower Traun
• Lippeaue Klostermersch
• Ruhr Binnerfeld

Useful references

Hunziger, L. (2004) Flussaufweitungen: Möglichkeiten und Grenzen. Wasser Energie Luft, 96, 243-249.

Hunzinger, L. (1998) Flussaufweitungen – Morphologie, Geschiebehaushalt und Grundsätze zur Bemessung. Mitteilung der Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie der ETH, 159, Zürich, 206 p.

Jähnig, S. C. & Lorenz, A. W. (2008) Substrate-specific macroinvertebrate diversity patterns following stream restoration. Aquatic Sciences, 70, 292 - 303.

Jähnig, S. C., Brunzel, S., Gacek, S., Lorenz, A. W. & Hering, D. (2009) Effects of re-braiding measures on hydromorphology, floodplain vegetation, ground beetles and benthic invertebrates in mountain rivers. Journal of Applied Ecology, 46, 406 - 416.

Jähnig, S. C., Lorenz, A. W. & Hering, D. (2009) Restoration effort, habitat mosaics, and macroinvertebrates - does channel form determine community composition? Aquatic conservation: Marine and Freshwater Ecosystems, 19, 157-169.

Requena, P., Bezzola, G. R. & Minor, H.-W. (2005) Aufweitungen in erodierenden Flüssen. Wasser Energie Luft, 97, 183-189.

Rohde, S., Schütz, M., Kienast, F. & Engelmaier, P. (2005) River widening: an approach to restoring riparian habitats and plant species. River Research and Management, 21, 1075-1094.

Weber, C., Schager, E. V. A. & Peter, A. (2009) Habitat diversity and fish assemblage structure in local river widenings: a case study on a swiss river. River Research and Applications, 701, 687 - 701.

Other relevant information