Remeandering straightened rivers

Design Guidance

Restoring the sinuosity of a river can be achieved in a number of ways.  As remeandering results in changes to channel processes, a thorough understanding of fluvial geomorphology is essential.  The following types of approach have been identified: 

Allowing a river to recover sinuosity naturally

Meanders can be allowed to develop naturally in a straightened channel.  This requires minimal intervention and therefore has few cost implications.  However, its use in many systems may be constrained by several factors:

  • Artificially channelised streams with stream powers per unit width greater than 35 W/m2 can develop their own sinuosity over time (Brookes, 1987).  However, most lowland channels tend to have lower energy and are therefore not likely to develop meanders without intervention.  This means that natural recovery may only be suitable for watercourses with relatively high stream power. 

Natural remeandering in an overwide urban channel, River Ravensbourne. A gravel point bar has formed and revegetated on the inside of meander bend. Both banks have timber toeboard

Removal of timber toeboarding on River Ravensbourne allowing natural adjustment on outside of meander bend and new depositional forms to develop.

  • Where conditions are suitable for meander development, it may take several decades for geomorphological processes to restore the desired level of sinuosity (Brookes, 1992).  Soar (2001) suggests that the rate of recovery is a function of the geomorphological potential to expend energy; therefore, upland systems may be able to recover, whereas lowland, low-energy streams, which experience formative events less frequently, may take centuries to recover naturally.
  • Artificial bank protection and structures are frequently encountered on channelised rivers.  The presence of reinforcement prevents natural recovery and limits the use of this measure to straightened earth channels, unless these structures are removed (see Rehabilitation of banks and riparian zone for further guidance). 

In many cases, it may be possible to remove bank protection where there is room for the river to remeander naturally.

Improving sinuosity within a straightened river

Bank material, substrate and vegetation on alternate sides of the bank can be removed to create a sinuous channel (Buisson et al., 2008) (see below).  This technique is generally not suitable for very narrow channels where the channel is confined laterally.  Care should also be taken not to remove vegetation which is stabilising the bank, or which is valuable in terms of its conservation value. 

Schematic diagram of sinuous flow introduced into a straight channel through the selective removal of vegetation and substrate. Adapted from source: Buisson et al. (2008), p61.

As an alternative, it may be possible to introduce an element of sinuosity in a straightened river channel through the installation of structures to vary flow patterns (RRC, 2002).  For example, current deflectors and artificial shoals can be introduced on alternate banks to improve geomorphological diversity and introduce sinuosity.  These structures ideally utilise soft engineering measures, and can be comprised of wooden posts and logs which are driven into the bed and backfilled with sediment (RRC, 2002) (see below).  These measures have been used on the River Skerne in Darlington, although the success was reported to be dependant on the size of the structure and the level at which it is constructed (RRC, 2002).  See also Rehabilitation of banks and riparian zone.

Schematic diagram of sinuous flow introduced into a straight channel through the installation of current deflectors. Source: RRC (2002), Section 3.1. 

Reconnecting remnant meanders

In many circumstances, it may be possible to divert a straightened channel into the course of a redundant channel, or cut-off meander.  The course of the old channel can often be identified by using aerial photos and historical maps, and in may cases may still be a recognisable feature on the floodplain.  Where remnant channels are clearly visible, reconnection can be relatively straightforward, although this is dependant on the degree of siltation that has occurred in the cut off and any incision or aggradation that has occurred in the main channel.  The bed levels of the two channels need to be consistent, and in some cases excavation or bed regrading works may be required (RRC, 2002). 

If bed levels are relatively similar, the meandering course can be reconnected by excavating the “plug” of bank material that blocks the entrance to the channel.  This measure was used successfully on the River Little Ouse near Thetford in Norfolk, in combination with the removal of some trees and minor bed regrading (RRC, 2002).  In cases where more significant bed level differences are observed, more invasive excavation may be required.  For example, remnant meanders on the Latrobe River, Victoria, Australia were re-excavated because a combination of bed scour in the straightened channel and siltation in the cut off meanders had created a difference in bed level (Rutherfurd et al., 2000).

Constructing a meandering channel adjacent to the straightened channel

This technique involves constructing a meandering channel adjacent to the existing straightened channel and then diverting the river into the new channel.  The existing channel can be backfilled with the excavated material from the new channel, or engineered to function as a backwater (see Create and restore backwater habitats) The new channel should be designed using geomorphological principles with the input of an experienced geomorphologist and ecologist. The flood risk implications of channel works should also be carefully planned and considered.

Key design considerations

Allowing sufficient space for meandering

The corridor required for a meandering system (b) will be much greater than for a straight system (a). This needs to take into account future adjustment through lateral migration and geomorphological assessment should inform design.

Grade control

The transition between the new and old channel needs to be carefully thought about in terms of scour and potential for headward recession (the bed progressively incises in an upstream direction sometimes causing instability of banks and changing bed grade).

Confluence between straightened river (to be backfilled) and new River Brent at Tokyngton Park still with old bank protection in place. A hard bed is needed for the transition from concrete to the new channel.

If areas of disconnected channel are reinstated, this may induce turbidity and fine sediment problems as material stored in the cut-off is remobilised.  If this sediment includes contaminants, this contamination could be released into the watercourse when the bends are reinstated.  Careful monitoring should be undertaken to ensure that the channel is recovering at an acceptable rate.

The new meandering channel should use geometry and plan form of natural sections of the river if these are present upstream or downstream as analogues. Design of the new channel should take into account existing guidance for designing new channels  (see Fluvial Design Guide, Environment Agency, 2010).

Riparian plants can be planted along the new channel to help stabilise the banks.  For further guidance on planting and managing riparian vegetation, please refer to Good Practice Vegetation Management.  Additional guidance on bank stabilisation measures is provided in Managing bank instability and erosion.  As well as controlling erosion, riparian vegetation can help improve water quality, restore the river’s hydrologic function, provide a source of nutrients and woody debris, and provide valuable habitat for fish and invertebrates.

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