1. CATEGORY

1.0 – River Training

2. DESIGN STATUS

Level II

3. ALSO KNOWN AS

Bendway restoration, re-meandering, channel reconstruction.

4. DESCRIPTION

Meanders are broad, looping (sinuous) bends in a stream channel. Meandering is a form of slope adjustment with more sinuous channel paths leading to decreased reach slope. Fluvial and ecological functions are integrally related to the highly diverse spatial and temporal patterns of depth, velocity, bed material and cover found in meanders. Meander restoration consists of reconstructing meandering channels that have been straightened by humans.

5. PURPOSE

Historically, thousands of channels have been straightened to facilitate construction of roadways, bridges, and other types of transportation facilities. In some cases, ecological functions may be restored by returning the channel to a new or previous alignment. Meandering alignments may also increase visual diversity and may be perceived as more natural.

If the existing channel is eroding, particularly due to excessive channel bed slope, relocation into a new, meandering alignment may ameliorate bed erosion processes. However, new erosion along the outer (concave) bank is likely, and site-specific problems are also possible, depending upon local flow and drainage patterns and soils encountered by the new channel.

6. PLANNING

Few natural channels are straight, and stream corridor communities are adjusted to the physical patterns typical of meanders (FISRWG, 1998). Deep pools usually occur along the concave bank near the apex of the bend, while shallow flows are found in riffles at points of inflection between bends and across the tips of point bars opposite concave banks. Coarse bed material collects at riffles in gravelly streams, providing essential habitat for many types of aquatic animals. Gradual erosion of concave banks injects large woody debris into the channel, while fresh sediment deposits on opposite bars are colonized by pioneering species, triggering successional sequences leading to high levels of plant diversity. Well-designed meandering channels are more stable, provide a greater variety of flow conditions and aquatic habitat diversity, and are visually more appealing than straight channels. The greater variety of flow conditions also enhances recreational boating and rafting opportunities.

8. HYDRAULIC LOADING

Not applicable.

Large-scale stream corridor restoration, various types of protection and revegetation measures, particularly on the outside of the new bend(s).

Meandering alignments are visually appealing, ecologically valuable, and may be more sustainable over the long term than straight alignments that are subject to erosion and deposition because they are not compatible with the hydrology of adjacent reaches.

Meander restoration is useful when land is available to re-create meandering alignments for channels that have been previously straightened, realigned, or relocated. Since channel response to meander restoration is difficult to predict with precision, application should be limited to sites where the probability of loss of life or severe damage to adjacent structures due to failure is very small. Furthermore, this technique is poorly suited to unstable (e.g., braided or incising) streams. In most cases, meander restoration should not be attempted unless resources are available for regular maintenance.

Earthmoving equipment or dredge plant is needed to excavate meandering channels.

If possible, construction should be scheduled during low flow periods, avoiding sensitive time windows such as spawning or migratory periods. If riparian vegetation is to provide bank stability for the new meandering channel, it may be advantageous to construct the restored meander in the dry and allow several growing seasons for bank vegetation to develop before diverting flow into the new channel.