Bypass channels

Design Guidance

Design guidance is not currently well established for bypass channels. They are not generally the subject of detailed hydraulic research or any specifically defined design criteria. Rather, they are built to mimic natural river channels and are thus heterogeneous and site specific. As such, available guidance focuses on issues such as bypass entrance design, slope and flow velocities. An overview of the key issues is provided here, with suggestions for further reading.

General channel geometry

As far as practicably possible, the bypass channel should resemble a natural side channel or tributary of the river system. In general, the closer the bypass channel matches the morphological characteristics of the natural habitat for the species present, the less likely hydraulic conditions will reach thresholds that limit fish passage.  If sufficient space is available, the channel should be sinuous or at least have some meanders. In some circumstances a straight channel may be necessary, although this is not preferable.  The channel morphology of a bypass channel typically takes one of two forms:

  • A pool / weir (sometimes referred to as pool / riffle) where energy dissipation is concentrated at local drops created by the weir (typically boulder bars –similar to the boulder bars of the rock ramps above), with pools in between each drop (see Figure overleaf), or
  • A rough channel where energy dissipation is less localised and occurs more or less regularly along the length of the channel, created by large boulders and or woody debris (see Figure 1).

In lowland rivers, vertical drops should not be used, as they are typically not natural features in such an environment. However, if necessary, very low drops can be created using woody debris fixed to the bed; these however should be highly heterogeneous and permeable throughout. In steeper rivers and streams, pool / weir sequences can be used. For some steep systems, a combination of the two can provide a good solution, with boulders and woody debris placed in between the localised drops to create roughness elements to dissipate excess energy and provide a heterogeneous habitat through which all occurring fish species can easily pass.

Figure 1: Schematic diagram of natural by-pass channel showing position of pool/weir and rough channel bypass sections (SNIFFER, 2007)

Slope

The slope of the bypass channel, though steeper than the main channel, should correspond to river reaches where particular communities of fish are found. As a general rule, the slope of the bypass channel should not exceed 5% (SNIFFER, 2007). 

By-pass entrance

The optimal location for the downstream entrance to the bypass channel is immediately downstream of the barrier, preferably within 1-2m. If this is not possible due to restrictions in terms of land-take or slope, this can partly be compensated for by augmenting the flow and increasing the attraction flow.  Fish must be able to access the bypass channel through a range of water levels. The profile of the entrance may be enhanced to assist fish entry over a range of flow levels. The by-pass channel immediately upstream from the entrance can also be steepened slightly to increase the attraction flow into the main river body. 

Bank / bed stabilisation and maximizing morphological diversity.

Where a new channel is constructed, the potential for bed and bank erosion should be considered and mitigated where necessary.  Where bank erosion control measures are required, ‘soft’ engineering solutions should be used, such as vegetation planting, biodegradable geo-textiles, faggoting, and willow mattresses. The construction of riffles, ledges, and placement of other features (boulders, large wood) common to the character of the river type will provide a foundation for the placement of fish habitat.  Opportunities for environmental enhancements, such a riparian planting, should be implemented where possible. See Use of soft engineering in bank stabilisation and protection and Management and use of large wood.

Post construction monitoring

Post construction monitoring should be undertaken where possible to monitor the impacts on fish passage and ensure that the bypass channel is working efficiently.

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