Good practice management of in channel vegetation

Design Guidance

Aquatic vegetation that is diverse and well structured provides excellent habitat for invertebrates and other wildlife. However, if one species becomes dominant, it can block waterways and suppress the growth of other species. In such cases, management is desirable but there are many factors to consider before carrying out works, e.g. when and how to carry out control and how much vegetation to remove.

The management objectives for watercourses should be determined before any works are carried out. Good planning will ensure that the right technique is used at the right time for the right reasons.

A number of comprehensive texts exist offering details on the techniques to achieve management aims while reducing adverse impacts and providing benefits to wildlife. Two important publications are The Drainage Channel Biodiversity Manual (ADA, 2008) and The New Rivers and Wildlife Handbook (RSPB, 2001).

The Drainage Channel Biodiversity Manual includes a range of guidance sheets for the maintenance of in-channel vegetation. Extensive guidance is provided on the selective removal of aquatic plants to permit re-colonisation of desirable species. Aquatic plants can be left in patches on alternate sides of the channel to give a clear sinuous route along the channel. Alternatively, a continuous strip of aquatic plants can be left uncut along one side of the watercourse. These approaches to vegetation management provide continuity of the plant community in the channel, allowing the establishment of an associated invertebrate and vertebrate animal community including, for example, dragonflies and fish.

The New Rivers and Wildlife Handbook (1995) (pp 237-240) identifies best practice methodologies for general vegetation management including undertaking rotational cuts and suitable timings for cutting. In general, control of in-channel vegetation consists of cutting, dredging or raking. Guidance on grazing and chemical control is discussed in Good Practice Management of Riparian Vegetation

Cutting should be accompanied by collection and removal to avoid blockages of downstream structures (e.g. culverts) and deoxygenation of water. Large quantities of cut vegetation should not be left on the bank as bankside plant communities can be smothered and die. Timing of cutting is important due to potential regrowth from roots and rhizomes, but also because of the potential impact to breeding birds and aquatic invertebrates. As a general rule, submerged plants should be cut in summer and emergents cut in autumn. When silt accumulates around aquatic vegetation, desilting (using a mechanical excavator) restores channel capacity more efficiently than cutting. As with cutting, alternate lengths can be desilted and left untouched, allowing rapid re-colonisation of aquatic plants and invertebrates.

In addition, the Environmental Options for Flood Defence Maintenance Works (Environment Agency, 2003) includes a series of cross-section and long-section drawings, which provide a visual picture of the desired environmental outcome required of vegetation management works.

Vegetation cutting will significantly alter habitat structure and may not always be the most appropriate methodology. Other methods such as the use of herbicides may be more suitable. The Centre for Aquatic Plant Management at CEH has produced online information sheets with details on how to manage certain native and non-native macrophyte species using mechanical, chemical, biological and environmental methods of control. The most effective techniques for each species are identified and described.

Broader ecosystem considerations should be built into any plan at the design stage, as species that rely upon the water environment can be vulnerable at different times of the year. For example, autumn removal of vegetation can reduce overwintering habitat for invertebrates while summer weed cutting can reduce fish habitat. An integrated approach to management of the wider system over the longer term is likely to provide the maximum potential both for biodiversity as well as for the management aims. Other factors that should be considered include the threat to nesting birds and fish spawning, shading of open waters, disposal of cuttings, the need for repeated cutting throughout the growing season and the risk of flooding to people and property under various management scenarios.

Before using machinery around watercourses, consideration must be giving to the timing of works, the type of machinery used and the operating skills required for the task. Machines working from the bank tend to be more selective than those working from the channel, but they must have access along the bank which can be a major limitation in more natural or urban areas. Machines working from the channel have less of an issue with access and the operator is in a better position to locate nests and avoid destroying them, but they have a seasonal limited use and are not suitable for any other tasks.

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