Create and restore backwater habitats

Design guidance

Site selection and defining function of new backwaters or bays

As with all restoration activities the key to successful backwater creation or restoration is site selection. The use of backwaters by fish and other wildlife depends on the presence of suitable habitat characteristics, thus new backwater habitats should be created where the proposed habitats will be self-sustaining under a new flow regime. The quantity, quality, and longevity of habitat will depend on the frequency, magnitude, timing, duration, and source of its flow.

Local topography and ground conditions should be used to determine the optimal locations for the new backwater. Aerial photography, historic mapping and site visits should also be used to assist in the decision making process. Understanding how the backwater will function to determine the type of hydraulic regime that could be achieved in the backwater will influence the habitat type.

Sometimes where it is not possible to reconnect meanders without considerable deepening (for example on the Lesser Teise in Kent where significant land drainage operations had reduced the depth of the main channel metres below the historic bed levels in the relict meandering channel, the more feasible option would be to create backwaters at several locations (without full reconnection of the meander system)).  If the backwaters are left unprotected natural adjustment may occur eventually leading to increased connectivity of old meander loops (assisted natural recovery).

Care is needed when seeking to reconnect old fluvial features such as meander cutoffs to the main channel as these features may already be supporting good quality habitats and may not benefit from reconnection to the main channel.

Re-connected meander at Shopham on the River Rother, West Sussex

Design guidance

The New Rivers and Wildlife Handbook (RSPB, 2001) (pp189-194) provides guidelines and case studies on backwater and bay creation.

Useful case studies can also be found in Section 2 of the ‘Manual of River Restoration Techniques’ (RRC, 2001).

The following data will be useful when understanding the current dynamics of the river and how the backwater will function within the river and floodplain system:

  • Topography and cross-sections of project area including river and floodplain.
  • High and low flow hydraulic characteristics of the main channel through the project reach and adjacent reaches. Recent high-water marks of the main channel
  • Geomorphological characteristics of the main channel as evidence of sediment regime through erosion or deposition.
  • Sources and paths of overbank flow or additional surface flow during heavy runoff events.
  • Evidence of standing water and/or wetlands in the project area as useful analogues.

[adapted from Washington Department of Fish and Wildlife document ‘Stream Habitat Restoration Guidelines' (2004). They provide comprehensive report on habitat restoration, though terminology may vary slightly from the UK principles and background literature are useful.]

Design of hydraulic regime within the backwater

  • Include a range of water depths to allow a range of vegetation to establish across the hydrological profile with maximum depths sufficient to prevent emergent plants colonising the whole area. Cross-section designs should be produced.
  • Backwater entrance conditions should be sufficiently wide (2m minimum) to allow access by fish and other fauna.
  • Consider how the backwater will function hydraulically under a range of flow conditions taking into account the sediment regime and longer term sustainability of the feature.

Where redundant channel lengths are to be used, such as in the River Skerne example (RRC 2.1 – Creation of Backwaters) the profile of the redundant channel may need to be changed (enlarged and reprofiled) to achieve full ecological potential. A normal water depth of 2m was used in the centre of the backwater on the Skerne to prevent emergent plants from occupying the whole water area. Conversely, shallow depths around the sides were needed to encourage both marginal and emergent plants. Where depth of the backwater is at a maximum, width will need to increase to allow the banks to be sufficiently well graded to allow profile allowing aquatics, emergent and marginal plants to colonise (also for health and safety reasons).  An example of a different intended marshy habitat (as opposed to open water) is provided on the River Colne where Backwater 2 designed with a bed level at just below normal water levels to create a marshy habitat.

A range of designs in demonstration projects on the Colne and Skerne to allow backwaters to perform different functions. Regular influence of higher flows into backwaters has been designed to promote flushing flows and maintain water levels. The backwater on the river Skerne has been designed to allowing progressive submergence of the backwater from flows in the main channel prior to the out of bank flows to promoting flushing of sediment and maintenance of an open channel at the confluence of the backwater and main channel. This was achieved on the Skerne profiling the land between the main channel and the backwater to allow flow to wash over into the backwater area before the whole meander corridor flooded.  The complex currents and slowing of flows that result at the confluence of the main channels with backwaters affect patterns of sediment deposition often resulting in sediments settling out and gradually closing off the backwater. On the Colne, the embankment adjacent to  redundant channel was removed to allow overspill of flows out of the backwater into the main channel once full. In other backwaters, embankments or levees can be retained to cutoff the backwater from flood flows so that it instead relies on water levels changing in the main channel and exchange of flows. This can create quiet areas of slow water, potentially more subject to sedimentation without flushing flows.

River Brent, backwater shown other side of the land which has been profiled to allow flows to spill over into the backwater to flush through

Maintaining flow during normal flow conditions

Backwaters can be used as part of river restoration to provide an outlet from existing drainage channels or surface water outfalls before drainage into the main channel. The drainage network can provide an independent discharge to maintain water levels. Backwater 2 in the Colne example shows a backwater at the end of a drainage channel which then links to the restored mill leat. The backwater  backs up during higher flows creating a reversal of flow and seasonal wetting up of the floodplain.

Provision of flow from ground water has been achieved by inclusion of gravels in the backfilled original channel (connected to the restored river) simulating a lens of gravel found in natural systems where the channel migrates laterally on the floodplain leaving behind gravel deposits. The purpose of the artificial lens is to encourage a small flow of river water to seep through to the backwater at all times. The amount of flow will be dependent upon the difference in water levels and the upstream river.

A shallow bay can be created by excavating an area adjacent to the river. In general, new bay areas should not be created on the outside of meanders or other high energy areas which are prone to erosion.

RRC Manual of Techniques River Colne 2.1

Planting

During excavation of the new channel, leave root systems in place as these provide valuable habitat.

Extensive overhanging trees should be avoided as they may cause natural enrichment of the static water and reduce the over-abundance of invertebrates by shading the backwater.

Where possible, aquatic vegetation should be allowed to colonise naturally within a newly created backwater. Wetland plants tend to readily colonise new areas as seed is carried by wind and water. If undesirable species are present within the catchment of the site (e.g. non-native invasive species), artificial planting may be required (see Dealing with Invasive Plant Species).

Alternative plants can be introduced from nearby sites. Plants of native origin should be used where possible. 

A buffer strip of longer vegetation can be created around the backwater channel to protect it from overgrazing and disturbance.

Maintenance

Existing or newly created backwaters should be monitored and maintained to ensure that they do not become overgrown with vegetation or gradually fill in with silt. If silt removal is required, not more than a third of the area should be cleared at any one time to prevent disturbance to resident aquatic flora and fauna. Maintenance work should be undertaken during autumn to minimise disturbance to wildlife.  Light grazing of the area immediately surrounding backwaters can promote varied vegetation growth if managed carefully. Intensive trampling can cause damage, and can be controlled by installing fencing.

prev   1  2  3  4   next