Increasing ecological value of structure

Design Guidance

Although there are several methods of increasing the ecological value of structures, there is little established design guidance. Design guidance in relation to ecological design features within transitional waters is provided in Airoldi et al (2005) and Moschella et al (2005).

Floodwalls / Hard bank protection

In some areas it is often unfeasible to either remove floodwalls or replace them with soft engineering solutions.  When vertical or near-vertical walls are the only option, techniques exist for enhancing the ecological interest of such walls, by timber cladding or creating of vegetated terraces in front of the structure.

Timber cladding

Most methods of timber cladding involve the use of wood coverings of one form or another on the tidal/riverward side of the structure. At one extreme, the whole wall may be panelled. The wooden panelling forms a relatively soft substrate for the colonisation of algae and invertebrates. Design guidance in relation to timber cladding within transitional waters is provided in Airoldi et al (2005), Moschella et al (2005) and Estuary Edges: Ecological Design Guidance (Environment Agency, 2008).

Ideally, a gap should be left between the wall and the timbers which can be back-filled with material of similar particle size distribution as the adjacent foreshore to form a vertical beach habitat. Complete covering with wood panelling may cause problems for inspection of walls, and hence partial panelling solutions are recommended. In places, a full height section of the wall should be left exposed for inspection and anchor bolt locations should be left uncovered. The space allowed between exposed sections will depend on the precise nature and construction of the wall. If considerations such as inspection requirements, aesthetics or other functional requirements preclude panelling, it may still be possible to install wooden timbering (this can be vertical and/or horizontal). Where timbering is used, this can be designed with a dual function, also being used intertidal planters that can be filled with rubble to provide habitat and support plant growth.

Horizontal timbers will offer most ecological value when located in the main intertidal plant growth zone. The intertidal zone is the area that is exposed to the air at low tide and submerged at high tide, and can often be identified as the area between tide marks. Timbers need not, however, be placed only in vertical and horizontal directions.

Deptford Creek, ecological enhancements, Environment Agency 2008

Following the installation of timber cladding, regular inspections should be carried out to check the integrity of intertidal timberwork. Treated timber should be avoided due to their risk to river life from toxic wood preservatives leaching into the river. Timbers with a relatively limited lifespan (such as untreated pine) may also be used but particular attention will need to be given to monitoring their condition for repairs. For more information, see Section 8 of the Estuary Edges: Ecological Design Guidance on aftercare and monitoring (Environment Agency, 2008).

Installation of fish refuge brushes

Other features that may be added to walls include plastic fronds or ‘brushes’ which can function as substrates for fish eggs. The brushes should be installed at a location which is not exposed to the air for more than a few hours each day to prevent damage to the eggs.

Fish brushes have been incorporated within several case studies, including Deptford Creek, South London and as part of improvement works to moorings along the River Thames. The refuges, which look like brush heads with horizontal fingers emanating from them, will be used by fish such as eel, smelt, flounder and sea bass to shelter from tidal currents.

Vegetated berms / terraces

Creating a vegetated berm or terrace in front of a hard engineered structure can provide space for marginal habitat. The design of the terrace will depend on several factors, including:

  • Available space - the more limited the space available, the steeper the terrace will need to be.
  • Sediment particle size distribution - steep slopes with cobbles and large gravel may not be suitable for vegetation growth, but may still provide refuge for invertebrates.

Design guidance on the creation of vegetated terraces within transitional waters is provided in Airoldi et al (2005) and Moschella, et al (2005) and in the Estuary Edges: Ecological Design Guidance (Environment Agency, 2008). In brackish and saline environments, where slopes of around 1:7 or less are achievable, terraces in which the substrate is stabilised solely by saltmarsh vegetation may be established fairly readily in the intertidal zone. The design of terraces will differ depending on their location or position within an estuary. The aim is to promote successful establishment of vegetation by providing a revetment that will trap and hold silt and water at the optimum tidal levels for plant growth, but not become waterlogged. Wherever possible, the aim should be to install at least 1m depth of gravel and growing medium, generally above a geotextile liner. A typical specification for a retained substrate might be: Lower 50cm 100% gravels from 25–10mm with a geotextile mat above to prevent rapid loss of fines. Upper 50cm 95% gravels from 25mm to dust with an additional 5% sand, topsoil and silt. Where space permits, a continuous sloping beach profile at a stable angle of repose between the new retreated flood defence wall and the truncated, capped remnant of the former wall can be considered. Such installations may have considerable value for intertidal and littoral fringe invertebrates.

One drawback with a stepped terrace form is that some flat fish (e.g. flounder, adult common goby) appear reluctant to cross up and over submerged terrace steps, and hence cannot access this valuable habitat. A possible solution is to ensure that terraces are sloping in two planes so that there is some point along the profile where the terrace height falls to zero to encourage to the passage of flat fish onto the terrace.

Greenwich peninsula, Environment Agency 2009

Vegetated berms or terraces can also be created in freshwater river environments in front of floodwalls or hard bank protection. Information on water levels and flow should be used to determine the optimum location for vegetated berms or terraces in fluvial environments.  Care should be taken to ensure that the berm or terrace is not placed too low in the water column so that is becomes waterlogged, or too high so that is does not remain wet during low flows.

Vegetated berm along the River Quaggy

Vegetation boxes

In fluvial environments, boxes may be created within the existing flood wall or hard bank protection to provide niche-habitat within which vegetation can grow in the wall.  Boxes within structures can also provide habitat for birds and help to improve the visual amenity of structures.

Vegetation boxes along the River Quaggy, Lewisham

Embankments

Guidance regarding environmental design of embankments is contained within the Environment Agency’s Landscape and Environmental Design Guidance (Environment Agency, 2007). Embankments can be specifically designed to accommodate trees by constructing them overwide to house planting on the landward side of an embankment.  Similarly opportunities can be sought to establish wildflowers and semi-natural grassland onto embankments.  However, flood embankments present certain constraints and conventional seeding techniques will need to be modified. It is important to ensure that there is an adequate depth of soil on the embankment to allow successful grass establishment.

Weirs

Where a weir cannot be removed or structurally altered to reduce impoundment, modifications are possible to restore some level of connectivity above and below the impounding structure.  These include incorporation of sluice gates and flow management structures, such as v-notches and pool and weir arrangements. These structures retro-fit the existing weir to ease fish passage. Such approaches may be useful for gauging station weirs that are required to be maintained for measurement purposes.

Flow focused through the addition of a notch

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