The following 17 points, if followed, should result in a successful feature:
- Avoid encroaching into the estuary (see Figure 1).
- Always choose the design with the best ecological value practicable within the sites limitations (see Figure 1). There is a presumption to vegetated intertidal terraces (as a minimum) if a wall is failing).
- Ensure you design to correct guidelines for either full set back/creek management (see Figure 2), a vegetated intertidal terrace (see Figure 3) or a wall option, paying careful attention to the zone where vegeation is likely to grow between Mean High Water Neap and Mean High Water Sprng tidal levels.
- Approach multidisciplinary consultant/contractors (who have knowledge of geomorphologists) and work with local organisations, where they exist, with local knowledge and expertise in order to achieve a good product.
- Consider masterplanning early on in the design of your site;
- Look to surrounding area to influence your design (see archaeology and heritage as well as education, aesthetics and art).
- Design to enhance wildlife, planting and greenspace, including maximising value to fish.
- Design for safety and navigation, considering the navigational use at the location in question and how users will interact with the proposed structure.
- Consider access to the feature for both navigation (e.g. ladders, grab chains) and for maintenance and monitoring (e.g. locked or hidden gates) to facilitate removal of litter.
- Design for geomorphology and litter.
- Ensure retaining walls are engineered for the appropriate ground conditions, geology and land based loadings (e.g. soil, water, buildings, vehicles, etc.,).
- Design for sustainability and adaptability. Don’t use wire or non-biodegradable materials (eg: plastics or geotextile) in otherwise natural designs, wire with faggot bundles hasn’t been successful, for example.
- The lifetime of the structure needs to be the same as the projected lifespan of the development (this is particularly important regarding timber).
- Choose construction materials which have the greatest surface roughness to enable stable colonisation by surface-dwelling algae and plants eg: textured concrete, groves in timber. Follow guidelines for the use of timber. If using concrete, consider use of concrete with a lower pH (eco-concrete).
- Do explore the use of innovative and differing materials and techniques using the general principles outlined in this document.
- Budget for monitoring and maintenance, including clearing of any collected litter.
- Include indemity/insurance. Note some parts of the design can have indeminity linked to one party and other parts can be linked to other parties.
Guidelines specific to full set back/creek erosion management
Full set back (e.g. Figure 2) is a minor managed realignment, which has the greatest benefits to ecology as it truly returns land to the estuary. It typically includes reconstruction of a flood wall or embankment many metres further inland, excavation and removal of soil in front of this to form a creek and then the use of brushwood to enhance and stabilise sediment deposition on the newly created shallow slopes (see currents under geomorphology). As it creates creeks of very low energy (both wave and currents), it involves little to no hard engineering. In the case of Dartford Creek, brushwood was used purely to control erosion on the outside of meander bends.
The following case studies in the Thames Estuary of full set back/creek management have been looked at in detail in Estuary Edges:
Guidelines specific to intertidal vegetated terraces
A vegetated intertidal terrace is a ledge formed within a hard engineered flood defence, typically backed by a flood defence wall and fronted by a shorter retaining wall which is then backfilled with material. It is a halfway house between naturalised set back and wall options. See figure 3 for an example of a successful intertidal vegetated terrace.
The aim is to promote successful establishment of vegetation through providing a revetment that will trap and hold silt and water at the optimum tidal levels for plant growth, but not become waterlogged. The main components of a successful backfilled terrace are location, size, height, slope and drainage and substrate.
Consider the location of the site within the estuary (see geomorphology and wildlife, planting and greenspace) and the surrounding habitats present in that part of the river. This will inform the type of habitat that can be recreated on the site and what to expect in terms of long term establishment.
The minimum width (horizontal distance from front to back) of a terrace should be 1m but in areas of high wave energy this should be wider (see geomorphology). Length along the river frontage will depend on site constraints, however it is better to have one long terrace than several shorter ones. The more limited the space available, the steeper the terrace will need to be. As a general rule, the wider the terrace, the more options will be available in terms of design to ensure that the terrace can establish and mature, and be maintained in the long term. Terraces do not have to be a uniform depth along their frontage, so can vary between wide and shallow areas to accommodate any specific constraints at a site.
The key zone for the growth of vegetation is between Mean High Water Neap and Mean High Water Spring tide levels and in many cases the aim should be to maximise the area of habitat created in this zone. However, the whole zone, from lowest tide mark to highest astronomical tide, is ‘foreshore’ and consider to be valuable habitat. The zone between 0.5 to 1 metre above highest astronomical tide, especially where there is a splash zone, is also of ecological importance and may support important shingle beach species. This zone can also provide valuable brownfield type habitat and include features such as nesting banks (See Deptford Creek Vertical Wall Renewal).
Naturally occurring saltmarsh in the UK is rarely stable at a slope greater than 1:7 and hence you should try to ensure that this slope is not exceeded in any area that you hope to support saltmarsh. This rule also applies to other vegetation establishment within an estuary as we would recommend creating terraces with a slope of 1:7 as a maximum slope (Figure 4). A shallower gradient will be more resilient to waves and currents (see geomorphology). The slope will to an extent also influence the drainage of the terrace. Drainage is important to avoid complete waterlogging and pooling on top of the terrace but needs to be slow enough to prevent rapid loss of fines through the terrace and ensure that silt can accrete within the terrace. Drainage can be achieved through a series of water outlets at the base of the terrace. It may also be an option to line the terrace with an entirely biodegradable liner, such as coir matting, to help filter the silt from the water as it drains from the terrace.
Wherever possible there should be a 1m depth of gravel and growing medium, potentially over a biodegradable liner such as coir matting. A typical specification to fill a terrace might be 95% gravels from 25mm-10mm in diameter with an additional 5% sand, topsoil and silt. Ideally, the terrace should be filled and left unplanted to colonise naturally and accrete naturally over time but this may be subject to waves and currents (see geomorphology).
There are a number of other factors that can be taken into account when designing a terrace to maximise its ecological value. 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. Avoid using gabions (metal wire baskets filled with rocks) as retaining walls for terraces as the lifespan of the wire when exposed to waves and saltwater rarely matches the lifespan of the rest of the feature. Failing gabions are observed in the case studies.
One drawback with a stepped terrace form is that flat fish such as Flounder and some other bottom dwelling fish such as 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 permit such species of fish to move onto the terrace (see fish).
The following case studies in the Thames Estuary of intertidal terraces have been looked at in detail in Estuary Edges:
- Wandsworth Riverside Quarter
- Battersea Reaches
- West India Dock Phragmites Beds
- Deptford Creek Vertical Wall Renewal (hybrid wall/terrace)
- Point Wharf Greenwich
- Green Peninsula Terraces North West
- Greenwich Peninsula Terraces North East
- Limo peninsula.
- Mill pool, River Roding
- Royal Wharf, Western Embayment
- Royal Wharf, Eastern Terrace
Guidelines specific to vertical wall options
Vertical Walls (Figure 5) have the least ecological value but when they are really the only option. They do contribute to the continuous habitat if well designed in the local context, the following guidelines can be followed to make then as good as possible:
- Choose materials for use in the wall which have the greatest surface roughness to enable stable colonisation by surface-dwelling algae and plants e.g. textured concrete, grooves in timber, fitting of textured tiles (high level design principle 14).
- If using concrete, consider use of concrete with a lower pH (eco-concrete) (also high level design principle 14).
- As with intertidal terraces, the areas expected to grow vegetation should be located between Mean High Water Spring and Mean High Water Neap water levels. Take care when calcuating these levels, especially when in tidal tributaries, as levels may differ from the main estuary.
- North facing or overshadowed walls (e.g. by tall buildings) may only gain limited vegetation.
- Always include some timber in your wall option as this has the greatest benefit ecologically. Even the vertical timbers placed on brick, concrete and metal rivers walls to prevent vessel damage are great for plant and invertebrate life. Wider options include:
- Panelling (Figure 5):
- Partial covering of walls is recommended (although this will depend on the precise nature and construction of the wall).
- A full height section of the wall should be left exposed for inspection and anchor bolt locations should be left uncovered.
- A gap can be left between the wall and the timbers that can be back-filled with material of fairly similar particle size distribution as the adjacent foreshore to form a vertical beach habitat.
- Panelling (Figure 5):
- Individual timbers:
- Horizontal timbers have the best benefit for biodiversity because they can accrete estuarine sediment. Recesses (Figure 6) as well as brushes/bristles can be added to improve the chance of this occurring. Horizontal timbers can be complimented with others at differing angles (Figure 7).
- If the wall is regularly used for vessels to dock against, individual horizontal timbers should be avoided due to the risk of vessels getting caught on them. See safety and navigation.
Generic advice for use of timbers in all estuary edge features can be found here.
The following case studies in the Thames Estuary of vertical wall options have been looked at in detail in Estuary Edges:
The process of building into or over the bed of the estuary (with or without infilling the space between or under the new structure and the river edge) is known as ‘encroachment’ and leads to higher water levels, increased flows and potential flooding. This leads to loss of areas of tidal foreshore, which are extremely valuable feeding and resting places for the rivers’ fish, birds and invertebrates. Foreshore areas provide precious open space for recreation and leisure and are often of considerable archaeological importance.
Climate change and increased sea levels will result in all the above effects worsening, including more foreshore loss.
The Environment Agency considers every case on its own merits but is generally opposed to works on estuaries that cause encroachment. The following considerations may be relevant:
- An inability to construct, renew and maintain sustainable flood defences and to take into account and adapt to climate change and increased flood risk;
- A reduction in the quality of life for people and the environment and in particular any reduction in opportunities for regeneration;
- Reduced storage volume of the tidal river or estuary;
- Change to the flow regime of tidal rivers and estuaries causing damage to the flood defences, flow carrying capacity, foreshore, banks and fisheries;
- Reduced or altered access to and along tidal rivers and estuaries;
- Loss of, or damage to, the ecological integrity of tidal rivers and estuaries and inter-tidal habitats;
- Reduced or altered river corridor open space on tidal rivers and estuaries and damage to local landscape character:
- Potential for pollution entering the wider environment;
- Potential for loss of, or damage to, archaeological heritage;
- Reduced opportunities for river-based recreational pursuits;
- Non-river dependent uses on river structures or within tidal rivers and estuaries.
Integrate the creation of inter-tidal habitat, terraces or wall features into the green space associated with development. This is so that the multiple benefits of more wildlife, river access and engagement with estuaries, creeks and riverside areas can add value to the development by providing the public with tangible improvements in the amenity value of the site.
- Integrate surface water drainage from sites through the intertidal feature by creating a creek feature devoid of plants. Design vegeated areas to form either side of this creek;
- Reduce flood risk by:
- Newly realigned flood walls/embankments having the same level of flood protection or better.
- Making space for more tidal water by creating naturalised set back or intertidal vegeated terraces.;
- Including surface water attenuation e.g. swales on the landward side of the floodwall to prevent tide locking/need for pumping and therefore save energy.
- Refer to any local flood risk strategies/plans e.g. the London one is called the Thames Estuary 2100 plan and can be obtained through the Environment Agency.
- Position buildings in your development so that they do not overshadow your estuary edge feature. Ideally set all buildings back from the edge and design open, green space which blends with the intertidal.
Archaeology and heritage
Heritage and archaeology of the local area and estuary as a whole are valuable features to consider when redesigning river frontages and including habitat features. Principles to follow:
- Check whether you are likely to affect any features of archaeological or heritage importance (such as those listed on the National Sites and Monuments Record or by local museums). In London the entire foreshore is considered to be of ‘high archaeological potential’ (Pers Comms Dr Fiona Haughey, Thames Archaeologist).
- Along many of our estuaries, historic wharves are protected by specific planning policies.
- The design of new features needs to be sensitive to aesthetics and the history of the area (Figure 12).
- Consult with appropriate bodies when considering features to listed structures.
- Seek opportunities to increase, recognise and celebrate the historical and heritage value of the site. For example, keep (or consider removal and reinstatement after works) of dockside fixtures, fittings and associated paraphernalia, including habitat features such a vertical timbers. Their loss impoverishes areas.
Education, aesthetics and art
Every project presents great opportunities to consider interpretation, education, art and aesthetics to help local communities connect to the site and the estuary (Figure 8 and 9):
- Look at opportunities to make design references to environmental issues and social history. Extending the landscape design from the intertidal into the water’s edge can produce striking landscapes.
- The shoreline offers opportunities for artistic and aesthetic expression in the design.
- Installations must have appropriate navigational safety signage and lighting, and designed to prevent inappropriate or unsafe mooring.
- Consider need for public awareness of ecology, habitat enhancement and safety signs. Interpretation boards or similar means of highlighted the wildlife present and the habitat features of a site are a way of drawing attention to the positive environmental improvements incorporated within a development.
- Consider encouraging better access to the intertidal with safe, wide steps or ramps to lower floodable levels.
- Encourage people to interact with each other at the site e.g. placement of picnic benches, gazebos for events.
- Consider interpretation on litter bins to tackle litter e.g. pictures of wildlife trapped in plastic wapping.
Wildlife, Planting and Greenspace
By creating intertidal features and including green infrastructure into riverside developments, opportunities will be created to help restore a more even balance for wildlife in the estuary. The main objective of creating such features is to partly replace the marginal habitat that was lost through gradual encroachment into the river by development and building of vertical flood defences. This marginal (edge) environment would typically vary throughout an estuary from predominantly freshwater plants in the upper (most inland) estuary, through to a mix of vegetation typical of brackish conditions in the middle estuary and finally to the saltmarsh and reedbeds found in the lower (most seaward) estuary. The type of habitat that can be expected will vary dependent on salinity, light, local landuse and quality of the design. All marginal habitats will provide essential refuges for wildlife in a heavily engineered environment, for fish, invertebrates and birds. These features can also provide niches for important and rare plants to establish and thrive. Principles include:
- Consider the energy in waves/currents (see geomorphology) the degree of salinity and the amount of litter around to determine the type of habitat that can be restored/created. This will vary from freshwater marginal habitat to marine/outer estuary habitats such as saltmarsh.
- Use specialist biodiversity advice from local charities and biological records centres.
- Consider habitat types in the local vicinity of the scheme to ensure continuity of habitat features.
- Ensure that full ecological surveys are carried out to identify any potential impacts of works on protected species, habitats or sites of nature conservation importance on or near the site. During works, undertake construction activities in line with a construction management code agreed by an ecologist.
- Consider the amount of shade present on the site. If the features will be shaded for the majority of the day, plants may not thrive and you can adjust the design to take this into account. Many light to medium shaded habitats can work well if adequately managed. but it may be appropriate to design to expect low colonisation by plants but seek to create more features for fish and invertebrates.
- Aim for natural colonisation by plants wherever possible. Planting should only be undertaken where absolutely essential. For instance, where rapid plant establishment is important to resist high wave wash.
- If your feature is to be naturally colonised, aim to complete it in the winter so that seeds can grow from the early spring. If planting, however, do so in early spring when the plants will be growing. These plants are best sourced from locally grown stock (e.g. the same creek) to increase the chance that they can survive the local consitions but also to reduce the chance of bringing in invasive non-natives. Planting from nurseries is one of the main pathways for invasive species.
- As per high level design principle 16, budget and plan for longer term maintenance. See the monitoring and maintenance section
- If planting rapidly growing plants such as phragmites, consider use of rhizome breaks. Otherwise such plants will take over and reduce the biodiversity of the site, impacting on invertebrates (although not necessarily fish).
- Do not expect north facing features to establish the same vegetation as other directions, if any within very saline environments. If shaded terrestrial levels are left unmanaged, then nettles and other invasive, shade-tolerant species such as buddleia may take over.
- If planting must be conducted on north facing sites, consider shade and water tolerant shrub species such as alder, and in drier spots elder, buckthorns and dogwood. Their success in establishment will be highly dependent on the salinity at the site.
- Match planting to know nearby successes, especially natural regeneration, using botanical surveys. (see ‘before construction’ under monitoring and maintenance).
- Planting schemes should aim to use native, locally (ideally same creek) sourced species in a wide variety. Typically suitable species could include: Sea Aster/Grey Clubrush/Sea Clubrush, hemp agrimony, water ragwort, water mint, water forget-me-not, water plantain. In saline habitats there is less choice but shrubby purslane/sea purslane, sea lavender, marsh mallow, could all be suitable. Don’t plant a single monoculture of reeds.
- Design the intertidal features to ensure that fish can access them as early in the flooding tide as possible, and safely egress them as late as possible on the ebbing tide.
- Avoid creating ponded areas or pools that don’t have a channel or drainage point that allows fish to escape when the tide goes out.
- Maximise the complexity of any new habitat features to create areas of refuge from the main tidal flow for juvenile fish.
- Intertidal vegetation that can be accessed by fish is of high benefit to a range of fish species and lifestages. Fish will use these areas for both refuge and foraging, and in the high intertidal zone they will often feed upon terrestrial invertebrates, as well as aquatic organisms.
- Small and large embayments, side channels and tidal creeks are heavily utilised by fish when they are inundated and should be preserved and recreated wherever possible.
- If space is available, avoiding edges and having a continuum of a gentle slope (for example by zig-zagging or having two sloping planes that coincide e.g. Wandsworth Riverside Quarter) will maximise the range of fish species that can access and therefore benefit from the habitat. This especially applies to benthic, bottom dwelling, species such as flounder and gobies which are reluctant to cross submerged terrace steps.
- Cobbles and large sized substrates in intertidal areas can provide a valuable shelter over the low tide period for species such as eels. These fish can remain out of the wetted channel, on the middle and upper foreshore over the low tide period, whilst remaining in wet areas under the large rocks and cobbles.
Safety and Navigation
- Consider the risk of hitting unexploded ordnance both landward and seaward of the previous river wall by consulting the unexploded ordnance register maps and conducting sonar surveys.
- Consider supplying (and maintaining) additional lifesaving equipment in the area.
- Features should have adequate lights, signs and marking on the front edge for the area of activity in the Thames, to avoid both unwanted mooring and being a hazard to navigation when covered by the rising or falling tide.
- Consider safety features for escaping from the water, such as grab chains.
- Navigational safety marking can double as a geomorphological or litter influence e.g. booms or groynes.
- Ensure that all elements of the design are secure, continue to be monitored and maintained throughout the lifetime of development and will not break free (e.g. timber), becoming a hazard to navigation.
- Construction of terraces behind cofferdams may improve safety for worker and river users as it limits their interactions (see Battersea Reaches).
- If a wall is regularly used for vessels to dock against only vertical wall options can apply and these must avoid horizontal timbers due to the risk of vessels getting caught on them.
Monitoring and maintenance
An important element of continued success of an estuary edge feature is continued monitoring and maintenance which may require appointment of an appropriate organisation. Do use third sector organisations with local expertise.
- You will to need to conduct ecological surveys/appraisal before a development is given planning permission. Building on previous methodologies and either utilising or collaborating with existing monitoring networks is recommended. Inadequate ecological surveys are likely to result in limited ecological value of proposed features
- Consider conducting (or approaching an appropriate organisation to do so) before and after social surveys to gain an understanding for the value of the feature to the occupants of the land/public.
After construction: short term (~Up to 5 years):
- Establishment of any landscape design project includes a defects liability period for any planting: an estuary edge features should be no different. Use this period to monitor the habitat development: sediment accretion and plant establishment (using a suitably qualified and ideally local ecologist) and litter accumulation.
- Remove litter regularly from sites. See litter section to help determine this frequency
- ‘Conduct post construction ecological and social surveys if done so before. Again consider doing this in collaboration with third sector and existing monitoring networks.
After construction: longer term (~5 to 10 years):
- Continue to remove litter regularly from sites.
- Continue to monitor sediment accretion if possible: foreshores in the Thames are typically eroding and early accretion of sediment of the feature may not continue into the longer term, particularly with sea level rise and climate change (see sustainability and adaptability).
- Consider any erosion in the context of whether that sediment formerly provided support to retaining walls. This also applies to water levels e.g. older sheet steel piles (Figure 10).
- Remove invasive non-natives species.
- Treat plants which typically take over e.g. remove spreading rhizones, cut back if appropriate. Note that cutting back some plants causes them to responsd more vigorously.
- Replace softwood which has degraded or fallen off as per guidelines for timber.
- Consider long-term support for post construction ecological and social surveys if conducted before. Again consider doing this in collaboration with third sector and existing monitoring networks.
If the feature requires a large reconstruction at any point in its design life, please contact the Environment Agency (or equivalent regulatory body) to discuss if a permit will required.
Modern urban estuaries are sadly collecting litter, and although this may improve with time (e.g. changes in availability of single use plastic to consumers, interventions to prevent litter entering rivers), this must be considered in the design:
In all cases it is good practice to incorporate access for maintenance and monitoring (e.g. locked or hidden gates) to facilitate removal of litter (as well as other habitat manangement) as per high level design principle number 9.
- Firstly, contact the relevant port or environment authority to ask if they have any information/local knowledge on litter densities at that point in the estuary. If it is thought to be high, consider the following measures in this order:
- Change the angle of entrances/constrict the entrance to reduce the chance of litter being swept in by waves (see waves under geomorphology). e.g. Barking.
- Subject to currents and wave action (see geomorphology) avoid frequent groynes/dividers across the feature which tend to trap litter in every pocket/cell.
- Innovate to develop a litter scoop/collector at the downdrift end of the feature with suitable access for removal.
- If phragmites is likely to dominant in this location (see Plants and Habitats section) consider lower level terraces which may not grow phragmites by default. Phragmites is particularly good at trapping litter as it is so tall.
- If planting (reminder: it advise to allow natural colonisation wherever possible) either plant smaller, lower growing species (not phragmites) OR cut phragmites stems back in the Autumn/Winter to facilitate litter clearance.
- Consider the use of booms to prevent entry into the feature e.g. Battersea. However, booms may cause a problem for fish access.
- Static litter collection barges have been used in the Thames estuary and could be used in conjunction with the feature. Consult the relevant port or environment authority about such initiatives.
Sediment accretion as opposed to erosion (the geomorphology) is key to the success of an estuary edge feature (as per design principle number 4) but it is not simple, involving knowledge of sediment availability, waves and currents. Your consultant should be able to interpret the below information (as per design principle number 4):’ Sediment availability to form habitat through deposition can therefore be a problem and it is important that this is adequately considered.
Unlike the parts of the Thames assessed in Estuary Edges, some estuaries are sediment poor or exporting sediment. Sediment availability to form habitat through deposition can therefore be a problem and it is important that point number 10 is followed to ensure this is adequately considered. Within the Thames, waves and currents present a range of differing geomorphological challenges but there are normally ways to tweak a design to ensure it functions under the required conditions. Where both combine together (which is very rare), accretion of fine sediment and therefore more typical estuarine habitat may be impossible to achieve and a cobble/gravel or bare timber substrate may be the only design achievable. Cobbles and large gravel may not be suitable for growing intertidal plants, but may still provide many refuges for invertebrates.
Consideration of waves:
- In wide open sections of estuary (or the mouths of creeks facing into such) (a rule of thumb is in excess of 100 m in width) waves (both wind and vessel generated) are important factors and good design can help avoid scour/promote accretion and prevent litter becoming a problem.
- Consider the prevailing wind direction (south south westerly in the south east of the United Kingdom) and align this with fetch lengths (open water) to give an idea of wave energy. A rule of thumb is that fetches in excess of 1 km could produce wind waves. These will effect both erosion and litter entering the feature.
- As vessels navigate on the right of a channel, waves generated by them will typically be felt against one end of any set back feature (due to reflection off the wall/embankment). In the Thames Estuary this on the downstream side of the feature on the right bank and on the upstream side on the left bank (see figure).
If either of the above two points are present, the following guidelines can be followed:
- Reduce the angle of the feature’s surface;
- Avoid sharp interfaces/corners which promote wave reflection;
- Plant the feature rather than rely on natural colonisation and do so with wave resistant plants at the front. Reminder: Do not plant the feature if the wave energy is not high!
- In locations with the greatest wave energy, place rocks/rip rap at the front of the feature before any plants.
- coir (coconut fibre) which can be purchased as mats or inside cylinder-shaped nets (‘coir rolls’) and for added resistance to erosion *.
- Consider interspersing rock and pre-planted coir. e.g. Royal Wharf.
- Try to make terraces deeper to avoid wave reflection from the back. A rule of thumb is that open estuary (>100 m widths) ledges must be deeper than 1 m to prevent wave reflection of the back wall.
- Consider innovation to intercept wave energy with wave absorbing booms/islands subject to navigation requirements.
- If the feature is fronted by a cobble/gravel foreshore which is only marginally beneath the level of the feature, consider groynes at the up drift and down drift ends to prevent movement of this material across the feature and smothering/eroding any plants (more regular groynes may trap more litter)
Consideration of Currents:
Currents are important, particularly in:
- More freshwater dominated sections of the estuary (upper estuary or tributaries);
- Encroachment has occurred significantly in the past (and the proposed feature is still partially proud of the normal river bank line) (e.g. Figure 6);
- On the extreme outside of a meander bend (e.g. Figure 6);
- Interaction with bridges, flood barriers, jetties and pontoons (e.g. speeding up of currents up or causing eddies).
Where currents are likely to be a problem, the following guidelines can be followed:
- Reduce the angle of the feature’s surface (also applies to high wave energy).
- Plant the feature rather than rely on natural colonisation and do so with plants resistant to faster currents. Reminder: Do not plant the feature if current energy is not high!
- Coir (coconut fibre) which can be purchased as mats or inside cylinder-shaped nets (‘coir rolls’) and for added resistance to erosion *.
- Place groynes on the feature, especially if it is marginally beyond the normal line of the river wall i.e. a large amount of encroachment has occurred in the past) e.g. Point Wharf, Greenwich. However, this design needs to be thought about carefully if waves are likely to wash litter into these groyne cells.
- In lower wave energy environments, use brushwood * (generally Hazel) bundled together with biodegradable cord to slow currents and gather sediment. This is particularly effective when entrenched in a woven pattern between closely placed driven poles to create robust low ‘fences’ g. Dartford and Barking Creeks.
*Any additional materials (e.g. brushwood, coir rolls, or matts) beyond fill and planting should be adequately anchored to the bank with timber stakes.
Steeper banks, faster currents and greater wave energy will require longer stakes. Willows, Hazel, Sweet Chestnut or Ash can be used for stakes. The length will vary from 1–1.5m and the diameter from 40–60mm. It is common practice to incline the stakes at about 60 degrees to the vertical.
Waves and currents will increase with sea level rise and climate change as outlined in sustainability and adaptability.
Sustainability and Adaptability
Try to use recycled materials in designs wherever feasible (avoiding plastics and following guidelines for timber), ideally reusing materials from the construction site to avoid transport. For example, the local post authority mayt have preused old hard wood from demolished jetties
Estuary edge features should be constructed so that flood defences behind them can still be adapted and raised. With naturalised set back this would mean ensuring there is enough space behind an earth embankment to widen it’s footprint landwards (thereby allowing an increase in it’s height) without effecting the fronting ecology. With intertidal vegeated terraces and wall options, this would generally mean avoiding supporting anything off the crest of the backing flood wall so that it can be alterated without effecting the fronting ecology.
Use locally robust habitat materials that are proven to work elsewhere on tidal banks. If using concrete then consider low carbon options.
In areas of the planet subject to sea level rise, the zone for plant growth between Mean High Water Spring and Mean High Water Neap tidal levels will gradually shift to a higher elevation. Areas of naturalised set back are likely to become increasingly less vegeted with time but the effects are likely to be felt the most on vegeated intertidal terraces and wall options as, without adaptation, vegeation may be lost completely. The retaining wall of intertidal vegetated terraces should be easily increased in height e.g. the ability to fit more boards so that sediment accretion can continue upwards. Similarly drainage should be able to be raised with time e.g. moving weep holes higher up. With wall options, timbers/roughed concrete should continue to elevations above the current Mean High Water Spring tidal level.
Sea level rise and increased fluvial flows from climate change will result in faster currents. Increased water depths and increased vessel traffic are likely to result in greater wave energy. Sites should be designed with this in mind (see geomorphology and litter) and a key part of this is good monitoring and maintenance.
Use of Timber
- Where wood is being used as a structural i.e. retains fill or sediment (Figures 13a, b and c), navigational or aesthetic element it should be durable enough to last the lifetime of the development. Hard wood is therefore advised. Structural/navigational/aesthetic elements can be made more attractive to wildlife through grooves and other features but if you can support regular replacement of timbers through monitoring and maintenance, a better alternative is to fit sacrificial ‘habitat’ softwood (e.g. untreated pine) on top which will be rapidly valuable to wildlife but has a limited (5 to 10 year) lifespan.
- Provide evidence in your application that the timber used is suitably durable for it’s purpose. Information on timber types and their longevity in the marine environment can be found in a 2011 Environment Agency report.
- Treated timber should be avoided due to their risk to river life from toxic wood preservatives leaching into the river.
Securing methods should be in the form of brackets surrounding the timber (Figure 14a), so that as its integrity deteriorates it is unlikely to break free and become a risk to navigation (Figure 14b).