Grid Reference TQ2624075602
1.1 Site Conditions Summary
|Salinity (during low fluvial flows)||~1% (Wade, Hawes and Mulder, 2017)|
|Date of construction||2005|
|Percentage of area accreted above design level||33 %|
|Degree of exposure to waves:||High|
|Degree of exposure to currents:||High|
|Max wave height||1.25 m|
|Slope direction||North-North West|
|Slope angle||1 in 4 normal to flow, horizontal parallel to flow.|
|Average Whole Structure lifespan||20-50 Years|
Table 1: Site conditions summary.
1.2 Site Characteristics
- Here the estuary is typically less than 1% of the salinity of seawater, but true saltmarsh plant species are known to occur in places.
- South bank of Thames just north of Wandsworth Bridge on the outer bend of a meander meaning exposure to currents is high.
- Wave energy is high with a significant fetch to the west allowing the prevailing south-south-westerly wind to generate waves
- The area is used by both commercial inland vessels as well as recreational users. Predominantly the recreational use is rowers, kayakers as well as motorboats.
- The upper estuary location means the channel is narrower and therefore vessel generated waves are significant.
- Adjacent to the Thames Path and a built up residential area, high visibility from passers by and residential flats.
- An Interpretation panel is present consisting of ecological information.
1.3 What the developers did
- Flood defence wall was rough cast face to encourage colonization although much of this was too high to do so.
- Designed for inter-tidal environment with 50 year design life.
- The design was similar to that of the Greenwich terraces, but due to the perceived erosion risk, a gabion mattress slope was built with pre-established coir pallets placed inside the lid.
- The lower slope was planted with inter-tidal species and the upper slope with more general freshwater bankside species.
- A rubber boom (approximately 1m deep) was installed along the edge of the terrace designed to float up on a rising tide and block flotsam and jetsam from entering the site.
The immediate area of the river in front of the terrace dries during some tides (see chart extract). During larger tides (spring) the terrace has water on it for over four hours while the top of the terrace is covered for one hour and fifty minutes. During smaller (neap) tides the terrace has water on it for two hours and forty minutes and the top of the terrace is only reached for 15 minutes.
This site is a few hundred metres east of Wandsworth Riverside Quarter and is of comparable size. The fluid dynamics and water chemistry should be equally comparable.
Very few fish were caught at this site in comparison to most other sites (although were similar to nearby Wandsworth Riverside Quarter).Species caught on the terrace included: 3 European Eels, 1 Bass, 1 Common Goby and 2 Roach. On the foreshore (control), 3 European Eels and 1 Roach were caught. Factors believed to contribute to this include:
- The eroded substrate and associated lack of vegetation result in a lack of habitat for fish to forage and shelter in.
- The vertical edge of the terrace may be discouraging some of the benthic fish species from accessing the intertidal area on the terrace.
- The rubber barrier hinders entrance and exit from the site and potentially cause fish to be stranded behind the boom.
- The seine netting during the survey was hampered by very large quantities of leaf litter, woody debris and some plastics.
Plant diversity was much lower than found at the adjacent site (WRQ) and is site is dominated (Table 1) by broad-leaved dock (Rumex obtusifolius) and the common Michaelmas daisy (Symphyotrichum x salignus), which is a hybrid aster and often considered invasive. Overall, the site has an unusual ecology which may merit further investigation.
- Both club rush species lacked fruiting material indicating poor health.
- Phragmites was poorly established except in the front sections which may be due to a lack of light in the middle and rear.
- The creeping buttercup (Ranunculus repens) here are unusual and have some morphological traits that lead London botanists to refer to this plant as the ‘Thames buttercup’.
|Planted||Natural Colonised||Dominant||Notable species|
|common reed (Phragmites australis),||remote sedge
|yellow loosestrife (Lysimachia vulgaris)|
|common reed (Phragmites australis),||pink water-speedwell
|common Michaelmas daisy (Symphyotrichum x salignus)|
|club-rush (Schoenoplectus sp.).||water ragwort
(Senecio aquaticus subsp. aquaticus)
Table 2: Showing the planted, assumed naturally colonised, dominant and notable species found at the site.
The invertebrate assemblage present suggests that the succession of this habitat has not progressed beyond the 2nd tier (Figure 7) of 5 on the Joint Nature Conservation Committee classification system. Despite the length of time since construction, this indicates that this site is not performing that well.
Figure 7: Joint Nature Conservation Committee classification system code for the habitat based on the invertebrate assemblage.
Just 33% of the accretable area has accreted with sediment above the design level (Figure 8). The underlying substrate within the gabion baskets has been washed away, to a depth ranging from 5cm up to 30cm in places. Much of the erosion is along the back of the terrace suggesting wave return off the back wall.
Contributing reasons for this include:
- Significant fetch to the west.
- Vessels travelling on the right of the channel generate wave wash up alongside the upstream facing wall preventing sediment accretion at the front of the terrace and depositing debris at the back.
- The baffle designed to prevent litter and debris ingress have done little to prevent wave energy washing out fill. Although there appears to be no further wash-out in 2017 than was observed in 2006 (Figure 9).
- Currents – outside of the meander bend.
- The slope is too steep.
4. Social, Litter, Safety and Navigation
12 people were interviewed (50:50 male:female; aged between 20-75 years old). On average people were positive about the terraces.
We asked them what the first word that came in to their head was when thinking of the Thames at the beginning and again at the end after the questionnaire. This showed a positive perception shift from neutral or negative to positive. This indicated that education and interpretation of the site would increase people’s connection to the terrace and the river.
A few pieces of litter were observed but the terrace was generally clear (Figure 14). However, it is possible that the site is cleared of rubbish regularly by the development.
4.3 Safety and Navigation
The structure does not create much risk to navigation and human safety, due to:
- it is set back into the river wall
- the depth allows the energy of the river to be dissipated
- the boom and timbers combined with the height of the terrace, relative to the tide levels, minimises the risk
- the access is limited and separation of the area from public space is robust, minimising the risk of trapped persons
- its height makes it unlikely as a point of egress from the river by those needing to exit
Signage would have been an additional enhancement to reduce risk further.
|Structural Element||Assessed Condition||Expected lifespan||Recommendations|
|Flood defence wall||20-50|
|Tidal Flap Valves||Good||20-50||Replace after 20 years (subject to manufacturers requirements)|
|River wall toe||Not able to assess*||20-50||Further inspection needed – see recommendations|
|Shingle foreshore||Levels acceptable||N/A|
|Reinforced concrete slab||Not able to assess*||20-50|
|Gabion revetment||Good (except fill loss)||20-50|
|Baffle||Moderate – some sections appear to need repair/replacement||<20||will require maintenance and periodic replacement (subject to manufacturers’ requirements)|
*where it was not possible to assess the element, assumptions have been made that it was designed for inter-tidal environment with a 50 year design life.
Table 3: Table showing individual engineering elements of Battersea Terraces and their expected lifespan.
Potential engineering improvements / refinements
- Cutting down pre-existing structures to provide toe details for intertidal terraces presents a risk due to uncertainties/ lack of available information on original date of installation, design details and design assumptions (with respect to issues such as loading and assumed minimum foreshore levels). It is therefore difficult to predict residual life of these vital structural elements.
- Avoid the use of vertical structures, in particular interfaces/ corners between vertical structures which result in wave reflection and localised erosion.
- Install rock rolls/ gabions immediately inshore/ riverward of toe and at interfaces/ corners between vertical structures to reduce scour due to wave reflection off vertical structures. Consider utilising materials which will bio-degrade once the intertidal habitat has established and rock infill which will be stable under wave exposure conditions without the requirement for gabions (gabions have a limited lifespan, due to the potential for corrosion/ abrasion).
- Install pre-grown intertidal habitat planting (which is tolerant to wave action) with suitable substrate (avoid use of gabion mattresses) and drainage.
- Establish a suitable monitoring and maintenance regime for the intertidal habitat/ structures/ foreshore (monitor foreshore/ nearshore levels to identify any future potential risk of lowering which could result in undermining of the river wall at the toe of the terrace).
6.1 Change to feature in the 3 years since construction (2005 to 2007):
- The presence of the baffle, the concrete capping and the edge of the gabions created a significant blockage to all fish and 2 edges which deter bottom dwelling fish (e.g. Flounder and Common Goby) from entering the feature.
- After two growing seasons the upper half of the terrace had established well. The coir pallets used in the upper terrace were more mature than those used in the lower areas and this may have affected their establishment.
- However, significant loss of fine sediment from within the gabion mattresses in the lower part of the slope occurred and few plants established. Little new sediment was deposited over the monitoring period. This sediment loss below the matting then led to movement of the matting and the plants were then pulled against the gabion mattress lids and damaged or severed. Fine substrate loss then continued up the terrace leading to failure of most of the planting.
- Grazing of plants by waterfowl and smothering of plants with tidal litter may also have also had adverse impacts on establishment.
- After two-and-a-half years there was sediment loss throughout the revetment, though significant natural colonisation by a variety of bankside species including Great Willow-herb, Gypsywort and a wide variety of ruderals.
- Fine sediment was beginning to accumulate in the lower parts of the terrace with reasonable growth of Sea Club-rush and Grey Club-rush.
- Alternatives here might have been to follow the Greenwich Peninsula technique more closely, not using gabion mattresses, ensure all coir pallets were better grown, and add further fine sediment trap features such as brushwood fascines and to remove flotsam and jetsam promptly.
6.2 Change in the feature in the last 10 years (2007 to 2017):
- Some parts of the baffle are not lifting properly making it even more difficult for fish to access.
- Poor fish, plant and invertebrate diversity indicate that this site is not performing well.
- Some new plant species have appeared including the creeping buttercup (Ranunculus repens) which makes this site unique and still important for biodiversity.
- Sediment accretion has not improved since the 2006 survey but is thought to be about the same.
- Concern of wire breaking free from gabions leads to new conclusions to avoid use of gabions in future designs.
- The gabion wire surface combined with the sedment washout has made maintenance very difficult.
- The site performs well for litter and for safety and navigation.
- Reducing the slope angle, adding taller groynes to help intercept currents, adding rock rolls (particularly to the rear of the terrace) to intercept wave energy and avoiding the large vertical wall at the back (to reduce wave reflection) could help.