Deptford Creek, Thanet Wharf
Grid Reference TQ3774177544
1.1 Site Conditions Summary
|Salinity (during low fluvial flows)||At creekmouth ~4.8‰ (Wade, Hawes and Mulder, 2017)|
|Date of construction||2009|
|Percentage of area accreted above design level||79 %|
|Degree of exposure to waves:||Low|
|Degree of exposure to currents:||Medium (fluvial currents)|
|Max wave height|
|Slope direction||South South East|
|Average Whole Structure lifespan||20-50 Years|
Table 1: Site conditions summary.
1.2 Site Characteristics
- Located approximately 540m upstream of the tidal mouth of the River Ravensbourne, this site is sheltered from wave energy.
- The River Ravensbourne flows predominantly through urban south east London but the headwaters are rural. It is therefore ‘flashy’ in its discharge and fluvial currents are at least medium.
- Salinity is semi-maritime at an average of around 2‰.
- A 37m stretch of old timber wall was in danger of collapse and therefore needed renewing. This was conducted by the Environment Agency.
1.3 What the Environment Agency did
- For the 37m long frontage, sheet steel piles were sunk approximately 3m landward of the original timber wall. Along the line of the original wall, ‘H’ piles sunk to mean high water neap tide level. These piles were fitted with softwood planks and backfilled with concrete to form a terrace.
- Between 0.6m and 1.8m beneath the front edge of the terraces, the softwood planks between the ‘H’ beams were used to form a recessed ‘ecological box’.
- For the eastern half of the terrace (approximately 18m), a further series of ‘H’ piles were sunk 2m in front of the new sheet steel piles terminating at a higher level of approximately 3.77m ODN. These piles were also fitted with softwood planks and backfilled with concrete, forming a double terrace.
- Both single and double terraces were topped with 0.3m of alluvium and 0.2m of sandy gravel.
- Vertical soft timbers were secured periodically to the sheet steel pile walls.
- No pre-planting was conducted.
The immediate area of river in front of the terrace is a silty canalised creek, with a small low flow channel in the centre.
During larger tides (spring) the terrace has water on it for over two hours and 48 minutes, while the top of the terrace is covered for just over one hour. During smaller (neap) tides the terrace has water on it for just over half an hour and the top of the terrace is only reached for 14 minutes.
Saxon Wharf was specifically chosen as a comparison site to Thanet Wharf due to the difference in light availability but similar construction. Whereas Thanet Wharf is South East facing, Saxon Wharf is directly opposite and North West facing.
Unfortunately, fish surveys were not conducted on Thanet Wharf and therefore a comparison with the fish findings at Saxon Wharf cannot be made.
Botanical surveys were not conducted on the terrace at Thanet Wharf and therefore no species information exists. However, the geomorphology surveys recorded the terrace coverage at 5% phytobenthos and 25% Macrophytes (the rest being bare mud). Although the percentages are higher than found at Saxon Wharf and this is likely due to the amount of light available, there is still not extensive botanical coverage. This is thought to be due to water logging.
The habitat type is Littoral Sandy Mud (Ls.Lmu.Mest.HedMac).The invertebrate assemblage present suggests that the succession of this habitat has progressed to the 5th level (Figure 7) of 6 possible biotopes on the Joint Nature Conservation Committee classification system for marine habitats. This suggests that the habitat is performing well.
79% of the accretable area has accreted with sediment above the design level (Figure 8). The sediment on the lower terrace is thought to be too water logged.
Contributing reasons for this include:
- 3 softwood timber boards forming the edge of the upper terrace have failed causing loss of fill (most of the 21% of washed out/disturbed sediment/no accretion) (Figure 9 and 10). A further board will soon fail (Figure 11).
- Sheltered from wave energy.
- Washed out sediment from behind the ‘H’ beams where drainage occurs (2% to 3%) (Figure 12).
- Limited depth of gravel/sand fill before concrete commences (explaining the waterlogging).
4. Social, Litter, Safety and Navigation
Questionnaires were completed adjacent to the estuary edge sites that had public access. The questions sought to understand how the Thames and intertidal green spaces were viewed and understood. These questions tell us about different value sets that can be measured. Surveys were conducted on Deptford Creek footbridge and are representative of all three Deptford creek estuary edges. Figure 13 shows us that residents and passers’ by (samples size:12 people; almost 1:1 male:female ratio; aged between 15-75 years old) were on average positive about the terraces and the Thames in general.
As part of the questionnaire, we asked them what the first word that came in to their head was when thinking of the Thames at the beginning of the interview and again at the end after they’d answered all the questions. 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.
Unlike Saxon Wharf, there is much greater coverage of vegetation, but this terrace has still gathered fairly limited litter (Figure 15). This may be because Deptford creek is so sheltered from wave energy and it is this force which washes litter into the vegetation.
4.3 Safety and Navigation
The terrace is set back well from the river, and marked with clear ladders, signage and cones. It would not necessarily appropriate on the main estuary as additional marking and lighting might be necessary, as well as life saving equipment.
|Structural Element||Assessed Condition||Expected lifespan||Recommendations|
|Sheet steel pile river wall||No obvious signs of distress.||20 to 50 years*|
|Steel ‘H’ piles||None given.||20 to 50 years*|
|Softwood timber||None given.||20 years||Replaced with hardwood instead.|
|Terraces consisting of 200mm thick layer of sandy gravel and 300mm thick layer of alluvium
above a concrete base.
|Water logged||Not specified.||Greater depth of alluvium and drainage on terraces.|
Table 3: Table showing individual engineering elements of Thanet Wharf and their expected lifespan.
*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.
Potential engineering improvements/ refinements
- Replace softwood with recycled hardwood throughout to increase residual life.
- 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 wall).
- Provide greater depth of gravel/ alluvium and drainage on terraces.
6.1 Within the first 9 Years since Construction (2009 to 2018):
- The good accretion of sediment on these terraces combined with the luxuriant vegetation growth and great habitat progression as shown by the invertebrate assemblage makes this site a great success.
- Softwood used to retain fill on the upper terrace has failed. Although there is no sign of this at Saxon Wharf, this is anticipated. This leads to high level design Principle 13: regarding the lifetime of the structure needing to be the same as the projected lifespan of the development (this is particularly important regarding timber). Timbers holding back fill should be hard wood.
- Water logging on the lower terrace is a minor problem, alleviated by the amount of sun this site gets. A greater depth of fill is required above any concrete. This is described in the guidelines specific to intertidal vegetated terraces.
- The site performs well for litter and safety and navigation.
- A monitoring regime is required.