Deptford Creek, Saxon Wharf
Grid Reference TQ3772477474
1.1 Site Conditions Summary
|Salinity (during low fluvial flows)||At creekmouth ~4.8‰ (Wade, Hawes and Mulder, 2017)|
|Date of construction||2009|
|Tidal Range||6.7 m|
|Percentage of area accreted above design level||85 %|
|Degree of exposure to waves:||Low|
|Degree of exposure to currents:||Medium (fluvuial currents)|
|Max wave height|
|Slope direction||North West|
|Average Whole Structure lifespan||20 to 50 years for steel piles and 20 years for softwood elements.|
Table 1: Site conditions summary.
1.2 Site Characteristics
- Located approximately 600m 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 58m 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
- At either end of this frontage for distances of 10m and 30m, sheet steel piles were sunk at the rear of the wall and ‘H’ piles sunk to the front to mean high water neap tide level. These piles were fitted with softwood planks and backfilled with concrete and topped with 0.3m of alluvium and 0.2m of sandy gravel to form a terrace varying between 2 and 3m wide.
- The central remaining stretch of 17m was fitted with sheet steel piles to full height only at the front (no terrace).
- Vertical soft timbers were secured periodically to the sheet steel pile walls.
- Between 0.6 m and 1.8 m beneath the front edge of the terraces, the softwood planks between the ‘H’ beams were used to form a recessed ‘ecological box’.
- 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 top of the terrace is covered for two hours and 48 minutes. During smaller (neap) tides the terrace has water on it for 14 minutes.
Thanet Wharf was specifically chosen as a comparison site to Saxon Wharf due to the difference in light availability but similar construction. Whereas Saxon Wharf is North West facing, Thanet Wharf is directly opposite and South East facing.
The largest amount of fish were found at this site, although with lower findings in diversity. Most of the fish recorded were not on the estuary edges feature, but the foreshore, emphasizing the importance of creek habitats. Species found included: Bass (Dicentrarchus labrax), Common Goby (Pomatoschistus microps), and Bream (Abramis brama). Only Bass and Goby were found on the estuary edges feature. Seine netting was also completed from the slipway in the water column at the Creekside Centre and is representative of the fish species which can be found and will utilise the estuary edges sites when water covers them from 1m depth. The high density of fish found, low levels of diversity, and greater number in the creek foreshore as compared to the estuary edge feature, was likely due to:
- Tidal creeks providing refuge for adult and juvenile fish in the estuary (Colclough et al, 2000)
- A small window of tidal access for fish to safely access the tidal walls for feeding and high risk of stranding due to a lack of gradient and drainage channels
No botanical surveys were conducted on the terrace at Saxon Wharf due to a lack of vegetation. General cover in algae can be seen but was not identified to species level. However, the geomorphology surveys recorded the terrace coverage at 19% phytobenthos and 1% Macrophytes (the 1% was likely to be just one plant). The low percentages are most likely to be due to lack of light on this north west facing terrace but the water logging also plays a part.
The habitat type is Littoral Sediment (LS.). The invertebrate assemblage present suggests that the succession of this habitat has not progressed beyond the 2nd level of a possible 6 biotopes on the Joint Nature Conservation Committee classification system for marine habitats. This suggests that the habitat is not performing well.
85% of the accretable area has accreted with sediment above the design level (Figure 8). The sediment on the terrace is thought to be too water logged.
Contributing reasons for this include:
- Sheltered from wave energy (Figure 9).
- Washed out sediment from behind the ‘H’ beams where drainage occurs (the 15%) (Figure 10).
- 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 12 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.
The intertidal terrace has not gathered litter (Figure 13). Although firstly thought to be due to the lack of vegetation (limited light – north facing), the near identical south facing site of Thanet Wharf, which has much more vegetation, has also gathered limited litter. It is therefore thought to be more a reflection of the sheltered nature: the lack of wave energy does not wash litter onto the terrace.
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 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 Saxon 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.
- 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 (2009-2018)
- The good accretion of new intertidal mudflat on this terrace which is utilised by fish makes this site a reasonable success but the lack of succession of the habitat as shown by the invertebrate assemblage and lack of vegetation limits biodiversity.
- As the site is north facing, vegetation growth will always be limited. The Wildlife Planting and Greenspace section stresses this expectation and measures to help work within the constriction. However, the presence of fish show estuary edges still have value in less optimal conditions.
- Water logging is the biggest problem. A greater depth of fill is required above any concrete. This is described in the guidelines specific to intertidal vegetated terraces.
- Although there is no sign of the softwood failing, this is anticipated as it has failed at Thanet Wharf. 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.
- The site performs well for litter and safety and navigation.
- The site performs well for social perceptions with people clearly noticing the green spaces and valuing the area.
- High social value suggest that improved interpretation could enhance their experience and educate passers-by.
- Little can be done to increase the vegetation to improve fish diversity using the estuary edges feature due to the lack of light.
- A monitoring regime is required.