UK Power Networks needed to install a new underground 33kV electricity line near Thamesmead in London. They chose two potential routes from Linton Mead to the Sewage Treatment Works and the final decision was to be based on the potential constraints including those relating to ecology.
ADAS was appointed to undertake a desk study and conduct a walk-over survey of both routes in late summer 2010 to identify potential or actual ecological constraints and recommend a preferred route.
The desk study revealed that much of the Crossness Southern Outfall Sewer Bank was a Site of Biological Importance, a locally designated site known as ‘The Ridgeway in Greenwich’.
There were local records of reptiles present on an adjacent site at the northern end of the route at Linton Mead.
The walk-over survey itself revealed habitats with potential to support reptiles and nesting birds particularly where large sections of the Southern Outfall Sewer Bank was covered in dense mature scrub with some open grassland.
There was also a large spoil embankment at Linton Mead which had become colonised by tall ruderal vegetation and scrub which was ideal habitat for reptiles. The proposed works involved levelling the embankment and this was clearly a potential risk to reptiles.
It was also possible that the Southern Outfall Sewer embankment could be used by reptiles, partly to hibernate under the dense scrub. This could potentially have resulted in reptiles being excavated or exposed during the trial pits and trench works proposed over the winter and early spring period.
However, since only a narrow (600mm) trench was required, provided this was excavated in suitable conditions, then risks to reptiles could be managed without recourse to detailed surveys and potential capture and translocation mitigation works which would almost certainly have delayed the start of the works.
Added to that was the presence of Japanese Knotweed in places along the banks and on the level ground through which the trench would be installed.
The main challenge was to ensure that all scrub and other vegetation were cleared before the end of October prior to reptiles going into hibernation, to clear the woody scrub and bramble outside the main bird nesting season and to prevent the potential spread of Japanese Knotweed. In particular, the potential risk of Japanese Knotweed being spread along the line of the trench works was considered to be high risk.
All of these constraints had to be addressed to ensure that all relevant EU/UK legislation was abided by and that the proposed works timescales could be met.
Added to this was the need to meet the requirements of Thames Water and ensure that vehicles accessing the area did not damage the old Victorian sewer pipe in the bank.
ADAS prepared detailed method statements for the works and preventing the spread of Japanese Knotweed.
A reptile survey was carried out in September 2010 at Linton Mead with the premise that if reptiles were found, the survey would be stopped immediately and a capture & relocation exercise would be undertaken to ensure that reptiles were removed off site before the end of October.
In the event no reptiles were recorded at Linton Mead and scrub and vegetation clearance could proceed unhindered solving not only the potential risk to reptiles but also the potential for nesting birds to be present in the spring when works were scheduled to begin.
Where the trench works were proposed, scrub was cleared slowly and progressively under the watchful supervision of an ADAS ecologist in October 2010 over a two week period. First all areas and individual plants of Japanese Knotweed within 7m of the proposed trench route were located and fenced off using high visibility tape and their locations recorded using a GPS with photographs.
A 10m wide swathe of mature scrub adjacent to the sewer bank was cut, sawn into logs with brash chipped on site. Log piles were created and chippings and brash piles were also left on site, but outside the working area, for the benefit of wildlife. In addition any arisings left over were raked and placed on the banks to leave bare ground along the working area.
The Japanese Knotweed stands were left uncut and treated with herbicide as part of a planned treatment programme to ensure that the integrity of the installation was protected. Although late in the season, it is hoped that the herbicide treatment would have some impact but a follow up treatment planned for the spring (when new growth began) ensured its demise with a contingency plan of further treatments over a couple of years.
During clearance of the dense impenetrable scrub, the bank was surveyed more thoroughly and two holes were discovered in the bank which was suspected of being a Badger sett. The ecologists placed sand outside each hole so that the activity around the sett could be monitored, as the presence of an active sett could have potentially delayed the works with the closed season fast approaching.
Despite finding evidence of fresh Badger and Fox prints in the sand no evidence to support current use of the sett was found over the period monitored and after discussions with Natural England, the sett was successfully closed and proofed without any delays to the works along both schemes.
Overall, the ecological and scrub clearance works were organised and completed within a tight timescale to give UK Power Networks the opportunity to conduct their works without delay, saving time and money and provided a ‘permanent’ solution to all the potential ecological constraints identified ahead of and during the works.
For more information please contact Ann Sherwood on 07774 141780 or email Ann.Sherwood@adas.co.uk.