|March/April 2015||Vol. 19
Environmental monitoring for a deepwater port
By Kevin Black1, Daniel Leggett2 and Katie Read3,
1Partrac Ltd, Scotland, 2DLEnviro Ltd, UK, (previously
Dredging International NV) and 3Dredging International
Dredge monitoring and oceanographic specialist Partrac Ltd was commissioned by Dredging International to deliver one of the largest marine monitoring networks associated with environmental compliance for the development of the London Gateway Port. This paper presents how matters relating to potential increases in suspended sediment concentrations were monitored, managed and mitigated for
As an island nation, an estimated GB£524 billion worth of goods are imported through British ports every year. The recently opened London Gateway Port development, located at the old Shell Haven site on the north shore of the River Thames and developed by global port infrastructure developer DP World, UAE, combines one of Europe’s largest logistics parks with the UK’s most advanced deep-sea port with an annual capacity of approximately 3.5 million TEU (twenty-foot equivalent units). Unique in its scale and ambition in the UK, it is positioned to contribute hugely to international trade and to the national economy.
The port will ultimately provide 2700 metres of quay, six deepwater berths with alongside depths of 17 metres and 24 giant quayside cranes. The total dredge, undertaken by marine dredge contractor Dredging International, was approximately 30 million square metres of material, and included silty-sand, sand, gravel and cohesive clay sediments. Approximately 16 million cubed metres of the dredged material was used in reclaiming (creating) land to construct the port platform, and five million cubed metres used beneficially in aggregates supply and land raising (for the logistics park behind the port). The dredging and reclamation commenced on 18 March 2010 and was completed on 8 March 2014, but the project included a phase (approximately one year) of pre-dredge monitoring.
Prior to the development, an environmental impact assessment (EIA) was carried out as part of the public inquiry process. The EIA identified the potential for increases to suspended sediment concentrations (SSC) in the Thames and associated changes in sedimentation, erosion or accretion (as well as the potential for reductions in the dissolved oxygen concentrations). Monitoring of the Thames and dredging and reclamation activities in respect of SSC was identified as a mitigation measure in the EIA and the real-time control of this aspect is the matter this article focuses upon. This is termed ‘compliance’ or ‘adaptive monitoring’, as it is predicated upon the implementation of limiting or ‘threshold’ SSC values to control the dredge processes. Such requirements are now a common facet of contemporary dredging activities.
The precautionary approach adopted for the London Gateway Port project resulted in the largest and most complex marine compliance monitoring programme ever seen in the UK and possibly Europe. This stretched over 32 kilometres of the estuary, and included an array of high-tech buoy-based systems delivering data every five minutes, characterisation of sediment plumes emanating from the work (for different sediment types, dredgers, activities and seasons), weir (water) boxes used to discharge water from the reclamation area, and an extensive water and sediment quality sampling programme. Over the five years of monitoring, more than 200,000 man-hours were dedicated to maintaining the monitoring system between the practical management of buoys, sensor platforms and instruments in the estuary, to sample analysis, website handling, to management of the operations as a consequence of the result, and to reporting daily to the client and regulators. Dredge monitoring and oceanographic specialist Partrac Ltd was ultimately commissioned to implement and deliver the programme alongside Dredging International, which managed the programme, real-time control of the works, identified analytical needs and interfaced with London Gateway Port, its advisors and the regulators overseeing the development.
One of the environmental mitigation measures was to place a network of 11 buoy monitoring stations around the dredging and reclamation activities, placed in-between the dredging and reclamation operations, and identified sensitive areas (including nature conservation sites, shell fisheries and industrial interests) to landward (Figure 1). This network was known as the ‘Red Line’, and each buoy recorded conductivity, temperature, dissolved oxygen concentration and SSC – some buoys included, in addition, current velocity (Figure 2). The measurements were taken one metre below the surface in the inner-most part of the estuary to reflect the body of freshwater flowing above the saline water in this location. To seawards, measurements were made one metre above the bed to reflect the potential of sediment disturbance there, to detect any increase in SSC due to discharges from the reclamation moving along the bed, and to measure the higher values in SSC found in that part of the water column. Certainly ambitious in its conception, the Red Line extended from upstream of the reclamation site axially down the estuary, and directly over the bed area to be dredged and deepened, to the outer estuary. The very nature of the monitoring –i.e., to adjust, change or stop the dredging process if SSC values increased above pre-determined thresholds – coupled to the stark commercial consequences of the slowing or cessation of dredge operations to Dredging International and ultimately London Gateway Port, meant that a truly robust network of monitoring stations was required, together with a hitherto unprecedented project management, data transfer and management, quality control and reporting framework.
SETTING THE THRESHOLD SSC VALUES
To control the works, an approach to thresholds was identified to be applied in real-time. The requirement was to identify a means of controlling operations in real-time and stopping works should a ‘Stop’ threshold be exceeded for more than 15 minutes. The issue of setting workable thresholds was not a trivial matter and considerable work on the issue was undertaken within the EIA process. It also took many months to determine a detailed working methodology through analysis and testing and regulators review. The Thames presents challenging environmental circumstances: it is a highly turbid estuary to start with, with significant spatial (the works zone spans the inner estuary which is far muddier and the outer estuary is sandier) and temporal (daily, spring-neap) variability in the SSC signal. Further, the concentration field is affected by seasonal freshwater inputs.
Thresholds for SSC were defined in a compound way. ‘Caution’ levels were defined at a level to provide pre-warning of a Stop level. Through analysis of the data that had been captured by the developer during the EIA process, and by the contractor pre-dredging, it was possible to determine the rate of change that generally might occur in the data. The setting of a Caution level aimed to allow sufficient time to be able to react to any changing SSC events whilst not being so sensitive that an unnecessary (spurious) amount of ‘cautions’ were triggered.
The Stop level was defined (pre-set) by the regulators for surface monitors to be 1000 mg l-1. The Caution level was set at 700 mg l-1 to provide the pre-warning although the analysis did show that in some circumstances natural changes would cause steps in SSC that exceeded this (i.e., steps direct from <700 to >1000 mg l-1). The Stop level is in the context of the Thames Estuary background levels, which is a notoriously turbid system, and measurements above 4000 mg l-1 exist in the baseline data (particularly at the estuary bed).
The bed monitoring positions all applied the 700 mg l-1 caution and 1000 mg l-1 ‘above background’. This above background concept took many months to determine. Through an extensive period, which involved detailed analysis of pre-dredge SSC time series data from various network locations (by HR Wallingford, UK, and Dredging International), it was determined that the average of the previous two tides yielded a reasonable measure of the ‘background’ conditions. It was then possible to extract that data for the point in time of the tidal cycle from the previous two tides by determining the point in time relative to an aspect of the tide.
The measurements (Figures 3 and 4 – alongside the many other measurements not presented here) made between March 2009 and March 2014 have demonstrated, in spite of slightly elevated SSC during Year One dredging, compared to the pre-dredge baseline year, that the levels of change in the environment fall within or below those identified as ‘potential impacts’ through the extended environmental impact assessment process (environmental statement, public inquiry, regulatory requirements). This programme (coupled with the much wider programme of monitoring not mentioned here) has formed a particularly valuable body of information, which is of use to regulators, dredging contractors and marine scientists. In particular, information like that collected here allows a judicious and objective assessment of the question “how much monitoring is enough?”, which is key to the management of similar projects on the part of the regulators. As such, the London Gateway Port project, which received a number of environmental management commendations, allows for more informed decisions and more realistic assessments of impacts to be made in the future.
© 2017 Underwater World Publications Ltd