Integrated Report on Environmental Aspects of Cork Harbour Dredging July 2013 Integrated Report on Environmental Aspects of Cork Harbour Dredging July 2013
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Integrated Report on Environmental Aspects of Cork Harbour Dredging July 2013 Integrated Report on Environmental Aspects of Cork Harbour Dredging July 2013 Volume 1 1.0 Executive Summary 1.1 General 1.2 Maintenance Dredging 1.3 Environmental Monitoring 1.4 Findings of all studies 1.4.1 Fluorescent Particular tracer study 1.4.2 Sediment pins 1.4.3 Multi-Beam echo sounding 1.4.4 In situ water sampling 1.4.5 Monitoring buoys 1.4.6 Sediment dispersion modeling 1.4.7 Benthic Impacts 1.4.8 Fisheries impacts 1.5 Conclusions 2.0 Introduction 2.1 Cork Harbour 2.2 Water Injection Dredging 2.3 Study 3.0 Results and Discussions on the Monitoring Programme 3.1 Fluorescent Particle Tracer Study 3.2 Sediment Pins 3.3 Multi-Beam Echo Sounding 3.4 In Situ Water Sampling 3.5 Monitoring Buoys 3.6 Sediment Dispersion Modeling 3.7 Benthic And Fisheries Impacts 4.0 Findings of Monitoring Programme 4.1 Fluorescent Particle Tracer Study 4.2 Sediment Pins 4.3 Multi-Beam Echo Sounding 4.4 In-Situ Water Sampling 4.5 Monitoring Buoys 4.6 Sediment Dispersion Simulation 4.7 Benthic Impacts 4.8 Fisheries Impacts 4.9 Conclusions 5.0 Draft Working Plan – Next Campaign Working plan Site conditions Soil Survey Productions CO2 emission lume 2 Appendices Appendix A: Water Injection Dredging Tracer Study – Cork – Environmental Impact and Sediment Transport Study utilising fluorescent particle tracers – Final Report, July 2012 – ETS Worldwide Ltd. Appendix B: Cork Harbour – Sediment Pin Survey (December 2011) – Aquatic Services Unit. Appendix C: In-situ Water Sampling – Van Oord Appendix D: Turbidity Buoys – Van Oord Appendix E: Hydrodynamic Modeling – RPS Volume 3 Appendix F: Assessment of Benthic Fisheries Impacts of Maintenance Dredging in Lough Mahon and the Lower River Lee (2011-2012) – Aquatic Services Unit – March 2013 1.0 EXECUTIVE SUMMARY 1.1 General The Port of Cork is the key seaport in the south of Ireland and has four distinct public facilities: City Quays, Tivoli Industrial and Dock Estate, Ringaskiddy Deepwater and Ferry Terminals and Cobh Cruise Terminal. Port of Cork recognise environmental management to be of equal importance to other prime business considerations and therefore commits itself to lead the wider port community to minimise environmental impacts through co-ordinated environmental management, respecting the principles of environmental sustainability. 1.2 Maintenance Dredging The Port of Cork previously undertook regular maintenance dredging operations by Trailing Suction Hopper Dredger (TSHD). During the tendering for a new dredging framework agreement in 2010, it became evident that there could be significant benefits in the migration from traditional dredging methods to Water Injection Dredging (WID). With its lower cost and lower carbon footprint it has the potential to provide a sustainable alternative. However the existing dredge license did not allow for the primary use of such method and data needed to be gathered to determine the effects of Water Injection Dredging on the estuary system and assist a new permit application. Stakeholders included the licencing authority i.e. Environmental Protection Agency (EPA), the Marine Institute (MI), the National Parks and Wildlife Service (NPWS) and Fisheries Ireland (FI). All responded positively to invitations to participate in a series of round table discussions with the Port Company, the Contractor (Van Oord), and its respective Consultants. In these meetings two main environmental concerns were identified related to the Water Injection Dredging in the River Lee Estuary namely: • Potential increase of turbidity levels in the estuary beyond a level the system could cope with • Increased siltation of sensitive mudflats to a rate that is above natural and that could cause smothering of fauna on them. It was accepted by all parties that a thorough testing and monitoring regime should be carried out and the Port of Cork instructed their Dredging Contractor to carry out Water Injection Dredging Trials in designated areas in the Estuary. 1.3 Environmental Monitoring The following studies were commissioned to monitor these trials: 1. Fluorescent Particle Tracer Study 2. Sediment Pins 3. Monitoring Buoys 4. Sediment Dispersion Modeling 5. Assessment of Benthic and Fisheries Impacts This document attempts to represent the integrated findings of the above studies and aims to overlay the findings with the dredging campaign. Timing of dredging, Testing and Monitoring 1.4 Findings Of All Studies 1.4.1 Fluorescent Particle Tracer Study Dredged material disperses rapidly and widely throughout the estuary due to tidal currents. This causes a temporary rise in turbidity levels until the sediment settles at depositional areas or is carried to sea. Dredged sediment is dispersed along the river estuary both in up-estuary and down-estuary direction. Hence, the dredged sediments are diluted by a large volume of water and the local effect on suspended sediment is low. The main depositional areas are the mudflats around Lough Mahon, Foaty channel and Monkstown creek. Hardly any sediment is found in the main navigational channels. The magnitude of deposition thickness is estimated to be in the order of millimetres. The dredged sediments come from the estuarine system and are not different from the existing sediments on the mudflats. The data clearly shows that there is a rapid dispersion of sediments throughout a large area. The depositions observed are spread quite well with no concentrated accumulations of sediment that would be harmful to the shellfishery and marine flora and fauna at these sites. The tracer study demonstrates that dredged sediments are dispersed along the estuary within only a few tidal cycles. The overall results of this study highlight that both natural and dredged silts are transported downstream and upstream. This pattern is quite common and in line with those found in other tide dominated Estuaries. 1.4.2 Sediment Pins The bed level of mudflats varies strongly in space and time due to natural erosion and deposition processes. These bed level changes are of a larger order than those associated with Water Injection Dredging (WID). Results from the sediment pin survey show that the levels of erosion / accretion strongly vary across the survey area and over the survey period. Generally, most of the sites at the upper harbour area exhibited erosion during the measurement period. Only a single site showed accretion, while in the Lower Harbour area approximately 1 cm of accretion was observed at all sites. If and how much the observed bed level changes are influenced by the WID alone cannot be determined. It does however seem unlikely that the dredging has caused both accretion and erosion in known depositional areas. Furthermore the observed bed-level changes are much higher than the bed level changes associated with WID (See Section 3.1 Fluorescent Particle Tracer Study). The differing rates of accretion and erosion in time and space are thus expected to be primarily caused by natural processes (tides, wind, varying river discharge etc.). The varying levels of erosion and accretion in space and time merely demonstrate that the estuary is a dynamic environment, in which erosion / accretion rates of several centimetres are part of the natural dynamics. 1.4.3 In-Situ Water Sampling Based on 17 samples, the relationship between the Total Suspended Solids (TSS) versus Buoy Sensor Median Turbidity (NTU) was determined as 1 NTU = 1.1mg / l (TSS). 1.4.4 Multi Beam Echo Sounding No areas exhibit bed level changes of more than 5cm 1.4.5 Monitoring Buoys Recorded turbidity levels rarely exceed 30 NTU, and are mostly less than 10 NTU. A limited data analysis shows that turbidity levels are related to the tidal currents; at high tidal ranges (spring tide) significantly higher NTU values were recorded than during low tidal ranges (neap tide). The NTU values are also related to the individual high and low waters. In general, recorded turbidity levels at high water are the lowest, while the highest values are at the outgoing tide. In situ water sampling correlates 1 NTU = 1.1 mg/l, consequently the above mentioned values of 10 and 30 NTU correspond to Total Suspended Solids levels of 31 and 10 mg/l. These values are considered quite common for an estuarine environment. The observed turbidity levels towards the end of the measurement campaign are somewhat higher than at the start of the measurements. These observations are influenced by the following factors: • Seasonal variations in weather (there are more storms in the fall and the winter) • Seasonal variations in river discharges • Dredging • Spring / neap tide The natural processes govern the turbidity variations on tidal (ebb, flood) as well as sub tidal timescales (spring, neap, seasonal effects). The contribution of the Water Injection Dredging activity cannot be discerned from the data. Measured and astronomical tide at Ringaskiddy Top: measured and the astronomical tide. Bottom: Instantaneous readings of the turbidity buoys. Peaks in turbidity readings are clearly related to the tides 1.4.6 Sediment Dispersion Modeling The model results clearly show that the dredged sediments spread widely and rapidly due to the tidal movement, and correspond reasonably well with the results from the tracer study. However, the model is not able to reproduce the observed levels of upstream transport to Lough Mahon. The mechanisms that are known to cause such upstream transport in estuarine environments are: • settling lag • scour lag • tidal asymmetry • residual gravitational circulation • internal tide asymmetry • fresh/salt water stratifications Though the first three could theoretically be reproduced by the current model, the model is based on the assumption of a logarithmic velocity profile which is known to not always hold for estuarine environments, especially when there is salt water intrusion. Due to the 2D nature of the model, it is not possible to include the effects of stratification on transport pathways of sediment.