DREDGING: MINIMISING IMPACTS, MAXIMISING BENEFITS

1. DREDGING: MINIMISING IMPACTS, MAXIMISING BENEFITS Minimising the impacts of dredging and disposal Introduction 1 3 2 Reducing the impacts of disposal The impacts of the disposal of dredged material can be reduced by: USE OF AN ALTERNATIVE DISPOSAL OPTION/LOCATION such as beneficial use; retention of the sediment within the system; or disposal to land or a confined disposal facility[6]. MODIFYING THE DISPOSAL TECHNIQUE and instead using bottom dumping; or pumping to the seabed[6]. USING SUPPLEMENTARY EQUIPMENT TO REDUCE DISPERSION OF SEDIMENT AND INCREASE DO LEVELS for example addition of silt curtains or aerators[6]. MODIFYING DISPOSAL TIMING TO REDUCE THE IMPACTS ON BIOLOGICAL RECEPTORS by considering local hydrodynamics and avoiding sensitive periods for animals such as breeding and migration periods[6]. Reducing the impacts of dredging The impacts of dredging can also be reduced by improvements in the technology used, for example by: MODIFYING THE DREDGING TECHNIQUE by changing the dredger type to a low impact dredger that is engineered to reduce over-dredging and minimise the suspension of sediment, e.g. scoop dredgers[7]. Alternatively the production rate can be reduced[6]. MODIFYING THE DREDGING EQUIPMENT by modification of the drag head; addition of a visor or silt screen (Fig.1); or constraining the overflow[6]. Reducing the dredging requirement The simplest way to reduce the impacts of dredging may be to reduce the dredging requirement. This can be done by: MODIFYING THE POSITION OF THE CHANNEL by realignment or creating a new channel.[6] IMPROVING THE MANAGEMENT PRACTICES FOR VESSELS through modified hull design; passage planning; or speed limits[6]. MANAGING TIDAL INFLOWS TO REDUCE SEDIMENT INPUTS through raising tidal weirs; or controlling the timing of impounding pumps[6]. MANIPULATING FLOW TO INCREASE NATURAL SEDIMENT TRANSPORT by constraining or directing flow; or constructing training walls[6]. REDUCING THE AMOUNT OF DREDGING by reducing the depth or size of the area to be dredged[6]. • During dredging, material is removed from the sea bed and disposed of at a different location, most commonly to allow passage of vessels through a port or harbour[1]. Dredging, and the disposal of dredged material, can have negative environmental impacts[1]. For example: • Reduction in water quality due to the increase in suspended solids; • Removal or destruction of habitats; • Smothering of marine life with particulate matter; • Changes to topography; • Erosion or deposition (2,3,4] • The severity depends on the ecology of the site, however if potential negative impacts are identified, measures must be taken to mitigate and/or offset them[5]. These measures aim to reduce the impacts to below the acceptable limits set out in the Water Framework Directive[6]. < Figure 1. A dredger with a silt screen attached to minimise sediment transfer throughout the water body [8] 4 Offsetting the impacts of dredging and disposal There are a number of ways to offset the impacts of dredging and disposal, such as: BENEFICIAL USE OF DREDGED MATERIAL TO REPLACE LOST SEDIMENT through trickle feeding; water column recharge; sediment bypassing; or intertidal placement[6]. BENEFICIAL USE OF DREDGED MATERIAL TO CREATE BANK PROTECTION to prevent bank erosion and slumping[6]. BENEFICIAL USE OF DREDGED MATERIAL in other ways such as in fertiliser; habitat construction[9]; or road construction material[10]. RE-OXYGENATION OF THE WATER COLUMN using an aerator[6]. RE-ESTABLISH CHARACTERISTIC BIOTA by restocking using habitat or species translocation; or seeding[6]. REFERENCES [1] Hill, MI. 2001. Guidelines on the impact of aggregate extraction on European marine sites. PosfordDuvivier. Peterborough. pp25-35. [2] Cooper, KM. Frojan, CRSB. Defew, E. Curtis, M. Fleddum, A. Brooks, L. & Paterson, DM. 2008. Assessment of ecosystem function following marine aggregate dredging. Journal of Experimental Marine Biology and Ecology. 366: 82-91. [3] Elsaeed, GH. 2011. The Impact of Dredging on Coastal Environments. Australian Journal of Basic and Applied Sciences. 5(2): 74-81. [4] Perkins, M. Pickering, L. Pihlajamaki, M. Poate T. Richardson, N. Roberts, M. Hansson, M. & Walsh, S. 2001. Dredging and Disposal in Estuaries. University of Southampton School of Ocean & Earth Sciences. [5] Bridges, TS. Gustavson, KE. Schroeder, P. Ells, SJ. Hayes, D. Nadeau, SC. Palermo, MR. Patmont, C. 2010. Dredging processes and Remedy Effectiveness: Relationship to the 4 Rs of Environmental Dredging. Integrated Environmental Assessment and Management. 4:619-630. [6] Environment Agency. 2010. Cleaning the waters: Marine dredging and the water framework directive, stage four: Identification and evaluation of measures. pp1-26. [7] Cohen, M. 2005. Dredging: The Facts. PIANC (The International Navigation Association), The Netherlands. [8]http://www.javeler.com/Equipment/Dredge%20System%20Components.htm. Cited 01/02/2011 [9] Ishii, R. Nakano, Y. Nakai, S. Nishijima, W. Okada, M. 2008. Benthic ecosystem development in an artificial tidal flat constructed from dredged spoil. Marine Pollution Bulletin. 12:2059-2066. [10] Siham, K. Fabice, B. Nor-Edine, A. 2008. Marine dredged sediments as new materials resource for road construction. Waste Management. 5:919-928. [11]http://islandnature.ca/2011/01/marvelous-mudflat-snails/. Cited 05/12/2011 < Figure 3. Macrobenthic species living at the bottom of the water column, such as the gastropod Batillaria, are at risk from dredging practices. [11] Case Study Use of dredged material to construct artificial tidal flats in Japan Dredged material from Ago Bay, Japan has been successfully used to create artificial tidal flats[9]. Dredge spoil (DS) is a good source of silt, clay and organic material, and can be mixed with mountain sand (MS) to create a medium for the creation of artificial tidal flats[9]. This is beneficial as it makes the composition of the artificial tidal flat closer to that of natural tidal flats. Figure 2 shows the effect of the artificial tidal flat on macrobenthos emergence Figure 2. Variation in emerging macrobenthic population in control and artificial tidal flats[9] The number of macrobenthos (Fig. 3) on the artificial flat increased, and the average number of macrobenthos in the artificial tidal flat was almost three times that in the control (that did not contain dredge spoil)[9]. 5 Reducing the uncertainty in dredging procedures In order to reduce the negative impacts it is important to know what is happening at the site, and the potential impacts of proposed projects. This is achieved by: MONITORING SITES regularly using the Environment Agency Water Framework Directive (EAWFD) monitoring programme[6]. USING TRIAL SITES to carry out monitored experimental applications prior to dredging operations[6].