Stochastic effects of dredge plumes: Development and application of a risk-based approach to assess ecological effects of dredge plumes on sensitive receivers

Dredge plumes and ecology have become increasingly intertwined in recent history; one of the more prominent reasons behind this is the influence of clouds of fine sediments released in the open water due to dredging (`dredge plumes') on for `sensitive receivers' such as coral, seagrass and mangroves. An Environmental Risk Assessment (ERA) in combination with a cost-benefit analysis could be a valuable instrument for deciding on ecological impact. When dealing with ecology, it is important to realize that nature is not a harmonious, calm state: in fact, it could be considered to be at best only approximately in equilibrium, where small scale changes could lead to a completely new equilibrium. In the case of dredging operations and ecological effects, this is even more relevant because ecological effects occur constantly; not just due to dredging. An ERA consists of a description of the system and its components, hazard identification, effects assessment, exposure assessment, risk characterization and an evaluation which provides feedback for a possibly updated system description. Globally following the steps in an ERA, exposures are looked at first. Relevant dredging exposures are suspended sediment concentrations and sedimentation. The source of this is the dredging equipment, which spills fine sediments in the open water. These sources lead to dynamic plume, which consists of interacting, complex three-dimensional fluid motions: sediment characteristics, bathymetry and dredging equipment type are just a few of the influencing factors. Because there motions are often still too complicated to model, the dynamic phase is captured in conservative estimates. These estimates serve as a source term for the passive plume phase. During the passive plume phase, advection and diffusion are dominant processes which are well understood processes. MIKE 21/3 or Delft3D, both hydrodynamic and transport models, can be used to schematize the passive plume. Once a passive plume reaches a sensitive receiver, in this document assumed to be coral, seagrass or mangroves, effects can occur. It is important to realize that these effects do not have to be induced solely by dredging: other background effects can also provoke effects in sensitive receivers. Effects can be classified in discrete classes, ranging from growth to mortality. To which extent a receiver exhibits a certain effect depends on the receiver characteristics, the shape of a temporal variable exposure and the type of exposure. In an attempt to properly catch the effect of temporal variable exposures, several models are discussed with most prominent being two originally toxicological models. Given their background these models appear to be limited to exposures which mimic a toxic exposure, although they are relatively easy to implement. At the other end of the spectrum are the energy budget models: models which try to describe important processes which together form an individual. While this is a more complete and holistic approach, there is concern regarding the uncertainty of the final answer given the large number of parameters (relative to the toxicological models) and the attributed uncertainties. Combining these exposure and effect models in dredging practice and explicitly stating their (combined) uncertainties leads to a risk assessment for dredging practice. A part of this risk assessment was applied to field data regarding coral cover for a number of coral reefs, but the field data showed no strong correlation between exposure and changes in coral cover. Nonetheless, for a quantitative assessment of ecological risks, a probabilistic analysis is invaluable. This can lead to a proper valuation of the risk of effects, which in turn can be used in a cost-benefit analysis to properly weigh these risks if they are acceptable or not when compared to the benefits.Hydraulic EngineeringHydraulic EngineeringCivil Engineering and Geoscience