**2. The Proposed Modeling Approach**

This paper provides an organic and comprehensive framework about the physical effects induced by sediments handling operations and proposes a broad-spectrum modeling approach intended to support contractors and controlling authorities in planning and managing such a kind of interventions. Hereinafter, the plume dynamics are intended to be related either to removal or disposal induced re-suspension/release of the fine fraction of the handled sediments, as well as to advection, deposition and sometimes re-suspension from the bottom due to environmental forcing. The whole sediment handling work cycle is described by different operational phases: removal (or excavation), loading, transport and disposal of handled sediments. Moreover, different environmental contexts (coastal areas, semi-enclosed basins and offshore areas) are considered as intervention areas.

Even if the water depth, respectively in shallow or deep offshore areas, may induce operational differences in excavation and disposal, these distinctions are not addressed in this paper. Indeed, as also suggested by Marine Strategy Framework Directive 2008/56/EC [27], the area of interest should

be defined taking into account the strict interaction between off-shore and near-shore hydrodynamics. Then, the area of interest could reach the national limits and beyond, of course with an appropriate and feasible spatial scale. Hence, all the main physical phenomena influencing the dynamics of the induced sediment plumes can be properly modeled.

Mathematical models, calibrated and validated through the use of literature and field data, are recognized as useful supporting tools to plan, design and manage sediment handling operations. In particular, they can support the comparative choice of the technical and operational alternatives based on the forecast of possible environmental issues. The reliable estimation of the physical processes characterizing the sediment plume dynamic during the whole handling cycle requires the selection of mathematical models able to reproduce the primary physical features of the intervention area, of the project goals, and of the environmental aspects. The proper model selection and implementation require the definition of the main hydrodynamic field and source term features driving the spatial and temporal variability of dispersal of the spilled sediment and contamination processes (if any). Similarly, the selected approach for the numerical solution of the governing equations for hydrodynamic and transport phenomena influences the burden in terms of resources, computational times and of required input data.

The proposed integrated modeling approach relies on standard numerical suites worldwide used to model the passive phase of plume dispersion, but the source term definition aimed at accounting for the near-field processes, at least from a macro-scale point of view. Three numerical modules, hereinafter referred to as the hydrodynamic module (H-M in Figure 1), source term module (ST-M in Figure 1) and transport module (T-M in Figure 1) are implemented in series. Basically, the transport module is used to estimate SSC and DEP resulting from dispersion of the sediment release (estimated by the source term module) due to the flow field variability (estimated by the hydrodynamic module). Then, an environmental assessment module (EA-M in Figure 1) provides standard methods and statistical parameters for the assessment of the physical environmental effects. A modeling–monitoring feedback system is then recommended and considered as an integral part of the proposed integrated modeling approach. With the aim to define a proper modeling setup, validation of modeling results and verifying when sediment spill exceeds specified limits. These limits, hereinafter referred to as reference levels, are intended to be defined with respect to the natural background conditions and to the environmental critical issues types (if any).

Lisi et al. [15] sugges<sup>t</sup> that the modeling studies should be performed in different steps, with increasing level of detail, and they give practical indications to optimize the work plan, with regard to environmental and operational site-specific objectives. The accuracy of quantitative estimates depends on the used modeling approach (modeling tools and scenarios), and it is a function of the expected results. Indeed, depending on the project phase (i.e., prior, during or after), different detail levels can be required. Furthermore, this also makes the proposed methodology feasible from an economic point of view (Figure 2). It is argued that expert judgment should be the first effort performed. A preliminary information phase aims at selecting reference conditions to assess when modeling approaches are needed and to define their accuracy level with respect to environmental expected effects. Within this phase, based on operative and environmental data collection, scientists with different expertise should be engaged within the framework of a holistic approach. When the preliminary information phase highlights that significant environmental effects are likely to occur, the implementation of modeling studies is recommended for their estimation. In these cases, the next step is the implementation of a preliminary modeling phase, in which simplified models are used to describe the key features of the plume dynamic and allow a fast estimation of its expected maximum extension area. The reader is referred to Section 5 for details on preliminary information phase and preliminary modeling phase. A detailed modeling phase (see Section 6 for details) is then suggested when the preliminary modeling phase confirms that the dispersion of spilled sediment can impact on water quality and on the site-specific environmental targets. It is intended to allow accurate evaluations even for complex conditions.

**Figure 2.** Flow chart of the different phases suggested for optimizing the modeling studies (thus the work plan and the related monitoring) to be performed with increasing level of detail during the different project phases.

The detailed modeling phase is always recommended when three-dimensional features (for source terms and/or hydrodynamic patterns) play a key role and when environmental critical issues are revealed and/or predicted by preliminary modeling phase. It has to be stressed that the preliminary modeling phase plays an important role even when detailed modeling is implemented, in order to evaluate the possible need and the nature of detailed analysis and to support the preliminary assessment of the worst-case scenarios (e.g., extreme events or mitigation failure). This is aimed at optimizing the work plan and the approval procedures, with regard to environmental and operational site-specific objectives. Figure 2 depicts the flow chart of the proposed general approach.

Some initial modeling assumptions (e.g., within the preliminary information phase and preliminary modeling phase) may vary during the project development (e.g., from the planning phase up to the execution). Indeed, it may be observed that key information could not well defined at the preliminary design phase of a sediment handling project, while it is (or should be at least) known at a later stage. According to the adaptive managemen<sup>t</sup> approach [17], a stepwise procedure can address uncertainties as the project progresses, incorporating flexibility and robustness into project design, and using latest information to instruct decision-makers as the project develops.
