**1. Introduction**

Interventions in marine and coastal areas often involve sediment dredging and disposal operations. The volume of sediments handling can vary in relation to the operations purposes, e.g., to maintain or improve the navigation depth of ports and harbors (e.g., [1]), for creating or improving facilities (e.g., [2]), for beach nourishment (e.g., [3]) and open-water disposal (e.g., [4]), to carefully remove and relocate contaminated materials (e.g., [5]) or morphological reconstruction in transitional areas. Moreover, the operational techniques (e.g., type and capacity of dredges) are key aspects to be accounted for when dealing with the assessment of physical effects due to sediment handling works (e.g., [6,7]).

Pre-approval from controlling authorities is typically required to verify environmental and economic compatibility of equipment, work plans and operational criteria prior to the initiation of the activities. The approval requirements include the evaluation of short-term effects occurring during the project phases (often referred to as process effects) and long-term effects caused by the final project layout (often referred to as project effects, e.g., [8]).

For the European Union, detailed environmental impact assessment (Directive 2014/52/UE, [9]) are aimed at selecting technical alternatives and designing appropriate mitigation measures and monitoring actions for ensuring environmental compliance, especially when either large quantities or polluted sediments have to be handled. Far from the intervention areas the dispersal and settling of plumes of spilled sediment can induce a broad range of effects, i.e., light reduction and sedimentation at sensitive receptors, changes in abundance, diversity and biomass of seabed habitats and benthic communities, contaminants and nutrients release. An efficient managemen<sup>t</sup> of sediment handling works requires knowledge both of the operational factors (i.e., extension of dredging/disposal areas, kinematic and geometric parameters of dredging/disposal techniques, duration and timing of operations) to assess the sediment release mechanisms, and of the site conditions (i.e., sediment type, water depth, currents and waves climates, thermohaline stratification, seasonal window) to assess the spatial dispersion and the settling time of the sediment plume during and after the end of operations.

In this framework, mathematical models are recognized as a valuable tool to forecast the plume dynamics and the areas interested by significant variations of suspended sediment concentration (SSC) and sediment deposition rates (DEP, e.g., [10–12]). Recent research (e.g., [13]) and international guidelines (e.g., [14]) often include the use of mathematical models to perform environmental studies needed to support decision makers (before, during and after execution) to optimize the interventions and monitoring actions with regard to environmental and project objectives [15], while maintaining desired production rates [16]. A major effort has been put to support contractors and controlling authorities to combine modeling and monitoring activities in a feedback framework [15,17,18]. This is aimed (i) at assessing and approving dredging equipment and work plans (prior to the operations start), and (ii) at introducing assessment procedures based on the application of environmental criteria, for ensuring that SSC remains below specified limits (during the operations) and for timely changing work plans and monitoring frequencies to prevent any potential short- and long-term environmental effects (during and after the operations).

Common modeling approaches involves hydrodynamic and transport models suitable to quantify and to compare the transport processes of the different spilled sediment, moving from the near- to the far-field (e.g., [11,12,19,20]). Nevertheless, technical and scientific literature highlights the lack of an organic and comprehensive methodology driving the selection of appropriate modeling tools and of accuracy levels needed for a reliable assessment of the induced physical effects in different environmental contexts and when environmental critical issues are involved (e.g., ecological sensitive receptor, water quality, sensitive habitat and species, fish farming facilities, regulatory constraints, etc.). In the context of national experiences, the Italian National Institute for Environmental Protection and Research (ISPRA) issued the Italian Guidelines dealing with the modeling approach that can be implemented in relation both to environmental and project objectives, promoting uniform procedures for different techniques, operational phases, and environmental contexts [15].

For ensuring the compliance with environmental requirements, the selection of a modeling approach must balance the accuracy of results related to strict environmental critical issues and operating criteria defined prior the initiations of the operations. Moreover, input data for the selected modeling scenarios should be appropriate for a reliable representation of the main physical processes variability driving the dynamic of the plume during the different operational phases, depending on the main characteristics of the selected techniques. It has to be stressed that past studies (e.g., [14,21,22]) found that results rarely focus on long-term effects of sediments dispersion, within either seasonal or annual time windows. Rather, they are focused on short-term scenarios usually related to either one or few tidal cycles or extreme events (e.g., [23–25]).

Increases of SSC and DEP away from the re-suspension source are mainly used to evaluate the extension of the area affected by plume dispersion, where the maximum SSC is usually expressed in relation to given thresholds. It has to be stressed that there is also a lack of tools that synthesize and make the modeling results useful for supporting decision system and environmental managemen<sup>t</sup> [22] and to give operational and environmental indications to optimize all the planning and managemen<sup>t</sup> phases of the sediment handling project.

GBRMPA [14] and Feola et al. [26] recommend that model results should be synthesized by means of maps showing statistical measures (i.e., maximum and mean) of the predicted SSC and DEP at different water depths, as well as by the synthetic parameters of the time series at different key sites, intended to be representative of the environmental context and of the duration of the project. It is suggested to analyze environmental effects in terms of the duration of the time windows during which given SSC thresholds are exceeded during the operations [16].

Starting from a literature review, this paper suggests standards for both setting up modeling and field studies and for analyzing and assessing modeling results with regards to: (i) areas of intervention (coastal areas, semi-enclosed basins and offshore areas), (ii) operational phases (excavation, loading/transport and disposal), (iii) operational techniques (hydraulic and mechanical dredges), and (iv) environmentally sensitive critical issues (if any). For sake of clarity, the key points of this paper are:


This paper is structured as follows. Section 2 aims at describing the proposed methodological approach. Sections 3 and 4 detail the rationale for the selection of scenarios and the source term definition respectively. Sections 5 and 6 illustrate the proposed integrated modeling approach for simulating sediment dispersion, intended as a general framework to assess the physical effects of sediments handling operation, thus by identifying areas interested by significant changes in terms of physical parameters (e.g., SSC and DEP) due to plume dynamics, and from which environmental risk can be derived. Also, the relationship between modeling and monitoring activities for proper implementation and verification both of modeling studies and of decision processes in different project phases are outlined (Section 8.1), and the importance of the managemen<sup>t</sup> and sharing of monitoring data is highlighted (Section 8.2). Concluding remarks close the paper.
