*8.1. Modeling and Monitoring Feedback System*

Modeling–monitoring interactions are often recommended in environmental impact assessment procedures (and other regulatory frameworks) for assessing the compliance of selected operational criteria with the established environmental requirements. Basically, the selection of the modeling accuracy, and of the type of data to be collected as well, heavily depends on the general requirements of the various monitoring phases (before, during and after execution monitoring, e.g., [18]), and on the short- and long-term effects to be verified. Different types of data must be collected for determining the dynamics and the composition of turbidity plumes. The most common parameters needed to validate models are: sedimentological, meteo–climatic, hydrodynamic, water quality, topo–bathymetric and descriptive (e.g., coastline features, the presence of infrastructures or specific land-uses) data. The objective of measuring physical parameters not directly related to water quality (e.g., currents, waves, water elevations) is to provide information on how long a plume can remain in a particular area and on the time required for its dispersion to adjacent waters, as well as considering factors that may increase turbulence in the water column causing additional turbidity and preventing sedimentation.

In order to optimize a modeling–monitoring feedback system, typical environmental questions to be answered in the early preliminary planning phases are: (i) what types of sediment spill sources could be expected/distinguished (e.g., single point-spill event, continuous point-spill over a certain period); (ii) whether suspended sediments will leave the dredge- or dump-site; (iii) where the material

will go and how much material will remain in the water column after a certain time. The most used parameter to characterize sediment plumes is turbidity, calibrated with in situ total suspended solids measures (TSS), defined as the total mass of material in a given volume of water, in mg/L. Nonetheless, establishing a reliable relationship between TSS and turbidity is not always possible because of the variation in characteristics of the suspended material. For defining turbidity values three aspects are usually considered: (i) turbidity level in the dredging area; (ii) the horizontal dispersion of the sediment cloud (for different hydrodynamics conditions); (iii) the settling time of the sediment cloud after cessation of operations. According to Pennekamp et al. [32], monitoring results allow the evaluations of the depth-averaged background concentration, of the characteristic increase of depth-averaged concentration at different distance from dredging/disposal activity, of the decay time of the re-suspended sediment after the execution of handling operations after which the turbidity return to background values, of the source term. In addition, chemical-physical parameters (temperature, salinity, conductivity and density conditions) are important to identify when sediment plumes significantly differ from the surrounding water. Dissolved oxygen and pH are commonly measured as indicators of water quality and of potential impact on biological resources (e.g., re-suspension of anoxic sediments may lower dissolved oxygen concentrations in the water column).

Fixed stations are required for comprehensive and regular monitoring over time. In fact, continuous time-series (single point or profiler) provide valuable information on the temporal variability of the monitoring variables along the water column. In particular, fixed stations allow collecting the background conditions during different environmental conditions before the execution of the works and to verify the selected reference levels during their execution.

During the execution phase, both fixed and mobile monitoring stations are required. Mobile stations (e.g., samplings from a vessel) are required when measurements at various locations over short-periods of time are needed, to one or more water depths, and to track the near-field plume through the water column. These types of measurements allows us to follow any change of the operational techniques that could influence sediment spill concentrations. Sampling time can be significantly increased depending both on site conditions (e.g., water depth and rate at which hydrodynamic conditions can vary) and on the purpose of the monitoring project phases.

The selection of the monitoring tools (i.e., fixed platforms, vessels, or towed vertical profilers) and of the sampling techniques is important to maximize the usefulness of the modeling–monitoring feedback system within the different phases of the project. Thus, understanding the advantages and limitations of the various available sampling techniques is important to determine the most cost-effective approach for sediment plumes monitoring. In general, using multiple instruments on the same platform reduces sampling time and provides synoptic measurements of the parameters being measured.

Based on the results of the monitoring and modeling analysis, it is possible to deepen the understanding of the system and its responses to pressures induced by changes on the involved environmental variables (chemical-physical and biological). Then it is possible to modify the design and monitoring choices. Mathematical models allow hydrodynamic and sediment transport evaluations (in time and space) for different selected spill scenarios, supporting the optimization of both work plans and environmental monitoring programs in the different project phases taking into account operational and environmental aspects. Monitoring data, on the other hand, are crucial to define input for near-field and far-field models, as well as for their calibration and validation and to verify the reliability of the modeling simplifications and results. For efficient managemen<sup>t</sup> of sediment handling works, the modeling–monitoring feedback system is recommended herein as part of the proposed integrated modeling approach. The main relationships between modeling and monitoring activities and the main details on various deployment platforms for data collection (fixed platforms, vessels, or towed vertical profilers) at different stages of project design are highlighted in the followings.

Figure 9 shows the main interactions within the modeling–monitoring feedback system that should be carried out to verify the feasibility and the environmental compatibility of interventions

in various design, execution and monitoring phases (i.e., before, during and after execution). In the preliminary design phase (i.e., before works execution), the modeling–monitoring feedback system serves as a supporting tool for the approval of work plans and for designing of suitable monitoring programs. In this context, the feedback between preliminary modeling spill scenarios and baseline-monitoring allows the selection of background conditions not related to the works execution. In particular, the determination of statistically reliable reference levels for the selected variables (e.g., SSC, DEP) will allow, during execution, to analyze monitoring and modeling results and to evaluate whether mitigation actions should be taken. Indeed, in the early stages of monitoring planning, site-specific information on types, distances and status of any sensitive objectives and receptors is needed for relating the significance of effects in terms of physical changes to the severity of impacts on critical targets (see Section 7).

**Figure 9.** Scheme of the modeling–monitoring feedback system in the different phases of design, execution and managemen<sup>t</sup> of handling operations. In the scheme, MMFS stands for modeling–monitoring feedback system; BE-BDS, DE-PDS and AE-PDS for project data sheets (PDS) before execution, during execution, and after execution, respectively; EIA for environmental impact assessment.

In fact, during and after sediment handling works, the modeling–monitoring feedback system is useful to verify (in the short-term) the fulfillment of the selected operational criteria and (in the short- and long-term) the compliance with the requirements of environmental protection mechanisms including legislation, contractor conditions and sustainability protocols [18]. In particular, changes of selected variables (e.g., SSC) can be compared with the selected (single or multiple) site-specific reference levels for water quality. For the short-term assessment, the surveillance monitoring is performed in the execution phase, trough the combined use of fixed and mobile (on-vessel) monitoring stations (see VBKO [85] and Aarninkhof et al. [86] for more details). This provides extensive sediment flux data useful for early warning to ensure that the amount of sediment re-suspension and dispersion is kept below the site-specific reference levels. Moreover, field data are crucial to validate the models to reduce the modeling uncertainties (i.e., the estimation of source terms). In addition, increasing knowledge on operational factors used to perform the sediment handling operations (e.g., more details on time schedule, production rate and type of dredges) should be considered for the set-up of more detailed modeling studies and for periodic reviewing of monitoring programs [17].

For interventions of considerable extension or in presence of very sensitive environmental critical issues (e.g., handling of pollutant sediments), the implementation of models in operational mode is useful to handle (mitigate or prevent) critical conditions that may occur during works execution (e.g., time windows of adverse weather and sea conditions, distribution of sediment concentration and sedimentation rates caused by natural or anthropogenic actions). In particular, the use of models in operational mode is recommended, as support to contractors and controlling authorities to promptly adopt proper alert procedures (e.g., interruption or modification of operations, implementations of mitigation measures) selected within a short term operation plan (STOP) aimed at ensuring that key parameters remain below specified alert levels (in term of both intensity and duration of exceedance), when these are forecast in one or more target points.

For the long-term assessment, a comprehensive and regular monitoring must be performed by means of fixed stations, including both the area involved by dispersal of re-suspended or spilled sediments and one or more periodic control points located in undisturbed areas (identified in advance trough the numerical modeling activities) and near environmental sensitive areas (if present). In this case, long-term scenarios should be implemented to forecast long-term effects and then verified through field measurements after the completion of the sediment handling operations. In particular, for a proper managemen<sup>t</sup> plan, after works execution, monitoring should be performed until undisturbed conditions or a new stable equilibrium of the marine ecosystem (based on environmental considerations and criteria provided by controlling authorities) are achieved.

### *8.2. Management of Monitoring Data and Information Flow*

According to the Adaptive Management approach (e.g., [7,17,87]), a sharing process between the contractor and the authority regarding the modeling–monitoring feedback system (to be foreseen before, during and after the conclusion of detailed studies) is desirable. The sharing process should include standard decision-making procedures and should be functional to optimize the work plan, the mitigation procedures (such as, modifications of dredging schedules, decrease of spill and overflow using special return pipes, closed grabs or clamshells, silt curtains or screens around dredgers) and the monitoring program (number, location and sampling frequency of the stations).

Moreover, within a modeling–monitoring feedback system, the implementation of an environmental information managemen<sup>t</sup> system (EIMS) is encouraged to constantly enrich the available and usable data-set for a better application of the proposed integrated modeling approach (before, during and after each design and monitoring phases). In particular, project data sheets (PDSs) are promoted for a systematic collection and adequate dissemination of environmental (e.g., climatic an hydrodynamic conditions) and operational data (e.g., details on dredging equipment and techniques, production rate, work time schedules, type and operating mode of any mitigation measures) to be organized in a specific standardized, homogeneous and easily manageable format. This is in order to maximize the usefulness of monitoring data within the various design phases of the same project and to support the initial phases of future projects characterized by similar environmental and operating conditions. Indeed, the availability of data can be useful to increase the reliability of the modeling hypotheses, in particular for the estimation of source term. Information sheets can be considered as guides for data collection. In order to maximize the usefulness of PDSs their compilations should be performed with a frequency suitable to represent the natural background turbidity (e.g., for representative weather and sea conditions, vessel traffic). During the works, it is desirable to compile them also when operational parameters and site-specific environmental conditions change. In particular, the use of standard methodologies for the compilation of project data sheets before execution (BE-PDS), during execution (DE-PDS) and after execution (AE-PDS) phases (see Figure 9) will allow a good efficiency for calibration and validation processes and reliability of the obtained modeling results.
