*J. Mar. Sci. Eng.* **2019**, *7*, 149

*\* Not a focus of this paper*

**Figure 7.** Scheme of the interactions between the significant physical effects related to the exceedance of reference levels (RLs) for SSC and sediment deposition rates (DEP) parameters and the severity of impact related to tolerance levels defined as a function of the status of sensitive receptors (if any).

A first approach for the definition of the significance of physical effects involves the use of a series of reference levels, the exceedance of which leads to the identification of intensities and durations. As an example, Figure 8 shows some indicative reference levels for intensity (SSC*RL*) and duration intervals (T*max* \_ *RL*), i.e., the maximum duration of time windows during which SSC exceeds the specific SSC*RL*. The significance of the environmental effects is then defined in terms of a combination of SSC*RL* and T*max* \_ *RL*. Of course, the same method may be performed by changing the reference levels as a function of the specific receptor or by using other pairs of meaningful parameters (e.g., frequency of occurrence, [78]) on the basis of project-specific and/or site-specific and/or receptor-specific evaluations. In particular, synthetic maps are defined for each reference level in terms of intensity (e.g., SSC*RL* = 10, 20, 50 mg/L in the example shown in Figure 8) storing at the single control point the significance value of the effect associated with the maximum duration of uninterrupted persistence of SSC above the specific value. From the overlap of the maps, the maximum value of registered significance is obtained for each specific control point. Maps can be overlapped to the location of sensitive habitats and ecological receptors in order to relate the sediment plume dynamic with different targets.

A second approach involves the use of a single index. Feola et al. [26] proposed to use the SSC number (SSC*num*; mg s/L, e.g., [82]) that gives integral information about intensity, duration and frequency of exceedance of reference level. Basically, it is defined, for each simulation scenario (*i*), as the sum of the products of the mean intensity above reference level (SSC*mean*\_*RL*,*<sup>i</sup>*) and the related duration (*tj*, *j* = 1 ... *Mi*, with *Mi* the number of the considered events). Then, it reads (e.g., [26,82,83]):

$$\text{SSC}\_{mm,i} = \sum\_{j=1}^{M\_i} \text{SSC}\_{m \text{can}\_{-}RL,i} t\_j \tag{6}$$

where SSC*num*,*<sup>i</sup>* is the SSC number related to the specific *i*-th scenario. Maps of this integrated index can be then evaluated by analyzing the time series for each control point and for different values of the reference level.

**Figure 8.** Example of maps of the significance of effects (based on intensity and duration) related to events of exceedance of reference levels (RLs). Significance classes are defined evaluating intensity and duration of exceedance of reference levels for increasing SSC (SSC*RL* = 10, 20, 50 mg/L). Final significance level is the integrated result obtained by maps overlapping (right map).

Further useful analysis should be suitable to quantify the spatial and temporal variability of the effects associated with the dispersion of the turbidity plumes as a function of the distance from the sediment source. For these purposes semi-variograms can be used (e.g., [84]). Basically, the semi-variogram is defined as half of the averaged squared difference of the parameter at hand (SSC in this case) between points located at different distances. Then, the analysis of the semi-variogram allows the direct measurement of the spatial scale of the transport phenomena resulting from the generation of the turbidity plume and can be used to quantitatively compare the extent of the areas affected by the sediment handling activities (influence zone) related to different scenarios of simulation. The use of the semi-variogram provides the estimate of the variance (alteration with respect to the undisturbed value) of the parameter at hand modeled as a stochastic variable, and the estimate of the physical intensity of the alterations with respect to the undisturbed value.
