The action of propeller-induced jets on the seabed of ports can cause erosion and the deposition of sediment around the port basin, potentially significantly impacting the bottom topography over the medium and long term. If such dynamics are constantly repeated for long periods, a drastic reduction in ships' clearance can result through accretion, or the stability and duration of structures can be threatened through erosion. These sediment-related processes present port management authorities with problems, both in terms of navigational safety and the optimization of management and maintenance activities of the port's bottom and infrastructure.
In this study, which is based on integrated numerical modeling, we examine the hydrodynamics and the related bottom sediment erosion and accumulation patterns induced by the action of vessel propellers in the passenger port of Genoa, Italy. The proposed new methodology offers a state-of-the-art science-based tool that can be used to optimize and efficiently plan port management and seabed maintenance.
The operational activities of harbors and ports are closely related to the local bathymetry, which must be sufficiently deep to guarantee the regular passage, maneuvering, and berthing of ships. However, ship clearance is often so limited that it threatens the safety of in-port navigation, and ships may even hit the seabed in extreme cases. Therefore, this is a critically important issue that often results in management and maintenance efficiency problems in terms of the bottom and a port's infrastructure in general (Mujal-Colilles et al., 2016; Castells-Sanabra et al., 2020).
The action of a ship's main propellers means that traffic in ports is responsible for generating intense current jets, as noted in Fig. 1. The high velocities induce shear stresses on the sea bottom, which can possibly result in sediment resuspension when they exceed the critical stress point for erosion (Van Rijn, 2007; Soulsby et al., 1993; Grant and Madsen, 1979). Before depositing back onto the seafloor the resuspended sediment may be transported widely around the basin by the combined effects of natural currents, such as those induced by tides, winds, or density gradients, and by vessel-related currents, such as those induced by the propellers or the movement and displacement of ships. Thus, the continuous traffic in and out of ports can result in the displacement of a huge volume of seabed material, which can then induce significant variations in the bathymetry over medium to long timescales. The formation of erosional or depositional trends in specific areas of port basins can potentially result from these variations.
Example of a propeller-induced jet of a moving ship (main propulsion without rudder).
If such dynamics are particularly pronounced and rapid (bottom accretion of the order of tens of centimeters per year or even higher), the port authorities must carry out dredging operations for the maintenance of the seabed, in order to fully recover the clearance and ensure the conditions necessary for undisturbed ship motion, maneuvering, and docking or undocking operations.
Most of the published literature about the effects of ships' propellers on port sediments and structures is experimental, and it has mainly been conducted in laboratories using physical models (Mujal-Colilles et al., 2018; Yuksel et al., 2019). Few practical instruments are available for port authorities that can provide robust and scientifically based analyses and predictions of the relevant processes. Such tools can enable managers to plan specific actions aimed at maintaining the seabed, thereby helping to guarantee the continuity of operational activities of ports and to optimize the use of economic resources. Unplanned maintenance activities usually involve additional costs due to the need to operate under emergency conditions and, in some cases, partially interrupt the service.
The integrated numerical modeling of hydrodynamics and sediment transport represents an important aid to port authorities and, more broadly, to port managers and operators, as suggested by Mujal-Colilles et al. (2018). This can reproduce and thus provide a better understanding of the seabed sediment dynamics induced by ships' propellers over short, medium, and long timescales, thereby establishing what tools are required to ensure the efficient operational maintenance of the seabed.
Propeller-induced jets have mainly been studied using empirical formulas based on specific characteristics of the ships and ports of interest, such as the bathymetry; propeller typology, diameter, and rotation rate; and the ship's draft. The most common approaches are the German method (MarCorm WG, 2015; Grabe, et al., 2015; Abromeit et al., 2010) and the Dutch method (CIRIA, 2007). The resulting induced velocities are usually only considered locally to inform the technical design of mooring structures and the protection of a port's infrastructure. Although various assumptions are introduced through empirical formulas, these approaches are limited and do not fully consider the three-dimensional evolution of the induced jet throughout the water column at any distance from the propeller or at any location of the port. Therefore, these tools are not suitable for the comprehensive management of ports.
We conduct a pilot study of the hydrodynamics and seabed evolution induced by ships' propellers in the passenger area of the port of Genoa (Fig. 2), where the marine traffic involves mainly passenger vessels (ferries and cruise ships, generally self-propelled) and in which the resulting sediment dynamics in terms of erosion and deposition rates are particularly significant: estimated to be of the order of several tens of centimeters per year (as directly estimated and communicated by the port operators and via an analysis of bathymetric surveys). In this study, we propose that the integrated high-resolution numerical modeling of three-dimensional hydrodynamics and sediment transport can be a robust and science-based tool for the optimization and efficient planning of port management and maintenance activities. We propose a new methodology that can be used in a delayed mode and can, thus, reproduce the historical major sediment processes over time, as in this study, or in a prediction mode through the potential implementation of real-time operational services.
The remainder of this paper is organized as follows: in Sect. 2, we introduce our methodology; the data available for the study are presented in Sect. 3; Sect. 4 describes the numerical models used; the results of the numerical simulations are presented and discussed in Sect. 5; and the summary and conclusions of the work are given in Sect. 6.
The study is based on the latest versions of the hydrodynamic and mud transport models MIKE 3 FM (DHI, 2017), which are described in detail in Sect. 3 and in Appendices A and B. A very high resolution was used in the numerical model to realistically reproduce the propeller-induced jet, both in the vertical and in the horizontal, at approximately 1–2 and 5 m, respectively. Together with a non-hydrostatic version of the hydrodynamic model, this enables the processes and dominant patterns of the current field generated by the ships propellers during the navigation and maneuvering inside the port to be reproduced very accurately.
As shown in Fig. 2, 12 docks have been included in the study (marked with orange or red lines indicating ferry or cruise vessels, respectively). The port authority mainly focused on passenger vessels, as they considered their effect on the seabed to be greater than other types of vessels that have much less frequent passage. Moreover, passenger ships generally self-propelled, whereas other vessel types are often driven by tugboats. Therefore, we only simulated passenger ships.
The turning basins in which arriving vessels undergo maneuvers for berthing are represented by the white dashed circles marked “a” and “b” in Fig. 2. Circle a refers to vessels berthing at docks T5 to T11, whereas circle b refers to vessels berthing at docks T1 to T3. Finally, the turning area for vessels arriving at docks D.L., 1012, and 1003 is at the entrance of the port and is not simulated in this study, as it is outside of our area of interest.
The general methodology can be separated into the following four phases:
We then conducted a semiquantitative calibration and validation of the modeling
results through a comparison of the seabed evolution reproduced using the
integrated modeling system and the various bathymetric maps derived from
surveys of the port topography at approximately 1-year intervals.
The proposed approach assumes that each hydrodynamic and sediment transport simulation uses the same bathymetry as the initial bottom condition. Although this assumption may have implications, as we explain in the results section, it does not compromise the main conclusions of the study.
Passenger port of Genoa. The colored lines along the docks refer to the typology of the operating ships: red lines indicate cruise vessels, and orange lines indicate ferries. The names of the docks (in white) are given next to the colored lines. The red dot represents the location of the station where sediment samples with physical information on the grains are available (see Sect. 4.2). The white dashed circles “a” and “b” represent the turning areas for vessels berthing at docks T5–T11 and at T1–T3, respectively. Land background from © Google Earth.
Most of the data necessary for this project were provided by the Port Authority of Genoa and Stazioni Marittime SpA, the main port operator in the area.
Several bathymetry surveys of the sectors of the port were available at
various resolutions. The dataset used for the simulations was obtained by
merging the latest available surveys (March–June 2018) of the inner sectors
of the port, delivered on a regular grid of 5 m resolution.
Figure 3 shows the merged bathymetry for the entire
port (left panel) as well as detailed information on Ponte Colombo and the surrounding
basin (right panel). The main area of interest for the study (from the line between
Calatà Sanità and Molo Vecchio to the end of the port, see Fig. 2)
measures approximately 0.60 km
Bathymetry of the port of Genoa. Entire passenger port (left panel) and a zoom in of Ponte Colombo and the surrounding basins (T5–T11, right panel). Land background from © Google Earth.
The bathymetry presented in the right panel of Fig. 3 follows the pattern of erosion and accumulation common to wet basins confined among docks. The propeller activity when vessels leave or approach the berth induces areas of erosion, identified by channels of deepened bathymetry (referred to with an “e” in the right panel of Fig. 3, and colored yellow and green) and areas of accumulation identified with tongues of shallower bathymetry (denoted by “a” in the right panel of Fig. 3, and colored brown).
Another survey covering approximately the same area as that of Fig. 3 is available for the May–June period in 2017. By comparing the topographical information of the two and integrating the information on dredging activities during the same period, we were able to reconstruct, in a semiquantitative fashion, the sediment dynamics occurring during this time window of approximately 1 year. This information was then used in the calibration and validation process for the numerical model of sediment erosion and transport, as detailed in Sect. 5.
The availability of information on sediment textures in the sea is limited.
We were able to access the MArine Coastal Information sySTEm (MACISTE;
In terms of marine traffic, the Port Authority of Genoa and Stazioni Marittime SpA considered 2017 to be a typical year. The traffic data were available on a daily basis and included information on the docks of arrival and departure as well as the names of the vessels involved. The entire year was considered in order to account for the typical seasonality of the traffic concentration, which is particularly significant for passenger vessels from the end of spring to the beginning of fall.
The characteristics of the vessels required for the modeling activity (i.e., length, width, tonnage, and draft) were obtained from information available through public sources. The outcomes of the analysis are presented in Sect. 4.1.
The non-hydrostatic version of the MIKE 3 HD flow model (DHI, 2017) was used to simulate the propeller-induced three-dimensional current along the port basin. The resulting hydrodynamic field was coupled with the sediment transport module MIKE 3 MT (DHI, 2019), which is suitable for fine-grained and cohesive material, in order to drive the erosion, advection and dispersion, and deposition of fine sediment along the water column.
The MIKE 3 FM flow model is an ocean circulation model suitable for
different applications within oceanographic, coastal and estuarine
environments at global, regional, and coastal scales. It is based on the
numerical solution of the Navier–Stokes equations for an incompressible
fluid in three dimensions (momentum and continuity equations), based on
the advection and diffusion of potential temperature and salinity and on the
pressure equation, which in the present non-hydrostatic version is split
into hydrostatic and non-hydrostatic components. The closure of the model is
obtained by the choice of a turbulence closure formulation with various
possible options within a constant value as well as a logarithmic law scheme or a
The domain of the present implementation of the model is presented in the
upper panels of Fig. 4. The images show two
examples of computational grids used for the simulations. Here, the docks
are T1 (left panel) and T10 (right panel) during inbound operations. The
grids are a combination of unstructured triangular and quadrilateral cells
with horizontal resolutions varying from 30 m in the furthest areas
from the ship trajectory to approximately 5 m within the closest area
to the ships' propellers. The mesh is rectangular in areas where the ships
are moving straight ahead, and the 5 m resolution covers a corridor with a width of
approximately 50 m. In the maneuvering areas, the mesh becomes
unstructured and the resolution is again 5 m. The red lines in the
middle of the 5 m resolution corridors of the upper panels represent
the routes followed by the ships inside the port. The lower panels of the
figure are snapshots taken from the web service
Table 1 shows the results of the traffic analysis within the port of Genoa for 2017 conducted using the daily traffic data provided by Stazioni Marittime SpA. The annual traffic is generally regular, and its frequency varies from basin to basin and depends on the season. Generally, the busiest docks are T5, T6, and T7, which account for almost 50 % of the total traffic. They follow an approximate daily frequency all year round, whereas the wet basins towards the end of the port, which mainly serve cruise vessels, show an evident seasonality, probably related to the Mediterranean cruise season (few and irregular passages from January to May and then regular and a much increased frequency from June to October or November).
Model domain and computational grids for docking routes for the T1 (left panel) and T10 (right panel) docks. In the lower panels, the corresponding actual routes are shown. Land background of the upper panels is from © Google Earth.
Analysis of ship traffic in the port of Genoa for the year 2017 and the main characteristics of the ships representative of each dock. The ships' length, width, draft, and propeller diameter values are expressed in meters.
In the vertical, the model is resolved over 10 evenly distributed sigma layers. The resulting layer depths vary from approximately 1 m in the berthing areas to approximately 2 m in the pits and in the areas closer to the port's entrance.
The propellers' maximum jet velocity was calculated based on the Code of
Practice of the Federal Waterways Engineering and Research Institute
(Abromeit et al., 2010) and the PIANC Report no. 180 (MarCom WG 180, 2015),
taking the German approach. The relevant parameters for the calculations are
shown in Fig. 1. The maximum velocity
As we had no direct information about the size of the ships' propellers, we
referred to the specific literature. For the propellers of the Ro-Ro ferries
that typically serve docks T1, T2, T3, T5, T6, T7, T9, T10, and T11, we
referred to report no. 02 of the “Mitigating and
reversing the side-effects of environmental legislation on Ro-Ro shipping in
Northern Europe” project (Kristensen, 2016), implemented by the Technical University
of Denmark (DTU) and HOK Marineconsult ApS. According to this study, the
relationship between the draft and the diameter of the ferry's propeller
is given by Eq. (4):
The water discharge was obtained by combining the diameter of the propeller and the intensity of the jet, which was discretized into a certain number of smaller discharges associated with various smaller sources of momentum in the numerical model. Thus, we realistically represented the propeller. The distribution of volume and momentum sources follows a spatially Gaussian (normal) distribution with a discretization step of 0.5 m and a constant rotation rate of the propeller.
Figure 5 shows the propeller-induced jet in the
hydrodynamic model. Panel a represents the plan of dock 1012, where a
large cruise ship is departing. The solid line in Fig. 5a is
the location of the vertical transect shown in Fig. 5b,
representing the jet velocity in the plane
Representation of the propeller-induced jet of the most
representative ship departing from dock 1012.
To preserve the water mass budget, we associated a sink to each source.
Sinks are prescribed in terms of negative equivalent discharge
(m
The choice of the vertical and horizontal resolutions of the hydrodynamic
model were the result of a thorough sensitivity analysis of the grid's cell
dimensions. We assumed that the most appropriate resolution for the model
allows the maximum (jet centerline) current produced by the combined
discharge and momentum sources in the model to reach the input maximum
velocity of
Model grid sensitivity analysis to the cell's dimension. The different colors correspond to different horizontal resolutions. Dashed lines indicate the configurations with 10 layers, and solid lines indicate those with 20 layers.
Figure 6 shows the sensitivity analysis of the grid
resolution. The resulting velocity at the propeller's axis is proportional
to the resolution, both in the vertical and the horizontal: the higher the
resolution, the higher the resulting velocity. The most appropriate grid is
that with a 1 m resolution and 20 vertical layers, which is the only
configuration of the model that allows the jet to reach the maximum speed
imposed as the input. However, this configuration would require
approximately 1 year of computational time to run the 24 simulations
implemented in this study in the same computational configurations, which is
obviously unrealistic. Therefore, we sought a compromise between acceptable
computational demand and realistic resulting velocity. The final
configuration took 5 m as the horizontal resolution and 10 vertical
levels. As these resolutions did not allow for the complete development of
the current speed, we introduced a correction to the input velocity of each
simulated vessel by increasing it by the necessary amount to reach the
empirically calculated
Due to the nature of the focal processes, we only account for the force of the propellers of the vessels. The jet induced by its motion is of an order of magnitude of several meters per second in the area surrounding the blades and when unconstrained it has a length of influence of at least 40–50 times the propeller's diameter behind the ship (Verhei, 1983). This is also an important source of toe scouring in the presence of a quay wall (Hamill et al., 1999). Natural forcing such as wind, density gradients, or tides are one to two orders of magnitude smaller and can, therefore, be neglected without introducing errors that can potentially affect sediment resuspension from the bottom. However, the Bernoulli wake may be responsible for currents of comparable intensity (Rapaglia et al., 2011), although smaller, and can be a forcing source in the system. In any case, we do not consider this due to technical complications and time constraints. Including such a process in further developments and analyzing its impact on the overall dynamics of ship-induced sediment transport would be of interest. Our final results prove satisfactory, suggesting that the governing processes for these dynamics are associated more with propeller-induced currents than with the motion of the ship itself, likely due to the limited speeds of vessels in this inner part of the harbor and to the relatively large volume of water available for each passing vessel.
The boundaries of the hydrodynamic domain are the docks around the basin and the port entrance, which is the only open boundary. Here, we imposed a Flather condition (Flather, 1976), assuming constant zero velocities and levels. This allowed us to minimize the boundary effects, albeit with some interference between the flux and the boundary line (not shown). However, due to the distance between the open boundary line and the berthing areas, such effects do not influence the results of the study. A zero normal velocity was imposed along the closed boundaries.
The hydrodynamic model was coupled with a sediment transport model – MIKE 3
MT FM – valid for fine-grained and cohesive sediment (diameter smaller than
63
The equations of the mud transport model are based on the advection and dispersion (AD) of the sediment concentration along the water column and are detailed in Appendix B. The AD equation is solved using an explicit, third-order finite difference scheme called ULTIMATE (Universal Limiter for Transient Interpolation Modeling of the Advective Transport Equations; Leonard, 1991).
The model consists of two areas: a water and a seabed environment. The
seabed is represented through a multi-bed layer and multi-fraction approach
in which the layers can exchange mass and only the top level is active, thereby
making it available for erosion. The different layers are defined by the
proportions of sediment in their composition, the degree of consolidation of
the sediment within each layer, and the thickness of the single layer. The
sediment proportions are described through their associated physical
characteristics, and are eroded and deposited proportionally to their
concentration both in the bed texture and along the water column.
Flocculation processes occur in the water environment of the model when a
certain concentration threshold is exceeded (here assumed to be equal to
0.01 g L
The specific equations and parameterizations referred to in the sediment model are summarized in Appendix B.
Three sediment surveys were conducted between June 2009 and July 2010. Table 2 presents the results of the surveys in terms of percentage and class of sediment per survey (right and center column, respectively). Given the nature of our study, our focus is on mud and fine sand; thus, grains coarser than 2 mm were not considered.
Sediment size data inside the port (see the station identified using the red dot in Fig. 2). Three different surveys were carried out between June 2009 and July 2010. (All times are given in local time.)
We assumed that the proportions of the samples with
The degree of consolidation of the seabed is both time- and depth-dependent.
The upper layer, which mostly contributes to the flux of resuspended
sediments into the water column, is composed of freshly deposited sediment
as it is subject to continuous reworking. The lower layers are more
consolidated, and the degree of consolidation increases by depth. This
vertical gradient in seabed properties is enhanced in a port environment, as
the upper layers are continuously influenced by the propeller-induced jets
(several times per day); hence, multilayer modeling of the seabed is
appropriate. Teisson et al. (1993) and Sandford and Maa (2001) also took this
approach. A single layer bed representation would imply an overestimation of
the bed's erodibility (soft mud and thus easily reworked), resulting in
unrealistic further overestimations of sediment erosion and concentration
along the water column. Therefore, a multilayer representation of the seabed is
required to account for the transition from unconsolidated to consolidated
material. Amorim et al. (2010) used a two-layer approach to model the seabed
with MIKE software, simulating the sediment transport in the navigation
channel of the port of Santos. However, as they suggested, a two-layer
representation of the seabed may produce an unrealistically abrupt
transition between erodible and hard bed layers; therefore, in order to consider a gradual
transition from freshly deposited to consolidated material, three bed layers
were defined here, representing the freshly deposited, slightly consolidated,
and fully consolidated sediments. The percentage of the fine particles in
the sediment texture was assumed to decrease proportionally to the depth of
the layers. Thus, the first layer contained 80 % of fine grains (50 % of
grains of
The main characteristics of the layers and sediment proportions implemented in the sediment transport model are presented in Table 3.
Finally, sediment input may also potentially come from six minor streams
that flow into the port area. These have very modest basins of approximately
1 km
Summary of sediment characteristics as implemented in the mud transport model.
The main results of the hydrodynamic and sediment transport model are presented in this section. Due to the large number of simulations carried out, only those regarding two docks are shown. However, the current and sediment concentration results corresponding to the other simulations are qualitatively similar. We focus on the simulations of docks 1012 and T7. dock 1012 is particularly important as it hosts the largest passenger vessels operating in the port, whereas dock T7 has a high frequency of passages.
Figure 7a and b show the propeller-generated current in
the bottom layer and at the depth of the propeller's axis, respectively, and Fig. 7c and d show the corresponding resulting suspended sediment
concentration in the same layers during the
departure of a cruise vessel from dock 1012. The characteristics of a vessel
representative of the traffic in the dock are given in
Table 1. When departing, the engine operates close
to full power, which we assume results at a rotation rate of 2 rps for the propeller. This induces a maximum velocity at the depth
of the propeller axis close to 9 m s
Various hydro and sediment dynamics occur during the inbound phase of
vessels maneuvering inside the port. Most of the maneuvering operations
(i.e., when vessels rotate within a turning basin and proceed backwards to
the docks) occur in the turning basins denoted by the dashed circles a and
b in Fig. 2. The engines operate at high power when
starting the maneuver to allow for the rotation of the ship. The vessel's
longitudinal axis then rapidly changes direction (from tens of seconds up to
a few minutes) and can span wide angles, depending on the specific
maneuver. The propeller-induced jet follows the same rotation along the
horizontal plane, resulting in a fan-like distribution of directions for
the associated currents. Such operations are realistically represented by
the model, as shown in Fig. 8, which refers to the
berthing of the vessel representative of dock T7. The currents shown in the
figure are those associated with the propeller's axis during four different
moments of the turning maneuver. Each panel refers to successive time
intervals of approximately 100 s. These successive instants are
presented in the following order: upper-left panel, upper-right panel, lower-left panel, and lower-right panel. In
the lower-right panel, the propeller has already changed rotation direction
and the vessel is now proceeding backwards. Thus, the induced current jet is
heading towards the center of the port and pushing the sediment towards this
area. The simultaneous seabed activity is shown in
Fig. 9. Although the jet-induced currents are
much weaker at the seabed than those at the depth of the propeller's axis,
they are still significant and may reach intensities of up to 1 m s
Results of the numerical models.
The current distribution at the seabed is much more chaotic than at the propeller's axis depth. This area of the port corresponds to the natural pit (which reaches approximately 22 m below the surface in the deeper part) in which the material dredged from the accumulation areas is often dumped during the sea bottom maintenance activities. The dashed line shown in the lower-right panels of Fig. 8 and Fig. 9 refers to the transect presented in Fig. 10, for the same instant (i.e., when the vessel has ended the maneuver in circle b and is approaching dock T7 backwards).
Results of the hydrodynamic model at the depth of the
propeller's axis. Each panel refers to a time interval of approximately 100 s from the previous panel. The temporal order of the panels is as follows:
A combined analysis of Figs. 8,
9, and 10
helps us understand the dynamics occurring in turning basin b during the
maneuvers when approaching docks T5, T6, and T7, and particularly the
overall sediment dynamics of the entire port, as these three docks account
for approximately half of the entire passenger traffic. The
propeller-induced velocities at the bottom of the natural pit during turning
maneuvers are variable and may exceed 1 m s
Results of the hydrodynamic model in the bottom layer. Each
panel refers to a time interval of approximately 100 s from the
previous panel. The temporal order of the panels is as follows:
The impact of the marine traffic on the bed thickness is illustrated in Fig. 11, which presents the erosion and deposition maps resulting from the simulations of one departure (left column) and one arrival (right column) of the representative passenger vessels of docks 1012 (top row) and T7 (bottom row). Blue represents areas of erosion, and red represents the accumulation of the sediment after an interval of time long enough for the resuspended sediment to completely settle. The left column Fig. 11 shows that a considerable amount of material tends to be eroded from the bases of the docks during the vessel's departure and then settles in the center of the mooring basins. This mechanism is clearly related to the vessel's departure (left column) rather than its arrival (right column). The erosion underneath the vessel's keel along its trajectory is evident, both during departure and arrival, thereby supporting previous experimental findings (Catells et al., 2018). The magnitude of the erosion and deposition of a single vessel's passage is of the order of a few millimeters in the areas most influenced by the vessel's activity.
Such an impact can become a real threat to the continuity of operations in large and busy ports such as Genoa over medium to long timescales. The few millimeters of accumulation and erosion can become several tens of centimeters after a few thousand annual passages. For the sake of completeness, the results of the impact on the bed thickness due to the activity of the other vessels not shown in the main body of the text are presented in Appendix C.
Based on the traffic analysis in Table 1, we projected each single marine passage to a 1-year duration and superimposed the effects of erosion and deposition of vessels that are representative of all of the passenger docks. Thus, we were able to reconstruct the annual port seabed evolution for the year of 2017. The effects of the single passages were weighted by the specific occurrences of that year, which resulted in 24 maps (one for each docking and one for each undocking), and the results were integrated to obtain a final map.
As the trajectories for reaching a dock (or departing from it) vary slightly from passage to passage, a Bartlett spatial filter was applied to the integrated results using the values of 4, 2, and 1 as weights. Figure 12 presents the results of this analysis. In the left panel, the results from the modeling system in terms of annual erosion (blue) and accumulation (red) are shown, and in the right panel, the observed seabed evolution is shown. The observed map was reconstructed using the outcomes of two bathymetric surveys carried out in the May–June 2017 and March–June 2018 periods. The difference in the bathymetries of the two surveys resulted in the evolution of the seabed during the approximate 1-year period, except for dredging operations. We indicated the areas where the most significant dynamics took place on the maps using numbers.
The area between the heads of Ponte dei Mille and of Molo Vecchio, identified as 1 in Fig. 12, was dredged during
the October–December period in 2017, and approximately 15 000 m
Erosion and deposition maps resulting from one departure (left column) and one arrival (right column) of the representative passenger vessels of docks 1012 (top row) and T7 (bottom row). Land background from © Google Earth.
Area 1 accounts for approximately 30–40
The central portions of the wet basins marked with number 2 in
Fig. 12 are areas of deposition, mainly due to the
departure phase of the ships. Again, the model can efficiently reproduce both
the accumulation along the central parts of the basins, where it may reach
20
The erosion underneath the vessels' typical routes (i.e., from the entrance to approximately the center of the port) is also well represented by the model (identified using the number 6 in Fig. 12). The model and the observations also exhibit good agreement in the deposition area (number 7), where a local gyre forms and entraps the suspended sediment. Finally, areas 3 and 4 are also subject to deposition, and qualitative agreement between the model and the various bathymetric surveys is evident from Fig. 12. The erosive print observed in the survey under these areas is most likely due to activities related to cargo vessels approaching and departing from dock Calata Sanità. These vessels were not the focus of our study, and Calata Sanità only operates container ships; thus, the model does not include the marine traffic in this area.
Annual erosion and deposition map reconstructed on the basis of the hydrodynamic and sediment transport simulations for the year 2017.
In general, the observed and the modeled annual evolution of the port seabed show very good agreement, which confirms the reliability and robustness of the hydrodynamic and sediment transport model and demonstrates the potential importance of an integrated modeling approach in optimizing the management of port activities.
The assumption of unvarying initial bathymetry conditions in the different
scenarios deserves some additional consideration, as it undoubtedly
introduces some inaccuracy into the results. This approach does not consider
the real order of vessels' passages or the impact that the evolving seabed
has on the hydrodynamics and sediment transport simulations. In particular,
the variable clearance distance between the propeller's tip and the seabed
due to the evolving erosion and deposition processes is not considered, although
this will increase the differences over time. However, the complexity of the
system requires the introduction of several approximations, such as the
dimension and rotation rates of the propellers, the typology and
distribution of the sediment, the layering of the sea bed, the shear stress
for erosion and deposition, or the constant initial bathymetry. A solution
for the bathymetry issue could be to implement the system in operational
mode and, thus, continually update the initial bottom boundary conditions
through the simulation iterations. However, this was not realistic in terms
of computational effort and was beyond the scope of the study, which was to
identify areas of erosion and deposition in the port and to evaluate the
order of magnitude of the corresponding evolution rates to support the port
management. Nevertheless, if we consider the most significant variation in
the seabed and the typical propeller-induced bottom velocities, which are of
the order of 50 cm (Fig. 12) and 1–2 m s
The impact of marine traffic on the seabed of the passenger port of Genoa was investigated through numerical modeling. The combination of a very high-resolution, non-hydrostatic, circulation model (MIKE 3 HD FM) with a sediment transport model (MIKE 3 MT FM), based on unstructured grids on the horizontal and on sigma levels on the vertical, enabled us to reconstruct the annual evolution of the port seabed. The final results of the modeling, in terms of maps of erosion and deposition inside the basin, were qualitatively supported by observational evidence. Our approach was to simulate only one arrival and one departure from each dock of the port and to analyze the impact of a single marine passage on the seabed in terms of sediment concentration, motion, and distribution.
From the traffic analysis in the port for a typical year (2017), we could obtain the detailed situation of the number of arrivals and departures for each dock as a starting point for the study. By superimposing the effects of single vessels weighted for the annual number of passages of the most representative vessel operating on each dock, an annual map of erosion and deposition was reconstructed and validated on a semiquantitative basis by comparison with various bathymetric surveys for the same period.
In general, the simulations showed that the velocity intensities on the
bottom induced by propeller-generated jets can reach almost 2 m s
Our findings showed how significant these deposition rates can be in a densely operated port, reaching values of several tens of centimeters per year in specific areas.
Our approach enabled us to minimize the computational time and also decompose the overall complex view of sediment transport of the entire port into several simpler views. Consequently, we were able to analyze the specific hydro and sediment dynamics for each dock and vessel, and to identify specific routes responsible for particularly serious erosion and accumulation, as historically reported by the management authorities of the port operations and traffic. The range of current intensities induced by the propeller action was identified along the water column, and this can be further used as a sound and scientifically based benchmark value for potential defensive actions on the seabed and port structures in order to guarantee the ongoing full operability of the port.
The most significant mechanisms for the port's hydro and sediment dynamics
that occur during vessel passages were identified and the subsequent
analysis identified how and why specific areas are subject to erosion and
other areas are subject to deposition as well as the extent of these mechanisms.
In particular, the mechanism of ongoing erosion along the docks' walls and of
deposition along the central portions of the mooring basins were identified
and explained, along with the ongoing deposition process in the area between
the heads of Ponte dei Mille and Molo Vecchio. Identifying and reproducing this process for the port
managers was particularly important, as it occurs at a very significant rate
of up to 40–50
The importance of this study is not only to confirm how integrated high-resolution modeling can reproduce the most significant and complex mechanisms of hydrodynamics and sediment transport occurring inside ports, which was successfully achieved, but it also suggests that it can be used as a tool for optimizing port management. It could be applied to regulate the marine traffic in ports and, thus, identify the most suitable schedule and routing in terms of sediment concentrations, bottom velocities, erosion, accumulation, and vessel drafts. It could also be used to identify the largest vessels that can potentially operate in the docks when planning future commercial traffic or to study the impact of increased port traffic on the seabed and on the port's structures. Finally, in recurring dredging operations, most busy ports must regularly face sediment accumulation problems, and our tool can inform awareness planning of such activities so that authorities are fully prepared.
Daily fully operational implementations of similar integrated systems can also be set up, as the daily schedule of the port is known. This would enable the continuous monitoring of the evolution of the seabed and allow authorities to be constantly and fully aware of the potential critical issues that they face.
Future research following on from this study should also consider the effect of the Bernoulli wake in combination with the propeller-induced jets on sediment resuspension, advection, and dispersion. This mechanism was not considered in the present version of the system. The current intensities caused by vessel-generated waves during and after their passages will be smaller than those induced by propellers along their axes, but they tend to penetrate along the water column and reach the bottom, thereby carrying a significant amount of energy and possibly resuspending a substantial amount of solid material (Rapaglia et al., 2011), which is likely to enhance vertical mixing and may induce the sediment to be suspended for longer periods and at higher depths.
The MIKE 3 Flow Model FM is based on the Navier–Stokes equations for an
incompressible fluid under the assumptions of Boussinesq. The governing
equations of the model are the equations of momentum (A1) and mass
continuity (A2), the equations of heat and salinity transport (A3 and
A4, respectively), and the equation of state (A5) based on the UNESCO
formula of 1981 (UNESCO, 1981a). Considering a Cartesian coordinate system
As we used the barotropic density mode, the only hydrodynamic equations used for the present work are Eqs. (A1) and (A2). The symbols used in the governing equations of the model are presented in Table A1.
Symbols used in the governing Eq. (A1).
The sediment transport module is based on the advection dispersion equation
for a passive tracer in an incompressible fluid. The tracer is the
concentration
The settling velocity for sediment is calculated through the Stokes' law
Eq. (B7).
Symbols used in Eq. (B1) to (B7) and the associated parameterizations of the sediment transport model.
The following matrices of plots (Fig. C1) present the results in terms of sediment erosion and accumulation for the scenarios for docks T1, T2, T3, T5, and T6 (top to bottom, left part of Fig. C1) and T9, T10, T11, DL, and 1003 (top to bottom, right part of Fig. C1). Undocking and docking phases are represented in the left and right panels, respectively.
Sediment erosion and accumulation for the scenarios of docks T1, T2, T3, T5, and T6 (left, top to bottom) and for the scenarios of docks T9, T10, T11, DL, and 1003 (right, top to bottom). Land background from © Google Earth.
The modeling dataset, including the simulations produced for the present study, comprises a data volume of more than 2 TB. Such a large amount of data raises an evident problem with respect to making them available on data repositories. Consequently, the output of the simulations will not be directly available. However, the model setup and all of the files necessary for their reproduction will be made available in MIKE FM format upon reasonable request from the corresponding author.
AG implemented the numerical models and simulations, post-processed the raw output, analyzed the results, and wrote the paper. Sina Saremi gave technical and scientific support during the implementation of the models, provided the code for modeling the propellers as input to MIKE, and supported the writing and finalization of the paper. AP first conceived the idea for the methodology adopted in the study, gave scientific support regarding the implementation of the models, and provided feedback during the writing of the paper. JHJ provided scientific support and advice regarding the driving mechanisms of marine-induced sediment dynamics. ST provided technical support for the model implementation and for the observed bathymetry analysis and reconstruction. CV and MV provided bathymetry data, sediment data, and information on dredging activities and general sediment-related issues. They also aided in the acquisition of the marine traffic data.
Caterina Vincenzi and Marco Vaccari are employees of the Port Authority of Genova (Autorità di Sistema Portuale del Mar Ligure Occidentale), which commissioned and funded the present study that was carried out by DHI, a private not-for-profit consultancy and research company in the field of water. Andrea Pedroncini, Silvia Torretta, Sina Saremi, and Jakob H. Jensen are DHI employees. Antonio Guarnieri was a DHI employee when the study was conducted; he is now employed at Istituto Nazionale di Geofisica e Vulcanologia (INGV).
This article is part of the special issue “Advances in interdisciplinary studies at multiple scales in the Mediterranean Sea”. It is not associated with any conference.
We are grateful to Stazioni Marittime SpA for providing the daily traffic data from the port of Genoa, which was the starting point for this study. We are particularly grateful to Captain Calcagno of Stazioni Marittime SpA for the qualified and experienced information he provided on the sediment and vessel dynamics in the port, which helped set up the numerical models and interpret and validate the final results.
We are also particularly grateful to Mujal-Colilles and the anonymous referee, who revised the first version of the paper, as their constructive criticism and comments helped us enrich and improve the final version of the article.
This paper was edited by Vanessa Cardin and reviewed by Anna Mujal-Colilles and one anonymous referee.