CNR-ISMAR

Consiglio Nazionale Ricerche - Istituto di Scienze Marine

Castello 2737/F, 30122 Venezia, Italia

Assessment of dumping material

potential spreading (bedload, suspended)

from Lithuanian dumping sites using 3-D

sediment transport model

Christian Ferrarin and Georg Umgiesser

ECODUMP RAPORT - I PART

June 2013

Contents

1 Introduction 4

2 Description of the models 5

2.1 The Hydrodynamic model 5

2.1.1 Computation of fluid density 8

2.2 The sediment transport model 8

2.2.1 Friction factor and bed shear stress 9

2.2.2 Non-cohesive sediments 9

2.2.3 Cohesive sediments 10

2.2.4 Sediment exchange with the bed 11

2.2.5 Morphodynamics 13

2.2.6 Bed representation 13

2.3 Transport and diffusion model 15

2.3.1 Transport and diffusion of salinity 15

2.3.2 Transport and diffusion of water temperature 16

2.3.3 Transport and diffusion of suspended sediments 16

3 Model simulation set-up 18

3.1 Numerical grid 18

3.1.1 Boundary conditions and forcing 18

3.1.2 Dumping sites and material 20

4 Simulation results 23

4.1 Model validation 23

4.2 Spread and deposition of dumping material 24

2

5 Conclusions 30

3

1 Introduction

This research activity focused on the assessment of the potential spread of

the dumping material (bedload, suspended) from existing dumping sites in

Lithuania using 3D sediment transport model. Modelling involve the spread

of dredged sediments during disposal and after the sediments being settled

with given scenario of multiple dumping events for one year. The numerical

investigations covered two dumping sites operating in Lithuanian territorial

waters: III (deepwater) dumping site, used mainly for dumping of till (morainic

deposits) and mud dredged from Klaipeda port area, located at 43-48 m depth;

IV dumping site, used for the disposal of sandy sediments (fine sand and silty

sand) at the depth of 28-34 m. The work was organized as follow:

• setting up sediment transport model with necessary resolution for given

task;

• calibration of the model using available data;

• simulations and analysis of the results;

• detailed description of simulation results and conclusions.

All necessary data for the simulations (dumping site locations, dumping mate-

rial amounts and properties, areas of interest for sediments spread, bathymetry

maps, maps of bottom sediment types, suspended sediments concentrations in

the area of interest, atmospheric forcing data, river discharges) were provided

by the Coastal Research and Planning Institute (CORPI), Klaipeda Univer-

sity.

4

2 Description of the models

The unstructured grid-based numerical model used in this study is a cou-

pled 3-D baroclinic and sediment transport model working simultaneously on

a common finite element grid. The 3-D hydrodynamic finite element model

SHYFEM (Umgiesser, 1997) solves the shallow water equations with a semi-

implicit algorithm that is unconditionally stable for gravity waves. At each

time step, the resulting 3-D model computes for every node of the numerical

domain the water level and the current velocities in each layer. Thereafter

the sediment transport rate model SEDTRANS05 (Neumeier et al., 2008)

computes the erosion and deposition rates and determines the suspended sed-

iment volume in the bottom model layers for several sediment sizes. Finally,

suspended sediment transport is computed by means of a transport and dif-

fusion module whereas the bedload sediment transport is computed by means

of a direct advection scheme.

2.1 The Hydrodynamic model

The hydrodynamic model SHYFEM here applied has been developed at ISMAR-

CNR (Institute of Marine Science - National Research Council) (Umgiesser

and Bergamasco, 1995; Umgiesser et al., 2004). The model uses finite elements

for horizontal spatial integration and a semi-implicit algorithm for integration

in time. The finite element method is highly flexible due to the subdivision

of the numerical domain in triangles varying in form and size. It is especially

suited to reproduce the geometry and the hydrodynamics of complex shal-

low water basins such as Venice Lagoon with its narrow channels and small

5

islands.

Velocities are computed in the center of each element, whereas scalars are

computed at each mode. Vertically the model applies Z layers with varying

thickness. Most variables are computed in the center of each layer, whereas

stress terms and vertical velocities are solved at the interfaces between layers.

The model resolves the primitive equations, vertically integrated on each layer.

The horizontal diffusion, the baroclinic pressure gradient and the advective

terms in the momentum equation are treated fully explicitly. The Coriolis

force and the barotropic pressure gradient terms in the momentum equation

and the divergence term in the continuity equation are semi-implicitly treated.

The vertical stress terms and the bottom friction term are treated fully im-

plicitly for stability reasons due to the very shallow nature of the lagoon. The

discretization results in unconditional stability which is essential for modelling

the effects of fast gravity waves, bottom friction and the Coriolis acceleration

(Umgiesser and Bergamasco, 1995). For the computation of the vertical diffu-

sivities and viscosities a turbulence closure scheme has been used. This scheme

is an adaptation of the k-ε module of GOTM (General Ocean Turbulence

Model) described in Burchard and Petersen (1999).

The equations, integrated on each layer, are:

∂Ul∂t

+ ul∂Ul∂x

+ vl∂Ul∂y

− fVl = −ghl∂ζ

∂x− ghl

ρ0

∂

∂x

∫ ζ

−Hl

ρ′dz + (1)

−hlρ0

∂pa∂x

+1

ρ0(τ top(l)x − τ bottom(l)

x ) + AH(∂2Ul∂x2

+∂2Ul∂y2

)

∂Vl∂t

+ ul∂Vl∂x

+ vl∂Vl∂y

+ fUl = −ghl∂ζ

∂y− ghl

ρ0

∂

∂y

∫ ζ

−Hl

ρ′dz + (2)

6

−hlρ0

∂pa∂y

+1

ρ0(τ top(l)y − τ bottom(l)

y ) + AH(∂2Vl∂x2

+∂2Vl∂y2

)

∂ζ

∂t+∑l

∂Ul∂x

+∑l

∂Vl∂y

= 0 (3)

with l indicating the vertical layer, (Ul, Vl) the horizontal transport at each

layer (integrated velocities), f the Coriolis parameter, pa the atmospheric pres-

sure, g the gravitational acceleration, ζ the sea level, ρ0 the average density of

sea water, ρ = ρ0 + ρ′ the water density, τ the internal stress term at the top

and bottom of each layer, hl the layer thickness, Hl the depth of the bottom

of layer l, AH the horizontal eddy viscosity.

The boundary conditions for stress terms are:

τ surfacex = cDρawx√w2x + w2

y τ surfacey = cDρawy√w2x + w2

y (4)

τ bottomx = cBρ0uL√u2L + v2L τ bottomy = cBρ0vL

√u2L + v2L (5)

where cD is the wind drag coefficient, cB is the bottom friction coefficient, ρa is

the air density, (wx, wy) are the zonal and meridional components of the wind

velocity respectively and (uL, vL) is the water velocity in the lowest layer.

At the lateral open boundaries of the domain, the water levels are prescribed

while at the closed boundaries the normal velocity is set to zero and the tan-

gential velocity is a free parameter. This corresponds to a full slip condition.

The model also simulates flooding and drying of the shallow water flats. This

is especially important in Venice Lagoon, because the intertidal area covers

about 15% of the lagoon at low water spring tide conditions. The flooding and

drying mechanism has been implemented in a mass consistent way, and spuri-

ous oscillations that are generated are quickly damped. When the water levels

7

fall below a threshold of 5 cm, the element is removed from the computation,

and water mass is conserved to compute water levels at every time step with

a laplacian interpolation. The element is reintroduced into the computation

when the interpolated water level values are higher than a second threshold

of 10 cm. The wet and dry mechanism and its implementation into the model

is fully described in Umgiesser et al. (2004).

2.1.1 Computation of fluid density

The contributions of salinity, water temperature and pressure on the water

density (ρ) is calculated in the model by the international Unesco equation of

state (Unesco, 1981).

The developed approach includes also the effect of suspended sediment on the

fluid density as:

∆ρ = Cs ∗(ρs − ρ)

ρs(6)

where Cs is the suspended sediment concentration (SSC) and ρs is the sediment

grain density.

2.2 The sediment transport model

The sediment transport model SEDTRANS05 (Neumeier et al., 2008) sim-

ulates erosion and sedimentation rates under either steady currents or the

combined and time-dependent influence of waves and currents.

The model adopts the Grant and Madsen (1986) continental shelf bottom

boundary layer theory to predict bed shear stresses and the velocity profile

in the bottom boundary layer. The velocity computed by the 3-D hydrody-

8

namic model in the bottom layer is used to calculate the bed shear stress.

SEDTRANS05 provides 5 formulations to predict sediment transport for non-

cohesive sediments: the methods of Brown (1950), Yalin (1963) and Van Rijn

(1993) predict the bedload transport; the methods of Engelund and Hansen

(1967) and Bagnold (1963) predict the total load transport.

Multiple grain sizes are used to track changes in seabed texture, and differen-

tial transport of material.

2.2.1 Friction factor and bed shear stress

The bed shear stresses and the velocity profile in the bottom boundary layer

are computed following the Grant and Madsen (1986) continental shelf bottom

boundary layer theory. This method accounts for current-wave interactions.

An explicit combined-flow ripple predictor is included in the model to provide

time-depended bed roughness prediction (Li and Amos, 2001). The model as-

sumes that total bed roughness (z0) is composed of grain roughness, bedform

(ripple) roughness as well as bedload roughness when sediment is in trans-

port. Bed roughness effects on boundary layer parameters are included in the

computation of friction factor and effective bed shear stress (τcs and τcws).

For cohesive sediment, a default friction factor (0.0022) and a default bed

roughness (0.0002 m) are defined according to Soulsby (1997).

2.2.2 Non-cohesive sediments

As bed shear stress increases, sediment particles will first be entrained from

their resting position and then start to move along the bed by more or less

9

regular jumps (bedload transport).

Five methods are proposed to predict sediment transport for non-cohesive

sediments. The methods of Brown (1950), Yalin (1963) and Van Rijn (1993)

predict the bedload transport. The methods of Engelund and Hansen (1967)

and Bagnold (1963) predict the total load transport.

Multiple sand grain size classes are considered to behave independently.

2.2.3 Cohesive sediments

The cohesive sediment algorithm was designed to model a full cycle of erosion-

deposition and possibly also the consolidation process.

The suspended sediment population is divided into several classes to represent

the natural variability of suspended sediment, each characterised by its settling

velocity ws(i) and concentration C(i). Each suspended particle is assumed to

have a characteristic ws(i), which is defined during the erosion process when

the particle is put into suspension. This ws(i) may be modified temporarily to

take into account flocculation and hindered settling.

If τ0 is higher than the critical shear stress of erosion of the bed surface τce(0),

then sediment erosion will occur. The mass erosion rate re is defined using

a standard formula for beds with variable τce (Van Rijn, 1993; Parchure and

Mehta, 1985):

re = E0 exp [Pe(τ0 − τce(z))0.5] (7)

where E0 is an empirical coefficient for minimum erosion, Pe is the propor-

tionality coefficient for erosion, and τce(z) the critical shear stress for erosion as

a function of erosion depth. For each time step, re is first computed with the

10

surface τce, then the eroded depth ∆z is computed (taking into account the

linear variation of ρdry with depth). If erosion occurs, a log-normal distribution

of seven ws-classes is put in suspension at each erosion step. The median of

this distribution depends on the erosion conditions (Neumeier et al., 2008).

Deposition of the suspended sediment of a ws(i)-class occurs only when the

bed shear stress τ0 is less than the critical shear stress for deposition τcd.

The deposition rate is computed as:

rd = Ciws(i) (1 − τ0/τcd) (1 − Ps) (8)

where Ps is a dimensionless probability coefficient of resuspension in the depo-

sitional state (ranging from 0 to 0.2 with a default value of 0). The deposition

of each class of suspended sediment is computed separately.

Further details are given in Neumeier et al. (2008).

2.2.4 Sediment exchange with the bed

Different approaches have been used to compute the net sediment flux between

the water column and the bottom (the benthic flux) for cohesive and non-

cohesive sediments. If ED(i) is positive erosion occurs, while if it is negative

deposition occurs.

For the cohesive sediment the flux term is given by the difference between the

erosion re and deposition rd rates, calculated by equations (7) and (8).

The net sediment flux between the bottom and the water column for non-

cohesive suspended sediments is computed as the difference between the equi-

librium concentration and the existing concentration in the lower level (Lesser

11

et al., 2004). The resulting expressions are:

Source = Ceq(i)(νv∆z

)

Sink = C(i)(νv∆z

+ ws(i))

(9)

with ws(i) the settling velocity, νv is the vertical mixing coefficient, Ceq(i) the

average equilibrium sediment concentration in the layer, calculated from the

near-bed equilibrium concentration Ceq(i) and assuming a logarithmic velocity

and SSC profile (Rouse like profile), C(i) is the existing suspended sediment

concentration in the layer and ∆z is the vertical distance from the center of

the layer and the bottom roughness height. This equation clearly indicates

that when the near bed sediment concentration is less than the equilibrium

value a net flux from the bed into the water column occurs. Likewise when

the concentration exceeds equilibrium, a net flux to the bed occurs.

Ceq(i), the sediment concentration at the reference height (bed roughness z0,

see section 2.2.1), is calculated using the formula of Smith and McLean (1977)

adapted to include the presence of multiple sediment fractions:

Ceq(i) = ηiγ0Cbτ∗/(1 + τ∗) (10)

where Cb = 0.65 is the volume concentration of bottom sediment, ηi is the

relative availability of the sediment fraction i at the bed, τ∗ = (τcws + τcr)/τcr

is the normalized excess shear stress, with τcws being the skin-friction combined

shear stress and τcr the critical shear stress for initiation of motion, and γ0 is

the empirical sediment resuspension coefficient (Li and Amos, 2001).

12

2.2.5 Morphodynamics

Modifications to the bed elevation are equal to the sum over the sediment

fractions of the net change due to erosion and deposition. The net sediment

change due to bedload is calculated using the sediment continuity equation,

which reads:

∂hb∂t

=1

1 − ε(∂qbx∂x

+∂qby∂y

) (11)

where hb is the change in sediment bed, ε is the sediment porosity and qbx and

qby are the volumetric bedload transport rate in x and y direction. A direct

advection scheme is used for the above equation.

Modifications to the sediment bed caused by resuspension and redistribution

of the suspended sediment are calculated as follow:

ρs(1 − ε)∂hb∂t

= −∑

ED(i) (12)

with ρs being the sediment density, ε is the sediment porosity and ED(i) is the

water column-bottom flux computed as the difference between resuspension

and sink (see section 2.2.4).

Water depth is updated every time step based on the net erosion and deposi-

tion.

2.2.6 Bed representation

The sediment bed model uses a three-dimensional grid underneath the hy-

drodynamic grid. Sediment within each class is exchanged between the bed

and the overlying water column through erosion and deposition. The bed could

have spatially different characteristics, such as grainsize composition, sediment

density and critical stress for erosion.

13

The bed is subdivided in several layers and levels. Each layer is considered

homogeneous, well mixed and characterized by its own grain size distribution

(fraction of each class of sediment considered). At each level are defined the

dry bulk density ρdry and the critical stress for erosion τce; it is assumed that

these variables vary linearly between two levels. If a layer is completely eroded,

it is removed and the remaining layers are moved upward. If a layer is only

partially eroded, the surface value of ρdry and τce are updated assuming a linear

variation in the uppermost layer. When deposition occur a linear variation in

the uppermost layer. When deposition occur the thickness of the uppermost

layer increases and the values of ρdry and τce are updated as freshly deposited

sediments. When the thickness of the uppermost layer reaches a defined limit

a new layer is added to the top.

At each location the uppermost layer has to be always greater or equal to the

surficial active, or mixed, layer that is available for suspension (Harris and

Wiberg, 2001). Active layer is considered to be the bottom roughness height

(defined as the sum of the grain roughness, the bedload roughness and the

bedform (ripple) roughness). The volume of sediment removed from the bed

for each size class during any time step is limited by the amount available in

the active layer.

Multiple sand grain size classes are considered to behave independently. Based

on laboratory and field experiments several researchers identified a transition

from non-cohesive to cohesive behaviour at increasing mud content in a sand

bed. A sand bed with small amounts of mud shows increased resistance against

erosion (Van Rijn, 1993). Finally, self-weight consolidation has been modelled

using a simplified, empirical numerical model (Neumeier et al., 2008).

14

2.3 Transport and diffusion model

The solute transport model model solves the advection and diffusion equation,

which, in the 3-D form, is given as:

∂Cl∂t

+ ul∂Cl∂x

+ vl∂Cl∂y

+∂(wl − ws)Cl

∂z=

∂

∂x

(Kh

∂Cl∂x

)+

∂

∂y

(Kh

∂Cl∂y

)+

∂

∂z

(Kvl

∂Cl∂z

)+ E (13)

where Cl is the concentration of any tracer (salinity, water temperature, or

conservative tracer) at layer l, ul, vl and wl are the velocities at layer l, ws is

the (positive) settling velocity (equal to zero in case of salinity and water tem-

perature), Kh and Kvl are respectively the horizontal and vertical turbulent

diffusion coefficients and E is a source/loss term.

The horizontal turbulent diffusivity was calculated using the model proposed

by Smagorinsky (1963), with a Smagorinsky parameter of 0.3. Vertical diffu-

sivities are calculated by the k-ε turbulence closure model. Fluxes through the

bottom were neglected here. The transport and diffusion equation is solved

with a first-order explicit scheme based on the total variational diminishing

(TVD) method.

This equation conserves sediment mass that is advected with currents, deposits

due to gravity and diffuses due to turbulence.

2.3.1 Transport and diffusion of salinity

In the case of salinity the source/loss term E in equation 13 represents the

difference between evaporation and precipitation through the water surface

(kg m−2 s−1). The evaporation rate is determined by the bulk aerodynamic

15

transfer method (Ham, 1999) using measurements of air temperature, relative

humidity, wind speed, air pressure and simulated water temperature.

2.3.2 Transport and diffusion of water temperature

In case of water temperature, the term E in equation 13 represents the heat

source through the water surface Q/ρcwhl, where ρ is the water density, cw

is the specific heat of water (cw=3991 J kg−1 ◦C−1) and hl is the depth of

fluid layer. Q is the heat flux (W m−2) between the atmosphere and the sea,

computed by the thermal radiative model as follows (Dejak et al., 1992):

Q = Qs +Qb +Qe +Qh (14)

where each term represents a physical process:

• Qs is the sun’s energy flux through the sea surface (short wave radiation);

• Qb is the net heat flux between the atmosphere and the sea (long wave

radiation);

• Qe is the heat flux generated by evaporation-condensation processes;

• Qh is the heat flux generated by conduction-convection processes.

The last three terms act on the air-water interface, and solar radiation is

adsorbed inside the water column over more than one layer, with an e-folding

depth of 2.0 m.

2.3.3 Transport and diffusion of suspended sediments

In case of suspended sediment, the term E in equation 13 represents the input

of dredged sediment from the top in the dumping sites. Therefore, the vertical

16

boundary conditions for the advection-diffusion equation are:

+Khtop∂Ctop∂z

+ wsCtop = E z=top of the surface layer

−Khbot∂Cbot∂z

+ wsCbot = ED z=bottom of the lowest layer

(15)

where ED is the net sediment water column-bottom flux, corresponding to

the difference between resuspension and deposition for each grain class (see

section 2.2.4).

17

3 Model simulation set-up

3.1 Numerical grid

The numerical computation has been carried out on a spatial domain that

represents the Klaipeda Strait and the Lithuania coastal area of the Baltic

Sea until the 70 m depth contour through a finite element grid. The grid

contains 4799 nodes and 8730 triangular elements. As shown in Fig. 1, the

finite element method gives the possibility to follow faithfully the morphology

and the bathymetry of the system and better to represent the zones where

hydrodynamic activity is more interesting and important, like the Klaipeda

Strait and the coastal area. Moreover, high spatial resolution has been used

to describe the areas of the dumping sites. In these areas the model resolution

(considered as the distance between two adjacent nodes) is about 200 to 400

m.

The water column is discretized into maximum 16 vertical zeta levels with

progressively increasing thickness varying from 1 m for the first 12 m to 18 m

for the deepest layer of the outer continental shelf.

3.1.1 Boundary conditions and forcing

The open boundaries of the considered system are the edges of the Baltic

Sea area and the Klaipeda Strait (Fig. 1). Open sea boundary water tem-

perature, salinity, water levels and water velocity were obtained by spatial

interpolation of 1 nautical mile spatial resolution forecasts by the operational

hydrodynamic HIROMB (Funkquist, 2003) provided by the Swedish Meteoro-

18

20.6

20.6

20.8

20.8

21.0

21.0

21.2

21.2

21.4

21.4

55

.0

55

.0

55

.2

55

.2

55

.4

55

.4

55

.6

55

.6

55

.8

55

.8

56

.0

56

.0

56

.2

56

.2

Klaipeda

NemunasRiver

Baltic Sea

Shallow dumpingsite

Deep dumpingsite

Curonian Lagoon

0 10 20 30 40 50

Bathymetry [m]

Figure 1. Computational finite element grid of the Lithuania coastal waters and the

Klaipeda Strait.

19

logical and Hydrological Institute. The temperature and salinity initial fields

were also spatially interpolated from data of model HIROMB while spatially

uniform water level was used for initial condition.

The Klaipeda Strait water fluxes, water temperature and salinity were ob-

tained by a numerical simulation of the SHYFEM model over a domain which

comprise both the Curonian Lagoon and the Baltic Sea (Zemlys et al., 2013).

Meteorological forcing fields were obtained by forecasts of the operational

meteorological model HIRLAM (www.hirlam.org) provided by the Lithuania

hydro-meteorological service. The simulations have been carried out with a

variable time step with a maximum value of 20 s for the time period between

1 January and 31 December of the year 2010.

Seven classes of sediment, ranging from clay to coarse sand, were considered

in this simulation.

3.1.2 Dumping sites and material

Two dumping sites were considered in this study: deep-water dumping site,

used mainly for dumping of till (morainic deposits) and mud dredged from

Klaipeda port area, located at 43-48 m depth; shallow-water dumping site,

used for the disposal of sandy sediments (fine sand and silty sand) at the

depth of 28-34 m. The locations of the dumping sites are illustrated in Fig. 1.

For each dumping event, the date and the amount of sediment discharged were

provided by CORPI. The amount and the properties of the dumping material

for each site are summarized in Table 1.

The number of ship passages per day for each of the dumping sites is illus-

20

Tab

le1.

Am

ount

and

pro

per

ties

ofth

ed

um

pin

gm

ater

ial

for

the

year

2010

.

Dum

pin

gD

um

pin

gT

otal

volu

me

[m3]

Mea

nse

dim

ent

conce

ntr

atio

n[m

gl−

1]

site

even

tsSan

dM

ud

Sum

<0.

002

0.00

2-0.

067

0.06

7-0.

10.

1-0.

250.

25-0

.50.

5-1.

0>

1.0

Dee

p93

035

089

6452

799

616

96.6

468.

424

6.9

164.

346

.06.

50.

4

Shal

low

8613

768

013

768

0.3

3.9

81.5

806.

011

3.8

19.6

2.2

21

trated in Fig. 2. Time-series of sediment discharge amount were used as surface

boundary conditions for each of dumping sites.

0

2

4

6

8

10

12

14

16

0 50 100 150 200 250 300 350 400

Nu

mb

er

of

du

mp

ing e

ve

nts

per

da

y

Day from 01/01/2010

Deep siteShallow site

Figure 2. Number of dumping events per day in the deep and the shallow dumping

sites.

22

4 Simulation results

4.1 Model validation

The numerical model SHYFEM was validated for the Curonian Lagoon and

the coastal area by Ferrarin et al. (2008); Zemlys et al. (2013). In these pre-

vious studies the model performance was tested using water level, sea surface

temperature (SST) and surface salinity measurements acquired in three sta-

tions inside the lagoon (Juodkrante, Nida and Vente), one in the Klaipeda

Strait and one in the Baltic Sea (Palanga, 25 km north of Klaipeda).

The statistical analysis results reported in the cited publications reveal that

the model catches the observed seasonal fluctuations in water level and de-

scribes well the seasonal cycle of the surface water temperature. Moreover, the

model reproduced correctly the salt water intrusion into the Curonian Lagoon.

Additional model validation was carried out for the year 2010 comparing the

model results against water temperature, salinity and velocity measured by

an offshore buoy located at approximatively 3 km from the entrance of the

Klaipeda Strait. Observed and simulated values are reported in Figs. 3 and 4.

The model well reproduces the dynamics of the water column close to the

Klaipeda Strait entrance which is generally characterized by a south to north

current. Due to this coastal current the jet of water coming from the Curo-

nian Lagoon is deviated northward. Occasionally, due to northerly winds (NN,

NW), the water circulation reverses and colder and fresher Curonian Lagoon

surface waters flow toward south-west.

23

Wate

r te

mpera

ture

Obs 1m

Mod 1m

Obs 15m

Mod 15m7

8

9

10

Salin

ity

Obs 1m

Mod 1m

Obs 15m

Mod 15m 4

5

6

7

8

31 Oct 4 Nov 8 Nov 12 Nov 16 Nov

Figure 3. Observed (continuous lines) and modelled (dashed lines) water temper-

ature (top) and salinity (bottom) at 1 m depth (red lines) and 15 m depth (blue

lines).

4.2 Spread and deposition of dumping material

The transport of the discharged materials depend on the circulation features in

the dumping site areas. We therefore present the general circulation patterns,

obtained averaging over the one-year long simulation, in the investigated area

in Fig. 5.

The model results show that the shallow dumping site is characterized by

an average surface current with intensity of about 4-6 cm s−1 and directed

northward. On the contrary, a southward current is identified in the bottom

layer. The deep dumping site results to be characterized by weak currents

(with average speed lower than 3 cm s−1) directed generally westward in the

24

Me

asu

red

ve

locity a

t 1

m

N

0.1 ms-1

0

Mo

de

lled

ve

locity a

t 1

m

N

0.1 ms-1

0

Me

asu

red

ve

locity a

t 1

5 m N

0.1 ms-1

0

Me

asu

red

ve

locity a

t 1

5 m

0.1 ms-1

0

31 Oct 4 Nov 8 Nov 12 Nov 16 Nov

Figure 4. Observed (black vectors) and modelled (red vectors) current velocity at 1

and 15 m depths.

25

20.6

20.6

20.8

20.8

21.0

21.0

21.2

21.2

21.4

21.4

55.0

55.0

55.2

55.2

55.4

55.4

55.6

55.6

55.8

55.8

56.0

56.0

56.2

56.2

Klaipeda

NemunasRiver

MatrosovkaRiver

Deima River

Minija River

Baltic Sea

Shallow dumpingsite

Deep dumpingsite

Curonian Lagoon

0 3 5 8 10

Current Velocity [cm/s]

20.6

20.6

20.8

20.8

21.0

21.0

21.2

21.2

21.4

21.4

55.0

55.0

55.2

55.2

55.4

55.4

55.6

55.6

55.8

55.8

56.0

56.0

56.2

56.2

Klaipeda

NemunasRiver

MatrosovkaRiver

Deima River

Minija River

Baltic Sea

Shallow dumpingsite

Deep dumpingsite

Curonian Lagoon

0 3 5 8 10

Current Velocity [cm/s]

Figure 5. Average simulated circulation patterns at the surface (left panel) and at

the bottom (right panel).

surface and southward in the bottom layer.

The sediment material discharged in the dumping sites settle in function of

the grain-size composition, the density gradient along the water column and

the ambient current velocity. Fig. 6 shows the maximum suspended sediment

concentration at the surface and at the bottom layers. SSC reach higher values

and have a wider spread in the deep dumping site with respect the shallow

one. This is due to the fact that in the deep site more sediment material is

26

discharged, the sediment is finer and due to the higher depth the sediment

particle take more time to settle. Suspended sediment concentration reached

values higher than 50 mg l−1 and the distribution of the suspension is oriented

from NW to SE in the deep dumping site and from N to S in the shallow one.

20.6

20.6

20.8

20.8

21.0

21.0

21.2

21.2

21.4

21.4

55

.0

55

.0

55

.2

55

.2

55

.4

55

.4

55

.6

55

.6

55

.8

55

.8

56

.0

56

.0

56

.2

56

.2

Klaipeda

NemunasRiver

MatrosovkaRiver

Deima River

Minija River

Baltic Sea

Shallow dumpingsite

Deep dumpingsite

Curonian Lagoon

0.0 2.0 4.0 6.0 8.0 10.0

Surface Max SSC [mg/l]

20.6

20.6

20.8

20.8

21.0

21.0

21.2

21.2

21.4

21.4

55

.0

55

.0

55

.2

55

.2

55

.4

55

.4

55

.6

55

.6

55

.8

55

.8

56

.0

56

.0

56

.2

56

.2

Klaipeda

NemunasRiver

MatrosovkaRiver

Deima River

Minija River

Baltic Sea

Shallow dumpingsite

Deep dumpingsite

Curonian Lagoon

0.0 2.0 4.0 6.0 8.0 10.0

Bottom Max SSC [mg/l]

Figure 6. Maximum suspended sediment concentration computed by the model at

the surface (left panel) and at the bottom (right panel). A zoom of the situation in

the shallow site is reported in the white window of each panel.

We could reasonably assume that in both the dumping sites the wind gen-

erated waves have little impact on the sediment resuspension. Therefore, the

water currents are the principal factor influencing the bedload transport of the

27

deposited dumping materials. The numerical model results show that due to

the weak current circulation close to the bed, most of the deposited sediments

tend to remain inside the dumping areas (Fig. 7. The deposited sediment forms

a layer of more than 20 mm in the central part of the damping site. Anyway,

part of the dumping material is spread by the currents and deposits outside

the dumping sites (pink area in Fig. 7).

28

20.6

20.6

20.8

20.8

21.0

21.0

21.2

21.2

21.4

21.4

55

.0

55

.0

55

.2

55

.2

55

.4

55

.4

55

.6

55

.6

55

.8

55

.8

56

.0

56

.0

56

.2

56

.2

Klaipeda

NemunasRiver

MatrosovkaRiver

Deima River

Minija River

Baltic Sea

Shallow dumpingsite

Deep dumpingsite

Curonian Lagoon

0.0 2.0 4.0 6.0 8.0 10.0

Deposition thickness [mm]

Figure 7. Spatial distribution of the accumulated sediments. A zoom of the situation

in the shallow site is reported in the black window.

29

5 Conclusions

In this study the potential spread of sediment discharged at two offshore dump-

ing sites was numerically investigated through the use of a 3-D finite element

model. The applied methodology allows to describe the principal processes

involved in the sediment spreading and deposition and to reproduce the fate

of sediments during disposal and after the sediments being settled.

The sandy sediment discharged in the shallow dumping site are distributed

along a north-south axis by the currents and tend to remain close the dumping

area after deposition. The fine sediments discharged in the deep dumping sites

are spread on a wider area respect to the shallow site case. Even if most of

the sediments discharged in the deep site deposit inside the dumping area, the

finest grains are transported by the ambient currents and deposit outside the

dumping site.

The adopted methodology is a powerful tool for investigating the fate of dump-

ing material and could be used to delineate a sustainable management of

dredged sediments.

30

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