sediment transport model

8/4/2019 Sediment Transport Model

1/11

Appendix 8

Sediment Transport Model


2/11

Sediment Transport Model

Dr. Michael Hartnett,Research Director,

Marine Modelling Centre, MRI,National University of Ireland, Galway

Abstract

A numerical model was developed to make predictions of sediment

transport throughout the Irish Sea; the objective of this component of the

project was to illustrate that a sediment transport model could be linked

with the hydrodynamic model of the Irish Sea.Sediment is mainly transported about the Irish Sea due to the forces of

tide, wind and wave induced currents. In this project only tide and wind

forces were considered; these are the primary long-term forces affecting

the movements of sediments throughout the Irish Sea.

The model selected to carry out the sediment transport analysis within the

Irish Sea was ECOMSED; this is an advanced sediment transport model

based on the Princeton Ocean Model (POM) hydrodynamic code. The

model was developed by HydroQual, USA and widely used throughout the

world. The transport and fate of both cohesive and non-cohesive

sediments can be simulated with ECOMSED. Resuspension, deposition and

transport of cohesive sediments, clays, silts and organic material, are

simulated using the SED module.

The model results show the spatial varying bed shear stresses throughout

the Irish Sea, and also how the shear stresses vary temporally for

changing tidal conditions. Results also show the areas within the Irish

Sea where sediment erosion and transport is most active. Selected

locations of high sediment transport activity are highlighted and a more

detailed investigation of the results at these locations is carried out.


3/11

A7.1 Sediment Model Introduction

A preliminary sediment was developed for the Irish Sea under the PRISM

project. The model was developed on the basis of the previously

developed hydrodynamic model of the Irish Sea. The numerical domain of

the Irish Sea is defined by the geographic region: -7E to 2.625E and

51N to 56N. The model consists of 176 and 301 cells in x- and y-

direction, respectively, and hence in total of 52,976 computational nodes.

A rectangular grid of dimensions 1/60 of latitude and 1/40 of longitude

is applied for calculations in horizontal plan. In the vertical, 34 levels in a

sigma coordinate system are used. The timesteps satisfying the numerical

stability conditions are chosen as t=8s and T=400s in external and

internal mode, respectively. The minimum bottom roughness height of 5

mm was assumed. Figure 2 presents a map of the domain and the

bathymetry of the region.

Figure 2


4/11

Sediment is primarily transported about the Irish Sea due to tide, wind

and wave induced currents. During this work only tide and wind forces

were considered; these provide the long-term sediment transport

pathways. The following two sections provide a brief review of sediment

distributions and movements throughout the Irish Sea.

Sediment distribution

The major features of sediment distribution in the Irish Sea largely mirrors

the distribution of tidal current speeds with gravels where the currents are

strongest and muds where they are weakest. Gravelly sediment is

widespread in the St. George's Channel and in Cardigan Bay and extends

northward in a broad ribbon through the central Irish Sea, past the Isle of

Man, tapering out in the North Channel. Sandy sediments flank these

gravels to east and west, covering much of the rest of the Irish Sea. Off

the Irish coast southward from Dublin lie a series of north/south

sandbanks (Kish, Wicklow, Bray, Arklow, Blackwater). Similarly, there is

an area of sandbanks to the northeast of the Isle of Man and in the

Solway Firth. There are three significant mud patches within the sandy

regions; two are in the areas of weak tidal currents to the southwest ofthe Isle of Man (the largest) and to the southwest of St. Bees Head, while

the third is in a deep area off Holyhead. Mud is also associated with most

estuaries.

Sediment Movement

The movement of sediment, both the path taken and the amount moved,

is very difficult either to measure or to predict. It is related to the extent

by which the near-bed current exceeds a certain threshold. In this respect

tides and waves are at least as important as residual currents. For sand

and gravel the threshold is proportional to the sediment's characteristics

(particle size and density). The sandy regions of the Irish Sea are

extensively covered by sand waves from which sand transport paths have

been deduced. Between Dublin and the North Channel, the Irish Sea is a

trap for sand, with sand moving inwards towards the Isle of Man and then

eastward into Liverpool Bay and the Solway Firth. For the eastern IrishSea this picture is, perhaps fortuitously, similar to that for near bottom


5/11

currents, see Figure 7. Along the North Wales coast it is supported by the

tendency for beach sand to move eastwards. South of Dublin the sand

moves southward out of the St. George's Channel and also northward into

Cardigan Bay.

Figure 2 Lagrangian Circulation in the north-western Irish Sea

Of more importance to the movement of contaminants, particularly of

heavy metals and of radionuclides like plutonium, is the behaviour of


6/11

mud, which is more difficult to estimate. Since mud sticks together, its

threshold is not just related to the sediment's characteristics but also to

its history at that location. The threshold which causes mud to start

moving is not the same as that which allows it to settle out. Once particles

are in suspension, primarily through the action of tidal currents and

waves, they will be transported by the currents in the water column.

The movement of mud at the patches referred to above is unknown, even

whether the patches are gaining or losing mud. Their ultimate source

material is glacial clays resulting from the last Ice Age. The mud's surface

layer is continuously being over-turned by the resident animals,

particularly worms.

Model Background

The model chosen to carry out the sediment transport analysis within the

Irish Sea was ECOMSED; this is an advanced sediment transport model

based on the Princeton Ocean Model (POM) hydrodynamic code.

The transport and fate of cohesive and non-cohesive sediments can besimulated with ECOMSED. Resuspension, deposition and transport of

cohesive sediments, which are composed of clays, silts and organic

material, are simulated using the SED module. The suspended transport

of non-cohesive sediments, i.e., fine sands, is calculated using the van

Rijn procedure. The effects of bed armoring due to particle-size

heterogeneity can also be included in non-cohesive sediment transport

simulations. Bed load transport is not considered here because it does not

significantly affect optical properties in the water column. The sediment

transport module can predict temporal and spatial distributions of: (1)

suspended sediment concentrations (cohesive and non-cohesive); (2)

sediment bed elevation changes; (3) fluxes at the sediment-water

interface; and (4) changes in sediment bed composition. The module can

accept as input: spatially-variable sediment bed properties and time

variable sediment loading at river discharges and open boundaries.

The basic differential equation used to solve for sediment transport isgiven below:


7/11

where Ck is the suspended sediment concentration; U, V and W represent

the three components of velocity as computed from the hydrodynamic

component. An important process to be considered in the development of

a sediment transport model is bed erosion. Material deposited on the bed

will resuspend when the shear stress at the seabed due to water currents

is above a particular value. In ECOMSED model the following formulationis used to compute the rate of erosion/resuspension as a function of shear

stress:

where = resuspension potential (mg cm-2); a0= constant depending upon

the bed properties; Td = time after deposition (days); b= bed shear

stress (dynes cm

-2

);c = critical shear stress for erosion (dynes cm

-2

);and m, n = constants dependent upon the depositional environment

Full details of the above model can be obtained from the ECOMSED

manual and are omitted here for clarity.

Model Application

The Irish Sea sediment model was set with the following characteristics:

cohesive (500 m) not included

tidal/wind forcing

simulation period ~34 days

non-cohesive sediments considered only

uniform initial distribution of D50 (500m, 75m)

In this model the critical shear stress for deposition (resuspension) were

selected as follows:


8/11

34.81 dyne/cm2 (500 m)

0.1089 dyne/cm2 (75 m)

The hydrodynamic model was run initially and the bed shear stress

distribution about the Irish Sea for both neap and spring tide conditions

are presented in Figure 3.

Figure 3 Bottom shear stress distribution throughout the Irish Sea

The above figure illustrates the regions of maximum bed shear stress

throughout the domain; these are areas from which material is likely to be

eroded from the seabed and transported to other locations.

Figure 4 illustrates the relatively low shear stresses that are induced in

the western Irish Sea; this is due to relatively low energy tidal activity and

deep water. The maximum predicted stresses here being in the order of

1.6 dynes cm-2. Figure 5 presents plots for bed shear stresses for the

Liverpool Bay, North Channel, St. Georges Channel and Arklow; the

maxima here range from 10-50 dynes cm-2. These variations are very

significant with respect to the critical values for shear stresses.


9/11

Figure 4 Bottom shear stress in the Western Irish Sea

At the end of the simulation the mode predicted changes to the elevations

and distributions of sediments throughout the Irish Sea. Figure 6

presents the results of these predictions; there are significant variations of

erosion and deposition through the Irish Sea and it can be sent that most

of the erosion/deposition occurs in regions of high bed shear stress as

shown in Figure 3. Figure 7 presents details of temporally varying bed

elevation changes at locations A-D as defined in Figure 6.

Bed shear stress

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25 30 35 40 45

Time [days]

tau[dynes/cm^

2]


10/11

Figure 5 Bottom shear stresses at four locations

Bed shear stress

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30 35 40

Time [days]

tau[dynes/cm^2]

Bed shear stress

-10

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40

Time [days]

tau

[dynes/cm^2]

Bed shear stress

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40

Time [days]

tau[dynes/cm^2]

Bed shear stress

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40

Time [days]

tau

[dynes/cm^2]

Liverpool Bay

North Channel

St. Georges Channel

Arklow


11/11

Figure 6 Bed elevation changes

Figure 7 Bed elevation changes at A-D

B

A

D

C

Bed elevation change [cm]

-25

-20

-15

-10

-5

0

5

10

0 5 10 15 20 25 30 35

Time [days]

thickness[cm]

A

B

C

D

sediment transport model

Documents