oil spill modelling

Oil Spill Modelling using Water Forecast Models.doc/NHP-2008-02-14 1

Oil Spill Modelling using Water Forecast Models

By

Niels Hvam Pedersen

DHI Water & Environment, Agern Allé 5, DK-2970 Hørsholm, Denmark,

Tel.: +45 4516 9200, Fax: +45 4516 9292, E-mail: [email protected]

ABSTRACT

The paper discusses the principles of assessing and controlling oil spills in surface waters using

Water Forecast models. The primary goals of such a system is to forecast the movement and

spreading of oil spills in order to make decisions on how to decrease the impact of the oil spill in

the affected area. The system can also be used to detect from where the oil spill originates by

backward tracing the observed oil spill.

1. INTRODUCTION

Oil spills are serious threats to the marine environment, and place enormous demands on the

national authorities responsible for the response and clean-up operations. In many cases, the

resources required are beyond the means of a single country.

In the last 30 years, around 5 million tonnes of oil have been spilled in the world's seas as a result of

nearly 10,000 accidents. The majority of the spills are small (less than 700 tonnes), and it is the

large spills that account for most of the amount spilled. Thus, in the period 1988-1997, 70% of the

oil spilled came from just 10 incidents. However, the history of oil spills has shown that in general

the amount of impact on the environment has rarely been correlated with the amount of oil spilled.

Many ecosystems potentially suffer from deterioration due to oil spills. Accidental spills together

with leaching of oil originating from oil production and transport activities may result in

contaminated water constituting a severe risk for the water environment, fish, birds or coral reefs

etc.

The impact depends on a number of factors, such as the ecological sensitivity of the impacted site,

type of oil and meteorological conditions (water temperature and weather). Once the spilling

incident has taken place, natural processes including weathering, evaporation, oxidation,

biodegradation and emulsification, will start taking place. They can reduce the severity of the oil

spill and accelerate the recovery of the affected area

The environmental impact of a given oil spill, however, will be influenced by many factors. The

topography of the system, evaporation, run-off events and tidal variations will determine the

magnitude and direction of flow. The position, duration and amount of oil spilled to the river/bay,

the transport/spread of the oil as well as self-purification processes in the waters will determine the

position of the oil slick and the concentration of oil constituents in the water phase.

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This complexity clearly demonstrates that in connection with setting up an operational forecast

system, or investigating the environmental impacts of spills, it is necessary to create a basic

understanding of the flow dynamics as well as the transport/fate processes of the oil.

In the following a water forecast system forming the basis for subsequent oil spill simulations will

be described.

2. WATER FORECAST SYSTEM

The basic hydraulic phenomena comprising water level and current conditions are calculated by

MIKE HD module. The model set-up comprises either a 2D or a 3D model, hosting in general a

local model of higher spatial resolution either in a nested grid or in a flexible grid. The regional

model will normally cover a large area spanning the area of interest. The local model area spans

those areas of special interest in much more detail. Figures 2.2 show the layout of a regional model,

in this case a model of South China Sea covering all the coastline of Vietnam. Local models can be

selected any place inside the regional model.

2.1 Bathymetry

The bathymetry is made according to available sources. In this case Mike C-MAP has been used to

create a bathymetry covering the South China Sea as illustrated on Figure 2.1.

Figure 2.1 The South China Sea shown in MIKE C-MAP, which has been used to create the regional model.

Based on the electronic sea charts a model bathymetry has been created with appropriate grid spacing in order to resolve

the overall current description in the area. Fine grid will be applied in areas of special interest (Rivers, straits etc). The

regional bathymetry for the South China Sea is illustrated on figure 2.2.


Figure 2.2 Regional model for South China Sea.

2.2 Boundary Conditions: Water Level Variations

The regional model is controlled by predicted tides along the open boundaries in the model. The

boundaries have been extracted from the KMS global tide model, which is based on satellite

measurements during the past 14 years from TOPEX/Poseidon satellite survey. The validity of this

model has been documented through numerous model studies around the world. A Global co-tidal

map of M2 is shown in Figure 2.3 and the tidal constituents for the Phase and the Amplitude for the

regional model is shown on Figure 2.4.


Figure 2.3 KMS Global Tide Model. Co-tidal chart showing the M2 constituents.

Figure 2.4 KMS Regional Tide Model showing Amplitudes and Phases for M2 constituents.

2.3 Wind The wind is very important and wind fields can be obtained from various sources – either national

or global. In this example the Global Forecast System GFS has been used, which among other

parameters calculates the Air Pressure, Wind Speed and Direction, Ice coverage, Air Temperature,

Precipitation and Cloudiness. The forecast is calculated four times per day (00 UTC, 06 UTC, 12

UTC, and 18 UTC) out to 384 hours. The horizontal forecast resolution is approximately ½ degree.

All GFS runs get their initial conditions from the Spectral Statistical Interpolation (SSI) global data

assimilation system (GDAS), which is updated continuously throughout the day. An example of the

applied wind field from the global wind is shown in Figure 2.5 and the regional wind field is shown

in Figure 2.6.


Figure 2.5 Global Wind Field from GFS (showing wind speed, direction and wind pressure)

Figure 2.6 Regional Wind Field from GFS (showing wind speed, direction and wind pressure)

2.3 Calculated Currents

The model can calculate generated currents from tide and wind, and two typical examples of the

calculated current pattern are shown in Figures 2.7.


Figure 2.7 Typical calculated water level variation and current speed

The regional model is calibrated towards numerous tidal and current stations.

2.5 Waves With the same model set-up waves can be calculated within the same grid and even used in the

hydrodynamic model if necessary. But normally waves will be forecasted in a water forecast model

in order to describe the wave influence for other purposes than Oil Spills, but in some cases it can

be important even for Oil Spills.

3. OIL SPILL SIMULATIONS

The Oil Spill Analysis model (OSA) simulates and predicts the spread/thickness of the oil slick on

the water surface as well as the concentration distribution of up to eight oil constituents in the water

column. These results can be further evaluated with respect to the environmental impact caused by

oil spills, including modelling of sensitive areas, assessment of ecotoxicological effects and

estimation of animal mortality, or being the core in an operational forecast system for oil spill

contingency planning. Figure 3.1 shows the conceptual diagram of the model.


Oil Spill Modelling

Hydrodynamic modelling

Transport and

Oil Weathering processes

Discharge

Rainfall runoff

Evaporation

Water levels

Fluxes

Wind

Oil spills

Diffuse sources

Leakage

Spreading

Dispersion

Evaporation

Emulsification

Dissolution

Transport/fate of oil slicks

Assessment of environmental effectsEcotoxico-

logical effects

Sensitivity

modelling

Mortality

estimation

Oil Spill Contingency Planning

Figure 3.1 Structure of the OSA model

The environmental modules of the MIKE system have been developed to determine the fate of

substances subject to transport such as oil constituents. The Oil Spill Analysis module simulates the

spreading and transformation of hydrocarbons in the aquatic environment under the influence of the

fluid transport and the associated physical and chemical dispersion processes such as

advection/dispersion, evaporation, mechanical spreading, dissolution and emulsion. These

processes, often referred to as weathering processes, are in the model calculated based on the

chemical and physical properties for the oil constituents separating the oil component into a number

of fractions with different chemical and physical characteristics, see Figure 3.2.

Figure 3.2 Weathering processes included in the OSA model


The modular structure of the MIKE modelling system provides flexibility to combine different

types of modules, e.g. hydrodynamics and waves to describe wave driven currents, and thus also in

general the option of adding new/improved descriptions to an existing set-up.

The biological, chemical and ecotoxicological expertise at DHI opens the possibility of adding

intervention scenarios to the operational oil spill model, so that rapid assessment of different

intervention scenarios involving, for example, chemical methods of intervention or biodegradation

can be made.

4. EXAMPLES ON OIL SPILL SIMULATIONS

The Oil Spill Analysis model has been used in various places some are described in the following.

Figure 4.1 Spill scenario in The Sound, Denmark

First example is related to a spill scenario in the Sound in the narrow strait between Saltholm and

Copenhagen. The current is strongly towards North and the dispersion of the Oil Spill is small.


Figure 4.2 Oil Spill in the Great Belt, Denmark (Picture from SOK, DK)

The next is from the Sound, Denmark. On the picture taken by SOK shortly after the spillage shows

that the spill is still visible. But later the oil changed properties and drifted around as a submerged

Oil Spill. Difficult to locate, but with a forecast system even a submerged spillage can be traced as

seen on Figure 4.3.

Figure 4.3 Simulation of Submerged Oil Spill in the Great Belt, Denmark


REFERENCES

DHI, 2005: Oil Spill Modelling Concept, June 2003

The GFS Atmospheric Model: http://www.emc.ncep.noaa.gov/gmb/moorthi/gam.html.

Andersen, O. B., Global ocean tides from ERS-1 and TOPEX/POSEIDON altimetry, J. Geophys

Res. 100 (C12), 25,249-25,259, 1995.

Andersen, O. B. The AG06 global ocean tide model, September 2006.

http://www.emc.ncep.noaa.gov/gmb/moorthi/gam.html

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