National Oceanic and Atmospheric Administration • NOAA Ocean Service 

Office of Response and Restoration • Hazardous Materials Response Division

Trajectory Analysis Handbook
Trajectory Analysis Handbook

National Oceanic and Atmospheric Administration

NOAA Ocean Service

Office of Response and Restoration

Hazardous Materials Response Division

7600 Sand Point Way NE

Seattle,Washington USA 98115

NOAA’s Hazardous Materials Response Division provides 

scientific support to the Unified Command for response to 

spills in the marine environment. For further information,

please visit our website at 

http://response.restoration.noaa.gov. This guide was 

developed with support from the U.S. Agency for International Development.

2
“Where will the oil go?” 

This is a critical question asked by the emergency 

responder during an oil spill. Knowing the trajectory of 

the spill gives decision-makers critical guidance in deciding 

how best to protect resources and direct cleanup. However, it is often very difficult to predict accurately the movement and behavior of an oil spill. This is due, in part, to 

the interaction of many different physical processes about 

which information is often incomplete at the start of a 

response. The modeler must thus continuously update predictions with new data and explore the consequences and 

likelihood of other possible trajectories, a procedure called 

“trajectory analysis.” The end product of trajectory analysis 

is often a map showing the forecast and probable uncertainty bounds of the slick movement.

This guide provides an overview of the physical processes 

that affect oil movement and behavior in the marine environment. Trajectory analysis is most often done using 

computer models to keep track of complex, interacting 

processes. Even without a model, you can estimate 

the time and length scale of an event using the information you’ll find here. The guide can help the responder 

and planner understand physical processes and potential 

uncertainties as they incorporate trajectory analysis into 

the response.

Introduction 3
4

Introduction

Trajectory Analysis Forecasting the movement of an oil spill is often hampered by insufficient input data, particularly in the first few hours 

of the release. Detailed spill data ( location, volume lost,

product type) are often sketchy and environmental data

(wind and current observations and forecasts) are often 

sparse or unavailable. Nonetheless, the modeler must 

examine the data and attempt to understand the physics 

and chemistry that will likely affect the oil movement and 

fate of the particular spill .

With an understanding of the physical processes, the mod-

eler can provide an analysis to the Unified Command 

as a forecast. If the initial forecast is inaccurate, which 

can occur due to erroneous spill data (e.g., location and 

amount of oil released) and/or erroneous environmental 

data (e.g., weather forecast) and model limitations, the 

modeling team reviews the new information and refines 

the forecast. In general, as the spill unfolds, the forecast of 

oil movement and fate improves because the quality and 

quantity of the on-scene observational data improves 

(while initial spill data becomes relatively less important).
Uncertainty

Trajectory analysis should include not only the “best guess”

of the oil movement and fate but also some representation 

of the uncertainty in the spill and environmental data used 

to make the forecast. The uncertainty in a trajectory forecast depends on the length and time-scale of the spill.

The following table indicates the uncertainty for input data 

required by most oil spill models. It should be noted that 

model output uncertainty is not necessarily directly proportional to model input uncertainty.

Introduction 5
6 Introduction

Uncertainty for input data required by most oil spill models

Category Parameter Uncertainty

Release details Spill location Low - Medium

Time of release Low - Medium

Type of oil (density, viscosity) Medium - High

Potential spill volume Low

Actual spill volume High

Release rate High

Oil weathering Light refined products Low

Intermediate Fuel Oils (IFO 180, IFO 380, Bunker C, High

Fuel Oil #6)

Heavily studied crude oils (Prudhoe Bay, Arabian, Low 

Ekofisk, Hibernia) 

Crude oils Medium - High

Winds Observations Low

24- to 48-hour forecast Low - Medium

48- hour to 5-day forecast Medium - High

Wind drift (typically 1 to 6%) Low

Surface currents River Low

Tidal areas with current stations (unless currents are Low

weak and variable)

Shallow-water lagoon Low - Medium

Shelf area (wind setup) Medium

Continental slope (e.g., Gulf Stream, California Low

Current)

Abyssal Plain High

Turbulence Spreading Medium

Horizontal diffusion Low - Medium
Trajectory Analysis

Ideally, the trajectory is displayed in a format that is easy 

to understand. It should indicate both the forecast and the 

uncertainty. In this example,the forecast “best guess”of the 

oil movement is overlaid on a map of the shoreline. The 

forecast is presented as light, medium, and heavy contours.

The scale at the bottom of the map represents the percent 

coverage of the surface oil within these contours. Plausible 

errors in the spill and environmental data were explored by 

the modeling team; the colored outer contour represents a 

90% confidence bound. This provides an indication of the 

uncertainty in the forecast.

Sample trajectory analysis map.

Introduction 7
 

8 Fate

Weathering Processes and Time Scales

The physical and chemical characteristics of petroleum 

change almost immediately when spilled in the marine 

environment due to evaporation, dispersion, emulsification, dissolution, oxidation,sedimentation, and biodegradation. All of these processes interact with each other and are 

collectively referred to as oil weathering. The table following describes some of the weathering processes and the 

time scales of those processes important for emergency 

response.

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Weathering processes and time scales important for emergency response

Weathering Process What is It? Why is it Important? Time Scales

Evaporation

Emulsification, or 

formation of mousse

Natural dispersion

Dissolution

Biodegradation

Formation of tarballs

Conversion of liquid to a gaseous 

phase.The lighter fractions in the oil 

are lost first.

Very small water droplets are mixed 

into the liquid oil. Water content 

often reaches 50-80%. Occurs on 

water, needs some wave action.

Breakup of an oil slick into small droplets that are mixed into the water by 

energy.

Mixing of the water-soluble 

components of oil into the water.

Breakdown of oil by microbes into 

smaller compounds, eventually to 

water and carbon dioxide.

Breakup of slicks of heavy crudes and 

refined oils into small patches that 

persist for long distances.

Major process that accounts for the loss of oil,

particularly light oils. At 15°C, gasoline evaporates completely over a 2-day period, 80% 

of diesel fuel evaporates, 40% of light crude,

20% of heavy crude, and only about 5-10% of 

Bunker C.

Can increase the amount of pollutant to 

be recovered by a factor of 2-4. Slows 

down other mixing processes.

Removes the oil from the water surface.

The most water-soluble components of oil 

are most toxic.

Rate depends on oil type, temperature, nutrients, oxygen, and amount of oil.

Tarballs are hard to detect, so the slick 

appears to be going away though it is still a 

threat.

< 5 days

Onset can be delayed for days 

but the emulsification process 

happens rapidly.

< 5 days

< 5 days

weeks to months

days to weeks

Fate 9
10 Fate

Evaporation

Percent evaporated over time for an instantaneous Evaporation can be a major mechanism for removing oil.

release of 100 barrels with winds at 10 knots and water The amount evaporated depends chiefly on the oil propertemperature at 20oC ties, the wind speed, and the water temperature.

% Evaporated Hour Generally, light refined products, like gasoline or jet fuel,

evaporate faster than heavier products, such as heavy Gasoline 94 1 crude oil. From the table, you can see that most of the 

Lagomedio 38 18 gasoline evaporates within a few hours. Lagomedio and 

Diesel fuel oil 37 18 Prudhoe Bay crude oils are more persistent in the environPrudhoe Bay 28 70

ment and have much lower evaporation rates, 38% and 

28%, respectively. After 120 hours, much of the product 

would be expected to remain on the water surface.
Breaking wave

Dispersion

Breaking waves can drive small droplets of oil into the 

water column. If the droplets are small enough (diameters 

less than 50-70 microns) natural turbulence in the water 

will prevent the oil from resurfacing, just as turbulence in 

the air keeps small dust particles afloat. The smaller droplets that stay in the water column are considered dispersed.

Dispersion can be a mechanism for removing oil from the 

water surface. The amount dispersed depends on the oil 

properties (the viscosity and surface tension, in particular) 

and water conditions.

Oil products with low viscosity, like gasoline or kerosene,

are more likely to disperse into the water with breaking 

waves than a high-viscosity oil, like an IFO 380 or Oficina 

heavy crude. Therefore, the dispersed fractions of gasoline 

or kerosene can be relatively large in heavy seas.

A possible treatment of oil spills is to spray the slick with 

chemical dispersant. Chemical dispersants enhance natural 

dispersion by lowering surface tension. This guide does 

not address subsurface oil movement because of the difficulty in developing a trajectory analysis of dispersed oil.

Fate 11
12 Fate

Dissolution

Dissolution begins immediately and is likely to continue 

throughout the weathering process.

The loss of petroleum product from dissolution is minor 

when compared to the other weathering processes.

Less than 0.1% (very heavy oil) to 2% (gasoline) of the 

spilled oil volume actually dissolves into the water column.

However, the components of the oil that dissolve into the 

water column are often more toxic to the environment.

Oil 

Unleaded gasoline

Diesel

Prudhoe Bay crude

Lagomedio

Aqueous Solubility (mg/L)*

260.9

60.4

20.5

10.0

Sample oil solubilities

*Jokuty, P., S.Whiticar, Z.Wang, M. Fingas, B. Fieldhouse, P.

Lambert, and J. Mullin. 1999. Properties of crude oils and oil 

products. EE-165. Ottawa, Ontario: Environment Canada.
Sample viscosities

Product Viscosity at room 

temperature (cP)

water 1

diesel fuel 10

Prudhoe Bay crude 46 

Prudhoe Bay crude after 250,000 

emulsification

Lagomedio 20

Lagomedio after 300,000 

emulsification

honey 10,000 

peanut butter 1,000,000

Emulsification

For many crude oils and some refined products, weathered 

oil is likely to reach a stage where water droplets are mixed 

into the oil, forming a water-in-oil emulsion, or “mousse.”

The ability to form an emulsion depends on water conditions and the chemical properties of the oil. For example,

oils with high wax and asphaltene content, such as Prudhoe Bay crude, emulsify easily if there are breaking waves.

Once the oil has emulsified, the viscosity can increase enormously (see table).

Generally, oils must weather a certain amount before forming an emulsion. Although the onset of emulsification may 

take several days, the emulsification itself can occur within 

a few hours.

The emulsion can be 70 to 90% water so that the combined 

volume of oil and water mixture may be much greater than 

the volume of the original spill.

Emulsions are often classified by their stability. In unstable 

emulsions, water and oil separate easily under calm conditions with warm temperatures. In stable emulsions, water 

remains in the oil for weeks to months.

Fate 13
14 Fate

A) 10,000 g/m3

B) 1000 g/m3

C) 100 g/m3

D) 10 g/m3

E) 1 g/m3

Different concentrations 

of fine dust in water.

Sedimentation

Sedimentation is defined as the adhesion of oil to solid 

particles in the water column. Oil can be sorbed onto sediments in the water column and may eventually be found in 

the bottom sediments.

Turbulent waters with high sediment load (~500 g/m3 ),

such as a fast-moving, muddy river, can move the oil 

through the water column within hours of the initial 

release.

Waters with low sediment load (< 5 g/m3), as in the open 

ocean, will allow oil to remain on the surface much longer 

(weeks), spreading the slick over a wider area.
Photo-oxidation

Sunlight changes the spilled oil’s chemical and physical 

properties.

This process is limited to the surface of the oil and can 

result in a thin, crusty ”skin” on slicks and tarballs.

The “skinning” of the oil is thought to limit evaporation 

because the lighter oil components can no longer diffuse 

through the surface of the slick.

Photo-oxidation may increase the ease of emulsification 

and is considered a long-term weathering process taking 

weeks to months.

Photograph showing a large patch of weathered oil 

with a crusty,“skin” layer on the surface. The white 

spots in the picture are 3-inch by 4-inch drift cards cast 

into the water to help track the movement of the oil.

Fate 15
16 Fate

Biodegradation

The spill is finally removed when the oil biodegrades. The 

microbes that degrade oil occur naturally in the environment.

The rate at which the organisms degrade the oil depends 

on the properties of the water and the oil and microbial 

activity. This process is thought to occur over time scales 

of weeks to years.

Kelp attached to weathered oil or “tarballs”
Oil Transport

Two major processes transport oil spilled on water: spreading and advection. For small spills (<100 barrels), the 

spreading process is complete within the first hour of the 

release.

Winds, currents, and large-scale turbulence (mixing) are 

advection mechanisms that can transport oil great distances.

In general, the oil movement can be estimated as the 

vector sum of the wind drift (using 3% of the wind speed),

the surface current, and spreading and larger-scale turbulence (diffusion).

Transport 17
18 Transport

Color-enhanced image of a 

Oil Spreading

The spreading process occurs quickly and, for most spills,

mostly within the first hour. In the open ocean, winds,

currents, and turbulence will quickly move the oil.

Spreading will occur quicker for lighter and for less viscous 

oils in warm water temperatures and for warm oils.

The slick does not spread uniformly but will often have a 

thick part surrounded by a larger, but thinner sheen. The 

figure shows a color-enhanced image of an experimental 

spill. The orange portion is the thick part of the slick and 

the pink area, sheen. Note that about 90% of the oil is 

found in 10% of the slick area (the orange portion of the 

figure).

test spill (<50 barrels).
 

Transport 19

very thick (black)

 thick (rainbow )

thin (gray or silver )

Oil Thickness

Oil slicks form very thin films on the open water and,

depending on the product, the thickness can range from a 

tenth of a micron to hundreds of microns. Since 1929, oil 

spill researchers have studied the relationship between oil 

thickness and the color of the film.

When direct light from the sun contacts a very thin oil film 

(<0.1 micron; μm), much of the light is reflected back to the 

observer (see diagram) as gray or silver sheen.

If the film is thicker (perhaps 0.1 to 3 μm), the light passes 

through the film and is reflected off the oil-water interface 

and back to the viewer. The observer will then see a film 

that can range from rainbow to darker-colored sheens.

For very thick films (> 3 μm), the light is absorbed and 

the slick will appear dark- colored (i.e., black or brown) 

to the observer. However, the viewer can no longer determine film thickness based on color. If the slick is darkcolored, the observer cannot tell whether the film is 3 μm 

or 100 μm thick.

Because sun angle, glare, sea state, view angle, and viewing 

through plexiglass windows can affect the appearance of 

the slick, estimating film thickness based on color is unreliable. Calculating oil volume also requires estimating the 

percent oil coverage, a very difficult task.

Very thin films: no phase shift or reflection and all 

frequencies reflected back (gray or silver)

Intermediate films: phase shift depends on wave length 

or color and distance traveled through the oil (rainbow)

Thick films: light is absorbed (brown or black)

Trajectory analysis does not typically provide good 

information on oil thickness.
20 Transport

Winds

Winds affect the trajectory in three major ways:

1) oil weathering 

2) surface effects on the water 

3) direct transport

Wind affects on oil trajectory

Wind Speed

LOW <5 knots HIGH >25 knots

evaporation increases

dispersion increases

patchiness increases

convergences natural Langmuir

observability good poor
Estimating Wind Speed From Helicopter

Wind speed Waves

0 to 5 knots Round or sinusoidal shape

5 to 10 knots Trochoid shape (peaks are pointed)

10 knots Breaking waves

15 knots Tops are coming off the waves

20 knots Streaks of foam trailing behind waves

>20 knots Difficult to observe surface oil

Wind Speed Scale

The Beaufort Wind Scale is named after Admiral Sir Francis 

Beaufort, who developed the scale in 1805 to estimate 

wind speed from observing the sea state. The table at 

left provides an alternative approach to estimating wind 

speed, particularly useful if you are observing from an aircraft. The following table shows the Beaufort Wind Scale 

for wind speed and the corresponding sea characteristics.

Transport 21
22 Transport

Wind Scales and Sea Descriptions (From Willard Bascom,Waves and Beaches, 1980)

Beaufort 

Scale

Seaman’s description 

of wind knots

Velocity

(knots)

International scale sea 

description and wave heights

Estimating wind velocities on sea

0 Calm <1 Calm, glassy 0 foot

1 Light air 1-3 

2 Light breeze 4-6 Rippled, 0-1 foot

3 Gentle breeze 7-10 Smooth, 1-2 feet

4 Moderate breeze 11-16 Slight, 2-4 feet

5 Fresh breeze 17-21 Moderate, 4-8 feet

6 Strong breeze 22-27 Rough, 8-13 feet

7 High wind 28-33 Very rough, 13-20 feet

(moderate gale)

8 Gale (fresh gale) 34-40 Very rough 13-20 feet,

Sea like a mirror

Ripples, no foam crests

Small wavelets, crests have glassy 

appearance and do not break

Large wavelets, crests begin to break.

Scattered whitecaps.

Small waves becoming longer. Frequent 

whitecaps.

Moderate waves taking a more pronounced long form; mainly whitecaps,

some spray.

Large waves begin to form extensive 

whitecaps everywhere, some spray.

Sea heaps up and white foam from 

breaking waves begins to be blown in 

streaks along the direction of the wind.

Moderately high waves of greater 

length; edges of crests break into spindrift.The foam is blown in well-marked 

streaks along the direction of the wind.
Wind Drift Observations of actual oil spills and controlled experiments 

indicate that the wind drift can range from 1 to 6% of the 

wind speed (most modelers use an average of 3%).The 

lower windage value of 1% reported may be due to some 

of the oil droplets being submerged by waves. Langmuir 

circulation can also result in wind drift variability.The oil 

within the windrows may move up to 5.5% of the wind 

speed.This hypothesis would account for the higher windage value of 6% reported at spills. While oceanographic theory predicts an angle between 

the surface current and the wind speed, observations of 

oil slick trajectories suggest that the actual angle is less 

than 10°.Wind direction predictions are typically not this 

accurate and few modelers include a rotation angle in their

Windrows

calculations.

Most modelers use wind speeds measured at 10 meters 

above the water surface. Observations at other heights 

are adjusted by standard techniques to the standard 10-m 

height. It should be noted that wind direction is commonly reported as the direction from which the wind is blowing 

and the surface current is reported as the direction toward 

which the water flows.This means that a north wind and a 

southerly current are moving in the same direction.

Transport 23
24 Transport

•

•

•

•

Drifters

Currents

The surface current is a mechanism for transporting oil.

Currents are an important factor in determining the length 

and time scale of a spill.

Ocean circulation can transport oil for thousands of 

miles in months to years

Ocean coastal flow can transport oil for hundreds of 

miles in weeks

Estuarine circulation can transport oil tens of miles in 

days

Rivers can transport oil tens of miles in hours to days
Tide Patterns

In a few coastal areas, there is a pattern of one high tide 

and one low tide per day. This tidal pattern is called a 

diurnal tide.

The dominant tidal pattern in most of the world’s oceans is 

two tidal cycles where the high water-low water sequence 

occurs twice a day. If the highs and lows in a semi-diurnal 

tide occur at different levels, the tide is referred to as a 

mixed, semi-diurnal tide.

The predicted astronomical tidal pattern is often modified 

by other factors. Winds acting on the sea surface and 

atmospheric pressure can modify the sea level. These types 

of events can be particularly important in shallow-water 

areas. Strong coastal storms can markedly modify the tidal 

patterns for a particular area.

Transport 25
26

Transport

SPRING TIDES

NEAP TIDE

Spring and Neap Tides Spring tide is the very highest and very lo

west tide, which 

occurs twice a month when the moon is either new or full. Neap tides are the opposite of the spring tide: the tidal range between high and low water is smallest and occurs 

near the time of the first and last lunar quarters.

Spring tides may be important for spill response as oil beached during this time is likely to remain stranded on 

the upper portion of the shoreline until the next spring tide 

(about 14 days) or storm event. If there is a storm surge 

during a spring tide, the oil can remain stranded for a much 

longer period.
Tidal Currents

Strongest tidal currents are found in shallow-water areas 

or through narrow channels that connect large bodies of 

water.

Currents in channels (i.e., entrances to bays and estuaries) 

are constrained to flow either up or down the channel. In 

open waters, the flow depends on the direction of the tide 

wave.

Along the outer coasts, the tidal currents and heights are 

more closely in phase (progressive wave).

Tidal currents are generally out of phase with tide heights 

for stations inside an enclosed bay (standing wave). Phase 

change can also be caused by bottom friction.

Tidal currents and heights at the entrance to Galveston 

Bay. Solid line is tidal heights and the dashed line is 

tidal currents. (top)

Tidal currents and heights at the entrance to Portland 

Harbor, Casco Bay, Maine. Solid line is tidal heights 

and the dashed line is tidal currents. Note max flood is 

about 3 hours earlier than high tide. (bottom)

Transport 27
�

�

28 Transport

Tidal excursion = �

To calculate the tidal excursion, let 

T = Time from low slack to low slack 

(high slack to high slack)

V = Maximum tidal current velocity

Tidal Excursion 

The trajectory analyst is often asked whether an offshore 

spill will move into a bay or estuary. To answer this question, one of the first things you should look at is the tidal 

excursion for the inlet. If the spill is anywhere near the 

extent of the tidal excursion, oil could move into the bay 

with the tide.

It is important to keep in mind that the tidal excursion is 

very much dependent on the bathymetry. In areas where 

the bottom is very broad and flat, the tidal influence will 

drop off quickly. In long, narrow channels, the tidal influence could be much larger.
Flushing

The volume of water exchanged between an estuary and 

the open sea during a complete tidal cycle is often called 

the tidal prism. In the figure, the difference between the 

volume of water at low tide (below the dashed line) and 

high tide (below the curved line) is the tidal prism.

The tidal prism method for estimating flushing assumes 

that the water entering on the flood tide is fully mixed 

with the water inside the estuary. It also assumes that the 

volume of river water and sea water during the flood tide 

equals the tidal prism.

Vertical profile of a simplified shoreline. Area 

between the curving and dashed lines indicates 

the tidal prism.

To calculate the flushing time, first measure the area of 

the estuary from a map. Second, calculate the volume of 

the estuary at low water, by selecting a depth that best 

represents the estuary. Third, calculate the volume of the 

estuary at high water,Vh. Finally, the flush time can be 

calculated from �� �

�

� �� ��� �

���� ��� ��

where t is the time of one tide cycle (from low tide c

to low tide) and Vl is the volume of water in the 

estuary at low tide

This is a general approach, but the method may underestimate flushing time due to incomplete mixing; fresh water 

at the head of the estuary may not move through the 

mouth of the estuary in one tide cycle and some water 

that escaped on the ebb is returned on the flood. Underestimating flushing times may mean that the oil remains in 

the estuary longer than predicted.

Transport 29
Exxon Valdez

30 Transport

Turbulent Mixing

Oil spilled into water is subjected to turbulent flow.

Oceanic turbulence is generated by winds and current, and 

by heating and cooling. Flow in the upper layers of water 

becomes more turbulent as the wind and current increases.

Turbulent diffusion, caused by random bulk movements of 

water, tears oil slicks into smaller patches that are distributed over a wider area.

The diffusion of oil occurs mainly in the horizontal direction. Horizontal diffusion of the surface water ranges from 

100 to 1, 000,000 cm2/s.

Diffusion in the vertical direction is much smaller by 

orders of magnitude than horizontal diffusion and generExxon Valdez oil spill oil spill ally decreases with depth.

Turbulent diffusion is not to be confused with mechanical 

dispersion (i.e., mixing caused by breaking waves).
Langmuir Circulation

Oil in widrows, or Langmuir cells.

Langmuir circulation is the result of the interaction 

between wind-driven surface currents and surface waves.

Though Langmuir circulation may be present in weak or 

no-wind situations, it is most often seen when the wind 

speed is 1.5 m/s or greater. Langmuir circulation is a major 

mechanism for breaking the slick up and may be important 

for transporting oil droplets into the water column. Predicting the onset and strength is difficult at best, but we do 

know the following:

1) The windrows, or streaks, tend to last from 5 to 30 

minutes, then dissipate and reform.

2) The surface current, stronger in windrows, can be up 

to 5.5% of the wind speed.

3) Downwelling (vertical) speeds at convergence range 

from 5 cm/s to 20 cm/s.

Langmuir cells in the mixed layer depth.

Reference: Special Issue Langmuir Circulation and Oil Spill 

Modeling. Spill Science and Technology Bulletin Vol. 6

Transport 31
32 Transport

Tidal Convergences

Convergences are natural collection areas for oil, especially 

tarballs. Because they are close together, the tarballs in a 

convergence may coalesce and form a cohesive slick.

Tidal convergences can be formed by water moving from 

shallow to deeper water (ebb tide) that is stretched. To 

conserve mass, the surface velocity decreases.

Flotsam, rafting birds, and oil can collect in these areas.

Under weak winds, the oil may not cross convergences.

Strong winds may rupture the convergence. However, tidal 

convergence can appear consistently in the same general 

Water column moves from shallow water area during ebb tides. to deeper water.
Freshwater-saltwater Interface

As with tidal convergences, the freshwater-saltwater interface is also a natural collection area for oil. However, this 

type of convergence is formed by river water flowing into 

the sea and spreading out over the seawater.

The fresh water is less dense than the seawater, creating a 

convergence at the surface.

Strong winds can rupture these convergences.

Freshwater-saltwater interface with oil sheen moving 

into the convergence.

Transport 33
34 Transport

Longshore Currents

Longshore currents are produced by waves approaching, at 

an oblique angle, a coastline having a gently sloping beach.

Speed and direction of the longshore current increase with 

wave height and with an increase in the angle of the wave 

front.

Typical speeds of longshore currents range from 0.3 m/s to 

1.0 m/s.

As the current approaches 1.5 m/s, a jet often forms that 

returns flow seaward in the form of rip currents.

This type of current is very important for trajectory purposes as it provides a mechanism for transporting oil in 

nearshore areas beyond the breakers and offshore.

Reference: Horikawa, K. 1978. An Introduction to Coastal 

Formation of longshore currents Engineering. New York: John Wiley and Sons. 403pp.
sport 35

Beaching/Refloating

Ocean currents cannot actually bring the oil into contact 

with the shoreline unless there is some kind of flow that 

penetrates the shoreline (e.g., marshes and mangroves).

The first photograph shows an oil spill moving into a marsh 

on a flood tide.

The second photograph shows streaks of oil moving along 

the shoreline. For the oil to beach, the wind must typically 

be blowing onshore.

Oil in marsh.

Oil streaks moving parallel to the shoreline.
36 Transport

Falling tide Beach face

Onshore wind

Seabreeze

Beach face

Heated air

Landbreeze

Beach face

Cooling air

Oil beaching with a 

falling tide and 

onshore wind.

Heating of the land 

and a sea breeze 

(from sea to land) 

during the day.

Cooling of the land 

and a land breeze 

(from land to sea) 

during the night.

Land/Sea Breeze

A falling tide with an onshore wind greatly increases the 

amount of shoreline oiling (see first figure). A sea breeze 

may strand the oil on the shore.

The land and sea breeze cycle occurs in many coastal areas 

and is caused by heating of the land during the day and 

cooling at night (see second and third figures). In some 

areas, the land and sea breeze can extend many miles offshore.
Summary

To develop a trajectory “picture,” the analyst must look at 

the components considered in any model and consider the 

proceses outlined in this guide. The major components of 

any model will be:

Spill Data

• location of spill

• type of oil

• volume lost

• time/type of loss (instantaneous or continuous? 

stationary? moving?)

Environmental Data

• wind

• currents (large-scale, tidal, river flow, etc.)

• tidal heights

• diffusion

Some of the processes in this guide are typically not modeled well and the modeler must account for these in the 

uncertainty included in the final trajectory analysis:

• oil thickness

• convergences

• local variations on astronomical tides

• small-scale currents (i.e., around piers, small groins,

or jetties)

• small-scale meteorology 

Summary 37
38 Summary

Beaching oil that comes ashore

Biodegradation breakdown of oil by microbes into 

smaller compounds, eventually to water and carbon 

dioxide

Convergence areas where surface waters “come 

together.” They are natural collection areas for oil, especially tarballs.

Diffusion large-scale turbulence that mixes spilled oil

Dispersion breakup of the oil into small droplets that are 

mixed into the water by sea energy. If the droplets are 

small enough, they remain in the water column.

Dissolution mixing of the water-soluble components of 

oil into the water

Diurnal tide coastal areas with one high tide and low tide 

each day

Emulsification small water droplets or water mixed into 

the liquid oil, thickening it to a “chocolate mousse” consistency. Water content often reaches 50-80%.

Evaporation conversion of liquid to a gaseous phase

Flotsam garbage, or detritus, on the water surface

Flushing turnover of water from an estuary or harbor

Freshwater-saltwater interface type of convergence 

formed when river water flows into the sea and spreads out 

over the seawater. Like tidal convergences, this interface is 

a natural collection area for oil.

Langmuir circulation wind induced water movement that 

leads to windrows, or streaks, of oil that dissipate and 

reform.This is a major mechanism for breaking up the slick 

and may be important for moving oil droplets into the 

water column.

Longshore currents produced by waves obliquely 

approaching gently sloping beaches.

Mixed, semi-diurnal tide two tidal cycles where the high 

water-low water sequences occur twice a day at different 

levels

Movement and fate the direction in which the spill 

moves, and the physical/chemical changes that occur to 

the oil over time

Neap tide the opposite of spring tides: the tidal range 

between high and low water is smallest and occurs near 

the first and last lunar quarters.

Observational data on-scene measurements (winds, currents, and oil location)
Photo-oxidation changes made by sunlight to a spilled 

oil’s physical and chemical properties

Progressive wave energy is transmitted through the 

water, but water particles move in an oscillatory manner.

Refloating oil that has come ashore and re-floated off the 

shoreline

Sedimentation adhesion of oil to solid particles in the 

water column

Semi-diurnal tide two tidal cycles where the high waterlow water sequences occur twice a day at the same level

Spring tide the very highest and the very lowest tide,

which occurs twice a month when the moon is either new 

or full

Standing wave as a tidal wave reaches the end of a bay or 

estuary, it is reflected back toward the entrance.

Surface tension tendency for molecules to stick together 

and present the smallest surface to the air

Tarballs weathered oil that has formed a pliable ball. Size 

may vary from pinhead to 30cm.

Tidal excursion degree of influence of the tides on movement of the oil

Turbulent mixing random bulk movements of water,

caused by high winds and currents, that tear oil slicks into 

smaller patches that are distributed over a wider area

Uncertainty “confidence limits,” or the degree to which 

the spill forecast may be relied upon to be accurate 

Viscosity a measure of fluids resistance to flow

Weathering changes in physical and chemical characteristics of spilled oil due to evaporation, dissolution, oxidation,

sedimentation, and biodegradation

Summary 39
40 Summary
Summary 41
Donald L. Evans

Secretary, U.S. Department of Commerce

Vice Admiral Conrad C. Lautenbacher, Jr., USN (Ret.)

Under Secretary for Oceans and Atmosphere and NOAA Administrator

Margaret A. Davidson

Acting Assistant Administrator for 

Ocean Services and Coastal Zone Management,

NOAA Ocean Service

January 2002

U.S. Department of Commerce • National Oceanic and Atmospheric Administration • NOAA Ocean Service

42