Mobile Example Problems

Try out these examples to learn the basics of modeling oil spills in Mobile Bay. Explore how the changing tides affect the trajectories of oil slicks, how wind can move an oil slick in a different direction from the currents, and how model and observation limitations can be overcome by considering both the “Best Estimate” and the “Uncertainty” solutions. This knowledge will help you in designing your own WebGNOME model runs.

Example 1

Tides are an important part of the circulation in Mobile Bay. To test this, you will start the same spill at two points in the tidal cycle: the beginning of a flood tide (2340 on July 1, 2000) and the beginning of an ebb tide (1215 on July 2, 2000). Place a spill at 30° 22’ N, 88° 1’ W (about halfway between the Gulf entrance to Mobile Bay and the Theodore Ship Channel) and observe the effects of tides on the spill trajectory and beach impacts.

Begin by selecting the Mobile Bay Location File which will launch the Wizard to guide you through setting up the scenario. Use the information in the following table as you advance through the Wizard.

Start time:	As above for flood or ebb tide.
Model duration:	1 day.
Uncertainty:	Not included.
River flow:	Low (30 kcfs).
Wind:	No wind (constant at 0 knots).
Spill type:	Instantaneous.
Time of Release:	Same as model start time.
Amount released:	1000 barrels (bbls)
Pollutant type:	Non-weathering.
Position:	30°22’ N, 88° 1’ W

Tips:

1. If you only want to change one spill parameter (like the start time of the spill in this example), there is no need to reload the Location File and step through the Wizard. Instead, use the buttons on the Menu Bar to switch from Map View to Setup View. In Setup View are various panels which allow you to edit the model setup. In the Spill panel, click on the edit (pencil) icon of the spill you created to edit the time of release.

When you change the start time of the spill, you will likely want to change both the spill start time and the model start time. If you change the spill start time first, WebGNOME will automatically prompt you to change the model start time to match the spill start time. So it is a good idea to always change the spill start time first.

2. To more easily visualize the difference between the spill impacts, consider taking a screenshot of the map at the end of the first run.

Answer:

When the spill starts just before the flood tide, most of the beached oil is on the fill island east of Deer River Point and some of the beached oil is on the western end of Theodore Ship Channel. When the spill starts just before the ebb tide, most of the spill is transported out of Mobile Bay towards the Gulf, with some oil beaching on Dauphin and Pelican Islands.

Example 2

The circulation in Mobile Bay is significantly affected by the flow rate of the Mobile River. In this example, you will look at a spill closer to Mobile in the springtime and examine the effects of low and high river runoff on the transport of the spill. Set the spill at 30° 37’ N, 88° 1’ W and set the run time to 0530 on March 14, 2000 (a flood tide is just starting at this time). Run the spill two times in GNOME, the first time with a low (30 kcfs) river flow and the second time with a high (300 kcfs) river flow. Change the model run duration to 3 days for this example problem.

What are the differences in beach impacts between these two scenarios?

Tips:

1. Once again, use the Spill panel in Setup View to change spill settings.

2. The run duration can be edited in the Model Settings panel.

3. The Mobile Bay User Guide explains how the currents due to the river are scaled based on the river flows into the Bay. Since this scaling is linear, an increase in the river flow from 30 kcfs to 300 kcfs implies the scaling should be increased by a factor of 10. If you click the edit icon (pencil) next to the River Currents in the currents panel, you’ll be able to edit the Reference Point Value. If you started with the low river flow case, you’ll be increasing this value from ~0.197 to 1.97.

Answer:

Changing the river flow rate changes the oil spill trajectory, leading to different beach impact areas. When the river flow rate is low, the spill moves further up into the bay on the flood tides, impacting the marsh areas surrounding Mobile, before slowly starting to move toward the bay’s entrances. When the river rate is high, the spill moves toward the entrances of Mobile Bay at a much faster rate, allowing less time for beach impacts enroute.

Example 3

Wind both moves the oil along the water’s surface and drives currents. Rerun the previous spill with the high river flow rate and add a 15-knot wind from the northwest. Run this spill scenario for 2 days.

How does the oil’s trajectory change from the previous example?

Tip:

You created a spatially constant wind at 0 knots in the initial model setup. It shows up in a list (currently with just one entry) at the bottom of the Point Wind panel. You can choose to edit this wind (by clicking the pencil icon in the list) or add a new constant wind by clicking the + icon at the top right of the panel.

Answer:

The wind dramatically changes the oil’s trajectory! Instead of quickly moving seaward, much of the oil beaches along the eastern shoreline of Mobile Bay, from Seacliff to Palmetto Beach.

Example 4

Forecasts of environmental parameters are inherently uncertain. For example, wind and weather forecasts can be “off” in the speed, direction, or timing of winds. GNOME supports an “Uncertainty” solution in addition to the “Best Estimate” solution that you have been running. The “Uncertainty” solution takes into account our uncertainty in wind, horizontal mixing, and currents. Now you will add the “Uncertainty” solution to see where else the spill might go.

Rerun the previous spill with a high river flow rate, but first make these changes: (1) change the wind to 15 knots from the east; (2) change the spill start time to 0100 on March 15, 2000; (3) reset the model duration to 1 day; and (4) include the “Uncertainty” solution.

“Zoom in” to your spill area and examine the difference between the “Best Estimate” (black) and “Uncertainty” (red) trajectories. Why do you think this type of information would be useful?**

Tip:

To include the “Uncertainty” solution, click the box labeled “Include uncertainty in particle transport” in the Model Settings panel in Setup View.

Answer:

The “Ucnertainty” solution shows where else the spill could go if the currents, winds or other model inputs were a little bit different. Although our “Best Estimate” solution does not show any oil impacts on the fill island, the “Uncertainty” solution shows that there could be oil contact. Responders use this information to make decisions about how they will allocate response resources. Sometimes a highly valued environmental resource (e.g. an endangered species) may be important enough to protect, even if it has a low probability of being oiled.

Example 5.

Different types of pollutants weather differently. In the previous examples, you were using a “non-weathering” pollutant that did not change with time. Now you are going to run a “What if?” scenario that compares the effects of different types of pollutants.

A barge carrying 10,000 barrels of product grounds at 0530, March 14, 2000 at the entrance to Mobile Bay, near Mobile Point (30° 13.49’ N, 88° 2.01’ W). The Mobile River is currently running low; winds are from the northwest at 8 knots.

Run the above scenario for a barge containing medium crude and a barge containing gasoline. At the end of your 24-hour prediction, write down the mass balance for each scenario in the table below.

	Medium Crude (bbls)	Gasoline (bbls)
Released	10,000	10,000
Floating
Beached
Evaporated
Dispersed
Off map

Tips:

1. You can make all the changes to the model setup in Setup View or start over with the Wizard by choosing “Select a Location File” from the New pull down menu on the menubar.

2. Use the ADIOS Oil Database link to open the ADIOS oil database. From the database interface you can select an oil that corresponds to a medium crude or a gasoline. It doesn’t matter which exact oil you select, as long as it falls into one of these broad categories. Download the oil and load the file into WebGNOME using the load oil drop box.

3. To view the mass balance for each scenario switch to the Fate View.

Answer:

Heavier oils remain in the environment longer than lighter, refined products. You can see that the beach impacts from the medium crude spill are more extensive than for the gasoline spill.