# Presentation: 183532

## Ohio Highway Accident - Interstate 70

- February 7, 2000
- Smoke Simulation Analysis
- Gary L. Achtemeier
- USDA Forest Service
- Athens, GA
- 11 June 2003

## Task Description

- The following set of slides summarizes a PB-Piedmont analysis of smoke movement from the burning of a pile of Christmas trees at the Village of Lewisburg, Ohio, between 8:00 AM and 3:00 PM on 7 February 2000 at which time the brush fire was thought to have been extinguished. It is alleged that residual smoke from this burn continued through the night and crossed Interstate 70 about one-half mile south of the burn site. This smoke, in addition to local dense fog, was implicated as a factor in a vehicle pileup on I-70 at approximately 10:55 PM on 7 February.

## About PB-Piedmont

- PB-Piedmont is an experimental model. PB-Piedmont DOES NOT model combustion rates nor smoke emissions. PB-Piedmont assumes that residual smoke of some unknown level of concentration is being discharged from a particular site. PB-Piedmont proceeds under the assumption that: “Given that smoke is emanating from a site, what is the likely location of that smoke at a given time within an entrapping layer of cooled air near the ground at night as it moves over complex terrain under varying synoptic weather conditions.”

## About PB-Piedmont

- As regards the Ohio I-70 accident on 7 February 2000, and the interpretation of the results to follow – no claim is made that smoke was being generated at the burn site and, if smoke was being generated at the burn site, no claim is made regarding the emission rate of combustion or whether resulting concentrations were sufficient to significantly impair visibility. PB-Piedmont tells us that, if smoke was being generated at the burn site, where the most likely destination will be.

## Weather

- Weather during the burn and the post-burn smoke movement period was dry with temperatures near 40F (4.4C) in early afternoon falling into the mid 20’s (-3C) by 11 PM. Winds were blowing from the west at noon LST. Winds shifted to blow from the northwest during the afternoon, from the north by evening and from the north-northeast by 10:55 PM – the time of the accident.

## Modeling Assumptions

- Nocturnal boundary layer “drainage layer” 10 m deep decouples from the free atmosphere at sunset.
- Drainage layer cools at rate of 5K/30min after sunset unless cooling rate modulated by cloudiness.
- Maximum lapse rate of temperature within the drainage layer cannot exceed 5K/30 m.
- Maximum lapse rate of temperature within the drainage layer decreases incrementally as airmass becomes saturated.
- Drainage winds form as weak internal pressure forces drive cooled air toward lower elevation.
- Synoptic boundary conditions: flow through for winds, synoptic temperatures, dew point temperatures, and pressure.
- Governing equations: time-dependent Navier-Stokes.

## Explanation of Cooling Rate

- Cooling of the shallow layer of air near the ground is what drives drainage winds. Observations of drainage wind formation in the South reveal that the transition from well-mixed conditions with relatively uniform winds over an area to drainage conditions with local winds impacting air movement occurs rapidly just after sunset. Increasing the empirical cooling rate speeds up the process while decreasing the cooling rate slows it down. Nevertheless, within about 2 hours, results are essentially the same. The cooling rate for dry soils of the Piedmont is probably less than that for snow cover. However, after the transition period, the differences in the model simulations should be minor. Therefore, PB-Piedmont has been run with the Piedmont cooling rate.

## Modeling Information

- PB-Piedmont Version 3.0-2003
- Grid spacing = 60m
- Domain size = 145x135 -> 8.7x8.1 km
- Visual domain = 1.7x1.3 km
- Weather data: Hourly METAR
- Elevation data: USGS 30 m DEM
- Bulk drag coefficient: 0.0025

## Explanation of 60-m Grid Spacing

- Elevation data are provided to PB-Piedmont on a dense grid with 30-m grid spacing. PB-Piedmont was run with a 30-m grid and a 60-m grid. Comparisons of the results showed no real differences except that the 30-m simulations take hours to run. This result was expected since the topography of the area is smoothly varying with few side streams with ravine-like features. Therefore, the results to follow were done with the 60-m grid.

## This is a map of the Lewisburg area showing key reference points for the analysis: I-70, SR-503, the Water Works and Twin Creek. The “X’s” mark where location information was collected for input into PB-Piedmont.

## This is the full 8.7x8.1 km domain for PB-Piedmont. It is included to show the terrain features that will impact the model simulations. The small red square identifies the location of the burn site.

## Two Simulations

- PB-Piedmont was run for two simulations. The first simulation was done with just the USGS 30-m DEM data. The second simulation was run with the elevation data modified to account for the I-70 embankment.

## PB-Piedmont observation points 0800 EST 16 January 2003

- 5: Light smoke (0811)

- 6: Smoke (0820)

- 7: Smoke (0825)

- 8: Smoke boundary (0832)

- 9: Smoke (0837)

- 10: Smoke boundary (0856)

- 11: Light smoke (0904)

- 12: Smoke (0908)

- 13: Smoke boundary south (0920)

- 14: Smoke boundary (0954)

- 15: No smoke (0800)

- Click left mouse button to view observation points.

- This is the viewing grid for the first simulation. Blue lines identify key features for the analysis. Click left mouse button to see these features.

- Burn Site

- I-70

- SR-503

- Clay St.

- Depot Ln

- Twin Creek

- Model Smoke at 5:00 PM

- Wind symbol
- Flag = 5.0 kt
- Barb = 1.0 kt
- Short barb = 0.5 kt

- Shading intervals are 3 m elevation

## The smoke plume at 7:00 PM EST. Cloudiness has retarded drainage wind formation. Nevertheless, the smoke is confined to the valley passing across I-70 east of the Twin Creek bridge.

## The smoke plume at 11:00 PM EST – the time of the accident on I-70. Drainage flows have concentrated the smoke to a narrow path west of the Twin Creek bridge. White contours identify lines of constant temperature. Fog conditions can occur where the temperature is less than 23F.

## SIMULATION TWO – The second simulation uses elevation modified for the I-70 embankment via aerial photographs of Preble County. Note the extension of high elevation into the Twin Creek valley. Grid points are 60 m. Smoke plume at 5:00 PM.

- SIMULATION TWO – The second simulation uses elevation modified for the I-70 embankment via aerial photographs of Preble County. Note the extension of high elevation into the Twin Creek valley. Grid points are 60 m. Smoke plume at 5:00 PM.

## SIMULATION TWO – Smoke tends to dam up behind the I-70  embankment. The plume becomes more diffuse and spread out. Smoke crosses I-70 from the Twin Creek Bridge westward. Smoke plume at 8:00 PM.

- SIMULATION TWO – Smoke tends to dam up behind the I-70  embankment. The plume becomes more diffuse and spread out. Smoke crosses I-70 from the Twin Creek Bridge westward. Smoke plume at 8:00 PM.

## SIMULATION TWO – Wind currents in the Twin Creek valley become light and variable as PB-Piedmont simulates weak eddies in the flow. At 9:00 PM, wind speeds are near zero at the burn site. This allows smoke to build up in high concentrations locally. 

- SIMULATION TWO – Wind currents in the Twin Creek valley become light and variable as PB-Piedmont simulates weak eddies in the flow. At 9:00 PM, wind speeds are near zero at the burn site. This allows smoke to build up in high concentrations locally.

## SIMULATION TWO – At 10:00 PM, the patch of dense smoke is observed moving southward toward I-70. 

- SIMULATION TWO – At 10:00 PM, the patch of dense smoke is observed moving southward toward I-70.

## SIMULATION TWO – At 11:00 PM, the patch of dense smoke is crossing I-70 from the Twin Creek Bridge westward. 

- SIMULATION TWO – At 11:00 PM, the patch of dense smoke is crossing I-70 from the Twin Creek Bridge westward.

- Accident Site at 1055 PM

## Summary: What PB-Piedmont tells us

- Summary: What PB-Piedmont tells us

- Synoptic forcing with winds blowing generally from the north combined with drainage forcing in a stream valley oriented north-south to confine the winds to blow southward from the burn site to I-70.
- SIMULATION ONE produced a narrow and steady smoke plume that crossed I-70 west of the Twin Creek bridge at 11:00 PM – just after the reported time of the accident.
- SIMULATION TWO, with the obstacle of the I-70 embankment, produced a broader, unsteady smoke plume that crossed I-70 from the Twin Creek bridge westward.

## Summary: Point of Clarification about SIMULATION TWO

- Summary: Point of Clarification about SIMULATION TWO

- PB-Piedmont produced patches of dense smoke as very weak valley flow behind the I-70 embankment broke up into eddy circulations.
- Although the formation of dense patches of smoke in weak flow may be physically realistic, the timing of the formation and subsequent movement down-valley is not predictable.
- Nothing can be claimed regarding the timing of arrival of the dense smoke at I-70 other than it is possible that dense smoke/fog (superfog) could have contributed to accident conditions at a particular time.

## Final Comments: The PB-Piedmont analysis suggests that a number of factors combined to produce hazardous visibility at the accident site on I-70. These are:

- Final Comments: The PB-Piedmont analysis suggests that a number of factors combined to produce hazardous visibility at the accident site on I-70. These are:

- Winds blowing from the west during the active part of the burn but shifting to blow from the north toward I-70 after dark.
- A north-south oriented stream valley that aided in channeling residual smoke from the burn site to I-70.
- A relatively moist ambient airmass that would support formation of valley fog by 11:00 PM.
- Obstruction to the valley winds presented by the I-70 embankment which dammed up the smoke-filled air and weakened valley winds thus allowing for residual smoke to build up in higher concentrations.
- Doing the burn in a valley bottom which increases the likelihood of smoke entrapment at night.
- Extinguishing the fire with water thus creating smoldering and cooling the combustion to allow the smoke to become entrapped in the drainage layer near the ground.