EPA/600/A-97/039
ADVANCES IN DENSE GAS DISPERSION MODELING OF ACCIDENTAL
RELEASES OVER ROUGH SURFACES DURING STABLE CONDITIONS
             G. Briggs1, R.E. Britter2, S.R. Hanna3, J. Havens4, S.B. King5, A.G. Robins6,
             W.H. Snyder7, and K.W. Steinberg8

             'NOAA, U.S. DOC, on assignment to U.S. Environmental Protection Agency,
               MD-80, Research Triangle Park, NC 27711
             University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, UK
             3EARTH TECH, 196 Baker Avenue, Concord, MA 01742, USA
             "University of Arkansas, 700 West 20th Street, Fayetteville, AR 72701, USA
             5Western Research Institute, 365 North 9th Street, Laramie, WY 82070-3380
                USA
             'University of Surrey, Guildford, GU2 5XH, UK
             7NOAA, U.S. DOC, on assignment to U.S. Environmental Protection Agency,
                MD-81, Research Triangle Park, NC 27711. Current address: University of
                Surrey, Guildford, GU2 5XH, UK
             8Exxon Research and Engineering Company, 180 Park Avenue, Florham Park,
                NJ 07932, USA
 INTRODUCTION

     A major, cooperative research project will be completed in 1997 from which an improved
 understanding will be gained about the dispersion of accidental, dense gas releases at industrial
 sites (i.e., high surface roughness) during low-wind stable meteorological conditions.  The
 plans for this project were presented by Hanna and Steinberg (1995). Most previous research
 was limited to releases over smooth surfaces in nearly-neutral conditions (Hanna et al., 1993).

     More specifically, the goals of this research program include the following:
 •    Determine  the effects of a wide range of surface roughness on dense gas dispersion
     (DGD),
 •    Explore the effects of a wide range of atmospheric conditions of most concern to DGD
     modeling, from near neutral stability with wind speed (at a height of 1 m) of about 5 m/s,
     to quite stable with wind speed of about  1.5 m/s  (corresponding to Pasquill "F"
     conditions),
 •    Determine  the effects of  wind shear and along-wind dispersion on  concentration
     magnitude and duration downwind of short-duration releases,
 •    Measure the effect of plume Richardson number, Ri" = 0 (passive plume) to about 20,

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 •    Determine whether there are significant atmospheric stability effects on vertical DGD by
     repeating some of the above in a stably stratified wind tunnel flow,
 •    Determine more precisely the Reynolds number and Peclet number limits of good
     simulations of full-scale scenarios in wind tunnels.

     The major elements of this research program include the following:
 •    Studies in several wind tunnels of vertical entrainment into dense gas clouds flowing over
     surfaces with roughness elements ranging in size from smaller to taller than the cloud
     height,
 •    Exploration of methods to simulate  dense gas releases under  a stable atmospheric
     boundary layer in an environmental wind tunnel  by achieving appropriate parameter
     ranges for a flow with a Monin-Obukhov length sufficiently small and a roughness length
     sufficiently large,
 •    Development of a demonstration data-set from a series of field experiments including both
     short duration and continuous dense gas release during neutral to very stable atmospheric
     conditions for surface conditions ranging from (1) smooth, (2) uniform roughness, to (3)
     a combination of uniform roughness and localized very large roughness.
 •    Modification of scientific algorithms (e.g., vertical entrainment parameterizations)  used
     in dense gas dispersion models and evaluations of the revised models.

 An integrated philosophy was used to coordinate the field and wind tunnel elements of this
 project in order to enhance the usefulness of the overall data sets. As the wind tunnel
 experiments are now largely complete, an overview of these experiments is given.  Also, some
 preliminary results from the completed neutral wind tunnel tests are provided, such as
 entrainment rate as a function of the plume Richardson number. The main field  experiment
 known as  "Kit Fox" was completed during the summer of 1995. A description of these
 experiments as well as a summary of the data collected are presented.
INTEGRATED EXPERIMENTAL DESIGN

    We believe that the scientific conclusions of dense gas diffusion (DGD) studies would be
greatly strengthened if both field and wind tunnel experiments were carried out in an integrated
fashion. Wind tunnel studies are cheaper, faster, and more controllable, but for DGD the low
tunnel speeds required for Richardson number (Ri) similarity impose simulation limits that are
not well defined. This is especially so for DGD in stable conditions, but we saw possibilities
of using advanced facilities for this purpose for the first time. Flow visualizations and plume
measurements over a range of Reynolds numbers, comparisons with similar field runs, and
comparison of measured dimensionless plume entrainment rates with field-validated values
could both increase confidence in wind tunnel DGD studies and better define the limits of this
tool.
    The first step was to use neutral wind tunnels, at speeds ensuring full turbulence, to design
two standard roughness arrays for use in both types of experiments.  A "uniform roughness
array" (URA) was developed in the wind tunnel at the U.S. Environmental Protection Agency
(EPA) Fluid Modeling Facility  (FMF)  by measuring wind and turbulence profiles  over
candidate arrays (Snyder, 1995). The object was to maximize z/Hr, tne ratio of roughness
length to element height, while maintaining low element density, since we needed to cover
37,000 m2 of field with the design. The URA is intended to represent the general effects of
non-smooth land surfaces on DGD.  An "equivalent roughness pattern" (ERP) was developed
in the Cermak-Petersen-Peterka (CPP) wind tunnel to represent the downwind effects of a
concentrated area of very large roughness, e.g. an industrial complex (Petersen and Cochran,

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 1995a and b).  Scale models of refineiy complexes were used to establish target values of
 downwind turbulence and passive diffusion. For maximum roughness efficiency, flat baffles
 facing the wind were chosen for both arrays.  In both the wind tunnel and the field experiments,
 a line of tall spires was set up perpendicular to the wind in the approach flow in order to
 generate additional turbulence (the spires were 5 m tall in the field).
     An earlier 1993 smooth-surface field experiment at DOE's Spills Test Facility provided
 experience with a dense array of collocated CO2 (real-time) sensors and bag samplers for CO2
 and SF6(Egami et al., 1995). The CO2 sensors were available with spans of 0.2 to 10% CO2,
 allowing practical arc distances of about 25 to 225 m for the planned release rates. The real-
 time sensors allowed us to make multiple releases during favorable conditions, to measure
 concentration fluctuations, and to detect the passage time and peak concentrations at each arc
 for short duration releases (20 s). The most relevant 1995 Kit Fox field experiments had three
 primary goals. The first goal was to study the effect of roughness on DGD using three different
 surface conditions: the baseline smooth desert, the URA alone, and the URA+ERP arrays
 together to simulate effects of an industrial release.  The second goal was to study DGD during
 much lower wind speeds and more stable conditions than attempted previously. The third goal
 was to study the effects of wind shear and along-wind dispersion on short-duration releases.
 The 1993 tests and continuous meteorological monitoring established that the best time to
 capture neutral to very stable conditions, with diminishing winds predominantly from a narrow
 sector, was one hour prior to and following sunset.  The 1995 Kit Fox series included releases
 with winds at a height of 1 m of 5 m/s down to 1 m/s and stabilities from Pasquill D to F and
 beyond, meeting or exceeding our expectations.
    Wind tunnels were used for simulations of full-scale,  point source releases and for
 idealized DGD studies. The CPP planning studies mentioned above can be compared with
 actual field tests, for both continuous and 10-s releases, made with u (1m) near 5 and 2.5 m/s.
 However, because of the scale-down of tunnel  speed required for Ri similarity, lower speed
 simulations were not possible because laminarization and flow instabilities would occur; this
 is a serious limitation in all DGD wind tunnel studies.  Measurements of continuous point
 source DGD were also made at four wind speeds  in the EPA FMF tunnel over a roughness
 array identical to the URA. Three series of idealized studies in three different tunnels complete
 our wind tunnel program. These focus on vertical entrainment because that is the most weakly
 supported element of present DGD modeling, especially with no previous studies over rough
 surfaces.  To reduce the  need for three dimensional measurements, which are very time
 consuming, and to maintain near constant plume Ri, a line source  spanning the tunnel was
 used. The EPA FMF wind tunnel studies focused on the URA array (Snyder, 1996).  Two
 scales were used, with elements 5 cm high and 5/6 cm high, to study entrainment for plumes
 both shallow and deep compared to  the roughness and to better establish the minimum
 Reynolds number required for full-turbulence simulation. A purposely similar program for the
5 cm elements was carried out in the  wind tunnel at the University of Arkansas Chemical
 Hazards Research Center (CHRC)(Havens et al., 1996); it used an identical physical setup but
 different instrumentation to check on the replicability of results. In addition, a series using
roughness of a  very different geometry was carried out  to test the generalizability  of
entrainment parameterizations, e.g., ones in terms of friction velocity. Finally, a similar series
 of measurements over a URA type array, 2 cm  high, is now in progress at EnFlo (University
 of Surrey) to study the effect of strong ambient stability on DGD. This is the first such attempt,
 and has required development of new instrumentation to measure surface heat flux.
FIELD EXPERIMENTS

    The primary objective of the field experiments was to capture a matrix of finite and

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 continuous duration releases under neutral to stable meteorological conditions over three
 different surface roughness configurations. To meet the objectives of the project, the field tests
 were designed to incorporate the results of the exploratory wind tunnel studies mentioned
 above, the 1993 CO2 dispersion experiments (Egami et al., 1995), and the predictions of
 expected plume concentrations and geometry by dense gas models.  It was necessary to
 measure gas concentrations, meteorological data, and source data at one second intervals prior
 to, during, and after release of the surrogate dense gas for three types of surface roughness: 1)
 the flat unobstructed desert surface, 2) the URA, and 3) the URA+ERP.  When the URA and
 ERP roughness elements were in place, a line of spires was placed 89 m upwind of the source,
 in order to enhance the development of turbulence in the boundary layer over the roughness
 elements. These so-called Irwin spires were also used in the wind tunnel experiments. Details
 concerning the locations and sizes of the spires and the ERP and URA elements are given in
 Table 1. Because the time window available for the experiments was limited, the sequence of
 the  surface roughness configurations used  for the experiments was ordered from high
 (URA+ERP) to low (smooth desert). As the roughness configurations were changed, the array
 of meteorological towers was kept fixed, but the 95 CO2 sensors had to be positioned lower and
 partially respanned for higher concentrations for anticipated  changes  in DGD during the
 smooth desert tests.
     The field tests were conducted at the U.S. Department of Energy (DOE) Spill Test Facility
 in Nevada. Storage tanks were filled with CO2 vaporized from a portable liquid tank to a
 maximum pressure of 8.85 atmospheres. A 329 m release line extended from the tank farm
 to a sub-surface box which had a quick-acting (<1 sec) sliding door that exposed a 1.5 m x 1.5
 m opening at ground level; this provided a low-momentum source. Since concentrations are
 inversely related to surface roughness, flow rates were adjusted to 4 kg/s for the URA+ERP
 surface, 1.5 kg/s for the URA  surface, and 1.0 kg/s for the  smooth surface, to keep arc
 concentrations within the optimum ranges of the sensors.
Table 1. Description of Locations and Sizes of Roughness Elements Used in Field Experiments.
Field
Roughness
Element

Spires

ERP
URA
Farthest
Upwind
Location
with
Respect to
the Source
89m

50m
89m
Spatial
Coverage

upwind edge
of URA

39m x 85m
120m x 314m
Number
of
Elements

36

75
6,600
Element
Width

0.458m
bottom
0.12m
top
2.4m
0.8m
Element
Height

4.87m

2.4m
0.2m
Element
Lateral
Spacing

3.25m

6.1m
2.4m
Element
Downwind
Spacing

not
applicable

8.5m
2.4m
    Solid slate infrared GO2 chemical sensors were deployed at four different downwind arrays
(25, 50, 100, and 225 m from the source).  The first three arrays each had three towers with
vertical arrays of five sensors  plus additional ground level sensors, while the 225 m array
consisted  of only  ground level  sensors.   Meteorological instruments,  consisting  of
propeller/vane anemometers and temperature probes, were located on three towers, one 20 m
in front of the spires, one 6 m in front of the ERP, and one 50 m downwind of the source.
Towers with sonic anemometers were located 20 m upwind of the spires, 6 m upwind of the
ERP, 7.5 m upwind of the source, and 50 m downwind of the source. An eight level 24 m

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 tower with propeller/vane anemometers and temperature sensors was located approximately
 100 m upwind of the source and 180 m off the centerline of the test grid.
    Releases were made during 13 evenings from August 22 to September 15.  At least
 partially successful data capture was obtained for 14 URA+ERP releases, predominately during
 "D" and "E" Pasquill stabilities, as determined from atmospheric Richardson numbers. For
 the URA surface, we count about 33 successful data captures, including about 6 each in the "E"
 and "F" stabilities. For the smooth surface, we count about 23 successful data captures; 7 of
 these were "F" stability and 3 can possibly be considered "G" stabilities. For each surface
 condition, about 1/3 of the releases were continuous (2 to 6 minutes) and 2/3 were short-
 duration (20 seconds).
    The analysis of the Kit Fox field experiments is only in a preliminary stage, since the data
 are still being calibrated and subjected to QA/QC procedures. However, one or two runs from
 each  stability class, roughness class, and source  duration class have been analyzed and
 compared with the predictions of an updated version of the HEGADAS model. The results of
 the analyses demonstrate that 1) vertical entrainment (and hence ground-level concentrations)
 are strongly affected by changes in the underlying roughness, 2) ambient wind speed has the
 largest effect on dense gas dispersion and subsequent distribution of concentrations, 3)
 theoretical  scaling relations developed in the wind tunnels and used in models such as
 HEGADAS are verified by the field observations, and 4) along-wind dispersion (for the finite
 duration releases) is enhanced by wind shear near the ground. The comparisons of limited
 observations  with  model predictions  suggest that the updated  algorithms in HEGADAS
 properly account for these effects.
VERTICAL ENTRAINMENT STUDIES IN NEUTRAL WIND TUNNELS

    At the time of writing, the two programs of vertical diffusion measurements in neutral
wind tunnels are complete, data reports are available (Havens et al., 1996: Snyder, 1996), and
data analyses are in progress.  The stable boundary layer program at the University of Surrey
is scheduled for completion by late spring of 1997. As described in the "Design" section, the
wind tunnel at the EPA FMF was used to investigate DGD over the "URA"  array at two
contrasting scales  with element heights Hr = 5/6 and 5 cm.  The small and large versions are
designated "WH4-12S"  and  "WH4-12L" (see Fig. 1),  with  "4" referring to  the ratio of
roughness element width to H, and "12" referring to the ratio of element spacing to H,.  Closely
coordinated experiments were run in the CHRC wind tunnel, which was especially designed
for dense gas studies. At CHRC, two significantly different roughness geometries were run,
one identical to the WH4-12L array above and one designated as the "WH1-8" array with Hr
= 3.8 cm. On the basis of earlier wind tunnel studies, the ratio zyH, for the 4-12 array is about
three times that of the  1-8 array; thus, the 4-12 array is far more  efficient for generating
increased turbulence intensity.
    It is expected that when the plume depth, h, is large compared with Hr, the roughness
effects may be parameterized through a single variable, z0.  However, when the elements are
the same height or larger than the plume depth, entrainment will be affected by the shapes,
sizes and spacings  of the individual elements. Hence, the dual-scale  experiments in the EPA
FMF wind tunnel were designed to study plume growth both when h » Hr and  when h < Hr
or h ~ Hr. The contrasting geometry studies in the CHRC wind tunnel focused on how the
geometry of the elements affect entrainment rates in the latter situation. The WH4-12L array-
was run at both laboratories with ostensibly identical flow rates and free flow speeds. The EPA
study included additional measurements to elucidate Reynolds number effects.
    In both facilities, a "line" source supplied a metered  rate of carbon dioxide  at negligible
vertical velocity through a bed of fine gravel contained within a rectangular box. This box

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stretched the entire width of the EPA tunnel. Because the CHRC tunnel was much wider than
the EPA tunnel, interior sidewalls were used to obtain the same effective width, and the line
source was identical.  Its width (streamwise direction) was 10 cm.  The flow structure was
measured with hot-wire anemometry (EPA) and laser-doppler anemometry (CHRC). A small
fraction of ethane (QH*) was mixed with the carbon dioxide (CO^ so that the downwind
concentration fields could be measured with flame ionization detectors.
     Prior to analyses of these wind tunnel and field data, an international group of scientific
advisors agreed on certain definitions and conventions. For example, in analyzing boundary-
layer profiles, we agreed to assume that the von Karman constant k = 0.4. We define effective
mean plume speed,  u, from  the vertical profile of wind speed, u(z), weighted by the
concentration profile: u = JuCdz-r/Cdz; when available, crosswind integrated or summed C
is used.  As a practical matter, u(z)  is measured outside the plume. A characteristic plume
depth is defined by h = /Cdz-fCj, where C§ is the surface concentration. Vertical entrainment
velocity is defined simply as we = d(uh)/dx. Thus, when  the mass flux of plume gas or tracer
is conserved (JuCdz = Q), then we = QdC/Vdx, a relatively simple  determination.  All
velocities are scaled by the friction velocity, u., which is measured outside the plume. The
basic Richardson number definition is Ri* = g'h/u*2, where g' is reduced gravity (gAp/pJ. For
the two-dimensional, line source plumes used above we can derive Ri* = B,/(u u*2), where B0
= go'Qo/C0 is the line-source buoyancy flux.,
     Some preliminary results, from just the EPA FMF wind tunnel, are shown hi Fig. 1; we
have plotted w,/u* versus Ri*.  One  encouraging result is that, at the passive limit (Ri* = 0,
plotted at 0.1 in Fig. 1), our data agree well with the best available field data, Project Prairie
Grass, when these data are analyzed exactly the same way: Wj/u* - 0.65. Previously published
values for this limit range from 0.4 to 1.0. Another encouraging result is the reasonably good
agreement between the small and large array, provided that the tunnel speed for the small array
was  not dropped below 1 m/s. When it was dropped further, concentration measurements
became erratic and the plume appeared to laminarize. Therefore, to maintain full-scale
similarity, it appears that the minimum roughness Reynolds number for laboratory studies over
sharp-edged roughness is Re*  =  u'z^/v = 1.5.  Compared to common DGD models, e.g.,
DEGADIS, these w,/u* are about 30% larger at small Ri*.  However, in a range where the
models are more frequently used, Ri* = 0.3 to 3, the agreement is reasonably good.
      1
     .1 --
    .01
                                       -TT+-
           D
           O
           A
    WH4-12S, EPA
    WH4-12L, EPA
    Prairie Grass
 Curves from DEGADIS.
 Upper k=0.40; Lower k=0.35

	1	.	. I I  I I I |	L.
        .1
                                         10
100
                                         Ri'
Figure 1. Entrainment velocities deduced from EPA FMF neutral wind-tunnel measurements.

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 PLUME STUDIES IN STABLE BOUNDARY LAYERS

     Entrainment studies with dense gas plumes in stable ambient conditions are being
 investigated in the University of Surrey's EnFlo stratified flow wind tunnel. The objective is
 the same as for the neutral experiments, except that the entrainment experiments are conducted
 in moderately stable conditions.
     Initial work concentrated on demonstrating that suitable, moderately stable boundary
 layers could be established, as determined by comparison of profiles of mean velocity and
 temperature, turbulence intensities, heat fluxes and temperature fluctuations with relationships
 based on measurements in the atmospheric boundary layer.
     In the wind tunnel, a standard 1 m high barrier wall and a vorticity generator system
 (spires) were mounted at the entrance to the working section.  The inlet heaters were set to
 provide a uniform temperature profile at the start of the cooled floor section (i.e., at x = 9 m)
 with the temperature difference between the free flow above the boundary layer and the cooled
 floor panels being held constant. For most of the work the tunnel was run at a free stream
 speed, u,^, of 1.35 m/s, with some additional runs at speeds of 1.2 and 1.5 m/s.  The floor was
 covered with a "WH4-12" configuration of roughness elements of height H, = 20 mm. This
 configuration is similar to the one used in the EPA and CHRC wind tunnels (see above), and
 used in the field as the URA roughness. A combination of IDA and cold-wire instrumentation
 was used to measure the full set of mean flow and turbulence profiles.
    A well behaved, moderately stable boundary layer was simulated with u^ set at 1.35 m/s,
 developing quite markedly to begin with, but slowly thereafter. Between x = 16 and 18 m, log-
 linear profiles were found to provide a very close fit to the mean velocity and temperature data
 up to a height of about z = L. The boundary layer depth, 6, was about 250 mm; this is shallow
 but sufficient for dense gas studies. The characteristic scaling ratios, 6/L, o,/u*, oju*, and
 o.p/0* are listed in Table 2, where L is the  Obukhov  (stability) length, ou, o^ and OT are
 standard deviations of  longitudinal and  lateral  turbulent  velocities and  temperature
 fluctuations, and 0.  is the  turbulent temperature scale.
Table 2.  Characteristic Properties of Stable Boundary Layer in EnFlo Wind Tunnel

       x(m)        6(rnm)      6/L         o,/u*       aju*
15
16
!7
18
200
250
250
270
0.78
1.48
1.38
1.57
1.8
1.7
1.6
1.5
-2.0
-2.0
-1.9
-1.9
1.2-
1.3-
1.2-
1.1-
1.4
1.4
1.3
1.3
1.7-
1.6-
1.7-
1.6-
1.9
1.7
1.9
1.8
    Since the scaling ratios in Table 2 satisfied the boundary layer criteria set forth by the
project steering group, the stable boundary layer flow was judged suitable for the dense gas
entrainment studies and the tunnel was then adapted for that phase of the work.  This involved
extending the fetch of cooled floor for sufficient plume development, and installing a supply
system for a mixture of carbon dioxide and propane. A source arrangement identical to that
used in the EPA and CHRC wind tunnels was also installed. The experimental program covers
both two and three dimensional plume studies, with attention focussed mainly on the former.
At conclusion, the entrainment velocity versus cloud Richardson number relationship will be
evaluated as a function of ambient stability conditions.

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 SUMMARY

     A multi-component cooperative research program consisting of both field and wind tunnel
 experiments was designed to answer basic questions concerning dense gas dispersion over a
 wide range of surface roughnesses and atmospheric stabilities. The field measurements are
 completed and the data should be cleared through the quality assurance process by late spring
 1997. Two series of experiments on vertical DGD have been completed in two neutral wind
 tunnels and data reports are available; partial analyses concerning Richardson and Reynolds
 number effects were presented here, while more finalized analyses are near completion.  A
 similar experiment in a stably stratified wind tunnel is near or at the end of the experimental
 phase, with a data report and data analyses due before the end of 1997. We believe that the
 totality of data collected under this program will be adequate to meet the goals stated at the
 outset of this paper.
ACKNOWLEDGMENTS

    This cooperative research is being sponsored by the Petroleum Environmental Research
Forum  (PERI7) Project 93-16, the U.S. Environmental Protection Agency, the  Western
Research Institute through its Jointly Sponsored Research agreement with the U.S. Department
of Energy, and the Department of Energy's support of the Chemical Hazard Research Center
at the University of Arkansas.
    Exxon Research and Engineering Company (ER&E) serves as contract coordinator for this
PERF 93-16 Project.  The other companies that are part of the Technical Advisory Committee
for this PERF Project, and their technical representatives, include Allied-Signal Incorporated
(Manny Vazquez), AMOCO Corporation (Doug Blewitt), Chevron Research and Technology
Company (Dave Fontaine), Mobil Research and Development Company (Frank Rogers), and
Shell Development Company (Dan Baker).
REFERENCES

Egami, R, Bowen, J., Coulombe, W., Freeman, D., Watson, J., Sheesley, D., King, B., Nordin, J., Routh, T.,
     Briggs, G., and Petersen, W., 1995, Controlled experiments for dense gas diffusion: experimental
     design and execution, model comparison. International Conference and Workshop on Modeling and
     Mitigating the Consequences of Accidental Releases of Hazardous Materials, AIChE, New York, 509-
     538.
Hanna, S.R., Chang, J.C., and Strimaitis, D.G., 1993, Hazardous gas model evaluation with field
     observations. Atmos. Environ., 27A:2265-2281.
Hanna, S.R. and K.W. Steinberg, 1995, Studies of dense gas dispersion from short-duration transient releases
     over rough surfaces during stable conditions. Air Pollution Modeling and Its Application XI, Plenum
     Press, New York and London, 481-490.
Havens. J., Walker, H.. and Spicer, T., 1996, Data report: wind-tunnel study of air entrainment into two-
     dimensional dense gas plumes. CHRC, University of Arkansas, Fayetteville, AR.
Petersen, R.L., and Cochran,  B.C., 1995a, Wind tunnel determination of equivalent refinery roughness
     patterns, CPP project 94-i 152, Tasks 1-5, Ft. Collins, CO.
Petersen, R.L.. and Cocliran,  B.C., 1995b, Wind tunnel testing of the 1995 Nevada test site field experiments,
     CPP project 94-] 152E; Task 6, Ft. Collins, CO.
Snyder, W.H., 1995, Data report, wind-tunnel roughness array tests. EPA, Research Triangle Park, NC.
Snyder, W.H., 1996, Duia report: wind-tunnel study of entrainment in two-dimensional dense-gas plumes,
     EPA, Research Triangle Park, NC.

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This paper has been reviewed in accordance with the U. S. Environmental Protection Agency's peer
and administrative review policies and approved for presentation and publication.  Mention of trade
names or commercial products does not constitute endorsement or recommendation for use.

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TECHNICAL REPORT DATA
^{WM/A-gy/osg
4. TITLE AND SUBTITLE
Advanced in Dense Gas Dispersion Modeling of Accidental Releases Over
Rough Surfaces During Stable Conditions
7. AUTHOR(S)
'Briggs, G.A., 2R.E. Britter, 3S.R. Hanna, "J. Havens, 5S.B. King, 6A.G. Robins,
W.H. Snyder, and 7K.W. Steinburg
9. PERFORMING ORGANIZATION NAME AND ADDRESS
'Same as Block 12 'Western Research Institute
365 North 9th Street
University of Cambridge Laramie, WY 82070
Trumpington Street
Cambridge, CB2 1 PZ, UK 'University of Surrey
Guildford
'EARTH TECH GU2 5XH, UK
196 Baker Avenue
Concord, MA 0 1 742 'Exxon Research & Engineering Co.
180 Park Avenue
'University of Arkansas Florham Park, NJ 07932
700 West 20th Street
Fayetteville, AR 72701
12. SPONSORING AGENCY NAME AND ADDRESS
National Exposure Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 277 1 1
3.F
5. RE PORT DATE
6.PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10.PROGRAM ELEMENT NO.
1 1 . CONTRACT/GRANT NO.
13.TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/9
15. SUPPLEMENTARY NOTES
A major, cooperative research project should be completed in 1 997 from which an improved understanding will be gained about
the dispersion of accidental, dense gas releases at industrial sites (i.e., high surface roughness) during stable meteorological
conditions. Most previous research focussed on releases over smooth surfaces in nearly neutral conditions. An integrated
philosophy was used to design this project with will enhance the usefulness of the overall data. As the wind tunnel experiments
arc now largely complete, an overview of these experiments including techniques, scope, and data quality checks will be give.
Also, some basic results from the completed neutral wind tunnel tests will be provided, such as entrainment rate as a function of
the plume Richardson number. The main field experiment known as "Kit Fox" was completed during the summer of 1 995. A
description of these experiments as well as a summary of the data collected will be presented.
16. ABSTRACT
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS ^IDENTIFIERS/ OPEN ENDED TERMS c.COSATI

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