Transcript Dispersion models

Modelling the environmental
dispersion of radionuclides
Jordi Vives i Batlle
Centre for Ecology and Hydrology,
Lancaster, October 2011
Lecture plan
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Dispersion models available in the ERICA Tool
Other types of dispersion models that are
available
And, along the way…
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Key parameters that drive dispersion models for
radioactivity in the environment
Applicability to different
scenarios/circumstances
www.ceh.ac.uk/PROTECT
What reasons to use models?
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Often the receptor is
not at a point of
emission but is linked
via an environmental
pathway (dilution)
Need to predict media
concentrations when
(adequate) data are
not available
Dispersion models are the tools required to make
this connection
www.ceh.ac.uk/PROTECT
Part I - Dispersion modelling in
ERICA
www.ceh.ac.uk/PROTECT
IAEA SRS Publication 19
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Designed to minimise underprediction (conservative
generic assessment)
A default discharge period of
30 y is assumed (estimates
doses for the 30th year of
discharge)
Currently being upgraded
www.ceh.ac.uk/PROTECT
Atmospheric dispersion
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www.ceh.ac.uk/PROTECT
Gaussian plume
model version
depending on the
relationship between
building height, HB
& cross-sectional
area of the building
influencing flow, AB
Assumes a
predominant wind
direction and neutral
stability class
Key inputs:
discharge rate Q &
location of source /
receptor points (H,
HB, AB and x)
Basic dispersion equation
A Gaussian plume model for an elevated release is as follows:
2
2


z

H

Q
y
S

C x, y, z 
exp

2
2
2u10 z  y
2 z
 2 y

where C = the air concentration (Bq/m3) or its time integral Bq.s/m3
Q = release rate (Bq/s) or total amount released (Bq)
u10 = wind speed at 10 m above the ground (m/s)
z = standard deviation of the vertical Gaussian distribution (m)
y = standard deviation of the horizontal Gaussian distribution (m)
HS = effective release height (m)
x, y, z = rectilinear coordinates of the receptors
Importance of Release Height
Effective stack height
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Conditions for the plume
(a)
a) H > 2.5HB (no building effects)
b) H 2.5HB & x > 2.5AB½(airflow in the wake
zone)
c) H  2.5HB & x  2.5AB½(airflow in the cavity
zone). Two cases:
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source / receptor at same building surface
not at same surface
Not generally applicable at > 20 km from
stack
www.ceh.ac.uk/PROTECT
(b)
(c)
Key parameters
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Wind speed and direction
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Release height
Precipitation
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10 minute average from 10 m wind vane & anemometer
10 minute total rainfall (mm)
Stability or degree of turbulence (horizontal and vertical
diffusion)
Manual estimate from nomogram using time of day,
amount of cloud cover and global radiation level
Atmospheric boundary layer (time-dependent)
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Convective and or mechanical turbulence
Limits the vertical transport of pollutants
www.ceh.ac.uk/PROTECT
R91 aerial dispersion model
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Based on the recommendations of the Working
Group on Atmospheric Dispersion (NRPB-R91, R122, -R123, -R124)
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Gaussian plume model
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Meteorological conditions specified by:
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Wind speed
Wind direction
Pasquill-Gifford stability classification
Implemented in PC CREAM and CROM
www.ceh.ac.uk/PROTECT
R91 - model limitations
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Model assumes constant meteorological and
topographical conditions along plume trajectory
Prediction accuracy < 100 m and > 20 km limited
Source depletion unrealistic (deposition
modelling & transfer factors are uncertain)
Developed for neutral conditions
Does not include
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Buildings
Complex terrain e.g. hills and valleys
Coastal effects
www.ceh.ac.uk/PROTECT
Surface water dispersion
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Freshwater
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Marine
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www.ceh.ac.uk/PROTECT
Small lake
(< 400 km2)
Large lake
(≥400 km2)
Estuarine
River
Coastal
Estuarine
No model for
open ocean
waters
Processes and assumptions
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Based on analytic solution of the advection diffusion
equation describing transport in surface water for
uniform flow conditions at steady state
Processes included:
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Flow downstream as transport (advection)
Mixing processes (turbulent dispersion)
Concentration in sediment / suspended particles
estimated from ERICA Kd at receptor (equilibrium)
Transportation in the direction of flow
No loss to sediment between source and receptor
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In all cases water dispersion assumes critical flow
conditions, by taking the lowest in 30 years, instead of
the rate of current flow
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The only difference between RNs in predicted water
concentrations as material disperses is decay by their
different radiological half-lives.
www.ceh.ac.uk/PROTECT
Rivers and coastal waters
Lz = distance to
achieve full vertical
mixing
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The river model assumes that both river discharge of radionuclides such
as water harvesting is done in some of the banks, not in the midstream
The estuary model is considered an average speed of the current
representative of the behaviour of the tides.
Condition for mixing is x > 7D and (y-y0)<< 3.7x
concentration in sediment is assumed to be
concentration in water x Kd
•
www.ceh.ac.uk/PROTECT
Kd = Activity concentration on sediment (Bq kg-1)
xxxxxActivity concentration in seawater (Bq L-1)
Small lakes and reservoirs
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Assumes a homogeneous concentration throughout the water body
Expected life time of facility is required as input
www.ceh.ac.uk/PROTECT
Limitations of IAEA SRS 19
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Simple environmental and dosimetric models as well
as sets of necessary default data:
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Simplest, linear compartment models
Simple screening approach (robust but conservative)
Short source-receptor distances
Equilibrium between liquid and solid phases - Kd
More complex / higher tier assessments:
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Aerial model includes only one wind direction
Coastal dispersion model not intended for open waters e.g.
oil/gas marine platform discharges
Surface water models assume geometry (e.g. river crosssection) & flow characteristics (e.g. velocity, water depth)
which do not change significantly with distance / time
End of pipe mixing zones require hydrodynamic models
www.ceh.ac.uk/PROTECT
Part II: PC CREAM as a practical
alternative for dispersion modelling
www.ceh.ac.uk/PROTECT
Collective dose model PC CREAM
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Consequences of Releases to the Environment
Assessment Methodology
A suite of models and data for performing
radiological impact assessments of routine and
continuous discharges
Marine: Compartmental model for European
waters (DORIS)
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Seafood concentrations => Individual doses => Collective doses.
Aerial: Radial grid R-91 atmospheric dispersion
model with (PLUME) with biokinetic transfer
models (FARMLAND)
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Ext. & internal irradiation => foodchain transfer (animal on pasture e.g. cow
& plant uptake models) => dose
www.ceh.ac.uk/PROTECT
Marine and aerial dispersion
Radial grid atmospheric model
Compartmental marine model
(continuous
discharge)
Irish Sea
North
Irish Sea
North West
Irish Sea
North East
Sellafield
Local
compart.
Cumbrian
Waters
Liverpool
And
Morecambe
Bays
Dundalk
Irish Sea
West
Dublin
www.ceh.ac.uk/PROTECT
Irish Sea
South East
Degree of improvement of the models
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Marine model (DORIS) => improvement
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Has long-range geographical resolution
Incorporates dynamic representation of water /
sediment interaction
Aerial model (PLUME) => no improvement
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Still a gaussian dispersion model unsuitable for long
distances > 20 km
Also assumes constant meteorological conditions
Does not correct for plume filling the boundary layer
www.ceh.ac.uk/PROTECT
Part III: Other alternative dispersion
models
www.ceh.ac.uk/PROTECT
Effects of using different models
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Uncertainty associated with the application of
aquatic SRS models:
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Models generally conservative.
From factor of 2 to 10 difference with respect to a dynamic
model.
Uncertainty associated with the application of a
Gaussian plume model for continuous releases:
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About a factor of 4 or 10 for a flat and complex terrain
respectively.
At distances < 2.5 times the square root of the frontal area
of the building, the model provides conservative results.
For distances of about 2.5 the above, the model tends to
underpredict for wind speeds above 5-m s-1.
www.ceh.ac.uk/PROTECT
Effects of using different models (2)
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For aerial, PC-Cream is no improvement to SRS 19
For marine, PC cream has a dynamic compartment
model
Effect of using such a fully dynamic model:
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In periods where concentrations in compartments increase,
dynamic model estimates of transfer will be lower than for
equilibrium model (‘build-up effect’)
In period where environmental concentrations decrease,
dynamic model estimates higher than equilibrium model
(‘memory effect’)
Diffcult to generalise, but differences could be up to
a factor of 10.
www.ceh.ac.uk/PROTECT
Aerial modelling
www.ceh.ac.uk/PROTECT
New-generation plume models
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Include deviations from idealised Gaussian plume
model
Include turbulence data rather than simplified stability
categories to define boundary layer
Include particulate vs gases and chemical interactions
Model includes the effects on dispersion from:
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Complex buildings
Complex terrain & coastal regions
Advanced models: ADMS, AERMOD
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Gaussian in stable and neutral conditions
Non-Gaussian (skewed) in unstable conditions
www.ceh.ac.uk/PROTECT
UK ADMS
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Modified Gaussian plume model
 Gaussian in stable and neutral conditions
 Skewed non-Gaussian in unstable conditions
Boundary layer based on turbulence parameters
Model includes:
 Meteorological preprocessor, buildings, complex terrain
 Wet deposition, gravitational settling and dry deposition
 Short term fluctuations in concentration
 Chemical reactions
 Radioactive decay and gamma-dose
 Condensed plume visibility & plume rise vs. distance
 Jets and directional releases
 Short to annual timescales
www.ceh.ac.uk/PROTECT
Types of output
NOx Concentration (ug/m3)
400
Stack with building
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
300
200
Concentration plot
Metres
100
0
-100
-200
-300
-400
0
100
200
300
400
500
Metres
www.ceh.ac.uk/PROTECT
600
700
800
900
1000
Marine modelling
www.ceh.ac.uk/PROTECT
Geographically-resolving marine
models
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Allow for nonequilibrium situations e.g. acute
release into protected site
Advantages:
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Resolves into a large geographical range
Results more accurate (if properly calibrated)
Disadvantages:
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Data and CPU-hungry (small time step and grid sizes
demand more computer resources)
Run time dependent on grid size & time step
Requires specialist users
Post-processing required for dose calculation (use as
input to ERICA)
www.ceh.ac.uk/PROTECT
Model characteristics
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Input requirements: Bathymetry, wind fields, tidal
velocities, sediment distributions, source term
Type of output: a grid map / table of activity
concentration (resolution dependent on grid size)
All use same advection/dispersion equations,
differences are in grid size and time step
Types of model:
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Compartmental: Give average solutions in compartments
connected by fluxes. Good for long-range dispersion in
regional seas.
Finite differences: Equations discretised and solved over a
rectangular mesh grid. Good for short-range dispersion in
coastal areas
Estuaries a special case: Deal with tides (rather than
waves), density gradients, turbidity, etc.
www.ceh.ac.uk/PROTECT
Model characteristics
Finite differences
www.ceh.ac.uk/PROTECT
Compartmental
Some commonly available models
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Long-range marine models (regional seas):
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Short-range marine models (coastal areas):
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POSEIDON - N. Europe (similar to PC-CREAM model but
redefines source term and some compartments - same sediment
model based on MARINA)
MEAD (in-house model available at WSC)
MIKE21 - Short time scales (DHI) - also for estuaries
Delft 3D model, developed by DELFT
TELEMAC (LNH, France) - finite element model
COASTOX (RODOS PV6 package)
Estuarine models
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DIVAST ( Dr Roger Proctor)
ECoS (PML, UK) - includes bio-uptake
www.ceh.ac.uk/PROTECT
DHI MIKE 21 model
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Two-dimensional depth averaged
model for coastal waters
Location defined on a grid - creates
solution from previous time step
Hydrodynamics solved using full
time-dependent non-linear equations
(continuity & conservation of
momentum)
Large, slow and complex when
applied to an extensive region
Suitable for short term (sub annual)
assessments
A post processor is required to
determine biota concentrations and
dose calculations
www.ceh.ac.uk/PROTECT
Marine Environmental Advection
Dispersion (MEAD)
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2 km grid
www.ceh.ac.uk/PROTECT
Applies advection
- dispersion
equations over
an area and time
Generates
activity
concentration
predictions in
water and
sediment
Has been
combined with
the ERICA
methodology to
make realistic
assessments of
impact on biota
MEAD input data - sediment
Distribution of fine grained bed
sediment
www.ceh.ac.uk/PROTECT
Distribution of suspended
particles (modelled)
MEAD output - Cumbrian coast
60Co
99Tc
in winkles
in crab
137Cs
in cod / plaice
241Am
in mussels
Could be used to derive CFs for use in ERICA
www.ceh.ac.uk/PROTECT
0.01
53.0
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
0.00
MEAD - Long-range results
55.0
1000.0055.0
5000.00
100.00
1000.00
50.00
54.5
100.00
54.5
10.00
10.00
54.0
5.00
Cumbria fishing
area
54.0
5.00
1.00
53.5
0.10
1.00
53.5
0.10
0.01
53.0
0.01
53.0
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
0.00
Predicted distribution of 137Cs
in seawater in 2000
1000.00
www.ceh.ac.uk/PROTECT
54.5
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
Predicted distribution of 137Cs
in bed sediments in 2000
5000.00
55.0
-6.0
100.00
0.00
More complex process models
Extra modules for extra processes
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More complex water quality issues e.g.
eutrophication
Wave interactions
Coastal morphology
Particle and slick tracking analysis
Sediment dynamics
Discharge
Influx_Pu_III_IV
Influx_Pu_V_VI
Reduction
Seawater_Pu_V_VI
Seawater_Pu_III_IV
OxidationInflux_Pu_particulate
Flushing_oxidised
Adsorption_susp
Desorption_susp
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ModelMaker biokinetic
models
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Dynamic interactions
with sediment
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Speciation
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Dynamic uptake in
biota
www.ceh.ac.uk/PROTECT
Flushing_reduced
Flushing_susp
Adsorption_coll
Suspended_load
Pelagic_fish
Adsorption_sed
Desorption_sed
Remobilisation
Deposition
Coagulation
Crustaceans
Surface_sediment
Molluscs
Bioturbation
Sedimentation
Burial
Benthic_fish
Middle_sediment
Bottom_sediment
Flushing_colloidal
Colloidal
Far_field
River and estuary modelling
www.ceh.ac.uk/PROTECT
River and estuary models
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Advantages:
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Large geographical range
Consider multiple dimensions of the problem (1 - 3D)
Considers interconnected river networks
Results more accurate (if properly calibrated)
Disadvantages - same as marine models:
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Data hungry
Run time dependent on grid size & time step
Requires a more specialised type of user
CPU-hungry (as time step and grid size decreases it
demands more computer resources)
Post-processing required for dose calculation (use as input
to ERICA)
www.ceh.ac.uk/PROTECT
Model characteristics
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Input requirements: Bathymetry, rainfall and
catchment data, sediment properties, network
mapping, source term
Type of output: activity concentration in water and
sediment, hydrodynamic data for river
All use same advection/dispersion equations as
marine but differences in boundary conditions
Generally models solve equations to:
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Give water depth and velocity over the model domain.
Calculate dilution of a tracer (activity concentration)
www.ceh.ac.uk/PROTECT
Common models
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Can be 1D, 2D or 3D models
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Off-the-shelf models:
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1D river models: River represented by a line in
downstream direction - widely used
2D models have some use where extra detail is required
3D models are rarely used unless very detailed process
representation is needed
MIKE11 model developed by the DHI, Water and
Environment (1D model)
VERSE (developed by WSC)
MOIRA (Delft Hydraulics)
Research models:
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PRAIRIE (AEA Technology)
RIVTOX & LAKECO (RODOS PV6 package)
www.ceh.ac.uk/PROTECT
Example - MIKE 11
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www.ceh.ac.uk/PROTECT
MIKE11 - Industry
standard code for river flow
simulation
River represented by a line
in downstream direction
River velocity is averaged
over the area of flow
Cross sections are used to
give water depth
predictions
Can be steady flow
(constant flow rate) or
unsteady flow
Use of cross sections can
give an estimate of
inundation extent but not
flood plain velocity
Catchment modelling
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Convert rainfall
over the catchment
to river flow out the
catchment
Represent the
processes
illustrated, however
in two possible
ways:
 Simple “black box” type model such as empirical
relationship from rainfall to runoff (cannot be used
to simulate changing conditions)
 Complex physically based models where all
processes are explicitly represented
 Example: DHI MIKE-SHE
www.ceh.ac.uk/PROTECT
Conclusions
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ERICA uses the IAEA SRS 19 dispersion models to
work out a simple, conservative source - receptor
interaction
SRS 19 has some shortcomings
PC-CREAM can be used as an alternative to the SRS19 marine model
There are further off-the-shelf models performing
radiological impact assessments of routine and
continuous discharges ranging from simple to complex
Key criteria of simplicity of use and number of
parameters need to be considered – must match
complexity to need
www.ceh.ac.uk/PROTECT
Summary of key points
SRS19 model
Marine
+ point in coast
PC Cream
DORIS
+ Large compartment box model
+ Dynamic transfer to water and
+ requires very few parameters
sediments
- no offshore dispersion
- requires more parameters
- very simple equilibrium model (Kd - Does not work well at fine
based)
resolution
Orther models
Marine
+ compartmental models for large areas
+ Grid models for fine resolutions (small
areas)
+ Dynamic / time-variable discharges
- parameter hungry (bathimetry,
gridding, etc)
River, lake, reservoir
+ very simple 1D model
- only models riverbanks
River, lake, reservoir
+ 2D - 3D models
+ Full representation of hydrodynamics
+ Can deal with tides, concentration
gradients
N/A
- Simple average flow conditions
- very simple equilibrium model (Kd
based)
- Simple linear river
Aerial
+ limited range 100 m to 20 km
+ constant meteorology
+ Gaussian plume, still conditions
www.ceh.ac.uk/PROTECT
+ Dynamic / time-variable discharges
+ Complex river networks
- parameter hungry (bathimetry,
gridding, etc)
PLUME
- Same as SRS19
AERMOD, ADMS, etc.
+ Non Gaussian for unstable conditions
+ Buildings and terrain
+ Solute modelling
+ Complex meteorology
Links to alternative models
Model
ADMS 4
AERMOD
Organisation
CERC
EPA
DELFT 3D
DELFT
Hydraulics
Cardiff
University
PML
HEC
(USACE)
IAEA
Halcrow
DIVAST
EcoS 3
HEC-RAS
IAEA SRS 19
ISIS
MEAD
MIKE11
MIKE21
MIKE3
MIKE-SHE
MOIRA-PLUS
PC CREAM 08
POSEIDON
PRAIRIE
R91
RODOS PV6
(COASTOX,
RIVTOX &
LAKECO)
TELEMAC 2 &
3D
VERSE
WSC
DHI
DHI
DHI
DHI
EU MOIRA
programme
HPA
CEPN
AEA
Technology
NRPB
EU RODOS
programme
Link
http://www.cerc.co.uk/environmental-software/ADMS-model.html
http://www.epa.gov/scram001/dispersion_prefrec.htm#aermod
(Freeware)
http://delftsoftware.wldelft.nl/index.php?option=com_content&task=blog
category&id=13&Itemid=34
http://hrc.engineering.cf.ac.uk/
http://www.pml.ac.uk/
http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html (Freeware)
www-pub.iaea.org/MTCD/publications/PDF/Pub1103_scr.pdf
http://www.halcrow.com/isis/isisfree.asp (Freeware)
http://www.halcrow.com/isis/default.asp (Professional edition)
http://www.westlakes.org (in-house model)
http://www.dhigroup.com/Software/WaterResources/MIKE11.aspx
http://www.dhigroup.com/Software/Marine/MIKE21.aspx
http://www.dhigroup.com/Software/Marine/MIKE3.aspx
http://www.dhigroup.com/Software/WaterResources/MIKESHE.aspx
http://user.tninet.se/~fde729o/MOIRA/Software.htm (Freeware)
http://www.hpa.org.uk/web/HPAweb&HPAwebStandard/HPAweb_C/11
95733792183
http://www.cepn.asso.fr/en1/logiciels.html
http://www.aeat.co.uk/
http://www.admlc.org.uk/NRPB-R91.htm
http://www.rodos.fzk.de/rodos.html (Freeware, password protected)
SOGREAH
http://www.telemacsystem.com/index.php?option=com_jdownloads&Itemid=31&task=
viewcategory&catid=3&lang=en (Freeware)
WSC
http://www.westlakes.org (in-house model)
www.ceh.ac.uk/PROTECT