Estuarine

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Estuarine Coastal Modelling Course
MODULE IX WATER QUALITY MODELLING PROCESS
Michael Hartnett, Environmental Change Institute,
NUI Galway
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• WATER QUALITY MODELLING PROCESS

OBJECTIVE Guidance on the major aspects in
developing hydrodynamic and water quality models
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• WATER QUALITY MODELLING PROCESS

CONTENT- I Boundary Conditions II Initial
conditions III Hydrodynamics IV Pollutant
loads V Water quality VI Heavy
metals VII Pathogens
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I BOUNDARY CONDITIONS
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I BOUNDARY CONDITIONS

• Boundary conditions (BCs)?
• Dominant processes along the boundaries
  (edges) of a model.
• BCs propagate throughout the interior of the
  model (Domain)
• resulting in some physical, chemical or
  biological effects.

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I BOUNDARY CONDITIONS

• Example
• Specification of a tidal boundary induces
  water level changes
• and currents within a model domain
• Driving forces behind a model
• Poor specification is a major source of
  modelling error

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I BOUNDARY CONDITIONS
BOUNDARY LOCATIONS
Boundaries located far enough away from region of
interest (ROI) Balance between accuracy and
speed Always ask Are the boundaries located
correctly?
ROI
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I BOUNDARY CONDITIONS

• Hydrodynamic BCs
• Tidal boundary elevations
• flows
• Streamline !
• River flows
• Wind

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I BOUNDARY CONDITIONS

• Hydrodynamic BCs
• Tidal elevation common
• Relatively easily obtained tide tables, surveys

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I BOUNDARY CONDITIONS

• Hydrodynamic BCs
• Tidal flows less common
• Difficult to obtain surveys, output from other
  model
• May be more accurate

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I BOUNDARY CONDITIONS

• Streamline BCs
• No friction/shear with water outside domain
• Sea boundary
• Free condtion

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I BOUNDARY CONDITIONS

• River flows
• Velocity V(m/s)
• Time varying
• Contributes volume and energy
• Note Q(m3/s) V x Area of flow

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II INITIAL CONDITIONS
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II INITIAL CONDITIONS

• Initial conditions?
• Models are run from time zero to simulation
  time.
• Initial conditions are the values specified
  for all model
• variables at time zero.
• Realistic examples
• Domain water level set to low water
• Water currents set to zero
• Dissolved Oxygen set to 3mg/l

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II INITIAL CONDITIONS
Example Orthophosphate initial conditions
Based on salinity Specified at each
model grid
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III HYDRODYNAMICS
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III HYDRODYNAMICS

• Examples of BCs
• Tides constant repeating tide e.g. spring tide
• repeating spring/neap 14 day cycle
• realistic tides from tidal
  constituents
• River annual/monthly averages
• flow recorded data
• Wind annual/monthly averages
• recorded data
• spatially constant (usually)

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III HYDRODYNAMICS

• Actual examples of BCs
• Dublin Bay
• Cork Harbour

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III HYDRODYNAMICS
Dublin Bay
MODEL BOUNDARIES _____ Streamline _____ Tidal
elevations _____ Flow
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III HYDRODYNAMICS

• Cork Harbour

River flow
River flow
River flow
Tidal elevation
Streamline
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IV POLLUTANT LOADS
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IV POLLUTANT LOADS

• Each pollutant source separately defined
• Number of pollutants
• Location
• Hydraulic flow rate Q(m3/s)
• Pollutant concentration C(mg/l)
• (C1, C2, C3Cn)
• Note Mass loading M(mg/s) Q x C

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IV POLLUTANT LOADS

• Load specification
• Constant
• Time varying
• Model inputs depends on available data
• Ideally time varying flow and load!

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IV POLLUTANT LOADS
Temporal variations
Recorded flow
Recorded nitrate nitrogen
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IV POLLUTANT LOADS

• Example Cork Harbour
• 20 discharges
• Sewage outfalls
• Industrial outfalls
• Rivers
• Marine source

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IV POLLUTANT LOADS

• Outfall loads
• Often based on IPC
• Sometimes limited measured data
• Inputs rarely completely known
• Ensure scenarios are conservative

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IV POLLUTANT LOADS

• Location of outfalls

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IV POLLUTANT LOADS

• River inputs
• Significant nutrient loads
• Fewer sources than outfall
• Highly variable seasonally
• Flow varies
• Nutrients vary
• Usually limited data

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IV POLLUTANT LOADS

• Location of rivers/diffuse

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IV POLLUTANT LOADS

• Cork Harbour

Flow data not always readily available
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IV POLLUTANT LOADS

• Cork Harbour

Nutrient data not always readily available
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IV POLLUTANT LOADS

• River Lee inputs

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V WATER QUALITY
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V WATER QUALITY
COMPLEX
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V WATER QUALITY

• Components
• Hydrodynamic boundary/initial conditions
• Pollutant boundary/initial conditions
• Phytoplankton cycle
• Reaction rates/constants

?
?
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V WATER QUALITY

• Simplified phytoplankton cycle

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V WATER QUALITY

• Phytoplankton Kinetics
• Species present
• Temperature
• Light
• Nutrients
• Transport processes

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V WATER QUALITY
Numerical Representation
CP phytoplankton population (mg Chl-a L-1)
GPI growth rate constant (d-1)
DPI death plus respiration rate constant
VS4 settling velocity (m d-1) D
total water depth
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V WATER QUALITY
Numerical Representation
GRTS Arrhenius equation GRNU Monod
equation Limiting Nutrient
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Nitrogen Modelling
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V WATER QUALITY

• Reaction rates/constants
• Determine the rate at which a process progresses
• Must specify rate values for various processes
• Example Phytoplankton growth

CP phytoplankton population (mg Chl-a L-1)
GPI growth rate constant (d-1)
DPI death plus respiration rate constant
VS4 settling velocity (m d-1)
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V WATER QUALITY

• Reaction rates/constants determination?
• Sources
• Experiments
• Texts e.g. Surface Water quality by Chapra
• Specialist literature e.g. Rates, Constants
  Kinetics, USEPA
• Journal papers
• Modelling manuals

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V WATER QUALITY

• Experiments
• Effects of light on growth rate

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V WATER QUALITY

• Reaction rates/constants
• Mostly from literature
• Often used to tune Water Quality model
• Can vary from report to report
• Sensitivity analysis to assess impacts
• Select conservative values !

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Reaction rates constants
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V WATER QUALITY
References
(1) Brown, L.C. and Barnwell, T.O., (1987), The
Enhanced Stream Water Quality Models QUAL2E and
QUAL2E-UNCAS Documentation and User Manual, U.S.
Environmental Protection Agency. (2) Bowie,
G.L., Mills, W.B., Porcella, D.B., Campbell,
C.L., Pagenkopf, J.R., Rupp, G.L., Johnson, K.M.,
Chan, P.W.H. and Gherini, S.A., (1985), Rates,
Constants, and Kinetic Formulations in Surface
Water Quality Modelling, Environmental Research
Laboratory, Office of Research and Development,
U.S. Environmental Protection Agency. (3)
Chapra, S.C., (1997), Surface Water Quality
Modelling, McGraw-Hill International Editions,
Singapore. (4) Falconer, R.A. and Liu, S.Q.,
(1988), Modelling Solute Transport Using the
QUICK Scheme, Journal of Environmental
Engineering, 114 (1) 3-20.
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V WATER QUALITY
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VI HEAVY METALS
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VI HEAVY METALS

• Toxic metals
• lead, mercury, zinc etc
• Linked to sediments and salinity

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VI HEAVY METALS

• Sediment dependency
• Heavy metal mass fractioned in water
• Some behave as solute
• Some attach to fine, cohesive sediments
• Thus must first model sediment transport
• Very difficult !
• Sediment transport module
• Validated/calibrated etc.

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VI HEAVY METALS

• Salinity dependency
• Heavy metal mass fractioned in water
• P (Particulate bound mass)/(dissolved mass)
• Partitioning coefficient (P)
• Function of water chemistry salinity
• Model salinity
• Determine P

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VI HEAVY METALS

• Salinity dependency
• Very poorly understood research stage
• P varies from estuary to estuary
• Models
• P constant
• P exponential decrease with salinity
• Locally derived function from measurements

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VI HEAVY METALS
Salinity dependency - exponential
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VI HEAVY METALS
Salinity dependency locally derived
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VI HEAVY METALS
Model predictions
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VI HEAVY METALS

• Stages
• Hydrodynamics
• Salinity
• Sediments
• Metal inputs
• Metal transport
• Very difficult !
• Considerable measurements required

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VII PATHOGENS
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VII PATHOGENS

• Stages
• Hydrodynamics
• Pathogen inputs
• Pathogen (solute) transport
• Die-off
• Modelling difficulties
• Loading
• Die-off rate

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VII PATHOGENS

• Die-off rate
• T90, time for 90 pathogens to die
• Typically around 15hours
• T90 range X X hours
• T90 is not a constant
• Temperature
• Sunlight
• Turbidity
• Sediments

?
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• WATER QUALITY MODELLING PROCESS

CONTENT- I Boundary conditions II Initial
conditions III Hydrodynamics IV Discharges/pollu
tant loads V Water quality VI Heavy
metals VII Pathogens
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Estuarine Coastal Modelling Course
MODULE IX WATER QUALITY MODELLING PROCESS
Michael Hartnett, Environmental Change Institute,
NUI Galway