EstProc 13102004

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EstProc 13/10/2004

• Waves in estuaries
• Andrew Lane David Prandle Judith Wolf

with
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Introduction

• Objectives
• POLs contributions to EstProc
• 1-D model of tides and sediments
• 2-D cross-section model, incl. morphology
• 3-D whole-estuary model
• wave modelling
• extreme events
• Summary

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Processes
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User-friendly 1-D z model

• Suspended sediment concentrations (Lagrangian)
• Depth 20 m, current amplitude 0.5 m s-1, ws 10-4
  m s-1
• See also www.gotm.net, General Ocean Turbulence
  Model

Fractional height
Tidal cycles
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2-D y-z cross-sectional model

• Morphology evolution (Langrangian)
• Depth 10 m, tidal amplitude 3 m, ws 10-3 m s-1

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Liverpool Bay model

• 3-D hydrodynamic model 120-m resolution
• Estuary bathymetry from LiDAR and echo sounding
• Tidal elevations and currents good agreement
  with observations   

m s-1
Current speed and direction during mid-flood
10 km
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Sediment in the Mersey

• Random-walk particle-tracking module
• sediment resuspension currents at bed, wind-waves
• vertical diffusion, advection vertical current
  profile
• settling ws and water depth

Suspended sediment at high water
Suspended sediment at low water
older sediment gt2 dys younger sediment
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Sediment in the Mersey

• Random-walk particle-tracking module
• sediment resuspension currents at bed, wind-waves
• vertical diffusion, advection vertical current
  profile
• settling ws and water depth

Suspended sediment at low water
Sediment at the bed at low water
older sediment gt2 dys younger sediment older
sediment gt2 dys younger sediment
Net accumulation
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Sediment in the Mersey

• Sediment transport is greatest in the deep
  channels at estuary entrances, peak value is
  about 10 tonnes s-1

HW
LW
LW
HW
HW
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Sediment in the Dee

• Sediment model of the Dee

older sediment gt2 dys younger sediment
Suspended sediment in the Dee estuary at high
water
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Wave models

• SWAN model and
• TOMAWAC model compared

Wave height and direction in the outer Thames
Estuary, 26 March 2002
SWAN model
TOMAWAC model
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Wave models

• SWAN model and parametric wave model compared

Wave height for 15 m s-1 wind in Liverpool Bay
SWAN model correct
Parametric wave model
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Wave models

• Parametric vs SWAN (small differences in
  estuaries)
• Wave height, peak period of fetch-limited
  locally-generated wind-waves ? stress at sea bed
  from waves

Mersey Dee
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Application of wave model

• Wave height and peak period calculated from wind
  speed, water depth and fetch
• Dissipation in nearshore region wave heights
  limited by shallow water and wave steepness
• Wave orbital velocity at sea bed, Ubed
  from linear wave theory
• Wave stress on sea bed from Ubed, bed friction
  coefficient (sediment type), near-bed wave
  orbital diameter

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Wave/current stress algorithms

• Sediment resuspension depends on stress at the
  sea bed
• stress from tidal currents
• stress from waves
• Combine wave and current stress?
• algorithms by Soulsby (1997), Soulsby and Clarke
  (2004)

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Recap effects of waves

• Waves increase bed stress
• Sediment more readily resuspended
• Sensitive to
• wind speed
• fetch, state of tide, wind direction
• Increase in sediment concentrations especially
  during extreme events

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Extreme events
HW
LW
LW
HW
HW

• Sediment concentrations, differences for N and E
  wind no surges yet!

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Summary

• Technical advances
• application of wave models
• wave-current bed stress algorithms
• Whole-estuary assessments
• Mersey, Dee, Ribble
• Tools for investigating
• morphology evolution, Global Climate Change,
  coastal management

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Ways forward

• Assess models and results
• POL Science Programme
• Coastal Observatory cobs.pol.ac.uk
• Operational modelling of estuaries, sediments,
  ecology
• Forecasts of 2050 scenarios
• Development of Estuary Morphological Models
  (Defra project FD2107) www.pol.ac.uk/estmorph

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Suspended sediment
older sediment younger sediment
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Sediment on the bed
older sediment younger sediment
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Website www.estproc.net