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Waves in estuaries

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					                       EstProc 13/10/2004

• Waves in estuaries
• Andrew Lane
  David Prandle
  Judith Wolf
                           with
                                         Introduction
• Objectives
• POL’s 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
                                                                     Processes
type:       1-D vertical            2-D section                3-D
            settling                as 1-D, plus               as 1-D, plus
            resuspension             horizontal diffusion       full horizontal
            vertical diffusion       and advection              diffusion and
                                                                advection
resolves:
            sediment                erosion/accretion          (bathymetric
             concentrations                                     evolution)
                                    (bedload)
depends on:
            currents at bed         vertical current profile   wind/waves
            sediment type, ws
            diffusion coefficient
            water depth
                                   ‘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
                                      2-D ‘y-z’ cross-sectional model
Morphology evolution (Langrangian)
Depth 10 m, tidal amplitude 3 m, ws 10-3 m s-1




                                                      initial profile
            Height (m)




                                             10 yrs
                         2½ yrs
                                  5 yrs



                         Transverse distance from centre (km)
                            Liverpool Bay model
                           3-D hydrodynamic model
        m s-1
                             ~120-m resolution

                           Estuary bathymetry from LiDAR
                              and echo sounding

                           Tidal elevations and currents
                              good agreement with
                              observations



10 km    Current speed and direction
         during mid-flood
                                               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
                                                          Mersey Narrows, 1.5 km


   Suspended                                          Suspended
   sediment at                                        sediment at
   high water                                         low water

                                                            Inner estuary
                                                            basin
                              older sediment >2 dys
                              younger sediment
                                                      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
                                                                 Mersey Narrows, 1.5 km


          Sediment at                                        Suspended
          the bed at low                                     sediment at
          water                                              low water

                                                                   Inner estuary
                                                                   basin
                                     older sediment >2 dys
                                     younger sediment
                                     older sediment >2 dys
Net                                  younger sediment
accumulation
                                                           Sediment in the Mersey
                       Sediment transport is greatest in the deep channels at
                         estuary entrances, peak value is about 10 tonnes s-1

                        Mersey Narrows, sediment transport up- and downstream
upstream downstream




                      10
                       5
                       0HW             LW             HW           LW           HW


                      10
                       5
                       0
                       0.0           0.5             1.0          1.5           2.0
                                                Tidal cycles
                                               Sediment in the Dee
Sediment model of the Dee


                                     older sediment >2 dys
                                     younger sediment




         Suspended sediment in the
         Dee estuary at high water
                                                                        Wave models
                                                                       Run Code                    H260302
                                                                       Water level (m)                 -1.28
                                                                       Wind Speed (m)                  7.50
       SWAN model and                                                  Wind Direction (deg N)            28

       TOMAWAC model compared                                          Offshore Hs (m)                  0.9
                                                                       Offshore wave dir (deg N)         27
                                                                       Measured Hs (m)                 0.73
                                                                       SWAN Hs (m)                     0.52
                                                                       TOMAWAC Hs (m)                  0.53
Wave height and direction in the outer Thames Estuary, 26 March 2002


                                                                                                          Hs (m)




                                                    1 km


SWAN model                                       TOMAWAC model
                                                                        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
                                                         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
                            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
                     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)
                             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
                    Sediment concentrations, differences for N and E wind
                                                                 800
                                                                                                                                  Extreme events
                     Sediment concentration (mg l )
                     -1
                                                                          M2 only
                                                                 600
                                                                                                Mersey Narrows
                                                                 400


                                                                 200

                                                                                                Inner estuary basin
                                                                   0
                                                                   HW                LW                HW             LW          HW
                     Sediment concentration difference (mg l )
                     -1




                                                                 200
                                                                 100
                                                                   0
                                                                 -100
                                                                          M2 + N wind 25 ms-1 (difference)
                                                                 -200
                                                                              25 ms-1 represents a once-in-a-hundred-year storm
                                                                 200
Sediment
                                                                 100
concentrations,
                                                                   0
differences for N
                                                                 -100                             -1
and E wind – no                                                           M2 + E wind 25 ms (difference)
                                                                 -200
surges yet!                                                         0.0             0.5              1.0              1.5          2.0
                                                                                                 Tidal cycles
                                          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
                                        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
Suspended sediment




                     older sediment younger sediment
Sediment on the bed




                      older sediment younger sediment
Website: www.estproc.net

				
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