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					Environmental Impacts on
Physical Processes:
Turbulence, Stratification,
Sediment Transport


Dr. Phil Hosegood, Dr. Daniel Conley
University of Plymouth
   DREEM & WHISSP
 [Dynamic Response to Energy
     Extraction and Mixing;
Wave Hub Impacts on Seabed &
     Shoreline Processes]
• Measurements of physical impacts resulting from
the development of Wave Hub.

• Necessary information for impact assessment (IA).

• Additional practical MRE significance in resource
assessment, array planning and device deployment.
PRIMaRE physical oceanography
capability: Water column
• Observational physical oceanographic measurements;
• 3D tracer distribution (i.e. temperature, salinity, chlorophyll, particle
size/distribution), current velocity, turbulence;
• Identify dynamic processes & predict response to MRE;
• Model initialisation, validation and improvement.




                      RDI 600 kHz acoustic Doppler
                                                     MSS microstructure profiler
MiniBat (Towed CTD)          current profiler
Physical impacts of energy
extraction: Significance
 • Extracted energy is otherwise available for turbulent mixing
 • The benthic ecosystem and sediment regimes depend on the
 details of the turbulence regime - difficult to model




                                      wind
                                                  Low
        Stratified                                productivity

                                                 High
                         wave                    productivity
Mixed




 • Changes to turbulent regime disrupt the balance between mixing
 and stratification that currently exists
 • Effects are site specific - planning must account for local effects.
Wave Hub: Dynamic Response to
Energy Extraction and Mixing (DREEM)
• Front responds quickly to changes in wind and wave field;
• Models currently unable to reproduce features;
• Ongoing field studies throughout summer 2010 to diagnose
dominant processes and effects of energy extraction.




                            6 days



                           Sea surface
                           temperature
Wave Hub frontal surveys:
Minibat, VM-ADCP, HF radar

• Mapping of 3-dimensional frontal structure;
• Input for numerical models;
• Impacts of energy extraction to be evident as changes in
thermohaline structure and regional flow.




  MiniBat (Towed CTD)
Wave Hub in-situ measurements:
Microstructure profiles, moored ADCP, drifters

• Turbulence microstructure tidal station reveal strong asymmetry
between flood & ebb cycle;
• Short-term increase in wind speed and wave conditions enhances
near surface turbulence – stratified → well mixed water column.
                                                                    epsilon [log(W/kg)]
                                                 0.0
                                                                                      Wind/wave mixing                          -6.50
                                                                                                                                -6.65
                                                 6.3                                                                            -6.80
                                                                                                                                -6.95
                                                                                                                                -7.10
                                                12.5                                                                            -7.25
                                                                                                                                -7.40
                                                18.8                                                                            -7.55
                                 press [dbar]




                                                                                                                                -7.70
                                                                                                                                -7.85
                                                25.0                                                                            -8.00
                                                                                                                                -8.15
                                                                                                                                -8.30
                                                31.3                                                                            -8.45

                                                37.5
                                                            Tidal mixing                                                        -8.60
                                                                                                                                -8.75
                                                                                                                                -8.90
                                                                                                                                -9.05
                                                43.8             Ebb tide                                                       -9.20
                                                                                                                                -9.35

                                                50.0
                                                                                                 Flood tide                     -9.50
                                                   6:02   8:25      10:47            13:10             15:32             17:55 [log(W/kg)]
   MSS microstructure profiler                                      Time [h] (09/07/2009)
                                                                        E:\projects\wavehub\wavehub_july_09\mss_wavehub_09july09\fast_eps\
Turbulence in tidal streams:
Resource assessment, sediment
transport and fatigue
• Turbulence generated at the sea bed occurs in coherent
structures;
• ‘Bursting’ is a depth dependent process, requiring full water
column measurements;
• Accounts for up to 96% sediment transport & strong shear



                                 Vertical current shear
  Hairpin vortices
Turbulence in tidal streams:
Resource assessment, sediment
transport and fatigue
• Departure from laminar flow essential to understand;
• Low speed bursts enhance shears across span of rotors → fatigue;
• Measurements discussed for Big Russell, Guernsey (2010) to
characterise turbulent flow and validate model predictions.


                                                     Mode 12
                                                      ADCP




                                Low speed burst

                                                         ADCP
Benthic Measurements
                         Measures
                         Currents,
                         Turbulence,
                         Bed shear stress,
                         Bed roughness
                         Suspended sediment
                         Bedload sediment
                         Bedforms and migration
                         Significance
                         Change in benthic habitat
                         Burial or exposure
                         Change of grain size




    UoP Benthic Tripod
                Shoreline Change
              DGPS
                            Monthly surveys of complete
                            beach profile at 4 beaches
                            (Godrevy, Portreath,
                            Porthtowan and Perranporth)
                            since April 2007. Continuous
                            coverage at 2 from Argus.
                            Necessary baseline for noisy
                            process.
Argus video
            In-Situ Measurements

                               •Provides wave
                               spectra (direction &
                               frequency), current
                               profiles.
                               •Provides calibration
Wave buoy                      and validation for
                               wave and circulation
                               models as well as HF
                               Radar.
                               •Point Data.
                               •Average data
                               archived.
                               •Project designed to
 ADCP              TRAWL       provide at least 1
                   RESISTANT   years baseline data
                   MOUNT
                   HF Radar Installation
 Deployed at 2 sites with each site
 requiring Tx and Rx array and
 electronics package. Installation is   Mean expected data coverage.
 non-permanent.                         Directional estimates exist for
Transmit                                intersections of colors. Wave coverage
Array                                   half of current coverage. Provides map
(4 element                              of independent estimates at 2 km
10 x 5 m)                               resolution.




 Receive Array (16 element, 150 m)
Rip current hazards
- WHISSP & RNLI
•68% of all incidents
recorded by the Royal
National Lifeboat
Institution (RNLI) on UK
beaches are due to rip
currents
•90% of those incidents
occurred on beaches
with identifiable rip
channel morphologies
Rip current hazards
- WHISSP & RNLI
•Assimilation of numerical model and video remote sensing for
near real-time Decision Support System
•Aid RNLI risk assessment process and resource allocation
•Model pollutant dispersal plumes within the surf zone
HYDRODYNAMIC PLANNING
 TOOL FOR TCT ARRAYS
• A GWR (Great Western Research) project which
   represents a partnership between International
   Power Marine Developments Ltd, Tidal
   Generation Ltd and the Universities of Plymouth,
   Bristol and Exeter*
• Project Goal: Create a decision-making tool for
  design of arrays of tidal current turbines (TCT) which
  optimize energy extraction and minimize
  hydrodynamic impacts. Modified an existing
  hydrodynamic model
* BST by Thomas Roc
                                                              Y



           TOOL DEVELOPMENT                                          X

Approach: Adapt the 3-D
Regional Ocean Modelling
System (ROMS) to TCT
planning application. Provides
local and regional capabilities.



                                         Cutout of velocity field around
                                           single TCT. Demonstrates
                                           need for 3-D representation.



 Wake structure from a linear array of
  TCTs (color represents velocity)
OPPORTUNITIES


•State of the art measurements and interpretation
in hostile ocean environments such as surf-zone,
tidal stream and shelf boundary layers.

•Resource assessment

•Planning

•Deployment

				
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posted:5/9/2013
language:English
pages:18