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Monitoring tidal dynamics by a high resolution current mapping system
47th International Liege Colloquium on Ocean Dynamics, 4-8 May 2015
Alexei SentchevLab. Océanologie & Géosciences, Université du Littoral, FRANCE
Max YaremchukNRL, Stennis Space Center, MS, USA
Motivation
OpenHydro turbine testing at Paimpol site in Brittany
SeaFlow (MST Ltd) first run 30 May 2003 SeaGen project (2006 to date) >2800MWh
more than 10 yr history of tidal flow energy conversion
Energy generation from tidal flow is growing activity in Europe (potential in France 3-4 GW, in UK 5-6 GW)
Site screening for Renewable Energy projects & reso urce characterization at tidal sites is an early stage p roject activity
- Site & Resource Assessment - Environmental Monitoring- Assessment of the performance of tidal turbines - Turbulence investigations- Impact of tidal devices on the flow properties, sediment transport, ecology, …
Underway velocity measurements by towed or vessel mounted ADCP is very efficient tool
High resolution current mapping of energetic tidal flow requiresan accurate space-time velocity interpolation
SeaGensite
Surface velocity south of Dover 13:20 – 14:20 on 5/07/13Current reversal at 13:40 ( HW + 3.5 hours)
The tidal flow change during surveying period often distorts the results of current mapping
Current mapping system
Low-cost, compact, towed ADCP profiling system “Koursk” with• measurements taken at 0.5m accuracy (onboard GPS) • GPS errors filtering • projection of recorded velocities on a regular space-time grid by Optimal Interpolation (OI) of velocity profiles using statistics from regional model runs
• assessment of the interpolation errors
(1) Broadband RDI ADCP (1.2 or 0.6 MHz)(2,3) High precision GPS and data acquisition system(4) onboard ADV and mini-CTD
Objectives:
� Assess the performance of a low-cost towed ADCP current mapping system
� Perform Optimal Interpolation of underway velocity meas in time and space
� Retrieve the evolution of tidal currents in limited size tidal basins
� Estimate the quality of current field’s reconstruction
OI: state & sample vectors, covariances, …
[ ]T***1 N
uu K=u Sample velocity vector
Model (background) velocity vector
Model and Obs covariances
mu
u State velocity vector
R B,
iH Operator projecting u onto i-thobservation point
Boulogne Harbour (BLH)
Boulogne
Dover
Study sites
Seaward area of Dover Harbour
Boulogne harbour : surface circulation and transport patternfrom arian image at HW-1 hour
Velocity surveys in BLH
[R] = σσσσ2ii Id
Covariance matrix of observations
Along-track velocity sample
Time
3D field
Time
3D field
Time
3D field
E n s e m b l e m e m b e r s
Space-time background covariance matrix (4-dimensional)
[B]
Local circulation model MARS-3D ∆x = 110 m; ∆t = 30 s; 20 σ-levels
nested into 1 km resolution regional model (Jouanneau et al., Oc.Dyn. 2013 )
Model domain & bathymetry
Current fields in the BLH: observations & model results
March 27, 12:00-14:00 GMT(HW at 13:30)
March 29, 07:00-08:25 GMT(LW at 07:25)
June 28, 17:40-19:20 GMT(HW at 17:20)
OI interpolation of velocity measurements
Velocity measurements interpolated for the mid time of the Survey 1 (March 27, 13:00, HW-0.5 hour) Crigging
e = 0.36e = 0.23
Interpolation errors in measurement locations
Crigging:OI:
OI interpolation in space and time
Low discrepancy with obs: e < 0.08
Discrepancy between the model and interpolated velocities (color shading)
is higher 0.04 < e < 0.30
Velocity observations (red) at the interpolation time (10 min window)
12:20 (HW-1.1h) 14:00 (HW+0.5h) 15:30 (HW+2h)
Model snapshots on March 27, around HW
Bottom mounted vs towed ADCP measurements
ADCPObs point
V component
U component
Towed ADCP survey
0.090.890.97
0.170.870.81
eCv
Cu
Space-time OI vs ADCP
Model vs ADCP
Interpolation errors
a posteriori error covariance matrix
Error reduction:1/2))()(( Bdiagtx, /2σγ =
for the 1st survey (composite map)
Conclusions
Advantages of experimental platform for high resolution underway velocity measurements in coastal tidal basins:
- low-cost, compact, easy to deploy and to do survey - not intrusive (suitable for sampling in the surface layer)- current profiling at low depth (estuaries, harbors, …)- in deep areas - profiling until bottom (or great depth)
OI technique is employed to retrieve the evolution of tidal currents from the survey data
Space-time correlations of the flow field are derived from numerical simulations of the tidal currents by MARS 3D model
Application of the current mapping system demonstrated significant reduction (30-60%) of the model-data misfit due to OI of the velocity observations in space and time
Interesting possibilities exist for reconstruction the 3D evolution of the velocity field, sediment dynamics, turbulence in the tidal flow at Renewable Energy sites, …