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, …