EFFECT OF BOTTOM LAYER MOTION ON ADCP MEASUREMENT USE OF DGPS. June 2004

Transcription

1 EFFECT OF BOTTOM LAYER MOTION ON ADCP MEASUREMENT USE OF DGPS June 2004

2 10 years of ADCP flow measurement in CNR 1- ADCP gives very reliable data in case of a motionless river bottom layer. 2- Combination ADCP/DGPS : a display of shiptrack and evidence of compass error. 3- Deterioration of data while using DGPS in the case of a motionless river bottom layer. 4- Flow measurement in the case of a moving bottom layer : - combining ADCP and DGPS, - ADCP only in BTM with correction

3 10 years of ADCP flow measurement in CNR 1- ADCP gives very reliable data in case of a motionless river bottom layer. 2- Combination ADCP/DGPS : a display of shiptrack and evidence of compass error. 3- Deterioration of data while using DGPS in the case of a motionless river bottom layer. 4- Flow measurement in the case of a moving bottom layer : - combining ADCP and DGPS, - ADCP only in BTM with correction

4 1- ADCP gives very reliable data in the case of a motionless river bottom layer. ADCP s main characteristics for measurement accuracy are given by the manufacturer : measure of depth, measure of velocity, internal compass. User main requirement is accuracy on flow rate measurement..which is not given.. CNR has carried out its own comparison tests thanks to well known method : propeller gauging

5 Case of motionless river bottom layer 21 gauging operations carried out from 1995 to 2004 in 14 different places by 3 different teams. Flow rates ranging from 0 to m 3 /s and velocities from 0.1 m/s to 2.5 m/s.

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12 Motionless river bottom layer : gauging operation with ADCP / GPS / propeller Gauging station ADCP GPS antennas ADCP 600 Propeller current meter

13 Motionless river bottom layer : results Flow rate graph comparaison ADCP et Moulinet écart en % (ADCP-Moulinet)/ADCP % 10% % 6% 800 4% ADCP en m3/s débit en m3/s médiane 5% -5% écart (adcp-moulinet)/adcp 2% 0% -2% -4% -6% -8% -10% - -12% débit Moulinet en m3/s In 80% of cases, the difference between ADCP and propeller flow rate is within +/- 4%

14 Motionless river bottom comparaison layer ADCP : results et Moulinet Velocity graph écart en % (ADCP-Moulinet)/ADCP % 10% % 6% % ADCP en m/s % 0% -2% débit en m3/s médiane 5% -5% écart (adcp-moulinet)/adcp -4% -6% -8% -10% % vitesse Moulinet en m/s In 80% of cases, the difference between ADCP and propeller velocity is within +/- 4%

15 1- ADCP gives very reliable data in the case of a motionless a bottom layer. KEY POINTS 21 gauging operations carried out from 1995 to 2004 in 14 different places by 3 different teams, Flow rates ranging from 0 m 3 /s to m 3 /s and velocities from 0.1 m/s to 2.5 m/s, In 80% of cases, the difference between ADCP and propeller flow rate is within +/- 4%, 20% of remaining cases can be explained by unstable hydraulic conditions during gauging operation.

16 10 years of ADCP flow measurement in CNR 1- ADCP gives very reliable data in case of a motionless river bottom layer. 2- Combination ADCP/DGPS : a display of shiptrack and evidence of compass error. 3- Deterioration of data while using DGPS in the case of a motionless river bottom layer. 4- Flow measurement in the case of a moving bottom layer : - combining ADCP and DGPS, - ADCP only in BTM with correction

17 Combination ADCP/DGPS : a display of shiptrack To be reliable, DGPS has to be operated in LRK mode (centimetric accuracy) Two beacons are required to guarantee the centimetric accuracy : - one on the moving ship, - one fixed on the bank.

18 Fixed station on the bank - GPS antenna - UHF antenna for DGPS correction transmission On board station - GPS antenna fixed on ADCP - UHF antenna

19 Flow direction Rhône à Ternay le 23 mai 2004 BB 1200kHz GPS shiptrack different to BTM shiptrack

20 Bottom-tracking Mode (BTM) Referenced on magnetic north Nord magnétique V bn L V bn-1 Est magnétique Ship velocity calculated by Doppler effect assuming a motionless river bottom : V ship/bottom = - V bottom/ship L 1 = ( Vb + V ) t n b Δ n 1 2

21 GPS mode Referenced on geographic north Nord géographique Y n Y n-1 L X n-1 Ship coordinates given by GPS X n Est géographique V b n = L Δt = ( Yn Yn 1)² + ( X n X n 1)² Δt

22 Combination ADCP/DGPS : evidence of compass error The gap between DGPS and BTM shiptrack is due to both magnetic deviation (angle between magnetic and geographic reference) and internal compass accuracy. By making a round trip while gauging, the magnetic deviation cancels itself. The residual error on shiptrack is resulting from the compass error only. information : magnetic deviation in Lyon is 3.35 Source :

23 Rhône à Ternay le 23 mai 2004 BB 1200kHz Motionless bottom Flow direction Length BTM [m] Length GPS [m] Equivalent length : both GPS and BTM mode are working well

24 Rhône à Ternay le 23 mai 2004 BB 1200kHz Motionless bottom Flow direction BMG-GMG mag 8.2 [m] (1) BMG-GMG dir [ ] (1) Length BTM [m] (2) Length GPS [m] (1)/(2) < 4%

25 Gauging departure point MG GPS Gauging arrival point (real = GPS) MG BTM (1) BMG GMG Gauging arrival point according to BTM On a round trip, with the same departure and arrival point, MG GPS << MG BTM : MG BTM close to BMG - GMG

26 Rhône à Ternay le 23 mai 2004 BB 1200kHz Motionless bottom Flow direction BMG-GMG mag 8.2 [m] (1) BMG-GMG dir [ ] Réf BTM Distance MG 7.15 [m] Distance MG BTM close to (1) (compared to L) 8.2 m ~ 7.15 m

27 Rhône à Ternay le 23 mai 2004 BB 1200kHz Flow direction BMG-GMG mag 33.2 [m] (1) BMG-GMG dir [ ] Length BTM [m] (2) Length GPS [m] Pb Length GPS Sans conséquence car 29.9 m ~ 33.2 m Réf BTM (1)/(2) < 5% (1) Distance MG [m]

28 Rhône à Ternay le 23 mai 2004 BB 1200kHz BMG-GMG mag 1.8 [m] (1) BMG-GMG dir [ ] Flow direction Length BTM [m] (2) Length GPS [m] (1)/(2) < 1% Réf BTM Distance MG 0.68 [m]

29 2- Combination ADCP/DGPS : a display of shiptrack and evidence of compass error. KEY POINTS Compass calibration is not very good according to the shiptrack, it must be taken into account in shiptrack given by BTM mode. Compass accuracy 2 (given by the manufacturer) : resulting error on MG = 3.5% of total length, According to CNR experience : - if MG < 5% of total length, river bottom is considered motionless and the BTM flow rate measure will be preferred, - if MG > 5% of total length, river bottom motion may have substantial effect on the flow rate measure and should be taken into account. So why not use GPS.

30 10 years of ADCP flow measurement in CNR 1- ADCP gives very reliable data in case of a motionless river bottom layer. 2- Combination ADCP/DGPS : a display of shiptrack and evidence of compass error. 3- Deterioration of data while using DGPS in the case of a motionless river bottom layer. 4- Flow measurement in the case of a moving bottom layer : - combining ADCP and DGPS, - ADCP only in BTM with correction

31 Deterioration of data while using DGPS in the case of a motionless river bottom layer. 10 campaigns of measure ADCP/GPS have been run, with stable hydraulic conditions, in 4 different places. The flow rates measured range from 200 m 3 /s et m 3 /s (which correspond to velocities ranging from 0.3 m/s to 1.5 m/s).

32 Results Flow rate graph (BTM GPS)/BTM within +/- 6% Comparaison des débits ADCPavec suivi BTM et GPS - FOND STABLE % % % % débit GPS en m3/s % 0% -2% Fonds fixes 600 Fonds fixes 1200 médiane +5% -5% écart (BTM-GPS)/BTM -4% -6% -8% - -10% débit BTM en m3/s

33 Results Velocity graph (BTM GPS)/BTM within +/- 6% Comparaison des vitesses ADCPavec suivi BTM et GPS - FOND STABLE % % % % vitesse GPS en m/s % 0% -2% Fonds fixes 600 Fonds fixes 1200 médiane +5% -5% écart (BTM-GPS)/BTM -4% -6% -8% vitesse BTM en m/s -10%

34 Flow rate calculation in DGPS mode N géo Flow direction Projected section s1 α Section S For one cell : β n GPS V water/ref.gps Plan view α Q GPS = V = V = V eau / réf. GPS eau / réf. GPS eau / réf. GPS. n GPS * s1 * S * S *cos( β α) with : = water course (combination GPS and compass), β = boat course (origin GPS) + 90 angles in reference to geographic north

35 3- Deterioration of data while using DGPS in the case of a motionless river bottom layer. KEY POINTS The results leads to the following remarks : Difference between BTM and GPS flow rate ranges within +/- 6%, with a greater dispersion for the BB 1200 khz (-2% to +4%), Despite the differences are not too large, BTM flow rate will be preferred to GPS flow rate.

36 10 years of ADCP flow measurement in CNR 1- ADCP gives very reliable data in case of a motionless river bottom layer. 2- Combination ADCP/DGPS : a display of shiptrack and evidence of compass error. 3- Deterioration of data while using DGPS in the case of a motionless river bottom layer. 4- Flow measurement in the case of a moving bottom layer : - combining ADCP and DGPS, - ADCP only in BTM with correction

37 Flow rate measurement in the case of a moving bottom layer : either ADCP combined with DGPS, taking into account the error related to the internal compass accuracy, or ADCP in BTM, making allowance for the effect of bottom layer motion. Treated on 2 examples on the Rhône river : in November 2002 and December 2003.

38 DGPS option - 2 references : the river bottom (moving) and DGPS (fixed) Utilisée : Pour déterminer la section en BTM Flow Pour déterminer la vitesse des paticules par rapport au fond V ship/bottom (measure BTM) V ship/reference GPS (calcul GPS) Utilisée pour calculer la vitesse des particules en absolu (GPS) V particles/bottom (calcul BTM) V particles/ship (measure ADCP) Utilisée pour calculer le débit en BTM Ideal case if the compass is well adjusted, otherwise V bottom/reference GPS (résultat calculs BTM et GPS) V particles/reference GPS (résultat calculs BTM et GPS) Utilisée pour calculer le débit en GPS

39 BTM option The shiptrack is moving upstream Flow and bottom layer motion direction GPS path (close to real path) L4 L3 Opposite of bottom layer velocity MG L4 L2 L3 BTM path (affected by the moving bottom) L1 L1 L2 Length = L1 + L2 + L3+ L4 MadeGood = MG = Longueur(Arrivée Départ) Plan view Vitesse_fond *Δt(Arrivé Départ) Note : MG can also be affected by the compass error ; According to tests, the compass error does not exceed more than 5% of the total length (same departure and arrival point)

40 BTM option - Water velocity and projected section Bottom layer motion direction Flow direction Plan view Bottom layer velocity reference to GPS BTM section n BTM GPS section Projected section onto perpendicular plan to V water V water/réf.fond (BTM) V bottom/réf.gps V water/réf.gps

41 Example 1 : BB 1200 khz November round trip Ship displacement (according to BTM positioning mode) : MG = 55 m in 560 s Length : 660 m (L) CNR criteria 5% x L = 33 m < MG = 55 m the river bottom is moving Le Rhône D = MadeGood (55 m in 560 s) Bottom velocity : 10 cm/s (55 m / 560 s) D Section : m 2 Correction of flow rate : Vf x S = 470 m 3 /s, +5.5% of total Q Plan view

42 Flow rate comparison : 1200 and 600 khz BTM and GPS mode Q 1200kHz BTM : m 3 /s Q 1200kHz BTM + Vf x S : m 3 /s Q 1200kHz GPS : m 3 /s Q 600kHz BTM : m 3 /s Q 600kHz GPS : m 3 /s ADCP BB 600kHz seems to be less sensitive to the moving bottom effect than ADCP 1200kHz (ratio MG/L < 5%) Correction ((MG/ Time) x S) allows us to get closer to the true value

43 Example 2 BB 1200 khz - December round trips Flow direction Ship displacement (according to BTM positioning mode) : MG = 346 m in s MG Length : m (L) CNR criteria 5% x L = 85 m < MG = 346 m the river bottom is moving Bottom velocity : 15 cm/s (346 m / s) Section : m 2 2 Correction of flow rate : Vf x S = 765 m 3 /s, +7.5% of total Q

44 Shiptrack comparison : 1200 and 600 khz BTM and GPS mode BB 600kHz 2 round trips Flow direction 2 MG X m in Y s (shorter than MG for ADCP 1200 khz)

45 Shiptrack comparison : 1200 and 600 khz BTM and GPS mode BB 600kHz single trip with 2 stops Flow direction Shiptrack BTM ~ Shiptrack GPS

46 Flow rate comparison : 1200 and 600 khz BTM and GPS mode Q 1200kHz BTM : m 3 /s Q 1200kHz BTM + Vf x S : m 3 /s Q 1200kHz GPS : m 3 /s Q 600kHz BTM : m 3 /s Q 600kHz GPS : m 3 /s ADCP BB 600kHz less sensitive to the moving bottom than ADCP 1200kHz (ratio MG/L < 5%) Correction ((MG/ Time) x S) allows to get closer to the true value

47 BB 1200kHz december 2003 Loss of bottom signal : comparison of BTM extrapolations with GPS

48 Loss of bottom signal : BAD with BTM

49 Loss of bottom signal : no BAD with GPS

50 Conclusion ACDP 600 khz and 1200 khz give very reliable data in case of a motionless river bottom layer. Estimate of the river bottom stability without GPS : MG/length < x % (x is specific to each ADCP example : BB 1200 khz 5% for CNR). Without DGPS, the additional flow rate resulting from the effect of bottom layer motion can be assessed by : ΔQ = (MG/time) x Surface ADCP BB 600kHz seems to be less sensitive to the moving bottom effect than ADCP 1200kHz (ratio MG/L < 5%). DGPS permits to evaluate the effect of a moving bottom on ADCP measurement.

51 EFFECT OF BOTTOM LAYER MOTION ON ADCP MEASURMENT USE OF DGPS June 2004