New acoustic system for continuous measurement of river discharge and water temperature

SEWater Science and Engineering, 2010, 3(1): 47-55doi:10.382/.issn.1674-2370.2010.01.005htp://ww.waterjournal.cne-mail: wse2008@vip.163.comNew acoustic system for continuous measurement of riverdischarge and water temperatureKiyosi KAWANISI*', Arata KANEKO', Shinya NIGO?, Mohammad SOLTANIASL',Mahmoud F. MAGHREBII. Graduate School of Engineering, Hiroshima University, 1-4-1, Kagamiyama,Higashi-Hiroshima 739-8527, Japan2. Ministry of Land, Infrastructure, Transport and Tourism, 2-4-36 Kitaku Sikadachou,Okayama 700-0914, Japan3. Civil Engineering Department, Ferdowsi University of Mashhad, P. O. Box 91775- 11, Mashhad, IranAbstract: In many cases, river discharge is indirectly estimated from water level or streamflowvelocity near the water surface. However, these methods have limited applicability. In this study, aninnovative system, the fluvial acoustic tomography system (FATS), was used for continuousdischarge measurement. Transducers with a central frequency of 30 kHz were installed diagonallyacross the river. The system's significant functions include accurate measurement of the travel timeof the transmission signal using a GPS clock and the attainment of a high signal-to-noise ratio as aresult of modulation of the signal by the 10th order M-sequence. In addition, FATS is small andlightweight, and its power consumption is low. Operating in unsteady streamflow, FATSsuccessfully measured the cross-sectional average velocity. The agreement between FATS andacoustic Doppler current profilers (ADCPs) on water discharge was satisfactory. Moreover, thetemporal variation of the cross-sectional average temperature deduced from the sound speed ofFATS was similar to that measured by a temperature sensor near the bank.Key words: streamflow; fluovial acoustic tomography; cross-sectional average velocity; unsteadyflow; water temperature1 IntroductionRiver discharge is an important hydrological factor in river and coastalplanning/management, control of water resources, and environmental conservation. Therefore,establishing the method and technology for streamflow measurement is a crucial issue.However, it is very difficult to measure cross-sectional average velocity in unsteady flows orduring extreme hydrologic events, such as flooding.For continuous measurement of water discharge, a few different pieces of equipment areavailable, e.g., acoustic velocity meters (AVMs) and horizontal acoustic Doppler current中国煤化工This work was supported by the Construction Technology ResearF of the Ministryof Land, Infrastructure, Transport and Tourism of Japan (NoMYHC N M H G.19-1212-005,21-1212-009).*Corresponding author (e-mail: kiyosi @ hiroshima -u.acjp)Received Sep.17, 2009; Accepted Dec.2, 2009profilers (H-ADCPs) (Ruhl and DeRose 2004; Wang and Huang 2005). The main drawback ofpreviously presented methods is that the number of velocity sample points in the cross-sectionof a stream is often insufficient for estimating cross-sectional average velocity. H-ADCPs canmeasure a horizontal profile of velocity over a range with sufficiently strong acousticalbackscatter. However, H-ADCPs do not provide any information for vertical velocity profiles.Moreover, the horizontal profile range of H-ADCPs decreases with increasing suspendedsediment concentration (SSC). In addition, H-ADCPs do not work well in estuaries because ofthe sound inflection.Although several methods have been introduced to estimate velocity distribution (Chiuand Hsu 2006; Maghrebi and Ball 2006), the results are disputable in complex flow fields suchas stratified tidal flows or unsteady flows. Thus, innovative methods and/or equipment forcontinuous measurement of river discharge are needed.In this study, the fluvial acoustic tomography system (FATS) was developed and utilizedo measure outflow rates from an estuary weir. FATS has advantages over competingtechniques: namely, the accurate measurement of the travel time of the transmission signalusing a GPS clock, and the attainment of a high signal-to-noise ratio (SNR) of signals due tomodulation by the 10th order M-sequence. As a result, FATS works well even during floodevents in which SSC and acoustic noise are very high (Kawanisi et al. 2010b). FATS alsoworks well in estuaries with saltwater intrusion (Kawanisi et al. 2009; Kawanisi et al. 2010a).2 Measurement principles and error analysisThe basic principle of FATS is similar to what is used in an AVM. In other words, thecross-sectional average velocity is calculated using the travel time method (Sloat and Gain1995). The authors have tentatively called FATS a next-generation AVM in a previous paper(Kawanisi et al. 2008). An old-fashioned type of AVM measures average velocity along atransverse line. Therefore, the AVM requires different strategies, the index velocity methodand the velocity profile method, for computing discharge. FATS is able to estimateCrOss- sectional average velocity using ray paths that cover the section, unlike an old-fashionedtype of AVM.The travel time along the ith reciprocal ray path r; between a pair of transducers in theflowing medium is formulated aslsJrac士u.n(i=1, 2,--,N)(1)where +/- represents the positive/negative direction from one transducer to another, c is thesound speed, ds is the increment of arc length along the ray path, u is the flow velocity,n is the unit vector along the ray path, and N is the number of ray paths. The path integralsare taken along rays. We assume that the two-way path geometry, is reciprocal and r; ≈I.The two-way travel time difference may be expressed as中国煤化工TYHCNMHG.48Kiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No. 1, 47-552u.n .2u.n2L,u,MI=5-5=1n2--(u.n)ads≈」,c"ds≈(2)where L, is the length of the ith ray path, and w, and G are the range average watervelocity and the sound speed along the ith ray path, respectively:元=一「. u. nds(3)云J,cds(4)c is calculated fromL+→+(+)-1.2=(imds≈(5)The cross-sectional average water velocity in the flow direction, V，is defined asiu1_1号v=-:os θ= cos0v2n=cosθ 2N个Lst;(6)where u is the component of the mean water velocity along the ray paths, and θ is theangle between the ray path and streamline.In order to estimate the cross-sectional average velocity v，it is preferable that the raypaths cover the cross- section as much as possible. The ray paths of FATS probably cover thecross-section in a freshwater environment. However, a salt wedge under the transducer causesray paths to be reflected, so those ray paths are not able to penetrate bottom layers (Kawanisiet al.2009; Kawanisi et al.2010a).In order to accurately identify the arrival time of a traveling sound mixed with noises, thetransmission signal is phase- modulated by applying the pseudo- random sequence (Simon et al.1985; Zheng et al. 1998). Fig. 1 shows the transmission signal modulated with the 3rd orderM- sequence as a typical example. The carrier signal is phase-modulated by taking a productwith the M- sequence. By transmitting this modulated signal, the SNR is increased significantly.In this study, the higher order (10th order) M-sequence is applied to get higher SNRs,increasing by 2" - 1, where n denotes the order of the M-sequence.3rd order M-sequenceCarrierwwwwwwwwwwwww.Modulated signalWWWWWWWwWMA。 AAAAAA中国煤化工Fig. 1 Phase- modulation of crrier signal byMYHCNMHGKiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No. 1, 47-5519The transmission signal with a phase-modulation is expressed ass()= M (t)A(t)sin(or)= A( )sin[ ot +y()](7)where M(t) is the 10th order M-sequence, and A(t) and y(t) are the amplitude andphase functions, respectively. The angular frequency 0 was set to 30 kHz in this study. Thereceived signal S(t) was processed using the cross-correlation between the received signaland the 10th order M-sequence. This process serves to identify the precise arrival time.Based on the total differential of Eqs. (5) and (6), the relative errors of c and w areestimated with Eqs. (8) and (9), respectively:8,_ δL_ δt(8)Cw(9)u，LOt;The average travel time error δT and the error of travel time difference δ(Ot) arenegligible when the pair of transducers is synchronized precisely with the GPS clock. As aresult, the relative error of the average sound speed δC/C， and the relative error of theaverage flow velocity 8u,/u, are equated with the relative error of the ray's length δL:/L，The term 8(Ot,)/Ot; on the right-hand side of Eq. (9) can be neglected if 8(Ot,) isinsignificant.3 Experimental site and methodologyAn experiment with FATS was carried out from June 8 to 25, 2009 on the Hyakken River.Fig.2 shows an aerial photograph of the experimental site. The array of sluice gates at themouth intermittently opens to discharge fresh water at low tides, so that saline water does notenter the river. The direct distance between S1 and S2 was 418.5 m. The cross- section alongthe ray path is shown in Fig. 3.A couple of broadband transducers were installed diagonally across the channel, asshown in Fig. 2. The central frequency of the transducers was 30 kHz. The transducers weremounted at a height of 0.45 m above the bottom, as shown in Fig. 3, where Z is the elevationrelative to the mean sea level. The acoustic pulses of FATS were simultaneously transmittedfrom the two omni directional transducers triggered every minute by a GPS clock. The anglebetween the ray path and the stream direction, θ，was assumed to be 35° when water wasdischarged through the gates.An upward-facing ADCP was located on the bottom in front of the sluice gates (Fig. 2).The ADCP was operated at 2 MHz and the bin length was set to 0.1 m. The profile interval,which describes how often the instrument collected the current profile data, and the averageinterval, which specifies how long the instrument would be actively collecting data withineach profile interval, were both 600s.中国煤化工Water level and temperature were measured eve:temperatureMHCNMHG50Kiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No. 1, 47-55sensor attached to the frame of a downstream transducer. The pressure-temperature sensor waslocated at a height of 0.4 m above the bottom.人zProcessinSiuicegates-二FlIoADCPTransducers2350 mFig. 2 Aerial view of study area and experimental setupZ(m)一号-0.5-1.0" SIS29--15Fig.3 Bathymetry along sound transmission line and locations of two transducers4 Results and discussion4.1 Cross-correlation between modulated signal and M-sequenceThe typical wave forms of cross-correlation and the amplitude function A(t) of thesignal received by the upstream transducer are shown in Fig. 4. The cross-correlation betweenthe modulated signal and the 10th order M-sequence has a sharp peak. Thus, we can determinethe accurate arrival time of sound from the sharp peak of the cross-correlation wave. As aresult, the cross- sectional average velocity was deduced from the arrival time of thetransmitted signal.一Signal received by upstream transducer三600[ ---- rsorelation电400272777928128328Travel time (ms)Fig. 4 Typical time plots of amplitude function and cross- correlation wave form4.2 Temporal variation of outflow rate中国煤化工The river discharge is calculated by FATS as followMYHCNMHGKiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No. 1, 47-5551Q=A(H )v sinθ= A(H )u tanθ(10)where A is the cross-sectional area in which sound paths travel, H is the water level, andθ is the angle between the ray path and stream direction. The outflow rate from the gates Qois deduced from the following equation:dVQ,=Q -It(11)where V is the water volume between the gates and the river cross-section along the soundtransmission line (ray path) of FATS.The temporal variation of the water level and outflow rate are illustrated in Fig. 5, whereH is the water level relative to the mean sea level. The water level varies due to intermittentdischarge, and the variation of the water level reflects the duration of gate opening each day(Fig. 5(a)). Unfortunately, the measurement fell into abeyance from June 12 to 16 owing tosome troubles with the downstream system (Fig. 5(b)). Specifically, we attached a newBluetooth device in order to improve the system on June 12, and its operation damaged aDC-DC converter. The DC-DC converter was fixed on June 16. As a result, the systemrecovered and the improvement enabled us to access the system without wires.-0.2是-04Mm-0.6WWM06-08 06-10 06-12 06-14 06-16 06-18 06-20 06-22 06-24 06- 26(a) Water level10006-08 06-1006-12 06-14 06-16 06-18 06-20 06-2206-24 06- 26Date(b) Outflow rateFig. 5 Time plots of water level and outflow rate for June, 2009The water discharge estimated by FATS indicates a considerable magnitude even when thegate is closed. This is probably caused by the change in flow direction. In order to estimate theflow direction, a four-station system with two crossing transmission lines is required.4.3 Relationship between ADCP and FATS dataIn this subsection, the outflow rates deduced from the ADCP and FATS are compared inorder to evaluate the performance of FATS. The relationship between the FATS velocity (V )and depth-averaged velocity of the ADCP (VADCP) is中国煤化工oW velocity(VADCP <0.1 m/s)，the correlation between v andMYHCNM H Gn! Fig. 6(a).Kiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No. 1, 47-55Conversely, there is high correlation in the higher velocity range, though there is large scatterwhen VADCP >0.8 m/s. Fig. 6(b) presents the data when VACP 20.1 m/s ; the solid linedenotes a regression line, v = 0.083 4 + 0.237VADCP . The standard deviations of the residualsfrom the regression line for VADCP and v are 0.096 m/s and 0.026 m/s, respectively. Thetwo instruments were installed in different places. Moreover, the definitions of average aredifferent: VADCP is depth-averaged and下is the cross-sectional average velocity. Thus, thelow correlation between VADCP and v does not indicate poor performance of FATS.0.:J)3 t).2 t0.1-0.2 0 0.2 0.4 0.6 0.8 1.0 .00.2 0.4 0.6 0.8 1.0Vaoer (m/s)VAncer (m/s)(a) Total velocity data(b) VAxcr20.1 m/sFig.6 Relationship between cross -sectional average velocity of FATS and depth-averaged velocity of ADCPFrom the regression line, we can see that the decay of the FATS velocity is moremoderate than that of the ADCP. The decay of the FATS velocity after the closing of the gatesis delayed because the cross-section of FATS is somewhere other than the gates.The relationship between outflow rates of the ADCP and FATS is shown in Fig. 7. TheADCP discharge (QADcp) is estimated as a product of the depth-averaged velocity and thestream cross-sectional area of the gates. At a low flow rate, the FATS discharge (Q) has apoor correlation with QADCP, like the poor correlation between the average velocities shownin Fig. 6(a). As shown in Fig. 7(b), however, correlation between both outflow rates is highwith larger flow rates, both outflow rates are comparable, and the regression equation for theoutflow rates is Q, =41.4+ 0.5QADCP . The standard deviations of residuals from theregression lines for QADCP and Q。are 19.3 m'/s and 11.8 m'/s, respectively.150 [150p00一要100自“自5050-5050 100 150 2001001500Ancr(m'/s)Qxcr(m/)(a) Total discharge data(b) Axcr >10m'/sFig. 7 Relationship between outflow rates of FATS discharge and ADCP discharge4.4 Temporal variation of water temperature中国煤化工The sound speedc is estimated by Medwin'sTYH.CNMHGfunctionofKiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No.1, 47-5553temperature T(°C)， salinity S， and depth D (m) (Medwin 1975) in the ranges of0≤T≤35°C，0≤S≤45, and 0≤D≤1000m:c=1 449.2 + 4.6T - 0.055T2 +2.9x10 4T3 +(1.34 - 0.01T )(S -35)+0.016D(12)Since there is no saltwater intrusion at the experimental site, the water temperature can beestimated from the sound speed.Fig.8 shows temporal variation of water temperatures obtained from the temperaturesensor at S2 and FATS. Both temperatures indicate diurnal variation, except on June 10, whenthe diurnal variation was not determined because of the rain. The temperature at S2 measuredby the temperature sensor is higher than the cross-sectional average temperature estimated byFATS because the location of the temperature sensor is shallow.30. Fromtemperaure sensor at S2一 From FATS242206-08 06-1006-1206-14 06-16 06-18 .06-2006-2206-24 06-26DateFig. 8 Temporal variation of water temperature for June, 20095 ConclusionsIn order to conduct continuous measurement of river discharge, an FATS that utilizes a .GPS clock and M-sequence modulation was developed and applied to shallow unsteady flow.Using a pair of transducers installed diagonally across the river, FATS was able to measure thecross-sectional average velocity. The cross-sectional average velocity of the river stream wasestimated from the travel time of the transmission signal obtained along the ray paths, whichcover the channel cross section. The sufficiently high signal-to-noise ratio was obtained owingto the 10th order M-sequence modulation of the transmission signal.In addition to measurement of flow rate, the cross-sectional average water temperaturewas deduced from the mean acoustic speed measured by FATS. This means that all of theseenvironmental factors are observational targets of FATS.AcknowledgementsWe would like to thank Dr. Noriaki Gohda of Hiroshima University/Aqua EnvironmentalMonitoring Limited Liability Partnership (AEM-LLP) for strong support in field work anddata processing.ReferencesChiu, C. L., and Hsu, s. M.2006. Probabilistic approach to mod中国煤化工in fluid flows.Journal of Hydrology, 316(1 -4), 28-42. [doi: 10.101 6/jhydn:MHCNMHG54Kiyosi KAWANISI et al. Water Science and Engineering, Mar. 2010, Vol. 3, No. 1, 47-55Kawanisi, K., Kaneko, A.. Razaz, M., and Abe, T.2008. Measurement of cross sectional average velocity in ashallow tidal river with a next-generation acoustic velocity meter. Proceedings of l6th AHR-APDCongress and 3rd Symposium of IAHR- ISHS, Vol. V: Hydraulic Structures for Water Projects, 1973-1977.Beijing: Tsinghua University.Kawanisi, K, Watanabe, S, Kaneko, A., and Abe, T. 2009. River acoustic tomography for continuousmeasurement of water discharge. 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