4th International Conference on Estuaries and Coasts
8-11 October 2012, Water Resources University, Vietnam

CONTINUOUS MEASUREMENT OF FLUSHING DISCHARGEFROM RESERVOIRSMASAO IKEDA1), KIYOSHI KAWANISI2), MAHDI RAZAZ2),
KAZUHIKO ISHIKAWA2) and NORIYUKI MIYAMOTO1)
1) Keisoku Research Consultant Co Ltd.
665-1, Fukuda1-chome, Higashi-ku, Hiroshima, 732-0029, Japan
2) Civil & Environmental Engineering, Hiroshima University
1-4-1 Kagamiyama, Higashi-Hiroshima, 739-8527, JapanAbstract
A Fluvial Acoustic Tomography System (FATS) was developed to enable continuous observation ofthe flow rates of rivers with tidal area, shallow and wide rivers, and flows during floods, which had beendifficult with conventional flow rate observation techniques. The ultrasonic waves used in FATS, unlike
those in the conventional ultrasonic flow meter AVM (acoustic velocity meter), propagate so as to covera horizontal cross section, making it possible to obtain the cross-sectional average flow velocity.
The observation site was located in 120 m wide strait reach in the Ghono River, Miyoshi City,
western Japan. In order to regrow new sphagnum, the water from the Haizuka and Haji reservoirs is
instantly discharged once a year. The flow rates estimated by the FATS which have 25-kHz braodband
transducers were compared with those measured by RC and ADCP. Comparing FATS discharge with
RC and ADCP discharge, FATS can mesure flow rate accurately under conditions difficult to measure
flushing discharge because of high -turbidity.
Keywords: acoustic tomography, flow rate, water temperature, flushing discharge, shallow and wideriver flow

1. INTRODUCTION
The first purpose of river planning is flood control. River channel plans are established so that flows
up to the flow rate (high water level) which is discharged into the river channels of the surrounding river
systems due to precipitation are conducted safely downstream. From the viewpoints of proper use ofrivers, maintenance of river functions, and environmental protection, securing a normal flow rate is also
one purpose of river planning. In these planning processes, flow rates are calculated from runoff analysis
based on actual past records of precipitation and water demand. However, reliable river flow rate data
are essential for verification of the appropriateness of the analytical model and parameters and forimprovement of the analytical accuracy.
Accordingly, establishment of a technology which enables continuous, accurate measurement of river
flow rates is an urgent issue. In flow rate measurements with the conventional techniques of float
measurement, ADCP, and propeller type flow velocity meters, only temporary flow rate measurements
are possible. Moreover, with the conventional flow rate observation methods, it was difficult to observe
flow rates under conditions such as (1) flows with salt wedge intrusion, as in rivers with tidal area, (2)shallow and wide rivers, and (3) during floods, which cause high turbidity. Therefore, the authorsdeveloped a new Fluvial Acoustic Tomography System (hereinafter, FATS) to overcome theseweaknesses, and realized automatic continuous flow rate measurement.
In previous work, Kawanisi (one of the authors) et al. succeeded in continuous observation of flow
rates in tidal areas and similar environments using FATS, and confirmed that practical river flow ratemeasurement is possible with FATS. In the present reserch, FATS which have 25-kHz braodbandtransducers was applied to streamflow measurement at the Ghono River (Ghono-kawa), which is agravel-bed river that flows through Miyoshi City, Hiroshima Prefecture, Japan, and the applicability ofFATS to flushing discharge was verified by comparison with RC and Acoustic Doppler Current Profiler(ADCP).

2. MATERIALS AND METHODS
2.1 PRINCIPLE OF FATS
The fundamental principle of FATS is similar to that of the acoustic velocity meter (AVM), in that theaverage cross-sectional flow velocity is calculated based on differences in propagation time. With AVM,
the average flow velocity of an acoustic ray traversing straight across the river is measured. Therefore,
observation to compensate for AVM is applied (for example, by IVM). Unlike AVM, the ultrasonicwaves in FATS are reflected between horizontal cross sections and do not require complex processing. In
cases where the flow of a river is unidirectional, the flow rate can be calculated simply by measuring one
part of the flow velocity component of the cross section. When there is a flow in the direction of themeasurement line between a set of 2 transducers positioned in the water, the flow velocity between thetransducers will be different in the ultrasonic waves propagating in the flow direction and in the
counter-flow direction, resulting in a difference in the propagation times obtained at the respective
transducers. With FATS, the average flow velocity between the transducers is obtained using thisdifference in propagation time.
Fig.-2.1 shows the geometrical relationship between a pair of FATS transducers installed in a river. The
transducers are set up at position A on the upstream side and position B on the downstream side.
Reciprocal ultrasonic waves are transmitted simultaneously by the two transducers at 1 minute intervals,
and the propagation times of the ultrasonic waves are measured.

upstreamATransducer

downstreamB
Transducer

Fig.-2.1 Installation of transducers in FATS

Assuming the mean speed of sound obtained and the mean flow velocity along an acoustic ray pathof length L are Cm and um, respectively, the propagation time t1 of an ultrasonic wave propagating fromthe upstream side to the downstream side and the propagation time t2 of an ultrasonic wave propagatingfrom the downstream side to the upstream side can be expressed as follows.
(1)(2)
From (1) and (2), the mean speed of sound Cm and the mean flow velocity um along the ray pathcan be obtained from equations (3) and (4).
(3)
∆(4)

/2
，∆
Where,
From Eq. (3) and (4), the relative errors of Cm and um are given by Eq. (5) and Eq. (6),respectively.
(5)
‐2

∆
∆

(6)

Because the error of mean propagation time δtm can be ignored if simultaneous signal transmission
and receiving can be performed in both directions, the second term on the right side can be eliminated(Kawanisi (one of the authors) et al., 2008). Accordingly, the relative error of the mean speed of soundis equal to the relative error of the distance of the ray path, and time accuracy is not important. On theother hand, because time error, which is the denominator of the third term on the right side of Eq. (5),is small, securing time accuracy becomes extremely important in the relative error of the flow velocity.
Normally, the distance accuracy necessary in flow velocity measurements can be sufficiently secured.
The speed of sound is a function of water temperature, salinity, and pressure. According to Medwin(1975), the speed of sound in water C can be expressed as shown by Eq. (7).
1449.2 4.6
0.055
2.9 101.34 0.01
35
0.016

(7)

where, T is water temperature (°C), S is salinity, and D is water depth (m).
The streamflow is calculated as follows.
(8)
where, A(H) is the area of the horizontal cross section through which the acoustic ray passes and is afunction of the water level H.To measure the mean cross-sectional flow velocity with greater accuracy, ultrasonic waves should be
propagated so as to cover the horizontal cross section as completely as possible. A stratification does notexist under shallow waters. Therefore, under this condition, the ultrasonic waves of FATS arepropagated so as to approximately cover the horizontal cross section.
In order to determine accurately the arrival time of ultrasonic waves, which contain noise, theM-sequence is used in the transmitted signal, and the transmitted wave is subjected to phase modulationby the M-sequence, which is a pseudo random noise signal. By sending a transmission wave with phasemodulation smaller than the frequency band width, SNR is increased in accordance with the relationship(2n – 1), where n is the order of the M-sequence. In this paper, an M-sequence with a order of 10 wasused. The M-sequence is also used in the received signal in the same manner was with the transmitted
signal, and the mutual correlation between the M-sequence used in the received signal and thetransmitted signal is obtained. By accurately measuring the propagation time using a GPS clock, theprocessing described above can be used to designate the arrival time. As a result, it becomes possible tomeasure the mean cross-sectional flow velocity and the river flow rate over an extended period of time,even in rivers affected by tides.

2.2 STUDY AREA
The river where field observation was carried out was the Ghono River, which flows through Miyoshi
City, Hiroshima Prefecture, Japan. Fig.–2.2 shows the observation points on the Ghono River. Theobservation points were set up 2.7 km downstream from the confluence of the Basen River and theSaijyo River, which are the two main tributaries of the Ghono River. The basin of the Ghono River has
an area of 3900 km2. The average annual flow rate at a gauging station 1.1 km downstream from theobservation points is 73 m3/s. The Ghono River is a habitat for ayu fish (Japanese trout). To stimulate
reproduction of the Sphagnum moss that grows on rocks in the river bed and is a source of food for ayu,
flashing discharge is performed once each year from the Haizuka Dam, which is located on a branch ofthe Basen River 26 km upstream from the observation points. The river is 115 m in width at theobservation points where the transducers were set up to measure the flow rate. The gradient of the river
bed between the transducers was 0.11%, and the Manning’s Roughness Coefficient was 0.03. The water
depth at low water level was approximately 0.6 m.
The streamflow measurements performed on March 22-23, 2012 were conducted during flushingdischarge from the Haizuka and Haji dams, which are located upstream from the observation points, inorder to verify the applicability of FATS to periods of flooding. Also preliminary observation wasconducted in October, 2011. In Fig.-2.2, T1 and T4 denote broadband transducers of FATS. In order tomeasure the water depth, which is necessary for obtaining the cross-sectional area of the horizontal
cross section, water level gauges were set at the same positions as the transducers on the two riverbanks.

Fig.–2.2 Location of observation points

3. RESULTS AND DISCUSSION
3.1 RESULTS OF CONTINUOUS MEASUREMENT OF FLOW RATE BY FATS
Fig.-3.1 and 3.2 show time plots of the flow rate and water temperature during the observation periodin October 2010. In Fig.-3.1, the flow rate measured by FATS is a 30 min moving average. The flow rateby the Rating Curve Methods was obtained from the telemeter water level at 1 hour intervals. In thehorizontal cross section between the two transducers, the minimum water depth during this observation
period was approximately 30 cm.
The results demonstrated that FATS functions without problems even under this type of shallow
water depth condition. In Fig.-3.2, the blue, green, and red lines show the water temperature as obtainedby FATS, the diver water level meter, and the telemeter water temperature, respectively. Comparing theresults of the water temperature measurements by FATS and the diver water level meter, it can beunderstood that continuous, high accuracy measurement of water temperature is also possible using
FATS.
Fig.-3.3 shows the results of flow rate measurements during the flashing discharge conducted on
March 22 and 23, 2012. FATS also functioned without problems under the high turbidity condition
during flashing discharge. The blue line shows the flow rate as obtained by FATS, and the red and green
lines show the flow rates calculated from the Rating Curve Methods and ADCP, respectively. Fig.-3.4
shows a comparison of the results by FATS and the Rating Curve Methods and ADCP. The red points
show the comparison of the flow rates given by FATS and the Rating Curve Methods, and the green
points show the comparison between FATS and ADCP. The respective correlation coefficients are0.9901 and 0.9782, demonstrating that FATS also functions without problems under high turbidity
condition during flushing discharge.

40Q(/s)

3530252015
10510/17FATS
RCADCP
10/19

10/21

10/23

10/25

10/27

10/29

10/31

date

Fig.-3.1 Time plots of flow rate
21T( )

191715131197510/17

FATS
Water level loggers
Telemeter water temprature
10/19

10/21

10/23

10/25

10/27

10/29

10/31

date

Fig.-3.2 Time plots of water temperature

Q(/s)

190
FATS
RCADCP

140
90
40
15:00

18:00

21:00

0:00

3:00

March,22March,23Fig.-3.3 Time plots of flow rate during flushing discharge

200RCADCP

150
100500050

100

150

200

Fig.-3.4 Comparison of results of flow rate measurements

6:00

T( )

8.0

FATS
T2
T3

7.6
7.2
15:00

18:00

21:00

0:00

March,22

3:00

6:00

March,23

Fig.-3.5 Time plots of water temperature during the flushing discharge

Fig.-3.5 shows time plots of the water temperature during the flushing discharge on March 22-23,
2012. The black, blue, and red lines show the water temperature as obtained by FATS, temperature
sensors near T2, and T3, respectively. For water temperature from FATS was the cross-sectional
average water temperature, the temperature sensors were near the river banks. This may cause thewater temperature variation’s time-lag between FATS and thetemperature sensors.

4. CONCLUSIONS
The fundamental principle of the Fluvial Acoustic Tomography System (FATS) is based onconventional AVM. However, with FATS, the average cross-sectional flow velocity of rivers can beobtained from ultrasonic waves with a ray path covering the horizontal cross-sectional area.
FATS worked without problems during the observation periods of preliminary observation, in spite of
the fact that the water depth at low water stage was approximately 0.6 m and the river width wasapproximately 115 m. Good agreement was also obtained between FATS and ADCP under highturbidity conditions during flashing discharge (relative error between FATS and ADCP was within±10%). Thus, these results demonstrated that FATS enables continuous observation of river flow rates
at shallow and wide rivers and under high turbidity conditions during flash discharge.
In previous research, Kawanisi (one of the present authors) et al. succeeded in long-term
continuous observation of a river with salt wedge intrusion at a diversion channel on the Ohta River inwestern Japan, and also succeeded in long-term continuous observation of fluctuations in salinity inaddition to the river flow rate and water temperature using FATS. These results demonstrate that theadvanced acoustic tomography system FATS is a revolutionary device which enables continuous
observation of river flow rates of (1) flows with salt wedge intrusion such as tidal rivers, (2) shallow andwide rivers, and (3) flows under conditions of high turbidity during flooding, all of which had beendifficult with the conventional Rating Curve Methods, ADCP, and other existing methods.

5. ACKNOWLEDGEMENTS
The authors wish to express their sincere appreciation to Mr. Kazumasa Saitoh, Construction Officerof the Miyoshi Office of River and National Highway, Chugoku Regional Development Bureau,
Ministry of Land, Infrastructure, Transport and Tourism (MLIT) and to all those concerned at Aratani
Civil Engineering Consultants Co., Ltd. for their generous cooperation in the field observations in thepreparation of this paper.

6. REFERENCES
Kawanisi K., Razaz M., Kaneko A., Watanabe S. (2010). Long-term measurement of stream flow andsalinity in a tidal river by use of the fluvial acoustic tomography system. In: Journal of Hydrology,
vol. 380(1-2), pp. 74-81.
Kawanisi K., Razaz M., Watanabe S. (2010). An innovative methodology/technology for streamflow
observation. In: JRiver Flow 2010, vol. 2, pp. 1741-1