Temporal variability of discharge rating curve investigated using shallow-water acoustic tomography
Kiyosi KAWANISI (Hiroshima University) and Mohamad Basel AL SAWAF (Hiroshima University)
Results
Figure 2(a) shows the time history of water level at the Ozekiyama gauging station from November 2015 toIntroduction
Acquiring continuous streamflow estimates is paramount in hydrological studies, extreme event analyses, andwater resources management. The streamflow is commonly estimated using stage-discharge relations
because it hard to do direct continuous flow measurements for years. The relation is called rating curve (RC).

May 2018. The rating curves were established from streamflow time series with high temporal density using
FAT system. The average of water level at both acoustic stations Hm were used as the water stage in rating
curves. For falling limbs of 16 hydrological events, which are shown by the red marks in figure 2(a), therating curves were analyzed. The stage–discharge and stage–water-slope relations did not show significant

It is believed that the RCs are liable to change depending on channel conditions such as cross-sectional

hysteresis loops at the survey site because of the steep bed slope. Figuers 2(b) and (c) show temporalshape, vegetation, and so on (Guerrero et al. 2012). However, a hydrological dataset sparse in time isvariations in the relative difference and correlation coefficient of between successive RCs, respectively. It isinsufficient to shed light on the temporal variability in the stage–discharge relation. This study examines thefound that the rapid changes of the rating curve occurs owing to large flood events.

minutes). The streamflow time series are obtained using a novel acoustic method: fluvial acoustic
tomography (FAT) developed by Hiroshima University. In this study, 16 hydrological events in a gravel-bed
river for 2016-2018 were analyzed, and the variable rating curves were assessed.

Overview of the fluvial acoustic tomography (FAT) system
The shallow-water acoustic tomography technique has been under development for a number of years at

Figures 3(a) and (b) present the stage–discharge and stage–water-slope relations, respectively. The slopeof water surface Sw is evaluated from the water levels at the both acoustic stations (T1, T2). The water slopeslinearly increase with increasing stage by H m ≈ 147 masl (masl: m above mean sea level). The increases of Sw
are insignificant while H m > ~ 147 masl. Because the water slope is milder than the bed slope of 0.11%, thesurvey site is located in backwater; the backwater is caused by the downstream sand banks (Figure 1).
HG (masl)

temporal variability in RC from streamflow time series with uniquely high temporal density (every 10(a)Hiroshima University (Kawanisi et al. 2018; Kawanisi et al. 2012; Razaz et al. 2013). A new travel-timetomography technique/system makes it possible to accurately measure the sound arrival times in shallow
can gauge the instantaneous section-average-velocity using multi-rays that cover the entire flow crossγm (%)

waters. Although the range averaged velocity is measured using the “time-of-travel method” , FAT system(b)

The received acoustic data at both riversides are transferred to the processing unit using the wireless LANbridge. The GPS receivers provide two accurate timing signals. One timing pulse (1 Hz) is used to ensure

γcor (%)

section unlike conventional acoustic velocity meters.

(c)that both systems run synchronously. Another signal of 10 MHz is used as the base clock of FAT system forFigure 2. (a) Time series of the water level at the Ozekiyama gauging station (masl: m above mean sea level). Red
marks denotes the analyzed periods. (b) Temporal variation in the relative difference of between successive RCs.
(c) Temporal variation in correlation coefficient between successive RCs.

high-precision transmitting and receiving signal processing.

Survay siteA 115-m wide straight reach of the Gono River, MiyoshiThe details of the study area are shown in (Kawanisi etal. 2012). The riverbed was consist of gravel

Hm (masl)

City, Japan was selected as the survey site (Figure 2).

(b)
Hm (masl)(a)

N(d50=27mm). The bed slope around the observationsite was approximately 0.11%, and the Manningroughness estimated from the stream velocity and thewater surface slope was approximately 0.03. As shown

Figure 1. Study area: installation points ofin figure 2, a couple of transducers (T1 and T2) weretransducers (30 kHz) and location of Ozekiyamadiagonally installed in the straight reach.
gauging station.

Q (m3/s)

Sw (%)

Figure 3. (a) Stage–discharge relations; black line denotes fitting curve; (b) Stage–water-slope relations.

Conclusions

A RC established from the sparse gauging in a hydrological year can result in large errors in thestreamﬂow estimation.

The novel FAT system allowed the unique high-frequency (every 10 minutes) streamﬂow gauging.

References

Guerrero et al., 2012, J Hydrol 446–447 doi:10.1016/j.jhydrol.2012.04.031
Kawanisi et al., 2018, J Hydrol Eng 23(2) doi:10.1061/(ASCE)HE.1943-5584.0001604
Kawanisi et al., 2012, Water Resour Res 48(5) doi:10.1029/2012WR012064
Razaz, et al., 2013, Water Resour Res 49(8) doi:10.1002/wrcr.20375

The uncertainty of RC reached around ±40% in the mountainous river. The streamﬂow records withuniquely high temporal density shed light on the irregularity in stage‒discharge relations. It shows thatthe power function is too simple as a real rating curve.