IMPROVEMENT OF DISCHARGE OBSERVATION ACCURACY IN ICE-COVERED RIVERS FOR RIVER MANAGEMENT

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1 in the Environment: Proceedings of the 16th IAHR International Symposium on Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research IMPROVEMENT OF DISCHARGE OBSERVATION ACCURACY IN ICE-COVERED RIVERS FOR RIVER MANAGEMENT Y. Suzuki 1, Y. Watanabe 1 and T. Kuwamura 1 ABSTRACT In Hokkaido, there are 13 rivers managed by the national government. These rivers are covered by ice during winter. On these rivers, there are 203 water level and discharge observation stations where river flow observation is conducted by the same method throughout the year even when the river is covered by ice. -covered rivers were surveyed using rotary portable current meters for different measurement durations and positions. It was found that 20-sec observation by 0.2D&0.8D method afforded the economic efficiency while maintaining sufficient measurement accuracy. A survey using an Acoustic Doppler Current Profiler (ADCP) was also conducted for a shorter duration of flow velocity measurement. Its accuracy was found to be lower than that of portable current meter because ADCP cannot measure at shallow points with high accuracy and, in this experiment, measurement was carried out at shallow points. It was confirmed, however, that prompt observation by ADCP is possible if a certain degree of error is permitted. INTRODUCTION The island of Hokkaido is at the northern end of Japan, an archipelago that stretches southwest to northeast. The island has four distinct seasons and its annual mean temperature is 6 to 10 C. It is covered with snow between December and March, and the temperature can drop to nearly 30 C in inland areas. In river flow observation, flow velocity is measured at 20 % and 80 % of the water depth at observation points of each observation section. Previous studies suggest that flow velocity must be measured for at least 20 seconds per point. Three observation durations (20, 60 and 120 sec) have been used, according to observation station, with the longer durations used at observation stations at the lower reaches. Measuring times were determined based on flow observation conducted in the summer of 1981, taking turbulence into consideration. Since then, the same measuring times have been used in both summer and winter. Because in Hokkaido most of the representative observation stations of their rivers are at the lower reaches, the duration of flow velocity measurement for these rivers is 120 sec. In winter, rivers in Hokkaido are covered by ice, the river flow is the lowest in the year, and flow fluctuations are relatively stable compared with summer. In this study, accurate flow observation, which involves detailed field survey of the flow velocity distribution in the 1 Civil Engineering Research Institute, Hiragishi , Toyohira-ku, Sapporo, Japan.

vertical direction, was conducted on an ice-covered river, and the most accurate and economical flow observation method was found by changing the measuring time and the position of observation points

FIELD SURVEY Outline of field survey observation stations Field survey observation stations were selected according to several conditions: the station is completely frozen over, there is water depth

2 vertical direction, was conducted on an ice-covered river, and the most accurate and economical flow observation method was found by changing the measuring time and the position of observation points in water depth. As an ADCP that is capable of measuring in shallow water has recently been introduced, its applicability to ice-covered rivers was considered. FIELD SURVEY Outline of field survey observation stations Field survey observation stations were selected according to several conditions: the station is completely frozen over, there is water depth sufficient for survey excluding the thickness of ice sheet and frazil ice, and the station is at a straight section of the river channel. Two observation stations Pompira Observation Station on the Teshio River and Moiwa Observation Station on the Tokachi River (Fig. 1) were selected accordingly. The Teshio and Tokachi rivers are among the largest rivers in Hokkaido, with lengths of 268 km and 156 km, respectively. Pompira Observation Station is 59 km and Moiwa Observation Station 21 km upstream from the river mouth. The river at both observatories is frozen over in January and February. When frozen over, they have an effective water depth of approximately 2 m excluding ice sheets and frazil ice. The river channels at the survey sections are roughly straight (Figs. 2 and 3). Fig. 1: Location of observation stations Teshio River Pompira Observation Station Tokachi River Moiwa Observation Station Fig. 2: Ground plan of Pompira Observation Station Fig. 3: Ground plan of Moiwa Observation Station Field survey method Surveys were conducted eight times in January and February Flow velocity measuring points on the cross-sectional plane numbered 12 (surface width: approx.

3 140 m) at Pompira and 10 (surface width: approx. 185 m) at Moiwa. Flow velocity was measured at depths that excluded ice sheet and frazil ice. For accurate measurement of flow velocity, both portable current meters and an ADCP were used. Observation using portable current meters was conducted at the effective water depths at intervals of 0.1 m and for the four measuring durations of 20, 40, 60 and 120 sec. Figures 4 and 5 show cross-sectional views of the observation stations. 標高 m Pompira Observation 誉平観測所 Station Elevation(m) 11 17/ 01/02 Flow velocity measuring point Frazil 10 標高 Elevation(m) m Moiwa Observation 茂岩観測所 Station 17/ 01/ Flow velocity measuring point m 6m 18m 30m 42m m 10m 30m 50m70m Frazil m Fig. 4: Pompira Observation Station Fig. 5: Moiwa Observation Station For observation, an ADCP of 2,400 khz was used. Observation was conducted for 120 sec concurrently with measurement by portable current meters. The interval of measurement was 0.1 m in the direction of depth. Observation equipment Ten portable current meters with propellers whose axis of rotation was parallel to the river flow were used. Also, to maintain validity of the regression formula for conversion of flow velocity, annual examination of current meters is required. The maximum error of the 10 current meters was 3 m/sec, according to the results of this examination. Two ADCPs were used for this observation, and the measurement error of the machines calculated from the accuracy formula presented by the manufacturer was approximately 2 m/sec. EVALUATION OF RIVER DISCHARGE USING PORTABLE CURRENT METERS To evaluate how differences in measurement duration and position affect the accuracy of river discharge measurement, it is necessary to determine a standard discharge by means of portable current meters. First, the standard flow velocity was found at flow velocity measuring points. The river flow rate calculated from the flow velocity was used as the standard river flow rate for comparison of the respective observation methods. Standard value of flow distribution at flow velocity measuring points It was presumed that the current profile in the vertical direction in an ice-covered river has a logarithmic profile in both the area above that of maximum flow velocity (i.e., the area influenced by ice sheet) and the area below that of maximum flow velocity (i.e., the area influenced by riverbed). The approximate logarithmic profile of flow velocity distribution in the water depth direction was calculated for the four observation durations (20, 40, 60, and 120 sec), and the correlation s and their standard deviation were evaluated to identify the case closest to the logarithmic profile as the standard flow velocity. Because the flow distribution is greatly influenced by ice sheet, frazil ice and

4 riverbed, the points for calculation of the standard value of flow distribution need to be deep enough. Five points at the Pompira Observation Station ( 6, 6, 18, 30 and 42 m) and four at the Moiwa Observation Station (10, 30, 50 and 70 m) were selected from those where an effective water depth of approximately 2 m could be secured. Figures 6 and 7 show representative cases of flow distribution in the depth direction at observed points. Table 1 shows the calculation results of mean values and standard deviation of correlation. Because this table shows that the value of the correlation was highest and standard deviation values were small in many cases of 120-second measurements, the case of accurate observation by 120-second measurements was set as the standard of flow velocity. The river flow rates calculated using the flow velocity obtained by accurate observation by 120-second measurements were also provided as the standard river flow rates, and the calculation results are shown in Table sec.accurate measurement(04/02/02) Flow velocity(m/s) sec.acuurate measurement(05/02/02) Flow velocity(m/s) Water depth(m) 20s 40s 60s 120s Water depth(m) s 40s 60s 120s Fig. 6: Flow distribution at the 18-m point Pompira Observation Station Fig. 7: Flow distribution at the 30-m point Moiwa Observation Station Table 1: Coefficients of correlation for measurements by portable current meters Table 2: Standard river flow rates Name of ststion Pompira Moiwa Division of Division of Measuring time of correlation influenced area 120sec 60sec 40sec 20sec Mean values of correlation Standard deviation of correlation Mean values of correlation Standard deviation of correlation Riverbed Riverbed Riverbed Riverbed Remarks Figures in bold boxes are cases with highest correlation. Figures in bold boxes are cases with smallest standard deviation Figures in bold boxes are cases with highest correlation. Figures in bold boxes are cases with smallest standard deviation Ponpira Observation Station(m 3 /s) 01/17/ /22/ /29/ /30/ /04/ /14/ /19/ /20/ Moiwa Observation Station(m 3 /s) 01/17/ /24/ /29/ /30/ /05/ /14/

5 Comparison of river flow rates by measurement time and position Comparison of river flow rates was made for the cases where the measuring time was 20, 40 and 60 seconds and 0.6D method (discharge is calculated by flow velocity measured at 60 % of effective water depth) or 0.2D&0.8D method (discharge is calculated by flow velocity measured at 20 and 80 % of effective water depth) measuring points. Difference (error) in river flow rates in different cases was found based on Table 2. Figures 8 and 9 show comparisons of mean values of error (absolute values) and standard deviation of mean values. These figures show that the mean value was smallest for 60 sec in the 0.2D&0.8D method and the standard deviation was smallest for 120 sec in the 0.2D&0.8D method at Pompira. At Moiwa Station, the mean value was smallest for 20 sec in the 0.2D&0.8D method and the standard deviation was the smallest for 60 sec in the 0.2D&0.8D method. These results show that the 0.2D&0.8D method with 60-sec measurement yields values closest to the standard river flow rate. Mean value(m3/s) M ean value ( absolute value ) Standard deviation 120sec 60sec 40sec 20sec 120sec 60sec 40sec 20sec Standard deviation Mean value(m3/s) M ean value ( absolute value ) Standard deviation 120sec 60sec 40sec 20sec 120sec 60sec 40sec 20sec Standard deviation 0.2D&0.8D method 0.6D method 0.2D&0.8D method 0.6D method Fig. 8: Mean value and standard deviation of measurement at Pompira Observation Station Fig. 9: Mean value and standard deviation of measurement at Moiwa Observation Station Instrument error of portable current meters The maximum instrumental error of the portable current meters used for field surveys was 3 m/sec. Because the value of 120-sec detailed observation, which was set as the standard flow rate, also included instrumental error, the error of flow rate by instrument error was calculated by multiplying the area of flow section by 3 m/sec. As a result, the minimum instrumental error of ±3.4 m 3 /sec was found for Pompira and ±7.1 m 3 /sec for Moiwa. Evaluation of flow rate observation methods Comparing the errors shown in Figs. 8 and 9 and the instrumental error, the errors originating in the shortness of measurement duration were within the range of instrumental error in all of the measurements by the 0.2D&0.8D method, while errors exceeding the instrumental error were evident in many measurements by the 0.6D method. Although the 60-sec measurement by the 0.2D&0.8D method was more accurate than those of the 0.6D method in terms of mean value and standard deviation, there was no major difference regardless of measuring duration in the 0.2D&0.8D method (Figs. 8 and 9), and the same level of accuracy was observed for every measuring duration. Since a shorter measurement duration is more economical, it is clear that the 0.2D&0.8D method with 20-sec measurement affords the best economical efficiency while maintaining sufficient accuracy.

6 ADCP FLOW VELOCITY MEASUREMENT ON AN ICE-COVERED RIVER Two sets of ADCP, both of which were designed for shallow water, were used. With ADCP, the flow velocity cannot be measured near the device itself and near the bed layer. Points where an effective water depth of approximately 2 m could be secured were therefore regarded as the subjects for evaluation, and five points at Pompira and four at Moiwa were chosen in the same way as in the examination of portable current meters. Comparison of flow distributions by difference in measuring duration While flow velocity measurements using ADCPs were conducted for 120 seconds in each observation, the flow distribution in the water depth direction in the cases of measuring for 20, 40 and 60 seconds were found from the 120-second measurement data to compare the difference in flow velocity by measuring time. Approximate logarithmic profile formulas of this flow distribution were calculated in each of the upper (area influenced by ice sheet) and lower (area influenced by riverbed) areas bordered by the section with the maximum flow velocity, in the same way as in the examination of portable current meters, and the s of correlation and their standard deviation were found. The results are shown in Table 3. It becomes clear that the case of 120-second measurement by ADCP has the highest value of correlation and smallest value of standard deviation. Table 3: Comparison table of s of correlation by ADCPs Name of ststion Pompira Moiwa Division of cofficient of correlation Mean values of correlation Standard cofficient deviation of correlation Mean values of correlation Standard cofficient deviation of correlation Division of Measuring time influenced area 120sec 60sec 40sec 20sec Riverbed Riverbed Riverbed Riverbed Remarks Figures in bold boxes are cases with highest correlation. Figures in bold boxes are cases with smallest standard deviation. Figures in bold boxes are cases with highest correlation. Figures in bold boxes are cases with smallest standard deviation. Comparison of mean flow velocities by difference in measuring time Mean flow velocities in 20-, 40-, 60- and 120-sec measurements using ADCP were compared. The difference in mean velocity by the difference in measuring time was found to be within the range of 0 to 5 m/s at both stations. In most cases it was approximately 2 m/s, which is the instrumental error of ADCP, except in the case of the 10-m point in the survey of January 30 at Moiwa, where a difference as large as 0.11 m/s was observed. Evaluation of measuring time The above results show that there was no considerable difference in observation results among the 20- to 120-sec measurements, and the flow distribution of the 120-sec measurement was close to the logarithmic profile. The 20-sec measurement method was therefore regarded as the most economical. The mean flow velocity is sufficiently accurate when the purpose, which is field observation, is considered. Thus, 20-sec measurement by ADCP is the most economical and accurate method. Although the minimum time was set at 20 seconds to match the measuring duration using for portable

7 current meters, it may be possible to obtain a flow velocity with the same degree of accuracy at a shorter measurement duration. Comparison of the reference river discharge and river discharge calculated according to measurements by portable current meters and ADCP Tables 4 and 5 show the sectional discharge calculated based on the 120-sec measurements by portable current meters and the 20-sec measurements by ADCP for the area from 6 m to 42 m at Pompira and from 10 to 70 m at Moiwa, respectively. These tables also show the difference (error) in flow rate between the 120-sec measurement and 20-sec measurement and the error of flow rate originating the instrumental error of the current meter (3 m/sec). These tables show that, in most cases, the flow rates as measured by ADCP exceeded the standard flow rate of portable current meters. Also, the error of ADCP measurement exceeded that of portable current meters by 9 % in mean and 18 % in maximum flow rate. Table 4: Comparison of sectional flow rates and standard river flow rates at Pompira Observation Station. Measuring method 01/17/02 01/22/02 01/29/02 01/30/02 02/04/02 02/14/02 02/19/02 02/20/02 (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) Portable cuurent meter 120sec ADCP20sec Error(ADCP-current meter) Flow rate of instrument error of current meter Percentage of error to flow rate of current meter 11.6% 4.9% 15.5% 11.1% 10.4% 9.0% 4.4% 2.7% Table 5: Comparison of sectional flow rates and standard river flow rates at Moiwa Observation Station. Measuring method 01/17/02 01/24/02 01/29/02 01/30/02 02/05/02 02/14/02 (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) (m 3 /s) Portable cuurent meter 120sec ADCP20sec Error(ADCP-current meter) Flow rate of instrument error of current meter Percentage of error to flow rate of current meter 1.4% 5.4% 9.2% 17.6% 5.8% 1% Appropriateness of flow measurement by ADCP on an ice-covered river ADCP cannot measure the flow velocity near itself and within a certain depth from the surface. This is not a problem on ice-covered rivers, where this depth tends to equal the thickness of ice sheet or frazil ice. However, the survey results revealed that the flow velocity became greater in measurements by ADCP that by portable current meters. ADCP measures the flow velocity using the Doppler effect of the main lobe of a sound wave emitted from a transducer oriented at some angle to the bed. Moreover, ADCP also emits the side lobe perpendicular to the bed. When the water is shallow, the reflected wave of the side lobe interferes with the reflected wave of the main-lobe, and an error arises in flow velocity observation. Although the measurement values for flow velocity tended to be greater than the actual values due to the shallow effective water depth, application of an ADCP to an ice-covered river is economical as flow velocity can be measured in a shorter time than with a portable current meter at a point around 2 meters in effective water depth, if an error of approximately 10 % is tolerable.

8 CONCLUSION In flow velocity measurements using portable current meters, the 20-sec measurement of 0.2D&0.8D method was found to be the most economically efficient while maintaining high accuracy. However, both the Pompira and the Moiwa observation stations, where field surveys were conducted, afford favorable observation conditions, as they are at roughly straight sections of river channels, have relatively little influence of water release from dams upstream, and have rather flat riverbeds. To obtain and verify additional data, it is therefore necessary to conduct similar field surveys at observation stations where flow rates are prone to rapid change from dams release or where the section is curved. It was concluded that portable current meters gave more accurate measurements than ADCPs in rivers with an effective water depth of less than approximately 2 m, which is the depth of those surveyed in this study. REFERENCES Japan Construction Engineers Association, Hydrologic Research Institute of the Ministry of Construction: Hydrologic observation, 131,196 (Nov. 1996). RD Instruments Inc.: Basic specifications.

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