SUSPENDED SEDIMENT CONCENTRATION MONITORING USING TIME DOMAIN REFLECTOMETRY

SUSPENDED SEDIMENT CONCENTRATION MONITORING USING TIME DOMAIN REFLECTOMETRY

Chih-Chung Chung^1*, Chih-Ping Lin², Yu-Chia Chang³

ABSTRACT

Due to geological weathering and climate change, slope sliding and soil erosion in watershed are becoming a serious problem in Taiwan. Most transportation of suspended sediment into river and reservoir occurs in a few torrential rain incidents. However, efficient and automated techniques for SSC monitoring are yet to be discovered. Time domain reflectometry (TDR) is a monitoring technique based on transmission lines, and wherein various TDR sensing waveguides can be designed to monitor different physical quantities, such as soil moisture content, electrical conductivity, and water level. By improving TDR method, a new travel time analysis method with temperature correction procedure is proposed.

The SSC accuracy is improved drastically to 1500 ppm and the measurement is insensitive to electrical conductivity and soil particle size. An extensive SSC monitoring program which includes TDR automatic monitoring and manual sampling is established at the Shihmen reservoir. SSC hydrographs are obtained for several typhoon events. In addition, the automatic monitoring station, featured by floating installation and multi-point measurements at depths, provides data for analyzing transportation velocity and thickness of venting density current.

Key Words: Time Domain Reflectometry (TDR), Suspended sediment concentration (SSC)

INTRODUCTION

Soil erosion in watershed is becoming a serious problem in Taiwan because of the geological weathering and dramatically clime changing. Monitoring of sediment movement is crucial to estimate sediment yield. It also plays an important role in land or reservoir management during heavy rainfalls. Therefore, accurate suspended sediment concentration (SCC) monitoring, especially for high SSC condition in a runoff event, is essential when studying catchment hydrology and land management. The SSC can be determined by directly taking samples for direct measurements or by using automated measuring techniques, such as optical and acoustic methods (Wren et al., 2002). However, the optical and acoustic methods are easily affected by particle sizes of suspended sediment or limited to a narrow measurement range (Sutherland et al., 2000; Thorne and Hanes, 2002). Moreover, the sophisticated instruments used in the automated methods are easily damaged during heavy runoffs by the

1 Postdoctoral, Disaster Prevention & Water Environment Research Center, National Chiao Tung University, HsinChu City 300, Taiwan, R.O.C. (*Corresponding Author; Tel.: +886-3-5712121 ext 55274; Fax:

+856-3-5714125; Email: chung.chih.chung@gmail.com)

2 Professor, Department of Civil Engineering, National Chiao Tung University, HsinChu City 300, Taiwan, R.O.C.

3 Deputy Engineer, Disaster Prevention & Water Environment Research Center, National Chiao Tung University, HsinChu City 300, Taiwan, R.O.C.

speedy flows and the rocks and debris entrained. Still further, the instruments are often too expensive to be deployed with wide spatial coverage.

Time domain reflectometry (TDR) is a measurement technique based on transmission line theory. A time domain reflectometer transmits an electromagnetic (EM) wave into a transmission line connected to a sensing waveguide and receives a reflected EM wave, which responds to the physical parameter to be monitored. It is widely applied in soil water content measurement (Topp et al., 1980), which is related to SSC measurement in principle. Unlike other techniques having a transducer with a built-in electronic sensor, TDR sensing waveguides are simple and durable mechanical device without any electronic components.

When connected to a TDR pulser above water for measurement, the submerged TDR sensing waveguide is rugged and can be economically replaced when damaged. Multiple TDR sensing waveguides can be connected to a TDR data acquisition system through a multiplexer and automated, hence increasing the system functions and spatial coverage (Chang, 2006).

This paper introduces the TDR approach for SSC measurement and SSC hydrographs obtained for several typhoon events.

THEORETICAL BACKGROUND TDR Principle

A TDR measurement setup is composed of a TDR device and a transmission line system. A TDR device generally consists of a pulse generator, an oscilloscope, a sampler, and the transmission line system which consists of a leading coaxial cable and a measurement waveguide (or probe) as Fig. 1 shows. The pulse generator sends an electromagnetic (EM) pulse along a transmission line and the sampler is used to record returning reflections from the measurement probe due to impedance mismatches. Over the last 20 years, TDR has become a valuable tool for measuring dielectric properties of soil and other materials.

Topp et al. (1980) showed that the apparent propagation velocity Va of an EM wave in a transmission line is related to the apparent dielectric constant εa. Furthermore, the apparent propagation velocity Va of the EM wave travelling through the probe can be obtained by determining the time difference between two reflections due to impedance mismatches of the probe. So the apparent dielectric constant εa proposed by Topp et al. (1980) can be formulated as

2 2

2 ⎟

⎠

⎜ ⎞

⎝

=⎛ Δ

⎟⎟⎠

⎜⎜ ⎞

⎝

=⎛

L t c V

c

a

εa

(1)

where c is light velocity at free space (2.998 × 10⁸ m s^-1), time difference Δt is between the arrivals of the two reflections as the round-trip length of the probe L as shown in Fig. 1.

Oscilloscope Step Generator

Coaxial Cable TDR Device

Sampler

Δt

L

Probe

Oscilloscope Step Generator

Coaxial Cable TDR Device

Sampler

Δt

L

Probe

Fig. 1 Time Domain Reflectometry (TDR) measurement setup and typical TDR waveform

Travel Time Analysis

To precisely determine the apparent dielectric constant εa as listed in Eq. (1), Heimovaara (1993) proposed a calibration method for obtaining the travel time between some apparently defined start time and the actual probe head start time (t0, as defined in Fig. 2) and probe length L. as the calibration method utilizes experiments on materials of known dielectric constant (typically air and water) and the following relation

c t₀ 2L _a

0 ε

τ = +Δ = +

Δ t t (2)

60 65 70 75 80 85 90 95

-1 -0.5 0 0.5

ρ

60 65 70 75 80 85 90 95

-0.01 0 0.01 0.02

ρ

Traveltime (ns) Open-end Shorted-end

Dual tangent line

t_{0 Δ}t

Apex of the derivative Δτ for tangent line method

Time mark

Δτ for derivative method

Time mark L

ρ’

(a)

(b)

(c) (in water)

Fig. 2 The schema of TDR probe in water (a) and corresponding waveform (b) and the derivative (c). The definition of travel time parameters are also shown in (b) for the dual tangent line method and (c) for the derivative method.

where Δτ is the measured travel time including t0 from time mark to the end of probe head and the round-trip travel time Δt of probe section. The probe constants including travel time

t0 and probe length L can be solved with air and water measurements once the Δτair of air and Δτw of water are obtained. The travel time is conventionally determined by the tangent line method in soil water content measurement. Chung and Lin (2009) showed that the derivative method is more clearly defined for automation and not affected by water salinity in non-dispersive media, which is supposed to be the case in sediment suspensions.

TDR SSC CALIBRATION, TEMPERATURE CORRECTION, AND MEASUREMENTS

The bulk dielectric permittivity of water with suspended sediments may be expressed as a function of SSC by the volumetric mixing model as: (Dobson et al. 1985)

ss w

SSC SS ε SS ε

ε =(1− ) + (3)

where εssc is bulk apparent dielectric constant of the water with suspended sediments; SS is the volumetric percentage of suspended sediment, which ranges from zero to 1, and εss is the apparent dielectric constant of the sediment.

Since the apparent dielectric constant of water is temperature dependent, a temperature correction method for TDR SSC measurements is proposed in this study. Based on the TDR travel time analysis as mentioned in the previous section, the TDR travel time in water containing suspended sediment at certain temperature can be rewritten based on Eqs. (2) and (3) as:

( ) [

(

) ( )

]

c t L t t

T ε_w ε_ss

τ ⎟ − +

⎠

⎜ ⎞

⎝ +⎛

= Δ +

=

Δ 2 ( )1

0

0 (4a)

( ) ( ) ( )

(

)

54 . 78 )

(T = ⋅ − ⋅ ⁻ T− + ⋅ ⁻ T− − ⋅ ⁻ T−

εw (4b)

in which the TDR travel time Δτ (measured in T Celsius degree) is composed of travel time between the electrical marker and start point of sensing waveguide t0 and the actual travel time Δt in the probe section. The Eq. (4b) shows how the apparent dielectric constant of water εw depends on temperature T (Pepin et al., 1995).

Therefore, a temperature correction method for TDR SSC measurements has the following steps (Chung, 2008):

A. Calibrate the system parameters L and t0 by measuring TDR travel times along the TDR sensing waveguide in air and in water and the water temperature.

B. Calibrate the dielectric permittivity of suspended sediment εss in Eq.(4a) with known concentrations.

C. Based on steps A and B, the volumetric percentage of suspended sediment can be determined by the equation:

( )

( )

(

)

2

) 2 (

0

c T L

c T t L T SS

w ss

w estimated

ε ε

ε τ

−

−

− Δ

= (5)

The SS can be transferred into the unit of ppm (or milligram per liter), which is commonly used in hydrology as

(

)

1 SS G mgl SS

ppm ^S

−

= ⋅

− (6)

where Gs is the specific gravity of suspended sediment.

RESULTS OF LABORATORY TESTINGS

Travel time - SSC Rating Curve

To evaluate the performance of TDR SSC measurements, various influence factors, such as soil type, and particle size, are systematically examined. A Campbell Scientific TDR100 device with SDMX50 multiplexer were used as a typical TDR measurement system, A 70 cm long TDR probe is connected via 25m CommScope QR320 cable to the SDMX50 multiplexer.

In addition, the sampling interval dt of each probe is chosen for the greatest resolution possible.

Shihmen clay (Gs = 2.73) and ChiChi silt (Gs = 2.71), and one man-made silica silt (Gs = 2.67) grinded from glass materials were selected for tests. The particle size of Shihmen clay is the finest. The mean particle size of ChiChi silt and silica silt are most identical, except that the ChiChi silt is composed of some sand and clay size particles. Calibration tests for the travel time – SSC rating curve are conducted on sediment suspensions with various SSC from 0 ~ 150,000 ppm.

Travel time – SSC rating curve of Shihmen clay was first established with the 70 cm probe.

TDR waveforms are recorded with various SSC from 0 to 150 000 ppm in water with two different EC (σ = 200, and 400 μs cm^-1), and the ravel times were determined by the derivative method.

The temperature corrected travel time Δτ (corrected to T = 25 in Celsius degree) are shown in Fig. 3. The apparent dielectric constant of the sediment estimated by regression has similar value for the two cases of different water salinity (εss = 8.47 for σ = 200 μs cm^-1, and εss = 7.53 for σ = 400 μs cm^-1). The difference in the resulting slopes of the rating curves is less than 3 percent, showing that water salinity has insignificant effect on SSC measurements.

-0.01 0 0.01 0.02 0.03 0.04 0.05 0.06

50.2 50.4 50.6 50.8 51 51.2 51.4 51.6 51.8 52

SS (-)

Δτ (ns)

Regression line (σ = 200 μS/cm) Regression line (σ = 400 μS/cm)

εss= 8.47

εss= 7.53

Fig. 3 Rating curve of travel time Δτ and Shihmen clay volumetric content SS. Error bars represent experimental data with 2 standard deviation.

Effect of Soil Type and Particle Size

The travel time – SSC rating curves for the silica silt and ChiChi silt were then established.

Due to limited amount of samples, the highest SSC for ChiChi silt was only 0.02 (50,000 ppm).

Fig. 4 shows the travel time – SSC rating curves for the three types of sediments (water with EC = 400 μs cm^-1), and Fig. 5 shows mean errors estimated from the difference between measured data and the regression lines of three different types of measured samples. The rating curve of ChiChi silt almost overlaps with that of Shihmen clay, showing no signs of particle size effect.

However, the calibrated εss of silica silt is 3.61, significantly different for that of ChiChi silt and Shihmen clay, resulting in about 14 % difference in the slop of the travel time – SSC rating curve. This difference may be attributed to different mineralogy of the silica from natural soils. It is believed that the bulk dielectric permittivity of the natural sediments does not vary significantly with time. Hence, it can be calibrated with a few actual SSC measurements. Comparing with particle size dependency in acoustic and optical methods, this sediment (mineralogy) dependency is not significant, meaning that the TDR SSC method would be more capable for SSC monitoring in a river especialy as source particle size changing during a strom case.

-0.02 0 0.02 0.04 0.06

50 50.2 50.4 50.6 50.8 51 51.2 51.4 51.6 51.8 52

SS (-)

Δτ (ns)

Shihmen clay Silica silt ChiChi silt

Silica silt

ε_ss^{= 3.61}

Shihmen clay

ε_ss= 7.53

0 0.005 0.01 0.015 0.02 51.4

51.6 51.8

ChiChi silt

Shihmen Silica

ChiChi silt

ε_ss^=6.92

Fig. 4 Rating curve of travel time Δτ with Shihmen clay, silica silt and ChiChi silt

0 5 10 15 x 10⁴ -2000

-1500 -1000 -500 0 500 1000 1500 2000 2500

SSC (ppm)

Mean error (ppm)

Shihmen clay Silica silt ChiChi silt

Fig. 5 The measurement error from rating curve of travel time Δτ with Shihmen clay, silica silt and ChiChi silt

IN-SITU SSC MONITORING DURING TYPHOON EVENTS

An extensive SSC monitoring program which mobilizes manual samplings during the typhoon event was established at the inflow and outflow locations of Shihmen reservoir in northern Taiwan. An automatic monitoring station using TDR sensing technology developed in this study was established in one outlet. This program started in summer, 2008, and SSC hydrographs were obtained for four typhoon events, including Kalmaegi, Fung-Wong, Sinlaku, and Jangmi.

To show the performance of TDR SSC measurements, the result of of Shihman outlet during Fung Wong typhoon is shown in Fig. 6. The hydrograph of TDR SSC generally agree well with that of manual sampling (bottle sampling of surface water). The difference is about

±2000 ppm, which is consistent with what was observed in the laboratory.

In the incoming of the shihmen reservoir area, a TDR SSC automatic monitoring station featured by floating installation and 8 multi-point measurements at depths was also established in this program, as shown in Fig. 7. This floating monitoring station is powerd by the solar panel, and communicates with the NCTU database center through the GPRS wireless modem. The main purpose of the floating monitoring station tries to characterize the venting density current behavior, including the SSC, thickness, and transportation velocity.

The hydrograph of TDR SSC of this floating monitoring station during Fung Wong typhoon is shown in Fig. 8. The duration of venting density current occurrence lasted almost 25 hours, and the highest SSC of venting density current was 15,000 ppm. Furthermore, the depth of venting density current (defined as SSC greater than 2000 ppm) was 17.5m (from EL.214 m ~ EL.231.5 m). Combined with pieces of SSC hydrograph information from stations at Loufu, Lungchuwan, and Shihman outlet, the hydrograph of TDR SSC of the floating monitoring station assisted the interpretation of venting density current transportation velocity, as shown in Fig. 9. The venting density current transportation velocity, which is defined by obtaining the arrival time of venting density current of each station, is 1.3 m sec^-1 from Loufu to the floating station and 0.3 m sec^-1 from the floating station to Lungchuwan or shihmen outlet.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

7/27下午 09:00

下午 11:00

上午 01:00 上午 03:00

上午 05:00 上午 07:00

上午 09:00 上午

11:00 下午 01:00

下午 03:00 下午 05:00

下午 07:00 下午 09:00

下午 11:00

上午 01:00 上午

03:00 上午 05:00

上午 07:00 上午 09:00

上午 11:00 下午 01:00

下午 03:00

下午 05:00 下午 07:00

下午 09:00 下午 11:00

Time

SSC,g/L

自動化量測含砂濃度 人工化量測含砂濃度

TDR SSC measurement

Direct sample

2008 7/28 00:00

2008 7/29 00:00 2008 7/28

12:00

2008 7/29 12:00

2008 7/30 00:00

SSC, ppm

Fig. 6 SSC hydrographs of Shihman outlet during Fung Wong typhoon,

Fig. 7 TDR SSC automatic monitoring station featured by floating installation and multi-point measurements at depths

Fig. 8 SSC hydrographs of Shihman automatic monitoring station during Fung Wong typhoon

0 5000 10000 15000 20000 25000 30000

27

日

日

日

日

i h

SSC, ppm

0 5000 10000 15000 20000 25000 30000

Turbidity, NTU

Loufu EL242m (manual) Floating station EL214m Shihmen outlet EL193.5m Lungchuwan (optical)

SSC, ppm Turbidity, NTU

2008 7/28 00:00

2008 7/29 00:00 2008 7/28

12:00

2008 7/29 12:00

2008 7/30 00:00

Time. hr

Fig. 9 Interpretation of venting density current tr tion velocity using hydrographs of four stations

during Fung Wong typhoon ansporta

CONCLUSIONS AND SUGGESTIONS

Time Domain Reflectometry (TDR) is a measurement technique based on transmission line with several advantages, such as durability, low cost transducers, and multiplexing, an innovative TDR SSC measurement method is developed in this study. The sensitivity and resolution of the TDR method were theoretically derived. Furthermore, the calibration method and temperature compensation method were developed accordingly.

These developments improve the TDR SSC accuracy drastically and the measurement is insensitive to electrical conductivity and soil particle size. Compared with traditional SSC method, the measurement range of the TDR method is theoretically unlimited, and the TDR probe is simply a waveguide which can be easily made to fit different environments. The field testing results further supported feasibility and great potential of TDR SSC measurement.

ACKNOWLEDGEMENT

The authors would like to thank the Water Resources Agency, Ministry of Economic Affairs, Taiwan, R.O.C., for supporting this research.

REFERENCES

Chang, Y.-C. (2006), Development of Monitoring Technique for High Suspended Sediment Concentration Using TDR, M.S. Thesis, Department of Civil Engineering, National Chiao Tung University, Taiwan, R.O.C., 100 Pages. (in Chinese)

Chung, C.-C. (2008), Improved Time Domain Reflectometry Measurements and Its Application to Characterization of Soil-Water Mixtures, Ph.D. Thesis, Department of Civil Engineering, National Chiao Tung University, Taiwan, R.O.C., 211 Pages.

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