Flood scour monitoring system using fiber Bragg grating sensors

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INSTITUTE OF PHYSICS PUBLISHING SMART MATERIALS AND STRUCTURES

Smart Mater. Struct. 15 (2006) 1950–1959 doi:10.1088/0964-1726/15/6/051

Flood scour monitoring system using fiberBragg grating sensorsYung Bin Lin1, Jihn Sung Lai2, Kuo Chun Chang3 andLu Sheng Li1

1 National Center for Research on Earthquake Engineering, 200, Section 3, Xinhai Road,Taipei 106, Taiwan2 Hydrotech Research Institute and Department of Bioenvironmental Systems Engineering,National Taiwan University, Taipei 106, Taiwan3 Department of Civil Engineering, National Taiwan University, Taipei 106, Taiwan

E-mail: yblin@ncree.gov.tw, jslai525@ntu.edu.tw, ciekuo@ntu.edu.tw and lslee@ncree.org

Received 30 December 2005, in final form 14 September 2006Published 13 November 2006Online at stacks.iop.org/SMS/15/1950

AbstractThe exposure and subsequent undermining of pier/abutment foundationsthrough the scouring action of a flood can result in the structural failure of abridge. Bridge scour is one of the leading causes of bridge failure. Bridgessubject to periods of flood/high flow require monitoring during those times inorder to protect the traveling public. In this study, an innovative scourmonitoring system using button-like fiber Bragg grating (FBG) sensors wasdeveloped and applied successfully in the field during the Aere typhoonperiod in 2004. The in situ FBG scour monitoring system has beendemonstrated to be robust and reliable for real-time scour-depthmeasurements, and to be valid for indicating depositional depth at the DaduBridge. The field results show that this system can function well and survivea typhoon flood.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Scouring around bridge piers/abutments remains a major causeof bridge failure induced by hydraulic deficiencies. Scourexcavates sediments around the foundations of a bridge causinga reduction in the safe capacity of the bridge. Bridge scourfailures have been reported all over the world. For example,there are approximately 590 000 highway bridges in the USNational Bridge Inventory [1, 2]. Of these, about 484 546bridges are over water and approximately 60% of them havebeen declared scour critical [1, 2]. Similar problems alsohappen in many East Asian countries, especially in thoseareas that every year are subject to repeated typhoon flooding,such as Taiwan, Japan and Korea. Scour failure tends tooccur suddenly, without prior warning or sign of distress tostructures. It is important to monitor the real-time scour-depth changes in order to prevent catastrophic failure of thebridge and possible loss of life. This is especially criticalfor those bridges that carry a high traffic load. These in situmeasurements can be used to study the maximum scour depth

and scour rate with various sediment properties under differentflow situations. This information is extremely useful inassisting engineers when designing better, safer and more cost-effective bridges.

An in situ monitoring system improves the safety of abridge, as well as being cost-effective by guarding againstpremature or unnecessary maintenance of a bridge. Inaddition, it should be able to be applied to bridges withdifferent foundations. However, around piers/abutments thecombined effects of the turbulent boundary layer, the vortexsystem, the time-dependent flow pattern and the sedimenttransport mechanism make the phenomenon of scouringextremely complex. Consequently, experimental studies mustbe conducted by considering only certain aspects as simplyconstants. A scour hole generally gets filled in as the flood flowdiminishes, and bridge inspections are not adequate to fullydetermine the extent of scour damage after a flood. Field dataare limited due to observational difficulties [1–4]. Therefore,in the traditional bridge design approach, the safety marginallowed for scour is usually large enough to compensate for any

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Flood scour monitoring system using fiber Bragg grating sensors

uncertainties, and consequently a lot of money may be wastedthrough over-design and unnecessary costs for protection.

However, at the same time, critical scour damage tobridges often cannot be repaired immediately, nor are bridgesthat are susceptible to scour being evaluated at appropriatetime intervals. Critical bridges subject to scour problemsrequire proper safety hazard monitoring. There are a numberof methods and equations in the literature for estimatingscour depth at bridge supports [3, 5]. Most of the scourstudies are limited to predicting the maximum scour depthbased on empirical equations. These empirical equationsand their derived simulation models are based mainly onlaboratory data from steady-flow experimental results whichmay not be accurate enough for field applications. Dueto the complexity of the flow and the sediment transportassociated with scour processes, there are a number of factorsinvolved that characterize the real-time scour-depth evolutionin a flood [6, 7]. Furthermore, the lack of development ofmeasuring instruments and data acquisition systems presentsdifficulties in the application of scour models for large-scalehydraulic and transportation structures in the field [3].

As mentioned previously, if the scour processes areunderstood, an appropriate bridge design or remedial measuresduring construction can be both efficient and cost-effective.However, these processes are complicated. Scour mostlyoccurs during high water stages and is often filled in asthe peak flow subsides. Unfortunately, acquiring in situscour-depth information using direct measurements is verydifficult as well as very dangerous in a typhoon flood whichis usually accompanied by high speed winds and torrentialdownpours. Many instruments have been developed formeasuring, monitoring and predicting the scour depth atbridge foundations [8–11], including sonar, radar, time-domainreflectometry (TDR) and an optical fiber sensor. However,most of these techniques have been found to have limitedapplication. For instance, both sonar and radar can receivesubstantial noise caused by the turbidity of the flow tomake those systems unreliable for real-time monitoring of thescour process. The TDR technique works by generating anelectromagnetic pulse which may attenuate and disperse itssignal due to the environment of the transmission line. Thisdrawback of the TDR technique reduces its ability to detectsubtle changes in the scour process. However, the opticalfiber sensor, specifically the fiber Bragg grating (FBG) sensor,has been proven to have excellent reliability for measuringboth strain and temperature [12–23]. According to previousexperimental studies [12–23], it has been found that theshift of the Bragg wavelength has a linear relationship tothe applied strain in the axial direction. Since an FBGsensor is absolute and linear in response, it is characterizedas having a low insertion loss as well as being interruptimmune. FBG sensors are highly attractive because of theinherent wavelength response and multiplexing capability in adistributive sensing network. In contrast to conventional straingauges, FBG sensors have immunity from electromagneticinterference. An FBG sensor is small and lightweight andis highly temperature and radiation tolerant. In practice it isflexible, stable and durable in harsh environments. Thus, itcan be multiplexed in a series of arrays along a single opticalfiber. For monitoring the dynamic responses of structures,

it has been demonstrated that an intelligent sensory systemcoping with optical fiber sensors is highly effective [13]. In thefield, however, the scour monitoring system faces the challengeof having to develop a real-time, reliable and robust systemwhich can be installed in a river bed near the bridge pier orabutment. Therefore it is necessary to develop an in situ systemfor monitoring and measuring scour-depth variations.

Two types of FBG sensors mounted on cantilevereddevices were developed and tested in the laboratory by Linet al [19]. These two sensors, Model I and Model II,could measure the strains representing both the processesof scouring/deposition and the variations in water level.Experiments were conducted successfully in the laboratoryflume demonstrating the applicability of the FBG sensors.However, these two models may not be durable and reliablein the field because after certain evaluations it was found thatthey may not be able to survive in a flood.

In the present study, an innovative scour monitoringsystem is proposed using button-like fiber Bragg gratingsensors utilizing real-time measurements in the scourprocesses. Installed at the Dadu Bridge in central Taiwan, thesystem is designed to withstand the strong currents and impactsof a flood. This FBG scour monitoring system was applied insitu during the Aere typhoon period in 2004, and the measureddata on the scour depth and deposition height were collectedand analyzed in this paper.

2. In situ scour monitoring system

For design purposes, it is well known that the establishedscour formula for estimating the maximum scour depth isessential. However, most of the data obtained to develop thescour formula are collected from the laboratory instead offrom the field. Practically, the field data collection of scourdepth is rather limited due to lack of durable and reliableinstruments, especially for recording data during flood events.Therefore, the main focus of this section is the collection offield data using our proposed scour monitoring system. Thesystem utilizing FBG sensors was applied and installed at theDadu Bridge. The following is a brief description of thefundamentals of the fiber Bragg grating sensor of the bridgesite, hydrologic data and field installation.

2.1. Fiber Bragg grating sensors

Using current technology, FBG sensors are easy to fabricateusing a side exposure technique. There are two typicalconfigurations: one consists of exposing a small portion of theoptical fiber to two interfering beams of ultraviolet (UV) lightand the other consists of having one UV beam focused througha phase mask. Due to a periodic modulation of the refractiveindex created in the core of an optical fiber, certain discreteoptical frequencies will resonate. If broadband light travelsthrough the optical fiber, the incident energy at that resonantfrequency will be reflected back. The Bragg wavelength shift�λB of an FBG sensor, subject to physical disturbance, can beexpressed as [12]:

�λB

λB= (1 − pe) ε + (α + ξ) �T (1)

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Figure 1. CFT segments with button houses implanted aremanufactured in the laboratory.

where pe, ε, α, ξ and �T are the effective photo-elasticconstant, axial strain, thermal expansion coefficient, thermaloptic coefficient and temperature shift, respectively. Generallyspeaking these coefficients depend on the type of opticalfiber used and the wavelengths measured. According toequation (1), any change in the periodicity of the refractiveindex modulation or the overall index of refraction will changethe Bragg wavelength. Consequently, any temperature orstrain-induced effect on the FBG can be determined by acorresponding shift in the center Bragg wavelength. Sincethere is only one sensing parameter, a wavelength shift isrequired in the sensor application, as the temperature andstrain cannot be measured simultaneously with one singlegrating. To separate the strain signal from the temperaturesignal, different compensation methods of temperature effectshave been reported in the literature [13–23]. In addition,discrimination techniques have been proposed in recent years,such as using the first-order or second-order diffraction formsto measure temperature and strain simultaneously [15], orusing a chirped fiber grating written in a tapered fiber tofabricate a temperature-independent fiber Bragg grating strainsensor [16]. In practice, using the matrix inversion technique,most of the applications utilize two superimposed FBGswritten at two different wavelengths to decouple strain andtemperature [17–23]. Based on the results of the experimentby Lin et al [19], the FBG sensors were identified as usefuldevices for measuring scour depth. Although the FBG sensorsdemonstrated their applicability in the laboratory, they cannotbe applied directly in the field because, after certain evaluationsin the present study, they were found to be unable to withstandthe conditions encountered in a flood.

Therefore, an innovative scour monitoring system capableof operating in the field and using FBG sensors is proposedand developed in this study. Based on the concept of a button-like mechanism, the FBG sensor in the system is coveredwith a waterproof rubber seal like a button housing. A stopbolt and a spring inside each button housing prevent damageto the FBG sensor in case of a flood flowing at excessivevelocity beyond the protection criterion, or if debris in theflow strikes with excessive impact force. Each sensor insideits own button housing is arrayed along a single optical fiber in

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Figure 2. The calibration of strain and loading for six FBG sensorsas examples.

series, and is implanted in a concrete-filled steel tube (CFT).The distance between each implanted FBG sensor is 1 m.Prior to installation in the field, the FBG scour monitoringsystem is designed with 1 Hz sampling rate and 1 με resolutionfor sufficient accuracy. The system was fabricated in thelaboratory at the National Center for Research on EarthquakeEngineering, Taipei, Taiwan. As shown in figure 1, severalCFT segments were manufactured. They were assembled inthe system and then installed in the field. The relationshipsbetween strain and loading, under a specific impact force, werecalibrated for six of the FBG sensors, to serve as an example,and they are plotted in figure 2.

2.2. Site description and hydrologic data

The Dadu Bridge spans the Wu River between two majorareas, Taichung County (northbound) and Changhua County

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P10P11P12P13

RiverbedRiverbed

PierPier

To Changhua County(Southbound)

To Taichung County(Northbound)

Filledwithsand

Groutedwith

concrete

Figure 3. The Dadu Bridge spans two major counties over the WuRiver.

Figure 4. The installation of the CFT for the downstream opticalfiber channel (DC).

(southbound), in central Taiwan. The bridge is subject to heavytraffic at peak hours. It is situated about 15 km from the mouthof the Wu River which flows westward to the Taiwan Strait.According to the hydrologic data, the upstream watershed ofthe Dadu Bridge in the Wu River has a natural drainage area of1981 km2. The bed slope in the reach of the Dadu Bridge isabout 0.1%. The mean annual discharge at the Dadu Bridge isestimated around 116 m3 s−1. The wet season usually occursbetween May and October. According to the Water ResourcesAgency [24], based on the frequency analysis of the floodprotection criterion for the Wu River, the 100 year return-period flood is analyzed to be 21 000 m3 s−1. As shown inthe photo of figure 3, the 1 km long Dadu Bridge is a simplesupport bridge with 25 spans, which was constructed in 1969;two more lanes were added in 1989. The main stream flowsunderneath the Dadu Bridge between pier 3 (P3) and pier13 (P13).

2.3. Installation on the Dadu Bridge piers

Although the FBG sensors were proven to be applicableand reliable for measuring scour-depth variations in thelaboratory [19], they are unlikely to withstand field conditions,especially the strong currents and impacts of a severe flood.

Figure 5. The layout of the FBG scour monitoring system on P12.

Figure 6. Flow features around P12 at the Dadu Bridge in the Aeretyphoon flood.

In an effort to assess the applicability of the FBG scourmonitoring system, it was designed to be installed on pier 12(P12). Pier 12 was selected because it was determined that itscaisson foundation was once exposed to approximately 10 mdepth during the Herb typhoon flood in 1996. The 10 m deepscour was observed and reported by the Directorate General ofHighways, Taiwan, while the repairs for the bridge foundationswere being carried out [24]. Figure 4 shows the installation ofthe FBG scour monitoring system encased in a pipe locateddownstream of P12. This system was completed on 15 July2004 before the next typhoon flood arrived. The side view ofP12, figure 5(a), shows that two channels using optical fibersconnected to the data logger in the FBG scour monitoringsystem are mounted both upstream and downstream of pierP12, namely pier UP12 and pier DP12, respectively. Theposition selected for the upstream optical fiber channel (UC)at UP12 was originally considered not to encounter the mostserious scour situation at the bridge pier nose. At UP12, theposition of the CFT was selected to be about 20◦ away from thepier nose of the caisson to reduce the risk of this FBG systembeing damaged by debris. Although the CFT was not installedat the pier nose, the major scour hole generated around pier P12is far larger than the small one created around the CFT. Being

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Figure 7. Strain readings against time along the UC during the Aere typhoon flood.

very close to UP12 the FBG sensors in the CFT can providethe indicative scour-depth measurement. At DP12, the positionof the CFT was installed about 10◦ away from the downstreampier face of the caisson. Section A–A corresponds to the layoutin figure 5(a) and is plotted in figure 5(b).

Each FBG sensor inside the button housing is arrayedin series. The FBG sensors for each optical fiber channelare implanted in the CFT. Along the UC the CFT, containing12 sensors, penetrates the river bed. The total length of theupstream CFT is 30 m. To keep the CFT in a stationary

position, concrete was grouted in the bottom part, and fineaggregate filled the area surrounding the FBG sensors section,as shown at the lower right-hand corner of figure 3. The toppart of the buried CFT protruded perpendicularly from the riverbed, and it was welded to the caisson of UP12 and protectedby a steel frame. As can be seen in figure 5(a), the firstFBG sensor on UP12 is buried 1 m beneath the river bed.In addition, in order to prevent the FBG sensors from beingdamaged by debris or large chunks of sediment being carriedalong with the flow, two pieces of L-shaped iron bumper were

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Figure 8. Strain readings against time along the DC during the Aere typhoon flood.

welded 30 cm away from the FBG sensor on the CFT (seefigure 1). For the downstream optical fiber channel (DC) onDP12, its CFT containing six sensors was also driven into theriver bed. As can be seen in figure 5(a), the first FBG sensoralong the DC is placed about 0.5 m higher than the river bedin order to distinguish between scour and flow signals. Thesecond sensor is placed along DC and buried 0.5 m below theriver bed.

If the river bed erodes down to the top-buried sensor onCFT, the exposed sensor receives the strain readings of theimpact of running water to measure the real-time scour depth.The exposed sensors continuously measure the increasingflow pressures as the flood stage keeps rising. During theflood recession period, impact strain gradually decreased,indicating that the sensors would be buried again because ofsediment deposition. In the time series, the data recordedfrom the adjacent locations of various FBG sensors indicatethe continuous scour-depth evolution, which is useful for thefurther understanding of the scour processes at bridges.

To satisfy the flood protection criterion in the Wu River,the FBG scour monitoring system is designed to withstand theharsh environments of a 100 year return-period flood attack.The resistance or impact from the running water, the drag force,acting on the FBG sensor can be written as follows [25]:

FD = CD ApρV 2

2(2)

where FD (kgf) is the drag force, CD is the drag coefficientrepresenting the pressure and friction drag effects, Ap (m2) isthe projected area of the button housing for the FBG sensor,ρ (kg m−3) is the water density and V (m s−1) is the flowvelocity. The CD coefficient related to the Reynolds numberis shown in figure 9.29 of [25]. The mean flow velocity inthe main stream along P2 can be estimated by using the HEC-RAS model [26]. In the HEC-RAS model, developed by theUS Army Corps of Engineers, the hydraulic characteristicssuch as geometry of the cross section at the bridge, river bed

roughness, and flood peak discharge, flood stage, etc are alltaken into consideration.

The diameter (D) of the contact plate devised with a FBGsensor inside the button housing is 2.5 cm; thus, the projectedarea is 0.000 491(=π ×0.0252/4) m2. The mean flow velocity(V ) at the Dadu Bridge is calculated to be 3.51 m s−1 underthe 100 year return-period flood criterion in the main streambetween P3 and P13. The Reynolds number can be calculatedas Re = V D/υ = 3.51 (m s−1) × 0.025 (m)/(0.873 ×10−6) (m2 s−1) = 1.005 × 105, where υ is the kinematicviscosity of water at 26 ◦C. The CD coefficient is thereforeabout 1.0 based on the shape of the button housing and thecalculated Reynolds number. The drag force impacting onthe sensor for a 100 year return-period flood is then estimatedusing equation (2), and it works out that FD = 3.02 (kgf).Each FBG sensor was tested and calibrated in the laboratory toestablish the relationship between strain and loading as shownin figure 2. The linear relationships under a drag force of3 kgf for the six sample FBG sensors indicate that the devicesachieve the required flood protection criterion.

3. Real-time monitoring and result analysis

The flow features during the Aere typhoon flood in 2004, atabout 8 am on 26 August around P12, are shown in figure 6.The real-time scour monitoring data recorded from the FBGsensors located at various levels are shown in figures 7 and 8in UC and DC, respectively.

3.1. Data readings from the FBG sensors along the UC

As shown in figure 5 along the UC the datum of the scour-depth measurement is set as the original river bed elevation,which is 3 m below the top of the caisson. Figure 7 showsthat along the UC, UC1 (located at −1 m), UC2 (locatedat −2 m) and UC3 (located at −3 m) were uncovered andwere then able to detect the flow drag force to reveal the

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Figure 9. Accumulation of floating debris upstream of UP12 in theflood.

P9

P8

Figure 10. The local scour holes around pier 9 and pier 8 after theend of the flood.

signal fluctuations. For instance, figure 7(a) shows the strainreadings of the originally buried UC1 sensor with varyingsignals which were recorded at about 17:10 on 25 Augustand then gradually increased. Those readings, varying withtime, indicate the scouring processes. From 21:00 the readingsfrom UC1 rose significantly, presenting a rising trend beforethe flood peak. The obvious fluctuations in the readingsshowed strong interactions between the FBG sensor and theflow turbulence/sediment transport around the peak flow in theperiod between midnight and 10:00 on 26 August. Some timeafter 22:50 on 30 August, as shown in figure 7(a), the UC1sensor took constant readings, indicating that it was once againburied due to sediment deposition. As mentioned previously,scour features are often filled in as the peak flow subsides.Similarly, as seen in figure 7(b), the UC2 sensor located 1 mbelow UC1 emerged at 21:20 on 25 August when the UC1sensor received a strong erosive impact. The river bed wasthen quickly eroded down to the position of the UC3 sensor,about 25 min after the UC2 sensor had emerged, as shown infigure 7(c). With strong impacts of both flow and sedimentuntil 10:00 on 26 August, aggradations occurred gradually onthe river bed due to recession of the flood. Figures 7(b) and (c)show the constant readings recorded from the UC2 and UC3sensors, indicating that both sensors were once again coveredby sediment. The UC3 sensor was buried first at 21:45 on 29

UP12

Figure 11. Upstream view of the Dadu Bridge at UP12 on 14October 2004.

DP12

Figure 12. Downstream view of the Dadu Bridge at DP12 on 14October 2004.

August, and the UC2 sensor was buried about half a day later at08:05 on 30 August. For the rest of the sensors, no fluctuatingresponses were recorded indicating that they did not emergeduring the Aere typhoon flood.

Our system, using button-like FBG sensor devices,survived in the field, and it is evident that the sensors werecapable of monitoring the scour/deposition processes duringthe Aere typhoon flood event. The above field data analysisindicates that the estimated scour depth at the CFT implantedwith UC sensors is between 3 and 4 m, say 3.5 m, becausethere were no fluctuating readings from other sensors belowthe UC3 sensor. Although the scour hole was filled up afterthe flood ceased, a total exposed depth of about 6.5 m wasestimated, from the top of the caisson. As a result the designedscour allowance from the caisson top for the UP12 pier is3 m. Therefore, the safety of the bridge pier must be carefullyexamined since a 6.5 m deep exposure was observed at UP12under peak flow conditions during the Aere typhoon flood. An

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Figure 13. Strain readings against time along UC during 12–14 October 2004.

effective scour countermeasure must be provided to ensure thesafety of the bridge pier.

3.2. Data readings from FBG sensors along DC

As shown in figure 5, along DC, two FBG sensors, namelyDC1 (located at +0.5 m) and DC2 (located at −0.5 m),received varying signals (see figure 8) while other sensorsshowed no fluctuation in their readings during the Aeretyphoon flood event. It must be noted, however, that the firstFBG sensor along DC was placed about 0.5 m higher than thedatum at the original river bed elevation. The strain readingsrecorded in figure 8(a) started to vary at 15:35 on 25 August,at the time that the rising water reached the DC1 sensor. After

the flood receded, the readings of the DC1 sensor were backto the everyday pattern of variations with a temperature higheraround midday and lower after 17:00. For the originally buriedDC2 sensor in figure 8(b), signals varied after 20:05 on 25August, revealing the emergence of the DC2 sensor, and thenits readings increased gradually. The DC2 sensor was found tobe exposed due to the general scour of the river bed. Therefore,the readings of the DC2 sensor consistently presented the samepattern of variations with the same temperature as that of theDC1 sensor after the flood receded. There were no fluctuatingresponses from the rest of the sensors below the DC2 sensor.

Along the DC, the scour depth exposed from the datumwas estimated to be between 0.5 and 1.5 m, say 1 m, in thisflood event. According to the scour characteristics around the

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Figure 14. Strain readings against time along DC during 12–14 October 2004.

bridge pier, the scour depth downstream of the pier is generallysmaller than that created upstream or in front of the pier. Themeasurements from the FBG sensors along the DC presentreasonable in situ data of the scour processes. Photos weretaken to show the debris trapped in the field and the local scourfeatures in front of the piers after the flood, and are shown infigures 9 and 10, respectively. Based on the above analysis it isevident that the FBG scour monitoring system can survive andfunction well through a flood event.

3.3. Data collection after the flood

For follow-up, data collection from the FBG scour monitoringsystem on regular sunny days was carried out for the periodof 12–14 October 2004. Two photos shown in figures 11and 12 were taken from the upstream and downstream piers ofP12, respectively. Since all of the FBG sensors along the UCwere buried under the river bed, no fluctuating signals wereobserved in figure 13. On the downstream side of DP12 alongDC, the DC1 and DC2 sensors were exposed to the air. Asshown in figure 14, these two sensors detected temperaturevariations which produced the same patterns as those describedin figure 8 after the flood had receded. Other sensors along theDC recorded constant readings without showing fluctuationsbecause they were still buried by sediments.

The above analyses demonstrate that the in situ FBG scourmonitoring system can successfully measure real-time scourdepth for the processes of scouring and deposition.

4. Summary and conclusion

Bridge scour is one of the leading causes of bridge failure.Undermining and exposure of the bridge pier foundationsdue to scouring action can result in structural failure of thebridge. Taiwan Island is subject to frequent floods as aresult of typhoons, and these floods may contribute to theloss of bearing support of bridges. Monitoring bridges isabsolutely crucial for prior warning or signs of distress ofbridge structures, especially because scour failure tends tooccur suddenly, particularly in a flood event. In recent years,

much effort has been dedicated to the development of real-time monitoring instrumentation for scour measurements. Itis important that scour-depth changes are monitored in situ inorder to prevent catastrophic failure of the bridge and possibleloss of life, especially during busy traffic periods.

Mounted on cantilevered devices, two models using FBGsensors were developed and tested in the laboratory, in order tomeasure the corresponding strains to represent the processesof scouring/deposition by Lin et al (2005). However, aftercareful evaluation in the present study it was found that thesetwo models are not durable and reliable enough for use in thefield, and that they would not be able to withstand the ravagesof a typhoon flood.

In order to be able to operate and survive in the fieldwe proposed an innovative scour monitoring system in thispaper. It uses button-like FBG sensors that were developedand utilized based on real-time in situ measurements. Installedat the Dadu Bridge across the Wu River in central Taiwan,the system is designed to work efficiently and to be ableto withstand the strong currents and impacts of a flood.Connected to the data logger, two channels using optical fibersin the system are mounted on the upstream and downstreampiers of P12. This system was applied during the Aeretyphoon period in 2004, and the measured data in the scourand deposition processes were collected and analyzed.

The field measurements taken upstream of P12 indicatedthat the scour hole that was created was entirely filled up afterthe flood created by the Aere typhoon ceased. About 6.5 mof exposed depth was measured from the top of the caissonaround the peak of the flood. Accordingly, the designed scourallowance from the top of the caisson at P12 is 3 m. In addition,the maximum scour depth generally occurs at the nose of thepier, which should be even larger than the measured data of6.5 m. Consequently, the safety of the bridge pier shouldbe carefully examined under peak flow conditions. On thedownstream side of P12, a scour depth of 1 m below theriver bed was estimated in this flood event. According toscour characteristics around bridge piers in general, the scourdepth downstream of a pier is usually smaller than that created

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Flood scour monitoring system using fiber Bragg grating sensors

upstream of the pier. The field measurements echo this andshow quite reasonable results for the scour processes.

Although three powerful typhoons of category 3 and abovemade landfall on Taiwan in 2005, fortunately all the flooddischarges of these typhoons were less than the 4 year return-period flood at the Dadu Bridge. Therefore, no significantscour monitoring readings were recorded by the system, andto date no scour damage has been reported. Nevertheless, aneffective scour countermeasure for the Dadu Bridge should beseriously taken into consideration to ensure the safety of thebridge pier before the next severe flood arrives.

Based on the analysis provided in this paper, it isevident that the proposed FBG system can function well andsurvive a flood event. The system is durable and reliableand has been demonstrated to be successful for real-timescour-depth measurements in the processes of scouring anddeposition. The field results indicate that this in situ scourmonitoring system using fiber Bragg grating sensors has greatpotential for practical applications in erosion problems aroundhydraulic works. Moreover, around bridge piers the combinedeffects of the vortex system, the turbulence flow pattern andthe sediment transport mechanism make the phenomenon ofscouring/deposition extremely complex. In the present study,the FBG sensors measured only the overall responses fromvarious components of loadings such as water static pressure,water flow dynamic pressure, soil pressure, etc. This showsthat the FBG sensors in the CFT can provide the indications forscour-depth measurements. If each sensor is installed as closeas possible, the data accuracy of the scour-depth measurementcan be improved.

The scour measurements obtained at the Dadu Bridgeare also valuable for studying the maximum scour depth andscour rate with various sediment properties under different flowsituations. The in situ information collected herein will be veryhelpful in assisting engineers to design more accurate, saferand more cost-effective bridges.

Acknowledgments

The authors gratefully acknowledge the Directorate General ofHighways, Ministry of Transportation and Communications,ROC, China Engineering Consultants, Inc. and the HydrotechResearch Institute of the National Taiwan University, Taiwan,for their financial support during the last two years. Theauthors would like to thank Mr S H Wu, National Centerfor Research on Earthquake Engineering (NCREE), andMr F Z Lee, Hydrotech Research Institute for their kindassistance in data collection.

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