Assessment of bridge scour in the lower, middle, and upperYangtze River estuary with riverbed sonar profilingtechniques

Shuwei Zheng & Y. Jun Xu & Heqin Cheng & Bo Wang &

Xuejun Lu

Received: 31 May 2017 /Accepted: 5 December 2017# Springer International Publishing AG, part of Springer Nature 2017

Abstract Riverbed scour of bridge piers can causerapid loss in foundation strength, leading to suddenbridge collapse. This study used multi-beam echosounders (Seabat 7125) to map riverbed surroundingthe foundations of four major bridges in the lower,middle, and upper reaches of the 700-km Yangtze RiverEstuary (YRE) during June 2015 and September 2016.The high-resolution data were utilized to analyze themorphology of the bridge scour and the deformation ofthe wide-area riverbed (i.e., 5–18 km long and 1.3–8.3 km wide). In addition, previous bathymetric mea-surements collected in 1998, 2009, and 2013 were usedto determine riverbed erosion and deposition at thebridge reaches. Our study shows that the scour depthsurrounding the bridge foundations progressed up to4.4–19.0 m in the YRE. Over the past 5–15 years, thetotal channel erosion in some river reaches was up to15–17 m, possessing a threat to the bridge safety in theYRE. Tide cycles seemed to have resulted in significantvariation in the scour morphology in the lower andmiddle YRE. In the lower YRE, the riverbed morphol-ogy displayed one long erosional ditch on both sides ofthe bridge foundations and a long-strip siltation area

distributed upstream and downstream of the bridgefoundations; in the middle YRE, the riverbed morphol-ogy only showed erosional morphology surrounding thebridge foundations. Large dunes caused deep cuts andsteeper contours in the bridge scour. Furthermore, thisstudy demonstrates that the high-resolution grid modelformed by point cloud data of multi-beam echosounders can clearly display the morphology of thebridge scour in terms of wide areas and that the sonartechnique is a very useful tool in the assessment ofbridge scours.

Keywords Bridge scour . Riverbed erosion . Channelmorphology. Bridge foundation .Multi-beam echosounders . TheYangtze River Estuary

Introduction

Riverbed scour surrounding bridge foundations is theprimary cause of bridge failures. The downflow andhorseshoe vortex systems are two main reasons leadingto the development of bridge foundation scours (Ungerand Hager 2006). A number of other factors can alsoplay a role in bridge scour, including suspended sedi-ment concentration (SSC) (Sheppard et al. 2004), flowdepth and velocity (Melville 1992), bed material (Ataie-Ashtiani and Beheshti 2006), bridge foundation typesand diameter (Melville 1992, Khosronejad et al. 2012),angle of the attack (Chang et al. 2011), channel geom-etry (Cardoso and Bettess 1999, Johnson et al. 2015),and bedforms (Noormets et al. 2006). Since scour depth

Environ Monit Assess (2018) 190:15 https://doi.org/10.1007/s10661-017-6393-5

S. Zheng :H. Cheng (*) :X. LuState Key Lab of Estuarine & Coastal Research, East ChinaNormal University, Shanghai 200062, Chinae-mail: hqch@sklec.ecnu.edu.cn

S. Zheng :Y. J. Xu (*) : B. WangSchool of Renewable Natural Resources, Louisiana StateUniversity Agricultural Center, 227 Highland Road, Baton Rouge,LA 70803, USAe-mail: yjxu@lsu.edu

and morphology are important to bridge safety, manyphysical and numeric modeling studies have been con-ducted over the past several decades to predict bridgescour (Deng and Cai 2009, Hosseini and Amini 2015).

Scour surrounding bridge foundations in unidirec-tional flow has been intensively investigated (Melvilleand Raudkivi 1977, Sheppard et al. 2004). However, theeffect of bidirectional flow in tidal channels on bridgefoundations is relatively little studied. Fluctuations inflow and water depth in a river channel affected by tidalcycle are more complex, which may lead to significantdifferences in the pattern of scouring. In a three-dimensional numeric modeling study on scour develop-ment in a three-pile group, Vasquez and Walsh (2009)found that the scour depth around the piers decreasedunder tidal conditions with flow reversal. Based onseveral previous datasets, Zanke et al. (2011) developeda universal formula for the estimation of equilibriumscour depth around a single cylindrical pile under theaction of steady currents, tidal and short waves. In aflume experimental study on scour development arounda vertical cylinder acting as an offshore wind turbinemonopole, McGovern et al. (2014) found a symmetricalshape of a score hole after two half-tidal cycles and thatthe score hole was both more shallowly and slower-developing than the scour hole in a unidirectional cur-rent test carried out in the same flume. These studieshave advanced our understanding of the foundationscour process in tidal channels. However, they are main-ly laboratory experiments and the prediction equationsdeveloped are little useful for field monitoring and as-sessment. This is especially the case for on-site bridgescour assessment in a large river estuary with highsediment load.

The Yangtze River Estuary (YRE) is one of the mostdeveloped and the most densely populated areas inChina (Gu et al. 2011). The estuary is China’s mostimportant waterway for good transport from inland tothe sea and is also one of the largest ports in the world.Understanding the change mechanism of bridge foun-dation scour and nearby channel morphology in theestuary is of great importance to transport and naviga-tion safety and long-term channel stability. A few stud-ies have been recently conducted on bridge structuralhealth monitoring or symmetrical shape of the bridgescore in the YRE (Lu et al. 2016, Wang et al. 2014).However, multi-beam echo sounders used as a tool inthe assessment of bridge scour morphology influencedby different tide processes in an estuary are rare.

This study aimed to utilize the state-of-the-art multi-beam riverbed scanning techniques to investigate bridgefoundation scours in a large estuarine river. Specifically,the study was aimed to (1) assess the current situation ofthe bridge scour in the Yangtze River Estuary, (2) inves-tigate the characterization of scour surrounding similarbridge foundations influenced by tides in this largeestuary, and (3) discern the possible factors affectingscour morphology changes in the Yangtze RiverEstuary.

Methods

Study area

The Yangtze River (or Changjiang in Chinese) is thethird longest river in the world with a drainage area of1.81million km2 (Cui et al. 2013, Yang et al. 2015). Theupper 4300-km reach from the headwater area to Yi-chang is commonly considered as the upper YangtzeRiver (YR), and the following 950-km reach from Yi-chang to Hukou is considered as the middle YR, whilethe last 930-km reach from Hukou to the river mouth istermed as the lower YR. Normally, the last 650-kmreach within the lower YR from Datong to the rivermouth is named as the Yangtze River Estuary (YRE)because of the influence of tides from the East ChinaSea (Fig. 1). The YR delivers a large quantity of sedi-ment to the estuary, forming many channel bars in theYRE (Gao et al. 2013, Zheng et al. 2016b). Some ofthese bars have formed into large islands (Fig. 1). Dur-ing the past 2000 years, the lowest section of the YREhas developed three-level bifurcation and four outletsinto the sea (Chen et al. 1979). Due to river engineeringin the past several decades, SSC in the river has declinedlargely (Dai et al. 2016). The change has a significantinfluence on the channel stability, dune and bar devel-opment, and estuary delta evolution (Dai et al. 2016,Zheng et al. 2016a, Deng et al. 2017).

The surrounding land area of the YRE is one of themost densely populated and highly developed industrialareas in China. The region includes the Yangtze RiverDelta (YRD), which has approximately 11% of China’stotal population and contributes to 22% of China’s totalGDP (Zhang et al. 2014). Since the 1960s, many largeriver bridges have been built across the YRE and thesebridges are important transportation links in the region.

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Basic information on the study bridges

In this study, we selected four major bridges from thelower YRE to the upper YRE to assess the current scourstatus of bridge foundations. They include the ShanghaiYangtze River Bridge (YRB) in the lower YRE, theSecond Nanjing YRB and the Dashengguan YRB inthe middle YRE, and the Tongling YRB in the upperYRE. Table 1 provides some basic information on thebridge foundations and river flow conditions.

Data collection

In this study, we created digital elevation model (DEM)datasets to assess morphological changes over time.Specifically, the north channel bathymetric data for2009 and 2013 were used to analyze the morphologicalvariation near the Shanghai YRB in the lower YRE.These data were surveyed by the Navigation GuaranteeDepartment of the Chinese Navy Headquarters

(NGDCNH) and the Maritime Safety Administrationof People’s Republic of China (MSAPRC), respectively.In addition, the navigation maps from Tongling to Nan-jing for 1998 and 2013 created by the Chang JiangWaterway Bureau (CJWB) were also collected to ana-lyze the morphological change near the Second NanjingYRB and the Dashengguan YRB in the middle YREand the Tongling YRB in the upper YRE (Table 2).

Bridge scour measurements

We used a multi-beam echo sounder (MBES; TeledyneRESON, SeaBat 7125) to survey the riverbed of the fourstudied river reaches. A boat mounting the soundertraveled the reaches several times during June 2015and September 2016. The sounder has an operationalworking frequency of 200/400 kHz. The theoreticaldepth resolution is 6 mm, and typical depth measure-ments are 0.5 to 150 m at 400 kHz and 0.5 to 400 m at200 kHz. The along-track transmitted beam widths are

Fig. 1 Geographical location of the Yangtze River Estuary (YRE)and the studied bridge scours in the river’s lower (a), middle (b, c),and upper (d) reaches of YRE. The bridges at a–d are,

respectively, the Shanghai YRB, the Second Nanjing YRB, theDashengguan YRB, and the Tongling YRB

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2° at 200 kHz and 1° at 400 kHz, and the across-trackreceived beam widths are 1° at 200 kHz and 0.5° at400 kHz. It has 512 beams at 400 kHz. This equipmenthas a maximum swath angle of 140° in equal-distancemode and 165° in equal-angle mode. During the survey-ing, a Trimble real-time differential global positioningsystem (DGPS) was used to control the position accura-cy at the decimeter level. The speed of the measuringboat was controlled at 1–3.5 m/s, and the threshold ofmaximum ping rate was 20 Hz which can automaticallyadjust with water depth. The equal-distant mode and aswath angle of 140° were chosen in acquisition moduleof the PDS 2000 control center. A flat seafloor with aslope in the downstream of the Shanghai YRB and in theNanjing reach of the YR was chosen for multi-beamcalibration of roll, yaw, and pitch, respectively. The finaldata were also corrected for sound speed and abnormalbeams. Four piers were selected to study the morpholog-ical characters and maximum scour depths in the YRE.

Estimation of erosion and deposition rates

The Chinese National Grid coordinates was used to geo-reference all collected bathymetric charts with ninebenchmarks in ArcGIS 10.4. Isobaths of water depths0, − 2, − 5, and − 10 m in the river channel were trans-formed into elevation points with a spacing of 50–80 mbetween two adjacent points on the isobaths. Points ofwater depth were also transformed into elevation. Thesepoint cloud data were relative to Beijing 1954 coordi-nates. Subsequently, the Kriging technique was utilizedto interpolate the digitized data into grid with 100 ×100 m resolution. Deposition or erosion of the channelwas estimated by subtracting the grid file of 1 year fromthat of another. An 18-km long river reach was chosen to

estimate the deposition and erosion rates near the Shang-hai YRB between 2009 and 2013. A 7.0-, 9.0-, and 5.0-km long river reaches were chosen to estimate the depo-sition and erosion rates near the Second Nanjing YRB,the Dashengguan YRB, and the Tongling YRB between1998 and 2013, respectively. The errors of the Krigingwere related to the accuracy of the collected depth datafrom the maps and the riverbedmorphology (e.g., slope).Previous studies have analyzed some of the river seg-ments of the lower Yangtze River and have reported alow gradient of the riverbed (Wang et al. 2009; Luo et al.2017). Especially, for the reaches of the Tongling YRB,Second Nanjing YRB, Dashengguan YRB, and Shang-hai YRB, the river gradient was estimated to be 1‰~1%(Chen et al. 2012). Luo et al. (2017) postulated that inusing the Kriging interpolation technique for such low-gradient riverbeds, errors could be assumed to be low (<

Table 2 Navigation charts used for creating the digital bathymet-ric data of the Yangtze River Estuary, China

Year Map title Scale Source

1998 Baguazhou waterway 1:40,000 CJWB

1988 Jiangxinzhou to Xinjizhou 1:40,000 CJWB

1998 Henggang to Chongwenzhou tail 1:40,000 CJWB

2009 Southern part of Changjiang Kou 1:130,000 NGDCNH

2013 Qixiashan to Zhongshan Harbor 1:40,000 CJWB

2013 Zhongshan Harbor to Qiudingzhou 1:40,000 CJWB

2013 Zhangjiangzhou to Hejiachang 1:40,000 CJWB

2013 Hejiachang to Nizhou 1:40,000 CJWB

2013 Changjiang Kou and approaches 1:150,000 MSAPRC

CJWB Chang Jiang Waterway Bureau, NGDCNH NavigationGuarantee Department of the Chinese Navy Headquarters,MSAPRC Maritime Safety Administration of People’s Republicof China

Table 1 Basic information on four major bridges in the lower (A),middle (B and C), and upper (D) Yangtze River Estuary, China.The letters A–D denote the river reach of the Shanghai YRB, theSecond Nanjing YRB, the Dashengguan YRB, and the Tongling

YRB, respectively. VLDS and SDS denote very large dunes (wave-length ≥ 100 m) (Ashley, 1990) and small dunes (wavelength ≤5 m), respectively

Name Built Foundation type Foundationdimension (m)

Riverbedconditions

Flowdepth (m)

Meanvelocity (m/s)

A 2009 Rectangular pile cap witha round head

37.2 × 72.2 Plane bed 16.3 –

B 2001 Cylindrical cofferdam 38.2 × 80.2 VLDS 32.3 1.23

C 2011 Rectangular cofferdamwith round head

36 × 36 SDS 15.2 1.21

D 1991 Cylindrical cofferdam 31 × 31 SDS 25.7 1.32

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10%). In our study, the volumes of erosion and deposi-tion were calculated through the method of Wheatonet al. (2009).

Net erosion volume was calculated as follows:

Enet ¼ Ev−Dv ð1Þwhere Enet is the net erosion volume, Ev is the erosionvolume, and Dv is the deposition volume.

Average erosion depth was calculated by

Edepth ¼ Enet=Stotal ð2Þwhere Edepth is the average riverbed erosion depth andStotal is the total surface area of the river reach near theShanghai YRB in 2009 or near the other studied bridgesin 1998.

Erosion rate is calculated as

Erate ¼ Edepth=n ð3Þwhere the Erate is the erosion rate and n is the number ofyears, which is 4 (2009–2013) for the Shanghai YRBand 15 (1998–2013) for the other three bridges.

Results

Channel erosion and deposition near bridges

Overall, the channel of the four studied reaches experi-enced erosion in different degrees over the past 5–15 years (Table 3). The Dashengguan YRB reach hadthe largest percentage of area eroded (94%), followed bythe Tongling YRB reach (85%), the Shanghai YRBreach (80%), and the Second Nanjing YRB reach(58%). The Dashengguan YRB reach also showed thehighest erosion rate (0.33 m/year), followed by the

Shanghai YRB reach (0.28 m/year), the Tongling YRBreach (0.15m/year), and the Second Nanjing YRB reach(0.03 m/year).

Scour morphology surrounding the bridge foundations

Bridge scour and riverbedmorphology grid models with1 × 1 m resolution were generated in PDS 2000 by usingthe raw data of point clouds. High-resolution data candisplay much more details of bridge scour, allowingmore accurate quantification of scour depth and length(Fig. 2 and Table 4). The results showed that the scourmorphology of the studied bridges in the lower, middle,and upper YRE showed different patterns. For the rect-angular pile cap bridge foundations, the riverbed mor-phology of the Shanghai YRB in the lower YRE had along erosional ditch distributed on both sides of thebridge foundation while two long-strip siltation areaswere distributed on the upstream and downstream of thebridge foundation (Fig. 2a). However, in the middleYRE, the riverbed morphology of the DashengguanYRB only showed erosional morphology without asiltation area surrounding the bridge foundation (Fig.2b). Furthermore, the maximum depth of scour sur-rounding the Dashengguan bridge foundation was about8.8 m, while the maximum depth of the Shanghai YRBwas only half (4.4 m) (Table 4). The longitudinal scour-ing depths at the Shanghai YRB and the DashengguanYRB were approximately 316 and 518 m (Table 4),respectively.

For the cylindrical cofferdam bridge foundations, thescour morphology shows a horseshoe shape in the mid-dle and upper YRE (Fig. 2c, d). The extent of thelongitudinal scouring was approximately 395 m at theSecond Nanjing YRB but was only about 227 m at the

Table 3 Changes in channel bathymetry near the four majorbridges in the lower (A), middle (B and C), and upper (D) YangtzeRiver Estuary, China. The letters A–D denote the river reach of theShanghai YRB, the Second Nanjing YRB, the DashengguanYRB, and the Tongling YRB, respectively. The erosion,

deposition, and erosion/deposition ratio (E/D rate) were calculatedusing the 2009–2013 bathymetric survey data for the ShanghaiYRB reach and the 1998–2013 bathymetric survey data for theother three YRB reaches. Erosion and deposition percentages werecalculated based on the area, and positive values indicate erosion

Erosionvolume × 106 m3

Depositionvolume × 106 m3

Erosionpercentage

Depositionpercentage

E/D rate Width Length

A 197.6 50.6 80 20 + 0.28 8.3 18.0

B 13.1 9.5 58 42 + 0.03 1.3 7.0

C 26.2 6.6 94 6 + 0.33 1.4 9.0

D 17.1 3.1 85 15 + 0.15 1.3 5.0

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Tongling YRB. The maximum depth of the scour wasabout 19.0m surrounding the Second Nanjing YRB, butwas at 15.4 m surrounding the Tongling YRB (Table 4).

Discussion

The flood-ebb tide process can change flow conditionsin terms of velocity, direction, and flow depth (Edgeet al. 1998, (Noormets et al. 2006). The highest tidelevel at Niupi Jiao (approximately 640 km downstreamthe Datong station) has been reported to be 4.5 m, whilethe highest tide at Jiangyin station (415 km downstreamof Datong, Fig. 3) has been reported to be 3.1 m. How-ever, the reaches below Jiangyin still experience theflood-ebb tide process while the upstream only has atide level change (Fig. 3). Our study shows that theflood-ebb tide process can cause significant variation

in bridge scour morphology. For example, the ShanghaiYRB and the Dashengguan YRB have similar bridgefoundations and flow depth (Table 1). Due to the Shang-hai YRB being significantly influenced by the flood-ebbtide process, where the preferential flows of ebb tideaccounted for approximately 66.5 to 79.2% (Lu et al.2016), its bridge scour showed two long-strip siltationareas distributed on the upstream (~4.3 m) and down-stream (~4.4 m) of the bridge foundations (Table 4). TheDashengguan YRB being located in the Nanjing Water-way (the highest tide level 1.2 m), 205 km upstream ofJiangyin, it is only influenced by the tide level change(Dai and Liu 2013) and the scour morphology showedno siltation area surrounding the bridge foundation.

In the flood-ebb tide-affected channels, the bridgescour showed two long-strip siltation areas distributedon the upstream and downstream of the bridge founda-tion (Fig. 2a). This phenomenon may have been a result

Fig. 2 Bridge scour morphology of the Shanghai YRB (a), theDashengguan YRB (b), the Second Nanjing YRB (c), and theTongling (d) in the Yangtze River Estuary with 1-m spacing ofdepth contours. The reference water levels are the water surfaceunder a river discharge of ~ 58,000 m3/s on June 25, 2015 (a) andof 23,100~28,000 m3/s during September 11–18, 2016 (b–d). Thevalues of white triangles show the locations with maximum waterdepths in the bridge scour scope, and the black triangle in a shows

the location of the water depth of the top point in the siltation area.The values near the white dots are the average water depths of theriverbed surface which was not affected by the bridge scour. Themaximum bridge scour depth was calculated as the differencebetween the maximum water depth and the average depth of theriverbed surface. All depth values in this figure were calculatedaccording to the reference water levels

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of the periodic change in flow velocity and direction. Asimilar situation has been reported by Vasquez andWalsh (2009) who simulated a three-pile group scourunder tidal flow using a three-dimensional hydrodynam-ic modeling. During the ebb tide, the riverbed surround-ing the bridge foundation is eroded by the downflowand horseshoe vortex, while a long-strip siltation areaformed downstream of the bridge due to the wake (Luet al. 2016). Similar to that, another long-strip siltationarea developed at the upstream of the bridge foundationduring the flood tide (Lu et al. 2016). However, in the

river reach without flood-ebb tide influence, the bridgescour morphology is different, i.e., there is no siltationarea surrounding the Dashengguan YRB foundation(Fig. 2b).

The development of very large dunes can significant-ly change the scour morphology. A previous study hasshown that dunes are of great significance to scour depthsurrounding bridge foundations (Noormets et al. 2006).The Tongling YRB and Second Nanjing YRB are lo-cated in the upper and middle reaches of the YRE,which can be influenced by tide only during low-flow

Table 4 Characteristics of riverbed scour surrounding the piers of the four bridges in the lower (A), middle (B and C), and upper (D)Yangtze River Estuary, China

Surveyed Max. SD (m) Max. DH (m) FD (m) Scour length(m) SD/D

Parallel Perpendicular

A 2015.06.25 4.4 4.3 16.5 316 137 0.11

B 2016.09.12 8.8 – 15.2 518 103 0.23

C 2016.09.11 19.0 – 32.3 395 214 0.53

D 2016.09.18 15.4 – 26.5 227 162 0.50

A Shanghai YRB, B Dashengguan YRB, C Second Nanjing YRB, D Tongling YRB, Max. SD maximum depth of the scour, Max. DH

maximum deposition height surrounding the bridge foundation, FD flow depth at the upstream of the bridge foundation, SD/D ratio of thescour depth and bridge foundation diameter

Fig. 3 Information on tide level and themaximum depth of bridgescours. The gray dashed line is the dividing line of the unidirec-tional flow and bidirectional flow. The river reach downstreamJiangyin has bidirectional flow influenced by flood-ebb tide pro-cess while the river reach upstream has unidirectional flow mainlyinfluenced by river discharge. Red triangles are the maximum

scour depth at the four studied bridges. Blue diamonds are thehighest tide level observed at the hydrological and tide stations.Tide level data for Datong and Nanjing were collected from thehydrological stations of Datong and Nanjing (Xu et al. 2012). Tidelevel data downstream of Jiangyin came from several local tidestations

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periods. The bridge foundations are similar in size andtype, and the scour morphology has a horseshoe pattern(Fig. 4); thus, they can be used to study how very largedunes affect the bridge scour morphology. The multi-beam data in our study show that very large dunes with awavelength greater than 122 m and a height greater than4.9 m developed on the riverbed of the Second NanjingYRB (Fig. 4a), indicatingmore complex scour morphol-ogy at the Tongling YRB (without very large dunesdeveloping in the bridge scour). At the same time,cross-sections parallel or perpendicular to the flow alsoshow the same results (Fig. 4).

Previous studies have reported that SSC plays asignificant role on the equilibrium of scour depth.Low SSC may lead to an increase of the time andmaximum scour depth around a bridge foundation(Sheppard et al., 2004). The SSC of the YangtzeRiver has largely declined since the late 1980s,resulting in a sharp reduction in the river’s sedimentyield, especially after the completion of the ThreeGorges Dam (TGD) (Fig. 5). As a highly SSC-ladenriver in the world, the YR used to have an averageSSC of 0.62 kg/m3 between 1956 and 1970,

followed by a reduced SSC (0.42 kg/m3) between1971 and 2002 (Dai and Liu 2016). In the pastdecade, the river showed a sharp decline in SSC,with an average of only about 0.18 kg/m3 between2003 and 2013 (Dai et al. 2016). It is difficult toquantify the effect of this reduction on the intensityof bridge scour and riverbed deformation. However,our results indicate that the reduced sediment loadmay be responsible for excessive bridge scour andriverbed erosion in the upper and middle YRE, aswell as for an insufficient deposition downstreamthe Shanghai YRB in the relatively strongly tidal-affected channel.

The YR has distinct seasonal and secular variabil-ity in river stage and discharge (Dai and Liu 2013).During 2014 and 2016, the river stage fluctuatedfrom 4.5 to 15.6 m, corresponding to a large flowvariation from 1.0 to 6.9 × 104 m3/s at Datong (Fig.5). Different river stage and discharge lead to thescour morphology constantly adjusting (Sheppardet al. 2004, Noormets et al. 2006). The river stageand discharge were also affected by the operation ofthe TGD. By the impounding and releasing of water,

Fig. 4 Bridge scour morphologyand cross-sectional profiles of theSecond Nanjing YRB (a) and theTongling YRB (b). A–A′ profilecrossed the large dunes whilethere were no large dunes thatdeveloped along the C–C′ profile.B–B′ and D–D’ profiles were thescour morphology on theupstream of the bridgefoundations

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the TGD increased the water discharge in the dryseason and reduced the flood peak discharge (Guoet al. 2012). Therefore, the natural seasonal and in-ternal variability of discharge and operation of TGDin the dry/flood season will continue to influence thescour morphology and scour depth in the future.

Channel erosion is a common problem in rivers thatbridge foundations have to experience during theirdesign life (Keshavarzi and Noori 2010, Johnsonet al. 2015). Even though bridge foundations can leadto local riverbed erosion, which may be a threat to thebridge safety, the channel erosion in a large scale inthe long run by nature and human activities also is oneof the most important factors for bridge safety in theYRE. The multi-beam data showed that the scourdepth surrounding the bridge foundations could prog-ress up to 4.4–19.0 m in the studied bridges. However,the river regime and cross-sections (Fig. 6) at theupstream and downstream (100 m away from thebridge foundation) of each bridge segment showedthat most of the riverbed at the main bridge founda-tions experienced erosion in recent years. For instance,the main channel of the Shanghai YRB segment hasexperienced a notable erosion from 2009 to 2013 (Fig.6a, b), and approximately 2.0–2.5 and 2.5–3.0 m ofthe riverbed were scoured at the upstream and down-stream of the Shanghai YRB, respectively (Fig. 7a, b).These results also occurred at the Tongling YRB seg-ment where about a depth of 1.0–2.0 and 4.5–5.5 m ofriverbed were eroded at the up- and downstreams of

the bridge foundation (Figs. 6g, h and 7g, h). Althoughthe studied bridge foundations at Dashengguan YRB(built in 2011) showed little erosion from 1998 to2013, the riverbed surrounding the two main bridgefoundations was eroded by about 15.0–17.0 m (Fig.7e, f). It is worth noting that although the SecondNanjing YRB river segment has experienced minorerosion (0.03 m/year), the up- and downstreams ofthe bridge foundations were scoured by 5.5–6.0 and3.5–4.0 m, respectively (Fig. 7c, d). Although localscour surrounding bridge foundations was part of thechannel erosion, the channel erosion in a large scalecan be considered as not being affected by bridgefoundations. As a result, the long-time channel erosioncould reach up to 15.0–17.0 m in the YRE which isalso one of the most important factors for bridgesafety.

Monitoring bridge scour is widely conducted byengineers to assess bridge foundation conditions. Cur-rent monitoring procedures focus on the bridge scour,lacking the ability to observe wide riverbed areas near abridge on operational time scales. It is, however, impor-tant that wide areas are observed up- and downstream ofthe bridges, especially in an estuarine channel whereerosional and depositional patterns in the wide areascan directly respond to the bridge foundations and theirerosion. Overall, this study demonstrates that the three-dimensional sonar profiling provides high-resolutionmulti-beam data over a wide area, which can be veryuseful to assess bridge scour and riverbed deformation.

Fig. 5 River stage and dischargeat the Datong station from 2015 to2017 (a) and river discharge andsuspended sediment load (SSL) atthe Datong station from 1953 to2013 (b). The long-termsuspended sediment load andriver discharge were obtainedfrom the Yangtze River SedimentBulletin (published onhttp://www.cjh.com.cn). TGD theThree Gorges Dam. The 2-yearriver discharge and water stagerecords (2015–2016) wereobtained from the Water Bureauof the Yangtze River Commission(data are available on http://www.cjxysw.com.cn)

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When compared with other monitoring approaches,such as using traditional sounding line or single-beamdata, the multi-beam sonar profiling can efficiently

provide higher resolution and more precise point dataof bridge foundations and nearby areas with greatscouring.

Fig. 6 Digital elevation model(DEM) of the river channel nearthe Shanghai YRB in 2009 (a)and 2013 (b). DEM of the riverchannel near the Second NanjingYRB in 1998 (c) and 2013 (d).DEM of the river channel near theDashengguan YRB in 1998 (e)and 2013 (f), and DEM of theriver channel near the TonglingYRB in 1998 (g) and 2013 (h) inthe YRE

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Fig. 7 Cross-sectional change of the river channels at the up-stream (a) and downstream (b) of the Shanghai YRB, at theupstream (c) and downstream (d) of the Second Nanjing YRB,at the upstream (e) and downstream (f) of the Dashengguan YRB,

and at the upstream (g) and downstream (f) of the Tongling YRBin the YRE. Cross-sections at the upstream or downstream of thesebridges are 100 m away from the bridges

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Conclusions

This study used multi-beam echo sounders to surveythe riverbed along four major bridges in the upper,middle, and lower Yangtze River Estuary duringJune 2015 and September 2016. The high-resolutiondata allowed detailed mapping of the river reaches,demonstrating the great usefulness of the techniquesin assessing bridge scours and riverbed deformationpatterns near the bridges. In addition, previous bathy-metric measurements in 1998, 2009, and 2013 wereutilized to determine longer-term riverbed erosionand deposition at the bridge reaches. This studyfound large scour depths at the bridge foundationsup to 19 m and extensive erosion of the riverbed nearthe bridges up to 0.33 m/year in recent years. Theflood-ebb tide seemed to have resulted in significantvariation in the scour morphology in the lower andmiddle YRE. In the lower YRE, the scour morphol-ogy and riverbed erosion were clearly reflected bytidal influence, displaying long erosional ditches dis-tributed on both sides of the bridge foundation andlong-strip siltation areas on the upstream and down-stream of the bridge foundations. In the middle ofYRE, the riverbed morphology only showed erosion-al morphology surrounding the bridge foundations.Very large dunes can significantly change the scourmorphology, leading to steeper contours of riverbedscour. Due to the operation of the Three Gorges Damupstream of the YRE, sediment inflow into the YREwill unlikely change in the future, and insufficientsediment offsetting the channel erosion may becomea serious problem for bridge safety in the YRE.

Acknowledgements During the preparation of this manuscript,Shuwei Zheng was supported by an award of the China Scholar-ship Council (File No. 201606140126). We would also like tothank an anonymous reviewer for reviewing the manuscript andofferingmany helpful suggestions, which have helped improve thequality of this paper.

Funding information This study was financially supportedthrough a grant from the Natural Science Foundation of China(Grant No. 41476075) and a grant from the Impact of MajorProjects on the geological environment of the Yangtze River(Grant No. DD20160246). The study also benefited from a USDepartment of Agriculture Hatch Fund project (Project No.LAB94230). The statements, findings, and conclusions are thoseof the authors and do not necessarily reflect the views of thefunding agencies.

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