Understanding Morphodynamic Changes of a Tidal River ... - MDPI

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Understanding Morphodynamic Changes of a Tidal River Confluence through Field Measurements and Numerical Modeling Qiancheng Xie 1, * , James Yang 2,3 , Staffan Lundström 1 1 2 3 4

*

and Wenhong Dai 4

Division of Fluid and Experimental Mechanics, Luleå University of Technology (LTU), 97187 Luleå, Sweden; [email protected] Division of Resources, Energy and Infrastructure, Royal Institute of Technology (KTH), 10044 Stockholm, Sweden; [email protected] Vattenfall AB, Research and Development (R & D), Älvkarleby Laboratory, 81426 Älvkarleby, Sweden College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China; [email protected] Correspondence: [email protected]; Tel.: +46-72-2870-381

Received: 20 August 2018; Accepted: 8 October 2018; Published: 11 October 2018

Abstract: A confluence is a natural component in river and channel networks. This study deals, through field and numerical studies, with alluvial behaviors of a confluence affected by both river run-off and strong tides. Field measurements were conducted along the rivers including the confluence. Field data show that the changes in flow velocity and sediment concentration are not always in phase with each other. The concentration shows a general trend of decrease from the river mouth to the confluence. For a given location, the tides affect both the sediment concentration and transport. A two-dimensional hydrodynamic model of suspended load was set up to illustrate the combined effects of run-off and tidal flows. Modeled cases included the flood and ebb tides in a wet season. Typical features examined included tidal flow fields, bed shear stress, and scour evolution in the confluence. The confluence migration pattern of scour is dependent on the interaction between the river currents and tidal flows. The flood tides are attributable to the suspended load deposition in the confluence, while the ebb tides in combination with run-offs lead to erosion. The flood tides play a dominant role in the morphodynamic changes of the confluence. Keywords: tidal river confluence; flow features; morphological changes; field measurements; numerical simulations

1. Background A river confluence is a key feature of a drainage basin in terms of hydrology and geomorphology, for geological records, as well as from a habitat point of view [1]. In the confluence, two merging run-off streams often result in enhanced turbulent mixing. This has a bearing on transported sediment and its amount delivered downstream. In a long-term perspective, the flow patterns govern the morphology changes of the confluence, e.g., the scouring and sediment deposition [2]. Many studies focused on hydrodynamic patterns and morphology changes in run-off confluences. Mosley [3] was a pioneer in the research, which was further developed by Best who defined six distinct hydraulic regions in a confluence [2,4]. Those included areas of flow stagnation, flow deflection, flow separation, maximum velocity, gradual flow recovery, and shear layers. With advanced instrumentation and novel experimental design, research of river confluences evolved and focused on the separation zone and the shear layer [5–8]. Yuan et al. [9] made a review of the state of the art in hydraulic research of run-off confluences. Best [2] examined principal morphological features Water 2018, 10, 1424; doi:10.3390/w10101424

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such as deep scour holes, bank-attached lateral bars, tributary-mouth bars, and a region of sediment accumulation in a river confluence. Other similar studies looked at the sediment and morphological aspects at non-tidal alluvial confluences [10–12]. They contributed to the understanding of the confluence scouring. A literature review shows that limited attention is drawn to tidal confluences with bi-directional flows [4]. In tidal environments, the confluence is also affected by tidal currents. As a result, the alluvial process in terms of erosion and deposition is different, which is an issue of concern for many practical applications, especially if the confluence is in an urban development area. Pittaluga et al. [13] investigated the morphodynamic equilibrium of alluvial estuaries, where river flows and tides meet. The complexity in a tidal confluence does not draw much attention [4,14,15]. In bi-directional flows, the shift of the dominant processes between river run-off and tides, featuring periodical changes in both magnitude and direction, induces more degrees of complexity in terms of flow patterns, sediment transport, and bed morphology change in the confluence. Ferrarin et al. [16] identified 29 scour holes at tidal channel confluences by examining their geomorphological characteristics and comparing them with scours in rivers. As a consequence of changes in the flow regime, their findings revealed, on a century scale, the morphological dynamics of scouring. Field studies [10–12] and laboratory experiments [17–20] are major tools for studying sediment transport and the hydromorphic process in a confluence. In some studies, field measurements were made with tides; their purpose was to examine transport of the bed load [21–23]. The study of suspended load in tidal confluences is limited. One reason is attributable to the fact that it is not easy to, in a controlled manner in the laboratory, produce flow conditions of suspended load in combination with tides. Due to the complexity of the process, erosion and deposition are still not well understood [18]. Hypotheses are usually made that the origin of mid-stream scour is related to high flow velocity, strong turbulence, and the effect of shear layer or curvature-induced helical circulation [2,18–20]. Numerical modeling allows duplications of complex boundary conditions and prediction of different scenarios in a short time [24–29], which is also true for the study of tidal river confluences. Previous attempts were made with three-dimensional (3D) models for simulations of secondary circulations and flow variations in the vertical direction [30,31]. For a shallow confluence with a large width-to-depth ratio, a depth-averaged model is an acceptable compromise if necessary corrections of cross-circulatory motion are made [32–35]. In this research, field measurements of flow and sediment were made in the study area including the confluence. A two-dimensional (2D) morpho-dynamic model was set up, in which the cohesive suspended sediment transport was simulated. The objective was, by means of field measurements and simulations, to provide insight into the physical phenomenon that governs flow features of the tidal confluence, to describe circulatory patterns of suspended load transport, and to predict the scour-hole evolution. This study reveals the relationship between the velocity and suspended sediment movement influenced by both the run-off and the tides. The results provide reference to behaviors of tidal currents, sediment transport patterns, and fluvial process in similar situations. The paper includes a description of the study area, field measurements of flow and sediment and data analyses, numerical formulation, model set-up with calibration and validation, major flow features, and morphological changes of the confluence. 2. Study Area The confluence in question is called Sanjiangkou, formed by the Fenghua and the Yao River in southeast China. The stream after the confluence is called the Yong River. It is approximately 26 km upstream from the mouth into the Pacific Ocean (Figure 1). The Yao River runs into the junction roughly at a 90◦ angle with the other two rivers. The confluence is significantly affected by combined actions of river run-off and tidal currents. The bed load is negligibly small; the sediment transport in the rivers is mainly in form of suspended load, a common feature of many fluvial rivers [36].

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Measured at the normal water surface, the width of the Fenghua River ranges from 90 to 180 m, Measured the normal surface, the width Fenghua River ranges from 90its to 180 m, and its and its averageatbed slope iswater 0.81%. The reach near of thethe confluence is almost straight; cross-section average slope is The 0.81%. Theaveraged reach nearrun-off the confluence straight; its cross-section is nearlybed U-shaped. yearly is 53.6 m3is/s;almost the annual sediment dischargeisisnearly 4.35 × 3 4 4 U-shaped. The yearly averaged run-off is 53.6 m /s; the annual sediment discharge is 4.35 × 10 tons [36]. 10 tons [36]. The normal width of the Yao Yao River m before before the the confluence, confluence, River ranges ranges from from 180 to 230 m. About 500 m a local constriction exists, with its water-surface width expanding from 140 to 180 m at at the the confluence. confluence. The daily daily run-off, run-off, as well well as as sediment sediment transport, transport, is controlled by the sluice sluice gates located 3.3 km km upstream upstream of of the the confluence; confluence; the the annual annual sediment sediment discharge dischargeisiscomparatively comparativelysmall. small.

Figure The water water system, system, locations locations of of the the river river confluence, confluence, and hydrological stations Figure 1. 1. The and hydrological stations of of water water levels levels (WL) and of flow and sediment (FS). (WL) and of flow and sediment (FS).

The The Yong Yong River River runs runs roughly roughly in in the the west–east west–east direction, direction, with with its its normal normal width width ranging ranging from from 150 150 3 /s to 250 m. The average river-bed slope is 0.117%. Its annual mean run-off is approximately 92 m to 250 m. The average river-bed slope is 0.117%. Its annual mean run-off is approximately 92 m3/s (annual 1099 m m33).). The The peak peak discharge discharge of of run-off run-off and and tides (annual average average run-off run-off 2.912 2.912 × × 10 tides occurs occurs normally normally 3 /s. The field recording stations for river during the second half of June, amounting to about 1800 m during the second half of June, amounting to about 1800 m3/s. The field recording stations for river water levels (WL) (WL) and andof offlow flowand andsediment sediment (FS) also marked in Figure 1. the At river the river mouth, water levels (FS) areare also marked in Figure 1. At mouth, e.g., e.g., at station WL6, the annual mean tidal range is 1.91 m; the flood and ebb durations are almost at station WL6, the annual mean tidal range is 1.91 m; the flood and ebb durations are almost the the same, approximately 6 h. The tidal asymmetry aggregates from the river mouth to the upstream. In the same, approximately 6 h. The tidal asymmetry aggregates from the river mouth to the upstream. In confluence, the ebb is 40 min than the flood At station theWL7, duration the confluence, the duration ebb duration is 40longer min longer than theduration. flood duration. AtWL7, station the difference becomes 60 min [36]. On the Yao River, the sluice gates stop the tidal propagation further duration difference becomes 60 min [36]. On the Yao River, the sluice gates stop the tidal propagation upstream. As for the location of the upstream limit oflimit current reversals along the Fenghua River, further upstream. As for the location of the upstream of current reversals along the Fenghua the tides affect approximately 15 km upstream of WL7/FS7; no records are available to show the River, the tides affect approximately 15 km upstream of WL7/FS7; no records are available to show run-off influence. The sediment in the river is mainly from the coastal area and is carried by the tides. the run-off influence. The sediment in the river is mainly from the coastal area and is carried by the tides.Local scouring is a typical morphological feature of the river confluence. Our concern of the bed morphology is is itsa scour-hole evolution (Figure Previous field measurements show that a Local scouring typical morphological feature 1). of the river confluence. Our concern of the bed scour hole, like a narrow andevolution deep crater along1). thePrevious Fenghua–Yong River, exists show in thethat confluence morphology is its scour-hole (Figure field measurements a scour and its deepest point is more than 10 m below the confluence bed elevation. During the earlier years, hole, like a narrow and deep crater along the Fenghua–Yong River, exists in the confluence and its scouring dominated the river sedimentation process. Since the 1980s, the study area was affected by deepest point is more than 10 m below the confluence bed elevation. During the earlier years, ascouring numberdominated of factors, the suchriver as the constructionprocess. of the sluice on the River and other human sedimentation Since gates the 1980s, theYao study area was affected by activities. These changes modified the hydraulic conditions and affected the erosion potential in the a number of factors, such as the construction of the sluice gates on the Yao River and other human water system, including the confluence. Bathymetric surveys, irregular fragmental, activities. These changes modified the hydraulic conditions and although affected the erosionand potential in the show that the morphology tends to shift from erosion to deposition.

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water system, including the confluence. Bathymetric surveys, although irregular and fragmental, show that the morphology tends to shift from erosion to deposition. 3. Field Measurements 3. Field Measurements 3.1. Data Collection 3.1. Data Collection To map the river and confluence topography and to record the tidal hydrological data, field surveys were the carried during two major periods, i.e., 2015 and 2016. The former was To map riverout and confluence topography andJune to record the January tidal hydrological data, field used for were this study. The river bathymetry used in the simulations was mapped from June 2015–January surveys carried out during two major periods, i.e., June 2015 and January 2016. The former was 2016, for which achieved using an HY1600 bathymetric profiler (SunNav Technology Co.,2015‒ Ltd., used this was study. The river bathymetry used in the simulations was mapped from June Tianjin, The hydrological included waterbathymetric level, flow velocity, flow discharge, sediment January China). 2016, which was achieveddata using an HY1600 profiler (SunNav Technology Co., concentration, grain-size distribution, water and water salinity. Ltd., Tianjin, China). The hydrological dataquality, included level, flow velocity, flow discharge, The water levels were monitored at seven cross-sections sediment concentration, grain-size distribution, water quality,(WL1–WL7), and salinity. five of which were along the Yong measure velocity suspended sediment, seven corresponding cross-sections The River. water To levels were flow monitored atand seven cross-sections (WL1‒WL7), five of which were along (FS1–FS7) were arranged, eachflow withvelocity three plumb lines (Figure 2). Theirseven distances to the confluence the Yong River. To measure and suspended sediment, corresponding cross(measured along the riverarranged, centerline) are with giventhree in Table 1. FS5 and FS6 are2).a few meters aparttofrom sections (FS1‒FS7) were each plumb lines (Figure Their distances the each other and are treated as the same section. Along each line, the sampling was made six depths confluence (measured along river centerline) are given in Table 1. FS5 and FS6 are aatfew meters from the water i.e.,are h =treated 0, 0.2Has 0.4H 1.0Hline, H0 (m) iswas the water apart from eachsurface, other and same section. Along the sampling made 0 , the 0 , 0.6H 0 , 0.8H 0 , andeach 0 , where depth at eachfrom line. All data were recorded one-hour at six depths the the water surface, i.e., h = 0,in0.2H 0, 0.4Hintervals. 0, 0.6H0, 0.8H0, and 1.0H0, where H0 (m) is the water depth at each line. All the data were recorded in one-hour intervals. Table 1. Distance of field measurement stations to the confluence.

Table 1. Distance of field measurement stations to the confluence. Water Level Station WL1 WL2 WL3 WL4 WL5 WL6

Water Level Station WL1 0.25 WL2 6.00 WL3 14.80 WL4 20.30 WL5 25.20 WL6 Distance to confluence (km) 2.20 Distance to confluence (km) 2.20 0.25 6.00 14.80 20.30 25.20 Flow and Sediment Station FS1 FS2 FS3 FS4 FS5 (FS6) Flow and Sediment Station FS1 FS2 FS3 FS4 FS5 (FS6) Distance to confluence (km) 2.20 0.25 9.70 18.10 24.70 Distance to confluence (km) 2.20 0.25 9.70 18.10 24.70

WL7

WL7 2.80 2.80 FS7 FS7 3.20 3.20

With aa four-beam RDI Workhorse Workhorse acoustic acoustic Doppler Doppler current current profilers profilers (ADCPs) (ADCPs) With four-beam 600/1200-kHz 600/1200-kHz RDI (Nortek They were (Nortek group, group, Rud, Rud, Norway), Norway), water-flow water-flow velocities velocities and and discharges discharges were were measured. measured. They were attached to the measurement vessels that were anchored on land. The uncertainty of the flow attached to the measurement vessels that were anchored on land. The uncertainty of the flow measurements was was below 5%.The TheYJD-1-type YJD-1-typepressure pressuresensors sensors(Tekscan, (Tekscan, Inc., Inc., South South Boston, Boston, MA, MA, measurements below ± ±5%. USA) were used for the water-level measurements and their accuracy is ± 1 cm. Major efforts were USA) were used for the water-level measurements and their accuracy is ±1 cm. Major efforts were made to to sample sample the the suspended suspended sediment, sediment, using using point-integrative point-integrative water water samplers samplers (Hoskin (Hoskin Scientific, Scientific, made Ltd., Saint-Laurent, Saint-Laurent, QC, QC, Canada). Canada). Samples Samples of of the the bed bed load, load, although although limited limited in in amount, amount, were were also Ltd., also taken with Shipek grab samplers (Envco, Auckland, New Zealand) and their amount was calculated. taken with Shipek grab samplers (Envco, Auckland, New Zealand) and their amount was calculated.

Sketch of measurement arrangement at a cross-section. Figure 2. Sketch

The grain-size distribution of the suspended load was analyzed using an automatic sieving Scientific Ltd.,Ltd., Nanjing, China) and anand automated laser particle-size analyzer device (SFY-D) (SFY-D)(Zhonghu (Zhonghu Scientific Nanjing, China) an automated laser particle-size (Mastersizer2000) (Malvern(Malvern Panalytical Ltd., Worcestershire, UK). Particle falling in the range analyzer (Mastersizer2000) Panalytical Ltd., Worcestershire, UK).sizes Particle sizes falling in between 0.0002 and 2 mm were identified. The obtained data are well suited for calibration and the range between 0.0002 and 2 mm were identified. The obtained data are well suited for calibration validation of numerical models. The field duringduring the wet season, i.e., thei.e., second half of and validation of numerical models. The data field acquired data acquired the wet season, the second June 2015, were analyzed to determine the sediment features in the study area including the confluence. half of June 2015, were analyzed to determine the sediment features in the study area including the They are also usedare foralso calibration validation. confluence. They used forand calibration and validation.

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3.2. Features of Suspended Sediment For each WL and FS station, the time series of the raw measurement data were analyzed. For each time period, the average value was first obtained for each plumb line with the six points. Based on the results of the three lines, the cross-sectionally averaged value was achieved using the weighted average method. According to the grain-size distribution, the sediment is classified as sand (0.05–2 mm), silt (0.005–0.05 mm), and clay (0.5

>0.5

Equation 2 n ∑i=1 (Oi − Pi ) ∑ (

E = 1−

𝐸 = 1∑−in=1∑(Oi −(O)2

) )

R2 =

R2

R2

1

1

>0.5 >0.5

n

n ∑ i =1

PBIASPBIAS

0

0

25% for rate flow rate ±25% ± for flow ± 55% for sediment ±55% for sediment

∑

(

(Oi∑−O)(

n ∑i=)1

!2

)(

𝑅q = ∑i=1 (Oi −2Oq)( Pi − P)

)

2

( P∑i − P)(

)

n ( Pi )×100 ) ∑i=∑ 1 (Oi − n ∑i=1∑Oi

PBI𝑃𝐵𝐼𝐴𝑆 AS = =

The river-bed river-bedroughness roughness(n) (n)isisa akey keyparameter parameter concern that is governed by factors, as The of of concern that is governed by factors, suchsuch as the the river-bed morphology, patterns, etc. Based trial anditserror, range wassetfinally set river-bed morphology, flow flow patterns, etc. Based on trialonand error, rangeitswas finally between between with 0.015–0.018 forchannel the main and for 0.018–0.030 the shore beach. 0.015 and0.015 0.030,and with0.030, 0.015–0.018 for the main andchannel 0.018–0.030 the shorefor beach. Figures 9–11 Figures the final calibration results of V FS3, and Sand at FS2, show the9–11 finalshow calibration results of V and S at FS2, FS4. FS3, and FS4.

Figure 9. Model calibration, numerical vs. field results at FS2 (spring tide, June 2015): (a) ||𝑉|; V |; (b) (b) S. S.


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Figure 10. Model calibration, numerical vs. field results at FS3 (spring tide, June 2015): (a) |𝑉|; (b) S. Figure10. 10.Model Modelcalibration, calibration,numerical numericalvs. vs.field fieldresults resultsatatFS3 FS3(spring (springtide, tide,June June2015): 2015):(a) (a)|𝑉|; |V |;(b) (b)S.S. Figure

Figure Model calibration, numerical field results FS4 (spring tide, June 2015): |V |(b) ; (b)S.S. Figure 11.11. Model calibration, numerical vs.vs. field results atat FS4 (spring tide, June 2015): (a)(a)|𝑉|;

|𝑉|; (b) S. were Figure 11. calibration, vs. field results at FS4 (spring tide, June 2015): (a)differences ForV,V,the theModel computed resultsnumerical were thethe measured ones; the For computed results were in ingood goodagreement agreementwith with measured ones; the differences negligibly smallsmall for all stations. For S,For despite certaincertain discrepancies between them, the matches were negligibly forthe all the stations. S,agreement despite discrepancies between them, the For V, the computed results were in good with the measured ones; the differences were generally satisfactory. The four S peaks within the tidal period were reasonably reproduced, matches were generally The four S peaks tidal periodbetween were reasonably were negligibly small forsatisfactory. all the stations. For S, despitewithin certainthe discrepancies them, the in terms of both magnitude and phase. Forand FS2,phase. FS3, and FS4, Table 6 shows theTable calibration results reproduced, in terms of both magnitude For FS2, FS3, and FS4, 6 shows theof matches were generally satisfactory. The four S peaks within the tidal period were reasonably the error parameters. All the values fell within the ranges required by the criteria. calibration results of theof error parameters. Alland the values the ranges required criteria. reproduced, in terms both magnitude phase. fell Forwithin FS2, FS3, and FS4, Tableby6 the shows the

calibration results of the error parameters. the values fell within the ranges by the criteria. Table 6. Error estimationsAll of model calibration (spring tide, June required 2015). Table 6. Error estimations of model calibration (spring tide, June 2015).

FS2 FS3 (spring tide, June 2015). FS4 Table 6. Error estimations of model calibration FS2 FS3 FS4

Parameter Parameter

3

3

3

(m/s) SS(kg/m V (m/s) (m/s) SS(kg/m SS(kg/m VV(m/s) (kg/m)3) VV(m/s) (m/s) FS4 (kg/m) 3) FS2 (kg/m3)) V FS3 Parameter NSE (>0.50) V0.96 0.96 0.82 3) 0.95 NSE (>0.50) 0.82 0.77 3) 0.93 0.76 3) (m/s) S (kg/m V0.95 (m/s) S0.77 (kg/m V0.93 (m/s) S0.76 (kg/m 0.97 0.80 0.95 0.79 0.94 0.78 R2 (0–1) 2 (0–1) R 0.97 0.80 0.95 0.79 0.94 0.78 NSE PBIAS (>0.50) 0.96 0.82 0.95 0.77 0.93 0.76 PBIAS (±25%) 1.23 −12.57 2.05 −2.86 3.56 8.80 1.23 − 12.57 2.05 − 2.86 3.56 8.80 2 R ((0–1) 0.97 0.80 0.95 0.79 0.94 0.78 ±25%) PBIAS (±25%) 1.23 −12.57 2.05 −2.86 3.56 8.80 5.2. Model Validation 5.2. Model Validation 5.2. Model Validation The model was validated against a neap tide that occurred during a 31-h period between 3:00 The model was validated against a neap tide that occurred during a 31-h period between 3:00 p.m. p.m. on 24model June and 10:00 p.m. on 25 June of thetide same year. In the validation, the procedure for result The occurred during a 31-h between 3:00 on 24 June and was 10:00validated p.m. on against 25 Juneaofneap the samethat year. In the validation, theperiod procedure for result evaluations was the same as for in the calibration. p.m. on 24 June onin25the June of the same year. In the validation, the procedure for result evaluations wasand the10:00 samep.m. as for calibration. The validation results arefor shown incalibration. Figures 12–14. The calculated V and S profiles matched well evaluations was the same as in the The validation results are shown in Figures 12–14. The calculated V and S profiles matched well with the measured data series. Table 7 shows the validation results ofVthe error parameters for the The results are shown Figures The results calculated and Sparameters profiles matched with the validation measured data series. Table 7 in shows the 12–14. validation of the error for the well three three stations. All the values met the requirements of the criteria. The model generated acceptable with the measured data met series. 7 shows of thethe validation results of the error parameters the stations. All the values theTable requirements criteria. The model generated acceptablefor results results and is suitable for prediction of flow and morphology changes of thegenerated area including the three stations. All the values met the requirements of the criteria. The model acceptable and is suitable for prediction of flow and morphology changes of the area including the confluence. confluence. results and is suitable for prediction of flow and morphology changes of the area including the

confluence.


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Figure 12. Model validation, numerical results vs. field data at FS2 (neap tide, June 2015): (a) |𝑉|; (b) S. Figure 12. Model validation, numerical results vs. field data at FS2 (neap tide, June 2015): (a) |𝑉|; (b) S. Figure12. 12.Model Modelvalidation, validation,numerical numerical results results vs. vs. field field data at FS2 (neap |; (b) Figure (neap tide, tide, June June2015): 2015):(a) (a)|V|𝑉|; (b)S.S.

Figure 13. Model validation, numerical results vs. field data at FS3 (neap tide, June 2015): (a) |𝑉|; (b) S. Figure13. 13.Model Modelvalidation, validation,numerical numerical results results vs. field field data at FS3 (neap |; (b) Figure (neap tide, tide, June June2015): 2015):(a) (a)|V|𝑉|; (b)S.S.

Figure 13. Model validation, numerical results vs. field data at FS3 (neap tide, June 2015): (a) |𝑉|; (b) S.

Figure14. 14.Model Modelvalidation, validation,numerical numericalresults results vs. vs. field field data data at at FS4 FS4 (neap |; (b) |𝑉|; Figure (neap tide, tide, June June 2015): 2015): (a) (a) |V (b)S.S. 7. Error estimations of model validation (neap tide,tide, JuneJune 2015). Figure 14. ModelTable validation, numerical results vs. field data at FS4 (neap 2015): (a) |𝑉|; (b) S. Table 7. Error estimations of model validation (neap tide, June 2015). Figure 14. Model validation, numerical results vs. field data 2015): (a) |𝑉|; (b) S. FS2 FS3at FS4 (neap tide, JuneFS4 Table 7. Error estimations of model validation Parameter FS2 FS3(neap tide, June 2015).FS4 Parameter V (m/s) V (m/s) S (kg/m3 ) 3 V (m/s) S (kg/m3 ) S (kg/m3 ) 3 3) (m/s)estimations ) V validation (m/s) FS3S(neap (kg/m V (m/s) S (kg/m ) Table 7.VError of model tide, June 2015). FS4 FS2S (kg/m NSE(>0.50) (>0.50) 0.96 0.83 0.940.94 0.81 0.81 0.97 0.97 0.78 0.78 Parameter NSE 0.96 0.83 V (m/s) FS2S0.84 (kg/m3) 0.93 V (m/s) FS3 S (kg/m3) 0.96V (m/s) FS4 S (kg/m3) 0.95 0.79 0.75 R2 (0–1) 2 (0‒1) R 0.95 0.84 0.93 0.79 0.96 0.75 Parameter NSEPBIAS (>0.50) 0.96 0.83 3) 0.94 0.81 3) V 0.97 0.78 3) V (m/s) −S12.99 (kg/m V3.05 (m/s) −5.63 S (kg/m (m/s) 9.66 S (kg/m 2.21 3.05 3.81 3.81 PBIAS (±25%) 2.21 −12.99 ‒5.63 9.66 2 25%) R (± (0‒1) 0.95 0.84 0.93 0.79 0.96 0.75

NSE (>0.50) 0.96 0.83 0.94 0.81 0.97 0.78 PBIAS (±25%) 2.21 −12.99 3.05 ‒5.63 3.81 9.66 2 R (0‒1) 0.95 0.84 0.93 0.79 0.96 0.75 6. 6. Typical Typical Flow Flow Features Features PBIAS (±25%) 2.21 −12.99 3.05 ‒5.63 3.81 9.66 6. Typical Flow to Features According the According to the statistics statistics of of the the whole-year whole-year water-level water-level data data at at WL6 WL6 during during 2015, 2015, the the tidal tidal frequency in the wet season (the 2nd half of June) was 10%, 40%, and 25% for the spring, mid, and 6. Typical Flow Features frequency in the wet season (the 2nd halfwhole-year of June) waswater-level 10%, 40%, and theduring spring,2015, mid, and According to the statistics of the data25% at for WL6 the neap tidal neap tides, respectively [45]. Under the combined action of the run-off and tides, the maximum flood tides, respectively [45]. Under the2nd combined of the run-off and tides, the maximum flood and frequency in thetowet season (the of action June) was 10%, 40%, and for the spring, According the statistics ofdominate thehalf whole-year water-level data at25% WL6 during 2015, mid, the flow tidal and ebb tides of the spring tides the sediment transport; they also characterize the ebb tides of the spring tides dominate the sediment transport; they also characterize the flow features neap tides,in respectively [45]. Under thehalf combined action of the40%, run-off and tides, thespring, maximum flood frequency the wet season (the 2nd of June) was 10%, and 25% for the mid, and features and they were selected to show the results. and they were selected to show the results. tides of the spring the sediment theflood flow neapebb tides, respectively [45]. tides Underdominate the combined action oftransport; the run-offthey and also tides,characterize the maximum features they were selected todominate show the results. and ebb and tides of Fields the spring tides the sediment transport; they also characterize the flow 6.1. Tidal Current Tidal features and they were selected to show the results. 6.1. Tidal Current Fields flood tide from the Yong River approached the confluence, the confluence When the maximum flow saw Current an increase in magnitude. The flow field at the maximum flood tide is shown in Figure 15a. 6.1. Tidal Fields

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When the maximum flood tide from the Yong River approached the confluence, the confluence flow saw an increase in magnitude. The flow field at the maximum flood tide is shown in Figure 15a. Currentreversals reversalsoccurred occurredbecause becausethe thetides tides were were stronger stronger than than the the river river run-off. run-off. ItItwas wasaaflow flow Current bifurcation. The surface flow pattern was relatively smooth in the confluence. In the Yong River, bifurcation. The surface flow pattern was relatively smooth in the confluence. In the Yong River, the the depth-averaged velocities were largest (0.6–0.8 m/s). In the Fenghua and Yaorivers, rivers,the thetidal tidal depth-averaged flowflow velocities were largest (0.6‒0.8 m/s). In the Fenghua and Yao currents were weak; the corresponding velocities amounted to 0.4–0.6 and 0.2–0.3 m/s, respectively. currents were weak; the corresponding velocities amounted to 0.4‒0.6 and 0.2‒0.3 m/s, respectively. Furthermore,the thetidal tidalcurrent currentflowing flowinginto intothe theYao Yao River River was was influenced influenced by by the the bend. bend. Due Duetotothe the Furthermore, centrifugalforce force[46], [46],its itsmainstream mainstreamwas wasclose closeto tothe theconcave concaveriver river bank. bank. centrifugal

Figure second half halfof ofJune June2015: 2015:(a) (a)At Atthe themaximum maximumflood flood tide; Figure15. 15.Tidal Tidalflow flow fields fields during the second tide; (b)(b) At At maximum ebb tide. thethe maximum ebb tide.

With the the incoming incomingmaximum maximumebb ebbtide tidefrom fromthe thetwo twotributaries tributariestotothe theYong YongRiver, River,the theflow flow With velocity reached its peak in the confluence. The tidal flow field at the maximum ebb tide is illustrated velocity reached its peak in the confluence. The tidal flow field at the maximum ebb tide is illustrated Figure15b. 15b. In the Yong rivers, the depth-averaged velocities were 0.7–0.9 and ininFigure In the YaoYao andand Yong rivers, the depth-averaged velocities were 0.7‒0.9 m/s andm/s 0.4‒0.6 0.4–0.6 m/s, respectively. In theRiver, Fenghua River,current the tidal was comparatively weak with m/s, respectively. In the Fenghua the tidal wascurrent comparatively weak with its velocity its velocitytoamounting 0.3–0.5 m/s. The simulations showed there a small zone of amounting 0.3‒0.5 m/s.toThe simulations showed that there was athat small zonewas of flow circulation flow to circulation close of to the left bank of The the confluence. The ascribed occurrence was ascribed mainly to close the left bank confluence. occurrence was mainly to the confluence the confluence geometry also toin themomentum difference in momentum between the two tributaries [47]. geometry and also to the and difference between the two tributaries [47]. The relative The relative strength of the two meeting role in the size of zone. the vortex strength of the two meeting streams playsstreams a role plays in the alocation andlocation size of and the vortex The zone. The separation zonepart occupies of the channel cross-section, thus to a reduction in the separation zone occupies of the part channel cross-section, thus leading toleading a reduction in the capacity capacity of river conveyance. It causes also sediment in the zone. of river conveyance. It causes also sediment depositiondeposition in the zone. Theflow flowvelocity velocityininthe theconfluence confluencewas wassmaller smallerthan thanthe thenearby nearbyvelocities velocitiesininthe theriver riverstreams. streams. The Thediscrepancy discrepancy was attributed momentum offsetting. If two flows merge withwith each The attributedto tothe theeffect effectofofthe the momentum offsetting. If two flows merge other, there is momentum exchange, which enhances the turbulent mixing, leading to energy dissipation. each other, there is momentum exchange, which enhances the turbulent mixing, leading to energy Moreover, the water depths in the depths confluence were larger, also explaining velocities. dissipation. Moreover, the water in the confluence were larger, the alsosmaller explaining the smaller In the confluence, comparison of the velocities at the maximum flood and ebb tides show that the velocities. latter 0.2 m/s larger than the former, which was due to the addition of the ebbthat tide Inwas the approximately confluence, comparison of the velocities at the maximum flood and ebb tides show to the run-off. The volume of0.2 them/s ebblarger tide was also than that of thedue flood while, for the latter was approximately than thelarger former, which was to tide, the addition ofthe theflood ebb tide,tothe and the tide were in opposite directions, thusthan offsetting The prevalence tide therun-off run-off. The volume of the ebb tide was also larger that ofeach the other. flood tide, while, for held that tide, the velocity of the ebbthe tide was higher than thatdirections, of the floodthus tide.offsetting each other. The the flood the run-off and tide were in opposite In summary, parting from theofwater levels, flow at of thethe maximum flood tide differed prevalence held that the velocity the ebb tide the wasconfluence higher than that flood tide. in both flow direction and magnitude from that at the maximum ebb tide, which depended the In summary, parting from the water levels, the confluence flow at the maximum floodontide tidal flow the ebband tide,magnitude a zone of flow existedebb close to left river bank. differed in direction. both flow At direction fromseparation that at the also maximum tide, which depended on the tidal flow direction. At the ebb tide, a zone of flow separation also existed close to left river bank.

6.2. Bed Shear Stress

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6.2. Bed Shear Stress →

For a location in question, τb relates the flow regime to deposition and erosion patterns. It is a

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function of a quadratic of U and the 2D Chézy coefficient C2D (m0.5 /s) or an equivalent roughness → length For 2D in depth-averaged flows, the τb isflow given by to deposition and erosion patterns. It is a For[48]. a location question, 𝜏 ⃗ relates regime coefficient C2D (m0.5/s) or an equivalent roughness function of a quadratic of 𝑈⃗ and the 2D Chézy → → → | U | ρ g U 0 length [48]. For 2D depth-averaged flows, τ𝜏 ⃗ =is given by , (10) b C2D 2 ⃗ ⃗ 𝑈 𝜌 𝑔𝑈 (10) , → → →𝜏 ⃗ = → where |U | (m/s) is the magnitude of U = U ξ + U η𝐶. C2D is expressed as 2D is expressed as where 𝑈⃗ (m/s) is the magnitude of 𝑈⃗ = 𝑈⃗ + 𝑈⃗ . C√ 6 H C2D = . (11) √𝐻 n (11) 𝐶 = . 𝑛 → Collins et al. [48] pointed out the variability of these parameters. In a tidal confluence, the τb Collins et al. [48] pointed out the variability of these parameters. In a tidal confluence, the 𝜏 ⃗ determination is complicated by factors such as confluence geometry, bed topography, and sediment. determination is complicated by factors such as confluence geometry, bed topography, and sediment. → Flow perturbations make it difficult to measure τ in the field. Therefore, numerical models are often b Flow perturbations→make it difficult to measure 𝜏 ⃗ in the field. Therefore, numerical models are used for obtaining τ in tidal environments. b often used for obtaining 𝜏 ⃗ in tidal environments. →

→ →

→

|⃗U𝑈⃗and is is also affected by by H. H. In the confluence, the |the U | and values were is proportional proportionaltoto|U𝑈 and also affected In the confluence, 𝑈⃗ H and H values 𝜏τ⃗b is between 0.4–0.6 m/s and m (exclusive of the hole). At the flood and and ebb were between 0.4‒0.6 m/s 6.0–17.3 and 6.0‒17.3 m (exclusive of scour the scour hole). At maximum the maximum flood → → τ τ tides, Figure 16 shows the distribution of the peak values. Their distributions of were similar, ebb tides, Figure 16 shows the distribution of the peak 𝜏 ⃗ values. Their distributions b b of 𝜏 ⃗ were with some local differences. For each tide, the spatial distribution and magnitude followed the pattern similar, with some local differences. For each tide, the spatial distribution and magnitude followed of the velocity gradient distribution. For a given river, decayriver, was exhibited fromexhibited the mainstream to the pattern of the velocity gradient distribution. For aagiven a decay was from the the bank. mainstream to the bank. → → affected by by both both run-off run-off and and tides. tides. The Thepeak peak𝜏τ⃗b values values 𝜏τ⃗b reflects reflects the velocity gradient and is affected occurred away Fenghua and Yong rivers, which was was true true for both occurred away from fromthe theconfluence confluenceininthe the Fenghua and Yong rivers, which for the bothflood the and ebb The occurrence of these areas areas was similar to the to situation with only run-off in the flood andtides. ebb tides. The occurrence of these was similar the situation withthe only the run-off rivers [1,4,9].[1,4,9]. in the rivers → Low 𝜏τb⃗ values At the the Low values occurred occurred in in such such areas areas as as along along the the Yao Yao River River and and in in the the confluence. confluence. At confluence, the Fenghua rivers run run almost alongalong a straight line, while Yao River intersects confluence, theYong Yongand and Fenghua rivers almost a straight line,the while the Yao River them at almost angle. As the tide comes Yongfrom River, tidesRiver, are bathymetrically intersects them aatright almost a right angle. As thefrom tide the comes thethe Yong the tides are → → bathymetrically constrained and the for bending accounts lowRiver. 𝜏 ⃗ values in the theflow Yao conditions River. 𝜏 ⃗ constrained and the bending accounts the low τb valuesfor in the Yao τb links links the flow conditions sediment transport, providing indications with sediment transport, with providing indications of morphology change. of morphology change.

→ Figure during the the second second half Figure 16. 16. Distributions Distributionsof of 𝜏τ⃗b during half of of June June 2015: 2015: (a) (a) At At the the maximum maximum flood flood tide; tide; (b) (b) At At the the maximum maximum ebb ebb tide. tide.

7. Morphological Changes With the typical confluence flow features, predictions were made to look at the potential pattern of morphological changes. The construction of the sluice gates on the Yao River interrupted the natural run-off downstream. Another factor is that many wading structures were built on both rivers upstream of the confluence, which also affected the run-off in the water system. This means that the tides interact with the run-off in a different way than before; the intrusion of the tidal waves is further

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7. Morphological Changes With the typical confluence flow features, predictions were made to look at the potential pattern of morphological changes. The construction of the sluice gates on the Yao River interrupted the natural run-off downstream. Another factor is that many wading structures were built on both rivers upstream of the2018, confluence, which also affected the run-off in the water system. This means that the tides interact Water 10, x FOR PEER REVIEW 15 of 20 with the run-off in a different way than before; the intrusion of the tidal waves is further upstream, which probably in more sediment deposition. were carriedwere out to estimate upstream, whichresults probably results in more sediment Simulations deposition. Simulations carried outthe to possible scenarios. estimate the possible scenarios. As shown earlier, the tidal currents, especially for the spring tides in wet seasons, dominate the sediment transport in the area. During the second half of June 2015, the 30-h spring tide was selected Along the the rivers rivers including including the the confluence, confluence, the the start start condition condition of of the the river bed for the purpose. Along 10:00 a.m. onon 17 June 2015. As shown in Figure 17, a 17, long, corresponded to to the thebathymetry bathymetryobtained obtainedatat 10:00 a.m. 17 June 2015. As shown in Figure a oval-shaped scour scour hole exists in the in confluence and extends into the Yong depth 10.8 m long, oval-shaped hole exists the confluence and extends into theRiver. Yong Its River. Its is depth is at the (the river is bed 6.5 m below the mean sea level, with with a beda elevation of −of 6.5−6.5 m). 10.8 mmaximum at the maximum (thebed river is 6.5 m below the mean sea level, bed elevation In the a morphological scalingscaling factor was used to accelerate the bed erosion anderosion deposition, m). Insimulation, the simulation, a morphological factor was used to accelerate the bed and a method ofacommon [39,49–51]. scaling factor was setfactor to 100,was implying that implying the prediction deposition, method practice of common practiceThe [39,49–51]. The scaling set to 100, that period coveredperiod 125 days. the prediction covered 125 days.

Figure 17. Contours of the confluence in June 2015 (T = 0).

Figure 18 18 illustrates, illustrates, in in the the confluence confluence and and its its close close vicinity, vicinity, the evolution from from T T == 0, 0, 30, 30, 60, 60, Figure the bed bed evolution 90, 120, and 125 days. The results show, as time elapses, that the three river reaches were subjected 90, 120, and 125 days. The results show, as time elapses, that the three river reaches were subjected to continuous continuous siltation. siltation. The The trend trend is is in in qualitative qualitative agreement agreement with with the the study study of of the the water water system system by by to Chen et al. [36], in which the cumulative influence of the wading structures, including the Yao sluice Chen et al. [36], in which the cumulative influence of the wading structures, including the Yao sluice gates, bridges, wharfs, waswas analyzed. TheirTheir resultsresults showedshowed that the that riversthe including confluence gates, bridges,and and wharfs, analyzed. rivers the including the also suffered from gradual siltation of suspended load. Figure 19 illustrates the change in scour-hole confluence also suffered from gradual siltation of suspended load. Figure 19 illustrates the change in depth and the averaged the riverof bed the around hole. Figure 20 shows, function scour-hole depth and theelevation averagedofelevation thearound river bed the hole. Figureas20a shows, asof a time, the of simulated profiles of the bed-level along each river the river confluence. function time, thelongitudinal simulated longitudinal profiles of thechanges bed-level changes alongateach at the Both the river bed and thebed scour hole tendency gradual deposition. The hole depth was confluence. Both the river and theshow scourahole show of a tendency of gradual deposition. The hole initially 10.8 m and became 9.25 m at T = 125 days. depth was initially 10.8 m and became 9.25 m at T = 125 days.

gates, bridges, and wharfs, was analyzed. Their results showed that the rivers including the confluence also suffered from gradual siltation of suspended load. Figure 19 illustrates the change in scour-hole depth and the averaged elevation of the river bed around the hole. Figure 20 shows, as a function of time, the simulated longitudinal profiles of the bed-level changes along each river at the confluence. Both the river bed and the scour hole show a tendency of gradual deposition. The Water 2018, 10, 1424 16 hole of 21 depth was initially 10.8 m and became 9.25 m at T = 125 days.


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Figure 18. 18. River-bed River-bed morphology morphology changes changes in inthe theconfluence: confluence:(a) (a)TT== 0; 0; (b) (b) TT == 30 days; (c) T = 60 days; Figure (d) TT == 90 days; (e) T = 120 days; (f) T == 125 (d) 125 days. days.

Both the flood flood and and ebb ebb tides tides contributed contributed to to shaping shaping the the confluence confluence scour scour hole. hole. The former gave rise to deposition, while while the the latter latterled ledto toerosion. erosion.However, However,the theflood floodtide tide played a dominant part played a dominant part in in process. a result, scour shrank upstream and downstream as time elapsed. thethe process. As As a result, the the scour holehole shrank bothboth upstream and downstream as time elapsed. Two Two factors accounted for the morphological feature. one hand,the thechange changewas wasassociated associated with the factors accounted for the morphological feature. OnOn one hand, sediment availability in each river. The field measurements indicated that the flood sediment availability in each river. The field measurements indicated that the flood tides tides carry carry a large large amount of suspended load load upstream. upstream. When Whenflow flowvelocity velocityfell fellbelow below0.8 0.8m/s, m/s, deposition occurred deposition occurred in in the confluence area. Using morpho-sedimentological and seismo-stratigraphic data, Silva et al. the confluence area. Using morpho-sedimentological and seismo-stratigraphic data, Silva et explained a similar similar phenomenon phenomenon of confluence confluence deposition deposition [15]. They found that the the river river sediment sediment was was deflected deflected back back into into the the river river by by the the flood flood tides, tides, thus thus producing producing the the sediment sediment deposition deposition on the the scour hole’s gentle side (downstream slope). On the other hand, the cumulative effect of river run-off scour hole’s gentle side (downstream slope). and and ebb ebb tides tides was was also attributable attributable to to the scour changes. changes. During the ebb tides, the deposited sediment in and resulted in slight bed bed erosion. The dominance of theof flood in the theconfluence confluencebecame becamere-suspended re-suspended and resulted in slight erosion. The dominance the tide eventually led to the sediment deposition along the rivers inclusive of the confluence. flood tide eventually led to the sediment deposition along the rivers inclusive of the confluence.

explained a similar phenomenon of confluence deposition [15]. They found that the river sediment was deflected back into the river by the flood tides, thus producing the sediment deposition on the scour hole’s gentle side (downstream slope). On the other hand, the cumulative effect of river run-off and ebb tides was also attributable to the scour changes. During the ebb tides, the deposited sediment in the confluence became re-suspended and resulted in slight bed erosion. The dominance 17 ofofthe Water 2018, 10, 1424 21 flood tide eventually led to the sediment deposition along the rivers inclusive of the confluence.


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Figure 19. 19. Change Change in in scour-hole scour-hole depth depth in inthe theconfluence. confluence. Figure

Figure 20. 20. Longitudinal bed-level changes in the confluence: confluence: (a) Distance along the river river centerline centerline (accounted from the the lowest lowestscour scourposition); position);(b) (b)Along Along the Yao River; Along Fenghua River; the Yao River; (c)(c) Along thethe Fenghua River; (d) (d) Along the Yong River. Along the Yong River.

The The Yao Yao River River also also features features gradual gradual siltation, siltation, which which changes changes its its river-bed river-bed slope slope and and lowers lowers the the sediment carrying capacity. This was also observed in the field [38]. Two plausible reasons account for sediment carrying capacity. This was also observed in the field [38]. Two plausible reasons account it. One is ascribed to the construction of the sluice gates and the wading structures upstream. As a for it. One is ascribed to the construction of the sluice gates and the wading structures upstream. As result, it not onlyonly intercepts the river but alsobut deforms the tidal the waves in the river. a result, it not intercepts the run-off, river run-off, also deforms tidal waves in According the river. to measurements [38], the mean high tidal level increased by 0.17 m; the mean low tidal level According to measurements [38], the mean high tidal level increased by 0.17 m; the meandecreased low tidal by 0.11 m. The flood tide duration became 9 min shorter and the ebb tide duration became 9 min level decreased by 0.11 m. The flood tide duration became 9 min shorter and the ebb tide duration longer. The other reason is due to the bending toward the confluence. The flow, either during the flood became 9 min longer. The other reason is due to the bending toward the confluence. The flow, either or ebb tides, fails to transport the sediment downstream, thus leading to deposition along the river. during the flood or ebb tides, fails to transport the sediment downstream, thus leading to deposition alongThe theperiodic river. changes in the tidal flow direction induce a complex morphological regime that does The not occur in unidirectional run-off Concerning the sedimentation pattern, the morphologic periodic changes in the tidal flows. flow direction induce a complex morphological regime that features migrate streamwise with run-off flows [3,49]. With tidal waves, the pattern migrates does not occur in unidirectional run-off flows. Concerning the sedimentation pattern,both the ways, which agrees with previous findings with of therun-off tides that both deposition and re-suspension take morphologic features migrate streamwise flows [3,49]. With tidal waves, the pattern

migrates both ways, which agrees with previous findings of the tides that both deposition and resuspension take place in the confluence [52]. The analysis showcases that the flow regime is the main driver of the confluence scour evolution. The morphological changes in the confluence subjected to strong tides are closely related to the interactions between the run-off and tidal currents. However, the latter plays a dominant role and governs the sedimentation pattern.

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place in the confluence [52]. The analysis showcases that the flow regime is the main driver of the confluence scour evolution. The morphological changes in the confluence subjected to strong tides are closely related to the interactions between the run-off and tidal currents. However, the latter plays a dominant role and governs the sedimentation pattern. 8. Conclusions In a river confluence subjected also to strong tidal currents, its flow and morphological changes are dependent on a number of factors, showing a complex pattern in both time and space. This study dealt with the typical features of such a confluence by means of field studies and numerical modeling. From the sea into the Yong River, the sediment is transported by the tidal currents, especially during the spring tides. During the selected period of two years, field measurements were made to examine the sediment behaviors. The data show that approximately 95% of the sediment in the study area is suspended load. From the river mouth to its upstream including the confluence, the flow and sediment changes are not always in phase with one another; the sediment movement is significantly modified by the tides. The peak values of sediment concentration occur during both the flood and ebb tides in the rivers. Tidal currents are essential for stirring sediment, modifying its concentration and transport. If there was no tide, the sediment concentration would be directly proportional to the river flow, with the sediment diverted only downstream. With the field measurements in the background, numerical modeling helps understand the alluvial features of the confluence. Two-dimensional simulations of suspended sediment transport were performed to simulate the sediment patterns. At the confluence, the flow at the maximum flood tides differs, in both flow direction and magnitude, from that at the maximum ebb tides. During the ebb tides, a small zone of flow circulations exists close to the left bank of the confluence. The bed shear stress is proportional to the water depth and flow velocity, and it is affected by the river-bed topography. Its distribution reflects the sediment erosion potential in the confluence. By means of a morphological scale factor, the scour formation in the confluence was predicted. The initial hole in the confluence, extending along the Fenghua and Yong rivers, becomes gradually deposited as time elapses. The shifting tidal directions induce a complex morphological pattern that does not exist in unidirectional run-off flows. The erosion and deposition migrate in both directions. The flood tides govern the sediment transport and deposition, while the ebb tides with run-offs lead to erosion. For the scour-hole development, the flood tides play a dominant role. Author Contributions: Q.X. was responsible for analyses of field measurement data and numerical simulations, with participation from S.L., J.Y., and W.D. The manuscript was written by Q.X. and J.Y. The research of river flow and sedimentation was supervised by J.Y. and S.L. Funding: Q.X. is supported by a four-year PhD scholarship from the Chinese Scholarship Council (CSC) and the Swedish STandUp for Energy project. The authors are members of the 111 Project “Discipline Innovation and Research Base on River Network Hydrodynamics System and Safety”, funded by the Ministry of Education and State Administration of Foreign Experts Affairs, China (Grant No. B17015), with Hohai University’s State Key Laboratory of Hydrology, Water Resources, and Hydraulic Engineering as the executive organization. Acknowledgments: The Hydrology and Water Resources Survey Bureau of the Lower Yangtze River is thanked for their support with the field measurements. Ahmed Bilal of Hohai University provided assistance with the model set-up and calibrations. The authors would like to thank Patrik Andreasson for his comments and the four anonymous reviewers for their suggestions that led to improvements in the quality of the article. Conflicts of Interest: The authors declare no conflicts of interest.

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