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Suspended sediment dynamics and influencing factors during typhoons in Hangzhou Bay, China

Suspended sediment dynamics and influencing factors during typhoons in Hangzhou Bay, China Hangzhou Bay is located in China on the south side of the Changjiang Estuary and is vulnerable to extreme weather, such as typhoons in the summer and autumn. In this study, a three dimensional suspended sediment numerical model was developed that considers the dynamic factors of advection, mixing, wave, and sediment-induced stratifi- cation to simulate and analyze the effect of typhoons on water and sediment transport in Hangzhou Bay. The model validations show that the model can sufficiently reproduce the variability of the suspended sediment concentration (SSC) during typhoon conditions. The simulation results show that the high SSC in the bottom layer was mainly dis- −3 tributed in the leading edge of the south coast, and generally exceeded 10 kg·m . During typhoons, the water and suspended sediment transport in Hangzhou Bay presented a pattern of "north-landward and south-seaward" circula- tion, which promoted the convergence of suspended sediment in the center part of the bay. During Typhoon Rumbia in 2018, the water and sediment flux across the section from Nanhui Cape to Qiqu Archipelago (NQ section) increased by 18.13% and 265.75%, respectively, compared with those before the typhoon. The wave-induced bottom shear stress during typhoons has a very significant impact on the bottom SSC. The sensitivity experiments show that the wave-induced bottom shear stress greatly promotes the sediment resuspension during typhoons, which indirectly makes the sediment-induced stratification stronger than the direct effect of waves on the vertical mixing. The strong winds brought by typhoons mainly enhanced the vertical mixing, which has a stronger effect on surface SSC than waves. The suppression of vertical mixing by sediment-induced stratification during typhoons should not be ignored, especially for high turbidity coastal waters, such as Hangzhou Bay. Keywords Sediment transport, Typhoons, Wave, Stratification, Hangzhou Bay, Numerical model an important mechanism that drives drastic changes in 1 Introduction suspended sediment concentration (SSC) in bays. The Suspended sediments are carriers of nutrients, organic East China Sea is affected by four typhoons every year matter, and pollutants that impede light transmission, on average (Lu et al. 2018).Very high waves are generated photosynthesis, and primary productivity and affect during typhoons, these waves propagate into nearshore the marine environment and ecological processes (Fang areas and induce phenomena such as wave reflection, et  al. 2016; Li et  al. 2016a; Zhao et  al. 2018). Though refraction, and fragmentation. In recent years, the inten- there are numerous factors, typhoons are undoubtedly sity and frequency of extreme events, such as typhoons, have increased, and the impact of typhoons on sediment *Correspondence: resuspension and redistribution in coastal waters has Jianrong Zhu gradually attracted increasing attention from oceanog- jrzhu@sklec.ecnu.edu.cn State Key Laboratory of Estuarine and Coastal Research, East China raphers (Bian et  al. 2010; Gong and Shen 2009; Lu et  al. Normal University, Shanghai 200241, China 2018; Palinkas et  al. 2014; Xie et  al. 2018). Analyzing © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 2 of 19 how waves influence the sediment transport and deposi - were strongest in winter and enhanced the sediment tion/resuspension processes in coastal bays is of critical deposition in the western shoal of the Pearl River Estu- importance to studies of coastal geomorphology or for ary. The effect of waves on SSC is more significant during research exploring the ecological effects of typhoons (Liu typhoons. Shen et  al. (2018) and Ren et  al. (2021) con- et al. 2022; Zhang et al. 2021). cluded that waves during typhoons greatly contributed to In recent years, the supply of fluvial sediments to estua - sediment resuspension in the Changjiang Estuary. Brand rine and coastal systems has been significantly reduced et  al. (2010) concluded that the wave effects increased −1 due to upstream dams (Syvitski et  al. 2009, 2005). With SSC from 30  mg·L   during calm periods to more than −1 the reduction of sediment entering the sea, tidal currents 100 mg·L  during turbulent periods at the shoals in the and waves play an important role in maintaining the southern portion of San Francisco Bay, USA. In addition, SSC in coastal waters. For example, Huang et  al. (2021) waves can change the vertical stratification of the estuary concluded that the main reason for the slow decline of and affect the distribution of suspended sediment and its SSC in the Changjiang Estuary and its adjacent waters transport direction (Green and Coco 2014). is related to wave and tide-induced sediment resuspen- Hangzhou Bay is located on the south side of the sion. Luo et  al. (2017) found that the bottom sediment Changjiang Estuary (Fig.  1), connects to the Qian- resuspension was mainly caused by tide-induced shear tang River in the west, and is characterized by a large stress and diffused to the surface layer by vertical mix - irregular and semidiurnal tide, strong flow, and high ing in winter. The waves increase the bottom shear stress SSC. Morphologically, Hangzhou Bay is an east‒west through wave-current interaction, thus effectively pro - oriented trumpet shaped bay. The bay has one of the moting sediment resuspension (Hsu et  al. 2006; Xu and highest SSC in the world. With the development of You 2017). For example, Xu and You (2017) studied the numerical models, some researchers have used numeri- effects of waves on sediment resuspension in the Oujiang cal models to carry out SSC research in Hangzhou Estuary, China. Zhang et  al. (2021) studied wave effects Bay. Xie et  al. (2009) developed a two-dimensional on water and sediment transport in the Pearl River Estu- suspended sediment numerical model in Hangzhou ary, China, and the results showed that the wave effects Bay to obtain sediment transport directions. Du et  al. Fig. 1 Map of Hangzhou Bay and Changjiang Estuary. Black triangle: the anchored ship stations; thick black line: the section from Nanhui Cape to Qiqu Archipelago; black dot: Chongming weather station Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 3 of 19 (2010) used a three-dimensional suspended sediment the different factors is presented in Sect.  4. Finally, the model to analyze the temporal variation in SSC during conclusion is presented in Sect. 5. the neap-spring tidal cycle in Hangzhou Bay. Xie et  al. (2013) used the Delft3D to find that sediment trans -2 Materials and methods port in Hangzhou Bay was affected by tidal asymme -2.1 Observations try. Because the bay mouth is occupied by Zhoushan The State Key Laboratory of Estuarine and Coastal Islands, it is not easy for waves from the outer sea to Research, East China Normal University, conducted con- enter the bay. The waves in Hangzhou Bay are weak tinuous field observations in Hangzhou Bay from August under normal weather conditions, with an annually 5 to 20, 2018. Two marine research ships collected averaged wave height of 0.2–0.5  m (Xie et  al. 2013). simultaneously observations at site A and site B (Fig.  1). However, Hangzhou Bay has large waves during Tripod-mounted observation systems were placed on typhoons and is affected by typhoons almost every year. the seafloor at each site. The instruments mounted in Li et  al. (2022a) concluded that the SSC in Hangzhou each tripod were positioned 0.2  m above the bottom. A Bay was most affected by waves during Typhoon Chan- 600  kHz acoustic Doppler current profiler (ADCP, RD hom based on FVCOM. Li et al. (2022b) concluded that Instruments) was used to measure the vertical current the combined wave-current bottom stress was the pri- profile; an optical backscatter sensor (OBS, D&A Instru - mary wave-current interaction that changed sediment ment Company) was used to measure salinity, tempera- resuspension and increased SSC. Due to the difficulty ture, and turbidity; a Sea-Bird SBE37 CTD was used to of actual observation data during typhoons, research measure temperature and salinity; and an electromag- on the SSC characteristics in Hangzhou Bay during netic current meter (Alec, Electronics, Tokyo) was used typhoon conditions is still relatively lacking. to measure the near-bottom current inaccessible by the Based on the improved ECOM-si (Estuary, Coast, and ADCP. The ADCP worked in upward-looking mode at a Ocean Model with semi-implicit), a high resolution three vertical resolution of 0.25  m and ensembles of 2  min at dimensional numerical model of suspended sediment in a 1-s ping interval. A diagram of the instrument posi- Hangzhou Bay is developed. The model takes into con - tions is shown in Fig. 2. Water samples were taken at the sideration advection, diffusion, settlement, sediment floc - sites and brought back to the laboratory for SSC meas- culation, waves, and sediment-induced stratification, to urements to determine the relationship between OBS simulate the sediment transport in Hangzhou Bay under turbidity and SSC. Surface data could not be obtained typhoon conditions. The model description and valida - during typhoons Yagi and Rumbia, which impacted the tion are presented in Sect.  2. In Sect.  3, the water and site in 2018. suspended sediment transport process in Hangzhou Bay At 23:00 on August 12, 2018, Typhoon Yagi made land- during typhoons are analyzed. The sensitivity analysis for fall on the coast of Zhejiang, China, with a maximum Alec ADCP OBS CTD Fig. 2 Tripod observation system and positions of the instruments Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 4 of 19 wind speed of 28  m/s; at 4:00 on August 17, 2018, to Typhoon Rumbia, the wind field in Hangzhou Bay was Typhoon Rumbia directly passed through Hangzhou mainly cyclonic (Fig. 5b). Bay and made landfall off the southern coast of Shang - hai with a maximum wind speed of 23  m/s (Fig.  3). The 2.2 Numerical model setup tripod observation systems at sites A and B continu- The improved three dimensional ECOM-si includes ously recorded the variations in benthic hydrodynam- a hydrodynamic model, a sediment module, and the ics and SSC during the typhoon period from August SWAN (Simulating Waves Nearshore) model, which pro- 12 to August 18, 2018. The weather station located on vides wave parameters for the hydrodynamic model and Chongming Island recorded wind speed changes during sediment module. The hydrodynamic model originated typhoons. Since Chongming Island is far from the center from the POM (Princeton Ocean Model) developed by of Typhoon Yagi, its maximum wind speed was approxi- Princeton University (Blumberg and Mellor 1987). The mately 13  m/s, while the center of Typhoon Rumbia model uses the “Arakawa C” grid configuration variables was close to Chongming Island, and its maximum wind (Arakawa and Lamb 1977). A nonorthogonal curve grid speed was approximately 20  m/s (Fig.  4). The impact of was used in the horizontal direction (Chen et  al. 2004), Typhoon Rumbia on Hangzhou Bay is more significant and the sigma coordinate was adopted in the vertical than that of Typhoon Yagi. The ERA5 (ECMWF Rea - direction. The level 2.5 turbulence closure model by Mel - nalysis v5) wind field data from the ECMWF (European lor and Yamada (1982) was used to calculate the vertical Centre for Medium-Range Weather Forecast, https:// mixing coefficients, and the parameter formula of the c d s . c lima t e. c op er nic u s . e u/ c d sa pp# !/ d a t a s e t/ r e ana ly si s - stability function was from Kantha and Clayson (1994), era5- single- levels? tab= overv iew) are consistent with the while the parameterization scheme of Smagorinsky actual wind speed and direction measured by the weather (1963) was used to calculate the horizontal mixing coef- station and can accurately reproduce the wind speed var- ficients. A wet/dry scheme was included to describe the iability during typhoons (Fig. 4). Figure 5 shows the wind intertidal flat with a critical depth of 0.2 m. To reduce the field distribution at 03:00 on August 13, 2018 (Typhoon numerical dissipation and improve the computational Yagi) and at 00:00 on August 17, 2018 (Typhoon Rumbia). accuracy in the material transport process, Wu and Zhu Due to the influence of Typhoon Yagi, the wind direction (2010) developed the high order spatial interpolation at in Hangzhou Bay was mainly southeast (Fig. 5a), and due the middle temporal level coupled with a TVD limiter 32°N Shanghai East 31°N 30°N China Zhejiang Typhoon Rumbia 29°N Sea Typhoon Yagi 28°N 119°E 120°E 121°E 122°E 123°E 124°E 125°E Longitude Fig. 3 Paths of Typhoon Yagi and Typhoon Rumbia in 2018 before and after landfall Hangzhou Bay Changjiang Estuary Latitude Speed (m/s) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 5 of 19 (a) (b) 15 m/s 8/10 8/12 8/14 8/16 8/18 8/20 Time (month/day) Fig. 4 Temporal variations in wind speed (a) and wind vector (b) at Chongming weather station. Red: observed data; black: ERA5 data to solve the advection term in the material transport were adopted from the ERA5 dataset, with a temporal equation. The barotropic pressure gradient force in the resolution of 6 h and a spatial resolution of 0.25° × 0.25°. momentum equation was solved by an implicit method, For a detailed introduction to the sediment module, and the continuous equation was solved by the semi- please refer to the author’s other article (Huang et  al. implicit method of Casulli and Cattani (1994). 2022). In the model, the sediment in the water body is The model domain covered all of the Changjiang Estu - considered cohesive fine grained sediment. The role of ary and Hangzhou Bay and adjacent seas from 117.5° E to waves in sediment resuspension cannot be ignored, espe- 125° E longitude and 27.9° N to 33.7° N latitude (Fig. 6a). cially during typhoon conditions. The improved model The model grid consisted of 396 × 522 cells in the hori- algorithm calculates the bottom wave orbit velocity with zontal dimension. Fifteen sigma levels were set in the sea surface wave parameters to obtain the bottom shear vertical direction with five logarithmically distributed stress generated by the waves. This algorithm needs to layers near the bottom (σ = − 0.929, − 0.964, − 0.982, − 0 use the significant wave height, average wave direction, .991, − 1.0) to distinguish the effect of high bottom SSC and significant wave period on the sea surface. The bot - on the bottom friction drag coefficient, and 10 layers tom shear stress under the influence of wave-current were established in the remaining layers (σ = 0, − 0.1, − 0 interaction is given by (Grant and Madsen 1979): .2, − 0.3, − 0.4, − 0.5, − 0.6, − 0.7, − 0.79, − 0.87). The grid 2 2 τ = |τ + τ | = (τ + τ |cosϕ|) + (τ sinϕ) w c w c c resolution ranges between 200  m at the top of t he bay, (1) approximately 600 m in the central part, and is 2 ⁓ 10 km τ τ c c = τ 1 + 2 |cosϕ| + τ τ w w near the open sea boundary (Fig. 6b). The open sea boundary condition was specified by the where τ is the bottom shear stress considering waves and tide and residual water level. The tidal signal was com - currents; τ is the maximum wave-induced bottom shear posed of 16 astronomical constituents, M , S, N, K, K , 2 2 2 2 1 stress; τ is the tidal-induced bottom shear stress; φ is the O, P, Q, MU, NU, T, L, 2N, J, M , and OO , which 1 1 1 2 2 2 2 2 1 1 1 angle between wave propagation and the current direc- were derived from the NaoTide dataset (http:// www. miz. tion. τ and τ can be calculated by: c w nao. ac. jp/). The residual water level and initial salinity field were derived from the results simulated by a large 2 τ = ρC U (2) domain model encompassing the Bohai Sea, Yellow Sea, and East China Sea (Wu et al. 2011). The river boundary ρf was driven by the river discharge at the Datong hydro- τ = U (3) logic station (Changjiang Water Resources Commission) for the Changjiang River and at the Fuchunjiang hydro- where ρ is the actual density of seawater with the sus- electric power station for the Qiantang River. Wind data pended sediment; U is the bottom current velocity; C is Wind speed (m/s) Wind vector Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 6 of 19 31.5 20 m/s 20 m/s 2018-08-13 03:00:00 2018-08-17 00:00:00 31.0 30.5 (a) (b) 30.0 121.0 121.5 122.0 122.5 123.0 121.0 121.5 122.0 122.5 123.0 Longitude (°E) Longitude (°E) Fig. 5 Wind field distribution during Typhoon Yagi (a) and Typhoon Rumbia (b) (a) (b) Datong Fuchunjiang station Fig. 6 Numerical model domain and grid (a) and an enlarged view of the model grid in Hangzhou Bay (b) the bottom drag coefficient; f is the wave fiction factor; 4 C 0.04C p p α = 15 exp − and U is the near-bed wave orbital velocity. A detailed CB (5) u u ∗ ∗ calculation procedure can be found in Wiberg and Sher- wood (2008). Waves affect not only the bottom shear where C /u is the wave age;C is the phase speed of p ∗ p stress but also the vertical mixing coefficient (Mellor and waves at the dominant frequency; u is the air side fric- Blumberg 2004; Terray et  al. 1999). Based on the level tion velocity; and u = 30u . When the effect of waves is ∗ τ 2.5 turbulence closure module, Mellor and Blumberg 2/3 2 2 not considered, q = B u at z = 0, B = 16.6 . When 1 1 (2004) considered the effect of waves breaking on the sea 2/3 2 2 the effect of waves is considered, q = (15.8α ) u CB boundary layer. The sea surface boundary conditions are: at z = 0. From measured wave heights and near-surface 2 dissipation data, Terray et al. (1999) used the turbulence ∂q K = 2α u ,z = 0 (4) q CB closure module to find the best fit between the turbulent ∂z mixing length ( l) and the measured data. where q is the turbulence kinetic energy;u is the water l = max(κz , l ), z = 0.85H (6) w z w s side friction velocity;α is a parameter related to the CB waves, with the following equation: Latitude (°N) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 7 of 19 where H is the significant wave height; l = κz ; κ = 0.4 s z X − X ( ) mod obs SS = 1 − is the von Karman constant. When the effect of waves (8) X − X + X − X mod mod obs obs is not considered, z = 0.K = qlS is the vertical eddy w m M viscosity; K = qlS is the vertical eddy diffusivity, where h H S and S are the stability function, please refer to Mel- M H (X − X ) mod obs lor and Yamada (1982). (9) RMSE = The SWAN model (Booij et  al. 1999) adopted an orthogonal mesh that covered the calculation range of where X is the variable and is the time-averaged value. the ECOM-si, with a spatial resolution of 2’ × 2’ and a The performance levels of the modeled results and time step of 30  min. The SWAN model outputted the observed results were evaluated by SS where SS > 0.65 significant wave height, significant wave period, and is considered excellent, 0.65–0.5 is very good, 0.5–0.2 is wave direction every 3  h, and these parameters were good, and SS < 0.2 is considered poor. interpolated to each time step in the sediment module The modeled current velocity, current direction, salin - to calculate the bottom shear stress and vertical mixing ity, and SSC were validated with observation data during coefficient under wave-current interactions. The wave neap tide and spring tide at site A and site B in Hangzhou parameters output from the SWAN model has been vali- Bay in August 2018; see Huang et  al. (2022). This study dated in related papers by our research group (Luo et al. used data that included typhoon periods to validate the 2017). model. We downloaded the real-time river discharge data for the Datong hydrological station and Fuchunjiang 2.3 Model validation hydropower station, as well as the real-time wind field The ECOM-si has been extensively validated in terms of data from ERA5 to drive the model. The model was cold water level, current speed and direction, salinity, and SSC started on 1 July 2018 and ran for 62 days. The compari - (Chen et al. 2019; Luo et al. 2017; Lyu and Zhu 2018; Qiu son between the simulated water level and the measured and Zhu 2013; Wu and Zhu 2010; Zhu et  al. 2015). This data from August 6 to 20, 2018, is shown in Fig. 7. Hang- study will further validate the model in Hangzhou Bay. zhou Bay is famous for its strong tides. In terms of tidal The following three skill assessments were used to quan - nature, the tides outside the mouth of the bay are regu- tify the validation of the model: correlation coefficient lar semidiurnal tides and inside the bay mouth, they are (CC), root mean square error (RMSE), and skill score irregular semidiurnal tides. After August 12, Hangzhou (SS) (Murphy 1988; Warner et al. 2005; Willmott 1981): Bay was impacted by Typhoon Yagi and Typhoon Rum- bia. Regardless of whether it was normal or typhoon con- X − X X − X mod mod obs obs CC = ditions, the simulated water level was consistent with the 2 2 2 measured value, and both the SS and CC exceeded 0.95 X − X X − X mod mod obs obs (Table 1), indicating that the model can successfully sim- (7) ulate the temporal variation in water level. (a) -2 -4 (b) -2 -4 6789 10 11 12 13 14 15 16 17 18 19 20 Time (Day) Fig. 7 Comparison of the simulated water level (black line) and measured data (red dot) at site A (a) and site B (b) in Hangzhou Bay from August 6 to 20, 2018 Elevation (m) Elevation (m) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 8 of 19 Table 1 Correlation coefficients (CC), root mean square error (RMSE), and skill scores (SS) for comparison of the simulated and observed data at the measuring stations Skill assessment Site A Site B CC RMSE SS CC RMSE SS Water level 0.99 0.25 m 0.99 0.99 0.24 m 0.99 −1 −1 Surface velocity 0.93 0.22 m·s 0.96 0.95 0.17 m·s 0.98 −1 −1 Bottom velocity 0.90 0.14 m·s 0.95 0.85 0.18 m·s 0.89 −3 −3 Bottom SSC 0.70 1.09 kg·m 0.83 0.50 1.07 kg·m 0.71 The simulated current velocity, direction, and SSC are 3.1 SSC compared with the observed data during typhoons in The distribution of the bottom SSC at four moments of Fig.  8. Due to the effect of bottom friction, the current the tidal cycle in Hangzhou Bay during Typhoon Rum- velocity in the bottom layer was smaller than that in the bia is shown in Fig.  9. At the maximum flood, the high surface layer. The average SSC in the bottom layer at site A SSC in the bottom layer was mainly distributed in the −3 was 2.78 kg·m , and the average SSC in the bottom layer southeastern part of Hangzhou Bay; at the flood slack, −3 at site B was 2.85 kg·m during typhoons. The CC, RMSE, the high SSC in the bottom layer was mainly distributed and SS of the simulated data and the observation data are in the south shore tidal flats and southeastern part of the −3 shown in Table 1. Note that the SS of the SSC is above 0.7. bay, generally exceeding 10  kg·m . At the maximum The model can successfully reproduce the variability in the ebb, the high SSC in the bottom layer was mainly located current velocity, direction, and SSC during typhoon condi- on the south coast and in the northern part of the bay; tions, and can be used to study the hydrodynamic and SSC at the ebb slack, the high SSC in the bottom layer was transport process in Hangzhou Bay. mainly distributed in the leading edge of the south coast, −3 and generally exceeded 10  kg·m . Through the com - 3 Results parison of the SSC distribution at four moments, it can To describe the transport of water and suspended sedi- be found that when the current velocity was high during ment, the residual unit width water flux (RUWF) and the the typhoon, the water mixing was stronger, and the bot- residual unit width sediment flux (RUSF) were used to tom SSC was generally lower; when the current velocity reflect the transport of water and suspended sediment, was low, the southeastern part of Hangzhou Bay near the which is defined as: south coast was the main area for sediment fall siltation, which was also the high value area for sediment. T h 1 −→ RUWF = V dzdt (10) 3.2 Water flux and sediment flux 0 h The distribution of RUWF and RUSF during Typhoon Rumbia is shown in Fig. 10. Typhoon Rumbia occurred T h RUSF = V · C · dzdt (11) during moderate tide and landed directly in Hangzhou 0 h Bay. The characteristics of surface RUWF (Fig.  10a) and RUSF (Fig.  10b) during the typhoon were mainly ��⃗ where V is the instantaneous horizontal velocity vector; related to the shape of the local wind (Fig.  5b), with h and h are the depths at the lower and upper bounda- 1 2 surface sediment transport in Hangzhou Bay extend- ries of a certain layer, respectively; T is one or more com- ing all the way to the top of the bay. And surface water plete cycles; C is the SSC; and the unit width is 1  m. In and suspended sediment entered from the north coast this study, three semidiurnal tidal cycles were used as an in Hangzhou Bay and was then transported to the averaging time window to remove the semidiurnal and sea from the south coast, forming a counterclock- diurnal tidal signals. The thickness of the surface and bot - wise circulation pattern in the bay. The bottom RUWF tom layers was one-tenth of the total water depth over (Fig. 10c) transported a longer distance along the north the tidal cycle. This method has been used in many stud - coast than the bottom RUSF (Fig.  10d), but they all ies (Chen et  al. 2019; Li et  al. 2016b; Lyu and Zhu 2018; converged in the central part of Hangzhou Bay. The Zhu et  al. 2015). For the convenience of research, this RUWF (Fig. 10e) and RUSF (Fig. 10f ) in the whole layer paper chooses Typhoon Rumbia that directly through showed the pattern of "north-landward and south-sea- Hangzhou Bay as a case study, and its impact on Hang- ward" in Hangzhou Bay, and the water and sediment zhou Bay is more significant than that of Typhoon Yagi. mainly came from the Changjiang Estuary, which was Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 9 of 19 3.2 (a) (b) 2.4 1.6 0.8 0.0 2.4 (c) (d) 1.6 0.8 0.0 (e) (f) (g) (h) (i) (j) 8/13 8/14 8/15 8/16 8/17 8/18 8/13 8/14 8/15 8/16 8/17 8/18 Time (month/day) Fig. 8 Comparisons between the observed data (red dots) and simulated results (black line) at study site A (left panel) and site B (right panel). a, b surface velocity; c, d bottom velocity; e, f surface direction; g, h bottom direction; i, j bottom SSC similar to the distribution of RUWF and RUSF dur- normal to the transect; T is three semidiurnal tidal ing the spring tide under climatic conditions (Huang c ycle s (~ 37 h). et  al. 2022). Although Typhoon Rumbia occurred dur- The water and sediment flux across the NQ section ing moderate tide, the water and suspended sediment before and during the typhoon are shown in Table  2. transport volume in the whole layer were significantly During Typhoon Rumbia, the NTWF and NTSF across greater than those during the climatological spring tide, the NQ section increased by 18.13% and 265.75%, while the surface water and suspended sediment trans- respectively, compared with those before the typhoon. port were mainly determined by the wind field of the It is worth noting that the sediment flux increased typhoon. more significantly than the water flux, indicating that The net transect water flux (NTWF) and the net tran - the typhoon greatly promoted sediment resuspension. sect sediment flux (NTSF) across a section are calcu - Typhoons such as Typhoon Rumbia, which made direct lated using the following equations: landfall in Hangzhou Bay, can promote more suspended sediment to enter Hangzhou Bay from the mouth of the T ζ L −→ Changjiang Estuary, which played an important role in NTWF = V dldzdt (12) 0 −H 0 the sediment exchange between the Changjiang Estu- ary and Hangzhou Bay. T ζ L −→ NTSF = CV dldzdt (13) 4 Discussion 0 −H 0 To discuss the effects of sediment-induced stratifica - tion, waves, and winds on suspended sediment during where ζ is the water level; L is the width of the tran- −→ typhoons, three numerical sensitivity experiments were sect ; C is the SSC; V is the velocity component 3 Direction (°) Direction (°) Velocity (m/s)Velocity (m/s) SSC (kg/m ) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 10 of 19 Fig. 9 Modeled bottom SSC at the maximum flood (a), flood slack (b), maximum ebb (c) and maximum ebb (d) during Typhoon Rumbia set up in this study. Exp 1 was without sediment-induced where ρ is the density of seawater without sediment, ρ w s stratification, Exp 2 was without waves, and Exp 3 was is the density of suspended sediment, and ρ is the actual without winds. The other dynamic factors were the same density of seawater with SSC. as the numerical model settings in Sect.  3, which was The comparison results of Exp 0 and Exp 1 at sites A called the control experiment (Exp 0). and B during the typhoon are shown in Fig.  11. After considering sediment-induced stratification, the simu - lated surface SSC at sites A and B decreased by 50.8% 4.1 E ec ff t of sediment‑induced stratification and 58.6%, respectively; the vertical eddy diffusivity ( K ) In high turbidity estuaries, sediment-induced stratifica - decreased by 41.0% and 36.0%, respectively, and the tion plays an important role in the trapping of bottom simulated bottom SSC decreased by 27.8% and 37.5%, sediment (Huang et al. 2022; Zhu et al. 2021). Compared respectively. The results show that the simulated SSC with other estuaries worldwide, the water in Hangzhou was much more consistent with the observed values after Bay is characterized by high turbidity. The formula for considering the sediment-induced stratification. the contribution of sediment to density is as follows The gradient Richardson number (R ) is often used to (Winterwerp 2001): estimate the relative strength of stratification and mix - ing in the water column (Galperin et al. 2007; Grant and Madsen 1986; Richardson 1920) and can be expressed as: ρ = ρ + 1 − C w (14) s Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 11 of 19 31.5 3 -1 -1 0.6 m ⋅s 0.3 kg⋅s 3 -1 -1 0.2 m ⋅s 0.2 kg⋅s 3 -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 m ⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (a) (b) 31.5 3 -1 -1 0.6 m ⋅s 0.3 kg⋅s 3 -1 -1 0.2 m ⋅s 0.2 kg⋅s 3 -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 m ⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (c) (d) 31.5 3 -1 -1 7.0 m ⋅s 1.5 kg⋅s 3 -1 -1 3.0 m ⋅s 1.0 kg⋅s 3 -1 -1 31.0 Nanhui Cape Nanhui Cape 1.0 m ⋅s 0.5 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (e) (f) 121.0 121.5 122.0 122.5 123.0 121.0121.5 122.0122.5 123.0 Longitude (°E) Longitude (°E) Fig. 10 The distributions of RUWF (left panel) and RUSF (right panel) in the surface layer (a, b), bottom layer (c, d), and whole layer (e, f) during Typhoon Rumbia g ∂ρ ρ ∂z Table 2 NTWF and NTSF across the NQ section. Negative values 0 R = (15) indicate sediment flow into Hangzhou Bay, and positive values (∂V /∂z) indicate sediment flow into the Changjiang Estuary where ρ is density; V is the vector horizontal velocity; g is Flux type Before Typhoon Rumbia Increasing the acceleration of gravity; ρ is the reference density; Typhoon rate (%) Rumbia z is the depth. Miles (1961) suggested that R has a criti- cal value of 0.25, above which stable stratification tends 9 3 NTWF (10 m ) -1.93 − 2.28 18.13 to occur, while below this value, stratification tends to NTSF (10 kg) -1.46 − 5.34 265.75 be unstable, and mixing may occur. In this study, we Latitude (°N) Latitude (°N) Latitude (°N) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 12 of 19 (a) (b) with sediment-induced stratification with sediment-induced stratification without sediment-induced stratification without sediment-induced stratification (c) (d) with sediment-induced stratification with sediment-induced stratification without sediment-induced stratification without sediment-induced stratification -3 (e) (f) with sediment-induced stratification with sediment-induced stratification without sediment-induced stratification without sediment-induced stratification 8/17 8/18 8/17 8/18 Time (month/day) Time (month/day) Fig. 11 Temporal variation in surface SSC (a, b), bottom SSC (c, d), and vertical eddy diffusivity (e, f) at sites A (left panel) and B (right panel) in Exp 0 and Exp 1 during the typhoon. Red dots: measured data, same below 0.4 Number of black dots: 33 Number of black dots: 6 Number of blue dots: 588 Number of blue dots: 615 0.3 0.2 0.1 (a) (b) 0.1 0.2 0.3 0.4 0.10.2 0.30.4 R (with sediment-induced stratification) R (with sediment-induced stratification) i i Fig. 12 Comparison of R at site A (a) and site B (b) with and without sediment-induced stratification during the typhoon. The blue dots: R at the i i current moment of Exp 0 is greater than Exp 1; the black dots: the opposite situation; the green line: the threshold of 0.25 calculate the instantaneous value of R at different times the suppression of vertical mixing by sediment-induced to indicate the variation in water mixing intensity. stratification during typhoons should not be ignored. The comparison of R between Exp 0 and Exp 1 at sites A and B during the typhoon are shown in Fig.  12. Each 4.2 Eecfft of waves point on the graph represents the relative position of R First, let’s study the difference in the effects of waves with and without sediment-induced stratification at a on the water column before and during typhoons. The certain moment. During the typhoon, the R in the water comparison results of Exp 0 and Exp 2 at sites A and B column is almost always increasing after considering sed- before the typhoon are shown in Fig. 13, taking August 5 iment-induced stratification (the number of blue dots is to 8, 2018, as an example. Before the typhoon, the aver- much greater than the number of black dots). Therefore, age significant wave height at sites A and B in Hangzhou K 3 h SSC (kg/m ) 3 SSC (kg/m ) R (without sediment-induced stratification) i Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 13 of 19 2.0 (a) with waves without waves (b) with waves without waves 1.5 1.0 0.5 0.0 (c) with waves without waves (d) with waves without waves -3 (e) (f) with waves without waves with waves without waves 2.0 (g) (h) with waves without waves with waves without waves 1.5 1.0 0.5 0.0 1.0 (i) (j) 0.8 0.6 0.4 0.2 0.0 8/6 8/7 8/8 8/6 8/7 8/8 Time (month/day) Time (month/day) Fig. 13 Temporal variation in surface SSC (a, b), bottom SSC (c, d), vertical eddy diffusivity (e, f), bottom shear stress (g, h), and significant wave height (i, j) at sites A (left panel) and B (right panel) in Exp 0 and Exp 2 before the typhoon Bay were 0.27 m and 0.26 m, respectively. After consider- effect of wave-induced bottom shear stress on sediment ing waves before the typhoon, the vertical eddy diffusiv - resuspension. ity ( K ) at sites A and B increased by 21.1% and 28.2%, The comparison results of Exp 0 and Exp 2 at sites respectively, and the simulated surface SSC increased A and B during the typhoon are shown in Fig.  14. by 11.8% and 44.3%, respectively, indicating that waves During Typhoon Rumbia, the average significant significantly increased the vertical mixing in the water wave height at sites A and B in Hangzhou Bay were column before the typhoon, increasing the surface SSC. 1.37  m and 1.17  m, respectively, which were signifi- After considering waves before the typhoon, the bottom cantly higher than those before the typhoon. After shear stress at sites A and B increased by 5.8% and 10.9%, considering waves during the typhoon, the simulated respectively, while the simulated bottom SSC decreased surface SSC at sites A and B increased by 4.7% and by 9.3% and 5.7%, respectively, indicating that the wave- 1.0%, respectively; the bottom shear stress increased induced bottom shear stress before the typhoon is a small by 41.5% and 52.5%, respectively; the simulated bot- amount compared to the tidal-induced bottom shear tom SSC increased by 89.6% and 74.1%, respectively; stress. The upward transport of bottom sediment caused while K decreased by 17.5% and 7.2%, respectively. by the enhanced vertical mixing by waves is greater than Compared with before the typhoon, it can be con- the increase in sediment resuspension caused by the cluded that the effect of waves on the bottom SSC wave-induced bottom shear stress, thus reducing the during the typhoon is more significant, while the bottom SSC. The results show that before typhoons, the effect on the surface SSC is less. And the effect of effect of waves on vertical mixing is stronger than the waves at site A is greater than that at site B during the -2 3 (N m ) SSC (kg/m ) Wave height (m) h 3 SSC (kg/m ) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 14 of 19 (a) (b) with waves without waves with waves without waves (c) (d) with waves without waves with waves without waves -3 (e) (f) with waves without waves with waves without waves (g) (h) with waves without waves with waves without waves 3.0 (i) (j) 2.4 1.8 1.2 0.6 0.0 8/17 8/18 8/17 8/18 Time (month/day) Time (month/day) Fig. 14 Temporal variation in surface SSC (a, b), bottom SSC (c, d), vertical eddy diffusivity (e, f), bottom shear stress (g, h), and significant wave height (i, j) at sites A (left panel) and B (right panel) in Exp 0 and Exp 2 during the typhoon typhoon, indicating that the effect of waves near the will indirectly cause sediment-induced stratification outside the estuary is more significant. enhancement to inhibit vertical mixing. The two is a Theoretically, the waves help to increase the vertical process of mutual cancellation. mixing in the water column, which is consistent with According to the above analysis, waves further affect the results before typhoons. It is an interesting find - vertical mixing indirectly by changing the vertical SSC ing that K decreases during typhoons when waves are gradient. Before typhoons, the direct effect of waves on considered. To show the effect of waves on SSC more vertical mixing is stronger than the effect of wave-induced graphically, a profile of the SSC at site A is presented, as bottom shear stress on the sediment resuspension. While shown in Fig. 15. Figure 15a shows that the surface and during typhoons, the wave-induced bottom shear stress bottom SSC differ greatly during typhoons, especially greatly promotes the sediment resuspension, which indi- the bottom SSC gradient, while Fig.  17b shows that rectly makes the sediment-induced stratification (mainly the vertical SSC gradient decreases significantly with - in the bottom layer) stronger than the direct effect of out waves. Section  4.1 concludes that the sediment- waves on the vertical mixing in the water column. induced stratification is highly significant, so it can be The distribution of suspended sediment transport deduced that the difference in the vertical SSC caused in Hangzhou Bay without waves in Exp 2 is shown in by waves during typhoons will intensify the sediment- Fig. 16a, c, e. Compared with Fig. 10b, d, f, it can be found induced stratification effect. On the one hand, waves that the waves did not significantly change the direc - will directly cause vertical mixing enhancement, and tion of suspended sediment transport, but significantly on the other hand, wave-induced bottom shear stress affected the amount of suspended sediment transport Wave height (m) -2 h 3 3 (N m ) SSC (kg/m ) SSC (kg/m ) 1.2 1.2 1.6 1.6 1.2 0.4 2.8 1.2 1.2 0.4 0.8 1.6 2.4 3.6 Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 15 of 19 -3 -3 SSC (kg m ) (a) -1 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -3 -3 SSC (kg m ) (b) -1 3.5 1 3.0 2.5 2.0 1.5 1.0 0.5 3.2 2.4 0.0 -3 -3 SSC (kg m ) (c) -1 3.5 0.4 1 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0369 12 15 18 21 24 27 30 33 36 39 42 Time (hour) Fig. 15 The vertical profile distributions of SSC at site A during the typhoon. a Exp 0; b Exp 2; c Exp 3 in the bottom and whole layer. The NTSF across the NQ bottom shear stress decreased by only 5.4% and 5.1%, section in Exp 2 decreased by 66.10% compared to Exp 0 respectively. According to the analysis in Sect. 4.2, after (Table  3), which greatly reduced the sediment transport waves were removed by Exp 2 during typhoons, the from the Changjiang Estuary to Hangzhou Bay, indicat- surface SSC decreased very little, while K increases ing that waves significantly affected sediment resuspen - instead. When winds were removed in Exp 3, the sur- sion during typhoons. face SSC, bottom SSC, and K all decreased, while the bottom shear stress increased slightly, indicating that 4.3 Eecfft of winds the strong winds brought by typhoons mainly enhanced To further illustrate the effect of strong winds brought the vertical mixing, thereby increasing the surface and by typhoons on the SSC in Hangzhou Bay, Exp 3 is set bottom SSC. From the SSC profile at site A (Fig.  15c), in this study. The comparison results of Exp 0 and Exp it can be seen that the vertical SSC gradient in Exp 3 is 3 at sites A and B during the typhoon are shown in obviously stronger than that in Exp 2, but both weaker Fig. 17. After considering winds during the typhoon, K than that in Exp 0, especially in the bottom layer. The at sites A and B increased by 41.4% and 9.6%, respec- results further indicated that the effect of waves dur - tively; the simulated surface SSC increased by 181.9% ing typhoons significantly enhanced sediment-induced and 33.9%, respectively; the simulated bottom SSC stratification, while winds had a stronger effect on sur - increased by 74.2% and 57.0%, respectively, and the face SSC than waves. 2.4 2.8 1.2 1.6 3.6 2.8 1.6 0.8 1.6 1.2 1.2 0.8 1.2 0.8 0.8 1.2 0.8 Depth (m) Depth (m) Depth (m) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 16 of 19 31.5 -1 -1 0.3 kg⋅s 0.3 kg⋅s -1 -1 0.2 kg⋅s 0.2 kg⋅s -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 kg⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (a) (b) 31.5 -1 -1 0.3 kg⋅s 0.3 kg⋅s -1 -1 0.2 kg⋅s 0.2 kg⋅s -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 kg⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (c) (d) 31.5 -1 -1 1.5 kg⋅s 1.5 kg⋅s -1 -1 1.0 kg⋅s 1.0 kg⋅s -1 -1 31.0 Nanhui Cape Nanhui Cape 0.5 kg⋅s 0.5 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (e) (f) 121.0 121.5 122.0 122.5 123.0 121.0 121.5 122.0 122.5 123.0 Longitude (°E) Longitude (°E) Fig. 16 The distributions of RUSF in the surface layer (a, b), bottom layer (c, d), and whole layer (e, f) in Exp 2 (left panel) and Exp 3 (right panel) during the typhoon Latitude (°N) Latitude (°N) Latitude (°N) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 17 of 19 Table 3 NTSF across the NQ section. Negative values indicate analyze the effect of typhoons on sediment trans- sediment flow into Hangzhou Bay, and positive values indicate port in Hangzhou Bay, in which advection, diffusion, sediment flow into the Changjiang Estuary flocculation, settlement, waves, sediment-induced stratification, and other factors were considered. The Type Exp 0 Exp 2 Exp 3 numerical model can sufficiently reproduce the varia- NTSF (10 kg) − 5.34 -1.81 − 2.04 tion in SSC during typhoons. The transport of water Declining rate (%) / 66.10 61.80 and sediment during Typhoon Rumbia in 2018 was simulated and analyzed. During typhoons, the water and suspended sediment transport in Hangzhou Bay The distribution of suspended sediment transport showed a pattern of "north-landward and south-sea- in Hangzhou Bay without winds in Exp 3 is shown in ward", which made the suspended sediment converge Fig. 16b, d, f. Compared with Fig. 10b, d, f, it can be found in the central part of the bay. During Typhoon Rumbia, that strong winds have significantly changed the amount the NTWF and NTSF across the NQ section increased and direction of suspended sediment transport in the by 18.13% and 265.75%, respectively, compared with surface and bottom layers of the bay, but the characteris- those before the typhoon, prompting more suspended tics of " north-landward and south-seaward " of the RUSF sediment to enter Hangzhou Bay. in the whole layer in Hangzhou Bay are still the same. For high turbidity waters, such as those in Hang- The NTSF across the NQ section in Exp 3 decreased by zhou Bay, the simulated SSC during typhoons was 61.80% compared to Exp 0 (Table  3), indicating that the much more consistent with the observed values strong winds during typhoons can promote sediment after considering the sediment-induced stratifica- transport from the Changjiang Estuary to Hangzhou Bay. tion. The suppression of vertical mixing by sediment- induced stratification during typhoons should not 5 Conclusion be ignored. The wave-induced bottom shear stress Based on the ECOM-si three dimensional numeri- during typhoons has a very significant impact on cal model, a three dimensional suspended sediment the bottom SSC, which greatly promotes sediment numerical model was established to simulate and resuspension. The strong winds brought by typhoons (a) (b) with winds without winds with winds without winds (c) (d) 9 with winds without winds with winds without winds -3 (e) (f) with winds without winds with winds without winds (g) (h) with winds without winds with winds without winds 8/17 8/18 8/17 8/18 Time (month/day) Time (month/day) Fig. 17 Temporal variation in surface SSC (a, b), bottom SSC (c, d), vertical eddy diffusivity (e, f), and bottom shear stress (g, h) at sites A (left panel) and B (right panel) in Exp 0 and Exp 3 during the typhoon -2 h 3 3 (N m ) SSC (kg/m ) SSC (kg/m ) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 18 of 19 References mainly enhanced the vertical mixing, which has a Arakawa A, Lamb VR (1977) Computational design of the basic dynamical pro- stronger effect on surface SSC than waves. Before cesses of the UCLA general circulation model. 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Computers Geosciences 34(10):1243–1262. https:// doi. org/ 10. 1016/j. cageo. 2008. 02. 010 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Anthropocene Coasts Springer Journals

Suspended sediment dynamics and influencing factors during typhoons in Hangzhou Bay, China

Anthropocene Coasts , Volume 6 (1) – Feb 6, 2023

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Springer Journals
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Copyright © The Author(s) 2023
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2561-4150
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10.1007/s44218-023-00019-5
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Abstract

Hangzhou Bay is located in China on the south side of the Changjiang Estuary and is vulnerable to extreme weather, such as typhoons in the summer and autumn. In this study, a three dimensional suspended sediment numerical model was developed that considers the dynamic factors of advection, mixing, wave, and sediment-induced stratifi- cation to simulate and analyze the effect of typhoons on water and sediment transport in Hangzhou Bay. The model validations show that the model can sufficiently reproduce the variability of the suspended sediment concentration (SSC) during typhoon conditions. The simulation results show that the high SSC in the bottom layer was mainly dis- −3 tributed in the leading edge of the south coast, and generally exceeded 10 kg·m . During typhoons, the water and suspended sediment transport in Hangzhou Bay presented a pattern of "north-landward and south-seaward" circula- tion, which promoted the convergence of suspended sediment in the center part of the bay. During Typhoon Rumbia in 2018, the water and sediment flux across the section from Nanhui Cape to Qiqu Archipelago (NQ section) increased by 18.13% and 265.75%, respectively, compared with those before the typhoon. The wave-induced bottom shear stress during typhoons has a very significant impact on the bottom SSC. The sensitivity experiments show that the wave-induced bottom shear stress greatly promotes the sediment resuspension during typhoons, which indirectly makes the sediment-induced stratification stronger than the direct effect of waves on the vertical mixing. The strong winds brought by typhoons mainly enhanced the vertical mixing, which has a stronger effect on surface SSC than waves. The suppression of vertical mixing by sediment-induced stratification during typhoons should not be ignored, especially for high turbidity coastal waters, such as Hangzhou Bay. Keywords Sediment transport, Typhoons, Wave, Stratification, Hangzhou Bay, Numerical model an important mechanism that drives drastic changes in 1 Introduction suspended sediment concentration (SSC) in bays. The Suspended sediments are carriers of nutrients, organic East China Sea is affected by four typhoons every year matter, and pollutants that impede light transmission, on average (Lu et al. 2018).Very high waves are generated photosynthesis, and primary productivity and affect during typhoons, these waves propagate into nearshore the marine environment and ecological processes (Fang areas and induce phenomena such as wave reflection, et  al. 2016; Li et  al. 2016a; Zhao et  al. 2018). Though refraction, and fragmentation. In recent years, the inten- there are numerous factors, typhoons are undoubtedly sity and frequency of extreme events, such as typhoons, have increased, and the impact of typhoons on sediment *Correspondence: resuspension and redistribution in coastal waters has Jianrong Zhu gradually attracted increasing attention from oceanog- jrzhu@sklec.ecnu.edu.cn State Key Laboratory of Estuarine and Coastal Research, East China raphers (Bian et  al. 2010; Gong and Shen 2009; Lu et  al. Normal University, Shanghai 200241, China 2018; Palinkas et  al. 2014; Xie et  al. 2018). Analyzing © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 2 of 19 how waves influence the sediment transport and deposi - were strongest in winter and enhanced the sediment tion/resuspension processes in coastal bays is of critical deposition in the western shoal of the Pearl River Estu- importance to studies of coastal geomorphology or for ary. The effect of waves on SSC is more significant during research exploring the ecological effects of typhoons (Liu typhoons. Shen et  al. (2018) and Ren et  al. (2021) con- et al. 2022; Zhang et al. 2021). cluded that waves during typhoons greatly contributed to In recent years, the supply of fluvial sediments to estua - sediment resuspension in the Changjiang Estuary. Brand rine and coastal systems has been significantly reduced et  al. (2010) concluded that the wave effects increased −1 due to upstream dams (Syvitski et  al. 2009, 2005). With SSC from 30  mg·L   during calm periods to more than −1 the reduction of sediment entering the sea, tidal currents 100 mg·L  during turbulent periods at the shoals in the and waves play an important role in maintaining the southern portion of San Francisco Bay, USA. In addition, SSC in coastal waters. For example, Huang et  al. (2021) waves can change the vertical stratification of the estuary concluded that the main reason for the slow decline of and affect the distribution of suspended sediment and its SSC in the Changjiang Estuary and its adjacent waters transport direction (Green and Coco 2014). is related to wave and tide-induced sediment resuspen- Hangzhou Bay is located on the south side of the sion. Luo et  al. (2017) found that the bottom sediment Changjiang Estuary (Fig.  1), connects to the Qian- resuspension was mainly caused by tide-induced shear tang River in the west, and is characterized by a large stress and diffused to the surface layer by vertical mix - irregular and semidiurnal tide, strong flow, and high ing in winter. The waves increase the bottom shear stress SSC. Morphologically, Hangzhou Bay is an east‒west through wave-current interaction, thus effectively pro - oriented trumpet shaped bay. The bay has one of the moting sediment resuspension (Hsu et  al. 2006; Xu and highest SSC in the world. With the development of You 2017). For example, Xu and You (2017) studied the numerical models, some researchers have used numeri- effects of waves on sediment resuspension in the Oujiang cal models to carry out SSC research in Hangzhou Estuary, China. Zhang et  al. (2021) studied wave effects Bay. Xie et  al. (2009) developed a two-dimensional on water and sediment transport in the Pearl River Estu- suspended sediment numerical model in Hangzhou ary, China, and the results showed that the wave effects Bay to obtain sediment transport directions. Du et  al. Fig. 1 Map of Hangzhou Bay and Changjiang Estuary. Black triangle: the anchored ship stations; thick black line: the section from Nanhui Cape to Qiqu Archipelago; black dot: Chongming weather station Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 3 of 19 (2010) used a three-dimensional suspended sediment the different factors is presented in Sect.  4. Finally, the model to analyze the temporal variation in SSC during conclusion is presented in Sect. 5. the neap-spring tidal cycle in Hangzhou Bay. Xie et  al. (2013) used the Delft3D to find that sediment trans -2 Materials and methods port in Hangzhou Bay was affected by tidal asymme -2.1 Observations try. Because the bay mouth is occupied by Zhoushan The State Key Laboratory of Estuarine and Coastal Islands, it is not easy for waves from the outer sea to Research, East China Normal University, conducted con- enter the bay. The waves in Hangzhou Bay are weak tinuous field observations in Hangzhou Bay from August under normal weather conditions, with an annually 5 to 20, 2018. Two marine research ships collected averaged wave height of 0.2–0.5  m (Xie et  al. 2013). simultaneously observations at site A and site B (Fig.  1). However, Hangzhou Bay has large waves during Tripod-mounted observation systems were placed on typhoons and is affected by typhoons almost every year. the seafloor at each site. The instruments mounted in Li et  al. (2022a) concluded that the SSC in Hangzhou each tripod were positioned 0.2  m above the bottom. A Bay was most affected by waves during Typhoon Chan- 600  kHz acoustic Doppler current profiler (ADCP, RD hom based on FVCOM. Li et al. (2022b) concluded that Instruments) was used to measure the vertical current the combined wave-current bottom stress was the pri- profile; an optical backscatter sensor (OBS, D&A Instru - mary wave-current interaction that changed sediment ment Company) was used to measure salinity, tempera- resuspension and increased SSC. Due to the difficulty ture, and turbidity; a Sea-Bird SBE37 CTD was used to of actual observation data during typhoons, research measure temperature and salinity; and an electromag- on the SSC characteristics in Hangzhou Bay during netic current meter (Alec, Electronics, Tokyo) was used typhoon conditions is still relatively lacking. to measure the near-bottom current inaccessible by the Based on the improved ECOM-si (Estuary, Coast, and ADCP. The ADCP worked in upward-looking mode at a Ocean Model with semi-implicit), a high resolution three vertical resolution of 0.25  m and ensembles of 2  min at dimensional numerical model of suspended sediment in a 1-s ping interval. A diagram of the instrument posi- Hangzhou Bay is developed. The model takes into con - tions is shown in Fig. 2. Water samples were taken at the sideration advection, diffusion, settlement, sediment floc - sites and brought back to the laboratory for SSC meas- culation, waves, and sediment-induced stratification, to urements to determine the relationship between OBS simulate the sediment transport in Hangzhou Bay under turbidity and SSC. Surface data could not be obtained typhoon conditions. The model description and valida - during typhoons Yagi and Rumbia, which impacted the tion are presented in Sect.  2. In Sect.  3, the water and site in 2018. suspended sediment transport process in Hangzhou Bay At 23:00 on August 12, 2018, Typhoon Yagi made land- during typhoons are analyzed. The sensitivity analysis for fall on the coast of Zhejiang, China, with a maximum Alec ADCP OBS CTD Fig. 2 Tripod observation system and positions of the instruments Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 4 of 19 wind speed of 28  m/s; at 4:00 on August 17, 2018, to Typhoon Rumbia, the wind field in Hangzhou Bay was Typhoon Rumbia directly passed through Hangzhou mainly cyclonic (Fig. 5b). Bay and made landfall off the southern coast of Shang - hai with a maximum wind speed of 23  m/s (Fig.  3). The 2.2 Numerical model setup tripod observation systems at sites A and B continu- The improved three dimensional ECOM-si includes ously recorded the variations in benthic hydrodynam- a hydrodynamic model, a sediment module, and the ics and SSC during the typhoon period from August SWAN (Simulating Waves Nearshore) model, which pro- 12 to August 18, 2018. The weather station located on vides wave parameters for the hydrodynamic model and Chongming Island recorded wind speed changes during sediment module. The hydrodynamic model originated typhoons. Since Chongming Island is far from the center from the POM (Princeton Ocean Model) developed by of Typhoon Yagi, its maximum wind speed was approxi- Princeton University (Blumberg and Mellor 1987). The mately 13  m/s, while the center of Typhoon Rumbia model uses the “Arakawa C” grid configuration variables was close to Chongming Island, and its maximum wind (Arakawa and Lamb 1977). A nonorthogonal curve grid speed was approximately 20  m/s (Fig.  4). The impact of was used in the horizontal direction (Chen et  al. 2004), Typhoon Rumbia on Hangzhou Bay is more significant and the sigma coordinate was adopted in the vertical than that of Typhoon Yagi. The ERA5 (ECMWF Rea - direction. The level 2.5 turbulence closure model by Mel - nalysis v5) wind field data from the ECMWF (European lor and Yamada (1982) was used to calculate the vertical Centre for Medium-Range Weather Forecast, https:// mixing coefficients, and the parameter formula of the c d s . c lima t e. c op er nic u s . e u/ c d sa pp# !/ d a t a s e t/ r e ana ly si s - stability function was from Kantha and Clayson (1994), era5- single- levels? tab= overv iew) are consistent with the while the parameterization scheme of Smagorinsky actual wind speed and direction measured by the weather (1963) was used to calculate the horizontal mixing coef- station and can accurately reproduce the wind speed var- ficients. A wet/dry scheme was included to describe the iability during typhoons (Fig. 4). Figure 5 shows the wind intertidal flat with a critical depth of 0.2 m. To reduce the field distribution at 03:00 on August 13, 2018 (Typhoon numerical dissipation and improve the computational Yagi) and at 00:00 on August 17, 2018 (Typhoon Rumbia). accuracy in the material transport process, Wu and Zhu Due to the influence of Typhoon Yagi, the wind direction (2010) developed the high order spatial interpolation at in Hangzhou Bay was mainly southeast (Fig. 5a), and due the middle temporal level coupled with a TVD limiter 32°N Shanghai East 31°N 30°N China Zhejiang Typhoon Rumbia 29°N Sea Typhoon Yagi 28°N 119°E 120°E 121°E 122°E 123°E 124°E 125°E Longitude Fig. 3 Paths of Typhoon Yagi and Typhoon Rumbia in 2018 before and after landfall Hangzhou Bay Changjiang Estuary Latitude Speed (m/s) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 5 of 19 (a) (b) 15 m/s 8/10 8/12 8/14 8/16 8/18 8/20 Time (month/day) Fig. 4 Temporal variations in wind speed (a) and wind vector (b) at Chongming weather station. Red: observed data; black: ERA5 data to solve the advection term in the material transport were adopted from the ERA5 dataset, with a temporal equation. The barotropic pressure gradient force in the resolution of 6 h and a spatial resolution of 0.25° × 0.25°. momentum equation was solved by an implicit method, For a detailed introduction to the sediment module, and the continuous equation was solved by the semi- please refer to the author’s other article (Huang et  al. implicit method of Casulli and Cattani (1994). 2022). In the model, the sediment in the water body is The model domain covered all of the Changjiang Estu - considered cohesive fine grained sediment. The role of ary and Hangzhou Bay and adjacent seas from 117.5° E to waves in sediment resuspension cannot be ignored, espe- 125° E longitude and 27.9° N to 33.7° N latitude (Fig. 6a). cially during typhoon conditions. The improved model The model grid consisted of 396 × 522 cells in the hori- algorithm calculates the bottom wave orbit velocity with zontal dimension. Fifteen sigma levels were set in the sea surface wave parameters to obtain the bottom shear vertical direction with five logarithmically distributed stress generated by the waves. This algorithm needs to layers near the bottom (σ = − 0.929, − 0.964, − 0.982, − 0 use the significant wave height, average wave direction, .991, − 1.0) to distinguish the effect of high bottom SSC and significant wave period on the sea surface. The bot - on the bottom friction drag coefficient, and 10 layers tom shear stress under the influence of wave-current were established in the remaining layers (σ = 0, − 0.1, − 0 interaction is given by (Grant and Madsen 1979): .2, − 0.3, − 0.4, − 0.5, − 0.6, − 0.7, − 0.79, − 0.87). The grid 2 2 τ = |τ + τ | = (τ + τ |cosϕ|) + (τ sinϕ) w c w c c resolution ranges between 200  m at the top of t he bay, (1) approximately 600 m in the central part, and is 2 ⁓ 10 km τ τ c c = τ 1 + 2 |cosϕ| + τ τ w w near the open sea boundary (Fig. 6b). The open sea boundary condition was specified by the where τ is the bottom shear stress considering waves and tide and residual water level. The tidal signal was com - currents; τ is the maximum wave-induced bottom shear posed of 16 astronomical constituents, M , S, N, K, K , 2 2 2 2 1 stress; τ is the tidal-induced bottom shear stress; φ is the O, P, Q, MU, NU, T, L, 2N, J, M , and OO , which 1 1 1 2 2 2 2 2 1 1 1 angle between wave propagation and the current direc- were derived from the NaoTide dataset (http:// www. miz. tion. τ and τ can be calculated by: c w nao. ac. jp/). The residual water level and initial salinity field were derived from the results simulated by a large 2 τ = ρC U (2) domain model encompassing the Bohai Sea, Yellow Sea, and East China Sea (Wu et al. 2011). The river boundary ρf was driven by the river discharge at the Datong hydro- τ = U (3) logic station (Changjiang Water Resources Commission) for the Changjiang River and at the Fuchunjiang hydro- where ρ is the actual density of seawater with the sus- electric power station for the Qiantang River. Wind data pended sediment; U is the bottom current velocity; C is Wind speed (m/s) Wind vector Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 6 of 19 31.5 20 m/s 20 m/s 2018-08-13 03:00:00 2018-08-17 00:00:00 31.0 30.5 (a) (b) 30.0 121.0 121.5 122.0 122.5 123.0 121.0 121.5 122.0 122.5 123.0 Longitude (°E) Longitude (°E) Fig. 5 Wind field distribution during Typhoon Yagi (a) and Typhoon Rumbia (b) (a) (b) Datong Fuchunjiang station Fig. 6 Numerical model domain and grid (a) and an enlarged view of the model grid in Hangzhou Bay (b) the bottom drag coefficient; f is the wave fiction factor; 4 C 0.04C p p α = 15 exp − and U is the near-bed wave orbital velocity. A detailed CB (5) u u ∗ ∗ calculation procedure can be found in Wiberg and Sher- wood (2008). Waves affect not only the bottom shear where C /u is the wave age;C is the phase speed of p ∗ p stress but also the vertical mixing coefficient (Mellor and waves at the dominant frequency; u is the air side fric- Blumberg 2004; Terray et  al. 1999). Based on the level tion velocity; and u = 30u . When the effect of waves is ∗ τ 2.5 turbulence closure module, Mellor and Blumberg 2/3 2 2 not considered, q = B u at z = 0, B = 16.6 . When 1 1 (2004) considered the effect of waves breaking on the sea 2/3 2 2 the effect of waves is considered, q = (15.8α ) u CB boundary layer. The sea surface boundary conditions are: at z = 0. From measured wave heights and near-surface 2 dissipation data, Terray et al. (1999) used the turbulence ∂q K = 2α u ,z = 0 (4) q CB closure module to find the best fit between the turbulent ∂z mixing length ( l) and the measured data. where q is the turbulence kinetic energy;u is the water l = max(κz , l ), z = 0.85H (6) w z w s side friction velocity;α is a parameter related to the CB waves, with the following equation: Latitude (°N) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 7 of 19 where H is the significant wave height; l = κz ; κ = 0.4 s z X − X ( ) mod obs SS = 1 − is the von Karman constant. When the effect of waves (8) X − X + X − X mod mod obs obs is not considered, z = 0.K = qlS is the vertical eddy w m M viscosity; K = qlS is the vertical eddy diffusivity, where h H S and S are the stability function, please refer to Mel- M H (X − X ) mod obs lor and Yamada (1982). (9) RMSE = The SWAN model (Booij et  al. 1999) adopted an orthogonal mesh that covered the calculation range of where X is the variable and is the time-averaged value. the ECOM-si, with a spatial resolution of 2’ × 2’ and a The performance levels of the modeled results and time step of 30  min. The SWAN model outputted the observed results were evaluated by SS where SS > 0.65 significant wave height, significant wave period, and is considered excellent, 0.65–0.5 is very good, 0.5–0.2 is wave direction every 3  h, and these parameters were good, and SS < 0.2 is considered poor. interpolated to each time step in the sediment module The modeled current velocity, current direction, salin - to calculate the bottom shear stress and vertical mixing ity, and SSC were validated with observation data during coefficient under wave-current interactions. The wave neap tide and spring tide at site A and site B in Hangzhou parameters output from the SWAN model has been vali- Bay in August 2018; see Huang et  al. (2022). This study dated in related papers by our research group (Luo et al. used data that included typhoon periods to validate the 2017). model. We downloaded the real-time river discharge data for the Datong hydrological station and Fuchunjiang 2.3 Model validation hydropower station, as well as the real-time wind field The ECOM-si has been extensively validated in terms of data from ERA5 to drive the model. The model was cold water level, current speed and direction, salinity, and SSC started on 1 July 2018 and ran for 62 days. The compari - (Chen et al. 2019; Luo et al. 2017; Lyu and Zhu 2018; Qiu son between the simulated water level and the measured and Zhu 2013; Wu and Zhu 2010; Zhu et  al. 2015). This data from August 6 to 20, 2018, is shown in Fig. 7. Hang- study will further validate the model in Hangzhou Bay. zhou Bay is famous for its strong tides. In terms of tidal The following three skill assessments were used to quan - nature, the tides outside the mouth of the bay are regu- tify the validation of the model: correlation coefficient lar semidiurnal tides and inside the bay mouth, they are (CC), root mean square error (RMSE), and skill score irregular semidiurnal tides. After August 12, Hangzhou (SS) (Murphy 1988; Warner et al. 2005; Willmott 1981): Bay was impacted by Typhoon Yagi and Typhoon Rum- bia. Regardless of whether it was normal or typhoon con- X − X X − X mod mod obs obs CC = ditions, the simulated water level was consistent with the 2 2 2 measured value, and both the SS and CC exceeded 0.95 X − X X − X mod mod obs obs (Table 1), indicating that the model can successfully sim- (7) ulate the temporal variation in water level. (a) -2 -4 (b) -2 -4 6789 10 11 12 13 14 15 16 17 18 19 20 Time (Day) Fig. 7 Comparison of the simulated water level (black line) and measured data (red dot) at site A (a) and site B (b) in Hangzhou Bay from August 6 to 20, 2018 Elevation (m) Elevation (m) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 8 of 19 Table 1 Correlation coefficients (CC), root mean square error (RMSE), and skill scores (SS) for comparison of the simulated and observed data at the measuring stations Skill assessment Site A Site B CC RMSE SS CC RMSE SS Water level 0.99 0.25 m 0.99 0.99 0.24 m 0.99 −1 −1 Surface velocity 0.93 0.22 m·s 0.96 0.95 0.17 m·s 0.98 −1 −1 Bottom velocity 0.90 0.14 m·s 0.95 0.85 0.18 m·s 0.89 −3 −3 Bottom SSC 0.70 1.09 kg·m 0.83 0.50 1.07 kg·m 0.71 The simulated current velocity, direction, and SSC are 3.1 SSC compared with the observed data during typhoons in The distribution of the bottom SSC at four moments of Fig.  8. Due to the effect of bottom friction, the current the tidal cycle in Hangzhou Bay during Typhoon Rum- velocity in the bottom layer was smaller than that in the bia is shown in Fig.  9. At the maximum flood, the high surface layer. The average SSC in the bottom layer at site A SSC in the bottom layer was mainly distributed in the −3 was 2.78 kg·m , and the average SSC in the bottom layer southeastern part of Hangzhou Bay; at the flood slack, −3 at site B was 2.85 kg·m during typhoons. The CC, RMSE, the high SSC in the bottom layer was mainly distributed and SS of the simulated data and the observation data are in the south shore tidal flats and southeastern part of the −3 shown in Table 1. Note that the SS of the SSC is above 0.7. bay, generally exceeding 10  kg·m . At the maximum The model can successfully reproduce the variability in the ebb, the high SSC in the bottom layer was mainly located current velocity, direction, and SSC during typhoon condi- on the south coast and in the northern part of the bay; tions, and can be used to study the hydrodynamic and SSC at the ebb slack, the high SSC in the bottom layer was transport process in Hangzhou Bay. mainly distributed in the leading edge of the south coast, −3 and generally exceeded 10  kg·m . Through the com - 3 Results parison of the SSC distribution at four moments, it can To describe the transport of water and suspended sedi- be found that when the current velocity was high during ment, the residual unit width water flux (RUWF) and the the typhoon, the water mixing was stronger, and the bot- residual unit width sediment flux (RUSF) were used to tom SSC was generally lower; when the current velocity reflect the transport of water and suspended sediment, was low, the southeastern part of Hangzhou Bay near the which is defined as: south coast was the main area for sediment fall siltation, which was also the high value area for sediment. T h 1 −→ RUWF = V dzdt (10) 3.2 Water flux and sediment flux 0 h The distribution of RUWF and RUSF during Typhoon Rumbia is shown in Fig. 10. Typhoon Rumbia occurred T h RUSF = V · C · dzdt (11) during moderate tide and landed directly in Hangzhou 0 h Bay. The characteristics of surface RUWF (Fig.  10a) and RUSF (Fig.  10b) during the typhoon were mainly ��⃗ where V is the instantaneous horizontal velocity vector; related to the shape of the local wind (Fig.  5b), with h and h are the depths at the lower and upper bounda- 1 2 surface sediment transport in Hangzhou Bay extend- ries of a certain layer, respectively; T is one or more com- ing all the way to the top of the bay. And surface water plete cycles; C is the SSC; and the unit width is 1  m. In and suspended sediment entered from the north coast this study, three semidiurnal tidal cycles were used as an in Hangzhou Bay and was then transported to the averaging time window to remove the semidiurnal and sea from the south coast, forming a counterclock- diurnal tidal signals. The thickness of the surface and bot - wise circulation pattern in the bay. The bottom RUWF tom layers was one-tenth of the total water depth over (Fig. 10c) transported a longer distance along the north the tidal cycle. This method has been used in many stud - coast than the bottom RUSF (Fig.  10d), but they all ies (Chen et  al. 2019; Li et  al. 2016b; Lyu and Zhu 2018; converged in the central part of Hangzhou Bay. The Zhu et  al. 2015). For the convenience of research, this RUWF (Fig. 10e) and RUSF (Fig. 10f ) in the whole layer paper chooses Typhoon Rumbia that directly through showed the pattern of "north-landward and south-sea- Hangzhou Bay as a case study, and its impact on Hang- ward" in Hangzhou Bay, and the water and sediment zhou Bay is more significant than that of Typhoon Yagi. mainly came from the Changjiang Estuary, which was Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 9 of 19 3.2 (a) (b) 2.4 1.6 0.8 0.0 2.4 (c) (d) 1.6 0.8 0.0 (e) (f) (g) (h) (i) (j) 8/13 8/14 8/15 8/16 8/17 8/18 8/13 8/14 8/15 8/16 8/17 8/18 Time (month/day) Fig. 8 Comparisons between the observed data (red dots) and simulated results (black line) at study site A (left panel) and site B (right panel). a, b surface velocity; c, d bottom velocity; e, f surface direction; g, h bottom direction; i, j bottom SSC similar to the distribution of RUWF and RUSF dur- normal to the transect; T is three semidiurnal tidal ing the spring tide under climatic conditions (Huang c ycle s (~ 37 h). et  al. 2022). Although Typhoon Rumbia occurred dur- The water and sediment flux across the NQ section ing moderate tide, the water and suspended sediment before and during the typhoon are shown in Table  2. transport volume in the whole layer were significantly During Typhoon Rumbia, the NTWF and NTSF across greater than those during the climatological spring tide, the NQ section increased by 18.13% and 265.75%, while the surface water and suspended sediment trans- respectively, compared with those before the typhoon. port were mainly determined by the wind field of the It is worth noting that the sediment flux increased typhoon. more significantly than the water flux, indicating that The net transect water flux (NTWF) and the net tran - the typhoon greatly promoted sediment resuspension. sect sediment flux (NTSF) across a section are calcu - Typhoons such as Typhoon Rumbia, which made direct lated using the following equations: landfall in Hangzhou Bay, can promote more suspended sediment to enter Hangzhou Bay from the mouth of the T ζ L −→ Changjiang Estuary, which played an important role in NTWF = V dldzdt (12) 0 −H 0 the sediment exchange between the Changjiang Estu- ary and Hangzhou Bay. T ζ L −→ NTSF = CV dldzdt (13) 4 Discussion 0 −H 0 To discuss the effects of sediment-induced stratifica - tion, waves, and winds on suspended sediment during where ζ is the water level; L is the width of the tran- −→ typhoons, three numerical sensitivity experiments were sect ; C is the SSC; V is the velocity component 3 Direction (°) Direction (°) Velocity (m/s)Velocity (m/s) SSC (kg/m ) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 10 of 19 Fig. 9 Modeled bottom SSC at the maximum flood (a), flood slack (b), maximum ebb (c) and maximum ebb (d) during Typhoon Rumbia set up in this study. Exp 1 was without sediment-induced where ρ is the density of seawater without sediment, ρ w s stratification, Exp 2 was without waves, and Exp 3 was is the density of suspended sediment, and ρ is the actual without winds. The other dynamic factors were the same density of seawater with SSC. as the numerical model settings in Sect.  3, which was The comparison results of Exp 0 and Exp 1 at sites A called the control experiment (Exp 0). and B during the typhoon are shown in Fig.  11. After considering sediment-induced stratification, the simu - lated surface SSC at sites A and B decreased by 50.8% 4.1 E ec ff t of sediment‑induced stratification and 58.6%, respectively; the vertical eddy diffusivity ( K ) In high turbidity estuaries, sediment-induced stratifica - decreased by 41.0% and 36.0%, respectively, and the tion plays an important role in the trapping of bottom simulated bottom SSC decreased by 27.8% and 37.5%, sediment (Huang et al. 2022; Zhu et al. 2021). Compared respectively. The results show that the simulated SSC with other estuaries worldwide, the water in Hangzhou was much more consistent with the observed values after Bay is characterized by high turbidity. The formula for considering the sediment-induced stratification. the contribution of sediment to density is as follows The gradient Richardson number (R ) is often used to (Winterwerp 2001): estimate the relative strength of stratification and mix - ing in the water column (Galperin et al. 2007; Grant and Madsen 1986; Richardson 1920) and can be expressed as: ρ = ρ + 1 − C w (14) s Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 11 of 19 31.5 3 -1 -1 0.6 m ⋅s 0.3 kg⋅s 3 -1 -1 0.2 m ⋅s 0.2 kg⋅s 3 -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 m ⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (a) (b) 31.5 3 -1 -1 0.6 m ⋅s 0.3 kg⋅s 3 -1 -1 0.2 m ⋅s 0.2 kg⋅s 3 -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 m ⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (c) (d) 31.5 3 -1 -1 7.0 m ⋅s 1.5 kg⋅s 3 -1 -1 3.0 m ⋅s 1.0 kg⋅s 3 -1 -1 31.0 Nanhui Cape Nanhui Cape 1.0 m ⋅s 0.5 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (e) (f) 121.0 121.5 122.0 122.5 123.0 121.0121.5 122.0122.5 123.0 Longitude (°E) Longitude (°E) Fig. 10 The distributions of RUWF (left panel) and RUSF (right panel) in the surface layer (a, b), bottom layer (c, d), and whole layer (e, f) during Typhoon Rumbia g ∂ρ ρ ∂z Table 2 NTWF and NTSF across the NQ section. Negative values 0 R = (15) indicate sediment flow into Hangzhou Bay, and positive values (∂V /∂z) indicate sediment flow into the Changjiang Estuary where ρ is density; V is the vector horizontal velocity; g is Flux type Before Typhoon Rumbia Increasing the acceleration of gravity; ρ is the reference density; Typhoon rate (%) Rumbia z is the depth. Miles (1961) suggested that R has a criti- cal value of 0.25, above which stable stratification tends 9 3 NTWF (10 m ) -1.93 − 2.28 18.13 to occur, while below this value, stratification tends to NTSF (10 kg) -1.46 − 5.34 265.75 be unstable, and mixing may occur. In this study, we Latitude (°N) Latitude (°N) Latitude (°N) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 12 of 19 (a) (b) with sediment-induced stratification with sediment-induced stratification without sediment-induced stratification without sediment-induced stratification (c) (d) with sediment-induced stratification with sediment-induced stratification without sediment-induced stratification without sediment-induced stratification -3 (e) (f) with sediment-induced stratification with sediment-induced stratification without sediment-induced stratification without sediment-induced stratification 8/17 8/18 8/17 8/18 Time (month/day) Time (month/day) Fig. 11 Temporal variation in surface SSC (a, b), bottom SSC (c, d), and vertical eddy diffusivity (e, f) at sites A (left panel) and B (right panel) in Exp 0 and Exp 1 during the typhoon. Red dots: measured data, same below 0.4 Number of black dots: 33 Number of black dots: 6 Number of blue dots: 588 Number of blue dots: 615 0.3 0.2 0.1 (a) (b) 0.1 0.2 0.3 0.4 0.10.2 0.30.4 R (with sediment-induced stratification) R (with sediment-induced stratification) i i Fig. 12 Comparison of R at site A (a) and site B (b) with and without sediment-induced stratification during the typhoon. The blue dots: R at the i i current moment of Exp 0 is greater than Exp 1; the black dots: the opposite situation; the green line: the threshold of 0.25 calculate the instantaneous value of R at different times the suppression of vertical mixing by sediment-induced to indicate the variation in water mixing intensity. stratification during typhoons should not be ignored. The comparison of R between Exp 0 and Exp 1 at sites A and B during the typhoon are shown in Fig.  12. Each 4.2 Eecfft of waves point on the graph represents the relative position of R First, let’s study the difference in the effects of waves with and without sediment-induced stratification at a on the water column before and during typhoons. The certain moment. During the typhoon, the R in the water comparison results of Exp 0 and Exp 2 at sites A and B column is almost always increasing after considering sed- before the typhoon are shown in Fig. 13, taking August 5 iment-induced stratification (the number of blue dots is to 8, 2018, as an example. Before the typhoon, the aver- much greater than the number of black dots). Therefore, age significant wave height at sites A and B in Hangzhou K 3 h SSC (kg/m ) 3 SSC (kg/m ) R (without sediment-induced stratification) i Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 13 of 19 2.0 (a) with waves without waves (b) with waves without waves 1.5 1.0 0.5 0.0 (c) with waves without waves (d) with waves without waves -3 (e) (f) with waves without waves with waves without waves 2.0 (g) (h) with waves without waves with waves without waves 1.5 1.0 0.5 0.0 1.0 (i) (j) 0.8 0.6 0.4 0.2 0.0 8/6 8/7 8/8 8/6 8/7 8/8 Time (month/day) Time (month/day) Fig. 13 Temporal variation in surface SSC (a, b), bottom SSC (c, d), vertical eddy diffusivity (e, f), bottom shear stress (g, h), and significant wave height (i, j) at sites A (left panel) and B (right panel) in Exp 0 and Exp 2 before the typhoon Bay were 0.27 m and 0.26 m, respectively. After consider- effect of wave-induced bottom shear stress on sediment ing waves before the typhoon, the vertical eddy diffusiv - resuspension. ity ( K ) at sites A and B increased by 21.1% and 28.2%, The comparison results of Exp 0 and Exp 2 at sites respectively, and the simulated surface SSC increased A and B during the typhoon are shown in Fig.  14. by 11.8% and 44.3%, respectively, indicating that waves During Typhoon Rumbia, the average significant significantly increased the vertical mixing in the water wave height at sites A and B in Hangzhou Bay were column before the typhoon, increasing the surface SSC. 1.37  m and 1.17  m, respectively, which were signifi- After considering waves before the typhoon, the bottom cantly higher than those before the typhoon. After shear stress at sites A and B increased by 5.8% and 10.9%, considering waves during the typhoon, the simulated respectively, while the simulated bottom SSC decreased surface SSC at sites A and B increased by 4.7% and by 9.3% and 5.7%, respectively, indicating that the wave- 1.0%, respectively; the bottom shear stress increased induced bottom shear stress before the typhoon is a small by 41.5% and 52.5%, respectively; the simulated bot- amount compared to the tidal-induced bottom shear tom SSC increased by 89.6% and 74.1%, respectively; stress. The upward transport of bottom sediment caused while K decreased by 17.5% and 7.2%, respectively. by the enhanced vertical mixing by waves is greater than Compared with before the typhoon, it can be con- the increase in sediment resuspension caused by the cluded that the effect of waves on the bottom SSC wave-induced bottom shear stress, thus reducing the during the typhoon is more significant, while the bottom SSC. The results show that before typhoons, the effect on the surface SSC is less. And the effect of effect of waves on vertical mixing is stronger than the waves at site A is greater than that at site B during the -2 3 (N m ) SSC (kg/m ) Wave height (m) h 3 SSC (kg/m ) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 14 of 19 (a) (b) with waves without waves with waves without waves (c) (d) with waves without waves with waves without waves -3 (e) (f) with waves without waves with waves without waves (g) (h) with waves without waves with waves without waves 3.0 (i) (j) 2.4 1.8 1.2 0.6 0.0 8/17 8/18 8/17 8/18 Time (month/day) Time (month/day) Fig. 14 Temporal variation in surface SSC (a, b), bottom SSC (c, d), vertical eddy diffusivity (e, f), bottom shear stress (g, h), and significant wave height (i, j) at sites A (left panel) and B (right panel) in Exp 0 and Exp 2 during the typhoon typhoon, indicating that the effect of waves near the will indirectly cause sediment-induced stratification outside the estuary is more significant. enhancement to inhibit vertical mixing. The two is a Theoretically, the waves help to increase the vertical process of mutual cancellation. mixing in the water column, which is consistent with According to the above analysis, waves further affect the results before typhoons. It is an interesting find - vertical mixing indirectly by changing the vertical SSC ing that K decreases during typhoons when waves are gradient. Before typhoons, the direct effect of waves on considered. To show the effect of waves on SSC more vertical mixing is stronger than the effect of wave-induced graphically, a profile of the SSC at site A is presented, as bottom shear stress on the sediment resuspension. While shown in Fig. 15. Figure 15a shows that the surface and during typhoons, the wave-induced bottom shear stress bottom SSC differ greatly during typhoons, especially greatly promotes the sediment resuspension, which indi- the bottom SSC gradient, while Fig.  17b shows that rectly makes the sediment-induced stratification (mainly the vertical SSC gradient decreases significantly with - in the bottom layer) stronger than the direct effect of out waves. Section  4.1 concludes that the sediment- waves on the vertical mixing in the water column. induced stratification is highly significant, so it can be The distribution of suspended sediment transport deduced that the difference in the vertical SSC caused in Hangzhou Bay without waves in Exp 2 is shown in by waves during typhoons will intensify the sediment- Fig. 16a, c, e. Compared with Fig. 10b, d, f, it can be found induced stratification effect. On the one hand, waves that the waves did not significantly change the direc - will directly cause vertical mixing enhancement, and tion of suspended sediment transport, but significantly on the other hand, wave-induced bottom shear stress affected the amount of suspended sediment transport Wave height (m) -2 h 3 3 (N m ) SSC (kg/m ) SSC (kg/m ) 1.2 1.2 1.6 1.6 1.2 0.4 2.8 1.2 1.2 0.4 0.8 1.6 2.4 3.6 Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 15 of 19 -3 -3 SSC (kg m ) (a) -1 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -3 -3 SSC (kg m ) (b) -1 3.5 1 3.0 2.5 2.0 1.5 1.0 0.5 3.2 2.4 0.0 -3 -3 SSC (kg m ) (c) -1 3.5 0.4 1 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0369 12 15 18 21 24 27 30 33 36 39 42 Time (hour) Fig. 15 The vertical profile distributions of SSC at site A during the typhoon. a Exp 0; b Exp 2; c Exp 3 in the bottom and whole layer. The NTSF across the NQ bottom shear stress decreased by only 5.4% and 5.1%, section in Exp 2 decreased by 66.10% compared to Exp 0 respectively. According to the analysis in Sect. 4.2, after (Table  3), which greatly reduced the sediment transport waves were removed by Exp 2 during typhoons, the from the Changjiang Estuary to Hangzhou Bay, indicat- surface SSC decreased very little, while K increases ing that waves significantly affected sediment resuspen - instead. When winds were removed in Exp 3, the sur- sion during typhoons. face SSC, bottom SSC, and K all decreased, while the bottom shear stress increased slightly, indicating that 4.3 Eecfft of winds the strong winds brought by typhoons mainly enhanced To further illustrate the effect of strong winds brought the vertical mixing, thereby increasing the surface and by typhoons on the SSC in Hangzhou Bay, Exp 3 is set bottom SSC. From the SSC profile at site A (Fig.  15c), in this study. The comparison results of Exp 0 and Exp it can be seen that the vertical SSC gradient in Exp 3 is 3 at sites A and B during the typhoon are shown in obviously stronger than that in Exp 2, but both weaker Fig. 17. After considering winds during the typhoon, K than that in Exp 0, especially in the bottom layer. The at sites A and B increased by 41.4% and 9.6%, respec- results further indicated that the effect of waves dur - tively; the simulated surface SSC increased by 181.9% ing typhoons significantly enhanced sediment-induced and 33.9%, respectively; the simulated bottom SSC stratification, while winds had a stronger effect on sur - increased by 74.2% and 57.0%, respectively, and the face SSC than waves. 2.4 2.8 1.2 1.6 3.6 2.8 1.6 0.8 1.6 1.2 1.2 0.8 1.2 0.8 0.8 1.2 0.8 Depth (m) Depth (m) Depth (m) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 16 of 19 31.5 -1 -1 0.3 kg⋅s 0.3 kg⋅s -1 -1 0.2 kg⋅s 0.2 kg⋅s -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 kg⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (a) (b) 31.5 -1 -1 0.3 kg⋅s 0.3 kg⋅s -1 -1 0.2 kg⋅s 0.2 kg⋅s -1 -1 31.0 Nanhui Cape Nanhui Cape 0.1 kg⋅s 0.1 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (c) (d) 31.5 -1 -1 1.5 kg⋅s 1.5 kg⋅s -1 -1 1.0 kg⋅s 1.0 kg⋅s -1 -1 31.0 Nanhui Cape Nanhui Cape 0.5 kg⋅s 0.5 kg⋅s 30.5 Andong shoal Andong shoal 30.0 Zhenhai Zhenhai (e) (f) 121.0 121.5 122.0 122.5 123.0 121.0 121.5 122.0 122.5 123.0 Longitude (°E) Longitude (°E) Fig. 16 The distributions of RUSF in the surface layer (a, b), bottom layer (c, d), and whole layer (e, f) in Exp 2 (left panel) and Exp 3 (right panel) during the typhoon Latitude (°N) Latitude (°N) Latitude (°N) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 17 of 19 Table 3 NTSF across the NQ section. Negative values indicate analyze the effect of typhoons on sediment trans- sediment flow into Hangzhou Bay, and positive values indicate port in Hangzhou Bay, in which advection, diffusion, sediment flow into the Changjiang Estuary flocculation, settlement, waves, sediment-induced stratification, and other factors were considered. The Type Exp 0 Exp 2 Exp 3 numerical model can sufficiently reproduce the varia- NTSF (10 kg) − 5.34 -1.81 − 2.04 tion in SSC during typhoons. The transport of water Declining rate (%) / 66.10 61.80 and sediment during Typhoon Rumbia in 2018 was simulated and analyzed. During typhoons, the water and suspended sediment transport in Hangzhou Bay The distribution of suspended sediment transport showed a pattern of "north-landward and south-sea- in Hangzhou Bay without winds in Exp 3 is shown in ward", which made the suspended sediment converge Fig. 16b, d, f. Compared with Fig. 10b, d, f, it can be found in the central part of the bay. During Typhoon Rumbia, that strong winds have significantly changed the amount the NTWF and NTSF across the NQ section increased and direction of suspended sediment transport in the by 18.13% and 265.75%, respectively, compared with surface and bottom layers of the bay, but the characteris- those before the typhoon, prompting more suspended tics of " north-landward and south-seaward " of the RUSF sediment to enter Hangzhou Bay. in the whole layer in Hangzhou Bay are still the same. For high turbidity waters, such as those in Hang- The NTSF across the NQ section in Exp 3 decreased by zhou Bay, the simulated SSC during typhoons was 61.80% compared to Exp 0 (Table  3), indicating that the much more consistent with the observed values strong winds during typhoons can promote sediment after considering the sediment-induced stratifica- transport from the Changjiang Estuary to Hangzhou Bay. tion. The suppression of vertical mixing by sediment- induced stratification during typhoons should not 5 Conclusion be ignored. The wave-induced bottom shear stress Based on the ECOM-si three dimensional numeri- during typhoons has a very significant impact on cal model, a three dimensional suspended sediment the bottom SSC, which greatly promotes sediment numerical model was established to simulate and resuspension. The strong winds brought by typhoons (a) (b) with winds without winds with winds without winds (c) (d) 9 with winds without winds with winds without winds -3 (e) (f) with winds without winds with winds without winds (g) (h) with winds without winds with winds without winds 8/17 8/18 8/17 8/18 Time (month/day) Time (month/day) Fig. 17 Temporal variation in surface SSC (a, b), bottom SSC (c, d), vertical eddy diffusivity (e, f), and bottom shear stress (g, h) at sites A (left panel) and B (right panel) in Exp 0 and Exp 3 during the typhoon -2 h 3 3 (N m ) SSC (kg/m ) SSC (kg/m ) Huang and Zhu Anthropocene Coasts (2023) 6:3 Page 18 of 19 References mainly enhanced the vertical mixing, which has a Arakawa A, Lamb VR (1977) Computational design of the basic dynamical pro- stronger effect on surface SSC than waves. Before cesses of the UCLA general circulation model. 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Journal

Anthropocene CoastsSpringer Journals

Published: Feb 6, 2023

Keywords: Sediment transport; Typhoons; Wave; Stratification; Hangzhou Bay; Numerical model

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