Modelling the Effects of Typhoons on Morphological Changes in The

Continental Shelf Research 135 (2017) 1–13

Contents lists available at ScienceDirect

Continental Shelf Research

journal homepage: www.elsevier.com/locate/csr

ﬀ Modelling the e ects of typhoons on morphological changes in the Estuary MARK of Beinan, Taiwan

Wei Po Huang

Dept. Harbor and River Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan

ARTICLE INFO ABSTRACT

Keywords: On average, Taiwan is subjected to three or four typhoon invasions each year. As a typhoon approaches, strong Sediment dynamics ocean waves from the sea, which are accompanied by flash floods on land, can considerably change the Estuarine morphology morphology of estuaries. Thus, knowing the effect of typhoons is important for coastal management. This study Suspended load transport uses simulations and field surveys to analyze the processes of sediment transport near the Beinan Estuary in Typhoon impact southeastern Taiwan. A model that is based on Eulerian and Lagrangian methods is used to simulate sediment Morphological changes transport from fluvial flows, waves, and currents. Typhoons of various intensities, different types of fluvial Wave effect hydrographs, and seasonal effects are used to study their effects on the bathymetry. The relationships among the fluvial sediments, the wave loads, and the evolution of the estuary are derived from the results. The results indicate that the morphology of the Beinan Estuary is governed by typhoon events and that the process of erosion, transport, and deposition of sediment are short. The amount of fluvial sediments that is required to cause changes in the estuary is determined based on these results, and this estimate can be used for erosion protection in the future.

1. Introduction interests. Numerical models with various degrees of sophistication have been developed, and measurements have been conducted either Estuaries form transition zones between rivers and the seas and on site or through remote sensing. Uncles (2002) (see also Hardisty, thus are subjected to complex hydrodynamic processes. These envir- 2007; Prandle, 2009) reviewed the progress in analyzing the physical onments require special attention because of their social, economic, processes of estuaries. and ecological importance. The morphology of an estuary is governed Generally, numerical models are used for one of two purposes, by both natural and anthropogenic factors. Natural factors include namely, either to predict the evolution of estuarine/coastal morpho- river discharges, sediment sources, tides, and wave forcing; anthro- dynamics (Bernardes, 2005; Dastgheib, 2012; Guo, 2014)orto pogenic factors include activities such as channel dredging, land evaluate possible changes that are caused by human activities reclamation, sea walls, or riverbank protection (Cook et al., 2007; (Gelfort et al., 2010). Numerical models for coastal processes have Dalrymple et al., 2012; Guo et al., 2012; Natesan et al., 2014; Yamada progressed substantially over the past several decades. Models are now et al., 2012). Montagna et al. (2013) noted that the interactions among available that cover the full spectrum of dominating factors, such as the climate, continental geology, and tidal regime make estuaries tidal currents, waves, wave-induced currents, and sediment transport. unique and different. Dalrymple et al. (1992) used three natural factors These models can be used in more complicated situations and provide to describe an estuary. Other definitions and classifications for more flexibility in considering a variety of loads that act on the seabed. estuaries have been presented in the past; however, none seems to Huan et al. (2010) used the wave, flow, and mud transport modules of cover all the characteristics of estuaries (Perillo, 1995; Simenstad and Mike 21 by DHI to simulate the sediment transport in the Bach Dang Yanagi, 2012). Because most of the base flows in the rivers of Taiwan Estuary, Vietnam. Mud transport under both fair and stormy weather are almost reduced to trickles during the dry season, the estuaries in conditions was modelled with reasonable results. A numerical model Taiwan can be categorized as ‘intermittent estuaries’ (Perillo, 1995). called CCHE2D-Coast was applied by Ding et al. (2013) to evaluate the With the increase in coastal threats, the interests of the scientific Touchien Estuary, western Taiwan. The hydrodynamic and morpho- community in solving the problems of estuaries/coasts have also dynamic responses to waves, tides and river flows were modelled. The increased in the past decades. The comprehensive references that were results were used to propose flood prevention and erosion protection recently published by Wolanski and McLusky (2012) confirmed these for the estuary.

E-mail address: [email protected]. http://dx.doi.org/10.1016/j.csr.2017.01.011 Received 7 September 2016; Received in revised form 20 December 2016; Accepted 22 January 2017 Available online 23 January 2017 0278-4343/ © 2017 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). W.P. Huang Continental Shelf Research 135 (2017) 1–13

Fluvial sediments were assumed to have the same grain size Taiwan. Both seasonal weather and typhoon invasions are considered. distribution as those in the shelf zone in the above-mentioned models. The calculated results compare favorably with data from field surveys. This scheme has an inherent drawback in that sediments that are A ‘first-order estimate’ of the amount of fluvial sediments to cause carried by large flows from a river during a flood can have grain size erosion/accretion is proposed according to these results. distributions that differ markedly from those in the coastal area. Larger grain sizes settle down closer to the river mouth, whereas finer 2. Study domain sediments are transported farther into the sea and distributed more or less evenly around the coast. Geleynse et al. (2010) showed that the 2.1. Geographical background formation of sandbars near the river mouth is related to the internal sedimentary composition and stream flow. The adjacent shoreline is The Central Mountain Range is located on the eastern side of undisturbed with distance from the river mouth. Thus, the morphology Taiwan and extends from north to south, which causes all the rivers in near an estuary has different evolutionary patterns than the adjacent eastern Taiwan to have short lengths, steep slopes, and high flow rates. coasts. Outflows from these rivers on ordinary days are typically small, and the A Lagrangian morphodynamic model can solve this problem much sediments mostly consist of fine materials. Their effects on the coastal more precisely by tracking the fluvial sediment that is deposited in the morphology are therefore of no importance. However, large amounts of coastal region. Black et al. (1999) noted that a Lagrangian model water may discharge from these rivers on severe weather days, assigns mass to particles of a precisely known position, and mass especially during the summer and autumn seasons, when torrential conservation is always guaranteed. Soulsby et al. (2007) (see also rainstorms or typhoons affect Taiwan. Under these circumstances, Huthnance et al., 2008) used Lagrangian models to assess the effect of high-speed, sediment-laden river outflows, which are occasionally changing estuarine morphology in the UK on flooding risk and combined with waves from the sea, can severely affect the estuarine habitats. Different types of scenarios were tested for future manage- morphology. ment. The accuracy in modelling changes in the bathymetries near The study area covers the coastal region of the Beinan Estuary estuaries can be enhanced by using both Eulerian and Lagrangian (latitude=22°45′53.70″ N; longitude=121°10′38.04″ E) in Taidong methods. County in southeastern Taiwan. Fig. 1(a) shows the geographic location However, even with the most advanced computational techniques, of Taiwan, and Fig. 1(b) shows where the measuring stations are not all the mechanisms that are involved in coastal and estuarine marked in the Beinan Estuary. The estuary is fan shaped with adjacent processes can be considered in detail. This concept is true irrespective beaches on each side, and the widths of the beaches decrease away of the degree of sophistication of the numerical model. Furthermore, from the estuary. Sandbars at the river mouth remain intact for some numerical errors may accumulate in the evolving morphology, render- time before they are breached and reshaped by streamflow during a ing the validity of long-term simulations uncertain (Ding and Wang, flood, which indicates that the sediment supply to the delta front 2008; Huthnance et al., 2007). Kristensen et al. (2013) noted that a 2D almost entirely originates from typhoons/flood events; therefore, model is only suitable for short- to medium-term simulations. One severe weather may greatly affect the morphology of the estuary. reason is that the computation can be time consuming. Another reason Only six bathymetric surveys were conducted in the past decade is that an accumulation of errors can eventually lead to erroneous because of financial constraints. Three surveys were conducted in 2005, results. A 2D model may not yield realistic results if drastic morpho- and one survey was conducted in 2006 as part of a construction project. logical changes occur. No major incidents were known to have occurred, so field surveys were Measured data are required to realistically model the evolution of conducted once every three or four years. Changes in the bathymetry shorelines. These data either serve as input for the model or are used to between 2009 and 2014 in this area are shown in Fig. 2. In this figure, check the outputs. Splinter et al. (2013) noted the need for data sets areas of accretion are marked in blue and areas of erosion are marked with high quality and adequate duration. Frequent field surveys were in red. Substantial erosion has clearly occurred near the Beinan proposed to enhance the accuracy of estimating shoreline evolution. Estuary. However, whether an underlying trend of erosion exists or if Ideally, field surveys should be frequent and cover as large a region as this erosion was a result of severe weather events that have occurred possible but quite often are limited in both number and extent because during these years is not clear. The coast may be subjected to severe of financial reasons. Large-scale field measurements appear only ocean climates because it is facing the Pacific Ocean and lies along the scarcely in the literature. Data of sediment in suspension, currents, tracks of typhoons. The four surveys in 2005 and 2006 covered all four waves, sea level, sediment size, and bedforms have been measured in seasons, so their data were used as basic information for this study. In recent years (Black et al., 1999; Bolaños and Souza, 2010). All these the following sections, we clarify the use of these data through data are very helpful to improve both our understanding of the relevant numerical modelling. These results can be used as a reference for mechanisms and the numerical modelling techniques. future erosion protection measures. The northwestern Pacific region is a basin with active tropical storms. The coasts in this region are under constant threat during the typhoon season. However, relatively few literatures have examined the 2.2. Hydrological background effects of severe weather on sandy beaches (Bouchette et al., 2013). As noted by many researchers, the effect of typhoons on sandy beaches or The Beinan River is 84.35 km long with a catchment of estuaries is ‘strikingly distinct from what occurs in systems forced by 1,603.21 km2. Its headwaters have an elevation of approximately more moderated wind/wave forcing’ (Campmas et al., 2014; Meulé 3295 m. The hydrological gauging station nearest to the river mouth et al., 2013). The base flows of most rivers in Taiwan are very low is located at a distance of 4.5 km from the river mouth and is during dry seasons, which is not the case for severe weather, such as positioned on the Taidong Bridge. This station is operated by the torrential rain during wet seasons or typhoon invasions. In these Water Resource Agency (WRA). Hourly discharge data from 1950 to circumstances, substantial amounts of water can flow down from 2014 are available. The annual mean river discharge is 93.09 m3/s. The inland areas. This sediment-laden flow is eventually discharged into observed hourly mean discharge varied from 0 m3/s under drought the sea. With strong waves coming from the sea, the conditions can conditions to 12,800 m3/s during the Typhoon Nora flood in 1973. become very complex. Therefore, knowing the possible effects of Monthly mean discharges also have a wide distribution and vary from torrential rain and/or typhoons on the coasts is very important. 17.53 m3/s in February to 229.66 m3/s in September. The data show In this paper, an integrated model is used to study the possible that the river flow has different characteristics during the dry and wet effects of severe weather on the morphology of the Beinan Estuary, seasons.

2 W.P. Huang Continental Shelf Research 135 (2017) 1–13

Fig. 1. Study area. (a) Geographic location of Beinan Estuary in southeastern Taiwan. (b) Enlarged scale where the locations of the sampling sites of the sediment, hydrological and wave data are marked. A wave rose diagram of the statistical results that were measured by the Taidong buoy is inserted. (c) Computational meshes.

2.3. Wave data Weather Bureau (CWB) since 2009. The second set of wave records was from the Taidong Buoy, which is operated by the WRA. The buoy is The wave data that were used in this paper originated from two located at 22°43′19.2″ N latitude and 121°08′24″ E longitude, where sources. The ﬁrst set was from an oﬀshore data buoy at 21°40′36.12″ N the water depth is 30 m. This buoy is located 3.5 km from the estuary latitude and 124°03′46.8″ E longitude, where the water depth is and has been operational since 2010. Time series of the wave heights, 5522 m. The distance between the buoy and the estuary was approxi- periods, and directions and the surface elevations are available from mately 318 km, and this buoy has been operated by the Central both sources. These data were used to calibrate the numerical model.

3 W.P. Huang Continental Shelf Research 135 (2017) 1–13

3.1. Measurement of sediment

Two types of sediment data were used: the suspended load of the Beinan River and the grain size distribution in the nearshore zone along the coast. The suspended load measurement involved sampling the water-sediment mixture to determine the mean suspended sediment concentration and particle size distribution, which was performed by using records from the Taidong Bridge gauging station. This station measured suspended loads and corresponding discharges from the Beinan River on a regular basis. The suspended loads were estimated by using a US DH-59 sampler, which is a depth-integrating sampler and is an isokinetic sampler (Diplas et al., 2007). Records from 2004 to 2015 were used for the sediment-rating curve (Biedenharn and Thorne, 1994) to estimate the estuarine sediment supply. The grain-size distribution data are from the results of sediment sample analyses. Fifteen locations were used to sample sediments along the coast with a spacing of 800 m between sample locations, which are marked in Fig. 1(b) as S01 to S15. All the sampling locations had six sampling points, except for the sampling location at the mouth of the main river channel. These six points at each sampling location Fig. 2. Bathymetric changes between 2009 and 2014: areas of accretion are marked with formed a line that was perpendicular to the shoreline, starting from the blue, and areas of erosions are marked with red. (For interpretation of the references to high-tide line seawards to a depth of 25 m. Two of these sampling fi color in this gure legend, the reader is referred to the web version of this article.) points were located on the beach at the high-tide (+0.61 m) and low- tide lines (−0.35 m), and the other four stations were at depths of 5, 10, ‘ ff ’ The former was used for the o -shore region , and the latter was used 15, and 25 m. Deeper than 25 m, the grain-size distributions were ‘ ’ for the near-shore region , which is described in the next section. A assumed to be the same as those at depths of 25 m in the later wave rose for the entire recorded period that was measured by the simulation. All the samples were dry sieved in the laboratory to Taidong Buoy is inserted in Fig. 1(b). The dominant wave directions at estimate the grain-size distribution. the Taidong Coast are E during winter and ESE during summer. The average wave height is 1.36 m, with a corresponding period of 6.2 s, 3.2. Sediment analysis and the tidal range is 0.96 m. Extreme waves can occur when typhoons approach. For example, the highest recorded wave during this period The observed fluvial suspended sediments consisted of sand and was 12.1 m with a period of 12.3 s. This wave occurred on September silt. The concentrations (SSCs) varied considerably at different times of 21, 2013, when Typhoon Usagi passed the Bashi Channel in southern the year, reaching as low as 50 mg/l under normal conditions and as Taiwan. high as 94,083 mg/l during flood events. During floods, the SSCs can be 100–1000 times greater than normal. These data were used to derive a sediment-rating curve, as shown in Fig. 4, which resulted in Eq. (1) for use in the following simulation. The coefficient of determi- 3. Methodology nation between the estimated and observed sediments r2 was equal to 0.87. A short description of the methods to estimate the sediment Q = 0.0001(Q )1.6586 (1) transport in the area is presented in this section. Also included are S descriptions of the field surveys, the numerical model set-up, and some where preliminary results. A flowchart of the study is shown in Fig. 3. QS is the suspended load (ton/s; t/s);

Fig. 3. Flow chart of the approach. The simulation began with waves and currents, and then the sediment transport and distribution were modelled. The corresponding numerical modules included Spectral Waves (SW), HydroDynamics (HD), Sediment Transport (ST), and Particle Tracking (PT).

4 W.P. Huang Continental Shelf Research 135 (2017) 1–13

records from the two buoys showed that no waves longer than 300 m occurred in the region of interest, at least between the years 2009 and 2015. A water depth of 150 m was chosen to separate the offshore and nearshore regions, which is more than 0.5 times the length of typhoon- generated waves. Thus, the waves at the nearshore boundary could be treated as deep-water waves. The coupled morphological model package MIKE 21 FM by DHI was used to study possible changes. The effects of river discharges and wave-induced and tidal currents in the nearshore region were considered by using the Hydrodynamic (HD) module. The transport of non-cohesive sediments and changes in the seabed level from the actions of waves and currents were estimated by using the Sediment Transport (ST) module. The Particle Tracking (PT) module, which is based on the Lagrangian method, was used to model the flow of fluvial suspended substances into and out of the estuary. In the ST module, the bed materials are assumed to consist of different fractions of sediments whose sizes are log-normally distributed (Jonasz, 1987; Lambert et al., 1981). Thus, a grading curve at each sampling point could be built based on two parameters: the

Fig. 4. Empirical relationship between the discharge and suspended loads. The dots median grain size D50 and the geometric standard deviation. A spatial represent the measurements, and the line represents the regressive result. distribution was obtained through bilinear interpolation by using data from the sampling points. By using ‘graded’ sediment, the effect that 3 ff Q is the discharge of the Beinan River (m /s). presence of di erent grain sizes has on the amount of bed load and The results of the grain-size distribution showed that the median suspended material and on the total rate of sediment transport can be grain size was 0.54 mm at the mouth of the Beinan River, while the estimated. median grain sizes were between 0.31 and 10.47 mm on the adjacent The PT simulation was used to model the suspended substances beach. The sizes on the beach surface from S7 to S8 and from S12 to that were discharged into coastal areas from Beinan River. The S13 ranged from 7.56 mm to 10.47 mm, which were larger than those suspended load that was discharged from the river was considered at all the other sampling locations. These two regions are affected by particles that were advected with the surrounding water body and the construction of armored rubble on the shore. Elsewhere, the grain dispersed. The representative particle, or grain, sizes used in the model sizes ranged from 0.30 to 5.97 mm at depths from 0 to 5 m and from were D75, D60, D30, D25, and D10 (Das, 2006). These grain sizes were 0.29 to 3.16 mm at depths from 10 to 25 m. The grain sizes at the then used as input in the PT module to trace their transports and fates sample locations decreased with increasing water depth. around the coastal areas. The movements of particles drifted because of fl The gradation of sediments was also estimated by calculating the gravity and the current ow, which were calculated in the HD module. The amount of sediments was tracked during the simulation and then uniformity coefficient Cu (Eq. (2)). When Cu≤4, the grain-size distribution of the sample was considered to be poorly graded or uniform added to the bathymetric change that was derived from the ST module. By using the law of mass conservation, the results from the ST and PT (Das, 1994). The results showed that the Cu values at locations S03 to fi S07 within the Beinan Estuary were greater than those at the other modules could provide a nal estimate of the changes in the bathymetry. sampling locations. The largest Cu, 31.2, was measured at a depth of 15 m at S04, whereas the value varied from 1.73 to 8.01 for the Each of the modules used distinctive time steps to satisfy the remainder of the measuring sites. The grain-size distribution exhibited stability criteria. All the time steps within the simulation for the a wider range near the river mouth than farther away, which was modules were then synchronized at an overall discrete time step of caused by the sediment inflow from the river. Large amounts of 1 h. The bed levels were updated at each hour, and the new bathymetry discharge and high flow velocities can cause landslides and the was used for the next time step. This approach improved the realism of scouring of the riverbanks and/or riverbeds in floods. Thus, the the results. Detailed descriptions of the modules and the governing diameters of the fluvial sediments were distributed over a wide range, equations that were used can be found in the Mike 21 user manual which consisted of gravel, sand, silt, and clay. The grain sizes of the (DHI, 2012). sediments at the river mouth differed from those on the adjacent Unstructured, non-uniform, triangular meshes with arbitrarily fi coasts, so different mechanisms may be involved. Thus, a more realistic spatially dependent sizes were used to t the irregular coastlines of modelling of the sediment transport near the Beinan Estuary must the Beinan Estuary as closely as possible. The sizes of the meshes in the fi include the effect of fluvial sediments. evaluated nearshore region were also re ned. Regions with elevations higher than 5 m were neglected because these areas are less likely to be CDDu =/60 10 (2) affected by marine forces, except for locations around the Beinan River. The boundary of the river channel began from Taidong Bridge because where the discharge and suspended load were recorded at this location. D is the grain diameter at 60% that passes the No. 20 sieve; 60 Each module was calculated with the same constructed meshes. D is the grain diameter at 10% that passes the No. 20 sieve. 10 Several test runs with different mesh sizes were conducted. The optimal mesh size was determined when no more substantial changes in the 3.3. Numerical modelling solution could be achieved. The final unstructured meshes consisted of 13,265 nodes and 25,941 cells, with a maximum segment length of The area under study was divided into 'offshore' and 'nearshore' 300 m in the open sea boundary and a minimum segment length of 5 m regions. The offshore region lies between 117°E to 126°E longitude and near the coastlines of the Beinan Estuary. Fig. 1(c) shows the 18°N to 28°N latitude and is shown in Fig. 1(a). As shown in Fig. 1(c), computational meshes. the nearshore region is the area between 121°06′ N and 121°11′ N The simulations were activated when typhoons traveled into the longitude and between 22°42′ E and 22°47′ E latitude. Wind-generated offshore region and ended when these events left the region of interest waves can have lengths from 0.1 to 1000 m (Thomson, 1981). Our to ensure that all the effects from typhoons were considered. The

5 W.P. Huang Continental Shelf Research 135 (2017) 1–13 offshore wave field was simulated by using the information for the typhoon events, including their tracks, central pressures, maximum wind speeds and radii, which can be found in the Typhoon Database at the CWB (2005). Simulated offshore wave fields were used as boundary conditions for the nearshore region, where bottom effects had to be considered. The measured bathymetry and observed wave data were used during this stage to include bottom effects and crosschecking, as well as for the simulations of the nearshore current fields. Outflows from the Beinan River, as measured at the gauging station, and astronomic tidal data were used for this purpose. Analyses of the coastal sediment transport could be conducted with these wave and current fields and updated morphological data. The sediments from the Beinan River were evaluated by using the measured discharges with the aid of the sediment-rating curve. The resulting morphological changes in the nearshore region could then be compared with historical bathymetry data.

4. Results

Typhoon Matmo, which made landfall on July 23, 2014, was used for model calibration. The bathymetric changes after its passage were compared to observed data for validation. The eﬀects of diﬀerent types of typhoons were then evaluated.

4.1. Model calibration

Typhoon Matmo was used to adjust the values of the parameters. This storm formed southwest of Guam, traveled northwestwards and made landfall on Taidong County in its path. During its lifetime, this typhoon had a minimum pressure of 965 hPa and a maximum wind speed of 36 m/s. Some of the revetments and seawalls along the coast of eastern Taiwan were damaged. Comparisons of the measured and simulated wave heights, wave periods, wave directions, and tidal elevations are shown in Fig. 5, beginning with the first panel and continuing downwards. The relative errors between the measured and Fig. 5. Comparisons of the measured and simulated results (from the top panel calculated results of the four items at each time step were less than downwards) of the wave heights, periods, directions, and water surface elevations at 10%. the observation site for Typhoon Matmo, July 2015. The observed data are represented Typhoons Haitang, Talim, and Longwang, which all occurred in with red filled circles, and the simulated results are represented with solid lines. (For 2005, were used to estimate possible bathymetric changes from these interpretation of the references to color in this figure legend, the reader is referred to the events. Typhoons Matmo and Fung-Wong from 2014 were then tested web version of this article.) to examine the erosion of the Beinan Estuary in its present stage. In the following sections, the shorthand notations of H (Haitang), T (Talim), these regions are steeper, so the waters are deeper than those else- L (Longwang), M (Matmo), and F (Fung-Wong) are used for simplicity where. Thus, strong offshore waves could propagate more closely to the when referring to these five typhoons. coast with little energy dissipation, exerting greater forces on these areas. 4.2. Model performance The current fields of these three typhoons that correspond to these wave fields are shown in Fig. 6(d)–(f). The driving forces in the current The typhoons from 2005 all formed near Guam and made landfall field simulations include the tidal elevations at open boundaries, river on northern Taiwan within 120 km of the study area. The intensities of discharges from the Beinan River, and the wave radiation stresses from these typhoons were all categorized as ‘severe’ (CWB). However, the the nearshore wave field simulations. The figures indicate that strong hydrographs that were recorded at the Taidong Bridge gauging station offshore currents occurred at the times of the maximum offshore wave showed that the typhoons had very different rainfall intensities. heights along the coast at depths from −10 m to −15 m. The currents Typhoon H had the heaviest amount of rainfall, with torrential rain were driven by oblique incident waves and collapsing breakers. The that lasted for almost 48 h. Extensive flooding and landslides occurred figures also show that the characteristics of the currents in the (CWB). On the other hand, Typhoon T exhibited concentrated rainfall. nearshore region were noticeably different from those in the offshore Most of this rainfall occurred within six hours. In contrast, Typhoon L region. The directions of the nearshore currents matched those of the brought only moderate amounts of rain during its passage. The peak nearshore waves. Farther offshore, the radiation stresses were weaker, discharges of these three typhoons at the station were 2908 (H) m3/s, and the directions of the currents were governed by tidal currents. 2007 (T) m3/s, and 316 (L) m3/s. Thus, the magnitude of the littoral sediment was governed by wave Fig. 6(a), (b), and (c) are the simulated wave fields at the times of intensity at the time of typhoon invasion. the maximum offshore wave heights for Typhoons H, T, and L, These waves and currents were then used as dynamic forces to drive respectively. These times were shortly before these typhoons made the sediment transport. The bathymetry data from May 2005 were used their landfalls. The eyes of all three typhoons were north of the Beinan as the initial condition. Fig. 7(a) shows the measured results of the River, so the predominant wave directions were all northwest. Areas morphology changes from May to Sept. 2005 for the entire estuary. with higher wave heights along the shoreline are marked with circles, Two typhoons, namely, H and T, made landfall during this period. For and the river mouth is among these locations. The bottom slopes in comparison, the simulated results from these two typhoons are plotted

6 W.P. Huang Continental Shelf Research 135 (2017) 1–13

Fig. 6. Simulated wave and current fields along the Beinan Coast. (a) and (d) Typhoon Haitang; (b) and (e) Typhoon Talim; (c) and (f) Typhoon Longwang. These figures are the results shortly before the typhoons made landfall, when the wave fields were most severe. In (a), (b) and (c), the contours are the distributions of the wave heights, and the wave directions are indicated by the directions in which the arrowheads point. In (d), (e) and (f), the distributions of the water surface elevations are shown as contours. The current directions are shown as heads, and the current velocities are indicated by the lengths of the arrow shafts. in Fig. 7(b). Measurements were performed only up to a depth of 50 m of sediments downstream. When these discharges encountered massive (Fig. 7(a)). The areas that are surrounded by the green lines denote water bodies, such as the sea, the flow velocity decreased and the where surveys were conducted. sediments settled to the bottom. Three submerged sandbars formed. During floods, high-velocity river discharges carried large amounts Comparing the two figures indicates that the simulated results were

7 W.P. Huang Continental Shelf Research 135 (2017) 1–13

Fig. 7. Comparison of the observed (6(a) and 6(c)) and simulated (6(b) and 6(d)) bathymetric changes around the Beinan Estuary. 6(a) and 6(b) are after the passages of Typhoons Haitang and Talim, and 6(c) and 6(d) are after Typhoon Longwang. Accretion occurred after Typhoons Haitang and Talim, whereas Longwang caused erosion. smaller than the measured results. these three typhoons were then crosschecked by comparing the results The simulated bathymetry after the passage of Typhoons H and T at all the mesh nodes to the corresponding measured changes. This was then used as input for Typhoon L, and the result is shown in comparison is shown in Fig. 8. Excluding nodal points with bathy- Fig. 7(d), which can be compared to the observed bathymetry in metric changes of less than 0.05 m, the mean absolute error for all the Fig. 7(c). Most of the sandbars that previously formed following the other nodes was 0.78 m, with a standard deviation of 0.92 m. The two passage of the previous two typhoons were now eroded. The outflow of dashed lines in Fig. 8 define the 95% confidence interval. Most of the the river during Typhoon L approximated the monthly mean discharge. estimated results were well within this interval. Therefore, the simula- However, the maximum offshore wave height reached 10.95 m, which tion could be considered satisfactory. is 25 times the normal wave height during summer. The dominant longshore sediment transport was from NE to SW during Typhoon L. 5. Discussion Without a sufficient supply of sediments from the river, the estuary eroded under the action of high waves. The results showed that the effects of typhoons on estuaries could The calculated bathymetry changes in the estuarine region from be profound. The measured sandbars at the river mouth were larger

8 W.P. Huang Continental Shelf Research 135 (2017) 1–13

sand bars that formed during the typhoon season were eroded and transported to deeper regions. Table 1 lists the changes in the volume of the bathymetry based on a series of surveys between May 2005 and May 2006. The bathymetric changes that occurred between these surveys were separated into summer (wet) and winter (dry) seasons according to the date of the survey. During the first period of the wet season from May to Sept. 2005, the total volume of deposited sediment was 2 million m3. Typhoons H and T both made landfall during this period. Based on the simulated results, the volume of sediments that were transported to the estuary because of normal daily rainfall events contributed less than 10% of the total change in volume. During the second period from Sept. to Oct., the total eroded volume was 634,628 m3. Typhoon L, which had minimal rainfall compared to the other two typhoons, made landfall on Oct. 2, 2005. The volume of erosion from Typhoon L was 651,481 m3 based on the results of the simulation, which indicates that the supply of sediments from rainfall on ordinary days may have added to accretion, although the amount was marginal. During the third period from Oct. 2005 to May 2006, which was within the dry season, the total eroded volume 3 3 Fig. 8. Comparisons of the observed and simulated bathymetric changes at the nodal was 733,219 m . The estimated monthly erosion rate was 91,652 m / points. The solid line indicates the 1:1 line, and the dashed lines define the 95% month, which was greater than the rate of accretion during the wet confidence interval. season. However, as shown in the last column of Table 1, the net result of the bathymetric changes that year was accretion because of the than those in the simulations, so some other factors that contributed to excessive amount of sediments that were supplied by Typhoons H and the characteristics of these sandbars are discussed in the following T. On the other hand, the amount of erosion during the entire dry sections, including the sediment content in the flow during both dry season was only 13% larger than the amount of erosion that occurred and wet seasons. The possible ways of sediment transport in the during the single event of Typhoon L; therefore, major changes in the nearshore region during typhoon invasions are also discussed. A bathymetry were governed by typhoon events. tentative relationship between bathymetric changes and sediment supply is provided based on the results of the studied cases. 5.2. Sediment transport during typhoons

5.1. Factors that contributed to the sediment supply When a typhoon approaches, it is often accompanied by heavy or even torrential rain. The typhoon wave field and fluvial sediment flux The sediment supply to the estuary solely originated from the changed rapidly in the time domain in the study area, as did the Beinan River. Possible reasons for the underestimation of the sandbars bathymetry in response to these forces. The distribution of the could be that sediments from monsoon rains and sediments on normal sediments at different time steps from different forcing is worth days were not considered in the previous simulations. Both simulated examining. Fig. 10(a) and (b) are the distributions of the sediment and measured data were used below to include these two types of fluxes at the time steps of the maximum offshore wave height and the sediment supply. Normally, the wave climate during the wet season peak discharge, respectively, for Typhoon H. The colored areas in the from May to Oct. is notably mild. The 90% exceedance probability of figures show changes in the bathymetry, and the contour lines are the the wave height is 0.44 m, with a corresponding period of 5.5 s and a water depths near the coastal area. The directions and strengths of the direction of ESE. Records showed that the largest monthly mean sediment fluxes are shown as arrowheads and the lengths of the shaft, discharge was 229.66 m3/s in September over the last five decades. The respectively. The maximum wave height occurred prior to the peak wave conditions and the discharge were then used to estimate the discharge. possible maximum sediment supply during the wet season. Simulations In Fig. 10(a), the discharge from the river approximated its normal were conducted for an entire month, and the results are shown in rate of 115.8 m3/sec, whereas the offshore wave height was 8.94 m. Fig. 9(a). The range of the color bar scale in Fig. 9(a) has been The majority of the sediments were transported within water depths of shortened for clarity because the changes were marginal. Small 10–20 m. This region was the breaker zone at the time, which indicates sections of sandbars can be seen at the river mouth in the figure. The that the sediment transport was governed by waves. The bed slope of total amount of sediments that were deposited within the estuary was the estuary is rather steep, approximately 1:12 within 0 to −30 m. The estimated to be 42,000 m3. The supply of sediments during the wet estuary is also convex in shape, so oblique incident waves can directly season is considered negligible compared to the amount of sediments approach the shore with only minor refraction, causing a large amount from Typhoon H (1,200,000 m3) or Typhoon T (720,000 m3). of littoral transport, which can cause erosion with an insufficient In addition to rainy days during the wet season, the effect of the sediment supply. However, the coast along the southern bank of the East Asian Winter Monsoon (EAWM) during the winter (dry) season is estuary, which is concave in shape, was supplied with longshore worth considering. The most prominent surface feature of the EAWM sediment that was transported from the river mouth sandbar and thus is strong northeasterlies along the eastern flank of the Siberian High retained its shape. Pressure System (SH) and the coasts of East Asia (Chen et al., 2000; The conditions changed after Typhoon H made landfall: the wave Wu and Wang, 2002). height began to decrease and the river discharge began to increase. As The average wave height can reach 1.6 m with a mean period of shown in Fig. 10(b), the deposition shifted towards the seaward side 6.0 s during the EAWM. In addition, the mean monthly discharge of and increased in volume through the supply of the sediment-laden the Beinan River can be as low as 17.53 m3/s. The measured changes in river discharge. However, the velocity of the outflow from the river the bathymetry from Oct. 2005 to May 2006 are shown in Fig. 9(b). The mouth was high and the adjacent continental shelf was narrow, so most coasts near the estuary eroded from wave action during winter because of the fluvial sediment flowed out of the so-called ‘closure depth’ the supply of sediments from the river was low. Most of the submerged (Hallermeier, 1983) of 20 m. Thus, the supply of sand to the adjacent

9 W.P. Huang Continental Shelf Research 135 (2017) 1–13

Fig. 9. Bathymetric changes during the (a) summer (wet) and (b) winter (dry) seasons. The former was estimated by simulations, which showed marginal deposition. The latter was calculated based on the survey data, which showed erosion.

3 Table 1 370,000 m of sediment was eroded at elevations from +5 to −50 m Measured changes in the bathymetry between 2005 and 2006. in the estuarine zone after the passage of these two typhoons. Typhoons usually cause coastal erosion because of their high waves Change in the bathymetry (m3) and strong currents. However, accretion can occur if the volume of Period Summer (wet season) Winter Annual fluvial sediments is too large to be transported by waves and currents. (dry season) changes Any thresholds that separate erosion and deposition in estuaries would Elevation (m) 2005/May to 2005/Sept. to 2005/Oct. to be interesting to identify. Such a ‘criterion’ would be very helpful to 2005/Sept. 2005/Oct. 2006/May determine whether erosion has occurred and/or if remedies should be fl 0to−5 135,611 −53,443 −79,743 2,425 implemented after the passage of a typhoon. The accumulated uvial −5to−10 287,358 −117,647 −77,258 92,453 load, the volume of bathymetric changes near the estuary, and the wave −10 to −20 430,766 −330,697 −329,852 −229,783 loads were plotted in Fig. 11 to explore this potential. The plotted −20 to −30 402,921 −75,203 84,080 411,798 − − − − results are from Typhoons H, T, L, M, and F. A vertical solid blue line 30 to 50 751,444 57,637 330,446 363,361 fi Sum 2,008,100 −634,628 −733,219 640,253 divides the gure into two parts, with the left side for erosion and the right side for deposition. In Fig. 11, the fluvial suspended loads were estimated by using hourly discharge data from Eq. (1). Bathymetric coasts was poor. changes near the estuary from each typhoon were determined from the As discussed previously, field surveys cannot be conducted on a simulations. The areas that were used for the calculations were the regular basis because of limited budgets. Surveys were conducted at same as those for Fig. 7(b). Furthermore, the wave loads were specific times, such as at the end of the winter monsoon or after calculated by using Eq. (3), as proposed by Dette and Führböter typhoon invasions. Measured results, such as those in Fig. 7(a) and (c), (1976). The calculated wave load can be regarded as the total wave can only be used to identify the evolution of the morphology, whereas energy that acts on the coast during a typhoon event: the dominant driving mechanisms are unclear. The numerical simula- n tions bridged the gap and indicated that the driving force of the coastal ͠ 2 NcΔtH= 1.225⋅ ⋅ ∑ 1/3i processes at the site were typhoon events and that the transgressive- i=1 (3) regressive cycle of the bathymetry changes in the Beinan Estuary were where short lived. N͠ is the wave load (kWh); fi H1/3i is the signi cant wave height at the i-th h. 5.3. Assessment of the existing erosion c = gd (4)

This study attempted to clarify why the shorelines near the Beinan d is 150 m, which is water depth at the near shore boundary; River have been retreating in recent years. Eq. (1) was used to estimate g is the gravitational acceleration; the total volume of suspended loads both annually and from each Δt is the time interval (1 h) typhoon event from 2009 to 2014. The results are listed in Table 2.As Each filled circle in Fig. 11 shows the suspended load that is related shown in the table, a sudden decrease occurred in the annual amount to the volume of the bathymetric changes for a specific typhoon. of suspended load in the year 2014. Deposition clearly increased with increasing fluvial suspended load. In 2014, two typhoons, M and F, made landfall in Taiwan (CWB). The linear fit of the data points in Fig. 11 indicated a threshold of The peak discharges were 1019 m3/s for Typhoon M and 1500 m3/s for 938,307 t. When this ‘threshold’ is exceeded, the estuary begins to Typhoon F. However, the hydrographs for these two typhoons showed accrete. The filled triangles in Fig. 11 are the wave loads that are that the flood durations were very short and that the sediment supply associated with changes in the bathymetry near the estuary from the was marginal. The numerical results indicated that approximately typhoons. No apparent relationship between these two parameters

10 W.P. Huang Continental Shelf Research 135 (2017) 1–13

Fig. 10. Sediment flux distribution at the maximum offshore wave height (9(a)) and the peak discharge (9(b)) of Typhoon Haitang. The former showed strong longshore sediment flux that was induced by waves, and the latter showed flux into the Beinan Estuary because of the large amount of discharge. The colored areas in the figures show changes in the bathymetry. The contour lines are the water depths near the coastal area. The directions and strengths of the sediment fluxes are shown as arrowheads and the lengths of the shaft, respectively.

Table 2 Suspended loads and associated bathymetric changes from 2009 to 2014.

Period Suspended Load Typhoon Suspended Load Accretion (a)/ (tons/year) Name (t/event) Erosion (e)

2009 33,557,440 Linfa 773,526 e Molave 2,358,145 e Morakot 18,607,854 a Patma 2,672,186 a

2010 11,491,193 Lionrock 1,436 e Namtheun 1,111,814 a Meranti 2,025,698 a Fanapi 2,329,257 a Megi 701,154 e

2011 30,388,674 Aere 47,878 e Songda 280,058 e Meari 389,761 e Muifa 198,091 e Namadol 7,467,581 a

2012 35,512,003 Talim 7,343,799 a Doksuri 2,228,345 a Fig. 11. Relationship among the ﬂuvial suspended load, the wave load, and the change Saola 321,766 e in the bathymetry around Beinan Estuary. The 'threshold' that predicts accretion or Haikui 119,679 e erosion after typhoons is shown as a solid blue line. (For interpretation of the references Kai-Tak 195,854 e to color in this ﬁgure legend, the reader is referred to the web version of this article.) Tembin 6,181,230 a Jelawat 158,536 e could be established. The maximum and second-largest total wave

2013 28,444,769 Soulik 755,719 e loads were caused by Typhoons H and L; however, these two typhoons Cimaron 139,616 e induced opposite bathymetric changes. Typhoon H resulted in accre- Trami 354,533 e tion, but Typhoon L resulted in erosion. The results clearly show that Kong-Rey 307,442 e the volume of sediment that was transported by the river was the Usagi 11,519,049 a dominant factor that governed the evolution of the Beinan Estuary. Fitow 456,211 e Although only a limited number of typhoons were considered here, 2014 4,464,759 Matmo 654,442 e almost all categories of typhoons were covered. Furthermore, diﬀerent Fung-Wong 825,581 e types of hydrographs were included in this study. The volume of 938,307 t of ﬂuvial suspended load can thus be regarded as a threshold

11 W.P. Huang Continental Shelf Research 135 (2017) 1–13 for erosion and/or accretion within the river mouth. This threshold was Hwang, K.-S., Sous, D., Mohammadi, B., Schuster, M., Manna, M., Campmas, L., Hwung, H.-H., the POC Team in Toulouse, 2013. Modelling storms and extreme derived from typhoon events, so it is suggested using this value to events impacting sand beaches in Taiwan. In: Proceedings of the EAGER (Extreme estimate the possible effect of typhoons on the morphology. The events Archived in GEological Records) Workshop, Taiwan, p. 3. bathymetric changes from each typhoon event from 2009 to 2014 Campmas, L., Bouchette, F., Meule, S., Petitjean, L., Sous, D., Liou, J.-Y., Leroux- Mallouf, R., Sabatier, F., Hwung, H.-H., 2014. Typhoons driven morphodynamics of were then reexamined, and the net changes are shown in the last the Wan-Tzu-Liao sand barrier (Taiwan). In: Proceedings of the 34th Conference on column of Table 2. Coastal Engineering, Seoul, Korea, p. 13. In the years 2013 and 2014, only Typhoon Usagi produced Chen, W., Graf, H.F., Huang, R.-H., 2000. The interannual variability of East Asian sufficient rain and, therefore, suspended load to exceed this threshold. winter monsoon and its relation to the summer monsoon. Adv. Atmos. Sci. 17 (1), 48–60. The remaining seven typhoons from 2013 and 2014 created lower Cook, T.L., Sommerfield, C.K., Wong, K.C., 2007. Observations of tidal and springtime suspended loads than the threshold and thus likely resulted in erosion. sediment transport in the upper Delaware Estuary. Estuar. Coast. Shelf Sci. 72, – Table 2 also shows that the volumes of both the annual fluvial sediment 235 246. CWB, 2005. The Central Weather Bureau. Typhoon Database. 〈http://rdc28.cwb.gov.tw/ supply and the typhoon-induced sediment supply substantially de- TDB/〉. creased beginning in the year 2013. Thus, erosion near the Beinan Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual basis Estuary has become a matter of concern since 2013. and stratigraphic implications. J. Sediment. Petrol. 62 (6), 1130–1146. Dalrymple, R.W., Mackay, D.A., Ichaso, A.A., Choi, K.S., 2012. Processes, Morphodynamics, and facies of tide-dominated estuaries. In: Davis Jr, R.A., 6. Conclusions Dalrymple, R.W. (Eds.), Principles of Tidal Sedimentology. Springer, The Netherlands: Dordrecht, 79–107. ff Das, B.M., 1994. Principles of Geotechnical Engineering third edition. PWS Publishing This paper assessed the e ects of typhoons on the bathymetry of the Company, Boston, USA, 672. Beinan Estuary. The river discharges, the river sediment load, and the Dastgheib, A., 2012. Long-term Process-based Morphological Modeling of Large Tidal effects of waves and currents are the main factors that govern the Basins. CRC Press/Balkema, Leiden, The Netherlands, 179. Dette, H.H., Führböter, A., 1976. Wave climate analysis for engineering purpose. In: bathymetric evolution of an estuary. The results of this study indicate Proceedings of 15th Conference on Coastal Engineering, Honolulu, Hawaii, ASCE, that severe weather such as typhoons can considerably affect the pp. 10–22. bathymetry of the Beinan Estuary. DHI, 2012. MIKE 21 and MIKE 3 Flow Model FM Hydrodynamic and Transport Module fi Typhoons can usually be damaging to a coast because of their Scienti c Documentation. DHI Water and Environment, Denmark, 52. Ding, Y., Wang, S.S.Y., 2008. Development and application of a coastal and estuarine strong winds, high waves, strong currents, and heavy rains. This, coasts morphological process modeling system. J. Coast. Res. 52, 127–140. are often eroded. However, when the streamflow velocity of an Ding, Y., Yeh, K., Chen, H., Wang, S., 2013. Coastal and estuarine planning for flood and estuarine river is substantial, as in the case of heavy and long-lasting erosion protection using integrated coastal model. In: Huang, W., Wang, K.H., Chen, Q.J. (Eds.), Coastal Hazards. ASCE, USA, 163–177. rainfall during a typhoon, an excessive volume of sediment can be Diplas, P., Kuhnle, R., Gray, J., Glysson, D., Edwards, T., 2007. Sediment transport injected into the estuary. The volume can be too great to be dispersed measurements. In: Sedimentation Engineering: Processes, Measurements, Modeling, by high waves and currents, and accretion results. However, if no and Practice. ASCE Manuals and Reports on Engineering Practice No. 110, Reston, Virginia, ASCE, pp. 307–353. additional supply of sediment originates from the river, these sedi- Geleynse, N., Storms, J.E.A., Stive, M.J.F., Jagers, H.R.A., Walstra, D.J.R., 2010. ments are eventually transported away from the estuary by waves and Modeling of a mixed‐load fluvio‐deltaic system. Geophys. Res. Lett. 37 (L05402), 7. currents. Finally, these sediments settle on the bottom at a depth of Gelfort, A., Ladage, F., Stoschek, O., 2010. Numerical modeling of morphodynamic changes in the Jade Estuary – Germany. In: Smith, J.M., Lynett, P. (Eds.), In: approximately 20 m, resulting in the erosion of the coasts. This Proceedings of the 32nd Conference on Coastal Engineering, Shanghai, China, pp. phenomenon is believed to have occurred in the Beinan Estuary in 2310–2320. recent years. Guo, L., 2014. Modeling Estuarine Morphodynamics under Combined River and Tidal Forcing (UNESCO-IHE Ph.D. Thesis). CRC Press/Balkema, Leiden, The According to the above results, a threshold volume of 938,307 t of Netherlands, 214. suspended load was used to predict whether accretion or erosion occurs Guo, Y., Wu, X., Pan, C., Zhang, J., 2012. Numerical simulation of the tidal flow and within the Beinan Estuary following the passage of a typhoon. Field suspended sediment transport in the qiantang estuary. J. Waterw. Port. Coast. Ocean – surveys are both time consuming and expensive; however, measuring Eng. 138 (3), 192 202. Hallermeier, R.J., 1983. Sand transport limits in coastal structure designs. In: the streamflow is relatively easy. The proposed 'threshold' should be Proceedings of Coastal Structures ’83, New York, ASCE, pp. 703–716. valuable as an initial indicator in assessing the possible consequences Hardisty, J., 2007. Estuaries: Monitoring and Modeling the Physical System. Blackwell of a typhoon invasion. Publishing Ltd, Oxford, UK, 176. Huan, N.M., Trinh, N.Q., Dat, P.T., 2010. Numerical simulation of sediment transport The results also showed that the total volume of suspended load has and morphology changes at the Bach Dang estuary. VNU J. Sci. Earth Sci. 26, 90–97. substantially decreased since 2014. However, with the exception of Huthnance, J.M., Karunarathna, G., Lane, A., Manning, A.J., Norton, P., Reeve, D., Typhoon Usagi, each of the typhoons in 2013 and 2014 brought a Spearman, J., Soulsby, R.L., Townend, I.H., Wright, A., 2007. Development of Estuary Morphological Models. The Institute of Mathematics and its Applications, minimal sediment-supply and may have caused erosion. Thus, the Essex, 10. critical conditions of erosion near the Beinan Estuary began in 2013. Huthnance, J.M., Karunarathna, H., Lane, A., Manning, A.J., Norton, P., Reeve, D.E., Soulsby, R.L., Spearman, J., Townend, I.H., Wolf, J., Wright, A., 2008. Development of Estuary Morphological Models. London, UK: Department for Environment, Food Acknowledgements and Rural Affairs, R & D Technical Report FD2107/TR, p. 57. Jonasz, M., 1987. Nonspherical sediment particles: comparison of size and volume This paper is partly aided by a project of the Ministry of Science and distributions obtained with an optical and a resistive particle counter. Mar. Geol. 78 (1–2), 137–142. Technology, Republic of China, Project no. MOST 104-2621-M-019- Kristensen, S.E., Drønen, N., Deigaard, R., Fredsoe, J., 2013. Hybrid morphological 006. Their support is deeply appreciated. modelling of shoreline response to a detached breakwater. Coast. Eng. 71, 13–27. Lambert, C.E., Jehanno, C., Silverberg, N., Brun-Cottan, J.C., Chesselet, R., 1981. Log- References normal distributions of suspended particles in the open ocean. J. Mar. Res. 39, 77–98. Meulé, S., Campmas, L., Bouchette, F., Le Roux-Mallouf, R., Sous, D., Liou, J., Bujan, N., Bernardes, M.E.C., 2005. Medium-Term (Months to Years) Morphodynamic Modelling Hwang, K.-S., Michaud, H., Larroudé, P., Sabatier, F., Hwung, H.-H., the KUNSHEN of a Complex Estuarine System. University of Plymouth, Plymouth, England, 103. partnership, 2013. The impact of typhoons on a sandy beach – insights from in-situ Biedenharn, D.S., Thorne, C.R., 1994. Magnitude-frequency analysis of sediment measurement in the Wan Tzu Liao barrier, south-westernmost Taiwan. In: transport in the lower Mississippi river. Regul. River.: Res. Manag. 9 (4), 237–251. Proceedings of EAGER (Extreme events Archived in GEological Records) Workshop, Black, K., Green, M., Healy, T., Bell, R., Oldman, J., Hume, T., 1999. Lagrangian Taoyuan, Taiwan, p. 2. modelling techniques simulating wave and sediment dynamics determining sand- Montagna, P.A., Palmer, T.A., Pollack, J.B., 2013. Hydrological Changes and Estuarine body equilibria. In: Harff, J., Lemke, W., Stattegger, K. (Eds.), Computerized Dynamics. Springer-Verlag, New York, 94. Modeling of Sedimentary Systems. Springer, Heidelberg, Berlin, 3–21. Natesan, U., Rajalakshmi, P.R., Ferrer, V.A., 2014. Shoreline dynamics and littoral Bolaños, R., Souza, A., 2010. Measuring hydrodynamics and sediment transport transport around the tidal inlet at Pulicat, southeast coast of India. Cont. Shelf Res. processes in the Dee Estuary. Earth Syst. Sci. Data 2, 157–165. 80, 49–56. Bouchette, F., Liou, J.-Y., Michaud, H., Rétif, F., Larroudé, P., Meulé, S., Bujan, N., Perillo, G.M.E., 1995. Definitions and geomorphologic classifications of estuaries. In:

12 W.P. Huang Continental Shelf Research 135 (2017) 1–13

Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries. Elsevier, Thomson, R.E., 1981. Oceanography of the British Columbia coast. Canadian Special Amsterdam, Amsterdam, 17–47. Publication of Fisheries and Aquatic Sciences, No. 56, Department of Fisheries and Prandle, D., 2009. Estuaries: Dynamics, Mixing, Sedimentation and Morphology. Oceans, Ottawa, Canada, p. 291. Cambridge University Press, Cambridge, 236. Uncles, R.J., 2002. Estuarine physical processes research: some recent studies and Simenstad, C., Yanagi, T., 2012. Introduction to classification of estuarine and nearshore progress. Estuar. Coast. Shelf Sci. 55 (6), 829–856. coastal ecosystems. In: Simenstad, C., Yanagi, T. (Eds.), Classification of Estuarine Wolanski, E., McLusky, D. (Eds.), 2012. Treatise on Estuarine and Coastal Science. and Nearshore Coastal Ecosystems. Elsevier, Amsterdam, 1–6. Elsevier, Amsterdam, 4590. Soulsby, R.L., Mead, C.T., Wood, M.J., 2007. Development of a Lagrangian Wu, B.-Y., Wang, J., 2002. winter arctic oscillation, Siberian high and East Asian winter Morphodynamic Model for Sandy Estuaries and Coasts. London, UK: Department monsoon. Geophys. Res. Lett. 29 (19). http://dx.doi.org/10.1029/2002GL015373. for Environment, Food and Rural Affairs, R & D Project Record FD2107/PR, pp. 55. Yamada, F., Kobayashi, N., Shirakawa, Y., Watabe, Y., Sassa, S., Tamaki, A., 2012. Effects Splinter, K.D., Turner, I.L., Davidson, M.A., 2013. How much data is enough? The of tide and river discharge on mud transport on intertidal flat. J. Waterw. Port Coast. importance of morphological sampling interval and duration for calibration of Ocean Eng. 138 (2), 172–180. empirical shoreline models. Coast. Eng. 77, 14–27.