Investigation of Morphological Changes in the Tamsui River Estuary Using an Integrated Coastal and Estuarine Processes Model

Home , Dahan River, Keelung, Tamsui River, Typhoon Aere (2004), Typhoon Jangmi (2008), Xindian River

water

Article Investigation of Morphological Changes in the Tamsui River Estuary Using an Integrated Coastal and Estuarine Processes Model

Tung-Chou Hsieh 1,* , Yan Ding 2,3 , Keh-Chia Yeh 1 and Ren-Kai Jhong 4

1 Department of Civil Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] 2 National Center for Computational Hydroscience and Engineering, University of Mississippi, University, MS 38677, USA; [email protected] 3 U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, Vicksburg, MS 39180, USA 4 Disaster Prevention and Water Environment Research Center, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-919-378-324

Received: 16 March 2020; Accepted: 8 April 2020; Published: 10 April 2020

Abstract: This study is to investigate morphological changes in the Tamsui River Estuary in Taiwan driven by multiple physical processes, such as river flows, tides, waves, and storm surges, and then to study the impacts of sediment flushing operated at the Shihmen reservoir upstream on the river estuary. An integrated coastal and estuarine processes model (CCHE2D-Coast) (Center for Computational Hydroscience and Engineering Two-Dimensional-Coast) was validated by simulating these physical processes in the estuary driven by three historical typhoons in 2008. The site-specifically validated model was then applied to simulate morphological changes in the estuary in response to reservoir sediment flush scenarios from the upstream. For the impact assessment of sediment flushing, a synthetic hydrological event was designed by including a historical typhoon and a typical monsoon. It was found that during the typhoon, the sediments will be mostly deposited in the estuarine river reach of Tamsui and the Wazihwei sandy beach. During the monsoon period, most of the sediments tend to be deposited in the second fishing port of Tamsui, the northern breakwater, and the estuary, while the Wazihwei sandy beach in the river mouth would be scoured by backflow. Simulations of the complex flow fields and morphological changes will facilitate the best practice of sediment management in the coastal and estuarine regions.

Keywords: coastal process model; wave; sediment transport; morphological change; coastal and estuary management

1. Introduction Heavy rainfall, waves, storm surges, and high tides during cyclone (typhoon or hurricane) periods cause hazards, such as flooding/inundation and shoreline erosion in coastal areas. A full understanding of the behavior of water bodies and their movement in estuarine areas during cyclone periods is essential for flood control, disaster mitigation, coastal erosion protection, and coastal structure preservation. Integrated coastal and estuarine process models (ICEPM) can simulate multiple physical processes, such as tides, waves, surges, river flows, wind, sediment transport, and morphological changes, e.g., [1–5]. They are applicable to study hydrodynamic and morphodynamic processes in response to what-if scenarios of coastal protection practices, and, therefore, facilitate safety assessment

Water 2020, 12, 1084; doi:10.3390/w12041084 www.mdpi.com/journal/water Water 2020, 12, 1084 2 of 20 Water 2020, 3, x FOR PEER REVIEW 2 of 20 of estuarine areas and the determination of coastal management measures to prevent or reduce disasters safety assessment of estuarine areas and the determination of coastal management measures to and to stabilize shorelines. prevent or reduce disasters and to stabilize shorelines. Surrounded by seas on all sides, Taiwan is subjected to 3–4 typhoons on average per year. The Surrounded by seas on all sides, Taiwan is subjected to 3–4 typhoons on average per year. The Tamsui River is the largest river in northern Taiwan, and variations of erosion and deposition on the Tamsui River is the largest river in northern Taiwan, and variations of erosion and deposition on the river bed are closely related to flows and sediment transport in the adjacent marine waters (Figure1). river bed are closely related to flows and sediment transport in the adjacent marine waters (Figure In recent years, owing to the construction of the Taipei Port, the littoral drift in the estuary and adjacent 1). In recent years, owing to the construction of the Taipei Port, the littoral drift in the estuary and coastal areas has been gradually changed, which affects the adjacent coastal terrain and landscape [6]. adjacent coastal areas has been gradually changed, which affects the adjacent coastal terrain and In addition, serious siltation occurs in the Shihmen Reservoir upstream of the Tamsui River, with landscape [6]. In addition, serious siltation occurs in the Shihmen Reservoir upstream of the Tamsui the total siltation in the Shihmen Reservoir reaching 1.86 million m3 by the beginning of 2017—a River, with the total siltation in the Shihmen Reservoir reaching 1.86 million m3 by the beginning of volume accounting for 32.6% of the designed water-storage capacity of the reservoir [7]. To alleviate 2017—a volume accounting for 32.6% of the designed water-storage capacity of the reservoir [7]. To the reservoir siltation problem, meet the industrial water demand, and extend the reservoir life, the alleviate the reservoir siltation problem, meet the industrial water demand, and extend the reservoir Water Resources Agency of the Ministry of Economic Affairs plans to add a desilting tunnel under life, the Water Resources Agency of the Ministry of Economic Affairs plans to add a desilting tunnel the water storage limit of the Shihmen Reservoir at the Amuping and Dawanping areas [8]. The under the water storage limit of the Shihmen Reservoir at the Amuping and Dawanping areas [8]. Amuping desilting tunnel is expected to be completed by 2022, and it will lead to changes in the amount The Amuping desilting tunnel is expected to be completed by 2022, and it will lead to changes in the of sediment transport in the Tamsui River and indirectly affect the sediment transport equilibrium amount of sediment transport in the Tamsui River and indirectly affect the sediment transport between the river and the sea. equilibrium between the river and the sea.

FigureFigure 1. Tamsui1. Tamsui River River catchment catchment of Taiwan.of Taiwan.

ForFor assessing assessing the the impact impact of sedimentof sediment management management plans, plans such, such as reservoir as reservoir sediment sediment ﬂushing, flushing on , theon sediment the sediment transport transport and morphological and morphological changes changes in the Tamsui in the RiverTamsui Estuary River andEstuary its adjacent and its coasts,adjacent it iscoasts, necessary it is tonecessary apply numerical to apply modelsnumerical to predict models the to spatiotemporalpredict the spatiotemporal redistribution redistribution of sediments of fromsediments the reservoir from intothe reservoir the areas downstream.into the areas Fordownstream. this impact For assessment this impact of sediment assessment ﬂushing, of sediment it is flushing, it is important to apply a validated numerical model, which can demonstrate its capability to reproduce flows, waves, sediment transport, and morphological changes in the estuary. This

Water 2020, 12, 1084 3 of 20 important to apply a validated numerical model, which can demonstrate its capability to reproduce flows, waves, sediment transport, and morphological changes in the estuary. This requires integrated coastal and estuarine processes modeling by taking into account all the hydrological forces, such as river flows, tides, waves, storm surges, and wind-induced currents, along with the movement of sediment in the river and that flushed from the reservoir. In the shallow coastal zone, the flow near the bottom, which drives sediment transport and causes bed changes, is composed mainly of wave motion and mean currents. The mean current in coasts and estuaries may be a combination of tidal flows, river flows, wave-phase-averaged currents, etc. Interaction of wave and current is a highly nonlinear process, particularly in coastal and estuarine areas. Since the 1970s, many studies have been conducted by experimental methods to explain the wave–current boundary layer interaction ([9–23]) on the theoretical side. Wave–current interaction can be simulated by using the so-called wave-phase-resolving numerical models [24,25], which are usually computationally costly. For long-term and large-scale simulations, the phase-averaged models [26,27] are often used to provide engineering answers to coastal problems, such as dredging, erosion, flooding/inundation, etc. It is important to simulate sediment transport and morphological changes by taking into account the combination of wave and current. The high shear velocity within the bottom boundary layer produces strong turbulence and large bed shear stresses impacting on the current, bringing increased bottom resistance in the presence of combined flow [28]. Hence, the combined wave-current flow is the basic element on sediment transport, mixing processes, diffusion, and other important coastal phenomena. Therefore, some researchers determine physical mechanisms by analyzing the combined flow over smooth or rough beds, both experimentally and numerically [29,30]. By developing the numerical models and experiments, researchers have analyzed that the sandpit morphodynamics might be caused by wave transformation, refraction, and diffraction induced by the pit itself. Van Rijn [31] developed a mathematical model to predict the sedimentation of dredged channels by waves and currents. Roos et al. [32] developed an idealized process-based morphodynamic model for the evolution of large-scale sandpits in a tide-dominated offshore environment. Benedet and List [33] use a numerical model to evaluate the effects of bathymetric features, such as dredged borrow areas, on the formation of erosion hot spots. Faraci et al. [34] conducted experimental to explain the possibility of exploiting sandpits at intermediate and shallow water depths. Most of the existing storm-surge models can be used to simulate the storm surge-induced coastal flooding, such as the POM (Princeton Ocean Model) [35], SLOSH (Sea, Lake and Overland Surges from Hurricanes) [36,37], and SHORECIRC (Quasi-3D NearSHORE CIRCulation Model) [38,39] models, but they are unable to consider sediment transport processes and bed variations when simulating coastal hydrodynamics. Since 1996, scholars have developed the ICEPM to simulate dynamic changes in coastal morphology, including shoreline changes, erosion, barrier breaching, and dam break [1,40–43]. This model is an integrated product of the wave spectrum, tidal current with wave effects (considering the radiation stress effect), sediment transport, and bed variation models with the computing process. By introducing an empirical sediment transport formula, the ICEPM allows calculating long-term morphological changes [2,3], whereas the computational period and relevant parameters need to be verified using actual data of morphological changes. Some researchers also estimate long-term changes in estuaries and lagoons using empirical semi-numerical approaches, such as tidal averaging and rapid morphologic assessment [4]. However, such types of tidal-averaging models mostly adopt the tidal phase averaging for computing, and thus, are unable to effectively simulate the maximum waves and surges caused by a typhoon and the resulting flooding event. Moreover, the abundant rainfall brought by a typhoon will lead to an increase in the upstream flood volume, which in turn will transport more sediment to the estuary, thereby making the estuarine flow field more complicated. To find maximum sedimentation and erosion is one of the important tasks in the impact assessment of hazardous typhoons and sediment flushing operations upstream the estuary. For the purpose, it is Water 2020, 12, 1084 4 of 20 necessary to simulate all the hydrodynamic and morphodynamic processes in the estuary, including wave dynamics, river hydrology, oceanography, and sediment transport mechanics. CCHE2D-Coast [5,44] is an integrated ICEPM model, capable of simulating these multiple coastal and estuarine processes. By using empirical sediment transport formulas, this model can simulate sediment transport driven by waves and currents in coasts and estuaries, and, therefore, enable the simulation of morphological changes in a wide region from the river upstream to the river mouth, and coasts. Through many years of development, validation, and improvement, this model demonstrated its applicability to help engineering planning and design of coastal flood and erosion protection by considering complex hydrological and morphological conditions in estuaries [3,5,45]. The wave model of CCHE2D-Coast has the following major features: (1) It simulates deformation and transformation of irregular waves in coastal and estuarine conditions, including wave breaking, refraction, diffraction, and interaction of wave and current at river mouth [27]; (2) it considers the effect of wave roller due to breaking to compute more accurate radiation stresses to include the effect of varying vertical mean current structure in the surf zone. The sediment transport model takes into account the effect of combined waves and currents in an estuary [5]. This model has been successfully applied to study the sediment transport and flood flows in the Touchien River Estuary, Hsinchu, Taiwan, and it has provided numerical results of flow and morphological changes under the conditions of various planning scenarios [5,45]. Therefore, this model was selected to simulate sediment transport and morphological changes in a river estuary such as Tamsui River Estuary, which are mainly driven by coastal waves, tidal currents, and river inflows. Since the construction of the Taipei Port in 1993, the Tamsui River estuary has undergone changes in the type of sediment movement. According to the statistics of Shihmen Reservoir’s discharge and sediment delivery ratio during 2006–2017, the sediment delivery ratio has been tending to increase since the desilting tunnel of Shihmen Reservoir was put into use for the first time in 2013 [8]. The Amuping desilting tunnel of Shihmen Reservoir is expected to be completed in 2022, and it will further change the conditions of the water-sediment at the Tamsui River estuary. Therefore, this study used CCHE2D-Coast to explore the changes in the scouring and aggradation of the Tamsui River estuary after the opening of the Amuping desilting tunnel. The rest of the paper is organized as follows: Section2 provides a brief introduction of the integrated coastal and estuarine processes model, CCHE2D-Coast. Section3 gives more information on the model for the study area, the Tamsui River estuary. Section4 is for model validation and model skill assessment results by simulating hydrodynamic and morphological changes through the 2008 typhoon season. Section5 provides simulation results and discussions for sediment flushing from the Shihmen reservoir. Finally, Section6 concludes the paper and gives some preliminary suggestions on the management of flood and sediment in the estuary.

2. Brief Description of the Integrated Coastal and Estuarine Model An integrated river/coastal/estuarine process modeling system, CCHE2D-Coast [5,44], was used to simulate the coupled hydrodynamic and morphodynamic processes driven by waves, tides, storm surges, and sediment transport for assessment of flooding and sedimentation in the Tamsui River Estuary (Figure2). CCHE2D-Coast contains a number of submodels for simulating the deformations and transformations of irregular/multidirectional waves, tropical cyclonic barometric pressure, and wind fields along storm tracks, tidal, and wave-induced currents, and morphological changes. For computing irregular waves, a multi-directional spectral wave action equation was adopted in the wave spectral module. It is capable of modeling wave deformation/transformation processes from deepwater to shallow water for nonbreaking and breaking waves, including wind-generated waves and whitecapping effects. For coastal structures, the wave model can take into account the transmission effect of wave energy through a permeable structure (e.g., a rubble mount breakwater). The flow model was used to simulate multi-scale hydrodynamic processes of free-surface water flows, such as river Water 2020, 12, 1084 5 of 20

ﬂows, tidal currents, nearshore currents, and storm surges. A non-orthogonal structured grid was usedWater for2020 all, 3, x simulation FOR PEER REVIEW models of waves, currents, sediment transport, and morphological changes.5 of 20

FigureFigure 2.2. CCHE2D-CoastCCHE2D-Coast is a fully integrated numerical model system for simulating waves, currents, andand morphologicalmorphological changes.changes.

InIn thethe current current module, module, the the two-dimensional two-dimensional (2-D) (2 depth--D) depth and˗ shortwave-averagedand shortwave˗averaged shallow shallow water equationswater equations with wave with forcing wave forcing were employed were emp toloyed simulate to simulate the flows the driven flows by driven wave by radiation wave radiation stresses, tides,stresses, storm tides, surges, storm river surges, inflows, river the inflows, Coriolis the force, Coriolis and turbulence force, and in surfturbulence zones. Thisin surf hydrodynamic zones. This modelhydrodynamic provides model users withprovides two optionsusers with to calculate two options the wave to calculate radiation the stresses: wave radiation one is the stresses: traditional one waveis the stresstraditional formulations wave stress by means formulations of the sinusoidal by means waveof the assumptionsinusoidal wave of the assumption linear small-amplitude of the linear wavesmall- theory;amplitude another wave is theory; the improved another radiationis the improved stresses radiation formulae stresses derived formulae from the derived non-sinusoidal from the wavenon-sinusoidal assumption, wave which assumption enables the, which three-dimensional enables the three (3-D)-dimens featuresional of vertical(3-D) features current structuresof vertical (e.g.,current surface structures rollers (e.g. or undertow, surface rollers currents) or undertow in the surf currents) zone to in be the taken surf into zone account to be taken [44]. into account [44]. In the sediment transport submodel, empirical sediment transport models were used to calculate the sedimentIn the sediment transport transport rates ofsubmodel, bed materials empirical and sediment the suspended transport sediment models underwere used the to conditions calculate ofthe combined sediment wavestransport and rates currents. of bed Amaterials unified and sediment the suspended transport sediment model [3 ]under was usedthe conditions to calculate of thecombined sediment waves flux and from currents. upstream A riversunified to sediment estuaries andtransport coasts model to consider [3] was the used sediment to calculate transport the seamlesslysediment flux from from the non-wave upstream environment rivers to estuaries at a river and to the coasts wave–current to consider interaction the sediment area at antransport estuary * andseamlessly a wave-dominant from the non coastal-wave zone. environment The local at sediment a river to transport the wave rate–current vector interactionq can be writtenarea at asan follows:estuary and a wave-dominant coastal zone. The local sediment transport rate vector can be written * * * τm τc * * as follows: q = q c + q w = − Ac u + AwFD u b (1) ρg 𝑞𝑞⃑ * * = + = ( + ) (1) where q c and q w are the local sediment transport rates due to current and wave, respectively. τm is the 𝜏𝜏𝑚𝑚 − 𝜏𝜏𝑐𝑐 maximum bottom shear stress which𝑞𝑞⃑ has𝑞𝑞⃑𝑐𝑐 been𝑞𝑞⃑𝑤𝑤 modified to𝐴𝐴𝑐𝑐 consider𝑢𝑢�⃑ 𝐴𝐴𝑤𝑤𝐹𝐹𝐷𝐷 the𝑢𝑢�⃑𝑏𝑏 difference of the stress in river where and are the local sediment transport𝜌𝜌𝜌𝜌 rates due to current and wave, respectively. is flows and nearshore currents, τc is the critical shear stress for moving bed materials, ρ is the water the maximum bottom shear stress which has* been modified to consider the difference* of the stress in density,𝑞𝑞⃑ g𝑐𝑐 is the𝑞𝑞⃑ gravitational𝑤𝑤 acceleration, u is the velocity vector of mean current, u is the wave𝜏𝜏𝑚𝑚 river flows and nearshore currents, is the critical shear stress for moving bed materials,b is the orbital velocity at the bottom, A and A are the empirical coefficient for sediment transport driven by water density, g is the gravitationalc acceleratw ion, is the velocity vector of mean current, is the 𝜏𝜏𝑐𝑐 𝜌𝜌 wave orbital velocity at the bottom, and are the empirical coefficient for sediment transport 𝑢𝑢�⃑ 𝑢𝑢�⃑𝑏𝑏 driven by mean currents and waves, respectively, is a direction function (= −1 for offshore, +1 for 𝐴𝐴𝑐𝑐 𝐴𝐴𝑤𝑤 onshore). By using computed mean velocity and wave orbital velocity, the maximum bottom stress 𝐹𝐹𝐷𝐷 is calculated as follows:

Water 2020, 12, 1084 6 of 20

mean currents and waves, respectively, FD is a direction function (= 1 for offshore, +1 for onshore). − By using computed mean velocity and wave orbital velocity, the maximum bottom stress is calculated as follows: * * 2 τm = ρC f u + u b (2) where C f is the bottom friction coefficient due to current and wave [44]. For detailed information on the sediment transport model, one may further refer to Ding and Wang (2007) [3] and Ding et al. (2016) [44]. Morphological changes were computed based on the mass balance of sediments, in which the diffusive transport for the anisotropic downslope gravitational effect was also included [41]. The wetting and drying process was properly modeled to handle water level variations and bed changes. This integrated processes model (CCHE2D-Coast) provides a variety of boundary conditions for computing wave actions, currents, sediment transport, and morphological changes. On the offshore boundary, wave spectral energy density was given by using the parametric wave spectrum defined by observed wave parameters in offshore buoys. Tidal levels were also specified on the offshore by using observed water level data or astronomical tidal level predictions. For river inflows upstream, hydrographs (time histories of discharge) can be given by observation data at hydrological stations or other model estimations. On the boundaries upstream for inputting river inflows, one may also give sediment transport fluxes based on measured (or planned) sediment release/flushing rates from hydraulic structures upstream, such as dams and reservoirs. The sediment transport fluxes can also be calculated by using a sediment transport formulation, such as Equation (1), by assuming the state of sediment transport through the boundaries is close to equilibrium. In general, it is assumed that no bed changes along the offshore boundaries occur through the computational period. More specific information on the boundary conditions for the present study in the Tamsui River Estuary is provided in the following sections. The simulations performed with this integrated model are also supported by a user-friendly interface (CCHE2D-GUI) [46], which assists users to generate computational grids, specify boundary conditions and model parameters, and monitor and visualize the simulation results. All the submodels work with the same computational grid. Thus, the wave and flow models run on the same computational cores, passing information between submodels through the local memory/cache. It does not need to switch any additional executable codes during computations. All simulations are performed efficiently on a PC. CCHE2D-Coast has been validated by simulating waves and storm surges induced by hurricanes in the Gulf of Mexico and investigating the impact of sea-level rise on morphological changes in the Touchien River estuary, Hsinchu, Taiwan [5]. The validated CCHE2D-Coast model was applied to the feasibility studies of coastal protection plans, such as those for river channel dredging and layout of new coastal structures in the Touchien River Estuary [45].

3. Study Area and Construction of the Numerical Model

3.1. Study Area The Tamsui River is the largest river in northern Taiwan, which is composed of major tributaries, such as the Dahan River, Xindian River, Keelung River, and Sanxia River, with a total drainage area of 2726 square km (Figure1). The main stream of the Tamsui River is 24 km long, with the Dahan River and Xindian River confluencing at Jiangzicui (on whose two banks the Xinzhuang and Manka districts are located), and the Keelung River confluencing downstream at Guandu. It forms sandbars under the Huajiang Bridge as well as at Guandu and Zhuwei along the flow path, which are the most important northern reserve for waterfowl and mangrove. The amount of sediment transport of the whole Tamsui River basin is approximately 9.28 million m3 per year. However, due to presence of the Shihmen Reservoir, Feitsui Reservoir, and sediment barriers, the actual amount of sediment transport in the middle reaches is approximately 1.85 million Water 2020, 12, 1084 7 of 20 m3 per year, of which about 85% corresponds to the suspended load transport—1.25 million m3 per year, whereas the bedload transport rate is 0.185 million m3 per year. The peak discharge at Tamsui River estuary can reach more than 2000 m3/s under typhoon rainstorms, making the Tamsui River one ofWater the 20 main20, 3, x sediment FOR PEER sourcesREVIEW of the coastal waters [6]. The river section from the estuary to the Xinhai7 of 20 Bridge of the Dahan River is a tideway, where the water level of the river changes with the rise and fall ofXinhai the tidal Bridge level of inthe the Dahan sea, accompanied River is a tideway, by a reciprocal where the movement water level of of the the river river water changes flow with the tidalrise and movement. fall of the There tidal is level a close in relationshipthe sea, accompanied between theby scouringa reciprocal and movement aggradation of balancethe river and water the sedimentflow with transportthe tidal movement. behavior of There the adjacent is a close coastal relationship waters. Eitherbetween the the invasion scouring of aand typhoon aggradation or the sedimentbalance and discharge the sediment of the transport upstream behavior reservoir, of they the alladjacent lead to coastal morphological waters. Either changes the ininvasion the Tamsui of a Rivertyphoon estuary. or the sediment discharge of the upstream reservoir, they all lead to morphological changes in theTo Tamsui explore River the e ffestuary.ect of the Shihmen Reservoir sediment discharge on the Tamsui River estuary, this studyT selectedo explore the the Guandu effect of Bridge the Shihmen on the Tamsui Reservoir River sediment as the upstream discharge boundary, on the Tamsui with the River selected estuary, sea areathis rangingstudy selected from the the north-bank Guandu TN08Bridge section on the of Tamsui the Tenth River River as Management the upstream Offi boundary,ce to its south-bank with the TS35selected section, sea area where ranging the total from shore the north length-bank was TN08 21 km. section The study of the area Ten extendedth River Management seawards from Office 3 to 5to km its (watersouth-bank bottom TS35 elevation section, of where20 m). the The total simulation shore length range was included 21 km. the The Tamsui, study Bali, area and extended Linkou − districts,seawards with from the 3 coastto 5 km comprising (water bottom a mixed elevation terrain ofof sand −20 andm). The rock. simulation There are manyrange concaveincluded bays the alongTamsui, the Bali coast,, and and Linkou beaches districts, are developed with the coast in the compris coves,ing such a mixed as the strip-shapedterrain of sand sandy and rock. beaches There of Qianshuiare many Bay,concave Zhouzi bays Bay, along and Shalun.the coast, Owing and beaches to the complicated are developed longshore in the currents, coves, such the sandas the shore strip is- susceptibleshaped sandy to beaches erosion. of In Qianshui the Tamsui Bay, River Zhouzi estuary, Bay, and the Shalun. southern Owing shore to between the complicated Bali and longshore Linkou is acurrents,ffected by the the sand development shore is susceptible of Taipei Port to erosion. and the In estuarine the Tamsui sediment River transport, estuary, the and southern as a result, shore the Balibetween section Bali of and the shoreLinkou in theis affected northern by section the development of the port is of currently Taipei Port subjected and the to siltation.estuarine In sediment contrast, thetransport, south side and ofas the a result, port (Linkou the Bali section) section showsof the shore an erosion in the trend. north Theern overallsection coastalof the port situation is currently within thesubject studyed areato siltation. is presented In contrast, in Figure the3 .south side of the port (Linkou section) shows an erosion trend. The overall coastal situation within the study area is presented in Figure 3.

S imulation region S tudy region Township region G auging s tation Landmark Dike

S outh boundary

Figure 3. StudyFigure region 3., simulationStudy region, region simulation, and relevant region, monitoring and relevant sites. monitoring sites.

3.2. Model Configuration The numerical elevation data used in this study were measured by the Institute of Harbor and Marine Technology, Ministry of Transportation and Communications [47]. The complete topo- bathymetric data was available for the years of 2006–2013, covering the river channel from the Guandu Bridge to the estuary and also covering the area that extended seawards approximately 3–5 km (water bottom elevation of −20 m); the selected sea area ranged from the north-bank TN08 section of the Tenth River Management Office to its south-bank TS35 section. If the monitoring data of this

Water 2020, 12, 1084 8 of 20

3.2. Model Configuration The numerical elevation data used in this study were measured by the Institute of Harbor and Marine Technology, Ministry of Transportation and Communications [47]. The complete topo-bathymetric data was available for the years of 2006–2013, covering the river channel from the Guandu Bridge to the estuary and also covering the area that extended seawards approximately 3–5 km (water bottom elevation of 20 m); the selected sea area ranged from the north-bank TN08 − section of the Tenth River Management Office to its south-bank TS35 section. If the monitoring data of this region were used to explore estuarine changes, the sea area would be too small to reflect the characteristics of waves and currents properly. Thus, the north shore of this sea area was extended to Linshanbi (with the corresponding topo-bathymetric data of the year 2008). The river flow path was adjusted using satellite image data, and the 2008 events of Typhoon Fung-Wong, Typhoon Sinlaku, and Typhoon Jangmi were included in this study as validation cases to verify the relevant parameters of the model. The simulation model boundaries shown in Figure3 cover the study area expanding 40 km and 30 km in the east–west and north–south directions, respectively. Non-orthogonal quadrilateral meshes were generated by using CCHE-GUI [46] with the total number of 156,349 nodes (289 541) × in the computational domain. The grid resolutions varied from 20 m at the river mouth to 500 m in the offshore. The eastern boundary crossed the Linshanbi station, while the western boundary lay approximately at the Zhuwei Fishing Port station. The southern boundary of the river was placed at the Guandu Bridge. The monitoring data at the east and west boundaries were measured by two tide-gauge stations of the Central Weather Bureau, the Linshanbi station and the Zhuwei Fishing Port station, respectively (Figure3). The monitoring data at the north boundary included wind speed, wind direction, and waves, all adapted from the monitoring data of the outside sea area of the Taipei Port. For the southern boundary, the data were obtained by simulating the hydrograph of flow and sediment concentration at the Guandu Bridge based on CCHE1D, a verified numerical model suited for modeling the scouring and aggradation in one-dimensional river channels. The upstream boundary data in CCHE1D were derived from the measured hydrograph of flow and sediment concentration at the Shihmen Reservoir; the downstream boundary data were the measured tidal level in the Tamsui River estuary [6]. The bed particle size was adapted from the survey led by the Tenth River Management Office titled “Evaluation of the Impacts of Tamsui River Sediment Transport on Adjacent Coasts” and taken as the median particle size d50 = 0.2 mm. For sediment transport, its total load was calculated. The Manning roughness coefficient was set to 0.02 in the outside sea area and 0.025 in the river area.

4. Numerical Model Validation and Statistic Error Analysis This study used the 2008 events of Typhoon Fung-Wong, Typhoon Sinlaku, and Typhoon Jangmi to carry out the model validation. The starting and ending times of each typhoon event are listed in Table1. The topo-bathymetric data used in the model were measured using single-beam echosounders in May 2008. The tidal levels at the west and east boundaries were given by measured tidal gauge data at the Linshanbi and Zhuwei Fishing Port stations. The wind data measured at the Taipei Harbor meteorological station were used to generate time-dependent and spatially-uniform wind fields. The peak flows in the three typhoon-induced flood events were 3798, 6476, and 7332 m3/s, respectively, whereas the sediment concentration was controlled by the sediment discharge operation of the stream Shihmen Reservoir, with the maximum concentration of 45,042 kg/s occurring during the period of Typhoon Sinlaku (Figure4). It was assumed that the flow at the Guandu Bridge was 50 m 3/s, and the sediment concentration was 0 kg/s in the absence of flood drainage at the reservoir. The simulation period ranged from 17:00 on 23 July 2008 to 00:00 on 1 October 2008, totaling 1675 hours. The model was spun up by computing the tide flows from 17:00 on 23 July 2008 before introducing Typhoon Fung-Wong, in which the time step was set to 30 s. After the spin-up, the wave–current interaction for modeling morphological changes was conducted for 66 days until the end of Typhoon Jangmi at 00:00 Water 2020, 12, 1084 9 of 20 on 1 October 2008. Then, the time step for computing flows was changed to 10 s; the time step for updating bed elevations was 100 s. The wave field was recomputed every one hour based on the latest flow field. Water 2020, 3, x FOR PEER REVIEW 9 of 20 Table 1. Starting and ending times and duration of the typhoon events used in model validation. Table 1. Starting and ending times and duration of the typhoon events used in model validation. Typhoon Name Starting Time Ending Time Duration (day) Typhoon Name Starting Time Ending Time Duration (day) Typhoon Fung-Wong 26 July 2008 7:00 30 July 2008 0:00 4 Typhoon Fung-Wong 26 July 2008 7:00 30 July 2008 0:00 4 Typhoon Sinlaku 10 September 2008 23:00 18 September 2008 0:00 7 Typhoon Sinlaku 10 September 2008 23:00 18 September 2008 0:00 7 Typhoon Jangmi 26 September 2008 23:00 1 October 2008 0:00 4 Typhoon Jangmi 26 September 2008 23:00 1 October 2008 0:00 4

FigureFigure 4 4.. TimeTime series series of of river river flow ﬂow and and sediment sediment concentration concentration at at the the Guandu Guandu Bridge Bridge (validation cases) cases).

TToo quantify the the difference diﬀerence between the the calculated calculated results results and and the the observed observed values, values, this this study study employedemployed the the four four parameters parameters,, such such as as the the root root-mean-square-mean-square error error ( (RMSERMSE),), the the Pearson Pearson correlation correlation coefficientcoeﬃcient ( (rr),), the the scatter scatter index index ( (SISI),), and and the the normalized normalized bias bias ( (NBNB)).. T Thehe calculation calculation results results of of these these parametersparameters at three stations (the(the locationslocations areare markedmarked in in Figure Fig. 3)3 )are are listed listed in in T Tableable 22.. TheThe RMSERMSE waswas usedused to to quantify the deviationdeviation betweenbetween the the simulated simulated and and observed observed values; values;r was r was used used to evaluateto evaluate the thecorrelation correlation between between the the simulated simulated and and observed observed values; values;SI SIwas was the the ratioratio ofof the standard deviation deviation betweenbetween the the simulated simulated and and observed observed values values to to the the mean mean of of the the observed observed values; values; the the indicator indicator NBNB evaluateevaluatedd the the extent to which thethe calculatedcalculated resultsresults were were overestimated overestimated or or underestimated underestimated relative relative to tothe the true true values. values.RMSE RMSE, r, rSI, SI, and, andNB NBcan can be be calculated calculated using using the the following following expressions: expressions: s P N E 2 = i=1 i (3) RMSE = 𝑁𝑁 2 (3) ∑𝑖𝑖=N1 𝐸𝐸𝑖𝑖 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 � P 𝑁𝑁 N ((m m))O( O ) = i=1 i i r = 𝑁𝑁 − − (4) q 𝑖𝑖=1 𝑖𝑖 𝑖𝑖 2 (4) PN∑(( 𝑚𝑚 −)2)𝑚𝑚�PN𝑂𝑂 (− 𝑂𝑂� ) 𝑟𝑟 i=1 mi m i=1 Oi O 𝑁𝑁 − 2 𝑁𝑁 − 2 𝑖𝑖=1q 𝑖𝑖 𝑖𝑖=1 𝑖𝑖 � �∑ 𝑚𝑚 −P𝑚𝑚� ∑ 𝑂𝑂2 − 𝑂𝑂 1 1 N E E N i=1 ( i − ) SI = P (5) = 1 𝑁𝑁N (5) N 1 i=1 Oi 2 � ∑𝑖𝑖=1 |𝐸𝐸|𝑖𝑖 −| |𝐸𝐸� 𝑁𝑁1 PN 𝑆𝑆𝑆𝑆 N i𝑁𝑁=1 Ei NB = P∑𝑖𝑖=1 𝑂𝑂𝑖𝑖 (6) 1𝑁𝑁1 N O N i=1| i| = (6) where E is the error between the simulated value1m and𝑁𝑁 observed value O in the simulation time i ∑i 𝑖𝑖=1|𝐸𝐸𝑖𝑖| i 𝑁𝑁 series, i.e., mi Oi; N is the total number of𝑁𝑁𝑁𝑁 the observations;𝑁𝑁 E is the average error of the simulated − PN 𝑖𝑖=1 𝑖𝑖 PN wherevalue expressed is the error as (1 between/N) i= 1theEi ;simulatedO is the average value observed∑ and𝑂𝑂 observed value expressedvalue in as the(1/ simulationN) i=1 Oi ;timem is | | P𝑁𝑁N series,the average i.e., simulated; is value the total expressed number as of(1 the/N )observations;= mi. is the average error of the simulated 𝐸𝐸𝑖𝑖 i𝑚𝑚1𝑖𝑖 𝑂𝑂𝑖𝑖 value expressed as (1/ ) | |; is the average observed value expressed as (1/ ) ; 𝑚𝑚𝑖𝑖 − 𝑂𝑂𝑖𝑖 𝑁𝑁 𝐸𝐸� is the average simulated value𝑁𝑁 expressed as (1/ ) . 𝑁𝑁 𝑁𝑁 ∑𝑖𝑖=1 𝐸𝐸𝑖𝑖 𝑂𝑂� 𝑁𝑁 ∑𝑖𝑖=1 𝑂𝑂𝑖𝑖 𝑚𝑚� 𝑁𝑁 𝑁𝑁 ∑𝑖𝑖=1 𝑚𝑚𝑖𝑖

Water 2020, 12, 1084 10 of 20

Table 2. Calculated results of the root-mean-square error (RMSE), the Pearson correlation coeﬃcient (r), the scatter index (SI), and the normalized bias (NB) for tidal level, wave height, and wave period in the typhoon events at three stations: Tamsui River Estuary tide gauge (a.k.a. Estuary Tide Gauge), Observation pile outside the Taipei Port (a.k.a. Observation Pile), and Third dock Inside the Taipei Port (a.k.a. Third Dock Gauge), of which locations are shown in Figure3.

Starting Ending Typhoon Monitoring Monitoring RMSE r SI NB Time Time Name Site Data Type (m) Estuary Tide Tidal level 0.10 0.99 0.16 0.04 Gauge Observation Tidal level 0.34 0.99 0.19 0.46 Pile 30 July Typhoon 27 July 2008 Third Dock 2008 Fung-Wong Tidal level 0.32 1.0 0.71 0.35 Gauge − Observation Significant 0.59 0.65 0.61 0.17 Pile wave height − Observation Significant 1.58 0.67 0.31 0.13 Pile wave period − Estuary Tide Tidal level 0.16 0.99 0.16 0.13 Gauge Observation Tidal level 0.26 0.99 0.19 0.28 Typhoon Pile 2 12 September Sinlaku, October Third Dock 2008 Typhoon Tidal level 0.35 0.99 0.71 0.34 2008 Gauge − Jangmi Observation Significant 0.42 0.93 0.33 0.13 Pile wave height Observation Significant 0.58 0.92 0.12 0.01 Pile wave period −

In the simulation range, tidal level gauges were installed in the Tamsui River estuary, the third dock in the Taipei Port, and the observation pile outside the Taipei Port, as shown in Figure3. Only at the observation pile outside the harbor at the estuary, were waves measured. In the Typhoon Fung-Wong event, the Shihmen Reservoir was operated for flood drainage and sediment discharge from 18:00 on 27 July 2008 to 23:00 on 29 July 2008, which resulted in a rapid increase in the water flow and sediment transport in the river channel. The overall r values shown in Table2 indicate a very strong correlation in tidal levels and wave parameters (wave heights and periods) between the observations and simulations at the four measurement stations. The simulation results in Figure5a show that the simulated water levels at the estuary tide gauge were quite close to the measured values (RMSE = 0.10 m; NB = 0.04). Figure5b presents the simulated and measured water levels at the same location from September 13 to October 1—covering the period of Typhoon Sinlaku and the period of Typhoon Jangmi (RMSE = 0.16 m; NB = 0.13). Although the RMSE values were small relative to the tidal range in the area (2.0–3.0 m), the differences in the water levels were due to the effect of river inflows. Water 2020, 12, 1084 11 of 20 Water 2020, 3, x FOR PEER REVIEW 11 of 20

FigureFigure 5. 5.Comparison Comparison ofof simulatedsimulated andand observedobserved waterwater levelslevels atat thethe EstuaryEstuary TideTide GaugeGauge duringduring ((aa)) TyphoonTyphoon Fung-Wong Fung-Wong as as well well as as (b ()b Typhoon) Typhoon Sinlaku Sinlaku and and Typhoon Typhoon Jangmi. Jangmi.

TheIn the simulated simulation water range, levels only at theone other buoy two was gauges installed were near also the close Taipei to thePort measured at a site with data. water The maximumbottom elevationRMSE was of 0.35approximately m at the third −10 dock m (Fig gaugeure inside3) for thewave harbor, parameter calculated measurement. during the periodFigure of 6 Typhoonspresents the Sinlaku simulated and Jangmi. and actual The valuesresults of ofNB wavevaried height from and0.34 wave at the period third dockduring gauge the insideperiod thes of − harborTyphoon to + Sinlaku,0.46 at the and observation Typhoon Jangmi. pile outside The theresults harbor. show It meansthat the that wave the height model reached underestimated 4 m during the waterthe period levels of inside Typhoon the harborJangmi but butoverestimated varied between outside. 0.4 and The0.6 m reason in the fornon the-typhoon errors couldperiod. be However, because twoduring gauges the transition are too close period to breakwaters from Typhoon of theSinlaku harbor. to Typhoon So the measurements Jangmi (Figure of 6 the), the water RMSE levels between may containthe simulated the effect and of observed reflections values of the of structures. the significant wave height was only 0.42 m, the r value was 0.93,In an thed the simulation SI value was range, 0.33 only, with one the buoy simulated was installed values tending near the to Taipei be slightly Port atoverestimated a site with water (NB = bottom0.13); the elevation RMSE, ofr, SI approximately, and NB between10 mthe (Figure simulated3) for and wave observed parameter values measurement. of the significant Figure wave6 − presentsperiod were the simulated0.58 m, 0.92 and, 0.12 actual, and results−0.01, respectively, of wave height with and the wavesimulated period values during slightly the periodslower than of Typhoonthe observed Sinlaku, ones and. TyphoonAs expressed Jangmi. by The the results above show quantitative that the wave indicators, height reachedCCHE2D 4 m-Coast during could the periodeffectively of Typhoon reproduce Jangmi the changes but varied in betweenwater level, 0.4 andwave 0.6 height m in,the and non-typhoon wave period period. during However, typhoon- duringinduced the flooding transition periods period and from non Typhoon-typhoon Sinlaku periods. to Typhoon Jangmi (Figure6), the RMSE between the simulated and observed values of the significant wave height was only 0.42 m, the r value was 0.93, and the SI value was 0.33, with the simulated values tending to be slightly overestimated (NB = 0.13); the RMSE, r, SI, and NB between the simulated and observed values of the significant wave period were 0.58 m, 0.92, 0.12, and 0.01, respectively, with the simulated values slightly lower than − the observed ones. As expressed by the above quantitative indicators, CCHE2D-Coast could effectively reproduce the changes in water level, wave height, and wave period during typhoon-induced flooding periods and non-typhoon periods. The topo-bathymetric data used in the analysis were measured by using the single-beam echosounders in 1 May 2008 and 31 October 2008. It may accurately reflect the morphological changes caused by Typhoon Fung-Wong, Typhoon Sinlaku, and Typhoon Jangmi. The total discharge of the Shihman Reservoir in three typhoon-induced flood events, respectively, were 142.83, 622.24, and 237.36 million m3 with the total sediment discharge of 3.45 million tons. Figure7a shows that the changes in the nearshore seabed did not comply with the sediment transport mechanism due to the anthropogenic disturbance. However, the morphological changes in the river estuary were from reasonable patterns of deposition and erosion. As indicated in Figure4, the sediment was transported in a large quantity to the estuary and channel bed, where it was deposited due to the large river flow

Water 2020, 3, x FOR PEER REVIEW 11 of 20

Figure 5. Comparison of simulated and observed water levels at the Estuary Tide Gauge during (a) Typhoon Fung-Wong as well as (b) Typhoon Sinlaku and Typhoon Jangmi. Water 2020, 3, x FOR PEER REVIEW 12 of 20 In the simulation range, only one buoy was installed near the Taipei Port at a site with water bottomFigure elevation 6. Comparison of approximately of simulated −and10 mobserved (Figure results 3) for of wave (a) significant parameter wave measurement. heights and (Figb) ure 6 presentssignificant the wavesimulated period ands at the actual Observation results Pileof wave. height and wave period during the periods of Typhoon Sinlaku, and Typhoon Jangmi. The results show that the wave height reached 4 m during theT periodhe topo of- Typhoonbathymetric Jangmi data but used varied in betwethe analysisen 0.4 and were 0.6 measuredm in the non by-typhoon using the period. single However,-beam echosoundersduring the transition in 1 May period2008 and from 31 TyphoonOctober 2008. Sinlaku It may to Typhoon accurately Jangmi reflect (Fig theure morphological 6), the RMSE changes between causedthe simulated by Typhoon and Fungobserved-Wong, values Typhoon of the Sinlaku, significant and wave Typhoon height Jangmi. was only The 0.42total m discharge, the r value of the was Shihman0.93, an dReservoir the SI value in threewas 0.33 typhoon, with- theinduced simulated flood values events tending, respectively to be slightly, were 142.83, overestimated 622.24, (andNB = 237.36 million m3 with the total sediment discharge of 3.45 million tons. Figure 7(a) shows that the Water0.13);2020 the, 12 ,RMSE 1084 , r, SI, and NB between the simulated and observed values of the significant12 ofwave 20 changesperiod inwere the 0.58nearshore m, 0.92 seabed, 0.12, anddid −not0.01, comply respectively, with the with sediment the simulated transport values mechanism slightly due lower to thethan anthropogenicthe observed disturbance.ones. As expressed However, by the the morphological above quantitative changes indicators,in the river CCHE2D estuary -Coastwere fromcould reasonableduringeffectively the patterns typhoon reproduce of periods. deposition the changes The and farthest in erosion. water deposition Aslevel, indicated wave site had heightin Figure the, waterand 4, thewave bottom sediment period elevation was during transported of typhoon10 m; - − inmeanwhile,induced a large quanti flooding the significantty to periods the estuary morphological and non and- typhoonchannel variation bedperiods., where occurred it was within deposited water due bottom to the elevation large river of flow5 m. − during the typhoon periods. The farthest deposition site had the water bottom elevation of −10 m; meanwhile, the significant morphological variation occurred within water bottom elevation of −5 m. Figure 7b indicates the bed changes simulated by the CCHE2D-Coast model through the same period from May to October of 2008. Comparing it with the observations in Figure 7a, the simulated bed changes have a good agreement with the observed one. The morphological changes in the tidal river reach from the river upstream of the Guandu Bridge to the river mouth have consistent variations with the field measurement. The similar depositions which occurred in the river bend were close to the concave bank and a large area of the river mouth. Although the model captures the features of the scouring and erosion close to the center of the bend, the simulated deposition at the river mouth was overestimated; the field observation showed more settling of river sediments at the south side. The simulated results also showed the less sedimentation inside the Taipei harbor. The dredging may be conducted near the breakwater in this period; however, the proposed model did not incorporate the effect of the anthropogenic activities. Moreover, the obvious difference in the bed change at the north shore of the river mouth could be found. The simulated erosion was driven by the simulated tidal currents and nearshore currents due to waves, while the sea bed was assumed as erodible due to lacking data. Overall, the numerical model produced reasonable results on the morphological changes in a wide range of the Tamsui River estuary. The current developed numerical models for simulating the sediment transport and morphodynamic processes are still challenging, a few factors, e.g., using spatially-varying bed materials, inclusion of dredging, sand miningFigure activities, 6. Comparison and fractional of simulated sediment and transport observed load, results would of (a) be significant further waveconsidered heights in and the ( bfuture) work. significant wave periods at the Observation Pile.

(a) Measured bed changes in the time period (b) Simulated bed changes through the same from 1 May 2008 to 31 October 2008 period

Figure 7 7.. ComparisonComparison of measured and simulated topographytopography ofof thethe TamsuiTamsui River River estuary. estuary. (DZ (DZ= =bed bedchange, change, contour contour line line is bed is elevation.bed elevation. The The unit unit is meter is meter for both).for both.)

5. SimulationFigure7b Results indicates and the Discussion bed changes on simulated Scenario byof Sediment the CCHE2D-Coast Flushing model through the same period from May to October of 2008. Comparing it with the observations in Figure7a, the simulated bed changes have a good agreement with the observed one. The morphological changes in the tidal river reach from the river upstream of the Guandu Bridge to the river mouth have consistent variations with the field measurement. The similar depositions which occurred in the river bend were close to the concave bank and a large area of the river mouth. Although the model captures the features of the scouring and erosion close to the center of the bend, the simulated deposition at the river mouth was overestimated; the field observation showed more settling of river sediments at the south side. The simulated results also showed the less sedimentation inside the Taipei harbor. The dredging may Water 2020, 12, 1084 13 of 20 be conducted near the breakwater in this period; however, the proposed model did not incorporate the effect of the anthropogenic activities. Moreover, the obvious difference in the bed change at the north shore of the river mouth could be found. The simulated erosion was driven by the simulated tidal currents and nearshore currents due to waves, while the sea bed was assumed as erodible due to lacking data. Overall, the numerical model produced reasonable results on the morphological changes in a wide range of the Tamsui River estuary. The current developed numerical models for simulating the sediment transport and morphodynamic processes are still challenging, a few factors, e.g., using spatially-varying bed materials, inclusion of dredging, sand mining activities, and fractional sediment Watertransport 2020, 3 load,, x FOR would PEER REVIEW be further considered in the future work. 13 of 20

55..1 SimulationEffect of Sediment Results Concentration and Discussion on the onTopography Scenario of Sediment Flushing

5.1. EAccordingffect of Sediment to the Concentration 2012 Report on “Hydraulic the Topography model studies for sediment sluicing and flood diversion engineering of Shihman Reservoir” released by the Water Resources Planning Institute, the According to the 2012 Report “Hydraulic model studies for sediment sluicing and flood diversion sediment delivery ratio (outflow sediment volume/inflow sediment volume of the Shihmen engineering of Shihman Reservoir” released by the Water Resources Planning Institute, the sediment Reservoir) of the reservoir after the construction of the Amuping desilting tunnel could be as high as delivery ratio (outflow sediment volume/inflow sediment volume of the Shihmen Reservoir) of the 55.2% [48]. In addition, the statistics of the sediment discharge amount from the afterbay of the reservoir after the construction of the Amuping desilting tunnel could be as high as 55.2% [48]. In Shihmen Reservoir from 2006 to 2017 revealed that in 2013, a total of 2.987 million tons of sediment addition, the statistics of the sediment discharge amount from the afterbay of the Shihmen Reservoir was discharged from the reservoir during the Typhoon Soulik period, which was the highest from 2006 to 2017 revealed that in 2013, a total of 2.987 million tons of sediment was discharged from discharge amount of sediment in recent years [49]. Therefore, this study adopted the case of the the reservoir during the Typhoon Soulik period, which was the highest discharge amount of sediment Typhoon Soulik as the base scenario with the sediment delivery ratio of 35% according to field in recent years [49]. Therefore, this study adopted the case of the Typhoon Soulik as the base scenario observation. with the sediment delivery ratio of 35% according to field observation. Typhoon Soulik impacted the study area from 01:00 on 12 July 2013, to 23:00 on 19 July 2013. Typhoon Soulik impacted the study area from 01:00 on 12 July 2013, to 23:00 on 19 July 2013. Therefore, the scenario from 00:00 on 1 July 2013, to 00:00 on 12 July 2013, was considered here to Therefore, the scenario from 00:00 on 1 July 2013, to 00:00 on 12 July 2013, was considered here to serve serve as a starting scenario, in which the time step for flow field calculation was set to 30 s, and the as a starting scenario, in which the time step for flow field calculation was set to 30 s, and the time step time step for wave field calculation was set to 7200 s. The situation from 30:00 on 12 July 2013, to 23:00 for wave field calculation was set to 7200 s. The situation from 30:00 on 12 July 2013, to 23:00 on 19 July on 19 July 2013, was the case scenario, in which the time steps for flow field calculation and wave 2013, was the case scenario, in which the time steps for flow field calculation and wave field calculation field calculation were set to 30 s and 7200 s, respectively, and the time step for bed calculation was were set to 30 s and 7200 s, respectively, and the time step for bed calculation was set to 100 s. The set to 100 s. The topo-bathymetric data used in the model for the case scenario was measured in May topo-bathymetric data used in the model for the case scenario was measured in May 2013. The west 2013. The west and east boundaries were the tidal level data measured at the Linshanbi and Zhuwei and east boundaries were the tidal level data measured at the Linshanbi and Zhuwei Fishing Port Fishing Port stations. For the south boundary, the current scenario without use of the Amuping stations. For the south boundary, the current scenario without use of the Amuping desilting tunnel desilting tunnel and the case scenario with the use of the Amuping desilting tunnel are shown in and the case scenario with the use of the Amuping desilting tunnel are shown in Figure8. Figure 8.

Figure 8Figure. Time series 8. Time of seriesriver flow of river and ﬂow sediment and sediment concentration concentration at the Guandu at the GuanduBridge. Bridge.

3 InIn the the current current scenario scenario with with Typhoon Typhoon Soulik, Soulik, the instant instant the the estuary estuary flow flow reached reached 5652 m 3/s/s was was whenwhen the the high high tide tide began began to to ebb (15:00 (15:00 on on 13 July 2013 2013).). At At this this time, time, the the flood flood elevation elevation reached reached its its maximummaximum in in each each area. area. The The water water levels levels in in the the river river channel channel were were approximately approximately 1.2 1.2–1.3–1.3 m m with with a a significantsignificant wave height ofof 0–0.40–0.4 m;m; thethe waterwater levelslevels in in the the outside outside sea sea area area were were approximately approximately 1–1.2 1–1.2 m mwith with a significanta significant wave wave height height of of approximately approximately 0.6–1.2 0.6–1.2 m, m, andand thethe estuaryestuary was subject to a a very very complicatedcomplicated flowflow fieldfield (Figure(Figure9 )9 due) due to to waves, waves, tides, tides, and and the the upstream upstream flood. flood. By 00:00 By 00 on:00 15 on July 15 2013,July 2013, Typhoon Soulik had moved away from the study area. When the flow in the estuary was 257 m3/s, the water levels in the estuary and the outside sea area were between 0–0.2 m, and the significant wave height was approximately 0–0.4 m, as shown in Figure 10.

Water 2020, 12, 1084 14 of 20

Typhoon Soulik had moved away from the study area. When the ﬂow in the estuary was 257 m3/s, the water levels in the estuary and the outside sea area were between 0–0.2 m, and the signiﬁcant wave height was approximately 0–0.4 m, as shown in Figure 10. WaterWater 20 202020, ,3 3, ,x x FOR FOR PEER PEER REVIEW REVIEW 1414 of of 20 20

3 FigureFigure 9 9Figure. .The The flow flow 9. The was was ﬂow 56 56552 was2 m m3/s 5652/s at at 15:00 15:00 m3/s on aton 15:001 133 July July on 2013 2013 13 July, ,when when 2013, the the when tide tide began thebegan tide to to began ebb. ebb. to ebb.

(b) Distribution of significant wave height ((aa)) Distribution Distribution of of water water level level and and flow flow (b) Distribution of significant wave height and significant wave direction velocityvelocity and significant wave direction Figure 10. The flow was 257 m3/s at 00:00 on 15 July 2013 when the tide was slack. Figure 10Figure. The flow 10. wasThe ﬂow257 m was3/s at 257 00:00 m3/ son at 1 00:005 July on 2013 15 Julywhen 2013 the whentide was the slack. tide was slack.

InIn response response to to Typhoon Typhoon Soulik, Soulik, the the Shihmen Shihmen Reservoir Reservoir was was operated operated to to tentatively tentatively discharge discharge the the 3 floodwaterfloodwaterfloodwater at at 21:00 21:00 on on 1 12122 July JulyJuly 2013 2013.2013. . The The flow flowflow at at the thethe Guandu GuanduGuandu Bridge BridgeBridge was waswas approximately approximatelyapproximately 273 273 mm33/s.//s.s. At At thisthis time, time, the the sediment sediment flux fluxflux of of the the estuary estuary was was dominated dominated by by li littorallittoralttoral drift, drift, which which moved moved in in the the same same directiondirection as as the the the longshore longshore longshore currents currents currents (Fig (Fig (Figureureure 1 11 111aa).).a). At At At12:00 12:00 12:00 on on 1 on133 July 13July July 2013 2013 2013,, ,due due dueto to the the to flood flood the flood control control control and and 3 dischargeanddischarge discharge operationoperation operation ofof thethe of Shihmen theShihmen Shihmen Reservoir,Reservoir, Reservoir, thethe the flowflow flow atat atthethe the GuanduGuandu Guandu BridgeBridge Bridge reachedreached reached 57865786 5786 mm 3/s,/s,/s, togethertogether with with the the sediment sediment flux fluxflux of of 33,443 33,443 kg/s kgkg/s/s (Figure (Figure 88). 8).). At At this this moment moment,moment, ,the thethe estuarine estuarineestuarine sediment sediment flux fluxflux waswas dominated dominateddominated byby thethe river-transportedriverriver--transportedtransportedsediment sedimentsediment (Figure (Figure(Figure 11 11b11bb).).). The TheThe sediment sedimentsediment was waswas then thenthen transported transportedtransported by bytheby thethe upstream upstreamupstream river riverriver flood floodflood to thetoto thethe estuary, estuary,estuary, reaching reachingreaching as far asas far asfar the asas the otheffshore offshoreoffshore around aroundaround the theareathe areaarea with withwith the bedthethe bedelevationbed elevationelevation of of25of −25−25 m. m. Them. TheThe high highhigh sediment sedimentsediment load loadload was waswas mainly mainlymainly concentrated concentratedconcentrated on onon the thethe river riverriver channel channelchannel close closeclose to − totheto thethe right rightright bank. bank.bank. At AtAt this thisthis flood floodflood peak, peak,peak, the thethe computed computedcomputed unit-width unitunit--widthwidth sediment sedimentsediment transport transporttransport flux fluxflux can cancan reach reachreach up upup to −3 3 to2.1to 2.1×10 2.1×1010 3 −m3 m m3/3/ss/s/m,/m/m,or ,or or5.57 5.5 5.577 kg kg/s kg/s/s/m./m./m. × − AsAs shownshown inin FigFigureure 1122, , thethe bedbed changeschanges werewere betweenbetween −−0.360.36 andand 0.480.48 m,m, andand thethe maximummaximum scouringscouring depth depth of of − −0.80.8 m m was was present present at at a a site site 6500 6500 m m far far a awayway from from the the Guandu Guandu Bridge. Bridge. Aggradation Aggradation occurredoccurred atat aa distancedistance ofof 68006800 mm toto thethe GuanduGuandu Bridge,Bridge, withwith thethe aggradationaggradation heightheight reachingreaching 1.41.4 m.m. DueDue toto thethe floodflood drainagedrainage ofof thethe ShihmenShihmen ReservoirReservoir andand thethe flowflow pathpath characteristics,characteristics, thethe channelchannel ofof thethe river river section section 2500−4000 2500−4000 m m away away from from the the Guandu Guandu Bridge Bridge was was scoured scoured by by 0.25 0.25 m m on on average, average, while while thethe rightright bankbank (the(the TaTamsuimsui mangrovemangrove reserve)reserve) waswas subjectsubject toto aggradation.aggradation. TheThe rightright bankbank ofof thethe estuaryestuary (5600−5800 (5600−5800 m m away away from from the the Guandu Guandu Bridge) Bridge) was was also also subjected subjected to to scouring, scouring, with with an an average average scouringscouring depth depth of of 0.12 0.12 m. m. Scouring Scouring and and aggradation aggradation alternated alternated in in the the region region from from the the left left bank bank of of the the

Water 2020, 3, x FOR PEER REVIEW 15 of 20 estuaryWater 2020 to, 12 the, 1084 north breakwater of the Taipei Port, with parts of the sediment transported by15 ofthe 20 backflow to the sand spit of the Wazihwei area.

Water 2020, 3, x FOR PEER REVIEW 15 of 20

estuary to the north breakwater of the Taipei Port, with parts of the sediment transported by the backflow to the sand spit of the Wazihwei area.

(a) 12 July 2013 21:00 (b) 13 July 2013 12:00

Figure 11. ComparisonComparison of of sediment sediment flux ﬂux ( (qqs)) at at different diﬀerent times times..

As shown in Figure 12, the bed changes were between 0.36 and 0.48 m, and the maximum − scouring depth of 0.8 m was present at a site 6500 m far away from the Guandu Bridge. Aggradation − occurred at a distance of 6800 m to the Guandu Bridge, with the aggradation height reaching 1.4 m. Due to the flood drainage of the Shihmen Reservoir and the flow path characteristics, the channel of the river section 2500 4000 m away from the Guandu Bridge was scoured by 0.25 m on average, − while the right bank (the Tamsui mangrove reserve) was subject to aggradation. The right bank of the estuary (5600 5800 m away from the Guandu Bridge) was also subjected to scouring, with an average − scouring depth of 0.12 m. Scouring and aggradation alternated in the region from the left bank of (a) 12 July 2013 21:00 (b) 13 July 2013 12:00 the estuary to the north breakwater of the Taipei Port, with parts of the sediment transported by the backflow to the sand spitFigure of the11. WazihweiComparison area. of sediment flux (qs) at different times.

(a) Bed scouring and aggradation in the (b) Bed scouring and aggradation in the range estuary A-A'

Figure 12. Simulated bed changes caused by Typhoon Soulik in the current situation. (DZ = bed change).

However, after the construction of the Amuping desilting tunnel, the sediment delivery ratio will be increased by 20.2% compared to the current situation. The sediment amount transported downstream will not be significantly increased if the flow is maintained unchanged. The river channel was divided into three zones for comparison. The results showed that the amount of 3 3 sediment deposition would, respectively, increase around 2627 m and 170 m in ZONE I and ZONE II, whereas(a) Bed that scouring would anddecrease aggradation 240 m3 in theZONE III(,b compared) Bed scouring to the and current aggradation situation in as the indicated range in Figure 13. estuary A-A' The change in the sediment volume is minor with the operation of the Amuping desilting tunnel. However,Figure the 12.12. operationSimulatedSimulated bed ofbed changesthe changes tunnel caused caused leads by Typhoontoby theTyphoon Reservoir Soulik Soulik in the Hydraulic in current the current situation. Slag situation.Removal (DZ = bed (DZ volume change). = bed from 1.13 ×change) 106 m.3 to 1.78 × 106 m3. It implies that the consideration of the Amuping desilting tunnel is helpfulHowever, for alleviating after the the construction Shihmen Reservoir of the Amuping siltation desiltingwith a minimal tunnel, disturbance the sediment of deliverythe estuarine ratio environment.will beHowever, increased after by the 20.2% construction compared of to the the Amuping current situation. desilting tunnel, The sediment the sediment amount delivery transported ratio downstreamwill be increased will not by be20.2 signiﬁcantly% compared increased to the ifcurrent the ﬂow situation. is maintained The sediment unchanged. amount The river transported channel wasdownstream divided intowill threenot be zones significantly for comparison. increased Theif the results flow showedis maintained that the unchanged. amount of The sediment river depositionchannel was would, divided respectively, into three increase zones around for comparison. 2627 m3 and The 170 results m3 in ZONEshowed I and that ZONE the amount II, whereas of thatsediment would deposition decrease 240would m3,in respectively ZONE III,, compared increase around to the current2627 m3 situation and 170 m as3 indicatedin ZONE inI and Figure ZONE 13. II, whereas that would decrease 240 m3 in ZONE III, compared to the current situation as indicated in Figure 13. The change in the sediment volume is minor with the operation of the Amuping desilting tunnel. However, the operation of the tunnel leads to the Reservoir Hydraulic Slag Removal volume from 1.13 × 106 m3 to 1.78 × 106 m3. It implies that the consideration of the Amuping desilting tunnel is helpful for alleviating the Shihmen Reservoir siltation with a minimal disturbance of the estuarine environment.

Water 2020, 3, x FOR PEER REVIEW 16 of 20 Water 2020, 12, 1084 16 of 20

FigureFigure 1 13.3. SimulatedSimulated bed bed changes changes caused caused by by Typhoon Typhoon Soulik Soulik when when the the Am Amupinguping desilting desilting tunnel tunnel is is inin operation. operation. The change in the sediment volume is minor with the operation of the Amuping desilting In summary, aggradation during the Typhoon-induced flood period occurred mainly at sites tunnel. However, the operation of the tunnel leads to the Reservoir Hydraulic Slag Removal volume 4000–5300 m away from the Guandu Bridge (Figure 12b) and in the outside sea area of the second from 1.13 106 m3 to 1.78 106 m3. It implies that the consideration of the Amuping desilting fishing port× of Tamsui; scouring× occurred mainly at sites 2500–4000 m and 5600–5800 m away from tunnel is helpful for alleviating the Shihmen Reservoir siltation with a minimal disturbance of the the Guandu Bridge, and in particular the Tamsui Ferryboat Wharf was in the latter range. estuarine environment. 5.2 MorphologicalIn summary, Changes aggradation during the during Flood the and Typhoon-induced Monsoon Periods flood period occurred mainly at sites 4000–5300 m away from the Guandu Bridge (Figure 12b) and in the outside sea area of the second fishingChanges port of in Tamsui; estuarine scouring scouring occurred and aggradation mainly at sites are 2500–4000 closely related m and 5600–5800to the sediment m away transport from the behaviorGuandu of Bridge, the adjacent and in particularsea area. The the currents Tamsui Ferryboatgenerated Wharfby waves was and in the tides latter provide range. a driving force for coastal sediment transport. The littoral drift at the mouth of the Tamsui River is affected by the southwest5.2. Morphological monsoon Changes in summer, during and the Floodthe longshore and Monsoon currents Periods move northward with the littoral drift; in winter, the region is under the influence of the northeast monsoon, and the longshore currents Changes in estuarine scouring and aggradation are closely related to the sediment transport move southward with the littoral drift. When typhoon rainstorms arrive, the upstream Shihmen behavior of the adjacent sea area. The currents generated by waves and tides provide a driving force Reservoir has to perform flood discharge operations for flood control and aggradation prevention. for coastal sediment transport. The littoral drift at the mouth of the Tamsui River is affected by the Therefore, there is a large amount of rainfall and reservoir-discharged floodwater in the upper southwest monsoon in summer, and the longshore currents move northward with the littoral drift; in reaches, and the energy generated by the upstream flood discharge is much larger than the ocean winter, the region is under the influence of the northeast monsoon, and the longshore currents move current energy, making the upstream flow the main factor responsible for morphological changes southward with the littoral drift. When typhoon rainstorms arrive, the upstream Shihmen Reservoir and water level rise in the Tamsui River estuary. has to perform flood discharge operations for flood control and aggradation prevention. Therefore, To understand the difference between the monsoon period and the Typhoon-induced flood there is a large amount of rainfall and reservoir-discharged floodwater in the upper reaches, and period in causing morphological changes in the Tamsui River estuary, this study simulated the bed the energy generated by the upstream flood discharge is much larger than the ocean current energy, changes with the scenarios of the monsoon period from 1 December 2009, to 28 February 2010, and making the upstream flow the main factor responsible for morphological changes and water level rise Typhoon Aere in 2004. The former scenario caused the largest water level in the Tamsui River Estuary in the Tamsui River estuary. tide gauge while the latter one led to the largest recorded flood event in the study region. Thus, such To understand the difference between the monsoon period and the Typhoon-induced flood period consideration would be helpful to explore the difference between the monsoon period and the in causing morphological changes in the Tamsui River estuary, this study simulated the bed changes Typhoon-induced flood period in causing morphological changes in the Tamsui River estuary. This with the scenarios of the monsoon period from 1 December 2009, to 28 February 2010, and Typhoon typhoon caused the largest recorded flood event in the study region, with the maximum discharge Aere in 2004. The former scenario caused the largest water level in the Tamsui River Estuary tide gauge rate of Shihmen Reservoir reaching 8366 m3/s, between Q50 and Q100. Notice that Q50 and Q100 denote while the latter one led to the largest recorded flood event in the study region. Thus, such consideration the 50-year and 100-year return periods of discharge, respectively. Additionally, the values of Q50 and would be helpful to explore the difference between the monsoon period and the Typhoon-induced Q100 at the cross-section No. T075 were respectively 7800 m3/s and 8400 m3/s. The cross-section No. flood period in causing morphological changes in the Tamsui River estuary. This typhoon caused T075 is located about 10.6 km downstream of the Shihmen Reservoir. As shown in Figure 14a, during the largest recorded flood event in the study region, with the maximum discharge rate of Shihmen the monsoon period, the upstream3 sediment source was reduced in strength owing to the small river Reservoir reaching 8366 m /s, between Q50 and Q100. Notice that Q50 and Q100 denote the 50-year flow, and thus, the morphological changes in the estuary were mainly determined by littoral drift. and 100-year return periods of discharge, respectively. Additionally, the values of Q and Q at Under the influence of the northeast monsoon, littoral drift would form a northeast50–southwest100 - the cross-section No. T075 were respectively 7800 m3/s and 8400 m3/s. The cross-section No. T075 is oriented rod-shaped aggradation pattern to the north of the second fishing port, and the sediment

Water 2020, 12, 1084 17 of 20 located about 10.6 km downstream of the Shihmen Reservoir. As shown in Figure 14a, during the monsoon period, the upstream sediment source was reduced in strength owing to the small river flow, and thus, the morphological changes in the estuary were mainly determined by littoral drift. Under Waterthe influence 2020, 3, x FOR of PEER the northeast REVIEW monsoon, littoral drift would form a northeast–southwest-oriented17 of 20 rod-shaped aggradation pattern to the north of the second fishing port, and the sediment will be will be gradually transported into the second fishing port owing to the impact of the tide. When the gradually transported into the second fishing port owing to the impact of the tide. When the currents currents flowed to the north breakwater of the Taipei Port, backflow was generated due to the flowed to the north breakwater of the Taipei Port, backflow was generated due to the breakwater breakwater obstruction, transporting the sediment of the Wazihwei sand spit to the left bank of the obstruction, transporting the sediment of the Wazihwei sand spit to the left bank of the estuary. estuary.

(a) End of the monsoon period (b) End of the typhoon period

FigureFigure 14. Diagram 14. Diagram of morphological of morphological change change in the in Tamsui the Tamsui River River estuary. estuary. (DZ (DZ = bed= bedchange) change).

During the Typhoon Typhoon-induced-induced flood flood period, owing to the rainfall and the flood flood discharge of the Shihmen Reservoir, the sediment was transported by the flood.flood. Concave banks were dominant in the river channel 2500 2500–4000–4000 m away downstream from the Guandu Bridge and the the flow flow was fast in this section, which was subject subjecteded to to scouring; the rive riverr channel 6000 6000–7000–7000 m away from the bridge was also subjectedsubjected toto scouring,scouring, mainlymainly owingowing to to the the bed bed type. type. However, However, the the riverine riverine section section within within 2500 2500 m mdownstream downstream of of the the Guandu Guandu Bridge Bridge and and the the estuarine estuarine section section within within 25002500 mm seawardseaward from the river mouth (7500–10,000(7500–10,000 mm downstreamdownstream of of the the Guandu Guandu Bridge) Bridge) were were mainly mainly subjected subject toed aggradation. to aggradation. The Thesediment sediment in the in outside the outside sea area sea was area a ffwasected affected by the seaby currents,the sea currents, and parts and of theparts sediment of the sediment spread to spreadthe Wazihwei to the Wazihwei sand spit owingsand spit to longshore owing to currentslongshore (Figure currents 14b). (Figure A comparison 14b). A comparison of the simulation of the simulationresults under results the twounder scenarios the two of scenarios Typhoon of Soulik Typhoon and Soulik Typhoon and Aere Typhoon revealed Aere that revealed the growth that the of growththe Wazihwei of the Wazihwei sand spit wassand a spitffected was by affected the amount by the of amount flood dischargeof flood discharge during the during typhoon-induced the typhoon- inducedflood period, flood while period, the while aggradation the aggradation in the second in the fishing second port fishing was mainly port was attributed mainly to attributed the littoral to drift the littoralduring drift the monsoon during the period. monsoon period.

66.. Conclusions Conclusions TToo evaluate the trend of riverbed change by numerical modeling, the relevant relevant model parameters were first first validated using the 2008 events of Typhoon Typhoonss Fung-Wong,Fung-Wong, Sinlaku,Sinlaku, and Jangmi, allowing the CCHE2D CCHE2D-Coast-Coast model model to to effectively effectively repro reproduceduce the complex flow flow field fieldss and sediment transport processprocesseses at the Tamsui River estuary, such as flooding,flooding, erosion erosion,, and aggradation along along the the coast, coast, along with scouringscouring ofof thethe estuarine estuarine sandbar. sandbar. Next, Next, this this model model was was used used to to explore explore the the eff ectseffects that that the theoperation operation of the of Amupingthe Amuping desilting desilting tunnel tunnel of Shihmen of Shihmen Reservoir Reservoir would would have have on the on downstream the downstream river riverchannel, channel, and to and compare to compare the sediment the sediment transport transport characteristics characteristics in the Tamsuiin the Tamsui River estuary River estuary during duringtyphoon-induced typhoon-induced flood and flood a period and a withperiod the with monsoon. the monsoon. The res resultsults showed that for sedimentsediment transport d duringuring the typhoon-inducedtyphoon-induced flood flood period, the upstream flood flood transported a a large amount of of sediment to to be be deposited deposited in in the the estuary, estuary, and parts of sediment were transported by currents to the Wazihwei sand spit where they were deposited. Under the condition that the flow is kept unchanged and the sediment concentration is increased (after the construction of the Amuping desilting tunnel), the sediment transport capacity of the river water will be reduced, which in turn would increase the degree of aggradation in the river channel while slowing down of aggradation in the estuary. During the monsoon period, sedimentation was mainly determined by ocean currents, and the transport was also subject to coastal topo-bathymetry, coastal structures, and tidal currents. The sediment is deposited in the outside area of the second fishing port

Water 2020, 12, 1084 18 of 20 sediment were transported by currents to the Wazihwei sand spit where they were deposited. Under the condition that the flow is kept unchanged and the sediment concentration is increased (after the construction of the Amuping desilting tunnel), the sediment transport capacity of the river water will be reduced, which in turn would increase the degree of aggradation in the river channel while slowing down of aggradation in the estuary. During the monsoon period, sedimentation was mainly determined by ocean currents, and the transport was also subject to coastal topo-bathymetry, coastal structures, and tidal currents. The sediment is deposited in the outside area of the second fishing port of Tamsui, the north breakwater of the Taipei Port, and in the estuarine areas. The Wazihwei sand spit would be subject to aggradation due to backflow. As to whether these processes would further lead to long-term morphological changes in the Wazihwei sand spit, it is necessary to address the issue by incorporating numerical modeling with topo-bathymetric data and satellite imagery in the future. Meanwhile, the Wazihwei area is in a nature reserve, and the effectiveness of conducting sediment management to maintain the sandbar area also can be evaluated by numerical modeling. This numerical simulation study reproduced the changes in hydrodynamics and morphodyanmics of the Tamsui River estuary and will facilitate the management of sediment transport in the Tamsui River Estuary driven by multiple coastal and estuarine processes, such as waves, tidal currents, and river flows.

Author Contributions: T.-C.H. collected the data, performed the statistical and simulating analysis, and drafted and revised the manuscript. Y.D. conceived of the study, performed model validation and simulations of design scenarios, participated in its design and coordination and drafted and revised the manuscript. K.-C.Y. participated in the design of the study and coordination, provided methods. R.-K.J. participated in the design of the study and performed river simulations using CCHE1D. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: Parts of the contents in this study are derived from the outcomes of a commissioned project of the Tenth River Management Office of the Water Resources Agency, Ministry of Economic Affairs. The authors would like to thank the anonymous reviewers and editor for their constructive comments which profoundly improve the manuscript. Conflicts of Interest: The authors declare no conflict of interest

References

1. Shimizu, T.; Kumagai, T.; Watanabe, A. Improved 3-D Beach Evolution Model Coupled with the Shoreline Model (3D-SHORE). Coast. Eng. 1997, 3, 2843–2856. 2. Reniers, A.J.; Roelvink, D.; Thornton, E.B. Morphodynamic modeling of an embayed beach under wave group forcing. J. Geophys. Res. Space Phys. 2004, 109.[CrossRef] 3. Ding, Y.; Wang, S.S.Y. Development and Application of a Coastal and Estuarine Morphological Process Modeling System. J. Coast. Res. 2008, 10052, 127–140. [CrossRef] 4. Roelvink, D. Coastal morphodynamic evolution techniques. Coast. Eng. 2006, 53, 277–287. [CrossRef] 5. Ding, Y.; Kuiry, S.; Elgohry, M.; Jia, Y.; Altinakar, M.S.; Yeh, K.-C. Impact assessment of sea-level rise and hazardous storms on coasts and estuaries using integrated processes model. Ocean Eng. 2013, 71, 74–95. [CrossRef] 6. Tenth River Management Office. Impact Assessment of Sediment Transport at Tansui Estuary on surrounding coast (1/2); The Tenth River Management Office, Water Resources Agency: New Taipei City, Taiwan, 2014. (In Chinese) 7. Northern Region Water Resources Office. Available online: https://www.wranb.gov.tw/public/Attachment/ 652011311785.pdf (accessed on 15 October 2019). 8. Northern Region Water Resources Office. Feasibility Study of Amouping Desilting Tunnel Project for Shihmen Reservoir Summary Report; The Northern Region Water Resources Office, Water Resources Agency: Taoyuan, Taiwan, 2015. (In Chinese) 9. Bakker, W.T.; Doorn, T.V. Near-Bottom Velocities in Waves with a Current. Coast. Eng. 1978, 1394–1413. 10. Kemp, P.H.; Simons, R. The interaction between waves and a turbulent current: Waves propagating with the current. J. Fluid Mech. 1982, 116, 227. [CrossRef] Water 2020, 12, 1084 19 of 20

11. Kemp, P.H.; Simons, R. The interaction of waves and a turbulent current: Waves propagating against the current. J. Fluid Mech. 1983, 130, 73. [CrossRef] 12. Visser, P.J. Wave basin experiments on bottom friction due to current and waves. Coast. Eng. Proc. 1986, 1, 61. [CrossRef] 13. Asano, T.; Nakagawa, M.; Iwagaki, Y. Changes in current properties due to wave superimposing. Coast. Eng. Proc. 1986, 1, 70. [CrossRef] 14. Simons, R.R.; Kyriacou, A.G.; Soulsby, R.L.; Davies, A.G. Predicting the nearbed turbulent flow in waves and currents. In Proceedings of the IAHR Symposium on Mathematical Modelling of Sediment Transport in the Coastal Zone, Copenhagen, Denmark, 1988; pp. 33–47. Available online: https://discovery.ucl.ac.uk/id/ eprint/1520924/ (accessed on 4 April 2020). 15. Sleath, J.F.A. Velocities and Bed Friction in Combined Flows. Coast. Eng. 1991, 450–463. 16. Arnskov, M.; Fredsøe, J.; Sumer, B. Bed shear stress measurements over a smooth bed in three-dimensional wave-current motion. Coast. Eng. 1993, 20, 277–316. [CrossRef] 17. Lundgren, H. Turbulent Currents in the Presence of Waves. Coast. Eng. 1972, 623–634. 18. Smith, J.D. Modeling of Sediment Transport on Continental Shelves; The Sea 6; Wiley Interscience: New York, NY, USA, 1977; pp. 539–577. 19. Grant, W.D.; Madsen, O.S. Combined wave and current interaction with a rough bottom. J. Geophys. Res. Space Phys. 1979, 84, 1797–1808. [CrossRef] 20. Fredsøe, J. Turbulent Boundary Layer in Wave?current Motion. J. Hydraul. Eng. 1984, 110, 1103–1120. [CrossRef] 21. Christoffersen, J.B.; Jonsson, I.G. Bed friction and dissipation in a combined current and wave motion. Ocean Eng. 1985, 12, 387–423. [CrossRef] 22. Coffey, F.C.; Nielsen, P. The Influence of Waves on Current Profiles. Coast. Eng. 1987, 82–96. [CrossRef] 23. Myrhaug, D.; Slaattelid, O.H. Combined wave and current boundary layer model for fixed rough seabeds. Ocean Eng. 1989, 16, 119–142. [CrossRef] 24. Chen, Q.; Madsen, P.A.; Schäffer, H.A.; Basco, D.R. Wave-current interaction based on an enhanced Boussinesq approach. Coast. Eng. 1998, 33, 11–39. [CrossRef] 25. Nwogu, O.G.; Demirbilek, Z. BOUSS-2D: A Boussinesq Wave Model for Coastal Regions and Harbors. In BOUSS-2D: A Boussinesq Wave Model for Coastal Regions and Harbors; Defense Technical Information Center (DTIC), 2001; Available online: https://erdc-library.erdc.dren.mil/jspui/handle/11681/7555 (accessed on 4 April 2020). 26. Booij, N.; Ris, R.C.; Holthuijsen, L. A third-generation wave model for coastal regions: 1. Model description and validation. J. Geophys. Res. Space Phys. 1999, 104, 7649–7666. [CrossRef] 27. Yan, D.; Sam, S.Y.W. Modeling of wave-current interaction using a multidirectional wave-action balance equation. Coast. Eng. Proc. 2011, 1.[CrossRef] 28. Lodahl, C.R.; Sumer, B.; Fredsøe, J. Turbulent combined oscillatory flow and current in a pipe. J. Fluid Mech. 1998, 373, 313–348. [CrossRef] 29. Simons, R.; Grass, T.J.; Mansour-Tehrani, M. Bottom Shear Stresses in the Boundary Layers under Waves and Currents Crossing at Right Angles. Coast. Eng. 1993, 604–617. [CrossRef] 30. Simons, R.; Grass, T.J.; Saleh, W.M.; Tehrani, M.M. Bottom Shear Stresses under Random Waves with a Current Superimposed. Coast. Eng. 1995, 565–578. [CrossRef] 31. Van Rijn, L.C. Sedimentation of Dredged Channels by Currents and Waves. J. Waterw. Port Coast. Ocean Eng. 1986, 112, 541–559. [CrossRef] 32. Roos, P.C.; Hulscher, S.J.M.H.; De Vriend, H.J. Modelling the morphodynamic impact of offshore sandpit geometries. Coast. Eng. 2008, 55, 704–715. [CrossRef] 33. Benedet, L.; List, J.H. Evaluation of the physical process controlling beach changes adjacent to nearshore dredge pits. Coast. Eng. 2008, 55, 1224–1236. [CrossRef] 34. Faraci, C.; Foti, E.; Marini, A.; Scandura, P. Waves Plus Currents Crossing at a Right Angle: Sandpit Case. J. Waterw. Port Coast. Ocean Eng. 2012, 138, 339–361. [CrossRef] 35. Blumberg, A.F.; Mellor, G.L. A description of a three-dimensional coastal ocean circulation model. In Oceanography of the Southeastern U.S. Continental Shelf ; American Geophysical Union (AGU), 1987; Volume 4, pp. 1–16. Available online: https://doi.org/10.1029/CO004p0001 (accessed on 4 April 2020). Water 2020, 12, 1084 20 of 20

36. Jelesnianski, C.; Chen, J.; Shaffer, W. SLOSH: Sea, Lake, and Overland Surges from Hurricanes; NOAA Tech Report; 1992; Available online: https://www.hsdl.org/?abstract&did=779560 (accessed on 4 April 2020). 37. SLOSH. 2008. Available online: http://www.nhc.noaa.gov/HAW2/english/surge/slosh.shtml (accessed on 16 January 2008). 38. Svendsen, I.A.; Haas, K.; Zhao, Q. Quasi-3D Nearshore Circulation Model SHORECIRC—Version 2.0; Technical Report; Center for Applied Coastal Research, University of Delaware: Newark, NJ, USA, 2003. 39. A Svendsen, I.; Qin, W.; A Ebersole, B. Modelling waves and currents at the LSTF and other laboratory facilities. Coast. Eng. 2003, 50, 19–45. [CrossRef] 40. Zyserman, J.A.; Johnson, H.K. Modelling morphological processes in the vicinity of shore-parallel breakwaters. Coast. Eng. 2002, 45, 261–284. [CrossRef] 41. Tuan, T.Q.; Stive, M.J.F.; Verhagen, H.J.; Visser, P.J. Process-based modeling of the overflow-induced growth of erosional channels. Coast. Eng. 2008, 55, 468–483. [CrossRef] 42. Kuiry, S.; Ding, Y.; Wang, S.S.Y. Modelling coastal barrier breaching flows with well-balanced shock-capturing technique. Comput. Fluids 2010, 39, 2051–2068. [CrossRef] 43. Faraci, C.; Scandura, P.; Foti, E. Bottom Profile Evolution of a Perched Nourished Beach. J. Waterw. Port Coast. Ocean Eng. 2014, 140, 04014021. [CrossRef] 44. Ding, Y.; Zhang, Y.-X.; Jia, Y.-F. CCHE2D-Coast: Model Description and Graphical User Interface; NCCHE Technical Report; The University of Mississippi: Oxford, MS, USA, 2016; 88p. 45. Ding, Y.; Yeh, K.-C.; Wei, S.-T. Integrated coastal process modeling and impact assessment of floating and sedimentation in coasts and estuaries. Coast. Eng. Proc. 2017, 1, 18. [CrossRef] 46. Zhang, Y.X. CCHE-GUI—Graphical Users Interface for NCCHE Models User’s Manual—Version 4.x; Technical Report No. NCCHE-TR-2013-01; 2013; Available online: https://www.researchgate.net/publication/ 283153483_CCHE2D-GUI-Graphical_User_Interface_for_the_CCHE2D_Model_User\T1\textquoterights_ Manual-Version_22 (accessed on 4 April 2020). 47. Harbor and Marine Technology Center. Available online: https://www.ihmt.gov.tw/ (accessed on 15 October 2019). 48. Water Resources Planning Institute. Hydraulic Model Studies for Sediment Sluicing and Flood Diversion Engineering of Shihman Reservoir; Water Resources Planning Institute, Water Resources Agency: Taichung, Taiwan, 2012. (In Chinese) 49. Water Resources Planning Institute. Sediment Transport Monitoring Facility Maintenance and Flood Event Observations in Shihmen Reservoir; Water Resources Planning Institute, Water Resources Agency: Taichung, Taiwan, 2016. (In Chinese)

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).