High-resolution X-Band radar measurements of currents,bathymetry and sea state in highly inhomogeneous coastal areas

In this paper, high-resolution wave, current and water depth fields derived by marine X-Band radar are presented for a coastal region of extreme tidal currents in the presence of inhomogeneous bathymetry at the south coast of New Zealand’s North Island. The current and water depth information for the presented location covers an area of approximately 13 km² with a spatial resolution of 225 m and an update rate of 3 min. The sea state data provides a spatial representation of coastal effects like wave shoaling and refraction forced by bathymetry and current interaction. The near-surface current measurements about 3 km off the coast show expected tidal current pattern with maximum northwest/southeast current of 1.5–2 m/s alongshore. This is in agreement with currents from the RiCOM hydrodynamic model. The spatial resolution of the observed current field exhibits in addition small-scale current features caused by the influence of the local bathymetry. These data demonstrate the insight to be gained in complex, high-energy coastal situations through the use of high-resolution remote sensing techniques.     

The tidal and wind induced hydrodynamics of the composite system Adriatic Sea/Lagoon of Venice

A shallow water hydrostatic 2D hydrodynamic numerical model, based on the boundary conforming coordinate system, was used to simulate aspects of both general and small scale oceanic features occurring in the composite system constituted by the Adriatic Sea and the Lagoon of Venice (Italy), under the influence of tide and realistic atmospheric forcing. Due to a specific technique for the treatment of movable lateral boundaries, the model is able to simulate efficiently dry up and flooding processes within the lagoon. Firstly, a model calibration was performed by comparing the results of the model, forced using tides and ECMWF atmospheric pressure and wind fields, with observations collected for a set of 33 mareographic stations uniformly distributed in the Adriatic Sea and in the Lagoon of Venice. A second numerical experiment was then carried out by considering only the tidal forcing. Through a comparison between the results obtained in the two experiments it was possible to assess the reliability of the estimated parameter through the composite forcing. Model results were then verified by comparing simulated amplitude and phase of each tidal constituent as well as tidal velocities simulated at the inlets of the lagoon and in the Northern Adriatic Sea with the corresponding observed values. The model accurately reproduces the observed harmonics: mean amplitude differences rarely exceed 1 cm, while phase errors are commonly confined below 15°. Semidiurnal and diurnal currents were correctly reproduced in the northern basin and a good agreement was obtained with measurements carried out at the lagoon inlets. On this basis, the outcomes of the hydrodynamic model were analyzed in order to investigate: (i) small-scale coastal circulation features observed at the interface between the adjoining basins, which consist often of vortical dipoles connected with the tidal flow of Adriatic water entering and leaving the Lagoon of Venice and with along-shore current fields connected with specific wind patterns; (ii) residual oscillations, which are often connected to meteorological forcing over the basin. In particular, it emerges that small-scale vortical features generated near the lagoon inlet can be efficiently transported toward the open sea, thus contributing to the water exchange between the two marine regions, and a realistic representation of observed residual oscillations in the area would require a very detailed knowledge of atmospheric as well as remote oceanic forcing.     

Tidal residual eddies and their effect on water exchange in Puget Sound

Tidal residual eddies are one of the important hydrodynamic features in tidally dominant estuaries and coastal bays, and they could have significant effects on water exchange in a tidal system. This paper presents a modeling study of tides and tidal residual eddies in Puget Sound, a tidally dominant fjord-like estuary in the Pacific Northwest coast, using a three-dimensional finite-volume coastal ocean model. Mechanisms of vorticity generation and asymmetric distribution patterns around an island/headland were analyzed using the dynamic vorticity transfer approach and numerical experiments. Model results of Puget Sound show that a number of large twin tidal residual eddies exist in the Admiralty Inlet because of the presence of major headlands in the inlet. Simulated residual vorticities near the major headlands indicate that the clockwise tidal residual eddy (negative vorticity) is generally stronger than the anticlockwise eddy (positive vorticity) because of the effect of Coriolis force. The effect of tidal residual eddies on water exchange in Puget Sound and its subbasins was evaluated by simulations of dye transport. It was found that the strong transverse variability of residual currents in the Admiralty Inlet results in a dominant seaward transport along the eastern shore and a dominant landward transport along the western shore of the inlet. A similar transport pattern in Hood Canal is caused by the presence of tidal residual eddies near the entrance of the canal. Model results show that tidal residual currents in Whidbey Basin are small in comparison to other subbasins. A large clockwise residual circulation is formed around Vashon Island near entrance of South Sound, which can potentially constrain the water exchange between the Central Basin and South Sound.     

Tidal dynamics of the Terminos Lagoon,Mexico: observations and 3D numerical modelling

The tidal circulation patterns in the Terminos Lagoon were studied based on the analysis of 1 year of measurements and numerical simulations using a baroclinic 3D hydrodynamic model, the MARS3D. A gauging network was installed consisting of six self-recording pressure–temperature sensors, a tide gauge station and two current profilers, with pressure and temperature sensors moored in the main lagoon inlets. Model simulations were validated against current and sea level observations and were used to analyse the circulation patterns caused by the tidal forcing. The numerical model was forced with eight harmonic components, four diurnal (K ₁, O ₁, P ₁, Q ₁) and four semi-diurnal (M ₂, S ₂, N ₂, K ₂), extracted from the TPX0.7 database. The tidal patterns in the study area vary from mixed, mainly diurnal in the two main inlets of the lagoon, to diurnal in its interior. The tidal residual circulation inside the lagoon is dominated by a cyclonic gyre. The results indicate a net flux from the southwest Ciudad del Carmen inlet (CdC) towards the northeast Puerto Real inlet (PtR) along the southern side of the lagoon and the opposite in the northern side. The results indicate two areas of strong currents in the vicinity of the inlets and weak currents inside the lagoon. The area of strong currents in the vicinity of the CdC inlet is larger than that observed in the PtR inlet. Nevertheless, the current analysis indicates that the highest current speeds, which can reach a magnitude of 1.9 m s^?1, occurred in PtR. A further analysis of the tide distortion in the inlets revealed that both passages are ebb dominated.     

Tidal and low-frequency currents off the Jaguaribe River estuary (4° S, 37° 4′ W), northeastern Brazil

This article characterizes the spatial and temporal current variations, in the subtidal and tidal ranges, during the rainy and dry seasons, at the continental shelf off the Jaguaribe River, through measurements of continuous current field data from an acoustic Doppler current profiler (ADCP) mooring during 124 days, from June 12 to October 14, 2009. To support this dataset, we collected corresponding data from a meteorological station located at the estuary. The spatial variation showed that highest current speeds occur near the coast, with an offset of a NNW coastal jet, decreasing intensity, monotonically, towards offshore up to 0.1 ms^?1. In the rainy season, small inversions of the wind field were observed, lasting 2 to 3 days on average and were accompanied by the direction of surface currents only. In the dry season, the period of reversal of wind fields and currents lasted 14 and 35 h, respectively. The analysis of empirical orthogonal functions in rainy and dry seasons showed that the continental shelf is predominantly barotropic, where the second and third modes explained only 7% of the total variance, during the dry season. The tidal currents are more intense in the direction normal to the coast, showing a semidiurnal tidal regime. Energy distribution between tidal currents and currents of longer periods showed that for the component parallel to bathymetry, subtidal frequency currents are dominant, contributing to more than 70% of the variance. For the normal component to the coastline bathymetry, there is a significant increase of power concerning tidal currents, at all depths, so they contribute with about 55% of the total variance.     

Comparison of artificial neural network models for hydrologic predictions at multiple gauging stations in an agricultural watershed

This paper reports on an evaluation of the use of artificial neural network (ANN) models to forecast daily flows at multiple gauging stations in Eucha Watershed, an agricultural watershed located in north‐west Arkansas and north‐east Oklahoma. Two different neural network models, the multilayer perceptron (MLP) and the radial basis neural network (RBFNN), were developed and their abilities to predict stream flow at four gauging stations were compared. Different scenarios using various combinations of data sets such as rainfall and stream flow at various lags were developed and compared for their ability to make flow predictions at four gauging stations. The input vector selection for both models involved quantification of the statistical properties such as cross‐, auto‐ and partial autocorrelation of the data series that best represented the hydrologic response of the watershed. Measured data with 739 patterns of input–output vector were divided into two sets: 492 patterns for training, and the remaining 247 patterns for testing. The best performance based on the RMSE, R² and CE was achieved by the MLP model with current and antecedent precipitation and antecedent flow as model inputs. The MLP model testing resulted in R² values of 0·86, 0·86, 0·81, and 0·79 at the four gauging stations. Similarly, the testing R² values for the RBFNN model were 0·60, 0·57, 0·58, and 0·56 for the four gauging stations. Both models performed satisfactorily for flow predictions at multiple gauging stations, however, the MLP model outperformed the RBFNN model. The training time was in the range 1–2 min for MLP, and 5–10 s for RBFNN on a Pentium IV processor running at 2·8 GHz with 1 MB of RAM. The difference in model training time occurred because of the clustering methods used in the RBFNN model. The RBFNN uses a fuzzy min‐max network to perform the clustering to construct the neural network which takes considerably less time than the MLP model. Results show that ANN models are useful tools for forecasting the hydrologic response at multiple points of interest in agricultural watersheds. Copyright © 2008 John Wiley & Sons, Ltd.     

Tidal and wind-induced circulation within the Southeastern limit of the Bay of Biscay: Pasaia Bay,Basque Coast

The Basque coastal area, in the southeastern Bay of Biscay, can be characterised as being more influenced by land climate and inputs, than other typically ‘open sea’ areas. The influence of coastal processes, together with the presence of irregular and steep topography, complicate greatly the water circulation patterns. Water movement along the Basque coastal area is not well understood; observations are scarce and long-term current records are lacking. The knowledge available is confined to the surface currents: the surface water circulation is controlled mainly by wind forcing, with tidal and density currents being weak. However, there is a lack of knowledge available on currents within the lower levels of the water column; likewise, on the main time-scales involved in the water circulation. This study quantifies the contribution of the tidal and wind-induced currents, to the overall water circulation; it identifies the main time-scales involved within the tidal and wind-induced flows, investigating difference in such currents, throughout the water column, within Pasaia Bay (Basque coast). Within this context, extensive oceanographic and meteorological data have been obtained, in order to describe the circulation. The present investigation reveals that the circulation, within the surface and the sub-surface waters, is controlled mainly by wind forcing fluctuations, over a wide range of meteorological frequencies: third-diurnal, semidiurnal and diurnal land–sea breezes; synoptic variability; frequencies, near fortnightly periods; and seasonal. At the lower levels of the water column, the main contribution to the water circulation arises from residual currents, followed by wind-induced currents on synoptic time-scales. In contrast, tidal currents contribute minimally to the overall circulation throughout the water column.     

Frequency analysis for predicting 1% annual maximum water levels along Florida coast,US

In the Coastal Flood Insurance Study by the Federal Emergency Management Agency (FEMA, 2005), 1% annual maximum coastal water levels are used in coastal flood hazard mitigation and engineering design in coastal areas of USA. In this study, a frequency analysis method has been developed to provide more accurate predictions of 1% annual maximum water levels for the Florida coast waters. Using 82 and 94 years of annual maximum water level data at Pensacola and Fernandina, performances of traditional frequency analysis methods, including advanced method of Generalized Extreme Value distribution method, have been evaluated. Comparison with observations of annual maximum water levels with 83 and 95 years of return periods indicate that traditional methods are unable to provide satisfactory predictions of 1% annual maximum water levels to account for hurricane‐induced extreme water levels. Based on the characteristics of annual maximum water level distribution of Pensacola and Fernandina stations, a new probability distribution method has been developed in this study. Comparison with observations indicates that the method presented in this study significantly improves the accuracy of predictions of 1% annual maximum water levels. For Fernandina station, predictions of extreme water level match well with the general trend of observations. With a correlation coefficient of 0·98, the error for the maximum observed extreme water level of 3·11 m (National Geodetic Vertical Datum) with 95 years of return period is 0·92%. For Pensacola station, the prediction error for the maximum observed extreme water level with a return period of 83 years is 5·5%, with a correlation value of 0·98. The frequency analysis has also been reasonably compared to the more costly Monte Carlo simulation method. Copyright © 2008 John Wiley & Sons, Ltd.     

Background water temperatures and power station discharges

An analylical treatment of the background or far-field temperature distribution associated with the cooling water discharge from a direct-cooling coastal power station is presented. Factors considered in the formation of this temperature field include residual tidal currents, horizontal mixing, the cooling water flow rate, separation of the outlet and intake and heat transfer across the water surface.A criterion is derived which specifies conditions under which the circulation of water between the outlet and the intake is expected to have a significant influence on background temperatures. As an illustration, the analysis suggests that under conditions typical of the U.K. coast in the southern North Sea this recirculation, while affecting the operation of the power station, has little effect on the far-field temperature for plant of less than 3–5 GW electrical capacity.At many coastal locations there will be a mean, or residual tidal current running parallel to the shore. The effect of this is to limit the off-shore extent of the temperature field while transporting the heat a considerable distance along the coast. These factors are quantified and it is demonstrated that the ‘area’ of the far-field is very sensitive to the threshold temperature which is taken to define its boundary.The results obtained with this model are greatly influenced by the values adopted for residual currents and mixing. The paper therefore stresses the need for reliable data, obtained from site surveys, on which the parameters must be based.     

Investigating prediction performance of an artificial neural network and a numerical model of the tidal signal at Puerto Belgrano, Bahia Blanca Estuary (Argentina)

In the present study we compare performances of the prediction of hourly tidal level variations at Puerto Belgrano, a coastal site in the Bahia Blanca Estuary (Argentina), by means of the MOHID model, which is a numerical model designed for coastal and estuarine shallow water applications, and of an artificial neural network (ANN). It was shown that the ANN model is able to predict the hourly tidal levels over long term duration with at least seven days of observations and with a better performance in respect to the numerical model. Our findings can be useful to implement ANN-based tools for future studies of the hydrodynamics of Bahía Blanca estuary.     

Migration and internal architecture of marine dunes in the eastern English Channel over 14 and 56 year intervals: the influence of tides and decennial storms

Submarine dune dynamics are controlled by tidal currents and wind forces. According to the relative influence of these forces and the nature of dune sediment, different bedform behaviors can be observed. The footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes. This paper is concerned with bedforms that compose the thick sediment wedge located in the eastern English Channel, off the Bay of Somme. This sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages. The dynamics of large submarine dunes, which are organized in fields, are studied thanks to bathymetric and seismic data over the periods 1937–1993 and 1993–2007. Dune morphology presents low lee and stoss side slopes (on average 8° and 3°, respectively) and dune migration rate is not very high. Dune movements are in the direction of residual tidal currents, i.e. toward the east, with mean migration rates around 0·8 to 5 ± 0·25 m yr^?1 and up to 6·6 ± 0·7 m yr^?1, respectively, at multi‐decennial and decennial time scales. The dune internal architecture is complex with superimposed eastward prograding units, displaying locally opposite progradation. Second‐order discontinuities (dip of 0·5°–4° perpendicular to dune crests) constitute dune master bedding. By counting the number of second‐order reflectors between 1937–1993 and 1993–2007, the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years. These time intervals coincide with the long‐term tidal cyclicities and also with the inter‐annual to decennial variability of storm activity in northern Europe. Two theories were made to interpret the dune internal structures: the second‐order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods (weak tidal and storm action), or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast. Copyright © 2010 John Wiley & Sons, Ltd.     

Effect of offshore waves and vegetation on the sediment budget in the Virginia Coast Reserve (VA)

The potential for rapid coastline modification in the face of sea-level rise or other stressors is alarming, since coasts are often densely populated and support valuable infrastructure. In addition to coastal submergence, nutrient-related water pollution is a growing concern for coastal wetlands. Previous studies found that the Suspended Sediment Concentration (SSC) of coastal wetlands acts as a first-order control of their sustainability, but SSC dynamics are poorly understood. Our study focuses on the Virginia Coast Reserve (VCR) Long Term Ecological Research (LTER) site, a shallow multiple tidal inlet system in the USA. We apply numerical modelling (Delft3D-SWAN) and subsequent analyses to determine SSC dynamics within the VCR. In particular, we consider two important controls on SSC in the system: vegetation (seagrass and salt marsh) and offshore waves. Our results show that vegetation colonies and increased wave energy lengthen water residence time. The reduction in the tidal prism decreases SSC export from the bay via tidal inlets, leading to increased sediment retention in the bay. We found that alongshore currents can enhance lagoon SSC by importing fine sediments from an adjacent inlet along the coastline. Our numerical experiments on vegetation seasonality can improve the understanding of wave climate impact on coastal bay sediment budget. Offshore waves increase sediment export from coastal bays, particularly during winter seasons with low vegetation density. Therefore, our study can help managers and stakeholders to understand how to implement restoration strategies for the VCR. © 2020 John Wiley & Sons, Ltd.     

Determination of aquifer parameters based on measurements of tidal effects on a coastal aquifer near Beihai,China

Measurements of the tide and groundwater levels in coastal zones are of importance in determining the properties of coastal aquifers. The solution to a one‐dimensional unsteady groundwater flow model in a coastal confined aquifer with sinusoidal fluctuation of the tide shows that the tidal efficiency decreases exponentially with distance and the time lag increases linearly with distance from the coast. The aquifer property described by the ratio of storage coefficient to transmissivity is determined if the damping constant of the tidal efficiency or the slope of the time lag with distance are obtained on the basis of tidal measurements. Hourly observations of the tide and groundwater levels at 10 wells on the northern coast near Beihai, China show that with distance from the coast, tidal efficiency decreases roughly exponentially and the time lag increases roughly linearly. The estimated ratio of storage coefficient to transmissivity of the confined aquifer ranges from 1·169 × 10^?6 d m^?2 to 1·83 × 10^?7 d m^?2. For a given transmissivity of 750 m² d^?1, the storage coefficient of the aquifer is 8·7675 × 10^?4 with the tidal efficiency method and 1·3725 × 10^?4 with the time lag method. The damping constant of the tidal efficiency with distance can be defined as the tidal propagation coefficient. The value of the confined aquifer is determined as 0·0018892 m^?1. Copyright © 2007 John Wiley & Sons, Ltd.     

A simple approach to adjust tidal forcing in fjord models

To model currents in a fjord accurate tidal forcing is of extreme importance. Due to complex topography with narrow and shallow straits, the tides in the innermost parts of a fjord are both shifted in phase and altered in amplitude compared to the tides in the open water outside the fjord. Commonly, coastal tide information extracted from global or regional models is used on the boundary of the fjord model. Since tides vary over short distances in shallower waters close to the coast, the global and regional tidal forcings are usually too coarse to achieve sufficiently accurate tides in fjords. We present a straightforward method to remedy this problem by simply adjusting the tides to fit the observed tides at the entrance of the fjord. To evaluate the method, we present results from the Oslofjord, Norway. A model for the fjord is first run using raw tidal forcing on its open boundary. By comparing modelled and observed time series of water level at a tidal gauge station close to the open boundary of the model, a factor for the amplitude and a shift in phase are computed. The amplitude factor and the phase shift are then applied to produce adjusted tidal forcing at the open boundary. Next, we rerun the fjord model using the adjusted tidal forcing. The results from the two runs are then compared to independent observations inside the fjord in terms of amplitude and phases of the various tidal components, the total tidal water level, and the depth integrated tidal currents. The results show improvements in the modelled tides in both the outer, and more importantly, the inner parts of the fjord.     

Performance of an artificial neural network on forecasting the daily occurrence and annual depth of rainfall at a tropical site

Performance of a feed‐forward back‐propagation artificial neural network on forecasting the daily occurrence and annual depth of rainfall at a single meteorological station is presented. Both short‐term and long‐term forecasting was attempted, with ground level data collected by the meteorological station in Colombo, Sri Lanka (79° 52′E, 6° 54′N) during two time periods, 1994–2003 and 1869–2003. Two neural network models were developed; a one‐day‐ahead model for predicting the rainfall occurrence of the next day, which was able to make predictions with a 74·3% accuracy, and one‐year‐ahead model for yearly rainfall depth predictions with an 80·0% accuracy within a ± 5% error bound. Each of these models was extended to make predictions several time steps into the future, where accuracies were found to decrease rapidly with the number of time steps. The success rates and rainfall variability within the north‐east and south‐west monsoon seasons are also discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Fine grain sediment transport and deposition in the Patos Lagoon–Cassino beach sedimentary system

Extensive mud deposits superimposed on the predominantly sandy inner continental shelf adjacent to the Patos Lagoon estuary, indicates that the Lagoon is a potential source of fine sediments to the coastal sedimentary system. The lagoon is large and shallow, and the water movement is mainly controlled by wind-driven set-up and set-down. The mean river inflow is around 2000 m³ s⁻¹, although peak flow rates exceeding 20,000 m³ s⁻¹ have been observed during El Niño periods. Though the tidal elevations are small, tidal velocities in the lagoon's inlet can be significant due to the large extension of the backwaters. Moreover, significant exchange flows can be generated between the estuary and coastal area due to barotropic pressure gradients established as a function of wind and freshwater discharge. The predominant net flow is seawards, but opposite near-bed flows due to pronounced vertical salinity stratification can also be observed. The coastal area is characterized by small tidal effects, large scale ocean circulation, wind-induced residual flows and wave-driven currents, where the waves originate from swell or are locally generated.     

SST observations of upwelling induced by tidal straining in the Rhine ROFI

In this paper SST imagery and a three-dimensional numerical model of a river plume were employed to detect upwelling induced by tidal straining in the Rhine ROFI (region of fresh water influence). Previous studies have shown that the Rhine ROFI in the North Sea exhibits strong cross-shore density gradients that compete with tidal and wind mixing to establish stratification. During neap periods with low mixing energy an area measuring 30 km offshore by 100 km alongshore becomes stratified. When the ROFI is stratified strong cross-shore currents are observed, with surface currents rotating anti-cyclonically and bottom currents rotating cyclonically. The cross-shore currents interact with the cross-shore density gradients to produce a semi-diurnal cycle of stratification. Due to continuity requirements imposed by the proximity of the coast, the offshore-directed surface currents and onshore-directed bottom currents should lead to coastal upwelling.     

Artificial neural network models for estimating regional reference evapotranspiration based on climate factors

Evapotranspiration (ET) is one of the basic components of the hydrologic cycle and is essential for estimating irrigation water requirements. In this study, an artificial neural network (ANN) model for reference evapotranspiration (ET₀) calculation was investigated. ANNs were trained and tested for arid (west), semi‐arid (middle) and sub‐humid (east) areas of the Inner Mongolia district of China. Three or four climate factors, i.e. air temperature (T), relative humidity (RH), wind speed (U) and duration of sunshine (N) from 135 meteorological stations distributed throughout the study area, were used as the inputs of the ANNs. A comparison was conducted between the estimates provided by the ANNs and by multilinear regression (MLR). The results showed that ANNs using the climatic data successfully estimated ET₀ and the ANNs simulated ET₀ better than the MLRs. The ANNs with four inputs were more accurate than those with three inputs. The errors of the ANNs with four inputs were lower (with RMSE of 0·130 mm d^?1, RE of 2·7% and R² of 0·986) in the semi‐arid area than in the other two areas, but the errors of the ANNs with three inputs were lower in the sub‐humid area (with RMSE of 0·21 mm d^?1, RE of 5·2% and R² of 0·961. For the different seasons, the results indicated that the highest errors occurred in September and the lowest in April for the ANNs with four inputs. Similarly, the errors were higher in September for the ANNs with three inputs. Copyright © 2008 John Wiley & Sons, Ltd.     

Coupled 1D–3D hydrodynamic modelling,with application to the Pearl River Delta

Within the hydrodynamic modelling community, it is common practice to apply different modelling systems for coastal waters and river systems. Whereas for coastal waters 3D finite difference or finite element grids are commonly used, river systems are generally modelled using 1D networks. Each of these systems is tailored towards specific applications. Three-dimensional coastal water models are designed to model the horizontal and vertical variability in coastal waters and are less well suited for representing the complex geometry and cross-sectional areas of river networks. On the other hand, 1D river network models are designed to accurately represent complex river network geometries and complex structures like weirs, barrages and dams. A disadvantage, however, is that they are unable to resolve complex spatial flow variability. In real life, however, coastal oceans and rivers interact. In deltaic estuaries, both tidal intrusion of seawater into the upstream river network and river discharge into open waters play a role. This is frequently approached by modelling the systems independently, with off-line coupling of the lateral boundary forcing. This implies that the river and the coastal model run sequentially, providing lateral discharge (1D) and water level (3D) forcing to each other without the possibility of direct feedback or interaction between these processes. An additional disadvantage is that due to the time aggregation usually applied to exchanged quantities, mass conservation is difficult to ensure. In this paper, we propose an approach that couples a 3D hydrodynamic modelling system for coastal waters (Delft3D) with a 1D modelling system for river hydraulics (SOBEK) online. This implies that contrary to off-line coupling, the hydrodynamic quantities are exchanged between the 1D and 3D domains during runtime to resolve the real-time exchange and interaction between the coastal waters and river network. This allows for accurate and mass conserving modelling of complex coastal waters and river network systems, whilst the advantages of both systems are maintained and used in an optimal and computationally efficient way. The coupled 1D–3D system is used to model the flows in the Pearl River Delta (Guangdong, China), which are determined by the interaction of the upstream network of the Pearl River and the open waters of the South China Sea. The highly complex upstream river network is modelled in 1D, simulating river discharges for the dry and wet monsoon periods. The 3D coastal model simulates the flow due to the external (ocean) periodic tidal forcing, the salinity distribution for both dry and wet seasons, as well as residual water levels (sea level anomalies) originating from the South China Sea. The model is calibrated and its performance extensively assessed against field measurements, resulting in a mean root mean square (RMS) error of below 6% for water levels over the entire Pearl River Delta. The model also represents both the discharge distribution over the river network and salinity transport processes with good accuracy, resolving the discharge distribution over the main branches of the river network within 5% of reported annual mean values and RMS errors for salinity in the range of 2 ppt (dry season) to 5 ppt (wet season).