Tide-surge Interaction Research Articles

Influence of wave setup and tide-surge interaction on storm surges in the Bay of Bengal

Influence of wave setup and tide-surge interaction on storm surges in the Bay of Bengal

Tide–surge interaction observed at Singapore and the east coast of Peninsular Malaysia using a semi-empirical model

Abstract. Tide–surge interaction plays a substantial role in determining the characteristics of coastal water levels over shallow regions. We study the tide–surge interaction observed at seven tide gauges along Singapore and the east coast of Peninsular Malaysia, focusing on the timing of extreme non-tidal residuals relative to tidal high water. We propose a modified statistical framework using a no-tide–surge interaction (no-TSI) null distribution that accounts for asymmetry and variation in the duration of tidal cycles. We find that our modified framework can mitigate false-positive signals of tide–surge interaction in this region. We find evidence of tide–surge interaction at all seven locations, with characteristics varying smoothly along the coastline: the highest non-tidal residuals are found to occur most frequently before tidal high water in the south, both before and after tidal high water in the central region, and after tidal high water in the north. We also propose a semi-empirical model to investigate the effects of tidal-phase alteration, which is one mechanism of tide–surge interaction. Results of our semi-empirical model reveal that tidal-phase alteration caused by storm surges is substantial enough to generate significant change in the timing of extreme non-tidal residuals. To mitigate the effect of tidal-phase alteration on return level estimation, skew surge can be used. We conclude that (1) tide–surge interaction influences coastal water levels in this region, (2) our semi-empirical model provides insight into the mechanism of tidal-phase alteration, and (3) our no-TSI distribution should be used for similar studies globally.

Wave–Tide–Surge Interaction Modulates Storm Waves in the Bohai Sea

Typhoons, extratropical cyclones, and cold fronts cause strong winds leading to storm surges and waves in the Bohai Sea. A wave–flow coupled numerical model is established for storm events observed in 2022 caused by three weather systems, to investigate how storm waves are modulated by wave–tide–surge interaction (WTSI). Wave response is basically controlled by water level change in coastal areas, where bottom friction or breaking dominates the energy dissipation, and determined by the current field in deep water by altering whitecapping. Wave height increases/decreases are induced by positive/negative water level or obtuse/acute wave–current interaction angle, leading to six types of field patterns for significant wave height (Hs) responses. For the three storm events, Hs basically changed within ±5% in central deep water, while the maximum increase/decrease reached 160%/−60% in the coastal area of Laizhou Bay/Liaodong Bay. Based on maximum Hs and its occurrence time, WTSI modulation is manifested as the superposition effect of wave–tide and wave–surge interactions in both space and time scales, and occurrence time depends more on tide than surge for all three storms. The enhancement/abatement of WTSI modulation happens for consistent/opposite changing trends of wave–tide and wave–surge interaction, with the ultimate result showing the side with a higher effect.

Temporal and spatial variation of typhoon-induced surges and the impact of rainfall in a tidal river

The Huangpu River, which flows into the Yangtze Estuary, runs through Shanghai Municipality in China. During the flood season, typhoon-induced surges result in extremely high water levels in the Huangpu River, which is the main factor threatening flood control safety in Shanghai. Based on observations of water levels at three stations along the Huangpu River from 2008 to 2021, together with wind and rainfall, this paper investigates: the trends of the annual highest storm surges; the differences in periodic variation and magnitude of surges during the spring-neap cycle; the longitudinal variation of surges; and the impact of rainfall. The results show that the annual highest surges at three stations all show a significant increasing trend, which is caused by the increasing wind speed, and the intensity and duration of rainfall during typhoon events, induced by the increasing frequency of landfall and close-to-YE (Yangtze Estuary) typhoon events, and strong and very strong typhoon events. The periodic variation and magnitude of surges show clear differences during the spring-neap cycle, resulting from the largest nonlinear surge-tide interaction during spring tide and the smallest during neap tide. Heavy rainfall can reduce the probability of 6–7 h and 2.7–3.3 h periods, and change the phase of the periodic variation, inducing partial main peaks and maximum surges during ebb or low tide. The longitudinal variation of storm surge along the Huangpu River can be divided into three types: decreasing upstream, first increasing then decreasing, and increasing upstream. Heavy and long-duration rainfall increases the water discharge from the basin to the river, amplifying the surge, with the largest impact in the upper reach and the smallest in the lower reach, which is the main factor inducing the latter two types, particularly the third type.

A Numerical Study on Storm Surge Dynamics Caused by Tropical Depression 29W in the Pahang Region

Amid mounting concerns about climate change’s impact on coastal areas, this study investigates storm surge dynamics induced by Tropical Depression 29W (TD 29W) using the MIKE 21 model. Comprehending the complex mechanisms behind storm surges is crucial considering gaps in understanding their combined influences, including tide–surge interactions, varying typhoon parameters, and changing storm tracks. The impacts of climate change, including accelerating sea level rise and its correlation with storm surge magnitudes, require detailed investigations for effective disaster management in vulnerable coastal communities. Through precise calibration, matching simulations with tidal gauge stations, this research uncovers the intricate interplay between landfall timing, diverse storm tracks, wind intensities, and the amplifying impact of rising sea levels. Findings indicate surge residuals ranging from −0.03m to 0.01m during TD 29W’s landfall, with higher surge residuals during rising tide phases. Moreover, an increase in TD 29W’s maximum wind speed moderately influences positive surges while significantly amplifying negative surge heights by 68% to 92% with wind speed increments. An analysis of typhoon track variations emphasizes the vulnerability of the Pahang coast to changing storm dynamics, underlining the need for tailored resilience strategies. Projections suggest a significant surge height increase by the year 2100, emphasizing the urgency of adaptive measures for the region.

Tide–Surge Interactions in Lingdingyang Bay, Pearl River Estuary, China: a Case Study from Typhoon Mangkhut, 2018

Tide–Surge Interactions in Lingdingyang Bay, Pearl River Estuary, China: a Case Study from Typhoon Mangkhut, 2018

The Impact of Coastline and Bathymetry Changes on the Storm Tides in Zhejiang Coasts

Coastal evolutions are expected to have a significant impact on storm tides, disproportionately aggravating coastal flooding. In this study, we utilize a nested storm tide model to provide an integrated investigation of storm tide responses to changes in coastline and bathymetry along the Zhejiang coasts. We selected coastline and bathymetry data from 1980 and 2016, as well as data from three typical typhoon events (i.e., Winnie, Haikui, and Chan-hom) for simulating the storm surge processes. The results indicate that changes in the coastline and bathymetry from 1980 to 2016 have resulted in an increase in storm tides in the northern part and a decrease in the central part of Zhejiang. Specifically, storm tides in Hangzhou Bay have increased significantly, with an average increase of about 0.3 m in the maximum storm tides primarily attributed to coastline changes. On the contrary, in smaller basins like Sanmen Bay, while reclamation itself has reduced peak storm surges, rapid siltation has consequently exacerbated the storm surge. By decomposing storm tides into astronomical tides and storm surges, we discovered that the change in tidal levels was twice as significant as the surge change. Moreover, the nonlinear tide–surge interaction was nearly four times that of the pure surge, significantly contributing to storm surge variation. Alterations in the momentum balance reveal that the water depth-induced bottom friction and wind stress increase contributes to the local enlargement of storm tides at the bay head, while the coastline changes exaggerate nearshore storm tides through an increase in the advection term.

Adding sea ice effects to a global operational model (NEMO v3.6) for forecasting total water level: approach and impact

Abstract. In operational flood forecast systems, the effect of sea ice is typically neglected or parameterized solely in terms of ice concentration. In this study, an efficient way of adding ice effects to the global total water level prediction systems, via the ice–ocean stress, is described and evaluated. The approach features a novel, consistent representation of the tidal relative ice–ocean velocities, based on a transfer function derived from ice and ocean tidal ellipses given by an external ice–ocean model. The approach and its impact are demonstrated over four ice seasons in the Northern Hemisphere, using in situ observations and model predictions. We show that adding ice effects helps the model reproduce most of the observed seasonal modulations in tides (up to 40 % in amplitude and 50∘ in phase for M2) in the Arctic and Hudson Bay. The dominant driving mechanism for the seasonal modulations is shown to be the under-ice friction, acting in areas of shallow water (less than 100 m) and its accompanied large shifts in the amphidromes (up to 125 km). Important contributions from baroclinicity and tide–surge interaction due to ice–ocean stress are also found in the Arctic. Both mechanisms generally reinforce the seasonal modulations induced by the under-ice friction. In forecast systems that neglect or rely on simple ice concentration parameterizations, storm surges tend to be overestimated. With the inclusion of ice–ocean stress, surfaces stresses are significantly reduced (up to 100 % in landfast ice areas). Over the four ice seasons covered by this study, corrections up to 1.0 m to the overestimation of surges are achieved. Remaining limitations regarding the overestimated amphidrome shifts and insufficient ice break-up during large storms are discussed. Finally, the anticipated trend of increasing risk of coastal flooding in the Arctic, associated with decreasing ice and its profound impact on tides and storm surges, is briefly discussed.

A regional analysis of tide-surge interactions during extreme water levels in complex coastal systems of Aotearoa New Zealand

Tide-surge interaction (TSI) is a critical factor in assessing flooding in shallow coastal systems, particularly in estuaries and harbours. Non-linear interactions between tides and surges can occur due to the water depth and bed friction. Global investigations have been conducted to examine TSI, but its occurrence and impact on water levels in Aotearoa New Zealand (NZ) have not been extensively studied. Water level observations from 36 tide gauges across the diverse coast of NZ were analysed to determine the occurrence and location of TSI. Statistical analysis and numerical modelling were conducted on data from both inside and outside estuaries, focusing on one estuary (Manukau Harbour) to determine the impact of TSI and estuarine morphology on the co-occurrence rate of extreme events. TSI was found to occur at most sites in NZ and primarily affects the timing of the largest surges relative to high tide. There were no regional patterns associated with the tide, non-tidal residual, or skew-surge regimes. The strongest TSI occurred in inner estuarine locations and was correlated with the intertidal area. The magnitude of the TSI varied depending on the method used, ranging from -16 cm to +27 cm. Co-occurrence rates of extreme water levels outside and inside the same estuary varied from 20% to 84%, with TSI modulating the rate by affecting tidal amplification. The results highlight the importance of investing in a more extensive tide gauge network to provide longer observations in highly populated estuarine coastlines. The incorporation of TSI in flooding hazard projections would benefit from more accurate and detailed observations, particularly in estuaries with high morphological complexity. TSI occurs in most sites along the coast of NZ and has a significant impact on water levels in inner estuarine locations. TSI modulates the co-occurrence rate of extreme water levels in estuaries of NZ by affecting tidal amplification. Therefore, further investment in the tide gauge network is needed to provide more accurate observations to incorporate TSI in flooding hazard projections.

Combined statistical and hydrodynamic modelling of compound flooding in coastal areas - Methodology and application

This paper presents a robust cost-effective framework for assessment of coastal-fluvial flooding due to compound action of multivariate dependent drivers. The methodology is an 8-step process that links statistical and hydrodynamic models to determine probabilities of multiple-driver flood events and associated hazards. The method involves individual and combined extreme value analysis, assessment of dependencies and interactions between flood drivers, multivariate joint probability determination accounting for dependencies, high-resolution hydrodynamic modelling of flood scenarios derived from multivariate statistical analysis, and ultimately mapping of inundation.Using Cork City, on the south coast of Ireland, as a study case, the research shows that the interactions and dependencies between tides, surges and river flows affect flood severity when they occur jointly. Tide-surge interactions have a damping effect on the total water level, while dependence between the surge residual and river flow amplifies the risk of flooding. The multivariate joint exceedance probability occurrence of high discharges and water levels represents a more realistic representation of the spatially variable water surface profiles then the combined univariate marginal scenarios. Multivariate analysis allows also considering multiple combinations of joint probability solutions along RP iso-curves. The results show that the quantification of compound flood impacts must be performed along the entire RP probability curve. This is because the physical/hydrological impacts of multiple-driver same-RP flood events can be very different leading to substantially different characteristics of flooding. The multi-scale nested flood model (MSN_Flood) was used to simulate flood wave propagation over urban floodplains for an ensemble of statistically derived flood scenarios. The hydrodynamic runs provide inundation maps that can be used to draw inferences about flood mechanisms and impacts.

The impact of nonlinear tide–surge interaction on satellite radar altimeter-derived tides

Both empirical and assimilative global ocean tidal models are significantly more accurate in the deep ocean than in shelf and coastal waters. In this study, we answered whether this is due to the quality of the models used to reduce tide and surge or the general approach to treat tide and surge as two separate components of the water level obtained from stand-alone models, which ignores the nonlinear tide–surge interaction. In doing so, we used tide gauge observations as partially synthetic altimeter time series, tide–surge water-level time series obtained with the 2D Dutch Continental Shelf Model – Flexible Mesh (DCSM), and tide and surge water-level time series obtained using the DCSM, FES2014 (FES) and the Dynamic Atmospheric Correction (DAC) product. Expressed in the root-sum-square (RSS) of the eight main tidal constituents, we obtained a reduction % when removing the DCSM tide–surge water levels compared to when we removed the sum of the DCSM tide and DCSM surge water levels. The RSS obtained in the latter case was only 3.3% lower than with FES and DAC. We conclude that the lower tidal estimates accuracy in shelf-coastal waters derives from the missing nonlinear tide–surge interactions.

Characteristics of astronomical tides and their modulation on sea level extremes along the Indian coast

In this study, long-term hourly sea-level records from 18 tide gauge stations during 1972–2007 were analyzed to study the characteristics of astronomical tides, sea-level trends, and extremes around India's mainland. The de-tided signals were used to estimate these parameters and study tide and surge interaction. The observed sea level depicts significant variability in daily, seasonal, and inter-annual time scales. Semidiurnal tides are the most dominant among the high-frequency tides, with an amplitude of up to 2.5 m for M2 and 0.75 m for S2 tides in the northwestern and northeastern continental shelf and reduced to 0.25 m in the south. The amplitude of diurnal tides (O1 and K1)is relatively weak (<0.25m) at all the stations. The annual harmonics dominate the seasonal cycle, with an amplitude of 0.7 m at Garden Reach in the northeastern continental shelf, decreases to 0.15 m at Tuticorin in the south, and remains small (<0.1 m) along the west coast of India. The amplitude of lunar nodal and perigee tides are significantly high (up to 25 mm) at several stations compared to the long-term global mean sea level trend (∼3.3 mm/y). The sea level trend is significantly positive (up to 4 mm/y) for Sagar Island, Diamond Harbor, Haldia, and Mangalore; negative (up to −3 mm/y) for Garden Reach and Okha, and in-significant (0.5 mm/y) for Mumbai, Vishakhapatnam, and Paradip. Interaction between the semidiurnal tides and surges was intense at most stations, with a high probability of surge peaks during the fall-tide conditions in the northern continental shelf, at rising-tidal conditions for the south-eastern and western peninsula, and co-occurring at high tide for southern stations. The degree of tide-surge interaction increases from south to north with an increase in tidal range and significant nodal and perigee tidal modulation.

Effects of tide-surge interactions on the temporal distribution of the peak residual in Hangzhou Bay, China

Hangzhou Bay has a trumpet-shaped geometry and is known for its extremely large tidal range. Understanding tide-surge interactions is essential for coastal defence applications during storm events. Analysis of long-term hourly sea level observation data at three sites and numerical experiments show that a remarkable cluster of peak residuals occurs 4 h before the high tide inside Hangzhou Bay (Zhapu and Ganpu Sites), while a weaker cluster occurs around the low tide outside the bay (Shengshan Site). Further analysis of various sea levels in Hangzhou Bay revealed that the non-linear level inside the bay was greater than that outside the bay, leading to the observed difference in frequency distributions between them. To understand the underlying mechanism, the contributions of each non-linear interaction factor to the tide-surge interactions were distinguished. It was found that quadratic bottom friction is the dominating factor inside Hangzhou Bay, which always peaks approximately 4 h before high tide due to wave deformation. Whereas, outside Hangzhou Bay, advective term and quadratic bottom friction are the main forces, which are much larger than wind stress, and they tend to have peaks around low tide. It is indicated that peak residual arrives synchronously with the peak non-linear interaction term.

Amplification of regional tides in response to sea level

The study investigates the temporal change in tides under the background of rapid sea level rise, for a highly vulnerable coastal region. It highlights the existence of non-astronomic tidal variability at seasonal and secular time-scales in the Ganga-Brahmaputra-Meghna (GBM) delta, along with increasing low and high tidal levels. The observed variability in semi-diurnal tides was found to be coherent to the mean sea level changes. M2 tide can be a proxy for sea level changes as they show a direct relationship. Steric changes were introduced in the barotropic ADCIRC (ADvanced CIRCulation) model and the results proved that change in water depth modulates the energetics of the tidal wave thereby causing amplification. A regionally varying tidal response was noticed, such that the regions associated with the GBM delta showed maximum amplification. The factors that can cause tidal amplification are region-dependent and coastal geomorphology plays an important role in it. Tidal prediction based on changing sea level was able to capture the observed seasonal modulation of tides and increasing trend in low and high tidal levels. Incorporating the effect of changing tides can improve the accuracy of tidal predictions and will help advance studies regarding regional sea level change, coastal flooding and tide-surge interaction.

Investigation of Storm Tides Induced by Super Typhoon in Macro-Tidal Hangzhou Bay

Typhoon-induced storm tides can cause serious coastal disasters and considerable economic losses. Understanding the mechanisms controlling storm surges helps the prevention of coastal disasters. Hangzhou Bay (HZB), a typical macro-tidal estuary, is located on the east coast of China, where typhoons frequently occur. The funnel-shaped topography makes this macro-tidal bay even more sensitive to storm tides. Super Typhoon Chan-hom was used as an example to study the characteristics and dynamic mechanisms of storm surges using a well-validated numerical model. The model considers the two-way coupling of waves and tides. The wind strength for the model was reconstructed using multi-source wind data and was refined by considering different rotating and moving wind fields. The Holland–Miyazaki model was used to reconstruct the local wind-field data with a good performance. The model results show that the total water level of HZB during typhoon Chan-hom was mainly dominated by tides, and the storm surge was closely related to the wind field. Surface flow was mostly influenced by winds, followed by tides. The spatial and temporal distributions of the significant wave height were controlled by the wind and local terrain. Wind stress was the largest contributor to storm surges (91%), followed by the pressure effect (15%) and the wave effect (5%). Both wind and wave-induced surges occurred during low slack waters. The tide-surge interaction changes (enhance or suppress) the surge by approximately 0.5 m during the typhoon, comprising approximately 50% of the total surge. Tides interacted with surges through various mechanisms, from the bay mouth (local acceleration and friction) to the bay head (friction and advection). The Coriolis force had a relatively minor effect. The findings of this study provide useful information for studies on sediment dynamics and coastal structures under extreme weather conditions.

A method for estimating the risk of fishery ports against typhoon: a case study on Dongsha fishery port

After standard seawalls have been built successfully, fishery ports become the structures most easily damaged during a typhoon. Estimating the risk of fishery ports against typhoons would be useful for identifying weaknesses and implementing corrective measures to protect fishing boats from a typhoon. This study describes a versatile methodology for conducting this type of quantitative assessment at fishery ports. The Dongsha fishery port in Zhejiang Province was selected as a case study to test the results derived from a high-precision Hydrodynamic Flexible Mesh model coupled with the Spectral Wave model. First, typhoon characteristics were assessed based on historical typhoons in the study area, and then, the wind, tide, storm surge, and waves were modeled and tide-surge interactions were investigated. Through comparisons of the destructive parameters from the typhoon assessment with the design and structural parameters of the fishery port, the level of the Dongsha fishery port against typhoons was determined to be 12, and the main weaknesses of the port’s defenses were found to be located near feature points T2, T3, T8, and T15. The results obtained demonstrate that the proposed methodology can be used to acquire valuable information on the risk of fishery ports against typhoons.

An estimation of water levels associated with a storm along the coast of Bangladesh using a non-central difference method of lines

In this paper, depth-averaged shallow water equations were solved using a non-central difference method of lines together with the classical RK(4,4) method to estimate water levels associated with a cyclone along the coast of Bangladesh. A 4-point backward finite difference method was adopted in discretizing spatial derivatives retaining time derivatives as continuous. Thus, the shallow water equations together with the boundary conditions were transformed into a system of initial value problems, which was solved using the RK(4,4) method. A nesting approach was employed to incorporate the land-sea interface and bottom topography effectively with economic cost. A stable tidal oscillation was generated in the domain applying the M2 tidal constituent along the southern boundary of the parent model and the surge model was then run to get water levels owing to dynamic tide-surge interaction. Numerical experiments were performed to simulate water levels due to tide, surge, and nonlinear tide-surge interactions associated with the cyclonic storm, AILA. The obtained results were in good agreement with those attained in some previous studies and observations and the WLs due to the nonlinear tide-surge interaction were in a reasonable agreement with respect to the root mean square error values and predictive skills.

Towards using state-of-the-art climate models to help constrain estimates of unprecedented UK storm surges

Abstract. Our ability to quantify the likelihood of present-day extreme sea level (ESL) events is limited by the length of tide gauge records around the UK, and this results in substantial uncertainties in return level curves at many sites. In this work, we explore the potential for a state-of-the-art climate model, HadGEM3-GC3, to help refine our understanding of present-day coastal flood risk associated with extreme storm surges, which are the dominant driver of ESL events for the UK and wider European shelf seas. We use a 483-year present-day control simulation from HadGEM3-GC3-MM (1/4∘ ocean, approx. 60 km atmosphere in mid-latitudes) to drive a north-west European shelf seas model and generate a new dataset of simulated UK storm surges. The variable analysed is the skew surge (the difference between the high water level and the predicted astronomical high tide), which is widely used in analysis of storm surge events. The modelling system can simulate skew surge events comparable to the catastrophic 1953 North Sea storm surge, which resulted in widespread flooding, evacuation of 32 000 people, and hundreds of fatalities across the UK alone, along with many hundreds more in mainland Europe. Our model simulations show good agreement with an independent re-analysis of the 1953 surge event at the mouth of the river Thames. For that site, we also revisit the assumption of skew surge and tide independence. Our model results suggest that at that site for the most extreme surges, tide–surge interaction significantly attenuates extreme skew surges on a spring tide compared to a neap tide. Around the UK coastline, the extreme tail shape parameters diagnosed from our simulation correlate very well (Pearson's r greater than 0.85), in terms of spatial variability, with those used in the UK government's current guidance (which are diagnosed from tide gauge observations), but ours have smaller uncertainties. Despite the strong correlation, our diagnosed shape parameters are biased low relative to the current guidance. This bias is also seen when we replace HadGEM3-GC3-MM with a reanalysis, so we conclude that the bias is likely associated with limitations in the shelf sea model used here. Overall, the work suggests that climate model simulations may prove useful as an additional line of evidence to inform assessments of present-day coastal flood risk.

Storm surge risk under various strengths and translation speeds of landfalling tropical cyclones

Landfalling tropical cyclones (TCs) frequently occur with strong intensity in most coastal areas, and storm surges are likely to occur in response to extreme sea level (ESL) growth. However, the level of ESL growth under various wind conditions, coastline geometries and tide-surge interactions has not been clarified. In the Pearl River Estuary and Daya Bay, observations of landfalling TCs have indicated an increasing frequency of intense and rapid landfalls in the 2010s as compared to the 2000s, accompanied by a noteworthy increase in storm surge. Based on a large ensemble (∼0.5 million storm surge events with various tracks, maximum wind speeds, maximum wind radiuses, translation speeds and tidal conditions) obtained from well-validated model simulations, the ESL growth in the study area is further quantified as follows: (a) ESL growth is more sensitive to the acceleration effect of landfalling TCs than to the strengthening effect of landfalling TCs since the effect of low acceleration (+3 m s−1) is comparable to that under notable strengthening (+10 m s−1); (b) ESL growth is strongly modulated by coastline geometry, especially in flared or arching coastline areas. ESL growth mainly occurs along flared coastline areas when landfalling TCs strengthen into severe TCs or typhoons but can also occur along arching coastline areas for stronger landfalling TCs, such as severe typhoons or supertyphoons; and (c) ESL growth could be increased or decreased by approximately 10% under the effect of tide-surge interactions. Both the large-ensemble method and the above ESL growth characteristics are worthy of attention in risk assessment and rapid prediction of storm surges in shallow waters.

Unsteady Linearisation of Bed Shear Stress for Idealised Storm Surge Modelling

The modelling of time-varying shallow flows, such as tides and storm surges, is complicated by the nonlinear dependency of bed shear stress on flow speed. For tidal flows, Lorentz’s linearisation circumvents nonlinearity by specifying a (steady) friction coefficient r based on a tide-averaged criterion of energy equivalence. However, this approach is not suitable for phenomena with episodic and irregular forcings such as storm surges. Here, we studied the implications of applying Lorentz’s energy criterion in an instantaneous sense, so that an unsteady friction coefficient r(t) adjusts to the temporal development of natural wind-driven flows. This new bed-stress parametrisation was implemented in an idealised model of a single channel, forced by time-varying signals of wind stress (acting over the entire domain) and surface elevation (at the channel mouth). The solution method combines analytical solutions of the cross-sectionally averaged linearised shallow-water equations, obtained in the frequency domain, with an iterative procedure to determine r(t). Model results, compared with a reference finite-difference solution retaining the quadratic bed shear stress, show that this new approach accurately captures the qualitative and quantitative aspects of the surge dynamics (height and timing of surge peaks, sloshing, friction-induced tide-surge interaction) for both synthetic and realistic wind forcings.