Narrow Tidal Channels Research Articles

Tidal connectivity modelling in wetlands associated with coastal lagoon of Punta Rasa natural reserve, Argentina

Tidal flows play a crucial role in connecting coastal lagoons to the sea through channels, in this way determining the hydrological dynamics of these environments. The Punta Rasa Natural Reserve (Ramsar site) is located in a transitional zone between the outer limit of the estuary of the Río de la Plata and the open sea. In this work, we study the northernmost lagoon at the end of the spit, which is connected to the sea by a narrow tidal channel. Wetlands are developing at the edges of the lagoon and in lower lying sectors. The aim of this study is to develop a conceptual model that explains the tidal connectivity of the coastal lagoon and to assess how it affects the marsh environments. Detailed topographic surveys were performed using Structure from Motion Multi-View Stereo from a UAV, while sea level data was analyzed using recordings at 20-min intervals over a five-year period. This dataset facilitated an analysis of the frequency of exceedances for threshold values of 0.8, 1.0, 1.2 and 1.4 m. The frequency analysis reveals that the highest exceedances of these thresholds occur during the summer months. When the water level reaches 1 m, the lagoon is fully connected to the sea via the tidal channel, resulting in complete inundation of the low marshland and the margins of the high marshland. At a tidal level of 1.4 m, the high marsh connected to the tidal channel and the coastal lagoon experience flooding. The results obtained in this study provides insights into modelling hydrological connectivity between marshes and the sea, as well as determining the frequency of tidal inundation in each wetland.

Detection and classification capabilities of two multibeam sonars

AbstractMultibeam sonars can be used to detect and classify marine fauna under conditions when optical sensors are ineffective. In this work, we compare the detection and classification capabilities of two multibeam sonars with different operating frequencies, the Tritech Gemini 720is (720 kHz) and the Teledyne BlueView M900‐2250 (2250 kHz). The two sonars were deployed with overlapping swaths in a narrow tidal channel with peak currents of approximately 2 m/s where seals, schools of fish, and diving birds were intermittently present. In comparing data concurrently acquired by both sonars, we observe differences in the appearance of detected targets and detection capabilities. The detected target tracks are classified using a random forest model as either individual biological targets (flora and fauna, including fish, diving birds, seals, and plant matter), fish schools, or nonbiological targets (including entrained air and sonar artifacts). Despite the observed differences in detection capabilities for the two sonars, automatic classification distinguishes between target classes with precision and recall above 0.7 and 0.9, respectively. These results suggest that while similar methodologies can be used for data analysis, some outcomes from environmental studies using multibeam sonars may be instrument‐specific.

Tidal response to sea level rise and bathymetric changes in the German Wadden Sea

Tidally dominated coasts are directly affected not only by projected rise in mean sea level, but also by changes in tidal dynamics due to sea level rise and bathymetric changes. By use of a hydrodynamic model, which covers the entire German Bight (South-Eastern North Sea), we analyse the effects of sea level rise and potential bathymetric changes in the Wadden Sea on tidal current velocities. The model results indicate that tidal current velocities in the tidal inlets and channels of the Wadden Sea are increased in response to sea level rise. This is explained by the increased ratio of tidal prism to tidal inlet cross-sectional area, which is due to the characteristic hypsometry of tidal basins in the Wadden Sea including wide and shallow tidal flats and relatively narrow tidal channels. The results further indicate that sea level rise decreases ebb dominance and increases flood dominance in tidal channels. This is, amongst others, related to a decreased intertidal area again demonstrating the strong interaction between tidal wave and tidal basin hypsometry in the Wadden Sea. The bathymetry scenario defined in this study includes elevated tidal flats and deepened tidal channels, which is considered a potential future situation under accelerated sea level rise. Application of these bathymetric changes to the model mostly compensates the effects of sea level rise. Furthermore, changes in current velocity due to the altered bathymetry are in the same order of magnitude as changes due to mean sea level rise. This highlights the significance of considering potential bathymetric changes in the Wadden Sea for regional projections of the tidal response to sea level rise.

On residual velocities in sigma coordinates in narrow tidal channels

In shallow coastal regions where water surface fluctuations are non-negligible compared to the mean water depth, the use of sigma coordinates allows the calculation of residual velocity around the mean water surface level. Theoretical analysis and generic numerical experiments were conducted to understand the physical meaning of the residual velocities at sigma layers in breadth-averaged tidal channels. For shallow water waves, the sigma layers coincide with the water wave surfaces within the water column such that the Stokes velocity and its vertical and horizontal components can be expressed in discrete forms using the sigma velocity. The residual velocity at a sigma layer is the sum of the Eulerian velocity and the vertical component of the Stokes velocity at the mean depth of the sigma layer and, therefore, can be referred to as a semi-Lagrangian residual velocity. Because the vertical component of the Stokes velocity is one order of magnitude smaller than the horizontal component, the sigma residual velocity approximates the Eulerian residual velocity. The residual transport velocity at a sigma layer is the sum of the sigma residual velocity and the horizontal component of the Stokes velocity and approximates the Lagrangian residual velocity in magnitude and direction, but the two residual velocities are not conceptually the same.

Hydrochemical dynamics of a wetland and costal lagoon associated to the outer limit of the Rio de la Plata estuary

Coastal lagoons and marshes are among the most fluctuating and productive ecosystems in the world, which provide a wide range of ecosystem. The objective of this work was to evaluate the factors and processes that condition the hydrogeochemical dynamics in salt marshes associated with the development of small coastal lagoons located at the southern end of the outer estuary of the Rio de la Plata, where it confluences with the Atlantic Ocean. Four samplings were carried out where major ions composition was determined in seawater, surface water of the lagoon and tidal channel, and in groundwater of the marsh, measuring in situ water pH and the electrical conductivity (EC). The chemical data were analyzed considering the conditions and processes associated with both water balance and tidal regime. The coastal lagoon develops within a sandy spit with dunes which is connected to the sea by a narrow tidal channel where the entry of the tidal flow is limited. The tide that enters has strong chemical variability, reflected in EC ranging from 19.1 to 37.5 mS/cm, depending on the dominant source (ocean or estuary). In periods with evapotranspiration domain, tidal water in the lagoon evaporates resulting in soluble salts precipitation such as gypsum and halite. The subsequent tidal flood dissolves these salts causing that in these periods both surface water of the lagoon and groundwater in the marsh present EC values between 8.4 and 57.1 mS/cm. Conversely, when evapotranspiration is low, surface water of the lagoon and tidal channel and groundwater of the marsh have EC between 3.5 and 18.1 mS/cm, and freshwater discharge from the dune sector where the rain preferentially infiltrate becomes relevant. The results obtained show the strong chemical variability that this type of coastal wetlands presents, providing the basis for its conservation and management of natural reserve areas.

Severe Weather-Induced Exchange Flows through a Narrow Tidal Channel of Calcasieu Lake Estuary

Exchange flows between estuaries and the coastal ocean are important for land-ocean interactions and ecosystem health. This study is aimed at resolving severe weather-induced exchange flows between the Calcasieu Lake Estuary and Gulf of Mexico. For that purpose, we use data from a long-term deployment of side-looking acoustic Doppler current profilers (ADCPs) and conductivity-temperature-depth sensors (CTDs) as well as flow velocity data from a boat operated survey. Regression between the transport measured from a boat mounted ADCP and the velocity data from a fixed side-looking ADCP is done to calculate a long-term transport along the Calcasieu Pass. Analyses have been done for the hydrodynamic response to 16 cold fronts passing the study area. Effects of six strongest cold fronts are discussed in more detail. Results have confirmed that the hydrodynamics is highly correlated with the frequent cold fronts. The highest correlation coefficient is r ~0.75 between the north wind and along channel transport. In general, winds from the southern quadrants push water into the estuary before each frontal passage; after the passage of the front, a rapid change of wind direction to the northern quadrants produces strong outward flows. A quasi-steady state balance between the wind stress and water level difference proposed in recent studies for different systems is further confirmed and discussed in this system. The quasi-steady state balance leads to a relatively high R2 value of greater than 0.8 between the modeled water level gradient and actual observed gradient. We have also applied a regression model, derived from the momentum balance requirement, for the subtidal exchange flow as a function of wind components and their squares which yield an R2 value greater than 0.7. With a confidence in the regression model, we further implement it for twelve years from 26 February 2007 to 10 April 2019. Four extreme events during this 12-year period of time are discussed–they include the Hurricane Ike (2008), Tropical Storm Lee (2011), a warm front, and a cold front. This hindcast of the exchange flows over multiple years can provide a useful tool for coastal management and research for estuarine channels where continuous observations of velocity are not always available.

Development of an Adapted Turbine Model for Hydrokinetic Turbines in 2d Shallow Water Solvers

Identifying optimum sites for hydrokinetic turbines in rivers and oceans in order to gain a maximum power output is more and more focused through the demand for sustainable energy production. This task may be supported by 2d shallow water solvers, which are able to simulate the flow in large areas with comparatively low effort. The main problem using them for turbine siting is that those solvers work with depth averaged flow quantities. Therefore, it is not possible to directly resolve the geometry of a kinetic turbine. A mathematical model is required representing the turbine characteristics in the flow field. Existing turbine models are usually based on the Linear Momentum Actuator Disk Theory which was derived for an ideal propeller in an infinitely large flow field. Regarding rivers and shallow or narrow tidal channels but also more complex types of hydrokinetic turbines - like diffuser augmented turbines - those assumptions are not fulfilled. This work presents the development of a turbine model adapted to those requirements and its implementation in 2d shallow water solvers.

Topographical Mapping of a Bare Tidal Flat Outside a Mangrove Area based on the Waterline Method and an Iterative Hydrodynamic Model: A Case Study of Yingluo Bay, South China

abstractA change in the elevation of bare tidal flats outside a mangrove area is an indispensable factor for the sustainable development of mangroves. Waterline extraction, as an effective and economical tool used in reconstructing the terrain of an intertidal zone, has been widely applied to open-coast tidal flats by constructing a digital elevation model (DEM). However, mangrove wetlands are usually located in wave-sheltered sites, such as estuaries and bays that have narrow tidal channels flanked by tidal flats. Changes in water level are affected by the dry-wet processes of complex landforms caused by tides. This article takes as a study case the area of Yingluo Bay, which covers the core region of the Zhanjiang and Shankou National Mangrove National Nature Reserve in southwestern China. Waterline extraction based on seventeen multisource and multispectral satellite images obtained from December 2014 to April 2015, combining the finite-volume coastal ocean model (FVCOM) hydrodynamic model in an iterative process, was used to generate a topographical map of the bare tidal flat outside the mangrove area in Yingluo Bay. The quality of the iterative DEMs was evaluated via six transects of a ground-based survey using Real - time kinematic (RKT) GPS in May 2015. The mean absolute error (MAE) and root mean square error (RMSE) of the DEM decreased with an increase in the number of iterations. In this study, the DEM in the third iteration was used as the final output because the difference from the previous iterative DEM satisfied an inversion-stopping criterion. The MAE and RMSE of the final DEM with the measured data were 0.072 and 0.09 m, respectively, without considering small tidal creeks. The method used in this study can be an effective and highly precise approach for detecting and reconstructing the historical terrain of a bare tidal flat outside a mangrove area. This work also has great importance regarding intertidal resource management and the sustainable development of mangroves facing the vulnerable coastal ecological environment.

The Ocean Circulation of Chatham Sound, British Columbia, Canada: Results From Numerical Modelling Studies Using Historical Datasets

Chatham Sound, a semi-enclosed inland sea located off northern British Columbia, is an important waterway due to many proposed industrial activities related to the Port of Prince Rupert, along with its high levels of marine productivity. The oceanographic forcing in Chatham Sound is complex due to a combination of large tides, seasonally strong winds, and large freshwater discharges. Although much oceanographic data have been collected over the past six decades, past studies of the oceanographic regime of the full Chatham Sound area have been very limited. Using these extensive forcing datasets, high-resolution three-dimensional numerical modelling using the Coastal Circulation model for Sediment transport was conducted to investigate the tidal and wind-driven currents in the stratified waters of Chatham Sound. The numerical study shows the progression of the river-dominated lower salinity near-surface waters being advected northward, especially on the eastern side of Chatham Sound, with more saline waters on the western side of the Sound, especially in areas of exchanges with the waters of Hecate Strait through side channels. These surface circulation results are in good agreement with the large-scale representation of the outer diffuse Skeena River plume as seen in high-resolution Landsat satellite imagery. The model was operated over a representative year with four seasonal model runs. Southern Chatham Sound was found to be dominated by large tidal currents. Seasonal variations of residual flow were also investigated. Significant correlation between non-tidal current speeds and Skeena River discharges was found in the Skeena River delta area and through narrow tidal channels in southern Chatham Sound. In other offshore areas, non-tidal currents were found to be constrained near the surface and driven mainly by winds.

Waterbird responses to regular passage of a birdwatching tour boat: Implications for wetland management

Participation in outdoor recreation can increase support for wildlife conservation, but may also disturb wildlife. We examined responses of wintering waterbirds to the regular passage of a small boat specifically dedicated to birdwatching tours in a coastal Ramsar site in northern Spain. Disturbances were measured during two separate periods: 2006–2008 and 2012–2015. Incidence and magnitude of disturbance events were compared by grouping species based on their interest to birders (target vs. non-target) and compared across sectors of the tour route. Flight-initiation distances (FID) were used to estimate species-specific buffer zones, which can be used to manage recreational disturbance to waterbirds. We further examined relationships between species-specific traits and FID, time flying, and distance flying following disturbance. A single boat tour disturbed on average 0.3% of non-target and 2.8% of target wintering bird populations within the wetland, with the effect being more pronounced on target species due to their smaller populations. Wing loading was positively associated with distance flying after disturbance. Based on measured FIDs, we calculated an overall buffer zone for all species of 100m, and species-specific buffer zones ranging from 41 to 211m. Disturbance incidence and the number of birds disturbed per tour were both greatest in narrow tidal channels (<200m), where boats were forced to pass within 100m of waterbirds. We urge caution in allowing boat passage through tidal channels in which boat operators cannot effectively maintain recommended buffer zones between their boat and waterbirds.

Dynamic habitat corridors for marine predators; intensive use of a coastal channel by harbour seals is modulated by tidal currents.

Previous studies have found that predators utilise habitat corridors to ambush prey moving through them. In the marine environment, coastal channels effectively act as habitat corridors for prey movements, and sightings of predators in such areas suggest that they may target these for foraging. Unlike terrestrial systems where the underlying habitat structure is generally static, corridors in marine systems are in episodic flux due to water movements created by tidal processes. Although these hydrographic features can be highly complex, there is generally a predictable underlying cyclic tidal pattern to their structure. For marine predators that must find prey that is often patchy and widely distributed, the underlying temporal predictability in potential foraging opportunities in marine corridors may be important drivers in their use. Here, we used data from land-based sightings and 19 harbour seals (Phoca vitulina) tagged with high-resolution GPS telemetry to investigate the spatial and temporal distribution patterns of seals in a narrow tidal channel. These seals showed a striking pattern in their distribution; all seals spent a high proportion of their time around the narrowest point of the channel. There was also a distinctive tidal pattern in the use of the channel; sightings of seals in the water peaked during the flood tide and were at a minimum during the ebb tide. This pattern is likely to be related to prey availability and/or foraging efficiency driven by the underlying tidal pattern in the water movements through the channel.Significance StatementTo maximise foraging efficiency, predators often make use of narrow constrictions in habitat to intercept prey using these corridors for movement. In the marine environment, narrow channels may act as corridors, and sightings of predators suggest that they may target these for foraging. Despite this, there is little information on how individual predators use such areas. Here, we investigate how individual harbour seals use a narrow coastal channel subject to strong tidal currents; results showed that seals spent the majority of their time at the narrowest point of the channel foraging during peak tidal currents. This highlights the importance of narrow channels for marine predators and suggests that this usually wide-ranging predator may restrict its geographic range to forage in the channel as a result of increased prey availability and/or foraging efficiency driven by water movements through the narrow corridor.Electronic supplementary materialThe online version of this article (doi:10.1007/s00265-016-2219-7) contains supplementary material, which is available to authorized users.

Near‐bed shear stress, turbulence production and dissipation in a shallow and narrow tidal channel

AbstractNear‐bed, highly resolved velocity profiles were measured in the lower 0.03 m of the water column using acoustic Doppler profiling velocimeters in narrow tidal channels in a salt marsh. The bed shear stress was estimated from the velocity profiles using three methods: the log‐law, Reynolds stress, and shear stress derived from the turbulent kinetic energy (TKE). Bed shear stresses were largest during ebbing tide, while near‐bed velocities were larger during flooding tide. The Reynolds stress and TKE method gave similar results, while the log‐law method resulted in smaller bed shear stress values during ebbing tide. Shear stresses and turbulent kinetic energy followed a similar trend with the largest peaks during ebbing tide. The maximum turbulent kinetic energy was on the order of 1 × 10− 2 m2/s2. The fluid shear stress during flooding tide was approximately 30% of the fluid shear stress during ebbing tide. The maximum TKE‐derived shear stress was 0.7 N/m2 and 2.7 N/m2 during flooding and ebbing tide, respectively, and occurred around 0.02 m above the bed. Turbulence dissipation was estimated using the frequency spectrum and structure function methods. Turbulence dissipation estimates from both methods were maximum near the bed (~0.01 m). Both the structure function and the frequency spectrum methods resulted in maximum dissipation estimates on the order of 4 × 10− 3 m2/s3. Turbulence production exceeded turbulence dissipation at every phase of the tide, suggesting that advection and vertical diffusion are not negligible. However, turbulence production and dissipation were within a factor of 2 for 77% of the estimates. The turbulence production and dissipation decreased quickly away from the bed, suggesting that measurements higher in the water column cannot be translated directly to turbulence production and dissipation estimates near the bed. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Tidal stream characteristics on the coast of Cape Fuguei in northwestern Taiwan for a potential power generation site

Sites of high tidal current power potential are mostly found at narrow tidal passages or channels. Not many coastal sites with unbounded ocean with high tidal current speed have been reported. The coastal sea near the Cape Fuguei in northwestern Taiwan is such a site with a high tidal power potential. The purpose of this study is to investigate the tidal current speed at this location for assessing the tidal current power potential. In-situ and remote sensing instruments were deployed simultaneously to measure the temporal and spatial distributions of tidal currents. An Acoustic Doppler Current Profiler (ADCP) was deployed at a 26m depth site and an X-band marine radar was installed at a coastal building. Two measurements lasting over one month were done. Both instruments showed the maximum current speed at the surface was more than 3.5m/s in the measurement period. ADCP measurements showed that the time mean speed in the upper layer (−1.7 to −6m) was 1.5m/s. The current speed decreased with depth slightly. It was also found from both ADCP and radar measurements that in the surface layer, the probability of duration for current speed exceeding 1m/s was more than 70%. In addition, radar measurement showed that the average surface current speed at the area from 2km offshore out to 4km (radar offshore limit) was more than 1m/s. The size of this area is more than 15km2. Our investigations indicated that this area has a high tidal power potential for current turbine deployments.

Numerical analysis of stratification and destratification processes in a tidally energetic inlet with an ebb tidal delta

AbstractStratification and destratification processes in a tidally energetic, weakly stratified inlet in the Wadden Sea (south eastern North Sea) are investigated in this modeling study. Observations of current velocity and vertical density structure show strain‐induced periodic stratification for the southern shoal of the tidal channel. In contrast to this, in the nearby central region of the channel, increased stratification is already observed directly after full flood. To investigate the processes leading to this different behavior, a nested model system using GETM is set up and successfully validated against field data. The simulated density development along a cross section that includes both stations shows that cross‐channel stratification is strongly increasing during flood, such that available potential energy is released in the deeper part of the channel during flood. An analysis of the potential energy anomaly budget confirms that the early onset of vertical stratification during flood at the deeper station is mainly controlled by the stratifying cross‐channel straining of the density field. In contrast to this, in the shallow part of the channel, the relatively weak cross‐channel straining is balanced by along‐channel straining and vertical mixing. An idealized analytical model confirms the following hypothesis: The laterally convergent flood current advecting laterally stratified water masses from the shallow and wide ebb tidal delta to the deep and narrow tidal channel has the tendency to substantially increase cross‐channel density gradients in the tidal channel. This process leads to stratification during flood.

Modeling the Effects of Tidal Energy Extraction on Estuarine Hydrodynamics in a Stratified Estuary

A 3-D coastal ocean model with a tidal turbine module was used in this paper to study the effects of tidal energy extraction on temperature and salinity stratification and density-driven two-layer estuarine circulation. Numerical experiments with various turbine array configurations were carried out to investigate the changes in tidally averaged temperature, salinity, and velocity profiles in an idealized stratified estuary that connects to coastal water through a narrow tidal channel. The model was driven by tides, river inflow, and sea surface heat flux. To represent the realistic size of commercial tidal farms, model simulations were conducted based on a small percentage (less than 10 %) of the total number of turbines that would generate the maximum extractable energy in the system. Model results show that extraction of tidal in-stream energy will increase the vertical mixing and decrease the stratification in the estuary. Installation of in-stream tidal farm will cause a phase lag in tidal wave, which leads to large differences in tidal currents between baseline and tidal farm conditions. Extraction of tidal energy in an estuarine system has stronger impact on the tidally averaged salinity, temperature, and velocity in the surface layer than the bottom layer even though the turbine hub height is close to the bottom. Finally, model results also indicate that extraction of tidal energy weakens the two-layer estuarine circulation, especially during neap tides when tidal mixing is weakest and energy extraction is smallest.

A numerical study of tidal asymmetry in Okatee Creek, South Carolina

The Okatee River, South Carolina is characterized by a narrow tidal channel and an extensive area of intertidal salt marshes. Current measurements in the upstream portion Okatee Creek show that tidal flow features an asymmetric pattern: ebb current is stronger than flood current. The ebb dominance is mainly caused by deformation of the dominant astronomical tidal constituent M 2. An unstructured grid, finite volume coastal ocean model (FVCOM) with wet-dry point treatment method is applied to examine physical mechanisms of M 4 overtide generation. Model experiments show that mean absolute amplitude and phase errors are 3.1 cm and 1.7 ° for M 2 elevation, 2.4 cm s −1 and 0.8 ° for M 2 current major axis, 2.1 cm and 1.8 ° for M 4 elevation, and 2.1 cm s −1 and 24.6 ° for M 4 current major axis. The overall pattern of tidal asymmetry is qualitatively reproduced. Various sensitivity experiments suggest that the generation of M 4 overtide is a result of nonlinear interaction of tidal currents with irregular creek geometry and bottom topography. Consistent with the classical view, the large volume of intertidal water storage is the major reason for ebb dominance in the creek. However, the zero-inertia assumption (i.e., negligible advective terms) is probably not valid for the entire tidal cycle. Besides the pressure gradient force and the bottom friction force, terms related to lateral shear of the along-estuary velocity (i.e., advective inertia and horizontal eddy viscosity) may also contribute in horizontal momentum balance. Exclusion of the flooding-draining processes over the intertidal zone will severely underestimate tidal currents in the river channel and make the tidal asymmetry less prominent.

Estimation of drag coefficient in James River Estuary using tidal velocity data from a vessel‐towed ADCP

A phase‐matching method is introduced to calculate the bottom drag coefficient in tidal channels with significant lateral variation of depth. The method is based on the fact that the bottom friction in a tidal channel causes tidal velocity to have a phase difference across the channel. The calculation involves a few steps. First, the observed horizontal velocity components are analyzed to obtain the amplitude and phase of the velocity at the major tidal frequency. The phase of the longitudinal velocity is then fitted to a relationship derived from the linearized momentum balance. The drag coefficient is then calculated. This method is applicable to narrow (approximately a few kilometers wide) tidal channels without strong stratification and where the cross‐channel variation of surface elevation is negligible compared to tidal amplitude. This analytic approach is easy to implement when appropriate observational data are available. It allows a spatial variation of the drag coefficient, and the resolution is only limited by that of the observations. The method is validated by identical twin experiments and applied to tidal velocity data, obtained in the James River Estuary, using an acoustic Doppler current profiler during spring tides and neap tides in October–November 1996. The obtained bottom drag coefficient ranged from 1.2 × 10−3 to 6.9 × 10−3 at different positions along two cross‐channel transects each 4 km long and 2 to 14 m deep. The maximum drag coefficient is found in the shallowest water near the banks of the estuary, while the minimum values occur between 9 and 12 m in the center of the channel. The friction of the lateral boundary may have contributed to the apparent increase of the bottom friction on the banks. The transverse mean values of the drag coefficient ranges between 2.2 and 2.3 × 10−3 for the spring and neap tides, respectively.

3D Analytic Model for Testing Numerical Tidal Models

A 3D analytic solution is presented for tides in channels with arbitrary lateral depth variation. The solution is valid for narrow channels in which the lateral variation of the amplitude of tidal elevation is small. The error introduced by the solution is on the order of a few percent in a tidal channel of a few kilometers in width. The solution allows an arbitrary lateral depth variation and thus provides a wide choice of depth functions, especially those with large bottom slopes. The largest amplitude of the along-estuary velocity appears on the surface in the deepest water. The depth-averaged velocity is the largest in the deepest water. The time of flood (ebb) in deep water lags that in shallow water. The time of flood (ebb) on the surface lags that at the bottom. Since this solution is simple and allows arbitrary lateral depth variations, it can be used to demonstrate the first-order tidal flow in narrow tidal channels of variable depth, and to test high-resolution numerical models with large depth gradients.

Tides, tidal currents and their effects on the intertidal ecosystem of the southern bay, Inhaca Island, Mozambique.

Sediment characteristics and tidal currents were studied in the 1500 ha intertidal area south of Inhaca Island,Mozambique. The tide is semi-diurnal with a range at spring of about 3 m. The area connects directly to the oceanthrough the Ponta Torres Strait and (indirectly) through several narrow tidal channels ending up in Maputo Bay.Velocities of up to 0.75 m s-1 were measured in the Ponta Torres strait. After Low Water, Indian ocean water startsentering the tidal flats, while entrance from the Maputo Bay side is delayed by one to several hours. With onlyone tide gauge and one current meter at hand, we found indications of a higher mean sea level on the oceanic side(probably due to ocean wave set up) and a mean flow in the Ponta Torres Strait directed towards Maputo Bay. Astorm from the SW had a particularly strong influence on the sea level of Maputo Bay, and for a period of a fewdays the current flowed constantly towards Maputo Bay. Measurements in the innermost Saco channel indicatedhigher velocities during ebb than during flood. The resulting residual current will increase sedimentation in the bay,a process which is confirmed by aerial photographs and could be linked to the sanding of the coral reefs and themangrove extension in the area. Extensive sandbanks are situated around both inlets as a result of the higher currentvelocities in these areas, thereby inhibiting the settlement of finer sediments and organic matter. Where the twoflows meet, formation of mudflats and seagrass beds are favoured. Mudflats are found mainly below High WaterNeap, exposed only during spring tides. This longer inundation period favours the settlement of finer material andorganic matter. The area and location of the substrates have great influence on the richness and abundance of thebenthic fauna, as grainsize and organic matter have been correlated with richness and abundance in other studies.

Remote sensing application in the study of migration and offset of coastal inlets

The coastal zone of Kerala with moderate energy, monsoonal-storm-dominated wave climate, together with a microtidal range, falls under the major tectonic class of the Amerotrailing edge coasts. In order to have an effective management of this coastal zone, one requires detailed information on various types of coastal landforms and the related processes acting on them. Coastal inlets are one among such landforms, which are relatively short and narrow tidal channels connecting bays and lagoons to the ocean. Coastal inlets and the related landforms form an important type of coastal feature. An examination of coastal maps from any area in the world will reveal that coastal inlets are of migratory and offset in nature. In the present study the migration pattern and offset behaviour of some of the inlets of Kerala coast have been examined using IRS-IA LISS II data and Survey of India topographic sheets. The study reveals that geocoded IRS-IA LISS II data could provide accurate geometrical information which may considerably minimise the field check.