Tidal Prism Research Articles

Managing Tidal Inlets

Sandy inlets are in a dynamic equilibrium between wave-driven littoral drift acting to close them, and tidal flows keeping them open. Their beds are in a continual state of suspension and deposition, so their bathymetry and even location are always in flux. Even so, a nearly linear relationship between an inlet’s cross-sectional flow area and the inshore tidal prism is maintained - except when major wind and/or runoff events act to close or widen an inlet. Inlet location can be stabilized by jetties, but dredging may still be necessary to maintain a navigable channel. Armoring with rock large enough to resist erosion can protect an inlet bed or river mouth from excessive storm flow erosion. Armoring can also be used as a stratagem to close inlets.

Evaluations of submarine groundwater discharge and associated heavy metal fluxes in Bohai Bay, China

Submarine groundwater discharge (SGD) has been recognized as an important source of dissolved heavy metals to the coastal ocean. Bohai Bay, the second largest bay of Bohai Sea in China, is subjected to serious environmental problems. However, SGD and SGD-derived heavy metal fluxes in the bay are seldom reported. In this study, we present mass balance models considering the radium losses caused by recirculated seawater to estimate water age, SGD and SGD-derived heavy metal fluxes in Bohai Bay during May 2017. The water age is estimated to be 56.7–85.0 days based on tidal prism model. By combining water and salt mass balance models, submarine fresh groundwater discharge (SFGD) is estimated to be (3.5–9.3) × 107 m3 d−1. The SGD flux estimated by the radium mass balance models is (3.2–7.7) × 108 m3 d−1, an order of magnitude larger than the discharge of the Yellow River during the sampling period. SGD-derived heavy metal fluxes were estimated to be (0.2–6.0) × 107 mol d−1 for Fe, (1.2–2.7) × 107 mol d−1 for Mn, (3.0–8.2) × 105 mol d−1 for Zn, (2.7–7.4) × 104 mol d−1 for Cr and (0.6–1.8) × 103 mol d−1 for Cd, which are significantly higher than those from local rivers. This study reveals that SGD is a significant source of heavy metals (Mn, Zn and Fe) into Bohai Bay, which may have important influences on the metal budgets and ecological environments in coastal areas.

Flow Reorganization in an Anthropogenically Modified Tidal Channel Network: An Example From the Southwestern Ganges‐Brahmaputra‐Meghna Delta

We examine variations in discharge exchange between two parallel, 1‐ to 2‐km‐wide tidal channels (the Shibsa and the Pussur) in southwestern Bangladesh over spring‐neap, and historical timescales. Our objective is to evaluate how large‐scale, interconnected tidal channel networks respond to anthropogenic perturbation. The study area spans the boundary between the pristine Sundarbans Reserved Forest, where regular inundation of the intertidal platform maintains the fluvially abandoned delta plain, and the anthropogenically modified region to the north, where earthen embankments sequester large areas of formerly intertidal landscape. Estimates of tidal response to the embankment‐driven reduction in basin volume, and hence tidal prism, predict a corresponding decrease in size of the mainstem Shibsa channel, yet the Shibsa is widening and locally scouring even as the interconnected Pussur channel faces rapid shoaling. Rather, the Shibsa has maintained or even increased its pre‐polder tidal prism by capturing a large portion of the Pussur's basin via several “transverse” channels that are themselves widening and deepening. We propose that an enhanced tidal setup in the Pussur and the elimination of an effective Shibsa‐Pussur flow barrier are driving this basin capture event. These results illustrate previously unrecognized channel interactions and emphasize the importance of flow reorganization in response to perturbations of interconnected, multichannel tidal networks that characterize several large tidal delta plains worldwide.

A modeling study of light extinction due to cohesive sediments in a shallow coastal lagoon under well mixed conditions

Cohesive sediments play an important ecological role in coastal systems, affecting light penetration in the water column and consequently the local biogeochemical processes. The main aim of this work is to study the water-column light extinction coefficient (k) dependence on cohesive sediment dynamics within Ria de Aveiro lagoon under sea level rise and extreme freshwater discharges events. A coupled estuarine hydrodynamic and transport model is explored, following previous modeling frameworks developed for the Ria de Aveiro. Three scenarios were simulated: a reference scenario (SC1), wherein present hydrodynamic, mean sea level and present freshwater discharges were used to simulate present hydrodynamic and cohesive sediments; a scenario considering local projections of sea level rise of 0.42 m (SC2); and a third one corresponding to maximum freshwater inflow for the lagoon's tributaries (SC3). Results show higher concentrations of cohesive sediments near the rivers mouths, gradually decreasing toward the lagoon's inlet. From SC2 scenario was found that the effect of sea level rise in sediment dynamics and in water-column light extinction coefficient is not significant. However, the tidal prism increase in this case induces a dilution of the sediments concentration from fluvial origin and consequently a slightly k decrease. Contrarily, in SC3 scenario, the extreme freshwater inflow enhances the cohesive sediment concentration in the entire lagoon, limiting the light penetration in the water column. In summary, extreme river discharge events have a highest impact on water-column light penetration in Ria the Aveiro than predicted sea level rise.

Recent evolution of the tidal prisms at the inlets of the western sector of the Ria Formosa, south coast of Portugal

The Ria Formosa is a coastal lagoon comprising six tidal inlets. The western sector encompassing three inlets, Ancão, Faro–Olhão and Armona is the most important one since it represents approximately 90% of the total tidal prism. The Ancão inlet is characterized by a cyclic eastward migration pattern, but until now the knowledge of the real impact of its dynamics in the last 20 years in terms of tidal prisms and interconnectivity between these three inlets is scarce. This work presents the most exhaustive data set acquired so far, comprising 25 field surveys led in the period 2004–2013 on the western sector of Ria Formosa. This provides a long-term analysis of the recent temporal evolution of the tidal prisms from the three inlets during a single migration cycle (18 years) of the Ancão inlet. It determines the relative contribution of the three inlets to the total prism of the western sector of Ria Formosa before the last artificial opening of the Ancão inlet. Results show that at Ancão inlet the tidal prism decreased in spring tide mainly due to its eastward migration. The other natural inlet, Armona inlet, depict a progressive narrowing responsible for the decrease of the tidal prism in neap tide while during the same tidal phase, at the Faro–Olhão inlet, the most important one in terms of exchanges, the tidal prism increased. Those alterations at the three inlets could be related to morphodynamic changes and morphological evolution and confirmed the strong interconnectivity between them to compensate the variations of their relative contributions to the total prism. The tidal prisms errors were low for all cases (< 10%) and confirm that, globally, there was an increase of the prisms with time.

Impacts of human interventions on the evolution of the Ria Formosa barrier island system (S. Portugal)

Human interventions on sandy barriers disturb natural barrier dynamics, to the extent of having become key forces in modifying geomorphological evolution. This work identifies natural and human-induced drivers and analyses their importance to the multi-decadal evolution of the Ria Formosa barrier island system, in South Portugal. Aerial photographs from the last six decades and historical maps are used to assess changes in cross-shore rates, morphological characteristics (barrier and dune widths, inlet morphology and migration) and barrier areas, through systematic methods that can be easily transferred to other barrier systems. Interventions, and especially hard engineering ones (jetties, inlet stabilisations), affected barrier evolution trends. Shore-perpendicular works increased shoreline progradation updrift and initiated coastal retreat downdrift, with strongest erosive impacts along the edges of the system. Inlet stabilisations changed tidal inlet hydrodynamics and initiated ebb-shoal attachment to the barriers on either side of a non-migrating inlet that experienced loss of tidal prism. This shoal attachment was the main factor for the increase in total barrier area of Ria Formosa during the 60 years of analysis. Barrier growth after 2005 was slower, which could indicate that the system is reaching morphodynamic stability.

Impacts of Sea Level Rise and River Discharge on the Hydrodynamics Characteristics of Jakarta Bay (Indonesia)

Jakarta city has been vulnerable to sea level rise and flooding for many years. A Giant Seawall (GSW) was proposed in Jakarta Bay to protect the city. The impacts of sea level rise and river discharge on the tidal dynamics in Jakarta Bay and flooding areas in Jakarta city were investigated using the finite-volume coastal ocean model (FVCOM). Model results showed that the bay is diurnally dominated by the K1 tidal component. The diurnal tides propagate westward, while the semidiurnal tides propagate eastward in the bay. The rise of sea level increases the diurnal tidal component and the inundation areas due to the increased tidal forcing: when considering a sea level rise of 0.6 m, the K1 amplitude increases by ~1% (0.25 cm) near the coastline and the current magnitude increases by 16.6% (0.05 m/s). The inundation area increases with the sea level rise in the low land elevation areas occurring near the coastlines: the inundation area increased by 29.68 km2 (7.1%) with a sea level rise of 0.6 m. The increase of river discharge amplified the diurnal tidal component as well as the inundation areas at the river mouth due to increased fluvial forcing: if 10 times the mean river discharge occurs, the K1 amplitude increases by ~1% (0.25 cm) and the current magnitude increases by 100% (0.4 m/s), and the inundation areas increase by 26.61 km2 (6.2%). The K1 tidal phase remains almost unchanged under both the sea level rise and river discharge conditions. The combined increase of sea level rise and the river discharge amplifies the inundation areas and the tidal currents due to increased tidal and fluvial forcing. The construction of GSW would decrease the tidal prism and dissipation effects of the bay, thus slightly increasing the K1 amplitude of the tidal level: by less than 1% (0.2 cm). There would be no significant change of phase lag for the K1 component. Although this study is site specific, the findings could be applied more widely to any open-type bays.

Anatomy of a transgressive tidal inlet reconstructed through high-resolution seismic profiling

Although the morphology and morphodynamics of modern tidal inlets have been analysed in a wealth of studies, these systems can be considered representative only for highstand conditions, while little is known about the characteristics and evolution of their transgressive counterparts. Through the analysis of high-resolution seismo-acoustic profiles, this work investigates a rare example of a preserved filled tidal inlet formed during the early Holocene on the continental shelf of the northern Adriatic Sea. The quantity and quality of the available data allowed a detailed reconstruction of the morphology and internal architecture of this feature. This 1.2 km-long, 17 m-thick channelized scour formed in a microtidal environment between 9.0 and 9.5 kyr cal BP. The tidal prism is estimated to have been ca. 4.4 × 107 m3. The internal geometry of the filling material shows no evidence of migration of the inlet, which lasted only for a few centuries and was soon after drowned by the sea transgression. This rapid evolution is in contrast with the millennial-scale lifespan of most of the modern highstand lagoon systems. This work provides a new outlook on the evolution of tidal inlets during phases of marine transgression and in relation to variable rates of sea-level rise. In many cases, paleo tidal inlets are the only witnesses of the presence of transgressive barrier-lagoon systems which had been completely eroded by the wave action after their submersion. These features represent therefore an important proxy to infer the position of the related coastline and to constrain the past relative sea level.

Tidal strait to embayment: Seismic stratigraphy and evolution of a rock-bounded embayment in the context of Holocene sea level change

Elongate, rock-framed Santa Catarina Island partly encloses a low-energy marine embayment that communicates with the ocean via narrow, rock-bounded inlets with ebb and flood-tide deltas. The bay shoreline consists of low-energy sandy beaches, strandplains, mangroves and bayhead deltas while the exposed Atlantic shoreline comprises a series of headland-embayment littoral cells in a linked, north-directed longshore drift system.New and previously published seismic stratigraphic investigations enable assessment of the palaeogeographic/geomorphic evolution of the area since the early Holocene. At a sea level of ca. −50 m a sandy mainland-attached shoreline complex (dune and beach) developed on a coastal plain setting some 6–7 km offshore of the present shoreline. North of the island, a barrier island and tombolo were formed in a coastal re-entrant adjacent to the northern entrance to Florianopolis Bay. The open coast beach/dune system was overstepped by rapid sea-level rise during MWP-1B whereas the barrier island and tombolo continued to accrete until MWP-1C when they too were overstepped.The back-island embayment contains up to 40 m of sediment overlying an irregular bedrock surface. It was initially flooded during MWP-1C and the early muddy sedimentation suggests a distal, sheltered marine embayment. The subsequent development of many large bioherms on bedrock and mud suggests a change to more fully marine conditions as sea-level continued to rise. In the North Bay, these were subsequently enveloped by tidal dunes migrating north from the mid-bay. The vertical stacking of 3-D and 2-D sandy bedforms suggests that a tidal strait had developed with strong tidal currents radiating from a constriction in the vicinity of modern Florianopolis City. The strong tidal currents may be linked to the Holocene sea-level high that is well documented for this region.A fall in sea level from this highstand, coupled with accumulation of tidal strait sediments, reduced the tidal prism and strength of tidal currents in the bay. At the same time, regressive beachridge plains developed in open coast and bay environments. This coincided with development of the contemporary tidal inlet complexes at the north and south outlets of the bay. As a consequence, the upper surface of the tidal strait sand deposits experienced tidal ravinement and the bay switched to a sheltered back-barrier environment dominated by deposition of mud and silt which blanket the contemporary bay floor. The tidal inlets sequestered sand from the littoral drift system in ebb and flood-tide deltas, which created breaks in the longshore drift system, causing littoral sediment to accumulate at the north of the island in a series of spits, and promoting strandplain development south of the island.

The role of porewater exchange as a driver of CO2 flux to the atmosphere in a temperate estuary (Squamish, Canada)

Porewater exchange is an important yet poorly understood component of the coastal carbon cycle. Here, a high-resolution automated radon (222Rn, a natural porewater tracer) and CO2 time series was conducted in the Squamish Central Estuary (Canada) over eight consecutive tidal cycles to assess the relative importance of porewater exchange on estuarine carbon dynamics. Radon and CO2 observations revealed a clear tidal trend which is indicative of porewater exchange driven by tidal pumping. A radon mass balance indicated an average porewater exchange rate of 14.9 cm day−1 (4.3% of the tidal prism). The estuary was a net source of CO2 to the atmosphere (average 212 ± 19 mmol m−2 day−1). Porewater exchange accounted for 9%, 5% and 30% of net dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and CO2 exported out of the Squamish Central Estuary, respectively, while porewater inputs of free CO2 accounted for 38% of the atmospheric evasion. These flux estimates as well as strong correlations between pCO2 and 222Rn suggest that porewater exchange has a strong influence on CO2 concentrations in the estuary even though they are a small contributor to overall DIC fluxes.

Flow Division Dynamics in the Mekong Delta: Application of a 1D-2D Coupled Model

The Mekong Delta constitutes a complicated multi-channel estuarine system, exchanging water with a delta-wide irrigation system. A 1D–2DH coupled numerical domain is calibrated and validated for water level and discharge during the dry season. This approach benefits from the simplicity of a 1D network within the estuarine and irrigation systems, while maintaining the interaction with the spatial tidal dynamics of the 2DH coastal domain. First, the role of the irrigation system on tidal dynamics is quantified; then, tidal propagation, freshwater budget, and the effect of offshore subtidal water level on discharge division are investigated. The results show that the complex irrigation system, in a friction-like manner, reduces the tidal amplitude up to 25%. The channels aggregate to 1% of the total water volume in the delta, while accommodating up to 10% of the tidal prism. Tidal amplitude reduces upstream, while subtidal water level is highly sensitive to upstream discharge, spring–neap cycles, and wind-generated offshore surge. Although cumulative discharge division within the estuarine network is consistent, temporal discharge division can be significantly sensitive to offshore wind-surge. During the dry season, it can reverse the expected subtidal discharge division within the time-scale of a few days and potentially influence salt intrusion.

Hydrodynamic changes due to coastal reclamation activities in Daya Bay

Based on the geometry and surface in Daya Bay, we artificially divided the reclamation projects into three periods to analyze the influences and changes on hydrodynamic conditions as a result of the reclamation projects. Three periods of tidal current fields, tidal prisms, and water exchange capacity are simulated by the Finite-Volume, primitive equation Community Ocean Model and the characteristics and trends of hydrodynamics in Daya Bay are discussed. The combination of observation and simulation in this paper gives a good description on the tidal dynamic system in Daya Bay. As indicated by model results, the tidal current velocity in the Bay totally decreases after numerous activities associated with reclamation construction. The decreasing current velocity region is mainly distributed near the Xiachong and Gangkou chain islands. The current velocity in 2015 decreases by approximately 5 cm s−1 compared with velocities before 2000. Future reclamation activities will exacerbate these decreasing current velocity trends in some regions. Compared with 2015, the tidal prism has significantly decreased by 1.3622 × 107 m3 due to planned reclamation. The half-water exchange times for Daya Bay in 2015 and after planned reclamation are 178.9 and 177.4 days, respectively. The water exchange capacity in Fanhe Harbor is weaker than other water fields throughout Daya Bay.

Three Decades of Sea-Ice Variability in Jiaozhou Bay Revealed by Landsat Observations

In this study, we used Landsat images and meteorological data to examine the spatiotemporal distribution and variability of sea ice in Jiaozhou Bay (JZB) between 1986 and 2016. The results show that JZB is not always covered by sea ice in winter, but in some extreme cases, sea ice has covered more than one-third of the sea area of the bay. Sea ice in JZB has generally formed between January 1 and February 5, primarily along the coast, and gradually expanding to the central area of the bay. Both meteorological and artificial factors have played important roles in modulating the sea ice distribution. We found sea ice coverage to have been strongly correlated with the accumulated freezing-degree days nine days before the occurrence of sea ice (R2 = 0.767). North-northwest surface winds have dominated the freezing period of sea water in the JZB, and wind speed has exerted a more significant influence on the formation of sea ice when the sea ice coverage has been generally small. Additionally, artificial factors began to affect the expansion of sea ice in JZB since 2007. The construction of the Jiao-Zhou-Bay Bridge (JZBB) is believed to have retarded water flow and reduced the tidal prism, thereby leading to the formation of an ice bridge along the JZBB, which effectively prevents the southward expansion of sea ice.

Tide propagation and salinity distribution response to changes in water depth and channel network in the Guadalquivir River Estuary: An exploratory model approach

This study analyzes the management alternatives that are currently considered by public administrations in the Guadalquivir River Estuary, which is a well-mixed and rather channelized estuary in the southwest of Spain. A linearized multichannel exploratory model that operates at tidal and averaged scales is used. The model extends the previous developments for calculating the tidal variations of salinity and provides elevations, currents and salinity. The effects of the change in mean depth of the main navigation channel, the reconnection of a secondary tidal channel (Brazo del Oeste) and the recovery of a marsh in the lower part of the estuary are the three alternatives analyzed. The model outputs indicate that the channel deepening would produce an increase in the amplitudes of the elevations, tidal salinities, and tidal prisms, as well as the extent of the saline intrusion. Similar effects could be alternatively obtained with the reconnection of the secondary channel with suitable design parameters. The increase in tidal elevations due to channel deepening should therefore be considered in any dredging project, because lower sediment volumes need to be dredged to attain the projected water column height at high water. The recovery of marshes notably influences the tidal wave propagation when the connecting width with the navigation channel exceeds one hundred meters. Its most remarkable effect is to increase the tidal amplitudes and to reduce the tidal currents upstream the connection point in proportion to the degree of connectivity with the main channel. A combination of alternatives could potentially serve to mitigate the specific effects of individual alternatives. The effects of channel deepening on the amplitudes of the tidal elevation and the currents could be partially compensated for through the recovery of tidal marshes.

Reconstructing basin-scale drainage dynamics with regional subsurface mapping and channel-bar scaling, Aptian, Western Canada Foreland Basin

An extensive subsurface dataset from the Lower Cretaceous McMurray Formation of northeastern Alberta, Canada, is leveraged to provide new insights into the Aptian drainage arrangement of the Western Canada Foreland Basin. Regional-scale stratigraphic mapping is supplemented with paleodrainage area estimates obtained using quantitative relationships between fluvial bankfull flow depth and drainage area in order to reconstruct paleocatchments, including those of a continental-scale river system and its tributaries. Using channel geometries delineated in high-resolution 3D seismic time slices, we test for evidence of a tidal prism within Aptian-aged drainage systems in order to assess the applicability of fluvial scaling relationships to the McMurray Formation.Refined paleogeographic reconstructions incorporate catchment estimates for the southerly-sourced trunk drainage system of the McMurray Formation, which corroborate the claim that the unit was associated with a continental-scale drainage network. Drainage area estimates for southwesterly-sourced tributary systems are, however, up to an order of magnitude smaller. These tributaries likely had headwaters in the North American Cordillera. This work demonstrates the applicability of combining regional mapping, sedimentological analysis, and fluvial scaling relationships to reconstruct sediment-routing systems from source-to-sink in the deep time record.

Field Measurements for Investigating the Dynamics of the Tidal Prism during a Spring-Neap Tidal Cycle in Jiaozhou Bay, China

Xiao, K.; Li, H.; Song, D.; Chen, Y.; Wilson, A.M.; Shananan, M.; Li, G., and Huang, Y., 2019. Field measurements for investigating the dynamics of the tidal prism during a spring-neap tidal cycle in Jiaozhou Bay, China. Journal of Coastal Research, 35(2), 335–347. Coconut Creek (Florida), ISSN 0749-0208.The tidal prism is a crucial environmental index for evaluating the water self-purification capacity in semi-enclosed bay systems, but quantitative studies linking spring-neap tidal variations are few. In this paper, field measurements of the tidal level and tidal flux were conducted in Jiaozhou Bay (JZB) to estimate the hydrodynamic and tidal prism variations during a spring-neap tidal cycle. JZB is located on the south coast of Shandong Peninsula, China, with a narrow inlet of 3.1 km width, which was accessible for continuous shipboard Acoustic Doppler Current Profiler measurements over 15 days. Unfortunately, due to the violent sea induced by typhoon “Chan-hom,” the measurements were suspended for 2 days. However, the fluxes during the temporary suspension were reproduced using the significant linear relationship between the inlet fluxes and the changes in tidal level (r2 = 0.94, P < 0.001), without considering the storm surge. The relationship between tidal range and tidal prism was also linearly correlative (r2 = 0.69, P < 0.001), indicating that the tidal prism was primarily controlled by the tidal range in JZB. These linear relationships are well verified by historical data in JZB. Compared with previous studies in JZB, the tidal prism has decreased 26% from 1928 to 2008. The decreased intertidal areas, caused by large-scale land reclamations, were likely responsible for the decrease in the tidal prism, which in turn weakened the self-purification capability and triggered the chronic pollutant enrichment. However, the tidal prism increased 4% from 2008 to 2015 as the restoration of water area through protective policies formulated by government. These findings will contribute to designing maintenance strategies for reducing human activity impacts on the health of JZB in the future.

Nearfield erosion at the steart marshes (UK) managed realignment scheme following opening

The ingress and egress of water from managed realignment sites in macrotidal estuaries commonly causes erosion at the breaches and the formation of channels across the fronting intertidal areas. However, the rate of development of these features and their final dimensions have not been well documented. This case study paper details the development of the breach area and the erosional processes in the main and exit channels associated with the Steart Marshes managed realignment site in south west England. The paper shows type of data, along with its accuracy, which is currently available for this type of study. It uses a combination of UAV and LiDAR data that was collected as part of a monitoring campaign to record the development of scheme over a period of 3 years and 7 months following site opening. This data would normally reside within consultancy reports and thus be unavailable to the wider scientific community. The paper demonstrates that further work is needed to improve the prediction of erosion rates associated with managed realignment schemes.The monitoring data at Steart Marshes show the development of a large exit channel along the line of the former channel that led across the existing intertidal mudflats. The highest rates of erosion occurred in the first 3 months following opening. After this time rates declined markedly and, approximately 7 months after opening, the breach and exit channel region began to show periods of accretion as well as erosion. The non-linearity in erosion rates is similar to previously reported sites, but the rates of development were more rapid due to higher flow speeds generated by large tidal prism and the single breach design.Although the volume of the exit channel region appears to have stabilised, a secondary channel is starting to form, which suggests that further changes are likely. Within the site, further recession of the step feature is expected before a stable condition is reached. The paper documents these changes in more detail and draws out those behaviours that are likely to apply to other schemes.

Sediment transport trends and cross-sectional stability of a lagoonal tidal inlet on the Central Coast of Vietnam

Lagoonal tidal inlets are a typical morphology of the Central Coast of Vietnam. Recently, navigation channels in these inlets have become increasingly threatened by siltation. This study analyses the relations between sediment distribution and transport trends (using the technique of Sediment Trend Analysis-STA®) in the lagoonal system of the De Gi inlet and then proposes appropriate countermeasures against sand deposition in the navigation channel. The STA identified three types of transport trends in the De Gi inlet, namely dynamic equilibrium, net accretion, and net erosion. Processes associated with the tidal prism have resulted in trends of sediment transport and deposition across the flood and ebb tidal shoals, which maintain a present cross-sectional area of about 1000 m2. However, longshore sediment transport from north to south resulting from northeast waves cause additional sand deposition in the channel. In addition, the effects of refraction associated with a nearby headland and jetty also increase sedimentation. These processes provide the main reasons for sediment deposition in the De Gi inlet. Short term and regular dredging helps to maintain the navigation channel. A system comprised of three jetties (north, south, and weir) is necessary to ensure the longterm cross-sectional stability of the navigation channel.

EFFECTS OF STORM SURGE ON WATER LEVELS OF THE LAGOON SYSTEM OF JACAREPAGUÁ – RIO DE JANEIRO, BRAZIL

The Lagoon System of Jacarepaguá is the most vulnerable coastal area of Brazil due to its high population density and important economic activities. Severe meteorological conditions due to climate changes are more likely to affect the lagoon system in the future, increasing the exposure of the area and the probability of flooding of the low-lying surrounding areas. To determine the vulnerability of the area to diverse agents, this work addressed the impact of different combinations of sea level rise, heavy rainfall and storm surges. The study cases considered two different bathymetry conditions, the actual silting bathymetry, and the resulting bathymetry after a planned dredging project. Tidal prism, the maximum water elevations and the time of occurrence were analyzed. The main results showed that storm surge has the most impact on the maximum water elevations, overcoming the impact of an increase in the sea level, river flow and changes in bathymetry. The results of time lags comparing the time of occurrence of maximum elevation recorded at the open sea and the time of occurrence of maximum elevation on the north margin, the most populated area, of the lagoon system showed a time lag of 13-17h. The benefits of the planned dredging project would be mostly to allow a better water renovation in the lagoons, due to a higher tidal prism.

A SEMI-ANALYTIC MODEL OF TIDAL INLETS AND THEIR EVOLUTION

The water exchange between the sea and a lagoon or bay through an inlet due to tides is a classical topic that has been investigated in a large number of studies (Keulegan, 1967; O’Brien and Dean, 1972; O’Brien and Clark, 1974; Escoffier, 1977). In this paper, a simple semi-analytic model of the flow induced by tides through an inlet connecting the sea to a lagoon or bay is developed. The model is employed to derive explicit expressions for key parameters associated with inlet flows such as bay water level amplitude, tidal prism, maximum inlet velocity, and mixing (retention) time. Also, the inlet flow model is combined with a sediment transport model to determine the conditions for equilibrium as well as the evolution of the inlet cross-sectional area towards equilibrium or closure.