Tidal Prism Research Articles

Nutrients and particulate matter exchanges through the Ria Formosa coastal lagoon, Portugal

Cravo, A., Cardeira, S., Pereira, C., Rosa, M., Madureira, M., Rita, F., Luís, J. and Jacob, J., 2013. Nutrients and particulate matter exchanges through the Ria Formosa coastal lagoon.The Ria Formosa lagoon (south coast of Portugal) is a highly productive shallow mesotidal system of high water renewal through six permanent connections to the ocean. The three inlets in the western sector represent ~ 90% of the total tidal prism. To understand spatial and temporal dynamics of nutrients, chlorophyll a (proxy of phytoplankton) and particulate matter, data acquisition of these parameters along with current velocity was conducted hourly during two consecutive semi-diurnal spring and neap tidal cycles (~12.5 h) in the three inlets of Ria Formosa western sector, in late autumn of 2011. In all three inlets, nutrients ranges were typical for this time of the year, varying in antiphase relative to tidal signal, although less evident during the neap tide. Chlorophyll a and suspended solids concentrations were lower during the neap tidal cycle with less evident tidal variation than for nutrients. Net transport results show that Ria Formosa plays an important role in the mass export of nutrients to the adjacent coastal area, particularly during spring tides, with a greater contribution by the Faro-Olhão inlet, followed by the Armona and Ancão inlets. These two, regardless of the residual tidal prism, export nutrients while suspended solids were imported. The Faro-Olhão inlet presented different behaviour: it acts as a flood dominated inlet importing chlorophyll a and nitrate, and exporting ammonium and suspended solids. Nonetheless, values can change due to tidal variability, asymmetry and distortion, and interconnection of inlets inside the lagoon. To further understand the seasonal variation of this system more surveys will be conducted.

Storm surge impact in the hydrodynamics of a tidal lagoon: the case of Ria de Aveiro

Picado, A., Lopes, C.L., Mendes, R., Vaz, N. and Dias, J.M., 2013. Storm surge impact in the hydrodynamics of a tidal lagoon: the case of Ria de Aveiro.Storm surges are a hazardous phenomenon, since they may flood large coastal areas, causing socio-economical and habitation losses. Thus, the study of their characteristics and effects in coastal regions is crucial to prevent their negative consequences. This work aims at assessing the storm surges impact in the hydrodynamics of a tidal lagoon located in the north-western Portuguese coast (Ria de Aveiro). Storm surge amplitudes of 0.58 m, 0.84 m and 1.17 m for 2, 10 and 100 return periods, respectively, were determined adjusting the annual maximum amplitudes to a Generalized Extreme Value (GEV) distribution. To assess the hydrodynamic changes in the Ria de Aveiro under storm surge conditions, numerical modeling simulations were carried out, considering four scenarios: a single astronomical tidal forcing (reference) and astronomical tide plus 2, 10 and 100 years return period surges. Maximum levels and velocities for the entire lagoon and the tidal prism for the main cross-sections were determined and compared with the reference scenario. Generally, the model results suggest that during storm surge events the maximum levels increase in whole domain, with the largest increase found for the 100 return period storm surge scenario (1.17 m). The most significant changes occur at the main channels head for all scenarios, revealing that these regions are the most vulnerable to marginal flooding. Also, storm surges induce higher velocities and tidal prisms in the lagoon, increasing the marginal risk of erosion, as well as the salinization of the lagoon marginal lands.

Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa

Bond, J., Green, A.N., Cooper, J.A.G. and Humphries, M.S., 2013. Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 446-451, ISSN 0749-0208. The inlet dynamics of estuary and lake systems with ephemeral inlets have been little studied compared to systems with semi-permanent tidal inlets. Here we document the meso-scale dynamics of a barrier and its associated ephemeral inlet. The inlet is typically open during the rainy season and experiences closure during winter low flow periods. It does not migrate. The inlet is formed by fluvial overtopping of the barrier and when active, forms a small delta seaward of the channel and a small flood tide delta in the back-barrier. The tidal prism is insufficient to maintain the inlet and it is quickly sealed through wave-reworking of sand from the ephemeral delta when fluvial discharge diminishes. During a large ocean swell event coupled with abnormally high tides in 2007, a departure from this seasonal behaviour occurred. The barrier migrated 100 m landwards and formed a gently (1.74 o ) seaward dipping sand sheet. Whereas the barrier had previously been narrow, it was widened by 80 m and rose 1.5 m in elevation. A breach later occurred and rapidly migrated northwards, establishing a significantly deeper inlet. This closed following wave reworking of a large ephemeral delta. Since then the barrier has maintained its landward position yet the inlet has continued to function as it did prior to the storm surge. This episodic barrier retreat appears to represent crossing of a morphodynamic threshold that triggered historically unprecedented rates of barrier rollover, creating a new set of inlet morphodynamic processes. These were short-lived and the system appears to have reverted to the typical seasonal morphodynamic processes despite such rapid rollover.

Estuary classification method for considering climate change effects in South Korea

ABSTRACT Jang D., and Hwang, J. H., 2013. Classification method of estuary considering the climate change in South. Sea level rise attributed to climate change is expected to affect the estuaries of South Korea significantly. The seawater intrudes into the beds of river channels and into aquifers near the coastal zone, and such processes are expected to be magnified in the climate change regime. Further, flooding and erosion are expected to be intensified at the shorelines of rivers and in coastal zones. Such adverse effects are expected to occur, which will depend on the individual characteristics of different estuaries. Some of these characteristics include the volume of a tidal prism, the length of shoreline peripheral, and the bathymetries in the estuaries. Our present work modified the existing estuary classification systems to be more responsive to climate change; in particular, we have included the wave change impact. Many studies have proposed estuarine classifications recognizing the various feat...

Flooding assessment under sea level rise scenarios: Ria de Aveiro case study

Lopes, C., Azevedo, A., Dias, J.M., 2013. Flooding assessment under sea level rise scenarios: Ria de Aveiro case study In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 766-771, ISSN 0749-0208. Sea level rise is an important consequence of climate change with a significant impact on society and ecosystems. The present work aims to assess the sensitivity of Ria de Aveiro marginal flooded area to the sea level rise. A previous version of the hydrodynamic model ELCIRC was improved extending its numerical grid through the inclusion of the intertidal areas and the marginal topography. The present configuration was calibrated comparing model predictions with sea surface elevation data recorded at ten stations distributed throughout the lagoon. The root mean square error and the SKILL were computed, and generally an excellent/ good agreement between predicted and observed sea surface elevation data was found. Once calibrated, the model was used to simulate the lagoon flooded area under present mean sea level and under two local sea level rise scenarios (0.42 m and 0.64 m), considering mean and spring tide conditions. In average was found an increase of 22% and 35% of the lagoon flooded area, respectively, relatively to the present. Additionally, an increase of 15% and 23% was found for the tidal prism at the inlet, for sea level rise scenarios of 0.42 m and 0.64 m, respectively. Numerical results evidenced that sea level oscillations induce important changes in the lagoon flooded area as well as on the local hydrodynamics. Consequently some activities developed in the lagoon margins may be in jeopardy, i.e. the agricultural fields located at the margins of S.Jacinto channel will be inundated by saltwater, if these sea level rise projections are confirmed.

A STUDY ON THE EFFECTS OF TIDE ON SEDIMENTATION IN ESTUARIES OF THE NIGER DELTA, NIGERIA

Estuaries in the Niger Delta-Nigeria are influenced by tidal currents and wave actions due to their proximity to the Atlantic Ocean. Tidal current provides the energy required to move sediments in and out of the estuaries from the seashores, while wave actions curtails the deposition of sediments at the bottom of the estuaries as the bed get shallower, resulting in the modification and long-term regulation of siltation through erosion or accretion. The interaction between estuaries bathymetric shapes, tidal currents and wave actions can only be fully understood through analytical or mathematical relationships. Analytical methods are less accurate in predicting future tides, while none of the existing mathematical relationships can accurately predict tidal behavior in the Niger Delta region due to the fact that parameters governing tidal actions vary from region to region, hence the need for this study. This study shows the behavior of the estuaries in response to variation in tidal heights, currents and wave actions through mathematical modeling, a knowledge which is useful in planning and timing of marine activities that requires pre-knowledge of tidal levels, direction and current velocities. The models were formulated and calibrated using parameters generated from the hydrographic, hydraulics and geotechnical investigation, including local field observations and measurements conducted within the study area. Soil samples taken from the area are composed of peat, organic clays, silty clays and sand. Peat constitutes the dominant soil which is locally known as 'chikoko' with high compressibility and color ranging from dark brown to dark gray and texture from soft to firm. Typical bed material size (D50) is approximately 0.2mm. Tides in the region are mostly semidiurnal with tidal prism ranging from 0.4 to 1.5m. The tidal strength is strongest at the inlets and decreases with distance inland. The depth of the estuaries is controlled by the strength of the tidal currents. Areas very close to the ocean with stronger tidal effect are very deep; while shallow canals and creeks predominates the hub of the estuaries. The results obtained using the present models compared favorably with the field measurements. Average correlation coefficients of 0.9 were obtained in some cases. The results showed that the estuaries erodes, accretes, or remain stable, depending on the rate at which sediment is supplied or removed from them.

EVALUATION OF THE RELATIONSHIPS BETWEEN NAVIGATION CHANNEL DREDGING AND EROSION OF ADJACENT BEACHES IN SOUTHERN BRAZIL

São Francisco do Sul Harbor, located in the Babitonga Bay, Santa Catarina, is one of the main containership harbors in Brazil and has been in operation since 1955. Due to the increasing demand for ships with greater size and draught, the navigation channel was dredged to 10 m depth in 1980 and has been gradually deepened to 13 m in recent years to accommodate Post-Panamax vessels. During the 30 years of dredging operations, more than 10 million m³ of sediments were removed from the coastal system and dumped into an offshore disposal area (about 15 m water depth), whilst the downdrift beach has experienced severe erosion. The downdrift municipality, Itapoá, recently filed a lawsuit against the Port claiming that harbor dredging has caused much of the erosion observed on its beaches. In order to evaluate the downdrift effects of dredging and maintaining the deep draft navigation channel across the ebb shoal of Babitonga Bay, the Delft3D model was used. The model was calibrated and validated with field data (water level and currents, waves and morphological changes). To evaluate channel impacts on adjacent beaches, 10 years morphology change simulations were conducted for pre-dredging scenario and scenarios of the navigation channel dredged at 10 m and 13 m depth, using the dredge and dump tool in Delft3D to maintain the deep draft channel along the simulation. A conceptual model of the sedimentary dynamics of the area was developed utilizing results from numerical modeling and field observations. It was observed in the results that dredging and maintaining the deep draft navigation channel over the last few decades have reduced the amount of sediment bypass by about 13 %. Channel dredging has therefore contributed to the erosion of the adjacent beaches, however other man-made interventions on this coastal system that contributed to the erosion problem were also identified (bay closure and tidal prism changes as well as jetties and other structures built updrift). Thus, it cannot be inferred that all the erosion of the downdrift beaches is due to the dredging activities. To mitigate for erosion effects of channel dredging, beneficial sediment disposal strategies were investigated. Strategies evaluated included beach disposal and ebb-shoal disposal of dredged materials. Beach disposal was the alternative that produced direct benefits to the downdrift beach while ebb-shoal disposal produced benefits that were not directly detected on the beach during the time frame evaluated here. Due to benefit-cost considerations, beach disposal was recommended for maintenance operations where dredging volume is greater than 400.000 m³ (about 50 % of the dredging maintenance operations exceed this threshold). For smaller maintenance dredging events, ebb shoal disposal was recommended. This work is the first of this nature in Brazil driven by legal disputes between downdrift beaches and Port authority and sets the precedent for future beneficial use of dredging materials along the Brazilian coast.

Synthesis of benthic flux components in the Patos Lagoon coastal zone, Rio Grande do Sul, Brazil

The primary objective of this work is to synthesize components of benthic flux in the Patos Lagoon coastal zone, Rio Grande do Sul, Brazil. Specifically, the component of benthic discharge flux forced by the terrestrial hydraulic gradient is 0.8 m3 d−1; components of benthic discharge and recharge flux associated with the groundwater tidal prism are both 2.1 m3 d−1; components of benthic discharge and recharge flux forced by surface‐gravity wave setup are both 6.3 m3 d−1; the component of benthic discharge flux that transports radium‐228 is 350 m3 d−1; and components of benthic discharge and recharge flux forced by surface‐gravity waves propagating over a porous medium are both 1400 m3 d−1. (All models are normalized per meter shoreline.) Benthic flux is a function of components forced by individual mechanisms and nonlinear interactions that exist between components. Constructive and destructive interference may enhance or diminish the contribution of benthic flux components. It may not be possible to model benthic flux by summing component magnitudes. Geochemical tracer techniques may not accurately model benthic discharge flux or submarine groundwater discharge (SGD). A conceptual model provides a framework on which to quantitatively characterize benthic discharge flux and SGD with a multifaceted approach.

Influence of varying tidal prism on hydrodynamics and sedimentary processes in a hypertidal salt marsh creek

ABSTRACTRecent initiatives directing tidal power development in the Bay of Fundy have raised questions about far‐field environmental impacts related to energy extraction. It is understood that commercial scale tidal power installations in the Minas Passage will result in an overall decrease in tidal amplitude in the Minas Basin. Corresponding changes in sedimentation patterns may or may not be within the natural range of variability, and it is hypothesized that intertidal sedimentation rates will demonstrate a non‐linear response to modification of the tidal energy regime. This research considers acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) data from a sheltered tidal creek in the Minas Basin, for analysis of tidal characteristics in a hypertidal creek environment over spring and neap tidal cycles. Sediment deposition in the creek was also measured. Results show a first‐order control of topography on flow magnitude in the tidal creek, which impacts net sediment deposition through resuspension and removal of newly introduced material. This study demonstrates that tides which peak around the bankfull level show reduced early ebb stage turbulence and flow velocity and encourage an extended depositional period. The dynamics of marshfull tides may be responsible for the maximum sediment deposition in tidal creeks, providing large amounts of material that is eventually distributed to and deposited on marsh surfaces. Copyright © 2012 John Wiley & Sons, Ltd.

Long-term monitoring of a mercury contaminated estuary (Ria de Aveiro, Portugal): the effect of weather events and management in mercury transport

The main aim of this research was to assess the mercury transport from an estuarine basin with a background of anthropogenic contamination during a spring tidal cycle (year 2009) and compare it with two previous tidal cycles (years 1994 and 1999), as part of a long-term monitoring program. Results showed that effective mercury transport occurs both in the dissolved and particulate fractions (0.18 and 0.20 kg per tidal cycle, respectively), and despite an overall decrease in environmental contamination, results more than double previous findings on particulate transport in the system. These findings result essentially from changes in the tidal prism (net export of 2 million m3 of water), given that both dissolved and particulate concentrations did not increase over time. Hydrodynamic simulations were performed to evaluate the effect of physical disturbance (dredging) and weather events (increased freshwater flow) in these processes, and results suggest the increased freshwater flow into the system as the main forcing function for the mercury transport increment. These results highlight the importance of long-term monitoring programs, since despite an overall improvement in local contamination levels, the enhancement of transport processes through hydrological changes increases environmental pressure away from the contamination source. Copyright © 2012 John Wiley & Sons, Ltd.

A three-dimensional hydrodynamic and salinity transport model of estuarine circulation with an application to a macrotidal estuary

A three-dimensional semi-implicit finite volume numerical model has been developed and applied to study tidal circulation and salinity stratification in the region of Oujiang River Estuary, China. The model employs horizontally unstructured grids and boundary-fitted coordinate system in the vertical direction. Governing equations consisting of continuity, momentum, and transport equations are all solved in the integral form of the equations, which provides a better representation of the conservative laws for mass, momentum, and transport in the coastal region with complex geometry and bottom bathymetry. The model performance was firstly quantified with skill assessment statistics on the choice of different parameters and validated with observed tidal elevation, current velocity, direction and salinity data over a spring–neap tidal cycle collected in 2006. Numerical results show that the model with wetting–drying capability successfully simulated the tidal currents and salinity fields with a reasonable accuracy and indicate that the Oujiang River Estuary is a macrotidal estuary with strong tidal mixing. In addition, the model results also show that the Oujiang River Estuary is a well-mixed estuary during spring tide. Then, the numerical simulations were performed to compare the hydrodynamic process and salinity distribution before and after a river training, which was conducted by blocking the south branch of the Oujiang River mouth. The results reveal that with the only north access to the sea, the influence of the blocking project on the flood discharge capacity is limited and the incremental velocity is beneficial to the navigation channel maintenance, although it will cause some scour to the embankment. Furthermore, the redistribution of tidal prism passing in or out the north branch makes a little severe salinity intrusion during high tide or low tide. However, the salinity intrusion is still within acceptable range, although it can cause some adverse effect on water intaking of production and life. The variations of salinity levels in Yueqing Bay situated at the north of the river mouth are not obvious, so the blocking project will not bring damage to local aquiculture. However, significant changes of salinity happen inside or outside of the south branch, so enough attention need to be paid to the changes of environment caused by the salinity variation after the blocking project. Overall, by weighing advantages and disadvantages of the blocking project, it is feasible and the model can be considered as a tool for managing and studying estuarine circulation.

Impact of Back-Barrier Dams on the Development of the Ebb-Tidal Delta of the Eastern Scheldt

Elekema, M.; Wang, Z.B., and Stive, M.J.F., 2012. Impact of back-barrier dams on the development of the ebb-tidal delta of the Eastern Scheldt.The Eastern Scheldt tidal basin in the SW part of the Netherlands has seen dramatic changes due to human intervention in the past few decades. The construction of two back-barrier dams in 1965 and 1969 had a significant impact on tidal hydrodynamics and sediment transport. The effects of these interventions were still ongoing when the hydrodynamic regime was altered again by the construction of the storm surge barrier between 1983 and 1986.This research describes the hydrodynamic and morphological evolution of the Eastern Scheldt ebb-tidal delta between 1960 and 1983, before construction of the storm surge barrier started. The analysis focuses on how the back-barrier dams changed the tidal flow on the ebb-tidal delta and how these altered hydrodynamics influenced sediment transport and morphology. It starts with a description of the observed hydrodynamic and bathymetric changes. These observations are used as input for a process-based numerical model (Delft3D FLOW), which is used to simulate situations before and after channel closures.In the decades before 1965, the Eastern Scheldt exported large quantities of sediment towards the sea through its inlet. The implementation of the dams caused a significant increase in tidal prism, which was observable in the response of the bathymetry; the export and the rates of channel deepening and ebb-tidal delta growth both increased. Analysis of tidal-flow measurements and model output show a persistent trend for sediment transport towards and out of the Eastern Scheldt inlet. The construction of the back-barrier dams only amplified this export by increasing the tidal prism and pushing the basin out of equilibrium even more than it apparently already was.

Effects of the Large-Scale Reclamation Project on Hydro-Dynamic Environment in the Estuary

The hydro-dynamic environment will be changed after reclamation project building. It will affect the navigation, estuary regime, tidal prism, flood controlling, storm surge and so on. Taking the shore reclamation between Oujiang and Feiyunjiang estuary as example, a tidal current mathematical model is used to study the effects of the large-scale reclamation on the hydro-dynamic environment. Based on triangle grid and finite volume method, the shallow flow model is examined by 9 tidal level stations and 9 observed synchronous vertical lines. And the comparison between computed result and measured solution demonstrates very good agreement. After the project building, the high tide level reduces to a certain extent and the flow of front reclamation line reduces by a big margin. But it is a little influence to the core ports of Wenzhou. And the tidal influx through Feiyunjiang Estuary decreases. The result can provide important reference and inspiration for the effects of hydro-dynamic environment due to the large-scale reclamation projects.

Turning the tide: experimental creation of tidal channel networks and ebb deltas

AbstractTidal channel networks, estuaries and ebb deltas are usually formed over a period longer than observations cover. Much is known about their characteristics and formation from linear stability analyses, numerical modelling and field observations. However, experiments are rare whilst these can provide data-rich descriptions of morphological evolution in fully controlled boundary and initial conditions. Our objective is to ascertain whether tidal basins can be formed in experiments, what the possible scale effects are, and whether morphological equilibrium of such systems exists.We experimentally created tidal basins with simple channel networks and ebb deltas in a 1.2 by 1.2 m square basin with either a fixed or self-formed tidal inlet and initially flat sediment bed in the tidal basin raised above the bed of the sea. Rather than create tides by varying water level, we tilted the entire basin over the diagonal. The advantage of this novel method is that the bed surface slopes in downstream direction both during flood and ebb phases, resulting in significant transport and morphological change in the flood phase as well as the ebb phase. This overcomes the major problem of earlier experiments which were entirely ebb-dominated, and reduces the experiment time by an order of magnitude.Ebb deltas formed in sand were entirely bedload dominated whereas the lightweight plastic sediment was intermittently suspended. Channels bifurcated during channel deepening and backward erosion to form a network of up to four orders. For initially dry tidal plains, the tidal prism increased as more sediment eroded from basin to ebb delta, so that evolution accelerated initially. The rate of change, the size of the channels and the final length of channels and delta were very sensitive to the tidal amplitude, tidal period and initial water depth in the basin. Most experiments with sand terminated with all sediment below the threshold for motion, whilst lightweight sediment remained mobile in the inlet region and firstorder channels, suggesting that sustained morphodynamics are feasible in experiments. We discuss how this novel experimental setup can be extended to produce tidal deltas, estuaries and other tidal systems and study their dynamics as a function of their forcing.

A methodology for the classification of estuary restoration areas: A management tool

Planning the recovery of estuarine areas represents a major challenge for environmental managers, who must find a balance between the desired environmental restoration, understood as the return to natural conditions, and the different socioeconomic uses currently borne by the estuaries. This work presents a methodology to optimize decision-making in accordance with the objectives which might arise in projects for the hydrodynamic restoration of estuaries. Socioeconomic issues are not considered in this study. The new approach is based on a classification of the zones to be restored according to characteristics representing their hydrodynamic performance and the possible morphodynamic effects of the restoration on the rest of the estuary. To achieve this, the four following parameters were chosen: (1) changes in tidal prism induced by restoration of that zone (ΔΩ), (2) the distance between the concession and the estuary inlet (L), (3) the tidal wave phase lag (φ) and (4) the flood potential of the restoration area (I). The classification combines self-organizing maps (SOM) and the K-means algorithm. The methodology was applied in a total of 139 areas (concessions) on ten estuaries along the entire coast of Cantabria (Northern Spain) where a Spanish Ministry of the Environment Recuperation Plan is under way. The results classify the 139 areas of restoration into five clusters. Empirical relationships were used to estimate the effects the restoration of each cluster may have on the estuary's various morphodynamic elements (cross-sectional area of the estuary mouth, area of tidal flats, volume of tidal channels and volume of the ebb tidal delta), giving managers an overall view of the potential effects of the restoration in each zone and providing a basis on which to plan these actions.

Present not the key to the past in tidal environments?

Landforms in tidal environments such as estuaries and lagoons are molded by both past climate as well as modern conditions such as local changes in mean sea level. Hence, it is difficult to determine which landforms in a tidal environment retain signatures of past climate and which ones reflect more modern conditions. In a new study, Stefanon et al. used conceptual laboratory experiments to investigate how cyclic variations of the local mean sea level affect landforms in tidal environments and if tidal landscapes retain signatures of past conditions. For the experiment the researchers created a synthetic lagoonal environment and periodically changed the mean sea level relative to the land. The researchers found that the total volume of water passing through the lagoon between each cycle of high tide and low tide, also called the tidal prism, strongly influenced the drainage pattern as well as the transport of water, sediments, nutrients, and pollutants in the lagoon.

Use of geomorphic, hydrologic, and nitrogen mass balance data to model ecosystem nitrate retention in tidal freshwater wetlands

Abstract. Geomorphic characteristics have been used as scaling parameters to predict water and other fluxes in many systems. In this study, we combined geomorphic analysis with in-situ mass balance studies of nitrate retention (NR) to evaluate which geomorphic scaling parameters best predicted NR in a tidal freshwater wetland ecosystem. Geomorphic characteristics were measured for 267 individual marshes that constitute the freshwater tidal wetland ecosystem of the Patuxent River, Maryland. Nitrate retention was determined from mass balance measurements conducted at the inlets of marshes of varying size (671, 5705, and 536 873 m2) over a period of several years. Mass balance measurements indicate that NR is proportional to total water flux over the tidal cycle. Relationships between estimated tidal prism (calculated water volume) for spring tides and various geomorphic parameters (marsh area, total channel length, and inlet width) were defined using measurements from air photos and compared to field data. From these data, NR equations were determined for each geomorphic parameter, and used to estimate NR for all marshes in the ecosystem for a reference spring (high) tide. The resulting ecosystem NR estimates were evaluated for (a) accuracy and completeness of geomorphic data, (b) relationship between the geomorphic parameters and hydrologic flux, and (c) the ability to adapt the geomorphic parameter to varying tidal conditions. This analysis indicated that inlet width data were the most complete and provided the best estimate of ecosystem nitrate retention. Predictions based on marsh area were significantly lower than the inlet width-based predictions. Cumulative probability distributions of nitrate retention indicate that the largest 3–4% of the marshes retained half of the total nitrate for the ecosystem.

Signatures of sea level changes on tidal geomorphology: Experiments on network incision and retreat

How do tidal networks respond to changes in relative mean sea level (RMSL)? The question on whether the morphological features of a tidal landscape retain signatures of past environmental forcings, or are in equilibrium with current ones, is critical to our prediction of the fate of residual tidal landforms. In the case of tidal networks, the issue is quite relevant owing to their fundamental role on landscape eco‐morphodynamic evolution. Here we explore the response of tidal networks to cyclic variations in RMSL triggering tidal prism changes on the basis of laboratory experiments carried out in a synthetic lagoonal environment. A decrease in the tidal prism leads to network retreat and contraction of channel cross sections. Conversely, an increase in the tidal prism promotes network re‐incision and re‐expansion of channel cross sections: Network retreat and expansion tend to occur within the same planar blueprint. Our results show that the drainage density of tidal channels is linearly related to the landscape‐forming prism, although this relation is speculated to hold with reasonable approximation as a statistical tendency rather than as a pointwise, instantaneous adaptation. Changes in tidal prism rapidly influence network efficiency in draining the intertidal platform and the related transport of water, sediments, nutrients and pollutants. This bears important consequences for quantitative predictions of the long‐term ecomorphological adaptation of the tidal landscape to RMSL changes.

Bio‐geomorphic effects on tidal channel evolution: impact of vegetation establishment and tidal prism change

ABSTRACTThe long‐term (10–100 years) evolution of tidal channels is generally considered to interact with the bio‐geomorphic evolution of the surrounding intertidal platform. Here we studied how the geometric properties of tidal channels (channel drainage density and channel width) change as (1) vegetation establishes on an initially bare intertidal platform and (2) sediment accretion on the intertidal platform leads to a reduction in the tidal prism (i.e. water volume that during a tidal cycle floods to and drains back from the intertidal platform). Based on a time series of aerial photographs and digital elevation models, we derived the channel geometric properties at different time steps during the evolution from an initially low‐elevated bare tidal flat towards a high‐elevated vegetated marsh. We found that vegetation establishment causes a marked increase in channel drainage density. This is explained as the friction exerted by patches of pioneer vegetation concentrates the flow in between the vegetation patches and promotes there the erosion of channels. Once vegetation has established, continued sediment accretion and tidal prism reduction do not result in significant further changes in channel drainage density and in channel widths. We hypothesize that this is explained by a partitioning of the tidal flow between concentrated channel flow, as long as the vegetation is not submerged, and more homogeneous sheet flow as the vegetation is deeply submerged. Hence, a reduction of the tidal prism due to sediment accretion on the intertidal platform, reduces especially the volume of sheet flow (which does not affect channel geometry), while the concentrated channel flow (i.e. the landscape forming volume of water) is not much affected by the tidal prism reduction. Copyright © 2012 John Wiley & Sons, Ltd.

Coastal Embayment Long-Term Erosion/Siltation Associated with P-A Relationships: A Case Study from Jiaozhou Bay, China

Liu, Y.; Wang, Y.P.; Li, Y.; Gao, J.; Jia, J.; Xia, X., and Gao, S., 2012. Coastal embayment long-term erosion/siltation associated with P-A relationships: A case study from Jiaozhou Bay, China. Journal of Coastal Research, 28(5), 1236– 1246. Coconut Creek (Florida), ISSN 0749-0208. The sedimentary environment of a large coastal embayment, Jiaozhou Bay, eastern China, in terms of erosion/siltation, has been assessed on the basis of seabed bathymetry and coastline data sets from sea charts (1936–2002) and Landsat-5 TM images (1986–2011). Nine types of sedimentary environment are identified according to the sedimentation rate. The results indicate that slow siltation was generally present from 1936 to 1963, with exceptions of some patchy, slightly eroding areas within the embayment. Transformation to a slow erosion pattern occurred in the main channels over the central embayment in 1963, and such a situation continued until 1982. Subsequently, since 1986, most of the coastlines extended toward the sea at a rate of 10 1 –10 2 my 21 ; this was mainly attributable to human activities, including reclamation over the NW tidal-flat and harbour construction along the east and south coastlines. Using the same data set, patterns of changes in tidal prism and deposition rate within the entrance channel were established. An equivalent friction coefficient (K) is proposed to evaluate the P-A relationship of the large tidal inlets in Jiaozhou Bay. The K value increases gradually over time, indicating a trend toward stable equilibrium in Jiaozhou Bay. This coefficient can be used to examine the status of morphological stability and equilibrium of coastal embayments.