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The development of a tidal creek system, in a low energy environment, Beaulieu Estuary, southern England
No full textThe saltmarshes within the Solent area have retreated significantly over the past 120 years. The
expansion of tidal creek systems contributes considerably to the net loss of these saltmarshes. The
development of a tidal creek system at Exbury Marsh was studied, as being representative of the
tidal creeks within the Beaulieu Estuary, with emphasis placed upon the evolution of the crosssectional
profiles. The present geomorphology of the studied tidal creek system was investigated,
together with the measurements on the bank sediment stability, the parameters of the root systems
(Sea Purslane and Sea Rush) and the hydrodynamics. This study aims to understand the
influences of various factors, e.g., root systems and bank sediment stability, on the
geomorphological evolution of a natural tidal creek system, within a low energy environment.
The results reveal that within this low energy saltmarsh, the erosion takes place at a low rate (1-2
cm a-1) on bank faces, greater than the vertical accretion rate of marsh surface (2.5 mm a-1). The
geomorphology of the tidal creek system is mainly characterised by the presence of cantilevers.
The bases of the cantilevers are found to be located at certain levels, related to
submergence/emergence cycles. The cantilever stability analyses suggest that lateral expansion
rate of the channel is approx. 2.9 cm a-1, caused by the periodic bank failure.
The root systems are central in increasing bank sediment stability, contributing a maximum of 6
Pa to the erosion threshold and a maximum of 7 kPa to the shear strength of the bank sediments.
The Sea Purslane root system is more effective than the Sea Rush root system, in retarding the
bank erosion.
The flow-induced bed shear stress (<0.1 Pa) is lower than the erosion threshold of the bank
sediments (>0.5 Pa), under normal weather conditions. The water level frequency analysis
suggests that the erosion below the cantilever bases can be achieved by submergence/emergence
cycles, aided by low tidal currents, over a long-term period.
The evolution of the cross-sectional profiles is controlled mainly by the transformation from
mudflat to saltmarsh, the presence of a gravel base, the stabilisation by root systems on bank
sediments and the erosion caused by submergence/emergence cycles (together with low tidal
currents), over a long-term period.
The main contributions of this study to practical application are: (1) root systems (e.g., Sea
Purslane) with high resistance to fluid-induced erosion are more efficient than those (e.g., Sea
Rush) with high resistance to gravity-induced mass failure, in stabilising tidal creeks. (2) The
retreat of the banks, in low energy environments, is associated with long-term processes, e.g.,
submergence/emergence cycles (together with low tidal currents)
Controls on creek margin stability by the root systems of saltmarsh vegetation, Beaulieu estuary, southern England
No full textThe retreat of cliffs (lateral expansion) within tidal creeks results in a net loss of saltmarshes, but this retreat process can be retarded by root systems. In order to understand the interaction between root presence and bank sediment, quantitative measurements of two saltmarsh species root systems (Atriplex portulacoides and Juncus maritima) were carried out in a saltmarsh in Southern England, and their relationships with bank stability were examined. Computed Tomography (CT) Scanning techniques were used to investigate three dimensional root architecture. The data obtained, e.g. root volume, diameter and distribution patterns of roots, were examined alongside more traditional root density measurements. The volumetric percentage, ratio between horizontal (lateral) and vertical roots (H/V ratio), and root diameter distribution are discussed in relation to their influence on bank sediment erosion threshold and shear strength. The results suggest that Atriplex portulacoides is more effective than Juncus maritimus, in stabilising banks. This is because root systems which provide a high resistance to flow-induced erosion are better than those which provide a high resistance to gravity-induced erosion in stabilising cliff banks. This conclusion is relevant to future saltmarsh protection and re-establishment
A comparison of biohydrodynamic interaction within mangrove and saltmarsh boundaries
Get PDFMangrove forests and saltmarshes are recognized for their roles in wave and current attenuation, although a comparison of in situ observations between woody and herbaceous plants is needed in order to understand the different mechanisms of bio-physical interaction within coastal wetlands. The aim of our study was to compare the mechanisms of flow reduction and energy dissipation by mangrove trees and saltmarsh grass in a subtropical area where tidal currents dominate. Fieldwork was conducted to measure the hydrodynamic processes occurring at the boundaries between a bare mudflat and vegetated tidal flat, as the flow transitions from a bare mudflat to either mangrove or saltmarsh. Synchronous ADV measurements at three sites revealed that the mangrove was more effective than the saltmarsh grass at flow reduction. In addition, a considerable rotation in flow direction was observed as the flow entered the mangrove trees, while rotation was considerably less pronounced within the saltmarsh edge. The mechanism for this difference was explained through a combination of changes in drag force and eddy viscosity over the two vegetation types. Although overall the mangrove was observed to dissipate energy more effectively than the saltmarsh, the relative efficiency of the vegetation at dissipating turbulent energy was found to vary with the maximum water level of tidal cycle. When the maximum water level remained below the mangrove canopy bottom (‘bio-line’), the energy dissipation ability of the mangrove was relatively low, a result of the presence of rigid, sparse trunks rather than denser saltmarsh grass found near the bed; when the maximum water level was sufficiently high to reach the mangrove canopy, the ability of the mangrove to dissipate energy was significantly increased, becoming more effective than the saltmarsh grass
Creek enlargement in a low-energy degrading saltmarsh in southern England
No full textA low-energy saltmarsh in the Beaulieu River Estuary, southern England was investigated. A geomorphological survey revealed that the creek bank geometry was characterized by the presence of cantilevers, a result of the protection provided by roots over the upper part of the bank. The saltmarsh deposits, consisting mainly of clay and organic matter, have a high resistance to flow-induced erosion. Short-term hydrographic deployments revealed that the tidal currents and waves recorded were insufficient to cause erosion of the bank surface. However, pin measurements over a 2-year period revealed that the tidal creeks within the saltmarsh did experience bank erosion, particularly below the cantilevers, in spite of this low-energy setting. Bank face erosion was therefore considered to be related to long-term processes. The bases of the cantilevers were found to be located at a precise level, in relation to the tides. Water level data over a 2-year period were examined against erosion measured below the cantilevers. The results suggest that the erosion on the bank faces, particularly below the cantilevers, is associated with the frequency of water level variations and long-term submergence/emergence cycles, which can decrease the bonds between the particles and enable erosion by low tidal and wind-wave currents within this low-energy environment. This mechanism is probably responsible for tidal creek enlargement via erosion of the bank face and subsequent bank failure, a process which acts over a long temporal scale (1–10 years)
Differential sediment trapping abilities of mangrove and saltmarsh vegetation a subtropical estuary
No full textSaltmarsh and mangrove are common coastal wetland types and their ability to enhance deposition has been investigated extensively, but rarely compared directly. This study carried out in situ observations to compare the sediment transport processes between a bare mudflat, a mangrove stand and a saltmarsh stand within a subtropical estuary. Turbidity variations over the latter portion of a spring tide were recorded, alongside measurements of flow data, to estimate sediment trapping by hydraulic forces under similar hydroperiods. In addition, vegetation was transplanted to compare the direct sediment trapping by high- and short-standing seedlings. The suspended sediment concentration (SSC) time series showed an overall reduction between the bare mudflat and the vegetated flats. Suspended sediment flux estimates revealed that a considerable amount of sediment was trapped by the saltmarsh and the mangrove edges. The flux estimates find that the saltmarsh edge is more efficient than the mangrove edge in trapping sediments transported normal to the edge. The sediment trapping mechanisms were considered based on two approaches: the hydrodynamic related sediment settling and direct trapping by vegetation. The calculation of deposition tendency showed that the presence of vegetation altered the flow direction and the tidal asymmetry of the deposition process, resulting in a higher deposition tendency during the flood phase to enhance sediment settling. In addition to sediment settling, vegetation surfaces were found to trap sediments directly. In combination with rinsing by precipitation, these trapped sediments accumulated on the bed and contributed to the deposition. Against the background of similar inundation periods, the saltmarsh grass showed a greater sediment trapping ability than the mangrove trees, in terms of both the hydraulic sediment trapping and the direct trapping by vegetation surface
Hydroacoustic measurements highlight channel impact on macrotidal mudflat morphodynamics
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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