Tidal Entrances Research Articles

Influence of different structural modulation scenarios on morphology change within tidal inlets (Case study: Bardawil Lagoon, Egypt)

This paper was specially written to complement the first part series published by (Nassar et al., 2019), to investigate the impact of anthropogenic changes around tidal entrances on sedimentary mass stocks in response to the successive pressures of decision-makers due to the sensitivity of the issue. The CMS package was therefore used to model coastal processes at the Bardawil Lagoon Western inlet1 and to forecast sediment-logical stocks each year such that the accumulation of sediments within the inlet can be mitigated by impeding the navigation of this inlet and by allowing water movement between sea and lake. The CMS package is based on a finite-volume approach. A CMS-Flow and CMS-Wave two steering modules have been introduced. In this analysis, unorthodox structural scenarios inside the tidal inlets were evaluated: extension of the jetties, flow-diverted wall (FDW), current deflector wall (CDW), sediment trap (ST), radial channels (RC) and lateral wide channel (LWC). Following the current posture of inlet 1, the use of CDW, ST or RC was considered to be reasonable compensation for the current dredging operation. This is mostly attributed to a lower infilling pace as well as the fewer dredging at year-end. Whereas the benefits generated from maintenance dredging can be perceived to be slightly costly, the long-term benefits of that expenditure can justify it.

Coastal morphodynamics and Chenier-Plain evolution in southwestern Louisiana, USA: A geomorphic model

Using 28 topographic profiles, air-photo interpretation, and historical shoreline-change data, coastal processes were evaluated along the Chenier Plain to explain the occurrence, distribution, and geomorphic hierarchy of primary landforms, and existing hypotheses regarding Chenier-Plain evolution were reconsidered. The Chenier Plain of SW Louisiana, classified as a low-profile, microtidal, storm-dominated coast, is located west and downdrift of the Mississippi River deltaic plain. This Late-Holocene, marginal-deltaic environment is 200 km long and up to 30 km wide, and is composed primarily of mud deposits capped by marsh interspersed with thin sand- and shell-rich ridges (“cheniers”) that have elevations of up to 4 m. In this study, the term “ridge” is used as a morphologic term for a narrow, linear or curvilinear topographic high that consists of sand and shelly material accumulated by waves and other physical coastal processes. Thus, most ridges in the Chenier Plain represent relict open-Gulf shorelines. On the basis of past movement trends of individual shorelines, ridges may be further classified as transgressive, regressive, or laterally accreted. Geomorphic zones that contain two or more regressive, transgressive, or laterally accreted ridges are termed complexes. Consequently, we further refine the Chenier-Plain definition by Otvos and Price [Otvos, E.G. and Price, W.A., 1979. Problems of chenier genesis and terminology—an overview. Marine Geology, 31: 251–263] and define Chenier Plain as containing at least two or more chenier complexes. Based on these definitions, a geomorphic hierarchy of landforms was refined relative to dominant process for the Louisiana Chenier Plain. The Chenier Plain is defined as a first-order feature (5000 km 2) composed of three second-order features (30 to 300 km 2): chenier complex, beach-ridge complex, and spit complex. Individual ridges of each complex type were further separated into third-order features: chenier, beach ridge, and spit. To understand the long-term evolution of a coastal depositional system, primary process–response mechanisms and patterns found along the modern Chenier-Plain coast were first identified, especially tidal-inlet processes associated with the Sabine, Calcasieu, and Mermentau Rivers. Tidal prism ( Ω) and quantity of littoral transport ( M total) are the most important factors controlling inlet stability. Greater discharge and/or tidal prism increase the ability of river and estuarine systems to interrupt longshore sediment transport, maintain and naturally stabilize tidal entrances, and promote updrift deposition. Thus, prior to human modification and stabilization efforts, the Mermentau River entrance would be classified as wave-dominated, Sabine Pass as tide-dominated, and Calcasieu Pass as tide-dominated to occasionally mixed. Hoyt [Hoyt, J.H., 1969. Chenier versus barrier, genetic and stratigraphic distinction. Am. Assoc. Petrol. Geol. Bull., 53: 299–306] presented the first detailed depositional model for chenier genesis and mudflat progradation, which he attributed to changes in Mississippi River flow direction (i.e., delta switching) caused by upstream channel avulsion. However, Hoyt's model oversimplifies Chenier-Plain evolution because it omits ridges created by other means. Thus, the geologic evolution of the Chenier Plain is more complicated than channel avulsions of the Mississippi River, and it involved not only chenier ridges (i.e., transgressive), but also ridges that are genetically tied to regression (beach ridges) and lateral accretion (recurved spits). A six-stage geomorphic process-response model was developed to describe Chenier-Plain evolution primarily as a function of: (i) the balance between sediment supply and energy dissipation associated with Mississippi River channel avulsions, (ii) local sediment reworking and lateral transport, (iii) tidal-entrance dynamics, and (iv) possibly higher-than-present stands of Holocene sea level. Consequently, the geneses of three different ridge types (transgressive, regressive, and laterally accreted) typically occur contemporaneously along the same shoreline at different locations.

Morphodynamic relationships for ebb and flood delta volumes at Florida’s tidal entrances

Tidal entrances constitute an important boundary condition for the coastal ocean regime. Based on data from 67 sandy entrances in Florida, morphodynamic relationships between tidal prism, entrance throat area, and ebb and flood delta volumes are revisited. The main source of sand in these deltas is the littoral zone as opposed to the river. It is found that, as a rule of thumb, the volume of a mature ebb delta is equal to one fifth of the prism at the spring range of tide. Data for the coasts of Florida taken together indicate that the volume of mature flood delta varies with one third power of the prism. The Atlantic Coast flood delta volumes by themselves do not show any correlation with the prism, presumably because entrance depths have been altered by dredging. A case study of the closure of a land barrier breach at Matanzas Inlet illustrates the application of the derived morphodynamic relationships between prism, throat area, and ebb delta volume. These relationships are useful for prediction of changes in the throat area and the ebb delta volume when morphologic changes occur over time scales consistent with the reestablishment of equilibrium.

Monitoring and comparison to predictive models of the Perdido Key beach nourishment project, Florida, USA

This paper summarizes the results of over 8 years of data describing the performance of a large beach nourishment project on Perdido Key, immediately adjacent to Pensacola Pass in Escambia County, FL, USA. As a result of a major excavation of the entrance channel to Pensacola Bay, over 7 million m 3 of beach-quality sand were placed along the easternmost 7.5 km of Perdido Key, adjoining the entrance channel at Pensacola Pass. The project included the placement of 4.1 million m 3 of sand directly upon the shoreline in 1989–1990, followed by the placement of an additional 3 million m 3 as an underwater berm just offshore of the beach nourishment project in water depths of roughly 6 m. Monitoring of the performance of the beach nourishment project and the offshore berm has been conducted since 1989, beginning with a pre-construction survey of the project area. Monitoring surveys have been conducted on an annual or biennial basis since that time, with the most recent survey occurring in July/August, 1998. Over 8 years of monitoring data indicate that the beach nourishment project has retained approximately 56% of the original volume placed within the 7.5-km project length. In addition, according to the latest monitoring survey, the dry beach width of the project, initially constructed as 135 m on average, is still 53 m wider than pre-project conditions. Approximately 41% of the originally placed dry planform area remains as of July 1998. The most recent monitoring surveys in 1995, 1997, and 1998 encompass the effects of two major storm systems, Hurricanes Erin (August 1995) and Opal (October 1995). Monitoring of the offshore berm area indicates only a slight landward migration of the berm, accompanied by a minor decrease in volume, over the entire monitoring period. The performance of both the beach nourishment project and the offshore berm appear to be significantly related to the two storm events, particularly Hurricane Opal, and the proximity of the project to the tidal entrance at Pensacola Pass. Comparison of the documented performance of the beach nourishment project to simple existing analytical models of beach-fill evolution have yielded encouraging results in terms of preliminary design aids for future beach nourishment projects in the vicinity of deep tidal entrances.

Initiation of sand transport over coarse beds in tidal entrances

A tidal entrance often acts as a source or a sink for sand in the littoral zone. At many entrances in biologically productive regions the bed in the vicinity of the throat section (minimum flow area) is composed of sediment containing coarse material including large shells, with sand occurring in the interstitial regions between the shell-defined roughness elements. A stochastic relationship giving the critical bed shear stress for the initiation of sand transport under these conditions is applied to hydraulic and sedimentary data obtained from two entrances in Florida. The results agree well with the critical bed shear stresses derived from data based on the observation of sand movement at the bed. The entrainment function based on observations is found to be approximately three times the value obtained from Shields' criterion. A practical implication is that the use of Shields' criterion in the computation of the rate of sand transport will in general overpredict the rate through the entrance.

MECHANISMS OPERATING AT A JETTIED RIVER ENTRANCE

Between October 1976 and July 1977 a northern rubble mound jetty was constructed at the mouth of the Hastings River, transforming the entrance from a single to a double jettied system. Prior to the jetty construction the entrance was characterised by the presence of a substantial swash bar (alternatively called an ebb delta marginal shoal) which was a continuous feature over 100 years of hydrographic survey records. However, construction of the northern jetty triggered an unprecedented onshore movement of the swash bar. This movement was well documented by a field monitoring programme incorporating hydrosurveys, aerial* photographs, tidal gaugings, sediment sampling, float tracking and nearby wave rider buoy information. A semi-quantitative model was developed to aid understanding and quantification of the macro sedimentary processes associated with this phenomenon. The model demonstrated that the sudden reduction of the swash bar was due to the disruption of a circulation of sand which had previously aided the dynamic stability of the bar. The quantitative predictions of the model agreed well with subsequent entrance behaviour. The philosophical development of the model and its findings are discussed in detail. In the literature there is a general lack of attempts to quantify the sediment transport relationships between the gross morphologic features of tidal entrances. This paper presents a methodology for assessing the sedimentary process at tidal entrances.

Design Procedure of Artificial Channels for Tidal Entrances

A generalized design procedure is proposed for artificial entrance channels for tidal inlets on sedimentary coasts. The artificial channels are classified into four kinds, according to the purposes to utilize them, each of which are demanded to furnish different requirements.Configuration of bottom topography of tidal entrance is examined to find three major characteristics. Probable cause of the deep appearing shoreward end of entrance is discussed. Application of the one-dimensional analytical method is introduced which can effectively predict tidal currents and bay tide. Existence of the depth of maximum channel velocity, which is one of the most important parameter for the channel design, is explained for an existing entrance.The criterion proposed recently by the writer for stable tidal entrances is discussed with taking account of littoral drift. It is found that an entrance affected by less littoral drift and by smaller sea tidal range has relatively greater flow area in equilibrium state. ...

Bed friction characteristics of three tidal entrances

The regimes of flow and sediment transport through a tidal entrance are contingent upon, among other factors, the temporal and spatial distributions of the shear stress at the channel bed, defined in terms of the depth-averaged velocity, a friction factor or a Manning's n and an equivalent sand bed roughness. Direct measurement of these parameters should, therefore, yield basic information on the flow and sediment transport characteristics of the entrance. Three entrances, located on the Gulf Coast of Florida, were selected for hydraulic measurements. In two of the three entrances, bed friction characteristics were derived from near-bed velocity measurements and at the third entrance the same were obtained from measured water surface slopes in the channel. The near-bed velocity profiles were found to be logarithmic, and the effect of flow inertia was found to be significant only for relatively short time periods close to slack water. In the fully rough range of flow the bed shear stress-velocity relationship was found to follow the square law with a characteristic friction factor and Manning's n for the entrance. The measured maximum friction velocity and the friction factor have been used to estimate the tidal prism—throat cross-sectional area ratio which is then compared with the same obtained from flow discharge measurements. A reasonable agreement is observed, thus attesting to the validity of the measured friction factors.

Depth of Maximum Velocity and Minimum Flow Area of Tidal Entrances

ABSTRACTTidal entrances on sandy coasts change their size and configuration at all times with scouring by tidal current and silting by incoming littoral drift. The minimum flow area of a long time stable or so-called equilibrium tidal entrance, however, is known to be usually in a relatively narrow range of it. Thus, it may be a unique function of external forces and the other channel properties.The objective of this paper is to investigate the stability requirements on the basis of analyses together with existing field data.Firstly, an explicit solution of channel current and bay tide is derived with one dimentional linearized approach.Secondly, the depth at which the channel velocity reaches its maximum, is obtained analytically based on the exiplicit solution. A graphical method to find the depth for trapezoidal and parabolic sections is given with applying figures prepared.The relationship between the minimum flow area and the tidal prism is investigated in the light of the present analytical approach...

Variations in the boundary-drag coefficient in the tidal entrance to Chesapeake Bay, Virginia

Use of the quadratic shear-stress law for estimating boundary drag requires specific knowledge of the magnitude of a drag coefficient, C D, and sectional mean velocity, u ̄ . In previous attempts to adapt the relationship for use in studies of marine-sediment transport, the flow measurement has been standardized at a level 100 cm above the bed. The particularized value of the drag coefficient has been designated as C 100. In the entrance area to Chesapeake Bay, Virginia, C 100 has been found to range through unacceptably wide limits. Two-thirds of the values obtained are between 3.5 · 10 −3 and 5.4 · 10 −2. Mean C 100 for the area is 1.3 · 10 −2 as compared to 3 · 10 −3 for tidal channels within Puget Sound, Washington. Present data suggest that, given a moveable bed, a size hierarchy of mobile bed forms, time-varying flow, and a lack of equilibrium between flow and bed, C 100 changes continuously with boundary shear stress. Accurate evaluation of boundary shear stress in tidal entrances with high flow rates and mobile beds presently requires measurement of velocity profiles.

HYDRAULIC CONSTANTS OF TIDAL ENTRANCES

Data contained in the Tide Tables, Current Tables and navigation charts of National Ocean Survey pertaining to tidal entrances along the coasts of the continental United States are analyzed to obtain flow coefficients defined by a simple hydraulic equation. The evaluation of the published data indicates that the data are sufficiently accurate and representative, despite some unexplained anomalies, to permit determination of the approximate discharge coefficients and, more importantly, to identify categories of flow regimes of inlets. A few conclusions are: 1. The Keulegan approach and similar analyses of inlet hydraulics provide a useful qualitative framework for ordering data but they apply quantitatively only to small inlets and lagoons with simple inlet channel geometry. 2. The lag of slack water in the entrance channel after HW and LW is a powerful tool in the analysis of the hydraulic regime. It is easily measured in the field and should become an identifying characteristic of each entrance. 3. The resistance coefficient F represents the overall impedance and exit losses, reflections from the lagoon shore, fresh water discharge and the configuration of the channel between the inner and outer bars. It may be determined from measurements of lag and maximum velocity at the throat and the range of ocean tide.