Coastal Engineering Journal

A mathematical model is formulated to describe incipient breaching of coastal barrier islands. The model is based on the assumptions of idealized breach morphology and is intended to describe the growth of breaches prior to possible closure by longshore sediment transport. The two coupled, non-linear equations governing breach width and depth are solved analytically for special cases. The analytical solutions explicitly exhibit an exponential behavior in breach dimensions and reveal that the macroscale process of breach growth is controlled by seven variables: initial width and depth of the breach, equilibrium width and depth of the breach, width of the barrier island, and maximum or initial net sediment transport rates at the bottom and sides of the breach. The literature is reviewed to compile general properties of coastal breaches, and sensitivity testing shows the model to be compatible with those observations. The model is applied to simulate the 1980 breach at Moriches Inlet, New York, and reasonable agreement is found.

Maintenance of navigable channels at coastal inlets typically requires sediment bypassing to preserve integrity of the adjacent beaches. Artificial bypassing mimics or preserves the pathways of sediment in the littoral zone and harmonizes the requirement for deepening navigation channels within the context of the natural sediment-sharing system of inlets and beaches. A range of techniques can be applied to perform bypassing. Customary on-demand channel dredging is not always the best solution because sediment shoaling does not necessarily follow a predictable schedule, potentially compromising navigation reliability between allowable dredging windows. If sediment can be directed to a location other than the channel, planning of dredge operations may be optimized and less expensive equipment feasible. A weir jetty system is one such solution. This paper presents progress in understanding of sediment bypassing through analytical prediction and takes a lessons-learned approach to design and modification of weir jetty bypassing systems. Results from recent physical model studies of spur jetties are also discussed with regard to their control on the nearshore current at coastal inlets and how spurs can be designed to alter sediment bypassing.

In recent years, numerous technological advances have made field studies and laboratory analyses of tidal inlets more time efficient while also substantially improved data quality. Mapping channel bathymetry was once a labor-intensive task that was accomplished by measuring water depths along a detailed network of channel profiles. Now aircraft-operated Light Detection and Ranging (LIDAR) can accurately map the bathymetry of most tidal inlets in a few hours' time. If greater detail of the channel bottom is required, then a multibeam acoustic system is utilized, although this technique is deployed from a boat and is less time efficient. The topographic version of LIDAR enables the construction of inlet shoreline maps and determination of volumes associated with shoreline gains and losses. In another advancement, side scan sonar mosaics now provide a picture of the bottom indicating different sediment types, bedrock outcrops, and bedform distributions. These surveys have also become essential for identifying scour holes next to rock jetties and assessing the susceptibility of these structures to collapse. The accretionary history and/or migrational trend of a tidal inlet can be determined from ground-penetrating radar transects on land and from shallow seismic surveys in water. These records provide a representation of the layers of sediment left behind as the inlet shifted positions. The hydraulic regime of a tidal inlet can be much more accurately determined using the Acoustic Doppler Current Profiler (ADCP). This instrument produces velocity profiles for an entire inlet cross section and provides almost instantaneous discharges for small and moderately sized inlets. In a slightly different mode, ADCP technology has been used for directional wave measurements. Finally, significant upgrades in the manipulation of information, including Geographic Information System (GIS) and image analysis methods, have greatly facilitated the analyses and interpretation of the field data. These advancements have provided scientists with more effective means of gathering data, testing numerical models, and finding solutions to coastal engineering problems.

Seasonally open tidal inlets usually occur in micro-tidal, wave-dominated coastal environments where strong seasonal variations of streamflow and wave climate are experienced. These inlets are closed to the ocean for a number of months every year due to the formation of a sand bar across the entrance. Because the estuaries connected to these inlets commonly serve as harbours or recreational facilities, the annual inlet closure is undesirable in most cases. Hence, there is increased interest in understanding the processes governing inlet closure and the hydrodynamics and exchange processes of these estuaries. This study was undertaken with the objectives of: (a) identifying the morphodynamic processes governing seasonal inlet closure, and (b) determining the effect of the seasonal closure of the inlet on the hydrodynamic characteristics of the estuary/lagoon connected to the inlet. Wilson Inlet, a typical seasonally open tidal inlet located along the southwestern coast of Australia, was selected as a case study. The inlet is usually closed to the ocean for 6–7 months every year.

Field experiments and numerical models were employed to examine inlet morphodynamics and estuarine hydrodynamics. As cross-shore processes, which the field data indicated to be important, have not been included in coastal area models to date, a quasi-3D morphodynamic model, which includes longshore and cross-shore processes, was developed to determine processes governing seasonal inlet closure. Application of the model to Wilson Inlet clearly indicates that onshore sediment transport during summer is the cause of inlet closure. Further applications show that summer streamflow and storm events also exert significant control on inlet closure depending on the intensity and timing of the event. The results of the estuarine hydrodynamic study indicate that the hydrodynamic regime of seasonally open estuaries changes distinctively when the inlet state varies from "open" to "closed."

The effects of large-scale human intervention on inlet dynamics are studied. In particular, the response of the largest of the Wadden Sea inlets (Texel Inlet) to the closure of a major part of its inner basin is re-analyzed. The re-analysis is based on datasets of bathymetry, discharges and water levels that have been obtained by intensive monitoring of both the inner basin and the ebb-tidal delta. Based on this re-analyisis and modern theoretical knowledge a conceptual model is postulated that describes the morphologic adjustment of tidal inlets due to large-scale human intervention. The kernel of this model is that the morphological adjustment towards a new overall equilibrium is split into two stages related to the existence of more than one temporal response scale.

At the Shiribetsu River mouth located in Hokkaido, Japan, a jetty has been constructed on the right hand side of the mouth from July to October 1999. In this study, the effectiveness of the jetty construction is studied by analyzing field data of river mouth morphology and water level variation. Before the jetty construction, water level rise due to wave set-up has frequently been observed every year from October to March. It is found that the height of the wave set-up increased in winter with the development of sand spit at the river mouth. After the construction, it is confirmed that the interruption of longshore sediment movement due to the construction of the jetty led to the stability of the river mouth opening. Furthermore, it reduces the wave set-up in the river mouth.