Inlet Geomorph Bibliography-Processes

Kraus, N.C., 2001. On Equilibrium Properties in Predictive Modeling of Coastal Morphology Change. Proceedings, Coastal Dynamics 01 Conference, ASCE, pp. 1-15.

This paper discusses the benefit of incorporating equilibrium properties in coastal morphology. Equilibrium properties of coastal systems exist over small, intermediate and large time scales. Kraus discussed that the equilibrium properties can constrain basic physics calculations which, for coastal processes calculations, may be very difficult to determine. Constraining these equations with the condition of equilibrium may make these problems solvable. Kraus includes definitions of key terms; closed/open system, steady state and static equilibrium, dynamic equilibrium, unstable equilibrium, asymptotic equilibrium (saturation) and liberation.

Kraus discusses equilibrium at beaches (profile equilibration) and equilibrium relationships for tidal inlets (with a tabulated literature review included). The author then extends upon the reservoir model (Kraus, 2000) and presents an example of its application at Shinnecock inlet and shows the sediment pathways which should be included within the reservoir model for this example inlet with focus on the use of equilibrium to describe transport processes at inlets.

Price, W.A. 1963. Patterns of Flow and Channeling in Tidal Inlets. Journal of Sedimentary Petrology, Vol. 33, No. 2, pp. 279-290.

Price examined the hydrodynamic nature of currents which flow through the inlet. Five types of tidal openings are investigated in this paper: tidal inlets with deltas; tidal inlets with one or both deltas absent but with bottom channeling, openings in coral reefs where channeling is present, mouths of wide shallow estuaries obstructed by bars, straights of continental shelves. All of these tidal opening types are slot shaped where their widths exceed their depths. A range of tidal inlets example locations with and without delta development are provided. Flow patterns through the deltas and inlet trough are discussed comparing ebb and flood flows. A discussion of flow reversals is also included. Price also presents information on the tidal jet, the nature of jet flow and associated sediment deposition in both deep and shallow passageways.

FitzGerald, D.M., 1982. Sediment Bypassing at Mixed Energy Tidal Inlets. Proceedings 18th Coastal Engineering Conference, ASCE Press, pp. 1094-1118.

FitzGerald examines inlet sediment bypassing through stable inlet processes and ebb delta breaching at six mixed energy (tide-dominated) coasts at non-structured tidal inlets. As an introduction, the work by Bruun and Gerritsen (1959) is discussed whereby the type of bypassing processes at an inlet can be determined by the ratio between longshore sediment transport and maximum discharge at the inlet under spring tidal conditions.

FitzGerald describes the new landward sediment transport due to landward directed currents over the ebb shoals terminal lobe which retard ebb currents and enhance flood currents. Additionally, the model of bar migration up the shore face with associated stacking of the swash bars is shown. Ebb tidal delta breaching caused by a dominant direction of longshore sediment transport is discussed. This, in turn, results in downdrift migration of the main ebb channel, eventually breaching of the shoal to form a new, more hydraulically efficient channel and bar migration onshore.

Examples of bypassing from five regions are discussed: Maine coast, central South Carolina, East Friesian Islands along the West German North Sea, the southern New Jersey coast, the Virginia coast and the Gulf of Alaska Cooper River Delta.

The location of the bar welding is discussed as it influences erosional and depositional patterns along the barrier island. The bar welding location is affected by the inlet size, wave versus tide dominance and channel orientation.

FitzGerald, D.M., 1996. Geomorphic Variability and Morphologic Sedimentologic Controls on Tidal Inlets. Journal of Coastal Research, SI 23, pp.47-71.

Geomorphic variability at tidal inlets is discussed in this paper. FitzGerald defines a tidal inlet as an opening in the shoreline through which water penetrates land, connecting the ocean and bays, lagoons, marsh and tidal creek systems. He describes that the main channel of the tidal inlet is maintained by tidal currents, distinguishing a tidal inlet from open embayments or rock bound passages where there is little or no mobilized sediment.

FitzGerald describes a history of morphologic models for inlet shoreline alignment, ebb tidal shoal processes, hydrographic regime and temporal changes at inlets and includes graphical depictions of the models described. FitzGerald then summarizes geomorphic and sedimentologic controls on tidal inlets. These include: sediment supply, basin geometry, regional stratography, occurrence of bedrock, riverine discharge and sea level changes. He also describes secondary controls with interaction of two or more of the factors previously mentioned.

Ebb tidal and inlet throat morphology is discussed as is the relationship to waves and tidal prism and the effects of deltas on the inlet shoreline. A table of tidal ranges and wave heights and prisms of mixed energy (tide dominated) shorelines of the world is included within this paper. Relationships between tidal prism and throat are and literature on the topic is discussed in this paper as is the dynamic relationship between tidal prism and inlet throat cross-sectional area.

Case studies of backbarrier processes in the East Friesian Islands, central Couth Carolina and in Chatham Harbor, Cape Cod are discussed. Additionally, a case study at the Saco River estuary and Kennebec River estuary in Maine are included in a discussion of estuary/inlet interaction and salinity effects.

Nummedal, D., Oertel, G.F., Hubbard, D.K., and Hine, A.C., 1977. Tidal Inlet Variability- Cape Hatteras to Cape Canaveral. Proceedings, 1977 Coastal Sediments Conference, American Society of Civil Engineers, pp. 543-562

A discussion of tidal range along the east coast study area begins this paper. It classifies North Carolina and northern South Carolina as microtidal wave dominated, southern South Carolina and Georgia as mesotidal tide-dominated, and the northeast coast of Florida as microtidal wave dominated. The paper builds on studies of tidal range to include wave energy, inner shelf slope and hydrologic properties of the inlets associated lagoon.

Physical parameters such as wave action and tidal current are illustrated on a graph with wave action increasing and tidal velocity decreasing from north to south. The authors then turn to a discussion of geological parameters. These include total lagoon area, open water area, percent open water area to total maximum throat depth, ebb tidal delta area, inner shoal area, maximum offshore distance of ebb tidal delta, distance to the 18 foot offshore depth contour and inner shelf slope. These values are provided for representative inlets in North Carolina, South Carolina, Georgia and Florida. Physical parameters are compared and discussed for the inlets in each of the states examined. The authors preformed a qualitative analysis of sediment transport mechanisms in a zone of wave and current interactions to improve understanding of tidal inlet process-response characteristics. A figure of states vs physical and geological parameters is included with the discussion.

They conclude that wave dominated inlets typically have small ebb-tidal deltas, pushed up against shore, wide throats with multiple sand bodies, and significant inner shoals. The tide dominated inlets are characterized by large ebb-tidal deltas extending far out from shore, well-defined deep main channels and inlet throats, and an absence of inner shoals, except where fresh water inflow induces stratification and landward bottom sediment transport. Further, the authors discuss that the regional variation identified in the paper is due to changes in the nearshore wave energy and tidal range from north to south. The tidal range increases and the wave energy decreases toward the apex of the Georgia Bight due to widening of the shelf and a decrease in nearshore slope and deep water wave action. The authors indicate that the ratio between open water and marsh in most Georgia and South Carolina inlets is such that inlet flow becomes dominant. The large open water areas relative to the marsh for North Carolina inlets may create flood dominance at these inlets. The existence of large inner shoals in ebb-dominated inlets can be attributed to the mechanics of wave-current interaction which produces higher suspended sediment concentrations at flooding than at ebbing tide.

Fitzgerald, D.M., Kraus, N.C., and Hands, E.B. 2001. Natural Mechanisms of Sediment Bypassing at Tidal Inlets. ERDC/CHL CHETN-IV-30, U.S. Army Engineer Research and Development Center, Vicksburg, MS.

In this paper, Fitzgerald, Kraus, and Hands present sediment bypassing at natural and modified inlets. They identify bypassing through examination of sequences of aerial photographs and bathymetric maps. The mechanisms identified include:

• Stable inlet processes – This is the case of stable inlets with non-migrating throats and stable main ebb channel position through the ebb delta. At these inlets bypassing occurs through formation of large bar complexes which migrate and attach to the downdrift shoreline.
• Ebb-tidal delta breaching – In this case, the throat is stable and the main ebb channel cyclically migrates downdrift.
• Inlet migration and spit breaching – In this case, throat constriction is caused by longshore transport and bar breaching reestablishes a new, more hydraulically efficient channel. This type of movement can be identified by the presence of an updrift spit and elongation of the tidal channel. The new inlet channel may be opened due to differences in the tidal phase and tidal range between the ocean and the back barrier. The new inlet may form during a storm. The old (migrated) inlet is increasingly less hydraulically efficient and closes.
• Outer channel shifting – This type of bypassing is limited to the seaward end of the main ebb channel and involves smaller sediment volumes than the ebb-tidal delta breaching model. The outer channel is deflected downdrift while the main channel remains fixed. As the outer channel becomes more and more deflected, it becomes hydraulically inefficient.
• Spit platform breaching – This type of bypassing occurs at migrating inlets where asymmetric channel configurations form due to the influence of the updrift barrier spit. The new channel is breached through the spit platform under this mechanism. This is analogous to flow through a river meander bend. In this form, secondary channels may be created.
• Bypassing at wave dominated inlets - Wave dominated inlets form arcuate ebb shoals close to shore and transport of sediment occurs continuously along the periphery of the delta over the shallow distal portion (especially at high tide).
• Jetty-weir bypassing - Jetty-weir bypassing occurs at inlets with one or two weirs and no settling basin. Transport into the weirs is most active during storms. The sediment in the weir can be transported seaward by ebb currents. This type of bypassing occurs most at inlets when ebb currents are strong enough to transport sands out of the channel.
• Jettied inlet bypassing – Sediment bypassing at jettied inlets occurs when excess sediment accumulated on the updrift beach. The amount of sediment accumulation and bypassing is dependent upon the jetty length, inlet size, channel depth, total current strength, and ebb shoal morphology. In this case, the jetties funnel ebb discharge and displaces the ebb shoal further offshore thus reducing the effects of waves retarding the formation of bar complexes. Transport along the outer bar by wave action occurs primarily during storms.
• Outer channel shifting at jettied inlets. In this form of bypassing deflection of the outer channel and shoal breaching, to produce a more hydraulically efficient channel. Sediment from the relic shoal is transported onshore due to wave action.

These types of bypassing mechanisms are discussed in this paper along with the volume of sediment transported through each of these mechanisms and bypassing frequency.

Riedel, H.P., and Gourlay, M.R. 1980, Inlets/Estuaries Discharging Into Sheltered Waters. Coastal Engineering, pp. 2550-2564.

The study was motivated by the design of a new international airport in Australia. During this design process an existing stable creek (Serpentine Creek) was reclaimed and flood waters were diverted into an artificial inlet (Moreton Bay). In order to design this reclamation and diversion, Riedel and Gourlay investigated characteristics of inlets and estuaries discharging into sheltered waters.

This area of Australia has a mild wave climate with low wave heights and small waver periods. Also, this area has low littoral drift rates. Although a literature review of the relationships previously derived for tidal inlets on open coasts are included in this paper, Riedel and Gourlay acknowledge that these relationships are not likely to apply in this case.

A literature search of Australian studies was performed for this research and a short discussion of this literature is included in this paper. Field studies were undertaken to obtain relationships. Tide, current and limited hydrographic data was obtained for four inlets and their estuaries in South East Queensland (Beelbi Creek in Hervey Bay, Tingalpa, Serpentine and Burpengary creeks in Moreton Bay). These were selected because of their similarity to the proposed artificial inlet (including sediment similarity). The data obtained consisted of: Tide records, tidal velocities, hydrographic surveys to define cross-sectional areas and tidal prisms.

Riedel and Gourlay identified that there are differences between the stability characteristics of small inlets discharging into sheltered waters and large systems connected through an exposed shoreline but that the difference is purely in turns of scale. For a given cross-sectional area of the inlet entrance the tidal prism for the exposed coast inlets is approximately 2 to 3 times those of sheltered inlets. Sheltered inlets have smaller littoral drift rates and cross sectional areas of sheltered entrances are larger than for exposed inlets for a given prism and velocities will be lower. Riedel and Gourlay also included a discussion of the relationship between cross sectional area and tidal prism for estuaries.

Hubbard, D.K., Barwis, J.H., and Nummedal, D., 1977. Sediment Transport in Four South Carolina Inlets. Proceedings, Coastal Sediments 1977. pp. 582-601.

This paper presents hydrographic studies at four South Carolina inlets (Fripp, Stono, Murrells, and Little River) to investigate sediment transport patterns through the inlet throat and across adjacent shoals. This work builds upon research on the variability in inlet types. Also included is a discussion of a model for ebb tidal delta circulation. As part of this research, current velocities and tidal lengths were measured hourly for 26 hours at each of the four inlets. During the studies the researchers noted the importance of wave induced sediment transport. A more detailed study was begun at Murrells inlet. Wave observations were taken over an 8 day period. Suspended sediment samples were collected over four days to determine sediment transport rates. Swash bar migration rates were also measured to estimate bedload transport. Tidal current processes in the main channel, swash platforms, swash bars and channel margin bars of the four inlets are discussed in this paper along with the sediment transport processes associated with each case.

The authors observed that degree of marsh development controls the ebb and flood dominance at the inlet and that the relative elevation of the water at the maximum flood and ebb flows effects channel flow through the inlet. The features of swash platforms and swash bars are described in this paper. Swash bar surfaces are dominated by landward flow. This dominance can result in time velocity asymmetry from topographic influences and wave input. Each influences are discussed. The authors also describe how wave processes effect sediment transport in tidal inlets. At Murrels inlet, the waves break in much shallower water (relative to wave heights) on flood than on ebb due to the effects of the currents. This process effects sediment transport on the bar. Previously established sediment transport rates from CIRC (1973) were compared to the measured data collected as part of this research. It was concluded that the theoretical relationships, based on fluvial or flume data may have questionable application in the tidal environment.

Dean, R.G., and Walton, T.L., 1973. Sediment Transport Processes in the Vicinity of Inlets with Special Reference to Sand Trapping. Estuarine Research, Volume II, pp. 129-149.

Dean and Walton focus on the sand trapped within the outer shoals of Florida inlets. They discuss flow processes at inlets and interaction between flow and inlet outer bars at inlets and the effects of wave energy on limiting shoal volumes. The material in the outré shoal can be thought of as being acted upon by (1) tidal forces which act offshore and (2) wave forces which act to return the material to the inlet. When these forces are balanced, the shoal has reached equilibrium. The authors also discuss the processes of migration as it relates to equilibrium of inner and outer shoal volumes. They provide examples of inlets with improvements (jetty construction and dredging) and discuss how these improvements effect sedimentary processes around inlets and modify the sediment budgets. The authors describe jetties with and without wiers. Jetties confine an inlet’s current and cause sand deposits to je “jettied” out to deeper water. Dean and Walton suggest that the areas of moderate wave action, where net littoral drift is substantial and the navigation channel is greater than 20 feet, natural processes are not likely to be effective in reestablishing natural bypassing after jetty construction. They describe examples of Hillsboro Inlet, Florida and Masonboro Inlet, NC. Dean and Walton also discuss dredging of the outer bar and the interruption to littoral drift and lowering of elevations in the entire bar formation that it causes. Specific examples of 23 Florida inlets are provided along with tables of the volumes of material deposited in the outer inlet and bay shoals. This paper concludes with an evaluation of the relationship between the volume maintained within the inlet shoals and the current erosion rate in Florida.

Walton, T. L. Jr. (2002). Tidal Velocity Asymmetry at Inlets, ERDC/CHL CHETN IV-47. U.S. Army Engineer Research and Development Center, Vicksburg, MS. http://chl.wes.army.mil/library/publications/chetn

In this paper the types of inlet asymmetries are discussed and specific focus is given to channel tidal velocity asymmetry which drives sediment transport. Two possible types of inlet tidal velocity asymmetry are presented here; flood dominant asymmetry and ebb dominant asymmetry. The relationship between bay tide, hb(t) and channel velocity, u(t) (from Kulegan 1967), u(t) =(Ab/Ac)*dhb(t)/dt where Ab is the cross sectional area of the bay and Ac is the cross sectional area of the channel leads to a discussion of tidal forcing by tidal harmonic constituents and asymmetry caused by them. He discusses the relationship developed by Boon and Byrne (1981), based on Kulegan (1967), who presented a bay tide relationship of hb=AM2cos(ωt)+AM4 cos(2ωt-gM4) where flood dominance exists if π ≤ gM4 ≤ 2π and ebb dominance exists if 0 ≤ gM4 ≤ π and the greater the ratio of AM4/AM2 the greater the flood or ebb dominance. Walton discusses other causes of tidal asymmetry as well, including asymmetry caused by friction (asymmetry generated by tidal interactions with estuarine/inlet channel geometry) asymmetry generated by basin hypsometry (the vertical distribution of bay surface area with wave height). He also presents asymmetry examples at five flood dominant inlets and four ebb dominant inlets.

Boon, J. D., and Byrne, R.J., 1981. On Basin Hypsometry and the Morphodynamic Response of Coastal Inlet Systems. In Marine Geology, 40 (1981), Elsever Scientific Publishing Company, Amsterdam, pp. 27-48.

The aim of the paper is to expand upon previous research into the tidal hydraulic processes which contribute toward flood or ebb dominance in inlet transport regimes. The authors introduce the concept of basin hypsometry which is the distribution of basin surface area with height in lieu of the term basin geometry which, as they discuss, refers to three special dimensions. Basin hypsometry involves only two special dimensions and id directly associated with the continuity equation which is used in tidal-flow computations. This allows for the simulation of the tidal hydraulic response in a basin and inlet system where the basin fills through sedimentation aided by marsh development.

Boone and Byrne utilize the INLET2 numerical model to examine the interaction between basin hypsometry and inlet channel hydraulics utilizing a large marsh basin complex near Wachapreague, VA and Swash bay, an individual marsh within the larger complex, as an example case. The authors show that both channel configuration and basin hypsometry are controlling factors in determining the characteristics of the mean vertical tide within the basin and the mean horizontal tide in the main channel:

(1) A mature or sediment-filled basin (ϒ = 1.8, 2.5) having adequate communication with the sea produces positive tidal duration differences. The latter are conductive of greater peak discharge and greater peak velocity during ebb.

(2) An open basin (ϒ = 3.5, 5.0) produces negative tidal duration differences associated with greater peak discharge during flood. Peak channel velocities are dependent upon the degree of tidal range reduction and position within the conveyance channel.

(3) Major reductions in channel cross-sectional area lead to a reduction in basin tidal range which tends to eliminate the effect of varying basin hypsometry. The tidal duration difference becomes strongly negative for highly restricted channels.

(4) A filled marsh basin (ϒ = 1.8) appears to reach a condition in which positive duration differences are progressively reduced as the channel cross-sectional area nears maximum values. It follows that the right side of the curve for ϒ = 1.8 may represent a region favoring dynamic equilibrium; namely, one in which the ebb transport potential or channel flushing capacity seaward varies inversely with channel cross-sectional area. The present Swash Bay system lies within this region.

(5) As a given basin fills with sediment, its potential tidal prism is continually made smaller. Thus the four basin configurations presented in Figure 8 this paper represent four different magnitudes of water volume seeking to pass through a given channel area indicated on the abscissa. The greater the volume, the greater the effect of channel impedance in reducing the portion that is admitted to the basin. The paper indicates that, for this reason, filled basins have a delayed reduction in tidal range as the channel cross-sectional area nears minimum values.

The authors discuss the tidal harmonic signatures M2, the fundamental harmonic period, taken as 12.42 mean solar hours, the period of the principle lunar semidiurnal constituent, and M4, the first-harmonic term representing the lunar quaterdiurnal constituent, a shallow water tide with a period of 6.21 solar hours. The addition of the M2 and M4 tidal harmonics produce a fixed distortion in the mean semidiurnal tide. Channel velocity and tidal harmonic relationships are then discussed in terms of ebb and flood dominance for three east coast basin and inlet systems. The authors use harmonic information to identify some of the morphodynamic differences seen at these inlets.

Dissanayake et al., 2009 Modeled Channel Patterns in a Schematized Tidal Inlet. Coastal Engineering 56 (2009) pp. 1069-1083.

This paper describes the process-based Delft3D (2DH) modeling performed for inlets in the Dutch Wadden Sea. The model used is forced by tides only and both short and long term simulations are run with morphology similar to Ameland Inlet.

The inlets in the Dutch Wadden Sea are mixed energy tide dominated. The Ameland inlet has a westward oriented main channel and ebb tidal delta. The reason for this orientation is hypothesized and then investigated using the model. The model domain is discussed along with details of model setup and sensitivity runs and model runs from a variety of different scenarios (inlet width, tidal asymmetry and direction, transport formulation and relative location of the tidal basin). Short term simulations were carried out over a few tidal cycles using M1 and M2 tidal forcing parameters.

The outputs of the short and long term model runs are discussed along with channel and ebb shoal asymmetry. The authors found that the direction of tidal forcing was the main parameter governing orientation of the main inlet channel and ebb delta.

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