Lagoon

Definition of Coastal lagoon: A shallow, semi-enclosed water body located at the land–sea interface, separated from the open ocean by a barrier system (such as sand spits, barrier islands, or reefs), but maintaining restricted hydraulic connectivity to the sea through one or more inlets. This is the common definition for Coastal lagoon, other definitions can be discussed in the article

Fig. 1. Lagoons along the low-mesotidal shore of Cape Cod (USA). Photo credit Spencer Kennard, Kelsey-Kennard Photo-Gallery. NASA's Earth Observatory.

Lagoons are a specific class of estuaries, characterized by their shallowness, narrow openings and limited freshwater input. The articles Morphology of estuaries and Estuary contain information that is also highly relevant for tidal lagoons. This article focuses exclusively on lagoons and particularly on the physical characteristics of lagoons in sandy coastal environments. An example of such a lagoon is shown in Fig. 1.

Lagoon development

Lagoons typically form behind a barrier that crosses a pre-existing depression or embayment such as a drowned river valley or coastal plain, and their planform reflects this original, often complex landscape. Protected from ocean waves, lagoons are low energy environments that provide accommodation space for sediment deposition. Erosion is mainly driven by tidal currents and/or internally generated wind waves. As sediment infilling makes the lagoon shallower, erosion and sediment transport increase. A dynamic equilibrium between basin morphology and energy regime may then establish: lagoon depths become largely independent of sediment supply. In tide-dominated lagoons, this equilibrium may be influenced by the development of tidal asymmetry during infill, as explained in the article Tidal asymmetry and tidal basin morphodynamics.

Fig. 2.Google Earth image of Lagoa dos Patos, Brazil – the world's largest 'choked' lagoon. The inlet channel is situated at the southern extremity. Cuspate megaspits have developed by wind-driven water circulation along the boundaries.

The barrier that constricts the lagoon inlet is typically formed by a sand spit aligned with the upstream coast. It forms as littoral drift extends the spit alongshore, across a former depression that gradually develops into a lagoon. The processes underlying sand spit development are exposed in the article Sand spit. Wave-induced onshore sand transport (especially by skewed/asymmetric swell waves, see Shoreface profile) and aeolian transport by onshore winds (see Dune development) contribute to building the sand spit into a barrier. The extension of the sand spit across the bay ceases when the ebb flow through the remaining narrow inlet becomes sufficiently strong to carry offshore the sediment supplied by littoral drift. The Appendix presents a simple model of this process, yielding a qualitative analytical expression for the equilibrium inlet cross-section.

Quantitively reliable predictions of lagoon evolution require numerical morphodynamic modeling, that account for site-specific hydrosedimentary features. These include shoals generated by flood flow deceleration inside the lagoon (Fig. 3). Ebb-tidal currents can deposit sand on the seaward inlet side, forming a subtidal ebb delta, though this is less developed on wind-dominated coasts. Wind-driven circulation can be important in lagoons with an elongate planform along the prevailing wind direction. These wind-driven circulations can generate cuspate spits along channel boundaries (Fig. 2).

Lagoons can have widely different sizes, from a few hectares to thousands of square kilometers. Large lagoons often have several inlet channels, which can persist when shoals limit hydraulic connections between them or when residual flow exists between the inlets. In many cases, inlet channels have been artificially stabilized with hard structures (jetties) to facilitate navigation or drainage. Non-stabilized inlet channels may shift in size and location in response to fluctuations in longshore and onshore sediment transport. On microtidal coasts, the size of lagoon inlets is largely dependent on runoff outflow. In periods of low runoff the lagoon inlet may temporarily close.

Lagoon occurrence

Lagoons are widespread coastal features where strong littoral drift (driven by obliquely incident waves, see Littoral drift and shoreline modelling) and abundant sand supply promote barrier formation. Lagoons are most common along low-gradient continental shelves, where wave-driven sediment transport can readily build and maintain barriers. Many lagoons are found along microtidal coasts (e.g. Gulf of Mexico, Baltic, Mediterranean coasts) and low-mesotidal coasts (e.g. American Atlantic and Pacific coasts), but they seldom occur on macrotidal coasts. Steep coasts tend to favor reflective shorelines rather than barrier–lagoon systems. Lagoons are also less well developed along low-energy coasts, muddy coasts and sand-starved rocky shorelines.

Fig. 3. Google Earth image of Ocracoke inlet of Pamlico Sound, USA – a restricted lagoon.

Kjerfve (1986) distinguished three types of lagoons: choked lagoons, restricted lagoons and leaky lagoons. Choked lagoons occur along coasts with high wave energy and significant littoral drift. They are connected to the sea by a single long narrow entrance channel and may consist of a series of connected basins. An example (Lagoa dos Patos, Brazil) is shown in Fig. 2. Restricted lagoons are generally large, shore-parallel, relatively wide water bodies with several inlets. Fig. 3 shows one of the inlets of Pamlico Sound, a large restricted lagoon on the US Atlantic coast. Leaky lagoons have multiple ocean entrance channels along coasts where tidal currents are strong enough to counteract wave action and littoral drift that would otherwise close the inlets. Examples include the Wadden Sea and the lagoons along the shore of Cape Cod (Fig. 1).

In response to sea-level rise, lagoons adjust through onshore barrier translation, with barrier overwash and aeolian transport playing key roles. Lagoon surface area may decrease where landward extension is constrained by highlands, but can expand over low-lying hinterlands. Lagoon infilling and consequent shrinkage are further influenced by the import of marine and terrigenous sediments, as well as by in situ peat formation.

Lagoon closure and barrier breaching

In lagoons with weak tidal flow through the inlet, littoral drift may close off the tidal inlet during dry periods. When wetter conditions return, runoff raises the lagoon water level until it breaches the barrier and creates a new outflow channel. Breaching is often preceded by strong groundwater seepage through the barrier and does not necessarily require scouring by barrier overwash. Susceptibility to breaching depends in particular on the ratio of the lagoon-ocean head difference to the beach barrier width. Barrier breach can also be triggered by storms waves and in some cases by lagoon water set-up in periods of strong offshore winds. In practice, however, barriers are often breached artificially to prevent flooding, to facilitate fish migration or to improve the lagoon water quality. Jetty construction is another common practice to maintain navigable inlets.

During periods of low fresh water runoff, evaporation may raise lagoon salinity. This typically occurs in warm, arid regions. If the inlet remains closed, evaporation can dry out parts of the lagoon, which are turned into sebkhas (salt flats).

During periods of high fresh water runoff, the salinity of lagoon waters can drop sharply. This promotes fast vegetation growth, which converts parts of the lagoon into swamps. These swamps can further evolve into peat marshes that remain emergent when runoff declines. This process results in gradual reduction in lagoon volume.

Human uses

Because of their shallowness and restricted mouth, lagoons generally support different human uses than most estuaries. They rarely host important harbors and are therefore usually less urbanized and industrialized. Natural values are often well preserved, allowing for important ecosystem services such as water purification, nutrient cycling, and carbon sequestration. Coastal lagoon habitats include wetlands, mangroves, salt-marshes and seagrass meadows. These habitats provide safe breeding and feeding areas for many marine organisms, including migratory species, birds, fish, and invertebrates Human uses include fishery, recreation and tourism. However, water renewal is typically slow due to the restricted inlet, making lagoons highly vulnerable to polluted effluents from agriculture, households and industry.

Appendix Tidal lagoon inlet equilibrium

Fig. A1. Schematic lagoon layout and symbols. Top: Longitudinal section along A-B. Bottom: Plan view.

This appendix considers lagoons on tidal coasts, where inlet morphodynamics mainly depends on a balance between offshore sand transport by ebb tidal currents through the inlet and alongshore sand transport driven by obliquely incident waves. The influence of wave-driven onshore transport, that can be a dominant process on microtidal coasts in periods of small littoral drift, is disregarded.

We consider a lagoon with negligible fresh water inflow and water exchange through the inlet dominated by an offshore semidiurnal tide with amplitude [math]a_0[/math] and radial frequency [math]\omega[/math],

[math]\eta_0(t) = a_0 \, \sin \omega t \, . \qquad (A1)[/math]

The water level in lagoons with an open connection to the sea generally exhibits water-level oscillations that follow the sea level at tidal or longer time scales. If the lagoon length is much smaller than the tidal wavelength, and if frictional losses at the lagoon mouth are negligible, the tidal prism [math]P[/math] of the lagoon can be approximated as [math]P = 2 a_0 S[/math], where the lagoon surface area [math]S[/math] has been assumed constant.

Observations from a large number of lagoons worldwide show that the lagoon tidal prism is approximately proportional to the tidally mean cross-sectional area of the lagoon mouth [math]A=hb[/math], where [math]h[/math] is the mean dept and [math]b[/math] the mean width. Considering cases where the depth [math]h[/math] is much larger than the tidal amplitude [math]a_0[/math] and where the width [math]b[/math] is approximately constant, the tidal prism can also be expressed in terms of the tidal velocity amplitude [math]u_0[/math] at the lagoon mouth as [math]P=(2/\omega) A u_0[/math]. The observed proportionality of tidal prism [math]P[/math] and cross-sectional area [math]A[/math] with a proportionality coefficient close to [math]2 / \omega[/math], implies that the tidal velocity amplitude is broadly similar among tidal lagoons and typically around [math]u_0 = 1[/math] m/s.

In reality, many lagoons experience significant frictional losses at the mouth, often due to constriction by sand barriers formed through littoral drift. The mouth width then depends on the strength of the ebb current needed to remove incoming sediment. As the mouth narrows, flow velocities increase. When velocities become sufficiently strong, the system tends toward an equilibrium in which the sediment discharge by the ebb flow through the mouth, [math]q_S[/math], balances on average the sediment supply [math]q_L[/math] from littoral drift. In the following, this equilibrium condition is examined more in detail using an approximate analytical model of the flow through the lagoon mouth.

We consider a lagoon with a constricted inflow channel of length [math]l[/math], according to the idealized geometry shown in Fig. A1. For lagoons much shorter than the tidal wavelength, the tidal elevation within the lagoon, [math]\eta(t)[/math], can be considered uniform throughout the lagoon. The one-dimensional depth-averaged flow equation through the inflow channel then reads

[math]\dfrac{\partial u}{\partial t} + c_D \dfrac{u |u|}{h+\eta} + \dfrac{g}{l} (\eta_0 - \eta) = 0 \, , \qquad (A2)[/math]

where [math]t =[/math] time, [math]c_D \approx 0.003 \sim[/math] seabed drag coefficient, [math]g =[/math] gravitational acceleration. Other symbols are indicated in Fig A1 and Table A1.

This equation is further simplified by considering [math]\; \eta \lt \lt h \lt \lt c_D u / \omega \approx 20[/math] m, giving

[math]c_D \dfrac{u |u|}{h} + \dfrac{g}{l} (\eta_0 - \eta) = 0 \, . \qquad (A3)[/math]

The validity of this equation is restricted to small tidal amplitudes and a limited depth range.

The velocity [math]u[/math] can be eliminated from the lagoon mass balance equation

[math]S \, \dfrac{\partial \eta}{\partial t} = b \, h \, u(t) \, , \qquad (A4)[/math]

giving

[math]\dfrac{\partial \eta}{\partial t} = a_0 \, \omega \, K \, \sqrt{|\eta_0-\eta|} \, sign(\eta_0-\eta) \, , \qquad K = \sqrt{\dfrac{g}{c_D a_0 \omega^2}} \, \dfrac{b h^{3/2}}{S \, l^{1/2}} . \qquad (A5)[/math]

The non-dimensional coefficient [math]K[/math] is called 'repletion' coefficient, as it represents the ratio of the tidal time scale to the time scale for filling the basin to the level [math]a_0[/math].

The nonlinear equation (A5) has no simple analytical solution. However, an approximate analytical solution can be obtained by assuming that the tidal level inside the lagoon oscillates with the same frequency as in the sea,

[math]\eta = a_0 a^* \, \sin(\omega t -\phi) \, . \qquad (A6)[/math]

Equation (A5) can then be rewritten as

[math]a^* \cos(\omega t - \phi) = K \sqrt{|\sin(\omega t) – a^* \sin(\omega t - \phi)|} \, . \qquad (A7)[/math]

Requiring that the left and right hand sides go to zero at the same time implies [math]a^* = \cos \phi[/math]. Requiring that the amplitude of the left and right hand sides of this equation are equal gives [math]a^* = \cos \phi = K \sqrt{\sin \phi}[/math]. We therefore have the approximate solution

[math]u = \sqrt{\dfrac{g a_0 h \sin \phi}{c_D l}} \cos(\omega t -\phi) \, , \quad \sin \phi = \dfrac{K^2}{2} \Big(\sqrt{1 + \dfrac{4}{K^4}} -1 \Big) \, . \qquad (A8)[/math]

The sediment transported by the ebb current [math]u[/math] is parameterized as

[math]q_S = 8 \, \dfrac{\rho}{\Delta \rho} \, \dfrac{c_D^{3/2}}{g} \, u^3 \, , \qquad (A9)[/math]

where [math]\Delta \rho =[/math] difference between sand grain density [math]\rho_{sed}[/math] and seawater density [math]\rho[/math]. Eq. (A9) is a simplified formula, assuming that the ebb velocity is most of the time much larger than the critical velocity for sediment entrainment.

The equilibrium cross-section of the lagoon mouth follows from equating the sand supply by littoral drift during a tidal period and the sediment transported by the ebb flow to the sea,

[math]T \, q_L = b \int_{\phi-\pi/2}^{\phi+\pi/2} \, q_S \, dt \, . \qquad (A10)[/math]

Evaluating the integral, using (A8) and (A9), gives:

[math]K_1 \, q_L = b^4 h^6 \, \Bigg[ \Big( 1 + \dfrac{K_2}{b^4 h^6} \Big)^{1/2} - 1 \Bigg]^{3/2} \, , \qquad K_1 = \dfrac{3 \pi}{4 \sqrt{2}} \dfrac{\Delta \rho}{\rho} \dfrac{(\sqrt{c_D}\omega S l)^3}{g^2}\, , \qquad K_2=\Big(\sqrt{\dfrac{2 c_D a_0 l}{g}} \omega S\Big)^4 \, . \qquad (A11) [/math]

The width of natural tidal inlets is generally about a factor of [math]\alpha \sim 100[/math] larger than the depth. Substitution of [math]b=\alpha h, \, A= \alpha h^2[/math] gives

[math]\alpha K_1 \, q_L = A^5 \, \Bigg[ \Big( 1 + \dfrac{\alpha K_2}{A^5} \Big)^{1/2} - 1 \Bigg]^{3/2} \, . \qquad (A12) [/math]

Fig. A2. Equilibrium curves for the inlet cross-section and velocity . Blue: right hand side of Eq. (A12); red: left hand side. Green: the velocity amplitude Eq. (A8). Parameter values are given in Table A1.

This relationship is shown in Fig. A2 for a particular lagoon with geometrical characteristics commonly occurring in nature, shown in table A1.

Table A1

Symbol	Description	Value
[math]a_0[/math]	offshore tidal amplitude	1 m
[math]b[/math]	width of the inlet channel	[math]\alpha \, h[/math]
[math]c_D[/math]	channel bed drag coefficient	0.003
[math]l[/math]	inlet channel length	1000 m
[math]q_L[/math]	littoral drift	0.005 m3/s (= 150,000 m3/year)
[math]S[/math]	lagoon surface area	107 m2
[math]\alpha[/math]	ratio channel width to depth	100
[math]\omega[/math]	tidal radial frequency	0.00014 s-1

Figure A2 indicates an inlet equilibrium cross-section of 1500 m², corresponding to a mean depth [math]h[/math] of about 3.9 m and a tidal velocity amplitude [math]u_0[/math] close to 1 m/s – a value within the range typically observed in tidal lagoons. Lower velocities are predicted in the case of smaller littoral drift and higher velocities in the case of larger littoral drift. However, these predictions are less reliable. In the former case, a large inlet cross-section and low tidal velocity amplitude tend to invalidate Eq. (A3), in which the velocity acceleration term is neglected relative to the friction term. In the latter case, the equilibrium depth becomes so small that the tidal variation of the inlet depth in Eq. (A3) can no longer be neglected. Figure A2 also shows that no equilibrium is possible if the littoral drift exceeds a certain limit – in the present example a littoral drift equivalent to about 3-4 million m³/year. In this case, the lagoon will be permanently closed from the sea if there is no net runoff from the catchment into the lagoon.

Fig. A2 suggests a second possible equilibrium, corresponding to very small values of the inlet cross-section and depth. This model prediction can be questioned because of the invalidity of the model assumptions mentioned above. Moreover, such an equilibrium would be instable because any small perturbation of the littoral drift implies a modification of the tidal velocity that either leads to inlet closure or to ongoing scouring of the inlet. The opposite holds for the first equilibrium solution, where an increase or decrease in the inlet cross-section by perturbation of the littoral drift lead to a negative feedback. An increase in the inlet cross-section leads to decrease in the tidal velocity, thus promoting accretion of the inlet cross-section. Conversely, a decrease in the inlet cross-section promotes scouring. The first equilibrium solution is thus predicted to be stable.

Another equilibrium criterion was proposed by Bruun and Gerritsen (1960), based the ratio [math]P/Q_L[/math], where [math]P =[/math] spring tidal prism and [math] Q_L =[/math] total annual littoral drift (both expressed in cubic meters). From analysis of a large number of tidal lagoons they infered that lagoon inlets have a high degree of stability when the ratio [math]P/ Q_L [/math] is larger than about 300. For [math]P/ Q_L [/math] ratios less than 100, they observed a predominant transfer of sand on (shallow) bars or shoals across the inlet entrance and less significant tidal currents. They therefore concluded that these inlets, which are often characterized by one or more narrow, frequently shifting channels, are generally less stable.

In the example shown in Fig. A2, the solution [math]A =[/math] 1500 m² is stable according to the simplified analytical model. The ratio [math]P/Q_L=[/math] 133, which does not guarantee stability according to the criterion of Bruun and Gerritsen. This indicates that the analytical model should be used with caution, because some potentially important processes are disregarded, in particular the occurrence of wave-induced onshore sand transport.

Related articles

Morphology of estuaries

Estuary

Sand spit

Littoral drift and shoreline modelling

References

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2026): Lagoon. Available from http://www.coastalwiki.org/wiki/Lagoon [accessed on 31-03-2026] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J