Lagoon
Definition of Coastal lagoon:
A shallow water body located at the land–sea interface, semi-enclosed by a barrier system (such as sand spits, barrier islands, or reefs), connected to the ocean by one or more restricted inlets which remain open at least intermittently[1][2].
This is the common definition for Coastal lagoon, other definitions can be discussed in the article
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Lagoons are often considered a class of estuarine or transitional waters, distinguished by their shallow depth, restricted connection with the sea and limited freshwater input. Some lagoons, however, can receive large episodic freshwater discharges. The present article focuses on lagoons themselves, with particular emphasis on their physical characteristics in sandy coastal environments. An example is shown in Fig. 1. The articles Morphology of estuaries and Estuary provide background information that is also relevant for tidal lagoons.
Coastal lagoons are commonly classified as 'transitional water systems'. This term was introduced in the context of the European Union Water Framework Directive to describe waters that are neither fully freshwater nor fully marine, but are brackish or hypersaline[3][4][5]. Transitional waters include rias, fjords, fjards, estuaries and lagoons. Lagoon tides range from almost absent to relatively strong, with many intermediate conditions.
In Europe, coastal lagoons are classified and protected under Directive 92/43/EEC on habitat conservation. Many lagoons also support breeding bird populations and are therefore additionally protected under Directive 2009/147/EC on bird conservation (see Birds Directive, Habitats Directive, NATURA 2000).
In temperate climate zones, coastal lagoons often support extensive salt marshes. Mangroves are characteristic habitats of many tropical and subtropical coastal lagoons. These habitats are discussed in the corresponding Coastal Wiki articles.
Contents
Lagoon development
Lagoons typically form behind a barrier that closes off, or partly closes off, a pre-existing depression or embayment, such as a drowned river valley or a low-lying coastal plain. Their planform therefore often reflects the inherited, irregular shape of the former landscape. Sheltered from direct ocean wave attack, lagoons are generally low-energy environments that provide accommodation space for sediment deposition. Erosion is mainly caused by tidal currents and internally generated wind waves. As sediment infilling reduces lagoon depth, erosion and sediment transport may increase, possibly leading to a dynamic equilibrium between basin morphology and the local energy regime. In tide-dominated lagoons, this equilibrium may also be affected by tidal asymmetry during infilling, as explained in Tidal asymmetry and tidal basin morphodynamics. A positive feedback to infilling is also possible due to horizontal and vertical extension of vegetated areas. In this case inlet closure may occur due to decrease of the tidal prism.
The barrier that constricts a lagoon inlet is commonly formed by a sand spit connected to the updrift coast. Littoral drift extends the spit alongshore across a former embayment, which gradually becomes a lagoon. The processes involved in spit formation are described in Sand spit. Wave-induced onshore sand transport, especially under skewed or asymmetric swell waves (see Shoreface profile), and aeolian transport by onshore winds (see Dune development) can further build the spit into a barrier. Spit extension across the bay ceases when the ebb flow through the remaining inlet becomes strong enough to export the sediment supplied by littoral drift. The Appendix presents a simple model of this process, leading to a qualitative analytical expression for the equilibrium inlet cross-section.
Quantitatively reliable predictions of lagoon evolution require numerical morphodynamic models that account for site-specific hydrodynamic and sediment-transport conditions. These include shoals formed by flood-flow deceleration inside the lagoon (Fig. 3). Ebb-tidal currents may deposit sand on the seaward side of the inlet, forming a subtidal ebb delta, although such deltas are generally less developed on wind-dominated coasts. Wind-driven circulation can also be important, especially in lagoons whose long axis is aligned with the prevailing wind direction. Such circulation patterns can generate cuspate spits along lagoon or channel margins (Fig. 2).
Lagoons vary widely in size, from a few hectares to thousands of square kilometres. Large lagoons often have several inlet channels, which may persist where shoals restrict hydraulic exchange between different parts of the basin[6] or where residual flow occurs between inlets[7]. In many lagoons, inlets have been artificially stabilized with hard structures such as jetties to improve navigation or drainage. Non-stabilized inlets may shift in size and position in response to variations in longshore and onshore sediment transport. On microtidal coasts, inlet size often depends strongly on freshwater outflow; during periods of low runoff, the inlet may close temporarily.
Lagoon occurrence
Lagoons are widespread where strong littoral drift, driven by obliquely incident waves (see Littoral drift and shoreline modelling), combines with abundant sand supply to promote barrier formation. They are most common along low-gradient continental shelves, where wave-driven sediment transport can readily build and maintain barriers. Many lagoons occur on microtidal coasts, such as those of the Gulf of Mexico, the Baltic and the Mediterranean, and on microtidal to low-mesotidal coasts, such as parts of the American Atlantic and Pacific margins. Lagoons seldom occur on macrotidal coasts, because strong tidal currents tend to maintain wide inlets, promote tidal-channel development and counteract spit growth across the inlet. Steep coasts generally favor reflective shorelines rather than barrier–lagoon systems. Lagoons are also less well developed along very low-energy coasts, muddy coasts and sand-starved rocky shorelines.
Kjerfve (1986[8]) distinguished three main lagoon types: choked, restricted 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 and narrow entrance channel and may consist of a series of connected basins. Lagoa dos Patos, Brazil, is shown in Fig. 2 as an example. Restricted lagoons are generally large, shore-parallel and relatively wide water bodies with several inlets. Fig. 3 shows Apalachicola Bay, Florida, a large restricted lagoon on the Gulf of Mexico.
Leaky lagoons have multiple ocean entrance channels along coasts where tidal currents are strong enough to prevent complete closure by wave action and littoral drift. Examples include the Wadden Sea and the lagoons along the shore of Cape Cod (Fig. 1). Their inlets are commonly deeply scoured by tidal currents. Tide-dominated inlets can also occur on microtidal coasts where the lagoon surface area is large enough for the tidal prism to exceed the annual longshore sand transport volume by a large factor (about 300[9], see the Appendix). Examples include Pamlico Sound on the North Carolina coast and Barataria Bay on the Louisiana coast. Tide-dominated inlets generally have well-developed ebb and flood tidal deltas[10] (Fig. 4).
In response to sea-level rise, lagoons may adjust by landward barrier migration, with barrier overwash and aeolian transport playing important roles. Lagoon surface area may decrease where landward migration is constrained by high ground or human infrastructure ('Coastal squeeze'), but may increase where low-lying hinterlands are inundated. Lagoon infilling and shrinkage are further influenced by the import of marine and terrigenous sediments and by in situ peat formation.
Lagoon closure and barrier breaching
In lagoons with weak tidal flow through the inlet, littoral drift may close the inlet during dry periods. When wetter conditions return, runoff can raise the lagoon water level until the barrier is breached and a new outflow channel is formed. Breaching is often preceded by strong groundwater seepage through the barrier and does not necessarily require scouring by overwash from the sea[11]. Susceptibility to breaching depends particularly on the ratio between the lagoon–ocean head difference and the barrier width[12]. Breaching can also be triggered by storm waves and, in some cases, by lagoon water-level set-up during periods of strong offshore wind[13]. In practice, barriers are often breached artificially to prevent flooding, facilitate fish migration or improve lagoon water quality. Jetty construction is another common intervention used to maintain navigable inlets.
During periods of low freshwater runoff, evaporation may increase lagoon salinity. This is especially common in warm, arid regions. If the inlet remains closed, evaporation may dry out parts of the lagoon floor, forming sabkhas or salt flats.
During periods of high freshwater runoff, lagoon salinity can drop sharply. This may promote rapid vegetation growth, converting parts of the lagoon into swamps. These swamps can subsequently develop into peat marshes that remain emergent when runoff declines. This process gradually reduces lagoon volume and may eventually contribute to inlet closure.
Human uses
Because of their shallow depth and restricted inlets, lagoons often support different human uses from those of larger estuaries. They rarely host major harbors and are typically less urbanized and industrialized, which can help preserve their natural value and the ecosystem services they provide, including water purification, nutrient cycling and carbon sequestration. Coastal lagoons include habitats such as wetlands, mangroves, salt marshes and seagrass meadows. These habitats provide important breeding and feeding grounds for marine and brackish-water organisms, including migratory birds, fish and invertebrates[14]. As transitional environments, coastal lagoons are naturally dynamic and subject to frequent environmental fluctuations and disturbances[15][16][1][17]. Human uses mainly include fisheries, mariculture, recreation and tourism. These activities may lead to habitat loss, physical alteration, pollution and overexploitation, as observed in many European lagoons[18].
Because water exchange is restricted, lagoons are especially vulnerable to polluted effluents from agriculture, households and industry, supplied by surface runoff or groundwater. Relevant background is provided in Threats to the coastal zone, Possible consequences of eutrophication and the articles referenced there. Lagoon flushing involves several processes, including the dispersion mechanisms described in Seawater intrusion and mixing in estuaries. Residence times can nevertheless be long, especially in choked and restricted lagoons.
Climate change
As ecotone systems connected to both freshwater and marine environments, lagoons are highly exposed to climate change. They are often considered 'sentinel systems' because they respond sensitively to environmental stressors. Climate change affects key lagoon conditions, including temperature, dissolved oxygen, salinity, nutrient concentrations and hydrology. Owing to their confined nature, lagoons are particularly sensitive to these pressures, especially in highly reactive interface zones such as the land–water, freshwater–marine, sediment–water and water–atmosphere boundaries[19].
Appendix Tidal lagoon inlet equilibrium
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[20], 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 depth and [math]b[/math] the mean width[21]. 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[22].
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- \eta_0) = 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- \eta_0) = 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} = \omega \, K \, \sqrt{ a_0 \, |\eta_0-\eta|} \, \rm{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]. [23]
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)|} \, \rm{sign} \left[\sin(\omega t) – a^* \sin(\omega t - \phi) \right] \, . \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]. With this harmonic approximation, the velocity inferred from the continuity equation can be written as[22]
[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[24]
[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)/\omega}^{(\phi+\pi/2)/\omega} \, 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]
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 unstable 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 perturbations of the littoral drift lead to 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[9]), based on 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 inferred 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 such 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
- Salt marsh
- Mangroves
- Sand spit
- Littoral drift and shoreline modelling
- Seawater intrusion and mixing in estuaries
References
- ↑ 1.0 1.1 Kjerfve, B. (Editor) 1994. Coastal Lagoon Processes. Elsevier Oceanographic Series 60
- ↑ Dean, R.G. and Dalrymple, R.A. 2002. Coastal Processes with Engineering Applications. Cambridge University Press, Cambridge, UK.
- ↑ Basset, A., Carrada, G.C. 2007. Editorial. Transitional Waters Bulletin 1: 1–2
- ↑ McLusky, D. and Elliott, M. 2007. Transitional waters: A new approach, semantics or just muddying the waters? Estuarine Coastal and Shelf Science 71(3):359-363
- ↑ Elliott, M. and Whitfield, A.K. 2011. Challenging paradigms in estuarine ecology and management. Estuarine, Coastal and Shelf Science 94(4):306-314
- ↑ van de Kreeke, J., Brouwer, R.L., Zitman, T.J. and Schuttelaars, H.M. 2008. The effect of a topography high on the morphological stability of a two-inlet bay system. Coast. Eng. 55: 319–332
- ↑ Salles, P., Voulgaris, G.and Aubrey, D. 2005. Contribution of nonlinear mechanisms in the persistence of multiple tidal inlet systems. Estuar. Coast. Shelf. Sci. 65: 475–491
- ↑ Kjerfve, B. 1986. Comparative oceanography of coastal lagoons. pp. 63-81. In: Estuarine Variability. (Wolfe D.A., ed.). Academic Press, New York
- ↑ 9.0 9.1 Bruun, P. and Gerritsen, F. 1960. Stability of coastal inlets. Trans. Am. Soc. Civ. Eng. 125: 1228–1259
- ↑ FitzGerald, D.M. and Miner, M.D. 2022. Tidal Inlets and Lagoons Along Siliciclastic Barrier Coasts. Treatise on Geomorphology, 2nd edition, Elsevier, Volume 8: 270-288
- ↑ Pierce, J.W. 1970. Tidal inlets and washover fans. J. Geol. 78: 230-234
- ↑ Kraus, N.C., Patsch, K. and Munger, S. 2008. Barrier beach breaching from the lagoon side, with reference to Northern California. Shore and Beach 76 :33-43
- ↑ Feyssat, P., Certain, R., Robin, N., Raynal, O., Aleman, N., Hebert, B., Lamy, A. and Barusseau, J.-P. 2022. Morphodynamic Behaviour of a Mediterranean Intermittent Estuary with Opening Phases Primarily Dominated by Offshore Winds. J. Mar. Sci. Eng. 10, 1817
- ↑ Newton, A. 2018. Assessing, quantifying and valuing the ecosystem services of coastal lagoons. Journal for Nature Conservation 44: 50–65
- ↑ Barnes, R.S.K. 1980. Coastal lagoons. Cambridge Univ. Press
- ↑ UNESCO, 1980. Coastal Lagoon Survey. Technical Papers in Marine Science 31
- ↑ Gamito, S. and Gilabert, J. 2005. Effects of changing environmental conditions on lagoon ecology. Diego, CM
- ↑ Borja, Á., Elliott, M., Carstensen, J., Heiskanen, A.-S. and van de Bund, W., 2010. Marine management – towards an integrated implementation of the European marine strategy framework and the water framework directives. Marine Pollution Bulletin 60: 2175–2186
- ↑ Ewel, K.C., Cressa, C., Kneib, R.T., Lake, P.S., Levin, L.A., Palmer, M.A., Snelgrove, P. and Wall, D.H. 2001. Managing critical transition zones. Ecosystems 4: 452-460
- ↑ Ranasinghe, R. and Pattiaratchi, C. 2003. The seasonal closure of tidal inlets: causes and effects. Coast. Eng. J. 45: 601–627
- ↑ Jarrett, J.T. 1976. Tidal Prism-Inlet Area Relationship. US Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS. GITI Report 3
- ↑ 22.0 22.1 Larson, M., Nunes, A. and Tanaka, H. 2020. Semi-analytic model of tidal-induced inlet flow and morphological evolution. Coastal Engineering 155, 103581
- ↑ Keulegan, G. H. 1951. Water Level Fluctuations of Basins in Communications with Seas. Third Progress Report on Tidal Flow in Entrances, U. S. Beach Erosion Board
- ↑ Meyer-Peter, E. and Mueller, R. 1948. Formulas for bed-load transport. Procs. 2nd Conf. IAHR, Stockholm, pp. 39-64
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