Tidal Environments

Definitions

A tidal environment is that part of a marine shore which is regularly submerged and exposed in the course of the rise and fall of the tide. Such environments exhibit particular physical and biological characteristics which, among others, play an important role in coastal dynamics, coastal ecology, coastal protection and engineering works, and integrated coastal zone management.

The coastal area affected by the ocean tides is known as the intertidal or eulittoral zone. Being a long-period wave, the tidal water level oscillates about a mean water level, which usually corresponds to the mean sea level. The vertical distance covered by the tide is known as the tidal range, whereas the part above or below the mean tide level is the tidal amplitude, which can hence have a positive or negative sign. In practice, a number of critical tide levels are distinguished on the basis of longer-term averages. Proceeding from high to low water levels, these are: MEHWS, mean equinoxial high water springs (highest astronomical tide);MHWS, mean high water springs; MHWN, mean high water neaps; MWL, mean water level (commonly mean sea level); MLWN, mean low water neaps; MLWS, mean low water springs; and MELWS, mean equinoxial low water springs (lowest astronomical tide). These mean tide levels are well correlated with various morphological and biological characteristics of tidal environments.

Besides the astronomical modulations, instantaneous tidal elevations can, in the short term, be substantially modified by water-level fluctuations induced by wind and/or wave set-up or set-down. The degree of wave and wind exposure can thus have a substantial influence on the nature of a tidal shore. Where such secondary and irregular fluctuations in coastal water levels are so frequent and strong that they completely mask the tidal signal, the coast is considered to be nontidal (e.g., the Baltic Sea).

In summary, tidal shores are highly variable environments which are not only influenced by the astronomically induced periodic rise and fall of the sea level, but also by numerous secondary processes. In combination, these factors define the local physical nature of a tidal environment (e.g., Davies, 1980; Allen and Pye, 1992; Allen, 1997; French, 1997).

Tidal forcing factors

The tides are essentially produced by the interactive forces exerted on the oceans by the sun and the moon. Since the motions of the sun and the earth-moon system are known with great precision, the tide-generating potential can be mathematically resolved into strictly periodic components. These components vary over time as the position of the sun and the moon relative to the position and orientation of the earth changes. The sum of all the tractive forces at any one time defines the total instantaneous potential. Doodson (1922) computed no less than 390 such harmonic components, of which about 100 are long period, 160 diurnal, 115 semidiurnal, and 14 one-third diurnal. Of these, only seven (i.e., four semidiurnal and three diurnal components) are of practical importance (e.g., McLellan, 1975). Indeed, only four of these are used to define the character of the tides around the globe (Figure T14), in which semidiurnal, diurnal, and two mixed tidal types, comprising a predominantly semidiurnal and a predominantly diurnal one, are distinguished (cf. Table T2). Since the tidal type has an important influence on the physical nature of tidal environments, the global distribution of these four types are illustrated in Figure T15.

Figure T14

Selected examples of tidal curves from different geographic locations illustrating the main tidal types (adapted from Dietrich et al., 1975). (A) semi-diurnal tides (e.g., Immingham, Great Britain); (B) mixed, predominantly semidiurnal (e.g., San Francisco, USA); (C) mixed, predominantly diurnal (e.g., Manila, Philippines); (D) diurnal (e.g., Do-Son, Vietnam).

Figure T15

Global distribution of the main tidal types (adapted from Davies, 1980). Note that the transitions between semidiurnal and diurnal tidal types (here represented by so-called mixed tides) are progressive and not abrupt.

As the sun, the earth, and the moon move along their elliptical orbits, they continually change their positions relative to each other. As a result, the total potential defining the height of the astronomical tide is modulated as a function of geographic location in the course of a day, a month, a year, and also on longer timescales. The most prominent tidal period is the fortnightly spring-neap cycle (synodic tide). Thus, spring tides coincide with full moon and new moon, whereas neap tides occur at the corresponding half-moon phases. These tidal ranges are further modulated in the course of a lunar month (anomalistic tides), the highest spring tides occurring when the moon is closest to the earth (perigee), the lowest when the moon is furthest away from the earth (apogee), the difference in distance amounting to roughly 13%. The solar component varies in similar manner between perihelion (currently coinciding with the winter solstice) and aphelion (currently coinciding with the summer solstice), the difference in distance between the two amounting to about 4%.

Another important astronomical feature influencing tidal environments on short timescales is the daily inequality of the tide (declinational tide). This feature results from the inclination of the earth’s axis relative to the plane of the ecliptic, and hence the tidal bulge. As the earth rotates around its axis, the position of a geographic locality continually changes relative to the tidal bulge, alternating between a maximum and a minimum tidal elevation every 12.42 h, a feature which affects the elevations of both successive high tides and low tides. On this, the declination of the moon is superimposed which, in turn, causes a progressive change in the daily inequality over time. Thus, with the moon over the equator, successive tides are equal in height (equatorial tides), whereas towards the position of maximum declination successive tides become progressively more unequal in height, dividing into a “large tide” and a “half tide” (tropical tides). The solar tide shows a similar inequality, being zero at the equinoxes (spring and autumn) and largest at the solstices (summer and winter).

The largest astronomical tides, that is, the greatest spring and lowest neap tides, in the course of a year occur at the vernal and autumnal equinoxes when the orbital path of the earth-moon system crosses the plane of the celestial equator (nodal points), that is, when the total potential of the tide-generating forces is largest because of their closest alignment.

In addition to these short-period astronomical cycles, the tide is also modulated by longer period phenomena which need to be considered when analyzing ancient tidal deposits (e.g., Archer et al., 1991; Williams, 1991; Oost et al., 1993; de Boer and Smith, 1994). Among these are the nutational motion or nodal cycle (≈8.6-year cycle) caused by the oscillation of the earth’s axis about its mean position, the precessional motion (≈23,000-year cycle) of the earth’s axis, the obliquity (≈41,000- year cycle) which defines the angle of inclination of the earth’s axis between 22° and 24.8° (currently 23.5°), and the eccentricity (≈100,000- year cycle) controlling the rate of change in the elliptical radius of the earth’s orbit around the sun. In addition, it has been shown that the length of the year has decreased from 420 to currently 365 days, while the length of the day has increased from 21 to currently 24 h in the course of the last 500 million years or so (e.g., Williams, 1991).

Besides the geographic variation in tidal type and tidal range resulting from astronomical modulations, the physical nature of a tidal environment is also influenced by numerous secondary factors. Among the more important of these are variations in tidal range with distance from the amphidromic point around which the tidal wave rotates, the Coriolis effect as a function of geographic location, the rate of change in water depth, the coastline configuration (plan shape and slope angle), as well as resonant effects resulting from the shape and depth of a tidal basin. Thus, at the center of an amphidrome the tidal range is considered to be zero, but it progressively increases in height with distance along the axis of the two opposing tidal bulges which rotate around the center offset by 180° or 6 h (tidal phases). The Coriolis force, in turn, forces the tidal wave to rotate clockwise in the Southern Hemisphere and anticlockwise

Table T2 The tidal character ( F ) as defined on the basis of the ratio between the sum of the lunisolar diurnal ( K ₁ ) and principle lunar diurnal ( O ₁ ) and the sum of the principle lunar semidiurnal ( M ₂ ) and principle solar semidiurnal ( S ₂ ) tidal component

in the Northern Hemisphere, but this principle can be upset near the Equator. The direction in which the tidal wave propagates along a shore thus depends entirely on the geographic location of the coast. Where tidal waves rotating around neighboring amphidromes meet, the tidal water motion can even be perpendicular to the coast.

In addition, the amplification of the tidal wave and the tidal type are strongly influenced by the configuration of the coastline and the rate of shoaling, the effect of this interaction being illustrated in Figure T15 and Figure T16. With decreasing water depth (h) the length of the tidal wave (λ) progressively decreases proportionally in the form λ ∝ h ^0.5.When propagating into funnel-shaped estuarine water bodies, the initial increase in tidal range due to convergence of the opposite shores is eventually compensated for by friction along the seabed as a result of which the tidal wave gradually decreases in height. This interplay between friction and convergence is proportionally related in the form a ₀ ∝ b−0.5 * h ^−0.25 where a ₀ is the amplitude of the tidal wave (m), b is the width (m), and h is the water depth (m) at a particular location along the estuary. This relationship can take on complicated patterns and in nature three basic modes of tidal wave propagation can be distinguished. Thus, in the case where friction dominates over convergence the tidal range progressively decreases in height up-estuary (hypersynchronic mode). In the opposite case, the tidal range increases in height (hyposynchronic mode), and where friction and convergence are in balance the tidal range remains constant (synchronic mode). Even neighboring estuaries will exhibit quite different modes of tidal wave propagation if they differ in shape and water depth (e.g., Borrego et al., 1995). In many estuarine environments a combination of two or even all three modes can be observed.

Figure T16

Global distribution of tidal shores based on tidal range according to the classification scheme of Hayes (1979).

In some cases, the entrance channel of an estuary or lagoon is so narrow and shallow that the propagation of the tidal wave is “choked” with the effect that the tidal range is dramatically reduced. This filtering mechanism, expressed by the so-called coefficient of repletion (K), has the numerical form of K=(T/2a ₀π) * (A _c/A _b) * [2a ₀ g/(1+2gln ² r ^−4/3)]^0.5 where T is the tidal period, 2a ₀ is the tidal range (a ₀ being the tidal amplitude), l is the length of the entrance channel, A _c is the cross-sectional channel area, A _b is the surface area of the water body, r is the hydraulic radius of the channel, g is the gravitational acceleration, and n is the Manning’s friction (0.01–0.10 s m^−1/3). This coefficient of repletion controls reductions in tidal range, phase shifts between ocean and lagoonal tides, non-sinusoidal variations of the lagoonal tides, and flow exchanges between the ocean and the estuarine or lagoonal water bodies (e.g., Kjerfve and Magill, 1989).

A final forcing factor which may affect the behavior of tidal waves in shallow water is resonance, a feature associated with standing waves. Clearly, a standing wave superimposed on a normal tidal wave would dramatically affect the physical nature of a tidal environment. In this context, we distinguish two types. In the case of a half-wave oscillator or seiche, the length of the water body is half the wavelength of the standing wave. The fundamental period (T) of a seiche is defined as T=2l/(gh)^0.5 where l is the length of the water body, h is the water depth, and g is the gravitational acceleration. Seiches are particularly common in lakes and marginal seas, where they are forced by wind stress, and hence of less importance in tidal environments. However, quarter-wave oscillators come into operation where the lengths of open-ended elongate gulfs or deep estuaries together with adjacent bays correspond to a quarter of the tidal wavelength (l=0.25T * (gh)^0.5). In this case, the period T=4l l/(gh)^0.5 with notations as above. If this condition is fulfilled, or nearly so, tidal amplification can be quite considerable (e.g., Bay of Fundy, Canada).

Coastal classification by tidal type and range

Besides classifying the world’s coastline according to tidal type (Figure T15), coastal tidal environments are also classified on the basis of tidal range, the scheme of Davies (1964, Davies 1980) having been the most

Table T3 Contrasting two existing classification schemes of tidal shores on the basis of tidal range

widely applied to date (Table T3). However, with only three subdivisions, this rather arbitrary approach prevents differentiation where it is most needed, that is, near the lower and upper limits of the potential tidal regime. For example, the Gulf of Mexico coast with an average tidal range of only 0.5 m is very different in character from the west coast of southern Africa where the tidal range averages at 1.6 m, yet both are classified as microtidal. In contrast to this, a more pragmatic classification, comprising five subdivisions (see Table T2), has been proposed by Hayes (1979). This latter scheme takes distinct, process-related geomorphic features into consideration, for example, the upper limit of barrier island occurrence at a tidal range of 3.5 m, which hence marks the transition between upper mesotidal and lower macrotidal regimes in this classification. To contrast the two classification schemes, the global pattern of coastal subdivision using the latter scheme is presented here for the first time (Figure T16).

Rocky versus sandy and muddy tidal environments

The coastlines of the world can be divided into four basic types, namely rocky shores, sandy shores, muddy shores, and bio-shores. In all cases, the character of a particular shore reflects the interaction between the substrate, the local wave climate, the tides, and the biology (e.g., Newell, 1979; Davies, 1980; Raffaeli and Hawkins, 1996). Geographic location, which controls climatic influences and biological species composition (e.g., Chapman, 1974), and the Holocene evolution of a coast (e.g., Bird and Schwartz, 1985) being important additional factors to consider. Rocky shores occupy the smallest overall area because the intertidal zone is commonly narrow as compared with the other shore types. Shore processes have received considerable attention by engineers for constructional purposes (e.g., Horikawa, 1989), whereas biologists have long been intrigued by the distinct faunal and floral zonation patterns along tidal gradients which evidently reflect high degrees of adaptation to the intertidal environment and the overprinting effects of competition between specific organisms (e.g., Lewis, 1972; Raffaeli and Hawkins, 1996). However, the interplay between biological and physical factors in defining zonation patterns is still not well enough understood to allow accurate predictions to be made (e.g., Delafontaine and Flemming, 1989). Different tidal environments are also characterized by different biogeochemical processes due to different climates, substrates, and biological community structures (Alongi, 1998).

The biological zonation pattern observed along the rocky shores of Great Britain as a function of tidal gradient and the degree of exposure to wave action is illustrated in Figure T17 (adapted from Lewis, 1972; Raffaeli and Hawkins, 1996). This basic scheme can be applied to most rocky shores of the world, the only difference being the species composition and distribution, a good example from subtropical Bermuda being shown in Thomas (1985). Important to note here is that the tidal gradient in Figure T17 is relative, expanding or contracting proportional to the tidal range.

Figure T17

Zonation patterns along rocky tidal shores as a function of exposure to wave action (adapted from Lewis, 1972; Raffaeli and Hawkins, 1996). Note that the scaling is relative to any particular observed tidal gradient.

In contrast to rocky shores, into which bio-shores can also be included, sandy and muddy tidal shores can attain shore normal extensions of many kilometers (e.g., Davis, 1994; Flemming and Hertweck, 1994). French (1997) distinguishes no less than seven intertidal coastal types based mainly on morphology and facies successions. The final transition between land and sea along sheltered coasts is characterized by sharp boundaries separating different floral zones, the typical pattern observed along the barrier island shore of the southern North Sea being illustrated in Figure T18 (adapted from Streif, 1990). In this example the zones are separated by the frequency of tidal submergence in the course of the year which is controlled by the elevation along the tidal gradient. In principle, this basic pattern should be applicable the world over, individual transition levels being dependent on local floral associations, the tidal range, the seasonal wave climate, and the difference in elevation between mean high tide levels at spring and neap tide (e.g., Chapman, 1974; Lugo and Snedaker, 1974). A systematic investigation of the factors controlling the upper and lower limits of occurrence of Spartina anglica relative to mean sea level along the coast of the United Kingdom has generated quantitative relationships between the spring tidal range, the fetch available for wave generation and propagation, the area of the tidal basin, and in the case of the upper limit also the geographic location (Gray, 1992). Thus, the lower limit (L _l) is defined as L _l=−0.805+0.366R _s+0.053F+0.135 * log_e A _b, where R _s is the spring tidal range (m), F is the fetch in the direction of the transect (km), and A _b is the area of the tidal basin (km²). The upper limit, by contrast, is defined as L _u=4.74+0.483R _s+0.068F − 0.199L°, where L° is the degree North Latitude expressed as a decimal. The correlation coefficients of r=0.97 for L _l and r=0.95 for L _u demonstrate the predictive potential of this approach.

Figure T18

Floral zonation pattern observed at the transition between upper intertidal flats and the terrestrial environment of the Wadden Sea (southern North Sea) as a function of tidal elevation and the frequency of tidal submergence in the course of a year (adapted from Streif, 1990).

A comprehensive overview of physical and biological processes active along sandy tidal shores is provided in McLachlan and Erasmus (1983).

Outlook

Many features of tidal environments are still poorly understood. Among these are the quasi-periodic, decadal to subdecadal fluctuations in the elevation of mean high tide and mean low tide levels. Being a worldwide phenomenon, one might assume that they result from variations in the astronomical factors defining the tidal potential. A clear correlation, however, is still lacking. As far as sandy tidal environments are concerned, accurate sediment budgets and transport pathways have remained elusive problems whose solution becomes more pressing in view of the predicted acceleration in sea-level rise. The distinction between strictly local features and others of global relevance requires more attention. A number of other unresolved issues have been addressed in the text.