9.2 Tidal processes and their geomorphic effects

Tidal Processes and Mechanisms

Tides are the rhythmic rise and fall of sea level driven by the gravitational pull of the Moon and Sun. These cycles do far more than just move water up and down a beach. Tidal currents transport enormous volumes of sediment, carve channels, build salt marshes, and shape estuaries. Understanding tidal processes is essential for seeing how waves, currents, and sea-level changes work together to produce the landforms you see along any coast.

Gravitational Forces and Tidal Bulges

The Moon's gravity is the primary driver of tides. Even though the Sun is far more massive, the Moon is much closer to Earth, and tidal force depends heavily on distance (it decreases with the cube of distance, not the square). This means the Moon's tidal influence is roughly twice that of the Sun.

Tidal forces arise because gravity isn't uniform across Earth's diameter. The side of Earth facing the Moon experiences a stronger pull than the far side. This difference stretches the ocean into two tidal bulges: one on the side nearest the Moon (direct gravitational attraction) and one on the opposite side (where reduced gravitational pull allows water to bulge outward due to inertia).

As Earth rotates beneath these bulges, most coastlines experience two high tides and two low tides per day. The orbits of the Moon and Sun add longer-period variations on monthly and annual timescales.

In the open ocean, tidal bulges don't simply sweep around the globe in a smooth band. The Coriolis effect deflects moving water, causing tidal waves to rotate around fixed points called amphidromic points. At an amphidromic point, tidal range is essentially zero; it increases with distance from that point. Each ocean basin contains several amphidromic points, which is why tidal range varies so much from place to place.

Factors Affecting Tidal Range

Even at the same latitude, two coastlines can have very different tidal ranges. Several factors control this:

• Coastal bathymetry: Shallow water slows and amplifies tidal waves, increasing range.
• Continental shelf width: Wide shelves tend to produce larger tidal ranges because the tidal wave has more shallow area to interact with.
• Coastline and estuary geometry: Funnel-shaped bays and narrowing channels concentrate tidal energy, amplifying range.
• Resonance: When the natural oscillation period of a semi-enclosed basin matches the tidal period, resonance dramatically amplifies the tide. The Bay of Fundy in Canada is the classic example, with tidal ranges exceeding 16 m because its basin length produces a resonant period close to the ~12.4-hour semidiurnal tidal cycle.

Coasts are classified by their tidal range:

• Microtidal: < 2 m range (e.g., much of the Mediterranean)
• Mesotidal: 2–4 m range (e.g., parts of the U.S. East Coast)
• Macrotidal: > 4 m range (e.g., Bay of Fundy, Bristol Channel)

This classification matters because tidal range strongly influences which coastal landforms dominate. Microtidal coasts tend to be wave-dominated, while macrotidal coasts are tide-dominated, with extensive tidal flats and salt marshes.

Types of Tides and Characteristics

Daily Tidal Patterns

The number and relative height of tides per day vary by location, depending on the geometry of the ocean basin and the declination of the Moon.

• Semidiurnal tides: Two high tides and two low tides of roughly equal height each day (~12.4-hour cycle). This is the most common pattern globally, found along much of the Atlantic coast.
• Diurnal tides: Only one high tide and one low tide per day (~24.8-hour cycle). These occur in areas like the Gulf of Mexico and parts of the South China Sea, where basin geometry suppresses one of the two daily tidal cycles.
• Mixed tides: Two highs and two lows per day, but with noticeably unequal heights. One high tide is significantly higher than the other, and one low is significantly lower. This pattern is common along the west coast of North America.

Monthly and Annual Tidal Variations

The relative positions of the Moon, Sun, and Earth create predictable longer-term cycles:

Spring tides occur during new and full moons, when the Sun, Moon, and Earth are roughly aligned (syzygy). The gravitational forces of the Sun and Moon reinforce each other, producing higher high tides and lower low tides. Spring tides repeat approximately every 14.8 days.

Neap tides occur during first and third quarter moons, when the Sun and Moon are at right angles relative to Earth. Their gravitational effects partially cancel, producing a reduced tidal range. Neap tides fall midway between spring tides.

King tides are exceptionally large spring tides that occur when the Moon is at its closest approach to Earth (perigee) and Earth is near its closest approach to the Sun (perihelion). Perihelion occurs in early January, so king tides in the Northern Hemisphere often happen during winter months. These events are increasingly used as previews of what routine tidal flooding may look like under future sea-level rise.

Unique Tidal Phenomena

Tidal bores are waves that travel upstream against a river's current as the incoming tide pushes into a narrowing estuary or river channel. They require a combination of large tidal range, a funnel-shaped estuary, and a shallow river channel. Notable examples include the bore on the Qiantang River in China (which can reach several meters in height) and the pororoca on the Amazon River.

Internal tides are tidal-frequency waves that propagate within the ocean's interior rather than at the surface. They form when tidal currents flow over underwater topography (such as mid-ocean ridges or continental shelf breaks) in a density-stratified water column. Internal tides are important because they drive vertical mixing of nutrients and influence deep-ocean circulation, though their surface expression is subtle.

Tidal Effects on Coastal Landforms

Tidal Influence on Sediment Dynamics

Tidal currents move sediment through repeated cycles of erosion, transport, and deposition. The specific landforms that result depend on tidal range, sediment supply, and the energy of the environment.

Tidal flats develop in low-energy, shallow coastal areas where fine sediments settle out during slack water (the brief period of no current between flood and ebb tides). They often show distinct zonation: mudflats in the lowest-energy areas closest to the shore, transitioning to sandflats where currents are slightly stronger.

Salt marshes form in the upper intertidal zone, where halophytic (salt-tolerant) vegetation colonizes the surface. Plant stems and roots trap sediment during tidal inundation, causing the marsh surface to gradually accrete vertically. This makes salt marshes important for coastal protection and carbon storage.

Tidal asymmetry is a critical concept for understanding net sediment transport. In many tidal systems, the flood tide (incoming) and ebb tide (outgoing) differ in duration and peak velocity. If the flood current is shorter but faster, it carries more sediment landward than the ebb carries seaward, creating a flood-dominated system that tends to infill with sediment. The reverse produces an ebb-dominated system that exports sediment.

Tidal Landforms and Features

• Tidal creeks and channels are carved by the repeated ebb and flow of tidal water through flats and marshes. They typically develop dendritic (branching, tree-like) drainage patterns and exhibit meandering similar to river channels, though their flow reverses with each tidal cycle.
• Tidal deltas form at the mouths of tidal inlets. A flood-tidal delta builds on the landward (bay) side of the inlet from sediment carried in by flood currents. An ebb-tidal delta builds on the seaward side from sediment carried out by ebb currents. The relative size of each delta depends on whether tidal currents or wave energy dominate.
• Barrier islands and spits are modified by tidal processes through the inlets that cut through them. Tidal currents maintain these inlets, and the size of the inlet is closely related to the tidal prism (discussed below). Storm surge combined with high tides can cause overwash, pushing sediment from the ocean side to the bay side of the barrier.

Tidal Erosion and Deposition Patterns

Tidal scour occurs where currents are forced through constrictions such as narrow inlets or bridge pilings. The accelerated flow deepens channels and excavates scour holes, which can be significant engineering concerns.

Tidal bars form where current velocity drops and sediment is deposited. These include linear sand bars aligned with flow, transverse bars perpendicular to flow, and point bars on the inner bends of meandering tidal channels.

Tidal rhythmites are layered sedimentary deposits that preserve a record of individual tidal cycles. Each layer pair typically consists of a coarser layer (deposited during stronger current) and a finer layer (deposited during slack water). Because tidal rhythmites record the spring-neap cycle, geologists use them to reconstruct paleotidal conditions and ancient Earth-Moon distances.

Tidal Prism and Estuary Formation

Tidal Prism Concept and Measurement

The tidal prism is the total volume of water that flows into and out of an estuary or back-barrier lagoon during a single tidal cycle. It's determined by two factors: the tidal range and the surface area of the water body.

A larger tidal range or a larger estuary area means a bigger tidal prism, which in turn means stronger tidal currents flowing through the inlet.

Tidal prism can be measured through:

1. Hydrographic surveys that measure current velocity and channel cross-section over a full tidal cycle
2. Remote sensing to map the area of intertidal zones that flood and drain
3. Numerical modeling that simulates tidal flow through the system

Tidal Prism Influence on Coastal Morphology

The tidal prism directly controls the size and stability of tidal inlets. Larger prisms require larger inlet cross-sections to accommodate the volume of water flowing through, and these larger inlets tend to be more stable over time.

This relationship is formalized in the O'Brien relation, an empirical equation:

$A = kP^n$

where $A$ is the inlet cross-sectional area, $P$ is the tidal prism, and $k$ and $n$ are empirically derived constants ($n$ is typically close to 1). This relationship is widely used in coastal engineering to predict how inlets will respond to changes.

Changes to the tidal prism cause morphological adjustments. Natural changes like sea-level rise increase the tidal prism by flooding more area, potentially widening inlets. Human interventions like land reclamation or dredging can either increase or decrease the prism, triggering inlet migration, shoaling, or erosion as the system adjusts toward a new equilibrium.

Tidal Prism and Estuarine Processes

The tidal prism governs how effectively an estuary flushes itself. A large tidal prism relative to the estuary's volume means rapid exchange with the open ocean, improving water quality by diluting pollutants and replenishing oxygen.

Tidal prism also affects the estuary's sediment budget. Depending on tidal asymmetry, the system may be a net importer or exporter of sediment. This has direct implications for whether tidal flats and marshes grow or erode over time.

The balance between tidal prism and freshwater river input determines the type of estuarine circulation:

• Salt wedge estuary: River flow dominates; a distinct wedge of saltwater intrudes along the bottom beneath fresher water (e.g., Mississippi River mouth).
• Partially mixed estuary: Moderate tidal energy creates some vertical mixing, but salinity still varies with depth (e.g., Chesapeake Bay).
• Well-mixed estuary: Strong tidal currents thoroughly mix the water column, so salinity is relatively uniform with depth but varies along the length of the estuary.