Seiche — Introduction
A seiche (pronounced /seɪʃ/) is a standing oscillation that occurs in an enclosed or partially enclosed body of water—lakes, reservoirs, swimming pools, bays, harbours, caves and even semi‑enclosed seas—when reflections from basin limits sustain a resonant wave. The phenomenon requires at least partial bounding so that incident and reflected motions interact persistently rather than radiating away.
Physically, a seiche can be described as the superposition of two waves of appropriate amplitude and phase travelling in opposite directions; their constructive and destructive interference produces fixed nodes (locations of minimal vertical motion) and antinodes (locations of maximal motion) within the basin. The spatial pattern and natural periods of these modes depend on the basin’s geometry, depth distribution and boundary conditions, so different lakes, harbours or caves exhibit distinct mode shapes and resonance frequencies.
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In coastal enclosures, seiches are often forced by long‑period (infragravity) motions that result from subharmonic, nonlinear interactions of wind‑generated waves; these infragravity waves have periods substantially longer than the wind waves that create them. The term seiche was popularized by François‑Alphonse Forel following his systematic observations on Lake Geneva in the late nineteenth century; the local Swiss‑French name, derived from Latin siccus (“dry”), alludes to exposed shoreline as water recedes.
Seiches are very long-period oscillations of enclosed or semi-enclosed water bodies whose vertical motions often unfold too slowly to be obvious to surface observers. They arise when a basin responds resonantly to an external disturbance—most commonly wind stress or spatial variations in atmospheric pressure, but occasionally seismic events or tsunamis. Gravity acts to re-establish a horizontal free surface, and that restoring force drives the subsequent oscillatory motion.
An initiating disturbance produces vertical harmonic motion that launches an impulse along the basin; the propagation speed of that impulse depends on water depth and the basin’s stratification. When the impulse reaches basin boundaries it is reflected, and the interaction of incident and reflected pulses leads to constructive and destructive interference. Through repeated reflections this process establishes standing-wave patterns characterized by nodes (locations of negligible vertical displacement) and antinodes (locations of maximum displacement).
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The spatial pattern and frequencies of these standing waves are set by the basin’s geometry—principally its length, mean depth and planform—and by water temperature, which influences density stratification and thus effective wave speed. The longest period is the fundamental mode, representing the lowest-frequency standing wave. For a rectangular basin with uniform depth the fundamental surface period may be approximated by Merian’s formula: T = 2L / sqrt(gh), where L is basin length, h is mean depth and g is gravitational acceleration. Higher-order modes appear at discrete, shorter periods equal to integer fractions of the fundamental (the nth harmonic has period T/n), producing a spectrum of modal oscillations.
Seiches are standing-wave oscillations in lakes, seas and harbours that result from interaction of incident and reflected waves to produce fixed nodes and antinodes. Their formation requires partial confinement of the water body—shorelines, peninsulas, sills or other boundaries that reflect energy and trap modes—rather than an unbounded surface. Regular basin geometry is not a prerequisite: even highly irregular basins, including harbours with complex shoreline indentations and uneven bathymetry, can support well-defined standing modes. In semi-enclosed coastal features the natural frequencies of these oscillations are typically stable and are governed mainly by the basin’s effective dimensions, depth distribution and boundary conditions (i.e., the locations of reflection and energy trapping) rather than simple geometric regularity. Geographically, seiches can therefore be expected wherever partial confinement produces persistent eigenmodes, making them a reproducible and regionally significant coastal and lacustrine phenomenon.
Lake seiches are low‑frequency standing oscillations of water level that occur routinely on larger lakes and in sheltered waters (harbours, bays, estuaries). Under typical wind‑wave conditions they are often obscured, but during calm periods or in confined basins seiches with characteristic periods from minutes to hours can be readily observed; small sheltered seiches commonly have amplitudes of only a few centimetres and periods of a few minutes, whereas larger basins support much longer, higher‑amplitude modes.
The resonant behaviour of a lake is controlled primarily by basin geometry, depth and the nature of the forcing. Longitudinal (along‑basin) and transverse (cross‑basin) modes can have very different natural periods in the same lake, and amplitudes scale with the strength of wind stress, storm set‑up or seismic excitation. Narrow, elongated basins amplify seiche response because greater effective fetch and alignment with prevailing wind directions enhance forcing and favour the longest longitudinal modes. Classic observations illustrate these controls: Lake Geneva supports a long longitudinal mode of about 73 minutes and a transverse mode near 10 minutes, while Lake Wakatipu shows a conspicuous 27‑minute, ~0.2 m oscillation tied to its geometry. On regional scales, semi‑enclosed seas also resonate—the North Sea commonly exhibits a lengthwise seiche with a period on the order of 36 hours.
Great Lakes examples demonstrate both spatial structure and hazard potential. Seiche‑driven water‑level differences along Lake Erie were recorded as opposing anomalies at the lake’s ends during events, and the lake’s shallow depth and northeast–southwest elongation make it particularly susceptible to strong wind‑driven seiches (extreme differences up to ~5 m have been reported). Rapid storm‑driven events have stranded boats and inundated shorelines (Lake Superior: ~1 m change in 15 minutes on 13 July 1995; Lake Huron: ~1.8 m change over two hours in the same storm system), and in 1954 a ~3 m seiche on Lake Michigan produced multiple drownings on the Chicago waterfront. Because modest seiche amplitudes can disrupt navigation and infrastructure, operational agencies issue guidance—for example, the U.S. National Weather Service flags seiche risk on the Great Lakes when amplitudes of ≈0.6 m (2 ft) or greater are likely. Storm‑generated seiches are analogous to coastal storm surge in that winds pile water against one shore, but the subsequent oscillation produces repeated reversals rather than a single residual rise.
Seiches are also triggered by seismic waves, often at great distances from the earthquake source. Ground motion can excite the natural periods of lakes, bays and even small basins such as swimming pools; notable instances include swimming‑pool sloshing during the 1994 Northridge event and after the 1964 Good Friday earthquake. Historical and modern earthquakes have produced far‑field seiches: the 1755 Lisbon quake induced oscillations in Scottish lochs and Swedish canals more than 2,000 km away; the 2004 Indian Ocean and 2005 Kashmir earthquakes generated seiches across wide regions of South and Southeast Asia; and the 2011 Tōhoku earthquake produced seiches as far as Norway (up to ~1.8 m in Sognefjorden). A long catalogue of earthquakes in the Indian subcontinent (e.g., 1803, 1819, 1842, 1905, 1934, 2001, 2005 events) documents recurrent seismic forcing of standing waters. More recent examples include a ~0.15 m seiche on Lake Pontchartrain from the 2010 Chile quake and widely shared footage of large seiches after the 2010 Baja California earthquake.
The combination of storm and seismic drivers has practical implications for lakes in tectonically active regions. Geological evidence at basins such as Lake Tahoe suggests prehistoric seiche or tsunami impacts with run‑ups on the order of 10 m, indicating substantial hazard potential. Consequently, seiche and related tsunami scenarios merit inclusion in regional emergency planning for vulnerable lake basins.
Sea and bay seiches arise when large-scale forcings excite long-period oscillations in semi-enclosed coastal basins; their amplitude and hazard are strongly controlled by the basin geometry and by the nature of the initiating disturbance. In marginal seas, propagating atmospheric lows can load a basin with excess water and set up low-frequency standing waves whose wavelengths may extend for hundreds of kilometres. When such waves enter constricted, shallow inlets they compress and amplify, producing hazardous surges—an archetypal example is the autumn flooding of the Neva embankments in Saint Petersburg, where North Atlantic cyclones driving anomalously high water into the essentially enclosed Baltic Sea generate long-period seiche modes that concentrate in Neva Bay.
Similar dynamics govern seiche-driven inundation in other historic port cities. In the Adriatic Sea, seiches contribute to the recurrent high-water events that threaten Venice; concern over seiche and storm surge hazards there motivated the MOSE scheme, a system of movable barriers erected at the lagoon entrances to reduce flood risk. In Nagasaki Bay, Japan, seiches most commonly occur in spring when an atmospheric low passes south of Kyushu; observed events have produced rapid water-level displacements of several metres (a documented 1979 event yielded roughly 2.8 m at the tide station and up to an estimated 4.7 m locally), with modal periods around 30–40 minutes and well‑recorded impacts on port infrastructure and fisheries (the phenomenon is reflected in the local term abiki, derived from amibiki, “dragging away a fishing net”).
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Seiches may also be excited by impulsive oceanic disturbances. Tsunamis can couple to a bay’s natural resonance and thereby amplify local oscillations; after the 1946 Aleutian tsunami, Hilo Bay experienced combined tsunami–seiche behaviour because the tsunami wave train contained components at about half the bay’s ~30‑minute resonant period, so every second wave arrived in phase and produced extreme runup (combined local heights approaching 8 m) and prolonged oscillations that exacerbated damage and casualties. Likewise, large mass movements such as subaerial landslides or glacier collapses can generate megatsunamis that evolve into persistent seiches within confined fjords: a September 2023 slide in Greenland produced an initial wave on the order of 200 m and subsequent seiche oscillations up to several metres that persisted for days and produced long-duration seismic signals.
Internal wave processes and tidal forcing provide another pathway for exciting coastal seiches. Large-amplitude internal solitary waves that form offshore and shoal at the continental shelf break can induce substantial shelf currents, which in turn force coastal seiche modes; documented occurrences include sites in Puerto Rico, the Philippines, Sri Lanka and the Bay of Fundy—the latter being notable for its extreme tidal range and strong internal wave activity. Finally, seiche motions need not be confined to the surface: in stratified waters analogous standing oscillations develop along density interfaces (thermoclines), producing subsurface seiches whose initiation mechanisms mirror those of surface seiches (atmospheric pressure changes, wind stress, tsunamis, internal waves and large mass movements), but which manifest as distinct vertical structure in observations and models.
Internal seiches in confined stratified basins take the form of subsurface waves that oscillate along the thermocline—the density interface separating an upper and lower layer—and constitute the fundamental oscillatory mode of a two-layer system, analogous to the surface seiche described by the Merian formula. For the fundamental mode the natural period T scales with basin length L and the internal long‑wave phase speed c according to T = 2L/c. The phase speed can be expressed (in squared form) as
c^2 = g·((ρ2 − ρ1)/ρ2)·(h1·h2/(h1 + h2)),
or, equivalently, c = [g′·(h1 h2/(h1 + h2))]1/2 where g′ = g(ρ2 − ρ1)/ρ2 is the reduced gravity and h1, h2 are the mean thicknesses of the upper (e.g., epilimnion) and lower (e.g., hypolimnion) layers. Because T ∝ L/c, longer basins have longer periods, while increasing density contrast or enhancing the effective coupling of layers (the harmonic combination h1 h2/(h1 + h2)) raises c and shortens T.
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Interactions with a sloping bed produce strong near‑bed effects. Thermocline excursions across a slope create a localized swash zone characterized by rapid, spatially confined temperature changes that can sharply alter habitat conditions for benthic and demersal organisms. Vertical motion of the thermocline is asymmetrical in its benthic impact: upward motions over a slope tend to destabilize the lower layer and drive convective overturning, increasing vertical mixing, oxygen delivery, and solute exchange at the sediment–water interface; downward motions strengthen stratification and suppress near‑bed turbulence and exchange.
As internal waves propagate along slopes they may steepen into non‑linear forms; shoaling and breaking of these waves at the bed inject substantial turbulence and vertical energy flux into the near‑bottom region. Such breaking events can resuspend sediments and thereby modify sediment transport pathways, turbidity, nutrient fluxes, and local geomorphology. Thus internal seiche dynamics provide a mechanistic link between basin stratification and wave mechanics on one hand and benthic turbulence, habitat conditions, and sedimentary processes on the other in stratified lakes and similar constrained water bodies.
Cave seiches
On 19 September 2022 Devils Hole, a water-filled karst feature in Death Valley National Park, experienced a pronounced seiche—an oscillatory standing wave—in which the pool’s water level varied by about 4 ft (1.2 m). Instrument records attribute this response to a magnitude‑7.6 earthquake in western Mexico whose seismic energy travelled roughly 1,500 miles (≈2,400 km) to perturb the cave pool, demonstrating long‑range, tele‑seismic coupling between a distant source and a confined subterranean water body.
Seiches in such cave pools arise when passing seismic waves displace water and excite the basin’s natural modes, producing sustained oscillations even in partially enclosed geometries. The amplitude observed at Devils Hole is notable for an underground pool and reflects the efficient transfer of seismic energy into the hydrodynamic modes of a small, resonant basin. Devils Hole has exhibited similar remote‑triggered seiches following large earthquakes in 2012, 2018 and 2019, indicating a recurrent sensitivity linked to its geomorphology and hydrogeologic setting.
These events highlight two scientific and management priorities: first, monitoring isolated karst and cave pools can yield insights into seismic wave propagation and basin resonance at continental distances; second, tele‑seismic disturbances may have ecological and conservation implications for subterranean aquatic habitats, warranting continued observation and study.
Engineering for seiche protection
Seiche oscillations must be explicitly considered in the design and siting of water-related infrastructure because their resonant water‑level fluctuations can compromise performance and safety. Critical assets requiring seiche-aware design include flood‑protection works (e.g., the Saint Petersburg Dam), large reservoirs and dams (e.g., Grand Coulee), potable water storage basins, harbours, and spent‑nuclear‑fuel storage basins.
The sensitivity of shoreline and nearshore elements to seiche-driven level changes is high. Engineered structures and beach–dune systems are particularly prone to damage during elevated stages, while wetlands can suffer ecological and geomorphic disruption from comparatively modest oscillations. Consequently, both historical records and projections of water‑level variability are essential inputs to coastal and reservoir engineering.
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The natural period of a seiche is determined principally by basin geometry: the horizontal scale (length or extent) together with average depth set the basin’s natural frequency and thus the timing of its oscillatory response. When externally forced wave trains contain periods close to that natural frequency, successive forcings add energy to the mode, amplifying water‑level excursions and producing more energetic, turbulent conditions within enclosed harbours or basins than in the adjacent open sea. Such resonant amplification increases navigational risk and places additional loadings on moorings and coastal structures.
For operational management and long‑term planning, seiche behaviour must be treated as a core component of hydrodynamic datasets alongside storm surge and tidal variability. These three factors—seiches, storm surges, and tides—are complementary drivers of extreme water levels and together inform risk assessment, design criteria, and the protection of both built and natural coastal systems. The Venice example, where storm surge, barometric surge and seiches combine to produce elevated high‑water events, illustrates the importance of accounting for interacting forcings in flood‑risk analyses.
Finally, the engineering literature identified in June 2008 indicates that additional empirical case studies, basin‑specific parameterizations (linking size and depth to modal behaviour), and explicit design guidance remain desirable to fully support practitioners in mitigating seiche hazards.