1.2 Tides and tidal processes

Tides shape coastal landscapes and influence marine ecosystems, making them crucial for coastal resilience engineering. Understanding tidal mechanics helps engineers design effective protection structures and predict flooding events. This knowledge is essential for developing robust coastal management strategies.

Tidal forces, driven by gravitational interactions between Earth, Moon, and Sun, create complex patterns of water movement. These patterns vary in frequency and amplitude, from daily cycles to long-term trends spanning years. Engineers must consider these tidal dynamics when planning coastal projects and assessing flood risks.

Fundamentals of tides

• Tides play a crucial role in coastal resilience engineering by influencing shoreline dynamics, sediment transport, and coastal ecosystems
• Understanding tidal mechanics helps engineers design effective coastal protection structures and predict potential flooding events
• Tidal forces shape coastal landscapes and impact marine life cycles, making them essential considerations in coastal management strategies

Gravitational forces and tides

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• Newton's law of universal gravitation explains tidal forces generated by celestial bodies
• Moon exerts stronger tidal influence on Earth due to its proximity, despite smaller mass
• Tidal bulges form on opposite sides of Earth due to gravitational pull and inertial forces
• Differential gravitational forces across Earth's diameter create tidal tractive forces

Lunar vs solar tides

• Lunar tides dominate with approximately 2.2 times greater magnitude than solar tides
• Solar tides contribute to overall tidal patterns and influence tidal range variations
• Synodic month (29.5 days) determines timing of lunar-solar tidal interactions
• Tidal range amplifies when lunar and solar tides align (spring tides)
• Tidal range diminishes when lunar and solar tides oppose each other (neap tides)

Tidal cycles and periods

• Semidiurnal tidal cycle occurs twice daily with period of approximately 12 hours 25 minutes
• Diurnal tidal cycle occurs once daily with period of approximately 24 hours 50 minutes
• Fortnightly cycle results from lunar phase changes, affecting tidal range over 14.77 days
• Annual cycle influenced by Earth's elliptical orbit around Sun, impacting tidal amplitudes
• 18.6-year nodal cycle caused by lunar orbital plane precession, modulating tidal ranges

Types of tides

Diurnal tides

• Characterized by one high tide and one low tide per tidal day (24 hours 50 minutes)
• Occur in regions where K1 and O1 tidal constituents dominate (Gulf of Mexico)
• Tidal range varies throughout the lunar month due to declination changes
• Diurnal inequality more pronounced near solstices and minimal near equinoxes
• Coastal areas experiencing diurnal tides require different engineering approaches for flood protection

Semidiurnal tides

• Feature two high tides and two low tides of approximately equal height per tidal day
• Prevalent in most ocean basins where M2 tidal constituent dominates (Atlantic Ocean)
• Tidal period of about 12 hours 25 minutes between successive high or low tides
• Slight variations in tidal heights occur due to lunar declination changes
• Semidiurnal tides influence timing of coastal construction activities and navigation schedules

Mixed tides

• Combine characteristics of both diurnal and semidiurnal tides
• Two high tides and two low tides per day with significant height differences
• Occur in regions where both diurnal and semidiurnal constituents have similar magnitudes (Pacific Coast of North America)
• Diurnal inequality more pronounced compared to semidiurnal tides
• Mixed tides create complex patterns of coastal inundation, challenging engineers to design adaptable coastal structures

Tidal components

Principal lunar semidiurnal

• Denoted as M2, represents the dominant tidal constituent in most ocean basins
• Period of 12.42 hours, closely matching the average semidiurnal tidal cycle
• Caused by gravitational attraction of the Moon and Earth's rotation
• Amplitude varies geographically due to ocean basin resonance and bathymetry
• M2 tidal predictions crucial for coastal engineering projects and navigation planning

Principal solar semidiurnal

• Represented as S2, second most significant tidal constituent in many regions
• Period of exactly 12 hours, driven by Sun's gravitational influence and Earth's rotation
• Interacts with M2 to produce spring-neap tidal cycle over 14.77 days
• S2 amplitude typically about 46% of M2 amplitude in open ocean areas
• Solar semidiurnal tides contribute to overall tidal patterns and coastal processes

Lunar elliptic semidiurnal

• Designated as N2, results from monthly variations in Moon's distance from Earth
• Period of 12.66 hours, slightly longer than M2 due to lunar orbital eccentricity
• Modulates M2 tidal amplitudes over the anomalistic month (27.55 days)
• N2 constituent important for accurate long-term tidal predictions
• Influences timing and magnitude of extreme tidal events (perigean spring tides)

Tidal datums

Mean sea level

• Average height of sea surface over extended period (typically 19 years)
• Serves as vertical datum for terrestrial elevation measurements and nautical charts
• Varies geographically due to oceanographic and meteorological factors
• Crucial reference point for coastal infrastructure design and flood risk assessment
• Long-term changes in mean sea level indicate climate change impacts on coasts

Mean high water

• Average of all high water heights observed over tidal epoch (usually 19 years)
• Used to define shoreline position for legal and regulatory purposes
• Varies with tidal range and local bathymetry
• Important datum for determining coastal setback lines and building elevations
• MHW datum helps engineers design appropriate freeboard for coastal structures

Mean low water

• Average of all low water heights observed over tidal epoch (usually 19 years)
• Serves as chart datum for nautical charts in many countries (United States)
• Critical for determining safe navigation depths and dredging requirements
• MLW datum used to calculate tidal range and assess intertidal habitat extent
• Influences design of coastal structures like piers, jetties, and breakwaters

Tidal currents

Flood and ebb currents

• Flood currents flow landward during rising tide, transporting water and sediment shoreward
• Ebb currents flow seaward during falling tide, carrying water and sediment offshore
• Current velocities generally strongest in narrow channels and tidal inlets
• Tidal current asymmetry can lead to net sediment transport in coastal systems
• Understanding flood and ebb patterns crucial for coastal erosion control and navigation safety

Slack water periods

• Occur when tidal current velocity approaches zero between flood and ebb phases
• Two slack water periods per tidal cycle in semidiurnal tidal regimes
• Duration of slack water varies with tidal range and local bathymetry
• Slack water periods important for timing of marine operations and sediment deposition
• Engineers consider slack water in designing coastal structures to minimize scour and erosion

Tidal current prediction

• Utilizes harmonic analysis of observed current data to determine tidal constituents
• Predictions account for astronomical forcing and local bathymetric effects
• Tidal current tables and software provide forecasts for maritime navigation
• Numerical models simulate complex tidal current patterns in coastal areas
• Accurate current predictions essential for coastal engineering projects and environmental assessments

Tidal range variations

Spring vs neap tides

• Spring tides occur during new and full moons when solar and lunar tides align
• Neap tides happen during first and third quarter moons when solar and lunar tides oppose
• Spring tides exhibit larger tidal range, while neap tides have smaller range
• Spring-neap cycle repeats approximately every 14.77 days
• Coastal engineers consider spring tide conditions for extreme water level scenarios

Equinoctial tides

• Occur near vernal and autumnal equinoxes (March and September)
• Sun positioned over Earth's equator, maximizing its tidal influence
• Can produce exceptionally high spring tides if Moon also near perigee
• Equinoctial tides increase risk of coastal flooding and erosion
• Coastal management strategies often focus on preparedness during equinoctial periods

Perigean spring tides

• Result when spring tide coincides with Moon at perigee (closest approach to Earth)
• Produce highest tidal ranges and potentially extreme high water levels
• Occur approximately every 7 months due to lunar orbit characteristics
• Perigean spring tides pose increased flood risk for low-lying coastal areas
• Coastal resilience planning must account for these infrequent but significant events

Tidal propagation

Shallow water effects

• Tidal wave speed decreases in shallow water, altering tidal timing and amplitudes
• Bottom friction in shallow areas can dampen tidal energy and reduce tidal range
• Shallow water harmonics generate additional tidal constituents (M4, M6)
• Tidal asymmetry develops in estuaries and coastal bays due to shallow water effects
• Understanding shallow water tidal dynamics crucial for coastal morphodynamics studies

Coriolis effect on tides

• Earth's rotation deflects tidal currents to the right in Northern Hemisphere, left in Southern Hemisphere
• Coriolis force influences formation of amphidromic systems in ocean basins
• Affects cross-shore distribution of tidal currents in wide estuaries and coastal seas
• Coriolis-induced current patterns impact sediment transport and coastal geomorphology
• Engineers consider Coriolis effect in designing large-scale coastal structures and predicting pollutant dispersion

Amphidromic points

• Locations where tidal range approaches zero and tidal phases rotate around
• Form due to interaction of Coriolis force and ocean basin geometry
• Tidal range increases with distance from amphidromic points
• Complex amphidromic systems develop in semi-enclosed seas (North Sea)
• Understanding amphidromic patterns important for regional tidal energy assessments and coastal management strategies

Tidal analysis and prediction

Harmonic analysis methods

• Decomposes observed tidal data into constituent harmonic components
• Utilizes least squares fitting techniques to determine amplitude and phase of constituents
• Requires long-term tidal records (preferably 19 years) for accurate analysis
• Accounts for astronomical forcing and local bathymetric effects
• Harmonic constants derived from analysis used for future tidal predictions

Tide tables and charts

• Provide predicted times and heights of high and low tides for specific locations
• Based on harmonic analysis of historical tidal data and astronomical calculations
• Include daily predictions and information on tidal datums and extreme levels
• Tidal current tables complement tide height predictions for maritime navigation
• Coastal engineers use tide tables for planning construction activities and assessing flood risks

Numerical tidal models

• Simulate tidal propagation and currents using hydrodynamic equations
• Account for complex bathymetry, coastline geometry, and bottom friction
• Global tidal models provide boundary conditions for regional and local models
• High-resolution coastal models capture fine-scale tidal processes in estuaries and bays
• Numerical models essential for assessing impacts of coastal interventions on tidal dynamics

Coastal impacts of tides

Tidal inundation zones

• Areas periodically flooded by tides, ranging from daily to annually
• Extent of inundation depends on local tidal range and coastal topography
• Tidal inundation influences soil salinity, vegetation patterns, and habitat distribution
• Mapping inundation zones crucial for coastal land use planning and ecosystem management
• Sea level rise expands tidal inundation zones, requiring adaptive coastal management strategies

Tidal erosion processes

• Tidal currents contribute to sediment transport and coastal erosion
• Strongest erosive forces often occur during spring tides and storm events
• Tidal range influences development of coastal landforms (tidal flats, salt marshes)
• Tidal inlet dynamics driven by interaction of tidal currents and wave action
• Understanding tidal erosion processes essential for designing effective coastal protection measures

Tidal wetland ecosystems

• Develop in intertidal zones with regular tidal flooding (salt marshes, mangroves)
• Tidal range and hydroperiod determine vegetation zonation and species composition
• Provide important ecosystem services (carbon sequestration, flood protection, habitat)
• Tidal wetlands vulnerable to sea level rise and changes in sediment supply
• Coastal engineers incorporate tidal wetland restoration in nature-based coastal defense strategies

Tides and coastal engineering

Tidal energy harvesting

• Utilizes tidal range or tidal currents to generate renewable electricity
• Tidal barrages capture potential energy of tidal range (La Rance, France)
• Tidal stream turbines harness kinetic energy of tidal currents (MeyGen, Scotland)
• Site selection based on tidal range, current velocities, and environmental considerations
• Tidal energy projects must address environmental impacts and navigation concerns

Tidal barriers and gates

• Protect coastal areas from tidal flooding and storm surges
• Operate by closing during high tides or extreme events (Thames Barrier, London)
• Design considers tidal range, sea level rise projections, and operational flexibility
• Environmental impacts include altered tidal dynamics and sediment transport patterns
• Tidal barriers often combined with other flood defense measures in integrated coastal management plans

Port and harbor design

• Accounts for local tidal range to determine quay wall heights and navigation depths
• Tidal currents influence channel alignment, turning basins, and berthing arrangements
• Lock systems may be required in areas with large tidal ranges (Port of Rotterdam)
• Sedimentation patterns affected by tidal dynamics, impacting maintenance dredging needs
• Port operations and ship scheduling optimized based on tidal predictions

Climate change effects

Sea level rise impacts

• Increases mean water levels, altering tidal datums and inundation patterns
• Amplifies tidal flooding frequency and extent in low-lying coastal areas
• May lead to tidal range changes due to altered basin resonance characteristics
• Affects tidal current velocities and patterns, potentially altering sediment transport
• Coastal engineers must incorporate sea level rise projections in long-term infrastructure planning

Changes in tidal amplitudes

• Climate change may alter tidal ranges due to changes in ocean stratification and circulation
• Some regions experience amplified tidal ranges, while others see reduced ranges
• Shifts in amphidromic point locations can significantly impact regional tidal patterns
• Changes in tidal amplitudes affect coastal flooding risks and ecosystem distributions
• Long-term tidal observations crucial for detecting and quantifying climate-induced changes

Coastal adaptation strategies

• Include both hard engineering solutions (sea walls, levees) and nature-based approaches (wetland restoration)
• Managed retreat considered in highly vulnerable areas with limited adaptation options
• Adaptive management frameworks allow for flexible responses to changing tidal conditions
• Integration of tidal dynamics in coastal zoning and building codes to enhance resilience
• Coastal communities develop long-term adaptation plans considering tidal changes and sea level rise projections