Dynamic Shoreline Why do we care? Loss of land Damage to structures Recreation
Coastal Water Movement Waves provide the energy Through breaking As waves shoal Speed decreases Height increases Wavelength decreases Period remains the same Become unstable Waves refract Bending of wave fronts toward shallow water as they shoal Waves generate currents Longshore Rip
Generation of Longshore Currents Longshore currents caused by waves or wave set-up By waves Angle of wave approach is the acute angle (less than 90 o ) This angle usually less than 10 o The greater the angle of approach, the stronger the longshore current
Longshore Currents
Longshore Currents by Wave Set-up Wave set-up process whereby waves pile water against the shore Larger waves produce more wave set-up than smaller ones Sea surface slope from where larger waves hit shore to where smaller waves hit shore causes pressure gradient This pressure gradient causes a current from high pressure to low pressure (Coriolis not a factor)
Generation of Rip Currents Where two opposing longshore currents collide (convergence), they form a a swift, narrow seaward rip current Rip current drains excess water from the surf zone
Rip Currents
Beaches The part of the land that touches the sea Parts of the beach Offshore seaward of where waves begin to break Nearshore from the offshore to where the waves wash back Breaker zone waves begin to break Surf zone waves expend most of their energy Swash zone waves wash back and forth Backshore land that adjoins the nearshore
Beaches Positions of the divisions of the beach vary with the tides Advances landward with high tide Retreats seaward with low tide Beach sediments are moved by currents and waves, especially breakers Beaches undergo seasonal changes and changes due to weather Sand moves offshore in winter and during big storms Sand moves onshore in summer and during calm weather
Beach Profile A cross-section of the beach along a line that is perpendicular to the shore By comparing beach profiles along the same line taken at different times, it is possible to see changes in the beach Beaches display seasonal cycles of expansion and contraction related to wave size
Swell Profile Made during calm weather Typical summer Concave upward with a wide, broad berm, relatively flat backshore, and steep intertidal beach face
Storm Profile Made when dominant waves are high and steep Made during storms Typical winter profile Finer sediment is transported seaward, leaving coarser sediment on beach Longshore bar forms, which migrates landward during calmer seas
Sand Budget Balance between sediment added to and eroded from the beach Input from rivers, sea cliff erosion, and on-shore sediment transport Removal by longshore currents, offshore transport, and wind erosion Steady state balance between gains and losses When Negative losses exceed gains When Positive gains exceed losses
Coastal Dunes Sand dunes are formed by winds blowing sand landward from the dry part of the beach Well developed dunes typically have a sinusoidal profile with the primary dune at the landward edge of the beach Secondary dunes can be located farther inland Dunes can extend 10 km into interior Swale - area between dunes Vegetation on dunes traps windblown sand and promotes dune growth and stability
Dunes Foredune Primary dunes Secondary dunes Back dune
Dune Blowouts Wind scoured breaks in the dune or depressions in the dune ridge Commonly occur when vegetation is destroyed With time blowouts can enlarge and eliminate the dune
Dunes Dunes are best developed if sand is abundant, onshore winds are moderately strong and persistent, the beach is wide, and the tidal range is large Sand saltates (bounces, nothing to do with salt) up the windward side of the dune, collects at the top in the wind shadow, and slides down the leeward side Erosion of windward side and deposition on leeward side results in dune migration
Importance of Dunes Dunes act as a natural barrier and prevent inland flooding Human activities can damage vegetation and lead to dune destruction by blowouts and washover by storm waves Washover forms a washover fan on the landward side of the dune
Barrier Islands Islands composed of sediment Parallel the coast Occur where sand supply is abundant and the sea floor slope is gentle (e.g. East coast of U.S.) Separated from mainland by shallow bodies of water, which are connected to the ocean through tidal inlets
Parts of a Barrier Island Nearshore zone Dune field Back-island flats washover fans deposited during storms Salt marshes protected areas where mud accumulates Barrier islands migrate landward
Creation of Barrier Islands Submergence of sand ridges on the coastal plain Sand spits that were breached during a storm Vertical growth and emergence of longshore bars
Cliffed Coasts Sea cliff- an abrupt rise of the land from sea level Erodes at its base Waves slam against the base, compress air in cracks, air expands violently as the water retreats Sediment is hurled against the cliff by waves Sea water can dissolve some rock types When sufficient material at the base is removed, the upper part of the cliff collapses
Erosion of Cliffed Coasts Rate at which cliff recedes is dependent on Composition and durability of the cliff material Joints, fractures, faults and other weaknesses in the cliff material Amount of precipitation Steepness of the cliff Wave-cut platform gentle sloping area in front of the sea cliff that was produced by sea cliff retreat
Erosion of Cliffed Cloasts
Deltas Emergent accumulation of sediment deposited at the mouth of a river as it flows into a standing body Nile and Mississippi are famous ones
Areas of a Delta Three major areas Delta plain flat low, lying area at or below sea level that is drained by a system of distributaries Delta front shoreline and broad submerged area of the delta that slopes gently seaward Prodelta far offshore area of the inner continental shelf that receives fine sediment from the river
Types of Beds Topset beds Flat lying beds of sand and mud on the delta plain Deposited by distributaries in their channels and in the interchannels Delta plain Foreset beds Thick silts and sands of the delta front that slope gently seaward The bulk of the delta Delta front Bottomset beds Flat lying silts and clays of the prodelta that settle out of suspension offshore prodelta
Delta Growth and Shape As sediment accumulates the delta expands seaward Foreset beds bury bottomset beds Topset beds bury foreset beds Shape of delta can be altered by: Tides Waves Rivers Reduction in supply of sediment to a delta results in delta erosion and subsidence as the sediments compact Louisiana 20 cm/yr, 1 cm/yr sea level rise
River, Wave or Tide domination River dominated In areas protected from large waves and small tidal range Delta has ideal triangular shape Wave dominated Only a slight protrusion at river mouth Waves and longshore currents erode most of the delta sediment Tide dominated Altered by ebb and flow of the tides Have long linear submarine ridges and islands that radiate from the river s mouth
Coastal Habitats Coastal has a broader meaning than shoreline Major coastal settings are: Estuary Lagoon Salt Marsh Mangrove Swamp Coral Reef Productive ecosystems Shallow and respond to waves, tides, and weather
Estuaries Semi-enclosed bodies of water where freshwater from the land mixes with sea water Examples: Lower Hudson River Narragansett Bay
Types of Estuaries
Estuaries and Mixing
Estuaries Due to the mixing, the environmental conditions fluctuate widely makes life stressful Extremely fertile due to nutrient inflow from land Stressful conditions and abundant nutrients result in low species diversity, but great abundance of the species present Phytoplankton blooms are irregular due to low light from turbidity Benthic fauna dependent on substrate
Salt Wedge
Lagoons Isolated or semi-enclosed, shallow, coastal bodies of water that receive little if any fresh water inflow Occur at any latitude Some polluted by man; were productive, now wasteland
Lagoons
Salt Marshes Intertidal flats covered by grassy vegetation Found in protected areas with a moderate tidal range Flood daily at high tide and drain through a series of channels with the ebb tide One of the most productive environments
Parts of Marshes Low Salt Marshes From the low tide mark to neap high tide High Salt Marshes From neap high tide to highest spring tide
Salt Marshes Low Salt Marshes More productive area Nitrate is commonly the limiting nutrient Plants die in autumn, partially decompose and supply abundant detritus Detritus either becomes food or accumulates and eventually forms peat High Salt Marshes More terrestrial than marine in nature More diverse flora and fauna
Life in Salt Marshes Distribution and density of organisms in salt marshes strongly reflects availability of food, need for protection and frequency of flooding Salt marshes serve as a nursery and shelter for juvenile organisms Many salt marshes damaged by man filled in
Mangrove Swamp Mangroves are large woody trees with a dense complex root system that grows downward from the branches Distribution of the trees is largely controlled by air temperature, exposure to wave an current attack, tidal range, substrate, and sea water chemistry Detritus from the mangrove forms the base of the food chain
Coral Reefs An organically constructed, waveresistant, rock-like structure created by carbonate-secreting organisms Composed of loose to well-cemented organic debris of carbonate shells and skeletons Living part is just a thin veneer on the surface
Location of Coral Reefs
Coral Corals belong to Cnidaris The animal is the coral polyp The body of the polyp resembles a sac with the open end surrounded by tentacles The coralite is the exoskeleton formed by the polyp
Coral Coral share a mutualistic relationship with the algae call zooxanthallae Zooxanthallae live within the skin of the polyp Zooxantallae can comprise up to 75% of the polyp s body weight The coral provides protection for the algae and supplies them with nutrients and CO 2 The algae supply the coral with oxygen and food Recycling of nutrients between the polyp and the algae allows the corals to thrive in the nutrient-poor tropical seas
Coral Survival Corals cannot survive in fresh, brackish or highly turbid water Corals do best in nutrient poor water because they are easily outcompeted by benthic filter feeders in nutrientrich water where phytoplankton are abundant
Atoll Formation
Lab time
Impact of People on the Coastline Humans try to stabilize the coastline Interfere with longshore sand transport Redirect wave energy to prevent erosion Build structures Beach nourishment adding sand to beach to restore beach Increase in sea level due to global warming will cause more land to be flooded and threaten more coastal buildings
Jetties Groins Breakwaters Seawalls Structures