CEE 1030 1st Edition Lecture 19 Land- Sea Boundary - common boundary between air, land, and ocean - constantly changing (modified Dynamic Interfaces shoreline- line that marks the contact between land and sea (constantly moving) coastline- the coast’s seaward edge; landward limit of the effect of the highest storm waves on the shoreMoving Shorelines- the shoreline migrates up and down each day as the tide rises and falls Changing coastlines - storms can result in abnormally high shorelines - coastline marks most inland evidence of ocean waves (the highest shoreline) - coastlines change mainly as result of storm erosion and sea-level change Waves- wind-generated waves provide most of the energy that shapes and modifies coastal zones Anatomy of a wave crest- high point of a wave trough- low point between waves Measuring waves wave height- distance between crest and trough wave length- horizontal distance between crest wave period- time interval between the passage of two successive crests Predicting wave size These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- wave size depends on: wind speed length of time wind has blown fetch (distance wind has traveled across open water) water depth Formation of Surface Waves - wind= force behind most ocean waves - as wind blows, energy is transferred from air to water by pressure and friction - water molecules are displaced in direction of wind, but snap back because of molecular cohesion- result: water particles travel in orbits as wave peaks and troughs “pass by” Oscillation waves= wave form advance as: water particles travel in circular orbits (like a toy boat at the surface) - Oscillation waves occur in open ocean (no blockages, deep water) Effect of water depth- water particle movement decreases with increasing water depth (i.e., radius of orbit gets smaller) - below half a wave length in depth, there is little to no wave action Effects of shallowing - deep water: water particles move in circular orbit from surface to depth - shallower water: water particles move in a circular orbit at surface but are increasingly elliptical toward ocean floor - shallows are slow wave speed, so wave length becomes shorter and crest heights increase - eventually crests become unstable and the steep wave front collapses - as wave crest breaks forward, turbulent water flow increases Translation waves translation wave: turbulent advance of water created by breaking waves in shallows Shore Zones offshore: seaward of where waves break at low tide nearshore: zone between where waves break at low tide to low-tide level foreshore: area between low-tide shoreline and coastline; most active area and typically wet backshore: area inland of foreshore, affected by storm waves and atypical high tides, but mainly wind Wave erosion - breaking waves exert great force - wave erosion is cause by: wave impact and pressure abrasion by rock fragments Beaches beaches: wave act to accumulate local sediment along the landward margin of an ocean or lake Beach formation - as breaking waves move water up the shore and then back down, sediment is moved and abraded - differential sorting and transport of sediment grains by wind and wave action - inland areas affected primarily by wind transport and occasional storm waves Foreshore feature- vary depending on: rockiness of shore currents wave intensity tectonic regime Foreshore features beach face: sloping section of beach below berm - exposed to swash of waves - composed of fine- grained sediment berm: nearly flat area inland of beach face - stays dry except at extreme high tide - sof dunes: hill or ridge of wind- deposited sand; fine grained relative to shore- dunes can extend inland for miles, or be interrupted by cliffsCostal features - change in sea level relative to land has significant impact on costal features - relative sea level can change slowly (i.e., geologic time scales) or rapidly (e.g., earthquakes) - falling sea level or rising coast results in an emergent coast - rising sea level or a sinking coast results in a submergent coast Emergent coasts - if coast rises or sea level drops, land once covered by sea emerges to form part of the landscape - waves erode surfaces previously below water - wave erosion at sea level undercuts cliffs, leading to collapse of cliff face - over time, cliffs retreat while beach extends at base Features of emergent coasts - jagged cliff face- wave- cut notch - wave-cut platforms - when erosion rates vary, undercutting may initially produce sea caves- when sea caves from opposite sides of a rocky headland meet, a sea arch may form - eventual weakening of sea arch may result in its collapse to form a sea stack Submergent coasts - if sea level rises or land subsides, land once exposed will be submerged by the sea - river valleys are inundated by ocean water and become estuaries - hilly terrains become islands Rising sea level - since 1900, sea level has risen 1-2 mm per year - as global sea level rises, many inhabited areas (including major cities) become submergent coasts - artificial solutions: levees, flood gates, stiltsCoastal erosion and deposition - factors affecting coastal erosion and deposition degree and type of tectonic activity topography and composition of the land configuration of coastline and nearshore prevailing wind and weather patterns proximity to sediment- laden rivers Differential coastline erosion - if coast is composed of rocks of different hardness, different areas will erode at different rates - result in an irregular coastline Bending waves= refraction waves - waves slow as they enter shallow water - where shoreline is curved, wave will “bend” and align with the underwater Wave refraction - consequences: wave energy is concentrated against sides and ends of headlands wave energy is spread out and weakened in bays Oblique waves- wind that do not blow perpendicular to the coast cause waves that reach the shoreline at an angle (= oblique waves) - oblique waves cause current to flow parallel to coast in the surf zone, called a longshore current Beach drif - oblique waves and longshore currents move sediment along beach in a zigzag patter (= beach drif) - longshore currents move fine sand in suspension and larger particles by saltation and
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