Lecture-15_shoreline&shoreface_v2 - Lecture 15:...

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Unformatted text preview: Lecture 15: GEO416M – Fall 2010 Shoreline and Shoreface Depositional Environments Significance of 1) Water Waves, 2) Longshore Currents, & 3) Tides on the erosion, transport and deposition of sediment in the shallow-marine setting ce Class reading from Boggs, Principles of Sedimentology and Stratigraphy: p.306 - 348. 1 Water Waves: Definitions Alternating rise and fall of the water surface. Produced by wind stress on water surface. • H = wave height • a = H/2 = wave amplitude • = L = wavelength ( ≥ 3.5H) • d =mean water depth Typical values for H: Open Ocean 2-5 m Highest Measured 34 m Typical values for c (wave speed): 3-30 m/s 2 MOVEMENT OF WATER IN A WAVE A parcel of water moves in an orbit or circular path as the waves passes. For a deepwater wave the particle essentially returns to its original position after the wave has passed. At water surface the diameter of the orbital path (OD) is equal to the height of the wave. 2 πz Deep-water waves Shallow-water waves ODz ODz surface e [z is measured downward from the water surface] λ 3 Deep water waves • In practice, considered deep-water or short waves when d > /2 •At depth of /2 waves are not feeling the bottom, uorbital = 0 Orbital Motions in Fluid Under Shallow Water Waves Orbital Velocity = f (wave height, length, speed, & water depth) Shallow water waves • In practice, considered shallow-water or long waves when d < /8 [orbital motion beneath waves begins to deform] • Waves are attenuated by bottom friction. • wave speed, c = (gd)1/2 4 With shallow water waves there is a net transfer of WAVE REFRACTION water mass in the direction of wave propagation. & LONGSHORE CURRENTS Most waves do not strike the shore directly, but rather meet it at an angle. When a wave approaches the coast in this manner, one end of the wave encounters shallow water first and slows down, while the rest of the wave travels faster, still in deeper water. This spatial variation in wave speed causes the wave crest to bend and is called REFRACTION. 5 The angled approach of waves to the shoreline sets up a LONGSHORE CURRENT. Longshore Drift: Transport of sediment by combination of swash & backwash on beach face + longshore current 6 Influence of wave refraction and longshore currents on patterns of coastal erosion and deposition. 1. Refraction can produce focusing of wave energy at protruding coastal headlands and reduce wave energy within coastal embayments. 2. Longshore drift helps set spatial structure of coastal sediment transport field (Erosion at headlands, deposition in bays). Site of sediment deposition from spatial convergence in longshore drift Site of sediment erosion from spatial divergence in longshore drift Site of sediment deposition from spatial convergence in longshore drift refracted wave crestline wave crestline 7 Spit = A long, narrow stretch of land extending from the shore into the sea. Produced by sedimentation tied to the longshore drift of sand. TX example: Bolivar Peninsula & Galveston Island 8 Barrier Island 2008 overwash deposits on Matagorda Peninsula, TX C. G. St. C. Kendall Coast Types Sequence Stratigraphy Defined Island Island Crest TX coastline Site is dominated by aeolian sediment transport Backshore: On Matagorda Peninsula the backshore is dominated by Hurricane Ike overwash deposits. 100-m long trench Sequential airborne LIDAR surveys confirm that sediment locally eroded from the beach built the Ike overwash fan deposits. 100–m long trench through Matagorda overwash fan Overwash deposit immediately behind island crest Overwash deposit at distal end where the fan tip enters the lagoon. Beaches: the Foreshore • in practice, shallow water waves steepen to the point of breaking when d < /20 • waves break when the apical angle reached 120 degrees. • high Froude number flow associated with the wave swash & backwash Beach deposit (predominantly upperplane bed stratification) 14 Connecting transport processes with environment and stratification waves field + irregular coastline sets up longshore current, producing greatest occurrence of strongly asymmetric combined-flow bedforms F Nearshore (Shoreface) sedimentation: large strongly asymmetrical bedforms 15 Combined Flow: Waves + Unidirectional Current Combined-flow bedforms: Superimposed bedforms produced by combined unidirectional and reversing currents (horizontal length = 1.1m). Constant unidirectional velocity = 0.10m/s with flow from right to left. Peak reversing current velocity = 0.60m/s (photograph provided by Simone Dumas). 16 Oscillatory Bedforms: Found further offshore on the inner shelf Velocity field at two points during wave cycle (Hansen et al., 1991) Suspended sediment at two points during wave cycle Oscillatory Ripples Superimposed bedforms developed under purely reversing currents (Peak velocity = 0.46m/s). 17 Relative contribution of unidirectional currents to sand transport diminishes with distance from shoreline. Hummocky bedded sands / Hummocky stratification: interpreted as forming beneath fair-weather wave base 18 Hummocky stratification: • identified and named in 1975 •observed in a lower shoreface/inner shelf position • typically interbedded with burrowed sediment • interpreted as being generated during storms • constructed predominantly through vertical accretion with minor evidence to systematic lateral migration (high volumes of suspendedsediment deposition) 19 Hummocky Stratification • many laminae thicken into swales (different from dune foresets that are thickest at crests and thin into troughs) •often associated with late-stage climbing ripples 20 Amalgamation of Storm/‘Event’ Beds on the Inner Continental Shelf 21 TIDES: alternating rise and fall of sea level within a day. The moon is the primary factor controlling ocean tides (sun is second). The moon produces tidal bulges at two locations of the earth’s surface: 1. Seawater moves up into a slight bulge on the side of the earth facing the moon because the moon’s gravity pulls the water towards it. 2. On opposite side of the Earth, another tidal bulge is produced by the centrifugal force pulling water outward from the center of the spinning earth-moon unit. Spring Tides When the moon & sun’s gravitational pull is in the same plane. During these days the two gravitational forces work together to make high high tides and low low tides. Neap Tides When the moon is in its first quarter or its last quarter, the sun’s gravitational pull is in perpendicular direction to that of the moon. The sun pulls water away from the areas of high tide to the areas of low tides, resulting in lower high tides and higher low tides. 22 The geometric relationship of moon and sun to the topography and bathymetry of Earth's surface results in creation of three different types of tides. 1. Diurnal tides - one high and one low water per tidal day (e.g., northern Gulf of Mexico and Southeast Asia) 2. Semi-diurnal tides - two high and two low waters per tidal day (e.g., Atlantic coasts of the United States and Europe). 3. Mixed tides - successive high-water and low-water stands differ appreciably (e.g., west coast of Canada and the United States). Magnitude of Tides 23 Transport of Sediment by Tidal Flows Flood- and ebb-tidal deltas Essex River, Ipswich Bay, MA 2000 ft San Luis Pass, Galveston Island, TX 24 Transport of Sediment by Tidal Flows Tidal channel with bidirectional bar forms. Andros Island, Bahamas Ebb current Flood current 25 Tidal Bundles repeat tidal cycle Rhythmic Bedding Associated with Tidal Flows 26 Plunging hyperpycnal river plumes Mulder and Alexander, 2001 Turbidity current generation (Hyperpycnal flow) River Reuss, Switzerland, 2005 What happens when these currents are modified by waves & tides? Tidal UT UT = tidal velocity UC = current velocity Plus waves Hyperpycnal Flow modulated by tides & water waves (Lamb et al., 2008) = Vertically aggrading oscillation ripples Coarser & Finer Sediment Associated with Tidal Flows 28 Questions You Should be Able to Answer 1. What is the shoreface? 2. How are trends in overall grain size, amalgamation of beds, and degree of bioturbation connected to the shoreface and to wave base? 3. What is wave base and how does it vary between normal and stormy conditions? 4. What is the height, amplutdue and length of a water wave? What are typical heights for ocean waves? 5. How does the orbital diameter of a water wave vary with depth below the sea surface? 6. What is the definition for a deep water or short wave? What is the associated orbital velocity at the bed beneath these waves? 7. What is the definition for a shallow water or long wave? What equation describes its wave speed. What is the associated orbital velocity at the bed beneath these waves? 8. What is wave refraction and how does it work? 9. What is a longshore current and how is it connected to wave refraction? 10. What is the longshore drift and its two components of sediment transport? 11. How do refracted waves act to erode shoreline promontories and fill in bays with sediment? 12. What is a spit and how is it built? Name a TX example of a spit. 13. What is a barrier island? 14. Where to aeolian deposits accumulate on a barrier island or beach complex? 15, What are aeolian deposits? 16. What is the backshore environment? 29 Questions You Should be Able to Answer 17. Did Hurricane Ike erode northern and central TX beaches and by about how much? 18. What is an overwash fan? Does it represent progradation or retrogradation of a barrier island? 19. What are the differences between spilling, surging, and plunging waves? 20. When at what water depth and apical angle do waves typically break? 21. What is wave swash and backwash? 22. What are characteristics of beach sedimentary deposits? 23. What is the foreshore? 24. Where do waves of translation occur and what types of bedforms and bedform deposits do they typically produce? 25. How are the nearshore and shoreface connected? 26. What is combined flow and combined-flow bedforms? 27. What are oscillatory bedforms and where do they develop relative to combinedflow bedforms? 28. What is a hummock? What is a swale? 29. What is hummocky stratification and where does it form? How is hummocky stratification connected to storms? 30. How is hummocky stratification connected to climbing ripples? 31. Describe the processes that lead to the amalgamation of storm/”event” bed on the inner continental shelf. 32. What is an ocean tide and what are the roles of the moon and sun in producing tides? 30 Questions You Should be Able to Answer 33. What is a spring tide and what causes it? 34. What is a neap tide and what causes it? 35. What are semi-diurnal, diurnal and mixed tides and what causes them? 36. What are flood and ebb tidal delta? Are they always the same size and if not, why? 37. What is the character of flow in a sinuous tidal channel? Are resulting sediment bars attached to the sides of channels? 38. What is rhythmic bedding? 39. What is a tidal bundle and how does it form? 40. What is a hyperpycnal versus hypopycnal flow? 41. How do vertically aggrading, rhythmic oscillation ripples form? 31 ...
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