Lecture 10 - Sedimentary notes PDF

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Lecture 10: EstariesFlow Conditions in Estuaries:- Estuaries are regions of the coastal ocean where salinity variationsin space are so large that they determine the mean circulation- Most estuaries are found at river mouths; they are thus long andnarrow, resembling a channel.- Can compare the flow o...

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Lecture 10: Estaries Flow Conditions in Estuaries: - Estuaries are regions of the coastal ocean where salinity variations in space are so large that they determine the mean circulation - Most estuaries are found at river mouths; they are thus long and narrow, resembling a channel. - Can compare the flow of river input (R) to the tidal input (V) - Grouped in to classes which include: 1) Salt wedge estuary: - The balance of forces that establishes a steady state in these flows involves advection of freshwater from a river and introduction of sea water through turbulent mixing by tidal currents - River volume R is very much larger than the tidal volume V, or there are no tides at all - The fresh water flows out over the sea water in a thin layer. All mixing is restricted to the thin transition layer between the fresh water at the top and the "wedge" of salt water underneath 2) Highly Stratified Estuary: - River volume R is comparable to but still larger than tidal volume V - The salty water is more dense, mixed up a little bit - There is a little mixing of the salty layer to the fresh - No mixing between freshwater to the salt water - There is a sharp variation in density as you go through the system — strong difference in when S = 0 and where S = 35 3) Slightly Stratified Estuary - River volume R is small compared to tidal volume V - This means that the fluid from V is dominating the situation - V comes in at the bottom of the estuary, this causes a larger turbulent flow which provides stronger mixing in the layers - Contain gradual mixing as you go down in depth - It is the ratio from the river and tidal volume is the difference 4) Vertically mixed estuary: - River volume R is insignificant compared with tidal volume V - In vertical columns density is pretty much the same - Tidal mixing dominates the entire estuary. Locally it achieves complete mixing of the water column between surface and bottom, erasing all vertical stratification - Provides large mixing due to the large turbulent flow from the bottom Sedimentation patterns - An estuary is a region where the flow patterns depend on the ratio between tides and river flow - Therefore, will see some variation during the day - See lots of sediments cycling around - Three combinations that alter sediment cycling can occur: storms, floods, tides (depends how strong the tides are depending on which day and which month) - If have cycling, sediment can be deposited - Where the stream flow drops, sedimentation can occur which is where the river and sea first interact as getting deeper and velocity decreases

Topset (uppermost region) - freshwater swamps, brackish water marshes, sandy channel floors - sediment accumulation controlled by sea-level rise - land surface sinks due to consolidation of underlying mud Foreset (middle region) - very high rates of sediment accumulation = thick, muddy deposits - sloped surface (few degrees) - gullies form from turbidity currents, landslides occur from slope failure Bottomset (deepest region) - forerunner of advancing delta - thin deposits of mud over inner-shelf sand Lobe of maximum sedimentation changes over centuries

- depression filled, and lobe switches to another location - Gullies also can form - Deltas can occur from estuarine deposition, where the deposition changes position where it deposits Case study: The Severn Estuary - It has a very large tidal range — second highest in the world - Tidal volume is significantly higher than river volume — vertically stratified - Estuary has a well developed funnel shape - Affect of the y change in the tidal depth, when the estuary narrows the depth change will be very great - Big tidal range relates to the funnel shape - Formation: - 1) Cambrian until end of tertiary (major regressions and transgressions of the sea led to the erosion and deposition of sediments which formed the solid rock of the region) - 2) The Pleistocene during which time glacial rivers eroded the present day river channels - 3) The Holocene period (10,000 years ago until the present). At the beginning of this period sections of the eroded channels were infilled with glacially derived sediments. - Today: dominated by medium/ fine grained sediment (sands, muddy sands, muds), leads to the formation of sand banks - Tidal bore are common in funnel shaped estuaries — result from the front of the rising tide steepening to a wave as a result of being channelled from wide estuary into a narrow — a wave

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As narrows down fast, see a fast tidal change Gentle changes occur at Weston Super Mare, a sharp change occurs as estuary narrows Occurs in conscience of the tide Speed and character of a bore depends on speed of flow/ tide which depends on the depth and gravity...