Tidally influenced environments By Alex Tkaczyk, Henrique Menezes, and Isaac Foli
Goals and aims Describe the role of tidal influence in depositional environments. - Deltas - Estuaries Provide an overview of tidal effects on deltas and estuaries. Examine a case study of the Backwater zone of an estuarine system located in Western Portugal.
What is a Tide? Tides are large scale rise and fall water level on an interval influenced by a number of far reaching factors, such as climate and the gravitational force of the moon on the Earth. Tides have cyclicity and variability in total extent (Spring Tide, Neap Tide) as well as daily occurrence and magnitude (Diurnal, Semidiurnal) Not to be confused with wave action; which is small scale (comparatively) local change in water level caused by wind
Tidally Influenced Deltas Delta: Landform created by the deposition of sediment carried by a river as the flow leaves the river mouth. Hydrodynamics and sediment deposition are influenced by both river discharge and tidal exchange, principally by tides in this case.
Tidally Influenced Delta Features Tides become channelized as they propagate upstream, focusing energy and accelerating tidal currents, allowing tidal influences to reach farther as a result of the delta s shape. Extensive tidal flats present, dissected by tidal channels. Funnel-shaped distributary channel with bidirectional sediment transport.
Tidally Influenced Delta Features Tidal sand bars are a major structural feature, aligned perpendicular to the coast and parallel to the tidal flow direction. Greater in relief and size when tidal discharge is greater than fluvial discharge. Relief decreases with increasing distance from the distributaries.
Estuaries A partially enclosed body of brackish water with one or more rivers or streams flowing into it and a free connection to the open sea. Found on the marine side of transition zones between fluvial and marine environments. Characterized by brackish water. Tidally dominated or influenced.
Types of Estuaries Coastal Plain: Rising seas (as a result of glacial melting) invade coastal river valleys. Tectonic: Subsiding plates, as a result of tectonic activity, create basins which are then filled with water. Bar-Built: Sandbars build up along coastline, creating a separate body of water from the ocean. Fjord: A valley cut by glaciers then invaded by the sea.
Tidal Influence in Estuaries As a result of being tidally influenced, mixing between the discharged freshwater and seawater is not consistent, varying with distance from the ocean. Maximum tidal influence found close to ocean, but tidal influence decreases upstream. Two resulting variables: Distance from ocean and tidal cycle. Current reverses direction with ebb and flood tides.
Tidal Energy in Estuaries Increasing tidal energy reduces vertical salinity gradients, resulting in more mixing between fresh and saltwater. Salt Wedge Estuary Partially Mixed Estuary Vertically Homogeneous Estuary Increasing Tidal Energy
Case Study: The Backwater Zone, Lusitanian Basin
- Brinkpoint Height The Backwater Zone Little literature as focused in on the impact of tides on bedforms in this specific part of estuarine systems. Backwater Zone: The most proximal point of the fluvial Tidal transition. (With respect to the river system) Tidal influence on bedforms: - Organic Concentration - Bottomset Thickness - Foreset Dip and Shape
Flow Velocity and Tidal influence
Tidal Velocity and Bore Tide has two effects on flow velocity in the backwater zone: - Ebb Tidal Drawdown : Regressing tide causes an increase in flow velocity. - Tidal Retardation: Rising (incoming tide) baffles outgoing flux decreasing flow velocity. Tidal Bore: Tidal Phenomenon in which the leading edge of the incoming tide forms a wave that travels up the river against the direction of the river s current.
Evidence of Tidal Modulation Four Key Repetitive features were observed at similar locations in the different field sites. The repetition and cyclicity of these features led to the interpretations of tidal influence on bedforms in the backwater zone.
Tidal Bore Passage of Tidal Bore marked by atypical drapes. Examination of fluid dynamics describes presence of irregular flow signals.
Conclusions Observed features on dune and bar scale bedforms indicate tidal modulation on net flow velocity. These features include: - Increasing/Decreasing organic particle concentrations. The tidal range present at the river mouth was sufficient to generate tidal bores which could potentially propagate at least 100 km upstream during the dry season and/or cause tidal modulation of flow during the entire year. - Increasing/Decreasing Bottomset thickness. - Increasing/Decreasing foreset dip and shape (Convex to Concave) - Increasing/Decreasing Brinkpoint height Modulations representative of one full cycle of semidiurnal flood ebb and flood, thus a twice daily variation in net seaward fluvial current velocity.
Thank You
References Martinius, Allard W., and Stuart Gowland. "Tide influenced fluvial bedforms and tidal bore deposits (Late Jurassic Lourinhã Formation, Lusitanian Basin, Western Portugal)." Sedimentology 58.1 (2011): 285-324. www.wikipedia.com http://sepmstrata.org/page.aspx?pageid=313 Wells, John T. "Tide-dominated estuaries and tidal rivers." Developments in Sedimentology 53 (1995): 179-205. http://classes.geology.illinois.edu/17sprgclass/geo440/geol%20440%202017%20lecture%2023web.pdf