Deltas and Estuary Classification ,Sedimentology and Protection

Deltas and Estuaries ,Environmental Sedimentology

1. Environmental Sedimentology Deltas and Estuaries Prepared By: Rabindra Kunwar(12) Applied Geology Central Department of Geology Submitted To: Dr.Naresh Kazi Tamrakar Associate Professor Central Department of Geology

2. Deltas and Estuaries Deltas and estuaries are dynamic and critical ecosystems where freshwater rivers meet the salty embrace of the ocean. These fertile regions are cradles of life, teeming with biodiversity and providing essential services to humans. Key Topics

3. Introduction Delta A river delta is a landform shaped like a triangle,created by the deposition of sediment the   deposition  that is carried by a river and enters slower-moving or stagnant water.This occurs at a river mouth, when it enters an  ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot carry away the supplied sediment . Figure 2:Lena River Delta,Russia Figure 1:San  Sacramento–San Joaquin River Delta Figure 3:San  Sacramento–San Joaquin River Delta at flood stage,2009,source:   Doc Searls . Figure 4: Nile Delta,Egypt

4. Formation Of A River Delta A river delta is formed when sediment in that river flows into another body of water, and begins to deposit that sediment there. As the river hits the width of a lake or ocean, the water flow quickly slows down, allowing for large amounts of sediment to drop from the water onto the river bottom. Over time, this sediment builds up, decreasing the gradient of the river bed and the floor of the river channel becomes closer to the floodplain. Delta Formation Process Figure 6: Delta formation in a lake Figure 5:General process of delta formation

5. Sedimentary Structure and Sedimentology Deltas have a fan shape, with a convex side towards the sea . Delta plain ( topset ): It is the part that is above water, divided into the upper delta (where fluvial processes take place) and lower delta (influenced by tides ). Delta front (foreset): It is the outer limit of the part above water . Prodelta ( bottomset ): The part found under the water . Mouth bar is created when the distributary channel ends and water speed decreases, resulting in deposition of sediment. Structure of Delta

6. Esturaine Delta River deltas can also be estuarine, which means they are partly fresh water and partly salt water. In these deltas the river does not flow directly into the ocean, or salt water, but rather into an  estuary  at the river mouth. Estuarine river deltas include the  Yellow River  of  China  and the estuary of the  Tagus River  in  Portugal . Figure:The estuarine delta of the Tagus River.Porugal Types Of River Deltas Deltas can be categorized based on their shape, formation, or location.Some of the most common ways to distinguish delta types are as follows : Inland Delta In rare cases the river delta is located inside a large valley and is called an inverted river delta. Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an  inland delta , and often occurs on former lake beds. Figure: Inner Niger Delta

7. Wave Dominated Deltas Wave-dominated deltas are created and shaped by the movements of waves within the body of water in question.The waves influence the shape and formation of the river delta by shaping the sedimentary deposits near the river’s mouth. The waves move the deposits from the mouth opening towards the shore, preventing a bar from building up. Nile Delta,Egypt Tide-Dominated Deltas Figure:The Sundarbans mangrove forest formed in the Ganges-Brahmaputra delta in India and Bangladesh. It is an example of a tide-dominated delta. Tide dominated deltas are somewhat similar to wave ones, as the movement of water in the delta helps to form the shape and type of river delta that is created, but the resulting shape is very different. In these cases, the rise and fall of the tide is what has the biggest impact.

8. Gilbert Deltas The third type of delta is the Gilbert river delta. This occurs when coarse sediments are deposited at the river mouth. The term is named after geologist Grove Karl Gilbert, who coined this type of delta. These types of deltas are most commonly formed by mountain rivers which flow into fresh water lakes. The water flow from the mountain tends to carry larger denser sediments which are washed along in the river where they stop, forming blockades at the river mouth. Essentially homopycnal flow by mountain streams of steep gradient into deep water immediately offshore. Occasional turbidity flows (mud-rock flows); hyperpycnal. Coarse gravels dominant in foreset beds. Fine sands and silts in bottomset beds, prograded locally to receding lake levels. Some of the deltas of Lake Bonneville are true counterparts of alluvial fans . Source:Daniel J. Jones Earth Science Curriculum Project, Boulder, Colorado Figure:Gilber Type Delta(a)Oblique View(b) Crosss -sectional View

9. Estuary An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea .  Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment .. The geomorphology of an estuarine basin is usually developed by one of three agents: (1) fluvial or glacial erosion, (2) fluvial and marine deposition, or (3) tectonic activity. The last of these involves the downfaulting of a coastal area or the broad local subsidence of a stretch of coastline, as in the case of San Francisco Bay.   . Formation

10. Plan view, energy distribution and spatial facies variation in a tide-dominated estuary (a)(modified after Dalrymple, R.W, Zaitlin , B.A and Boyd, R., 1992 Jour. Sed. Petrol. V. 62, Fig. 4, p.1136). (B)Peter Scanes Figure: Plan view, energy distribution and spatial facies variation in a wave-dominated estuary(a) (modified after Dalrymple, R.W, Zaitlin , B.A and Boyd, R., 1992 Jour. Sed. Petrol. V. 62, Fig. 4, p.1134 )(b)Peter Scanes . Types of Estuaries Based on Dominance of Wave and Tide

11. Sediment transport in tide-dominated estuaries Fine and coarse sediment enters the estuary from the catchment. The amount of sediment input varies regionally, depending on catchment and climatic conditions, and the volume of freshwater input.   Large quantities of suspended sediment are characteristic of tide-dominated estuaries; strong tidal currents continually resuspended and rework fine sediment in the channels, so that the water column is naturally highly turbid (Turner et al., 1994, Wells, 1995. Fine sediment undergoes both deposition and erosion on the extensive intertidal flats (Dyer, 1998, Woodroffe et al., 1999, Masselink et al., 2000 ). The sediment trapping efficiency of tide-dominated estuaries is moderate because they are generally highly energetic and turbid systems, and mixing of seawater and freshwater results in the loss of suspended sediment to the open ocean (Harris et al., 2003).

12. Sediment transport in wave-dominated estuaries Fine and coarse sediment enters the estuary from the catchment . Coarse sediment (i.e., gravels and sands) is deposited at the head of the estuary forming a fluvial bayhead delta. (Webster et al., 2002, Pasternack et al., 2002). Fine sediment (i.e., muds, clays, and organic material) is deposited on the fringes of the central basin by river processes, tides, and internally generated waves. ( Boorman et al., 1998, Brown, 1998, Saintlan et al., 1999, Temmerman et al., 2003, Laegdsgaard , 2001 ). Suspended sediment is transported into the central basin, where it is deposited in a low-energy environment. Flocculation, or particle aggregation due to changes in salinity, is also an important process that enables fine particles to settle out from the water column. Benthic micro-algae (BMA) assist in the stabilisation of fine sediment ( Wulff et al., 1997, Cahoon et al., 1999, Murray et al., 2002 ). At the entrance, tidal currents are locally accelerated in the constricted entrance, and form flood and ebb tidal deltas (Roy, 1984a). Sedimentary processes are dominated by the landward transport of coarse sediment derived from the marine environment (Green et al., 2001). .

13. Types of Estuaries Types Of Estuaries Based On M orphology Bar-Built Or Lagoon-Type Estuaries This type of estuary usually forms in tropical and sub-tropical locations. They are common near the tectonically stable continental edges or coasts of marginal seas. A lagoon-type estuary forms when the deposition of sediments forming barrier beaches partially cuts off a section of the sea water along the coast Drowned River Valleys Drowned river valley estuaries are common in areas with a temperate climate. They arise by the penetration of seawater into land and the subsequent occupation of coastal river valleys by the sea. These estuaries are usually wedge-shaped. As the distance from the sea reduces, the estuary widens and deepens. However, the depths of these estuaries rarely exceed 100 ft. With climate change-induced sea-level rise, the number of drowned river valley estuaries is expected to rise in the future Figure: The Chesapeake Bay ( center ) and Delaware Bay (upper right) are both examples of drowned river valley estuaries. (Credit: NASA)

14. Fjord-Type Estuary A fjord-type estuary forms in river valleys widened and deepened by glaciers. Such valleys are U-shaped. Sills, rocks, or bars formed by centuries of glacial depositional activities at the mouths of river valleys partially blocks the flow of water into the sea. Such estuaries are flanked by steep slopes on either side. They are also very deep but the depth reduces seaward. Figure: This satellite image of Glacier Bay, Alaska, shows long narrow fjords that have been carved out of the surrounding terrain by advancing glaciers.

15. Tectonically Produced Estuaries Tectonically produced estuaries are less common than other types of estuaries mentioned above. The formation of such estuaries happens due to the subsidence of land or increase in sea-level due to tectonic movements. Earthquakes, volcanic eruptions, and resulting landslides can lead to the formation of tectonically produced estuaries. Seawater inundation of the basin or depression created due to the tectonic movements or the rise in sea level due to such movements forms these estuaries. The San Francisco Bay is one such estuary.

16. Types Of Estuaries Based On The Circulation Of Water Salt Wedge When the flow volume and velocity of the river draining into the sea exceeds that of the tidal forces of the sea, the effect of tidal actions on the estuary is reduced to a minimum.  Figure:The   marina at the mouth of the Mississippi River. Partially mixed As tidal forcing increases, river output becomes less than the marine input. Here, current induced turbulence causes mixing of the whole water column such that salinity varies more longitudinally rather than vertically, leading to a moderately stratified condition. Examples include the Chesapeake Bay and Narragansett Bay.   Figure:A satellite image of Chesapeake Bay

17. Well-mixed Tidal mixing forces exceed river output, resulting in a well-mixed water column and the disappearance of the vertical salinity gradient. The freshwater-seawater boundary is eliminated due to the intense turbulent mixing and eddy effects.  Figure:Delaware Bay in Winter Inverse Inverse estuaries occur in dry climates where evaporation greatly exceeds the inflow of freshwater. A salinity maximum zone is formed, and both riverine and oceanic water flow close to the surface towards this zone.This water is pushed downward and spreads along the bottom in both the seaward and landward direction.An example of an inverse estuary is Spencer Gulf, South Australia. Figure:Spencer Gulf as seen from space

18. Intermittent Estuary type varies dramatically depending on freshwater input, and is capable of changing from a wholly marine embayment to any of the other estuary types. i ntermittent estuaries (or inlets) are small channels (<100 m wide, a few metres deep) which develop on narrow low-lying barriers, and which allow the connection of brackish coastal water bodies with the ocean. Sediment dynamics The wide variety of hydrodynamic conditions in an estuary also yields a variety of sedimentary systems, from sandy channel beds, via mixed intertidal flats to mudflats and muddy marshlands. Sediment originates from rivers, sea and the land surrounding the estuary (Day  et al ., 1989; McLusky & Elliott, 2004). Where saline water and sediment-laden river water meet and mix, the increasing salinity may cause sediments suspended in the fresh water to flocculate, resulting in an estuarine turbidity maximum (ETM) and a zone of intense sedimentation. Wind in estuaries, tidal basins and lakes not only causes locally generated wave fields, but also non-tidal circulations, especially in areas with significant depth variations. Floods and storm events at sea influence the amount of water, sediments and nutrients that are transported from the river into an estuary.

19. The Vital Importance of Estuaries and Deltas Habitabt / Land formation / Erosion protection/Sediment traps: / River Hydrology/Nutrient cycling/Water filtration:  . Natural and Anthropogenic Change :

20. Excavator dredger in Neeme harbour , Estonia (April 2023

21. Land Reclamation Practies Land reclamation in Santos estuary due to EMBRAPORT construction

22. Managing for Prevention: Protecting deltas and estuaries is crucial for their ecological and economic value. Sustainable management strategies include: Conservation and restoration: Protecting mangroves, salt marshes, and other habitats; restoring degraded ecosystems. Sediment management: Implementing practices to regulate sediment flow from rivers and mitigate the impacts of dams. Pollution control: Reducing agricultural runoff and industrial waste discharge to improve water quality. Adaptation and planning: Implementing measures to adapt to rising sea levels and other changes, including coastal protection infrastructure . Thank You very Much for Making This Far