Lecture 22- Estuarine geomorphology and processes II Flashcards Preview

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Flashcards in Lecture 22- Estuarine geomorphology and processes II Deck (32):
1

What is What is the classification of estuaries based on sediments? (2)

1. Tide dominated= macrotidal (tide more than 4m) 2. Wave dominated=microtidal (tide less than 2 m)

2

What are estuaries sediment sinks for? (2)

-both for marine and fluvial material

3

What is a sediments sink?

-area that hold sediments over time, for many years, can store chemicals and nutrients= implication for pollution in the system and biology of organisms

4

What movement of sediment is essential in estuaries?

-landward movement of sediment is essentual -in both types of estuaries there are these -driven by waves and tides -if only moving offshore then it would be a delta, estuary= sediment moves onshore

5

What is an estuary?

- The seaward portion of a drowned valley system which receives sediment from both fluvial and marine sources and which contains facies influenced by wave, tide and fluvial processes (facies= sedimental environment)

6

How is the fluvial, tidal and wave influence distributed in an estuary?

-divide each estuary into three zones: Marine dominated, mixed energy and river dominated

1. Marine dominated= waves and tide are the dominant forces

2. Mixed energy (the middle zone)= combination, of wave, fluvial and river

3. River dominated= fluvial influence is dominant -the relative sizes of each region differ in estuaries depending on what is the dominant force (if it is a wave or tide dominated delta)

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7

What are the characteristics of a wave dominated (microtidal) estuary?

• Wave energy dominant • Microtidal Tides (< 2m) • Mixed energy zone central basin

8

What are the relative forces of waves, rover and tides in wave dominated (microtidal) estuary?

-the three sections are of similar size, marine dominated being slightly larger

-in marine dominated part the waves are much stronger than tides, river has almost no influence here

-in the mixed energy part the total energy is low and a central basin forms

-in the river dominated part, river is dominant with the tides being of some importance and waves do not reach here

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9

What are the sediment environments (facies) of a wave dominated (microtidal) estuary?

-arise as result as a result of the dominant processes in the system= forming the sediment, 5 characteristic formations:

1. barrier= large sand barrier built up over the entrance= waves delivering sediment

2. washover= where extremely large waves overtop the barrier and bring sediments over

3. Tidal delta/flood tide= incoming tide, so more influence than the ebb

4. bay head delta= sediment from the river moving toward the ocean forming a triangular shape thing

5.basin= water forms there= fish can live there

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10

What are the characteristics of a wave dominated (microtidal) estuary- the long description?

-A wave-dominated estuary represents a coastal bedrock embayment that has been partially infilled by sediment derived from both the catchment and marine sources, in which waves are the dominant force shaping the gross geomorphology. In Australia, wave-dominated estuaries are most abundant on the south-east and south-west coasts, where they occur on exposed coastlines with a relatively small tidal influence (Roy et al., 2001, Cooper, 2001). Wave-dominated estuaries feature a supra-tidal (or sub-aerial) barrier at the mouth that encloses a broad central basin. The barrier creates a constricted entrance (which can be periodically closed) that allows the exchange of water between the central basin and the sea. Sediment in wave-dominated estuaries ranges from fine to coarse sands in the barrier and tidal inlet deposits, fine organic muds and sandy muds in the central basin, to coarse, unsorted gravels, sands and muds (mostly of terrigenous origin) in the fluvial bayhead delta (Nichol, 1991). Depending on the degree of sediment infilling, the central basin of wave-dominated estuaries may be irregularly-shaped, following the outline of the drowned bedrock valley (Riggs et al., 1995). In the case of wave-dominated estuaries formed in unconsolidated coastal deposits the central basin may be oval-shaped and oriented parallel to the coast (Chapman et al., 1982, Morrisey, 1995). At the head of a wave-dominated estuary is a fluvial bayhead delta that extends into the central basin and is comprised of vegetated and unvegetated levees, channels, and intertidal areas. The fluvial bayhead delta is constructed from terrigenous material from the catchment being deposited and the mouth of the river

11

What does a wave dominated (microtidal) estuary look like?

-

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12

What is the micro-tidal estuary classification (wave dominated) according to Cooper?

1. first decide if it is an estuary (need landward movement of sediment)

2. Then look at if the mouth is normally open or close

3. If open then look if it has a barrier or not, if it does then asses if it is wave or tide dominated

4. If the mouth is usually closed then asses the water level in the estuary, can be water level perched(higher than the sea level) or water at sea level

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13

What are the key features of wave dominated estuaries (microtidal)? (7)

1.A diverse range of both marine and brackish, subtidal, intertidal and supratidal estuarine habitats are supported. 2.Narrow entrance restricts marine flushing, only a small proportion of the estuarine water volume is exchanged each tide. 3.River flow typically high, and flooding may expel marine water and flush material from the estuary. 4.Turbidity, in terms of suspended sediment, is naturally low except during extreme wind or fluvial runoff events. 5.Central basin is an efficient 'trap' for terrigenous sediment and pollutants. 6.Long residence time encourages trapping and processing (e.g.denitrification) of terrigenous nitrogen loads. 7.'Semi-mature' in terms of evolution: morphology will rapidly change over time due to infilling, resulting in shallowing of the central basin, and expansion of the fluvial delta.

14

What is the other way of dividing microtidal (wave dominated) estuaries, according to Roy? (3)

-this model developed for NSW but can be applied elsewhere 1. Drowned river valley 2. Barrier estuary 3. Saline coastal lake

15

What are the characteristics of drowned river valley (microtidal- wave dominated estuary-Roy model)?

-system has an entrance that is always open to the ocean, wide and deep, has no tidal delta, very branching

-20-50 m deep

-colonised by mangroves, play a role in sediment deposition -no barrier -Port Hecking example

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16

What are the characteristics of a barrier estuary (microtidal- wave dominated estuary-Roy model)?

-has an entrance that is always open

-has a barrier but there is a gap

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17

What are the characteristics of a saline coastal lake (microtidal- wave dominated estuary-Roy model)?

-barrier that separates it from the ocean

-closed for most of the time

-smaller and shallower -the barrier can close -tides are attenuated as you move upward in the system

-moves through a narrow entry to the basin

-has to be closed most of the time

-can break the barrier during floods

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18

What are some example of saline coastal lakes?

-

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19

What are the characteristics of a tide dominated (macrotidal) estuary?

• Tide energy dominant

• Macrotidal Tides (> 4m)

• Limited wave influence

-funnel shape, wider at the ocean side

-have characteristic delta offshore

-characteristic of large tide ranges

-the tides can move far onshore (even 10ks)

-tidal currents comprise large portion of the energy

-marine dominated

-tidal currents are present even high up

-here waves do not have much of an effect (unlike wave dominated)

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20

What are the relative forces of tides, waves and the river in the tide dominated (macrotidal) estuary?

-the marine dominated section is much larger than the mixed energy and river dominated sections

-in the marine dominated section tides are the dominant force, waves not very strong, river is not an influence

-in the mixed energy section the tide and the river influences meet and tide is stronger till the first half and then river takes over

-in the river dominated section the river is the main influence and tide has some significance

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21

What is the sediment environment (facies) in the tide dominated (macrotidal) estuary?

• Mudflats

• Sandbars

•Meandering/ straight channel

•Salt marsh

•Sand flats

-mudflats= high nutrients= covered in high tide, exposed during low tide

-main species are grasses within saltmarsh

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22

What are the characteristics of tide dominated (macrotidal) estuaries?

-Tide-dominated estuaries generally consist of a landward-tapering funnel shaped valley, bounded by various intertidal sedimentary environments such as intertidal flats, mangroves, saltmarshes, and saltflats. Depending on the degree of sediment infilling, the boundaries of tide-dominated estuaries may follow the irregular outline of the drowned valley (Riggs et al., 1995), or, in more mature cases are smooth and intersected by small tidal creek dendritic drainage networks (Wells, 1995, Wolanski et al., 1992). -Major structural elements inside the estuary include elongate tidal sand banks, which occur in the wide entrance, oriented perpendicular to the coast and parallel to the direction of dominant tidal currents (Fitzgerald et al., 2000, Green et al., 2001). The tidal sand banks are usually dissected by deep channels containing strong tidal currents. Landward of the estuarine channels, the source river that feeds into tide-dominated estuaries often features a straight–meandering–straight river channel profile (see East Alligator River, below). This represents the point at which the convergence of seaward-directed water and sediment transport by the river, and landward-directed water and sediment transport by tides occurs (Dalrymple et al., 1992, Bryce et al., 1998). Due to strong tidal currents generated by large tidal ranges, tide-dominated estuaries are usually highly turbid.

23

What are the key features of tide dominated (macrotidal) estuary? (7)

1.A diverse range of both marine and brackish, subtidal, intertidal and supratidal estuarine habitats are supported. Intertidal and supratidal areas are often extensive, whereas turbidity may preclude seagrasses in some areas. 2.Large entrance promotes efficient marine flushing. 3.River flow is typically high, however the effects of floods are buffered by large water area, and large tidal exchange. 4.Turbidity is naturally high due to strong turbulence induced by tides. 5.Flanking environments such as intertidal flats, mangroves, saltmarshes and saltflats tend to trap terrigenous sediment and pollutants. Marine flushing results in loss of some material to the coastal ocean. 6.Tidal movement over flanking environments encourages the trapping and processing (e.g.denitrification) of terrigenous nutrient loads. Marine flushing results in loss of some material to the coastal ocean. 7.'Semi-mature' in terms of evolution: infilling by marine and terrigenous sediment will result in expansion of flanking environments, narrowing of channels, and seaward progradation.

24

What is the picture I don't get?(tide dominated macrotidal estuary)

-

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25

How does the energy in the environment of the tide dominated (macrotidal) estuary affect species and densit?

-Energy environment present affects the species diversity and density

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26

What are the three things we need to understand in estuaries to understand their evolution?

-evolution over time, how they store sediment -infilling= have wide basin, sediment accumulates, reduces depth of the estuary, provides medium for grasses etc to colonize 1. role of surface age= how old the sediment there are, can tell how long they have been there= tells rate of infill 2. underlying topography= the shape of the valley, if deep basin= then will be filled more slowly, type of rock is important on the rate of filling 3. recycling of sediments= how are they utilized by animals or plants, if likely to stay in system or move in and out

27

What is the 3 stage evolution of a drowned river valley?

1. stage 1. estuary is wide and deep with limited sand at the entry, the ocean tide range is constant through estuary

2.stage 2- bit of sediment moving into the mouth and shallowing the area, the river side there is sediment moving offshore by river discharge and runoff, black= mangroves are quote widespread and contribute to sediments storagre

3. stage 3- filled with sediemnt= being cahneged into a flood plain, mangroves have died off, tidal channel is along a meandering creek, the width increases as you move in

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28

What is the 4 stage evolution of a coastal lake and barrier estuary refill? (Roy model)

stage 1- limited infilling, waves and tides are limited through the entry

stage2- some infilling, towards the entrance of estuary, increase of marine sediments and area shallow

stage 3- further infill, much less water, colonized by vegetation, limited tidal range to one single channel

stage 4- full infiling of the system= grassy floodplain, swamp. only one channel from the sea, narrowing and shallowing of the estuary

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29

What is the evolution (3 stages) of a tide dominated estuary(macrotidal)?

1.Transgressive phase- tide dominated, youthful estuary, wide and deep,, part of transgressing phase= sea level up

2. Big swamp phase- mangroves are the yellow, trapping the sediment, shallowing the estuary, tidal channel has become much narrower

3. Highstand regressive phase-green= mangrove to floodplain and grass -mangorves only in the mouth where saline intrsuion. =high stand aggresive stage

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30

What is the role of mangroves in the Mary river infill?

-Mary river more infilled that the other river

-mary river= lot of sediment, reduced width and depth of channel, mangrove has played a role here stabilised the channel

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31

What are paleochannels?

-paleochannels, transition from wide deep estuary, to shallow -paleochannel= remnant river bed, represebńtative of the infill

32

When are infill rates marine and when fluvial?

- Infill rates initially marine (during transgression) then fluvial (after stillstand). Critical to understand when interpreting the ‘health’ of a system.