Estuaries and deltas. Flashcards
What processes do tides affect?
- the extent of wave action and mixing of estuaries, lagoons and bays.
- Biological activities and the zonation of organisms.
- Chemical processes such as dessication.
What is the difference between neap and spring tides?
Spring tides are the result when the moon and sun align, whilst neaps occur when the 2 gravitational forces are not aligned. alignment allows the 2 quantity’s of gravitational force to combine, whilst non-alignment causes them to interfere.
Springs occur around the New and Full moon, neaps occur in-between. It takes 2 weeks for spring - neap - spring.
Neaps are on average 40% lower than springs.
How do you measure the gravitational force?
using the distance and the mass of an object.
Why does the moon have a greater gravitational force than the sun?
Due to its close proximity to the earth.
How long is one lunar day?
24 hours and 50 mins.
What are estuaries?
- areas that drain river valleys
- are tidally influenced
- have a mix of fresh and salt water.
What are the three zones within an estuary?
- estuary head
- estuary margins
- estuary mouth
What are the types of estuary?
Ria - a drowned river valley from rising sea levels
Fjord - a drowned glaciated river valley
Lagoon - a partially closed estuary mouth
Calanque - Mediterranean limestone erosion from water movement.
Where do estuaries form?
at locations where the rate of sea level rise is faster than the rate of sediment accretion by riverine or marine sedimentation.
How does mixing take place in an estuary?
Takes place by molecular diffusion (slow) and turbulent mixing (advection/physically mixing) (fast)
Why is mixing difficult in estuaries?
because of the differences in densities of the fresh and salt water. Creates a boundary between 2 fluids, a stratified water column.
Salt water wedge or a freshwater lens may form.
What is the turbidity maximum?
This is the point where the mixing of different water densities is greatest. The mixing, especially around the nodal point, results in the mixing and increasing of sediment in the water column. Important for marsh formation.
What is a hysteresis loop?
this describes how the sediment concentration changes as the flow velocity and tides change.
How may an estuary become a delta?
if the estuary was filled in by riverine sedimentation, faster than the rate of sea level change.
Why do delta distributaries/channels continually change position?
due to the river flow, tidal flow, and wave action.
What are the 3 types of delta?
Fluvial, tidal and wave dominated.
Characterise the three types of delta?
fluvial - large river catchment areas, with a high river discharge and sediment supply into protected seas with little wave or tidal influence. extend outwards from the coast with river levees that define channels.
Wave - exposed to open ocean swell - creating alongshore and onshore transport, results in smooth shorelines.
Tidal - tidal prism greater than fluvial prism. Possess both tidal and river channels, creating finger like shorelines, enhanced by parallel orientated bed forms - tidal shoals, bars and inlets.
What are the parts to a delta?
delta plain - consisting of network of distributaries (dependant on the gradient, grain size and sediment load), delta front - where the delta is most actively accreting and the pro delta where the finest fractions of sediment are deposited.
What is a turbidity current?
These are currents that are negatively buoyant due to a high sediment load, they flow close to the bottom moving under their own weight. Due to high bedload movement, submarine channels may be carved out.
What controls the hydrodynamic behaviour of the river water as it enters the receiving basin?
- water velocity at river mouth (most important one for jet plumes)
- slope of the sea bed
- vertical density distribution of water column (most important one for density currents)
What are homo, hyper and hypopycnal condistions
Homopycnal - mixed water
Hypopycnal - salt water wedge
Hyperpycnal - turbidiity current.
What is an axial jet?
this is a jet plume that occurs when river water exits a distributary mouth at high speed into a deep water basin under homopycnal conditions, with no interaction between the jet and the seabed.
How do Deltas evolve?
Delta switching is primarily the result of stream capture upriver, this halts accretion at one site and increases it at another. Lobe switching refers tot eh channel switching and extensions that take place within a delta.
What force causes a tidal bulge due to gravitational forces to occur on both sides of the earth?
Centrifugal force.
What are semi diurnal, diurnal and mixed tides?
Semi diurnal = 2 highs and 2 lows a day e.g UK
diurnal = 1 high and 1 low a day e.g New Orleans
Mixed = between the 2. e.g San Francisco
How does the Coriolis force affect tides.
causes travelling water to curve to the right (N.Hemisphere). this results in an amphidromic motion and creates a amphidromic point. Higher tides are located the further from the point you go.
How is the tide influenced?
Gravitational forces, amphidromic systems, pressure, width of continental shelf and the shape of the coast.
How does the width of continental shelf affect the tidal range?
Wider shelves cause the waves celerity to be reduced, allowing the back of the tidal wave to catch up, increasing the water height, creating higher tidal limits.
narrow shelves tend to reflect tidal waves.
How does the shape of the Severn estuary create high tidal limits.
The funnel shape of the estuary causes water to be compressed, and pushes it further up, creating a bore upriver.
When is the maximum tidal velocity achieved?
At the flood and ebb tide mid points, this is when maximum sediment erosion and transport may take place.
What are sediment couplets?
due to the variability between tidal velocity, sand may be deposited in an area during one point, then overlaid with mud at a different point in the tidal cycle.
What are the three types of river mouth?
Inertia, Friction and Buoyancy dominated river mouths.
What is the halocene?
this is the interaction between the fresh and marine flows, it is here that the salinity changes abruptly.
How do salt marshes affect hydrodynamics of system?
They reduce the waters velocity causing coarse sediment to deposited early on, filtering sediment from the water.
What happens to pollutants within an estuary?
pollutants bind to deposited sediment, plants absorb the pollutants, or they may become mixed into mudflats.
