Deltas Flashcards
Compare deltas with sea level rise
Sediment supply is so large that is entirely outpaces sea level rise.
How did deltas receive their name?
Herodotus - Nile River alluvial plain similar in shape to greek letter “Delta”
Describe Gilbert’s findings on sediment layering.
Flat at the top (sand/topset), tilted deposit in the middle(silty/forset), flat at the bottom (clay/bottomset). Classic coarsening upward sequence.
Same as what you would find as a barrier island wash over.
What are the main four types of alluvial deltas?
- Braidplain deltas
- Scree Apron
- Alluvial Fan Deltas
- River Deltas
Braidplain Deltas
High Sediment /low water content
Scree Apron
Wet sediment - Largely gravity driven
Alluvial Fan Deltas
More water then the scree apron delta that allows it to flow better and further but still is a sediment base form
River Deltas
Almost all water. Formed by murky/muddy water.
Levee Formation
Elongates the channel far out into the ocean and allows the long channels such as the Mississippi delta.
Levee Formation Process
- Start out with a river and as the water starts to rise, the sides start to flood.
- When water has more area for the water to flood the water starts to slow down. Then the sediment falls out. The coarse sediment falls at the margins of the river.
- This forms a natural levee.
What happens if the levee breaches?
Crevasse Splays
Define Crevass Splays
The process allows the delta to expand to the sides and build out. This is called Crevasse Splays.
Crevasse Splays create new land. Levees contain the river and allow it to elongate.
What happens has the levee becomes longer and longer?
become hydraulically inefficient and so the river looks for a steeper, shorter channel.
Avulsion
This process by which the river changes its course and breaches its levee creates not just a Crevasse Splays but the whole river changes its course.
Avulsion & Barrier Island Process
When the active bird foot is abandoned. There are still some waves and tide that will rework the delta into a barrier island.
Think of it as cutting off one process and allowing the other two process to shape the shoreline.
Wave-Dominated Fluvial Deltas: Waves greater than sediment supply.
As soon as sediment makes it to the ocean gets swept out by waves
- Roughly no delta
- Repeated avulsions relocated river mouth
- Relatively small protuberance of the coast.
Wave Dominated Fluvial Deltas: Wave = to sediment supply
- Roughly a triangle-shaped delta
- Changing shoreline orientation, until LST inc to match delivery
- Avulsions and abandoned shoreline protuberances smooth the coast.
- See the striations that are old beach or dune systems.
Tide Dominated Deltas
- Tides are great or equal to river flow
- Delta development requires lots of sediment (typically tectonically active watershed -> many of biggest deltas in the world are tidally dominated)
Describe the morphology of a Tide Dominated Delta
- Funnel-shaped mouth with upstream taper – constrains tidal flow & enhances the tidal range
- A fan-shaped array of island
- Well developed channel bars and islands -also constrain tidal flow
Tide - Dominated Deltas : Influenced vs Dominated
You will have a tide-influenced system that has a plume of sediment pushed out with elongated sand sections.
If it is just a tide-dominated system then it looks like an estuary with elongated sand bodies.
