Deltas

Question 1

Compare deltas with sea level rise

Answer 1

Sediment supply is so large that is entirely outpaces sea level rise.

Question 2

How did deltas receive their name?

Answer 2

Herodotus - Nile River alluvial plain similar in shape to greek letter “Delta”

Question 3

Describe Gilbert’s findings on sediment layering.

Answer 3

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.

Question 4

What are the main four types of alluvial deltas?

Answer 4

1. Braidplain deltas
2. Scree Apron
3. Alluvial Fan Deltas
4. River Deltas

Question 5

Braidplain Deltas

Answer 5

High Sediment /low water content

Question 6

Scree Apron

Answer 6

Wet sediment - Largely gravity driven

Question 7

Alluvial Fan Deltas

Answer 7

More water then the scree apron delta that allows it to flow better and further but still is a sediment base form

Question 8

River Deltas

Answer 8

Almost all water. Formed by murky/muddy water.

Question 9

Levee Formation

Answer 9

Elongates the channel far out into the ocean and allows the long channels such as the Mississippi delta.

Question 10

Levee Formation Process

Answer 10

1. Start out with a river and as the water starts to rise, the sides start to flood.
2. 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.
3. This forms a natural levee.

Question 11

What happens if the levee breaches?

Answer 11

Crevasse Splays

Question 12

Define Crevass Splays

Answer 12

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.

Question 13

What happens has the levee becomes longer and longer?

Answer 13

become hydraulically inefficient and so the river looks for a steeper, shorter channel.

Question 14

Avulsion

Answer 14

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.

Question 15

Avulsion & Barrier Island Process

Answer 15

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.

Question 16

Wave-Dominated Fluvial Deltas: Waves greater than sediment supply.

Answer 16

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.

Question 17

Wave Dominated Fluvial Deltas: Wave = to sediment supply

Answer 17

1. Roughly a triangle-shaped delta
2. Changing shoreline orientation, until LST inc to match delivery
3. Avulsions and abandoned shoreline protuberances smooth the coast.
4. See the striations that are old beach or dune systems.

Question 18

Tide Dominated Deltas

Answer 18

• 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)

Question 19

Describe the morphology of a Tide Dominated Delta

Answer 19

1. Funnel-shaped mouth with upstream taper – constrains tidal flow & enhances the tidal range
2. A fan-shaped array of island
3. Well developed channel bars and islands -also constrain tidal flow

Question 20

Tide - Dominated Deltas : Influenced vs Dominated

Answer 20

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.