Chad Kiel: Hey guys, it's Chad Kiel with Learning Geology and Nature Reconnection,
and today we're learning about Alluvial Fans.
What is an alluvial fan?
And I'm going to tell you about some interesting data about this alluvial fan,
and the processes that created it, because it happens to be in
my backyard.
Let's go check it out.
So before we dive down the rabbit hole of alluvial fans, let's first take a
second to talk about weathering and erosion, because those are two factors
that are gonna come into play again and again when we're talking about this
alluvial fan.
Right here around 6,500 feet into the mountains, temperatures can vary from
20 degrees each day, causing the rocks to kind of expand and contract, and this
expanding and contracting's gonna cause fracturing.
Now these fractures are susceptible for water to percolate down into the rock, and
when the water freezes in the rock, we all know that ice has a larger volume than
water, it again fractures out that rock, and these fractured blocks are deposited
here in this active channel, and then we have the erosion factor that comes and
transports it down mountain.
So that's the beginning process of this whole alluvial fan story.
Alluvial fans are triangular-shaped deposits of water transported material
often referred to as alluvium.
There are examples of an unconsolidated sedimentary deposit, and tend to be
larger and more prominent in arid to semi-arid regions, like we are today.
These alluvial fans typically from in elevated or even mountainous regions
where there's a rapid change in the slope from a high to a low gradient.
The river or the stream carrying the sediment flows at a relatively high
velocity due to the high slope angle, which is why coarse material is able
to remain in the flow.
When the slope decreases rapidly into a relatively plane area or plateau, the
stream loses the energy it needs to move its sediment.
Deposition subsequently occurs, and the sediment ultimately spreads out, creating
the alluvial fan.
As the streams gradient decreases, it drops the coarse grain material.
It makes swagger of the channel, and forces it to change direction, and
gradually build up a slightly mounded or shallow conical fan shape.
The sediment that results from the erosion in this elevated mountainous region right
here, ultimately flows into the primary streams and the regions, where the streams
act as a drainage system, and carry the sediment to the alluvial plane, which is
down below us.
Once the sediment exits the feeder stream, the sediment is no longer confined to the
channel walls.
With this unconfinement, the sediment begins to fan out, the alluvial fan
becomes wider with increasing distance from the mouth of the canyon.
The biggest natural hazard on alluvial fans are floods, and debris flows.
Floods on alluvial fans are commonly flash floods, they occur with little to
no warning, usually have high velocities in sediment transporting capabilities, and
are relatively short.
Modern flood control infrastructure is essential to stop flooding, property
damage, and the loss of life from happening.
Successful development can and does occur on alluvial fans, even in rural
areas like ours, mitigation measures are utilized; building pads are elevated,
obvious drainage paths are left unobstructed, and in some cases,
flood walls are built.
Floodplain management principles of alluvial fans are part of everyone's
life that chooses to live in this special place.
I hope this helps you understand more about alluvial fans, and the processes
that create them.
Until next time, I'm Chad Kiel.