What is an Alluvial Fan? EXPLAINED | Learning Geology

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.