um but what I've been looking at is how bedrock rivers are transporting coarse
sediment and I'm looking at a series of rivers across Taiwan one of the things
that you can see in these drone videos is that the rivers are incredibly
variable so we have these incredibly coarse sediment this is up to 10 meters
in diameter um and actually I'm gonna come on scale on screen in a moment
first scale oh there I am and so you can see that there's spatial patterns in the
grain size we also have patterns and where it is relative to the banks and
patterns relative to bedrock exposure as we move down these different rivers so
one of the things that we're interested in is how these bedrock rivers are
transporting sediment and are roading the banks and eroding into the bedrock
and we need to look across the full range of variability and understand how
is this changing over space and also how is this changing over time so what are
the variability in the spatial locations of sediment transport capacity and also
how is this changing with different flows different flow events
so the way
that we're approaching this is with a series of drone surveys so on the left
is the drone survey from the field and on the right is the photo realistic 3D
model that we can actually make of the exact same site so these are not quite
exactly aligned but you can see that these are highly photorealistic they're
representing the full 3d geometry of the channel we're able to pull out the full
grain size distribution and actually see what's going on here
we can then go
through these different rivers to systems this is an animation of another
3D model and we can move up and down stream throughout these channels and map
just like we were in the field we can actually pull out things that are more
difficult to map in the field this is actually one that we've had to survey
from a trail it's not possible to get down into this Bank but we can go
through and we can map lithology like we would in the field we can pull out full
grain size distributions measuring every single grain that is in the channel we
can pull out things that would be really difficult to measure in the field like
the high watermarks that we're seeing in the channel breaks in that vegetation or
that weathered bedrock we can also pull out where that current
water is and then we can see how this changes at different points in the channe
l so this is an example of the kind of data density that we can get out
of this so this is a one kilometer reach in northeastern Taiwan and basically we
have measurements of the bed slope across the entire channel the high-water
marks the channel width at both high water and low flows. We can also measure
the relative area of sediment both coarse sediment and fine sediment in
Taiwan fine sediment is like less than a meter so different than other systems we
can measure the distribution of boulders throughout the system and what we're
actually starting to see here is that the spatial variability in these rivers
is actually pretty much due to the rock strength so we have variations in the
lithology of bedrock that is exposed at the surface and that is changing where
you're getting wide rivers changing the channel morphology so here for example
the black schist are relatively weakest rock is creating much wider river
channels and that river channel it's actually widening when we are parallel
to the foliation Ingle of the bedrock itself so you have structural
variability that is really creating the spatial variability in the system and
that's actually corresponding to where we're getting different sedimentation
patterns so cool to see it change over space but we can also measure it over
time so these are photorealistic surveys that have full 3d capability and so we
can take a series of repeat surveys this is further downstream of the same river
where we have for repeat surveys and we can actually compare them and see how
these rivers change with different storm events so we have a really unique data
set this is about a 200 meter reach in also in northeastern Taiwan where we
have a range of grain sizes anywhere from kind of 12 meter boulders don't
have kind of fine sand and these different surveys capture a range of
different discharge events so between 2015 and 2017 we actually captured a
Typhoon um between 2017 and 2018 we had a
high-flow year not a typhoon but still high flow and then between 2018 and 2019
there were relatively little storms so we can start to compare this kind of
data so we can fully map across these reaches the different distributions of
different grain sizes so this is between 2015 and 2017 that, that change we were
capturing the typhoon event and we can actually measure the differences in the
grain size distribution throughout so overall here we're not seeing a large
change in the overall grain, grain size distribution but we are seeing a lot of
change in 3D so we can actually measure aggradation and erosion at really high
resolution and we're seeing meter up to a meter of aggradation and a meter of
erosion throughout this system and it's non spatially uniform we can actually go
through and measure every single boulder that was in this channel and see whether
it moved or not so about half of the boulders in this system did move in this
time period and while it is more likely for the finer grains the 1 meter
boulders to move we are seeing up to an 8 meter boulder moving between these two
on two surveys if we move in from the typhoon year to the stormy year we have
less overall change we're seeing a net erosion throughout the system
we're transporting fewer boulders but we're still transporting up to a 5 meter
boulder in between that time but we're seeing a really big change in the grain
size distribution that we're actually seeing in the channel so all the sudden
we've lost a lot of our gravel and we're increasing all of our other size classes
so we're actually making the channel more complex and we're reorganizing the
channel and getting a lot more of a rougher surface that we're seeing there
and this is kind of that gravel is eroding out in these patches all
throughout the system between 2018 and 2019 where we have a relatively quiet
year we don't see much change in the overall bed state but we still see more
of that gravel going away and the other size class is increasing and so this
kind of spatial pattern is showing that complex to move these kind of flows with
this wide range of grain sizes but we can start to capture the full complexity
both in the surface measurements and in the actual full grain size distributions
so overall this has showed that we can use these kind of surveys to measure
things over both space and over time and see how the different sediment how
sediment is able to be transported differently wherever we are in the
stream and we can use these repeat surveys to see how it'll change over
time so thank you for having me here
I’m done