welcome to the introductory series of
videos on how to create a sediment
transport file in HEC-RAS this is the
second video on how to generate sediment
transport boundary conditions and the
sed file my name is Stanford Gibson
I am the sediment transport specialist
at HEC in charge of the sediment
capabilities in ras and in this video
I'll walk you through how to create a
sediment file for a 1d hec-ras model if
you haven't done the first part and
created a quasi unsteady flow file you
can do this standalone if you already
know how to do that we provide those
files or you can just build on to the
quasi unsteady flow file you created in
part one so a sediment transport model
and ras needs two basic types of data
you need bed data and boundary data the
bed data the sediment that goes into the
bed those are your initial condition
what is the control volume and gradation
of the sediment that are in your
bed at time zero and then the flux data
is the boundary conditions at at every
kind of upstream external node and so
you need to specify not only the
sediment mass that comes in at those
boundaries or concentration but also the
gradation of that material which can be
a challenging part the files we've
provided under sediment demo 2 we have
solution files in case you want to go in
and see what it looks like when it's all
done but the data files have a project
file a geometry file and then just the
quasi unsteady flow file that we
developed in the last video and then we
have sediment data and so we're just
going to go and press the open or the
file open and say sediment demo 2 data
files and select the project and so you
see we have a project file a geometry
file and a quasi unsteady flow file so
next we have to enter sediment data to
enter sediment data you go to the menu
edit sediment or you press the sediment
Data button now the sediment data editor
can look a little intimidating when you
first open it up because it has all
these blanks that you may not know how
to fill in but we're gonna fill these in
pretty quickly and zero in just on the
actual data we need and so there are
three tabs here initial conditions and
transport parameters boundary conditions
and then this optional Bstem tab that's
really only if you're going to do bank
filling analysis which will get its own
separate video really what we're gonna
focus on is the initial conditions which
is really the bed gradation in the bed
control volume and the equations and
then the boundary conditions which is
really about the flux at the upstream
boundaries or any lateral fluxes that
you want to add so let's start with the
initial conditions and transport
parameters you have to define bed
sediment for every cross-section and if
you come over here you can see the
profile plot or the individual cross
section and so each cross-section needs
a sediment reservoir or a sediment
thickness below the bed and so the way
that we're gonna do that is you can
either put in a maximum depth which will
be how deep the sediment goes below the
the thalweg or a minimum elevation a
minimum elevation is much less common
that's if you say no the bedrock
elevation and wanted to find that kind
of distinctly in the real world but
generally we just put in a max depth um
particularly if it's an alluvial channel
that can erode without much constraint
vertically so we're in SI so let's just
start out with a 5 meter maximum depth
and that means that we'll allow all of
these cross sections to erode 5 meters
we can update that to make it a bigger
sediment reservoir if we need it and
then the other question is what are the
extents that we're going to allow to
erode and so this you can go in and
define more carefully later but this
button right here use Bank extents
allows you to define your movable bed
limits as your banks and so now if you
come in here you'll see this cross
section has a sediment control volume or
sediment reservoir associated with it
full of errodable sediment and so the
next thing we need to do is define the
gradation associated with that sediment
now this again looks pretty intimidating
because every cross-section needs a
better gradation there's no way around
that we need bed gradation information
for every cross-section and you almost
never have bed gradation at every
cross-section but we have a couple of
options that allow you to interpolate or
to associate a bed gradation with
multiple cross-sections so if you come
in here to the sediment data you're
going to open the tutorial data and this
is the flow data we used in the last one
if you come down here to the next tab
bed sediment we've just defined a single
bed gradation bed gradations are usually
defined in the geotechnical cumulative
distribution function and so this is a
size and percent finer curve you're
going to do first is you're going to
enter all of the bed gradations you have
into this generalized bed gradation
database here in ras where it says
define and edit bed gradations you'll put
all of your bed gradations in here in
this case we only have one so we press
that button and then whenever you see
this new sheet of paper in ras
that's that's where you create something
new so we're going to create a new bed
gradation and we're going to call it
sample one and then I'll just come over
here and copy all of these and you're
going to copy starting at the top to
make sure they go in the right place and
then paste them here and you can see
we're using percent finer you can put
in the grain class percentage as a
discrete percentage of each grain class
and you can see it will let you convert
between them as well but we're going to
use the percent finer nomenclature
because it's a little bit more common
and a lot of times these are the data
you get back from the geotechnical lab
except sometimes they're not in this log
base to scale where there are every
grain class is twice the size of the one
below it and so in that case you have to
set it to fit or you can go in and
define your own grain classes right so
I'm going to say okay and so now I have
a bed gradation and so the next thing
we're going to do is we're gonna
associate that bed gradation with the
cross sections if you click over here in
the bed gradation field now you get a
drop-down menu
and in that drop-down menu it will be
all of the bed gradations that you have
defined so you can come here and say I'm
going to choose that sample there and
you can choose another sample downstream
and then you can press the interpolate
button to interpolate between them or
you can choose the sample hover over the
the lower right hand corner to get the
crosshairs click and drag and associate
the same sample with multiple
cross-sections now I'm going to come in
here and save the data I haven't saved
this yet so it's gonna prompt me for a
name and I'll call it sediment data and
we're on our way so you can see that
even though this sediment data editor
comes up with a lot of blanks and there
might be some cognitive resistance to
that we can fill them up pretty quickly
and then go back and work on them as we
develop the model really there's only a
couple other things you have to do here
and these are some of the biggest
decisions you're gonna make you have to
decide on your transport function which
is the equation that you're gonna use to
kind of turn your hydro dynamics into
sediment transport potential or capacity
and you have to choose your sorting and
armoring method which is the bed mixing
method that determines whether or not
you're going to develop an active layer
that will reduce your erosion we'll have
to deal with these in a different venue
or a different video because these are
relatively complex ideas but for now
we're going to select the Laursen Copeland
transport function and the Copeland or
Exner 7 mixing method why am i choosing
those well because I know that they're
going to work on the system a priori and
let me just say again in this video that
this system does not exist in the world
it's completely contrived and should not
be used for research or planning
purposes now all of my initial
conditions and my equations have been
selected what we need to do now is
define our sediment boundary conditions
and that's how is our sediment going to
change in time we basically need a
sediment flux and a gradation of that
flux for every time step but we almost
never have time series of sediment data
the way we have time series of flow data
so there are multiple ways to do this if
we go to boundary conditions
you'll notice that we require a boundary
condition at the upstream boundary of
our model and if we go and open the
geometry file you'll notice that it's a
this is a single reach and so we only
have one upstream node so ras only
populates one required boundary
condition so you'll notice there are
three possible upstream boundary
conditions you can define a sediment
load series which is just a time series
of sediment data that you can put in
manually or bring in from a DSS file
this again is pretty rare but sometimes
you know the USGS will develop a
sediment time series for your gauge
based on sediment data or rating curve
or sometimes you've got a sediment
wash-off model like SWAT or HMS and you
can develop a DSS time series and bring
that in the other option is equilibrium
load this is a pretty popular option and
is frankly way more popular than it
should be it's very easy because you
don't have to input data and it just
computes the equilibrium flux at the
upstream end but if you're doing a
sediment model it is unlikely that your
system is in equilibrium and this can
introduce a lot of data
the most common more credible method is
to define a sediment rating curve at the
upstream boundary condition that will
look at the incoming flows and compute a
sediment load for that upstream boundary
condition now I'm going to do an entire
video just on developing a sediment
rating curve there's some statistical
nuance to this but let's just say that
we've got one now and we're just going
to talk about how to put it into your
model so we're gonna select rating curve
what you see is we get a rating curve
with actually a lot of rows in it it
seems like there's more rows than there
should be
well here we're gonna put in our flows
and here we're gonna put in our sediment
loads but then that will give us our
mass flux and you'll notice down here
you can do this either in load or
concentration but that'll give us our
sediment flux usually it's a power law
generally this is some sort of power
function you know the the relationship
between load and flow is usually load as
flow to some power usually around to or
between 1.5 and 2.5 but then you're
going to have to break that flux down
into the different grain classes
which is where this gets particularly
challenging now you can define any
number of sets here and you would want
to define more than two sets if you have
a flow load relationship that isn't
linear in log space let's say that it's
supply limited so at the higher flows
you get less load or maybe vice versa or
if the gradations don't change linearly
and so we're going to open the sediment
flux boundary condition and here we've
actually defined four flow load gradations
and so I've actually included the power
function here I used to generate my flow
load curve and so this is the flow lower
load relationship in these top two rows
but then our sediment flux has to be
broken down into the individual grain
classes so for each of these flow load
pairs the loads are then broken down
into the percentage or fraction of each
grain class now you can pick these in as
percentages or you can put them in as
decimals
it doesn't matter to ras ras is
gonna add them up and then divide in
order to get an actual true ratio they
could actually just be masses but you
can see that here we have basically
forty percent very fine sand for the
smallest flow load pair and then we have
sixty five percent very fine sand for
the largest flow load pair and so a
valid question might be where will you
come up with these data well sometimes
there are load gradation measurements
associated with gauges or studies and so
you can look at how sediment changes
with flow does it get coarser does it
get finer um which would you expect
would you expect sediment in general to
get coarser or finer with flow it's
actually a pretty interesting and
complicated question it turns out that
it could do both in some cases as flow
increases the the sediment load gets
finer in other cases as flow increases
the sediment
gets coarser I've included a link on a
paper we've written on this topic that
actually we'll talk walk you through the
different mechanisms that could cause it
to but in this case we've got something
that works for the system and so we'll
just copy this and then paste it and
you'll notice that I actually went in
and I selected all the cells that I
wanted to paste into before I paste
because ras doesn't always work like
excel where it will automatically
populate everything out you actually
have to sometimes select the cells
before you paste into them and then you
can plot your rating curve and because we
just used the power function here it's
linear in log space which of course
means that the higher flows have
disproportionately higher load which is
the way that rivers work right we say
okay we go to file save and now you'll
notice we have a sediment data file here
and our sediment data are complete and
in the next video we will run this and
then look at some output and then think
a little bit about what those output
mean and we'll just have a couple of
comments about some of the things that
go wrong and some of the things that you
can do to improve your sediment
transport modeling at this point I'd
like to recognize that these videos have
been supported by the flood and coastal
storm damage reduction research and
development work unit of the Corps of
Engineers and a lot of the sediment
transport features in HEC RAS
were based on the sediment transport
features in a previous program called
HEC 6 which was developed by Tony
Thomas and much of the technology that
we built this program on is based on his
decades of work and insight all right
we'll see you in the next video