This is an HEC-RAS sediment user manual
video and in this video we're going to
show you how to add
sediment to a tributary with a lateral
flow series
boundary condition and a rating curve so
the model that we're working with here
is a very simple model it's got five
cross sections
constant slope constant flow of 2000 cfs
we're going to run it for 24 hours
a downstream stage boundary condition
and the sediment model has
maximum depth the removable bed limits
are in the station
and the bed gradation is all coarse sand
we are bringing in an equilibrium load
boundary condition
upstream not because you should use
equilibrium load you should use this
with extreme caution we do not recommend
it but
it will help us visualize the result
because it will result in a stable bed
so i'm going to run this model without
the tributary
and if we look at it
we can look at the flow this is the
generalized plotter in RAS
and each of the quasi-unsteady flows is
actually a steady flow
profile in RAS so you can go look at it
and we can look at the flow
you can see the flow is constant
throughout it's 2000 cfs
and then if we go look at the sediment
we're going to turn on profile and i ran
it for 24 hours so the cumulative should
give us a 24 hour
result and if we look at invert change
there is very little invert change
there's very little bed changes in
equilibrium models
so it should just be moving through i
mean there's a little bit of rounding
error at the boundary condition
but there's no invert change and if you
look at the cumulative
mass bed change it's trivial for this
run
okay so what we're going to do next is
we're going to add
a lateral flow to our quasi-unsteady
flow boundary condition let's save it as
and call it Trib
and then we're going to add a boundary
condition location it'll ask
us hey where do you want this Trib let's
put it right in the middle
and so we're going to come in here and
you'll see this isn't an upstream boundary
condition so we can't just put in a flow
series it's not a
downstream boundary condition but we
could put in a lateral flow a uniform
lateral and an internal stage we'll talk
about those other ones another time
but let's define a ladder flow and we'll
just
24 hours one hour computation
command let's just bring in 10 200 flow
and double that just to have coverage
okay file save
and so then if we run
our tributary flow
in the model
and we go look at the flow
you can see that the flow increases
and it increases downstream of the
cross-section you specified it at
this is a feature of both the unsteady
and the quasi-unsteady
flow model that when you bring flow into
the model
it actually comes in between the control
volumes which means it shows up at the
next
cross section downstream here's a little
schematic that shows
that you know in this case we've got the
flow coming in upstream and the flow
coming in
at 500 so what that's going to look like
is the flow and actually the load are
going to come
in downstream of that cross section in
quasi-unsteady
and unsteady which gives us the flow
profile that we see here
that you know it's 2000 um and then it
jumps up
after the cross section and this is kind
of an important point because steady
flow actually does the opposite and
previous versions quasi-unsteady
actually did the opposite
where if you define the flow at a cross
section the flow comes in
upstream of the cross section so it
shows up at that cross section but
that's not how
the sediment model works sediment model
works with unsteady and quasi-unsteady
so the flow will come in
downstream of the cross-section and show
up at the next one so if you want to
bring in a tributary
you should bring in the tributary to the
cross-section
upstream of where the tributary actually
comes into the system
so back to our model if we go look at
our sediment output
and we look at the cumulative mass bed
change
in profile at the final cross section
what you'll see here now is that
actually there's non-trivial erosion
at that cross-section downstream of
where we brought the flow in because
we've brought the flow in
without bringing any sediment so we've
increased the capacity over this
equilibrium state but we haven't given
it any
sediment to fulfill that capacity and so
it erodes downstream of the tributary
this is actually something that happens
in the real world and something that
happens numerically
it's a common numerical artifact you
know because sediment transport
functions are non-linear
if you add two flows that are both in
equilibrium
with the sediment that's coming in the
capacity will increase
more than the sum of the flows and so in
the approach that we use
to compute sediment transport you know
two
flows that come together in equilibrium
will actually form a scour hole
but in this case it isn't an equilibrium
we actually have
sediment coming in from the tributary
and so what we're going to do is we're
going to go to our sediment file
and we're going to add a boundary
condition
and so just like what we did with the
quasi-unsteady flow file we'll say add
sediment boundary locations
and we'll say yeah i want 500.
so now we get a sediment boundary
location and you'll see if we
click on this now we have
a couple of options we can do clearwater
which is new we can bring in a sediment
load series you can actually bring a
sediment load series at any cross
section because you don't need a flow
but you can only bring rating curves in
at cross sections
where you have flows because you need a
flow for the rating curve if we were to
say
go in and add a sediment boundary
location at 250
and then click on that you'll see the
rating curve disappears because you
can't add a rating curve to a cross
section that doesn't have a flow
let's delete that and so when we come
here
we'll add our rating curve
i have a contrived rating curve here and
this whole example is contrived
incidentally and should not be used for
project or research purposes and
we're going to just keep it all 100
coarse sand you can see for the
flow of 200 coming in we've got a load
of 200 tons per day coming in
say okay all right file save
and so now we have a rating curve
associated with that tributary
now if we go in and run
we can go look at our sediment output
and what's happened
to the mass bed change the cumulative
mass bed change which is
the total amount of mass that
is either added to or removed from the
control volume in time and you can see
that actually we've got
the opposite effect you can see that you
know we're in equilibrium
right up into the point where we define
the tributary and then the tributary
flow and load
come into the model downstream of the
cross section and so
we actually have more sediment than the
capacity can fulfill and so we end up
depositing downstream of the tributary
and most of it deposits there so by the
time we get to the downstream
cross-section it's equilibrated
and so if we look at the flux you know
we can look at
say the sediment concentration and you
can see the concentration hovers around
40 until you get to the tributary
tributary comes in between the cross
sections and jumps up to
the mid-50s and then deposits some
sediments so it drops a little bit
and if we actually look at the that's
the concentration we actually look at
the sediment flux
which here we're looking at the
cumulative mass to leave
the control volume over the whole
simulation
which is you know one day of flux in our
simulation
and so i don't really love this color so
let me go change that color
let's get a darker color and help make
it thicker
and what you can see is that you know
we've got this equilibrium load coming
in that just gets transferred downstream
because it's an equilibrium slope it's
at the same slope and
you know it's a uniform flow condition
but then
here you can see that the load coming in
at this point
is 218.37
tons over the course of the whole
simulation and if you look at what's the
load here
it's 330 tons and so that is
the increase in the sediment that's
being added to the system minus whatever
is deposited
all right so that is how you add
a tributary with a sediment load to an
HEC-RAS sediment model
with a lateral flow boundary condition
and a rating curve of course there are
other ways to do it
you can do it with a junction and
actually bring the sediment in
if you can avoid that you probably want
to because of some of the issues with
the non-linearity of the combination of
flows and loads i was talking about
earlier
junctions can be a little unstable so if
you
can bring in a tributary flow and load
with a lateral
series that's preferable but of course
you can just build a dendritic system in
1d RAS as well
my name is Stanford Gibson i'm the
sediment transport specialist at
HEC