reservoir sedimentation is probably the most
common application of sediment transport
models and there's a very good reason for that
reservoirs turn rivers into lakes
and tend to collect decades
of the river sediment load
in that impoundment
and reservoir managers need to decide
what to do with that
sediment and how to manage it
what the lifespan of the dam will be
if there are ways that can extend the lifespan
of the dam
there are ways to reintroduce sediment continuity
lots of questions that surround the
sediment engineering and sediment analysis
field really circle around the question of
the sediment issues associated with dams
so we're going to do a series on river sedimentation modeling
with HEC RAS
we're going to step through several ways to analyze
sediment transport in reservoirs
we'll start out with a couple of videos on how to
analyze deposition in reservoirs
and reservoir lifecycle
then we'll have a couple of videos
on reservoir sediment management
modeling reservoir flushing things of that nature
and then hopefully we'll do a video or two
on dam decommissioning and removal
just to kind of finish the dam lifecycle and
the types of applications HEC RAS gets applied to
this video will just introduce a couple of very basic
principles of reservoir sedimentation
and the two main ways that you can set up a
quasi unsteady reservoir model
then the next video will actually step you
through the process of setting up those two models
my name is Sanford Gibson
I am the sediment transport specialist
at HEC and in charge of the sediment
transport features in HEC RAS
now before you model any sediment process
it's really important that you understand
how that process works
the video won't go deep into sediment transport
processes in reservoirs
but let's just cover a couple of basic principles
that first thing is that most reservoirs form some
sort of delta
the coarser sediment deposits upstream
the finer sediment deposits downstream
and you develop a delta almost like a smaller
scaled version of as a river flows into the ocean
you end up with a prograding wedge
where you have a gradual top set slope
and then a steeper foreset slope
that is advancing into the pool
whereas you have finer sediment that deposit
beyond that foreset slope
and the shape of this delta is really dependent
on the gradation of the sediment flowing into
your reservoir this is a reproduction of a
classic image from the textbook Morris and Fan
where they classify different kinds of reservoir
deposition and you've got the classic delta
where you have the prograding wedge
into the pool
and then you have at the other end of the spectrum
you have a wedge where essentially all the
sediment is fine and it deposits from the bottom
up like filling a bathtub and then you have
tapering or uniform which really aren't their own
categories but are just points within the
continuum between those end points
and so different reservoirs depending on their
gradation behave in different ways
you know coarser reservoirs will form this very
distinct delta that will prograde into the pool
whereas reservoirs that bring in finer material
will have more of the tapering or uniform or
even wedge configuration
and so if you're interested in how quickly your
reservoir will fill a little bit of trap efficiency math
and a sediment rating curve you can estimate
the lifecycle of your dam
you want to get a model involved if you're
interested in what is the distribution of your
sediment in the reservoir what is the shape of
your delta and what are the feedbacks as your
sediment fills up will the trap efficiency change
and then finally if you want to do any sort of
reservoir sediment management flushing
drawdown things of that nature a sediment
model is almost compulsory so if you're
interested in sediment processes or sediment
management these are two great references
that are available online
what we're gonna do is we are gonna model
reservoir sedimentation in HEC RAS
this video will talk about two ways to model
reservoir sedimentation in quasi unsteady flow
so you can model sediment in HEC RAS in
either quasi unsteady or unsteady flow
unsteady flow is more precise but it's also harder
quasi unsteady flow is a lot more stable
you can usually run longer run times and so for
most sediment transport applications quasi
unsteady is the way to go the problem is that
quasi unsteady does not conserve water volume
which generally in a riverine environment you're
more interested in conserving sediment volume
than water volume and the quasi unsteady
approximation is fine that simplification
becomes more problematic in a reservoir so in
the next video we'll talk about how to use
unsteady flow to simulate sediment transport
in a reservoir but if you're going to use quasi
unsteady flow and because it is in fact still more stable
you can do that you just can't route flow through
you're reservoir you can't allow your reservoir to
actually route the water through it you have to
define the stage of your reservoir and so there
are two ways that we do that the first and
simplest way is just to define your downstream
boundary condition as your reservoir stage
what would this look like on a reservoir
well let's go to a reservoir in the
Pacific Northwest of the United States
this is Fall Creek Lake and files that come with
this video include a paper we wrote on modeling
this reservoir and basically the idea here is that
we are going to draw a cross section across the
upstream edge of the dam and we're going to
end the model there we're going to end the
model before we even get to the dam because
because dam operations will complicate things
if we know the stage of the reservoir we don't
have to route water through the reservoir we can
just operate the reservoir stage as a
downstream boundary condition and the
backwater is often a really good approximation
of the hydrodynamics of the unsteady flow
despite the fact that we're doing quasi unsteady
flow but there are some downsides of this one of
the downsides is you can't model the downstream
reach of the reservoir or you can't model
multiple reservoirs in series and so the second
method is to actually model the full reach and
include an inline structure the inline structure is
the way that we tend to model dams in HEC
RAS but you can't actually allow the structure to
route the water through it because this is a
quasi unsteady flow model we'll talk more about
the complexities of that in the next video
and so what you have to do to get an
appropriate result with quasi unsteady is you go
ahead and define an internal stage for your
reservoir and then you essentially to the same
thing you operate the reservoir by defining the
reservoir stage
now before we continue there's one important
note about inline structures in RAS sediment
simulations that's that inline structures dams
whether or not they have gates are pass through
nodes a pass through node in sediment
transport just means that all the sediment that
comes to the node leaves the node
they're not control volumes but more importantly
we don't decide how much sediment can pass
through them based on the geometry of the
structure or the location of the gates
you could imagine these three dams that I'm
showing right now would have pretty different
sediment behavior as far as what kind of
sediment would pass through them and we
ignore that in RAS right now you know we're
working on features but the reason is that in
general this doesn't turn out to be as egregious
an assumption as it seems like it would be
except for really small reservoirs or reservoirs
where the delta has approached the dam
you know by the time water reaches the
structure often the sediment that remains in the
water is washload is the kind of material that
could go through or over most gate
configurations but that's still an assumption that
you want to assess and make sure that you're
not overpredicting flux through the dam
in the next video I'll actually guide you step by
step through each of these modeling
approaches but just to show you how the
geometry will work this is the synthetic reservoir
dataset that I provided with these videos
and flows going from upstream to downstream
in the northwest direction and the dam it's gonna
be at station 10000 and so this is the existing
without dam geometry and so the way the two
different geometry files will look is we have one
geometry file where the downstream boundary
condition will be at the dam face so we remove
these downstream boundary conditions and
now the first cross section upstream of our dam
becomes the downstream boundary condition
for our reservoir stage
then the second way to do this is to add an
internal dam and so now you can see we've
added a dam an inline structure
at station 10000 and then we'll add an internal
stage boundary condition at that location
to specify the reservoir stage
but the next video we'll walk step by step
on how to set up both of those models