Modelling a glacial lake outburst flood

[emilyrose]

Hello,

I am completely new to HEC-RAS so any advice would be greatly appreciated.

To complete my thesis I need to model a glacial lake outburst flood, I would ideally like to produce a flood hydrograph and inundation map if possible. I have read a lot of papers etc. and seems most use a dam- breach model.

I am wondering if anyone could guide me as to whether I need to use a dam-breach model to produce a flood hydrograph which is based on the release of a large volume of water from a glacial lake or if this can be done with another technique, such as the unsteady flood simulation etc. I have read all the manuals and watched several youtube videos but unfortunately nothing is doing what I am trying to achieve.

Apologies if this a stupid question, as I said, I am a complete novice to HEC-RAS and flood modelling in general, but is the final step of my thesis so any guidance would be an immense help to me.

Many thanks.

[Jarvus]

Could HEC-RAS be used to model a glacial lake outburst. Yes. The software won’t, of course, tell you how fast the ice dam will collapse, but if you enter appropriate data, it could model everything else. You can enter a trapezoidal breach and have it grow at various, user entered rates.

There are many techniques for modeling flood hydrographs that involve various levels of effort, require different data, and have different accuracies. But if you want a flood hydrograph and inundation map, I would think a dam model like RAS would be a good choice.

However, for someone completely new to RAS, that sounds to me like a lot of work for a final step to complete a thesis. It is also not clear what your background is in river hydraulics and flood modelling. Which again, isn’t to say that one can’t learn how to do it, but it could take quite a bit of time, especially if you are tying to basically do this on your own.

I’ve read a little about glacier lake outburst, mostly from articles in the magazine New Scientist. Are you trying to model a huge historical event, say from thousands of years ago? Or is this a modern, smaller scale event? I’m wondering what you are going to use for your terrain model. If you can download a current terrain model, that would save a lot of work. If you are trying to recreate conditions that existed in the past, that sounds like a lot of work.

I’m not an expert on the terrain side of things, but dealing with the terrain could be more work than the river hydraulics.

RAS is free to download. The example data sets include a dam break model. You could play around with that to get a sense of what you might be getting into.

[emilyrose]

Thank you so much for your reply!

My thesis is assessing the hazard of a potential glacial lake outburst flood, so the rest of my thesis has assessed how the hazard has changed over the past decade, i.e. glacier area change, lake area and volume change etc., I have also assessed how the vulnerability of the downstream community has changed in terms of population and infrastructure at risk. So the final step would be to tie this together which an estimation of how far an outburst of that size may travel etc.

You are completely right, it is a lot to learn and I have very little knowledge on river hydraulics and flood modelling unfortunately, and neither does my supervisor but it is something I am being advised to do nonetheless – so I have been trying my best to teach myself the basics to complete this but it is rather complex to me.

Most research within the field does use a dam-breach model, however, these are more complicated and as the aim of my project is not to assess how the dam would breach I was hoping to do something as simple as possible. In terms of terrain, I have a modern DEM which I have produced a DTM in TIN format from (not sure if this is right), but using HEC-RAS I created a terrain from this which seemed correct.

I have currently been doing a 2D unsteady flow simulation, using a flow hydrograph for the upstream, and normal depth for downstream. As a moraine-dammed lake it is assumed that flood discharge would increase linearly to a peak and decrease linearly to 0 m3/s over a time span equal to that of the rising limb – so the hydrograph is assumed to be triangular in shape as has been applied to other GLOF studies. The peak discharge was calculated using Qmax= 0.72V^0.53, a relationship developed for moraine-dammed lakes.

I managed to get this simulation up and running but even giving the hydrograph a duration of 8 hours the result in RAS-mapper is showing the flood to travel only ~30km downstream, when I want it to reach ~90km downstream to a large town. I am certain it would reach this far as there has been a flood from a nearby lake which reached the town that was a fraction of the size of the lake that I am modelling. I have no idea how to resolve this at all, I may have carried out the whole simulation out wrong but I am unsure how to make the flood continue further.

I hope this makes some sense! Thank you for your time.

[Sauhardra Joshi]

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<[email protected]>
In-Reply-To: <[email protected]>
From: Sauhard Joshi
Date: Sun, 8 Nov 2020 08:46:06 +0545
Message-ID:
Subject: Re: Modelling a glacial lake outburst flood
To: “emilyrose [via HEC-RAS Help]”
Content-Type: multipart/alternative; boundary=”000000000000575c6c05b38faa01″

–000000000000575c6c05b38faa01
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hello,
actually I am doing thesis on GLOF as well. Is there anyway I could connect
with you? From what i read in your mail, I think you have a good
understanding of the hydrology of Glacier and having difficulty in
Hydraulic modeling. I have few queries about your inflow hydrograph . I
assume the lake to be reservoir of certain constant volume without inflow
and let it over top over the moraine dam. After reading your mail I think
your idea for inflow hydrograph seems more valid. Is you empirical formula
for discharge at outlet of the dam? or upstream of the lake?
Furthermore, if you need any information regarding my work, I will be happy
to share. Thank you.

On Sat, Nov 7, 2020 at 10:42 PM emilyrose [via HEC-RAS Help] < [email protected]> wrote:

> Thank you so much for your reply!
>
> My thesis is assessing the hazard of a potential glacial lake outburst
> flood, so the rest of my thesis has assessed how the hazard has changed
> over the past decade, i.e. glacier area change, lake area and volume chan=
ge
> etc., I have also assessed how the vulnerability of the downstream
> community has changed in terms of population and infrastructure at risk. =
So
> the final step would be to tie this together which an estimation of how f=
ar
> an outburst of that size may travel etc.
>
> You are completely right, it is a lot to learn and I have very little
> knowledge on river hydraulics and flood modelling unfortunately, and
> neither does my supervisor but it is something I am being advised to do
> nonetheless – so I have been trying my best to teach myself the basics to
> complete this but it is rather complex to me.
>
> Most research within the field does use a dam-breach model, however, thes=
e
> are more complicated and as the aim of my project is not to assess how th=
e
> dam would breach I was hoping to do something as simple as possible. In
> terms of terrain, I have a modern DEM which I have produced a DTM in TIN
> format from (not sure if this is right), but using HEC-RAS I created a
> terrain from this which seemed correct.
>
> I have currently been doing a 2D unsteady flow simulation, using a flow
> hydrograph for the upstream, and normal depth for downstream. As a
> moraine-dammed lake it is assumed that flood discharge would increase
> linearly to a peak and decrease linearly to 0 m3/s over a time span equal
> to that of the rising limb – so the hydrograph is assumed to be triangula=
r
> in shape as has been applied to other GLOF studies. The peak discharge wa=
s
> calculated using Qmax=3D 0.72V^0.53, a relationship developed for
> moraine-dammed lakes.
>
> I managed to get this simulation up and running but even giving the
> hydrograph a duration of 8 hours the result in RAS-mapper is showing the
> flood to travel only ~30km downstream, when I want it to reach ~90km
> downstream to a large town. I am certain it would reach this far as there
> has been a flood from a nearby lake which reached the town that was a
> fraction of the size of the lake that I am modelling. I have no idea how
> to resolve this at all, I may have carried out the whole simulation out
> wrong but I am unsure how to make the flood continue further.
>
> I hope this makes some sense! Thank you for your time.
>
> ——————————
> If you reply to this email, your message will be added to the discussion
> below:
>
> http://hec-ras-help.1091112.n5.nabble.com/Modelling-a-glacial-lake-outbur=
st-flood-tp9330p9336.html
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hello,

actually I am doing thesis on GLOF as well. Is =
there anyway I could connect with you? From what i read in your mail, I thi=
nk you have a good understanding of the hydrology=C2=A0of Glacier and havin=
g difficulty in Hydraulic modeling. I have few queries about your inflow hy=
drograph . I assume the lake to be reservoir of certain constant volume wit=
hout inflow and let it over top over the moraine dam. After reading your ma=
il I think your idea for inflow hydrograph seems more valid. Is you empiric=
al formula for discharge at outlet of the dam? or upstream of the lake?

Furthermore, if you need any information regarding my work, I will b=
e happy to share. Thank you.=C2=A0=C2=A0

On Sat, Nov 7, 2020 at 10:42 =
PM emilyrose [via HEC-RAS Help] <[email protected]> wrote:

<= blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-l= eft:1px solid rgb(204,204,204);padding-left:1ex">

Thank you so much for your reply!=20

My thesis is assessing the hazard of a potential glacial lake outbu=
rst flood, so the rest of my thesis has assessed how the hazard has changed=
over the past decade, i.e. glacier area change, lake area and volume chang=
e etc., I have also assessed how the vulnerability of the downstream commun=
ity has changed in terms of population and infrastructure at risk. So the f=
inal step would be to tie this together which an estimation of how far an o=
utburst of that size may travel etc.=20

You are completely right, it is a lot to learn and I have very litt=
le knowledge on river hydraulics and flood modelling unfortunately, and nei=
ther does my supervisor but it is something I am being advised to do noneth=
eless – so I have been trying my best to teach myself the basics to complet=
e this but it is rather complex to me.=20

Most research within the field does use a dam-breach model, however=
, these are more complicated and as the aim of my project is not to assess =
how the dam would breach I was hoping to do something as simple as possible=
. In terms of terrain, I have a modern DEM which I have produced a DTM in T=
IN format from (not sure if this is right), but using HEC-RAS I created a t=
errain from this which seemed correct.=20

I have currently been doing a 2D unsteady flow simulation, using a =
flow hydrograph for the upstream, and normal depth for downstream. As a mor=
aine-dammed lake it is assumed that flood discharge would increase linearly=
to a peak and decrease linearly to 0 m3/s over a time span equal to that o=
f the rising limb – so the hydrograph is assumed to be triangular in shape =
as has been applied to other GLOF studies. The peak discharge was calculate=
d using Qmax=3D 0.72V^0.53, a relationship developed for moraine-dammed lak=
es.=20

I managed to get this simulation up and running but even giving the=
hydrograph a duration of 8 hours the result in RAS-mapper is showing the f=
lood to travel only ~30km downstream, when I want it to reach ~90km downstr=
eam to a large town. I am certain it would reach this far as there has been=
a flood from a nearby lake which reached the town that was a fraction of t=
he size of the lake that I am modelling.=C2=A0 I have no idea how to resolv=
e this at all, I may have carried out the whole simulation out wrong but I =
am unsure how to make the flood continue further. =C2=A0

I hope this makes some sense! Thank you for your time.=20

=09
=09
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[Jarvus]

That approach sounds reasonable to me. If the large town is the main focus and it is ~90km downstream, then the exact mechanics of the dam breach are less important. The farther downstream it is, the less the dam breach itself matters. If the town was immediately below the dam, then it matters a lot.

So doing a triangular hydrograph seems reasonable. Nobody knows exactly how a glacial lake dam is going to fail anyway. If you get your model up and running, you can run several different shaped hydrographs of different durations.

If neither you nor your supervisor have a background in this, I’m not sure what suggestions I can really offer outside of trying to find someone who does have a background who would be willing to review your model.

There are lots of different things that could be wrong with your model that is causing bad results. Some of them may be very easy fixes and some of them may not.

For instance, if the terrain file has not previously been used for a river flood study, it may need minor various tweaks. Or it could need major work. Even if it has been used for a river flood study, if the glacial outburst flood is on a much bigger scale and the inundation is much higher, you may need to make adjustments to parts of the terrain that were not adjusted for previous studies.

[Chris G.]

Emily- I would happy to spend some time over the phone with you to discuss this topic. I have some experience with modeling GLOFs in RAS, especially modeling the Ice Age Missoula Floods. If you’re interested, shoot me an email and we can set something up. [email protected]

[Author]

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