Modeling Storage behind a culvert

[pmur3774]

I’m having a problem I hope someone can point me in the right direction. I am trying to model a detention basin in steady flow (unsteady is really not feasible due to steep stream gradient). I ran the basin in HMS and received the peak inflow/outflow values. I then inserted the outflows into the RAS model at the cross section
immediately downstream of the culvert outlet of the basin. When I ran the model I get results that do not match the inserted outflows. For example, at the 2yr event, the peak inflow to the basin is 78 cfs and the outflow from HMS is 65 cfs. The output in RAS shows the downstream flow to be 0.5 cfs. When I do not reduce the outflow and re-run the model the flows are the same (78 in and 78 out). This problem is similar
when I run all the frequency flows. The output flows do not match the steady flow inserted values.

Anyone run into this,or does someone have an answer as to how to fix this?

Thanks in advance.

[markwood]

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Thu, 19 Feb 2015 06:16:26 -0800
From: “Markwood, David”
To: “pmur3774 [via HEC-RAS Help]”
Subject: RE: Modeling Storage behind a culvert
Thread-Topic: Modeling Storage behind a culvert
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Date: Thu, 19 Feb 2015 14:16:26 +0000
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If I understand correctly, running a steady simulation (elevations computed=
from downstream to upstream), I believe you need to use the HMS outflows a=
t both the downstream and upstream face of the culvert, so that the culvert=
experiences a single discharge.

Should also be noted, if tailwater affects the performance of the outflow, =
you may need to iterate between RAS and HMS using outflow curves in HMS for=
the structure applying rating curves from RAS that capture the hydraulics/=
tailwater downstream of the culvert, until elevations (and discharges) matc=
hup between the two.

From: pmur3774 [via HEC-RAS Help] [mailto:[email protected]=
e.com]
Sent: Thursday, February 19, 2015 9:02 AM
To: Markwood, David
Subject: Modeling Storage behind a culvert

I’m having a problem I hope someone can point me in the right direction. I=
am trying to model a detention basin in steady flow (unsteady is really no=
t feasible due to steep stream gradient). I ran the basin in HMS and recei=
ved the peak inflow/outflow values. I then inserted the outflows into the =
RAS model at the cross section
immediately downstream of the culvert outlet of the basin. When I ran the =
model I get results that do not match the inserted outflows. For example, =
at the 2yr event, the peak inflow to the basin is 78 cfs and the outflow fr=
om HMS is 65 cfs. The output in RAS shows the downstream flow to be 0.5 cf=
s. When I do not reduce the outflow and re-run the model the flows are the=
same (78 in and 78 out). This problem is similar
when I run all the frequency flows. The output flows do not match the stea=
dy flow inserted values.

Anyone run into this,or does someone have an answer as to how to fix this?

Thanks in advance.

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If I understand correctly, ru=
nning a steady simulation (elevations computed from downstream to upstream)=
, I believe you need to use the HMS outflows at both the
downstream and upstream face of the culvert, so that the culvert experienc=
es a single discharge.

 

Should also be noted, if tail=
water affects the performance of the outflow, you may need to iterate betwe=
en RAS and HMS using outflow curves in HMS for the structure
applying rating curves from RAS that capture the hydraulics/tailwater down=
stream of the culvert, until elevations (and discharges) matchup between th=
e two.

 

 

From: pmur3774=
[via HEC-RAS Help] [mailto:ml-node+[email protected]]

Sent: Thursday, February 19, 2015 9:02 AM
To: Markwood, David
Subject: Modeling Storage behind a culvert

 

I’m having a problem =
I hope someone can point me in the right direction.  I am trying to mo=
del a detention basin in steady flow (unsteady is really not feasible due t=
o steep stream gradient).  I ran the basin
in HMS and received the peak inflow/outflow values.  I then inserted =
the outflows into the RAS model at the cross section

immediately downstream of the culvert outlet of the basin.  When I ran=
the model I get results that do not match the inserted outflows.  For=
example, at the 2yr event, the peak inflow to the basin is 78 cfs and the =
outflow from HMS is 65 cfs.  The output in
RAS shows the downstream flow to be 0.5 cfs.  When I do not reduce th=
e outflow and re-run the model the flows are the same (78 in and 78 out). &=
nbsp;This problem is similar

when I run all the frequency flows.  The output flows do not match the=
steady flow inserted values.

Anyone run into this,or does someone have an answer as to how to fix this? =

Thanks in advance.

---

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