Unpredicted Deep Scour Holes at Bridge with Tidal Influence

[Anonymous]

Hi Chris G.,
I’m working on a bridge replacement project on the Oregon coast that crosses an estuary. The estuary is about 300′ wide but the existing bridge opening is only 44′ wide. I’ve set up a HEC-RAS model using the extreme high tide stage data as the downstream condition of an unsteady model analysis and the 100-year constant flow for the upstream condition. The existing bridge is very old. It is estimated that it was constructed in the 1930’s and the channel under the bridge has been protected with grouted riprap. The riprap is at an elevation of 4.6′ and the lowest downstream channel elevation is 3.6′. The cross section data for the model is a combination of real survey data near the bridge and Lidar data further upstream. The model runs without errors and gives realistic results. Completing the scour calculations using the information from the existing model, predicts a scour depth of about 10′. However, on either side of the existing bridge are large round scour holes. One is 30′ deep and the other is 25′ deep. What I suspect is happening is a submerged hydraulic jump due to the grouted riprap acting like a weir/dam.
I’ve looked all over the internet for help and advise, but this seems to be a fairly unique situation with advanced fluid mechanics. If you could help, it would be much appreciated. Here are my questions:
1) Is there any other hydraulic principle that could be creating the deep scour holes on either side of the bridge?
2) If the proposed bridge does not use grouted riprap will the scour holes on either side of the bridge fill in because the condition for the submerged hydraulic jump will be removed?
3) The scour depth for the proposed bridge is 6′ (78′ opening). If we use ODOT Class 700 riprap (3′ thickness) with a golf club shape and filter rock and geotextile fabric, will the riprap be stable given the adjacent scour holes on either side have a slope of 3:1?

This is the website I’ve been reading/watching to learn about submerged hydraulic jumps. http://krcproject.groups.et.byu.net/

[Chris G.]

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Subject: Re: Unpredicted Deep Scour Holes at Bridge with Tidal Influence
References: <[email protected]>
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Date: Tue, 11 Aug 2015 16:58:51 -0700
To: “Karina [via HEC-RAS Help]”
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Hi Karina. Might be easier if you give me a call, being that you are local. 5=
03-946-8536.=20

Sent from my iPhone

> On Aug 10, 2015, at 18:42, Karina [via HEC-RAS Help] wrote:
>=20
> Hi Chris G.,=20
> I’m working on a bridge replacement project on the Oregon coast that cross=
es an estuary. The estuary is about 300′ wide but the existing bridge openin=
g is only 44′ wide. I’ve set up a HEC-RAS model using the extreme high tide s=
tage data as the downstream condition of an unsteady model analysis and the 1=
00-year constant flow for the upstream condition. The existing bridge is ver=
y old. It is estimated that it was constructed in the 1930’s and the channel=
under the bridge has been protected with grouted riprap. The riprap is at a=
n elevation of 4.6′ and the lowest downstream channel elevation is 3.6′. The=
cross section data for the model is a combination of real survey data near t=
he bridge and Lidar data further upstream. The model runs without errors and=
gives realistic results. Completing the scour calculations using the inform=
ation from the existing model, predicts a scour depth of about 10′. However,=
on either side of the existing bridge are large round scour holes. One is 3=
0′ deep and the other is 25′ deep. What I suspect is happening is a submerge=
d hydraulic jump due to the grouted riprap acting like a weir/dam.=20
> I’ve looked all over the internet for help and advise, but this seems to b=
e a fairly unique situation with advanced fluid mechanics. If you could help=
, it would be much appreciated. Here are my questions: =20
> 1) Is there any other hydraulic principle that could be creating the deep s=
cour holes on either side of the bridge?=20
> 2) If the proposed bridge does not use grouted riprap will the scour holes=
on either side of the bridge fill in because the condition for the submerge=
d hydraulic jump will be removed?=20
> 3) The scour depth for the proposed bridge is 6′ (78′ opening). If we use O=
DOT Class 700 riprap (3′ thickness) with a golf club shape and filter rock a=
nd geotextile fabric, will the riprap be stable given the adjacent scour hol=
es on either side have a slope of 3:1?=20
>=20
> This is the website I’ve been reading/watching to learn about submerged hy=
draulic jumps. http://krcproject.groups.et.byu.net/
>=20
>=20
> If you reply to this email, your message will be added to the discussion b=
elow:
> http://hec-ras-help.1091112.n5.nabble.com/Unpredicted-Deep-Scour-Holes-at-=
Bridge-with-Tidal-Influence-tp1902.html
> To unsubscribe from HEC-RAS Help, click here.
> NAML

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charset=utf-8
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Hi Karina. Might be easier if you give=
me a call, being that you are local. 503-946-8536. 

Sent from m=
y iPhone

On Aug 10, 2015, at 18:42, Karina [via HEC-RAS Help] &=
lt;ml-node+s109111=
[email protected]> wrote:

">

Hi Chris G.,

I’m working on a bridge replacement project on the Oregon coast that cro=
sses an estuary. The estuary is about 300′ wide but the existing bridge open=
ing is only 44′ wide. I’ve set up a HEC-RAS model using the extreme high tid=
e stage data as the downstream condition of an unsteady model analysis and t=
he 100-year constant flow for the upstream condition. The existing bridge is=
very old. It is estimated that it was constructed in the 1930’s and the cha=
nnel under the bridge has been protected with grouted riprap. The riprap is a=
t an elevation of 4.6′ and the lowest downstream channel elevation is 3.6′. T=
he cross section data for the model is a combination of real survey data nea=
r the bridge and Lidar data further upstream. The model runs without errors a=
nd gives realistic results. Completing the scour calculations using the info=
rmation from the existing model, predicts a scour depth of about 10′. Howeve=
r, on either side of the existing bridge are large round scour holes. One is=
30′ deep and the other is 25′ deep. What I suspect is happening is a submer=
ged hydraulic jump due to the grouted riprap acting like a weir/dam.

I’ve looked all over the internet for help and advise, but this seems to=
be a fairly unique situation with advanced fluid mechanics. If you could he=
lp, it would be much appreciated. Here are my questions:=20

1) Is there any other hydraulic principle that could be creating the dee=
p scour holes on either side of the bridge?

2) If the proposed bridge does not use grouted riprap will the scour hol=
es on either side of the bridge fill in because the condition for the submer=
ged hydraulic jump will be removed?=20

3) The scour depth for the proposed bridge is 6′ (78′ opening). If we us=
e ODOT Class 700 riprap (3′  thickness) with a golf club shape and filt=
er rock and geotextile fabric, will the riprap be stable given the adjacent s=
cour holes on either side have a slope of 3:1?

This is the website I’ve been reading/watching to learn about submer=
ged hydraulic jumps. http://krcproject.groups.et.byu=
.net/

=09
=09
=09

---

If you reply to this email,=
your message will be added to the discussion below:

http://he=
c-ras-help.1091112.n5.nabble.com/Unpredicted-Deep-Scour-Holes-at-Bridge-with=
-Tidal-Influence-tp1902.html

=09
To unsubscribe from HEC-RAS Help, click here.
NAML

=

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