HEC-RAS veloctiy profile 2

[Jajosk]

Have a few questions for HEC-RAS pertaining to the attached river schematics.
1. I need to know from where the "Main Channel Distance" appearing as the horizontal axis title in the second attachment is measured. Is it going from 8 to 19, or otherwise?
2. I have a bridge located about midway in the reach modelled. The reach is a natural channel, almost straight with no tributary or in-line or off-line feature. Does the velocity profile make sense with its somewhat peculiar troughs and crests?
Thanks
J

 

[Crystalct1]

1. It is measured from d/s to u/s always. So, this is your value in the reach length column on your geometric data window for each cross-section (XS). XS 8 should have 0 as the reach length because it represents the distance from XS 8 to the next d/s section. Since XS 8 is the most d/s section, the distance to the next one is considered 0. Looking at XS 9, you should see a positive distance there representing the distances along your x-axis there. XS 9's information on the graph will correspond to x= [reach length at XS 9].

2. You say you have modeled a bridge, but if so, that bridge would be seen on the plan view that you have attached. It appears that your bridge is not correctly modeled. Usually, velocities increase at bridges/culverts since flow must be squeezed through a smaller XS area than through the natural section of stream. Check that you have correctly encoded the bridge information. Look at which XSs you have modeled it between and the lengths in the bridge data. Check that the roadway crossing is modeled at the same location. You will definitely see a gray shaded "road" at your bridge location if modeled correctly.

 

[francesca]

A couple points from your post concern me.

1. As Crystal said, your bridge would show on the plan view if you were looking at the proposed condition. You don't seem to understand HEC-RAS well and need to get someone who does on board or read the Hydraulic Reference Manual.
2. The total reach length modeled is not sufficient to include enough fully-expanded flow on either side of the bridge (assuming that the plan view is to scale).
3. The velocities in the profile are very low, so the range doesn't bother me. However, you can't look at a velocity profile and infer anything about the quality of the model. I have never submitted a velocity profile for review. You need to review the velocity distribution at each cross-section and assess.
4. There is not a single ineffective flow station on the plan view. Even in an existing conditions/baseline/natural model, you would block out areas that are so shallow that the water is basically stagnant. In Indiana, you block out anything <0.5 fps AND <3 ft deep as ineffective flow. Since there are places in your model where the bank average velocity is in this range, I think that there would be substantial ineffective areas even in your natural model. And of course in your proposed model you need the expansion and contraction cones downstream and upstream of your bridge, respectively.
5. Given the location of the bank stations, the XS 1, 2, 3, & 4 (bridge XSs - refer to manual) are not long enough. Also, 1 & 4 are not far enough up/downstream to be up/downstream of the contraction/expansion reaches. (It's okay to have extra XSs in between.)

I suggest you (1) Read the Hydraulic Reference Manual and (2) download CheckRAS and run that on the model.

 

[Jajosk]

Thanks Crystal and francesca for taking the time to read and offer help.
First off, I do not have access to someone who is experienced in HEC-RAS to help me out. If I did, I would go to her/him rather than try to explain a model in words on an internet forum. I am new to modeling and I am willing to learn. I have read the hydraulics manual in parts, and will continue to read further. Everything I describe below is open to your criticism, which will only help me learn.
1) The bridge was not physically modeled. However, I had used expansion and contraction coefficients to simulate the effect. This is not a real channel, only an example exercise I have created. Question - what is the need for physically imposing a bridge if correct expansion and contraction coeff's have been used?
2) The reach length is 160 m approx. There are 12 X's and this is almost a straight reach with no tributary or in-line or off-line feature. @francesca - why would you suggest the number of XS's are not enough, and how many would you like to see?
3) Can you please explain how I can see velocity distribution at each XS? I can see only one velocity for one given flow that I am routing through the channel (attachment 1).
4. I am not sure how to recognise ineffective areas, or if this a purely judgment call. In the second attachment I have the major XS types I have in the reach. Can you point out on the XS's the areas that you think could be ineffective?
5. I don't have any XS marked 1-4. Could you please clarify.
6. What will be far enough u/s and d/s for a reach like this?
7. I have used contraction and expansion coefficients in the following manner (attachment 3):
i) U/s XS - 19 to 16: .1 / .3
ii) U/s XS 15: .3/.3 (approx 10 m from bridge)
iii) Bridge centreline XS 14: .2/.5
iv) D/s XS 13: .1/.4 (approx 15 m from bridge)
v) D/s XS 12: .1/.4
vi) D/s XS 11-8 except 10: 1./.3
vii) D/s XS-10: .3/.3 (there's a natural depression in the channel)
I am not sure if the Plan can be considered scale, but it stretches for a total of 160 m approx. The relative distances between XS are marked consistently. Comments about the coeffs please?
Thanks again!!

 

[Jajosk]

Gosh..E-T won't allow more than one attachment per message Here's Attachment 2.

 

[Jajosk]

...and Attachment 3. Thanks

 

[gbam]

You will need to introduce at least a contraction in your floodplain to represent your bridge. This can be acheived using either the bridge editor or the blocked flow option. Your model need to "know" where to place the increased contraction and expansion head losses due to the bridge/encroachment. I recommend that you model using the bridge routine in case of overtopping or pressure flow resulting in the vicinity of the bridge. Hydraulics will change in this region due to the affect of piers and the encroachment. Please read through the Chow Open Channel Hydraulics (1959) book or some other source of hydraulics book (Fundamentals of Hydraulic Engineering, Prashun) to become familiar with hydraulics in general and especially at bridges.

 

[francesca]

Okay, I'll go easier on you since you're trying to learn. Have a look at the worked examples in the Manuals. They indicate best practices. This is a very good resource:

http://www.in.gov/dnr/water/files/fp_guidelines.pdf

(Chapter 8 deals with HEC-RAS modeling.)

You do need to put the bridge in as a bridge - under some flow regimes you may get pressure flow, and all regulatory agencies will want to see the bridge in there. You need to block out the overbanks, change the contraction/expansion coefficients, and reduce the Manning's through the bridge (internal x-sections). You can also vary the channel under the bridge if your agency allows channel clearing.

I don't mean that there aren't enough x-sections, but rather that you haven't modeled a long enough section of the reach. You have to model far enough upstream that the backwater effects are all evident and have dissipated. In some instances, the WSE will drop immediately upstream of the bridge, giving the illusion that the bridge has not adversely affected the WSE - but further upstream you will see the backwater. You need to dissipate backwater within ROW or flood easement limits, but still need enough modeled reach to indicate that there are no backwater effects.

In order to get a velocity distribution on each x-section, you need to specify the number of devisions on the Flow dialog. I used to do 10 on each overbank. Blocking out stagnant water is entirely subjective, but HEC-RAS is a 1-dimensional model, and it's the more conservative approach. It's an iterative process to get everywhere <0.5 fps & <3' deep.

In your attachment showing the x-sections, there are a couple issues. Firstly, there is a HUGE variation in the cross-section. Is this really the case with the x-sections so close together? If so, you need to consider expansion/contraction coefficients to account for the rapidly varying flow. Secondly, the areas that have flow shown that are separated from the main channel by ridges need to be blocked out as ineffective.

 

[Jajosk]

Thanks gbam and franceska, and sorry for getting behind in getting back.
I will go over the Indiana guidance fran referred to. As for blocked areas, if I block out the meadows adjoining the overbank, will it not raise WSE? Physically, the water can get to the meadows, so blocking those areas seem unnatural. Unless, the stoarage ability of blocked areas implies that there is no change to WSE due to the blocking, just that the velocity would change. But how can that be true I wonder...? In other (hopefully simpler) words, what effect does not blocking the meadows have? Also, does the attached file describe the blocking of the areas as suggested in 27 March post?
Anyways getting to be warm here these days...and I can't wait to bring out my potjie and head for the lake...gosh lake reminds me of runoff modelling again

 

[gbam]

Jajosk - When I mentioned the use of blocked flow regions that was to represent the bridge and roadway embankment. I use this method as a quick and dirty approach to determine the backwater impact from a proposed structure. It is easy to code quickly for rough results.

If you have a pearched bridge condition (flow can overtop the road on each or one side of the bridge) then it can be modeled using the conveyance and the multiple opening option.

Don't forget to apply your ineffective flow boundaries within your cross sections.

 

[Jajosk]

Ok, I might have gotten a bit further along the road since the last posts. New sets of questions arise though
Attached file has all the info need. Basically I have added a bridge, and checked for WSP and velocity. Tried with subcritical first and noticed the WSP is critical at the most u/s point in reach, and a consequent increase in velocity. This seemed odd given the relatively uniform reach width and lack of any major bend. Tried mixed flow to see if there is a chance of a hydraulic jump downstream, but the WSP and velocity came out exactly same as subcritical, except the flow computation dialogue box showed a different number for "RS" (don't know what it is and if it has to do something here at all).
So my questions are -
1) why did the program find exactly same results for sub and mixed analyses when there is a critical flow in the reach?
2) does the boundary conditions (critical for both u/s and d/s have anything to do with it)?
3) why is the velocity increasing d/s?
4) is the demarcation of ineffective flow areas for the bridge appropriate?
5) for franceska - the Indiana guide ask for blocking out flow below a threshold. Is the only way to do that is to first run the model without any blocking, then inspect all the XS's and do a rerun with the ineffective areas manually blocked out? Or can this is be done first time round?
Thanks!

 

[francesca]

Boundary conditions downstream should be normal depth with the slope equal to the regression slope of the thalweg. You need at least 2 cross-sections before the first bridge x-section to let the calculation settle. (Actually, you need more to provide a good modeled length of reach.)

To do the <0.5 fps and <3 ft depth you do run the model iteratively to find the optimal station to place the ineffective flow area. (It takes less than an hour to run several iterations!) Your WSE will go up, yes. The reason you do this is because shallow and slow areas do not have any effective conveyance. HEC-RAS is a one-dimensional model; it doesn't have any way of determining the depth at any point in the x-section. Read the Hydraulic Reference Manual to understand how HEC-RAS comes up with the single "friction" value for each cross-section. You are over-estimating the conveyance if you don't block out these shallow and effectively stagnant areas in your cross-section.

I'm sorry, but I don't download Word Documents off the internet, so I'm not looking at the files you posted.

 

[Jajosk]

Thanks franceska! I will do the ineffective areas iteratively in future. I am uploading the pdf of the doc file I put up earlier today. Hope that will help you and others with the rest of my questions.

 

[francesca]

Figure 5: It takes 3-4 x-sections for the WSE calculation to settle. Change the downstream boundary condition from Critical Depth to Normal Depth (recommended in the Hydraulic Reference Manual and countless modeling guidelines) and the calculation will settle more quickly.

Figure 3 (upstream): Your x-section doesn't extend the full width of the flow area OR your bridge embankment doesn't extend down to the ground. (Is there really no embankment?) You have a pier within your bank stations. You only have one ineffective flow area at all - there should be one on each side of both cross-sections to handle your expansion and contraction reaches.

Figure 1: Your four bridge x-sections are not long enough. They need to extend the full valley width. If this figure is to scale, then I would say that your modeled reach is far too short to include the expansion/contraction reaches, let alone x-sections up/down stream of them. See page 6 of this:

http://www.hec.usace.army.mil/publications/TechnicalPapers/TP-151.pdf

or this blog post:

http://hecrasmodel.blogspot.com/

(Feb 27, 2012).

The rest of your figures are not really meaningful. A reviewer would want to see prints of your individual cross-sections showing the Manning's values.

 

[Jajosk]

Thanks for your comments Ms. F. Sorry I had to be away for some time. My responses to your comments are below:

1) Abt Fig 5 - I will do as you suggested;
2) Abt Fig 3 - The bridge is located on a underfit glacial stream. The bridge is supported by piers only, there is no embankment par se that is on the floodplain. I am working on getting the ineffective areas set up for the model. There are 2 cases here - blocking out the ineffective areas due to the pier (which can be done the first run of the model); and blocking out <5 fps and <3 ft depth areas (which can be done only after the first run has been completed - is that correct?). I am hesitant about the first case, as piers, unlike embankments do not block out all flow from edge of bank to end of the pier facing the main channel, i.e., there is some flow behind the piers too. Treating piers as embankments seems a bit too conservative. Thoughts?
3) Abt Fig 1 - Like I mentioned before, this is an undercut stream and the survey info does not go past the point where the terrain starts to flatten out (after the overbank area). Some of the XS's I have in the model appear extended which is because I simply extended the last elevation for an extra 100 m or so...this is an approximation, and I am not sure if it is a good idea to begin with. The idea was to see if the extra width will reduce the WSE (in reality the water will of course flow out on the flat areas, which is the logic behind modeling the extended floodplain). Does this make sense?
I went over the references, and indeed my contraction and extension reach lengths are too short. I am working on them, and will post results of XS's once done. I am still lost on what is the total length of reach that should be modeled here though...no reference I have come across yet talks about that. You mentioned 3-4 XS for equations to settle, but over how much length? Thanks again!!

 

[francesca]

Where you don't have survey, supplement with USGS survey (assuming you're in the US). Don't use ineffective areas for the piers. By coding them as piers, their flow constriction effects are considered.