I have not addressed anything like this specific problem in HECRAS, and used HECRAS just a bit.  So I am unsure if HECRAS can provide the solutions you seek. There are several questions though you must estimate, such as will the bar be submerged when debris is likely to accumulate, such as in flood?  Are there very heavy debris loads in stream (natural, upstream activity, trees contributing due to disease, wind damage, etc.)?  Is the debris substantially longer than spacing between or among piers or from past, causing accumulation during storms or floods?  Is there regular maintenance to clean up debris or is it allowed to accumulate?  Are BMPs needed to prevent excessive debris from logging operations?  Could adjustments to the placement of the bridge piers helped to reduce debris accumulations for projects in the future?

In HECRAS, cross sections are entered, so does the cross section reflect the sand bar, or was the cross section(s) away from bar?  If the cross section included the bar and any other bridge features, then HECRAS should be able to calculate the velocity changes as compared to upstream and downstream,  The tendency of bridge cross sections that fill in portions of the floodplain, is constraining the channel width and increasing velocity, both which may have a tendency to contribute to woody debris accumulations or issues.  

I have not tried to reorient woody debris on streams, but the problem with bridges and piers may be the log in travelling downstream perpendicular to the channel and catching on the bridge piers.  Structures such as J hooks or cross vanes that reduce bank stress and funnel more flow through the thalweg, may be able to reorient the debris to some extent so it is moving more parallel with stream due to differences in velocity.  Perhaps other measures can be installed to reorient debris before it hits the piers.  But if the stream is overloaded with debris, this may not work.  

On small streams, a debris rack is sometimes used above a culvert to catch the debris, but in one instance in Oregon, there was so much accumulation that small stream during flood was diverted down the road ditch, heavily damaging the road with deep gully until the flow was diverted across the road and down the fill slope to a stream below, substantially adding sediment and increasing channel instability.  

I am attaching a paper that addresses elements of this subject and has some nice pictures of examples, and some mitigation and management measures to consider.  It does discuss some calculations and considerations concerning bridge piers and debris, so perhaps you will find something useful in addressing your problem.  

t would be good if you could attach some image of your experimental setup. It is somewhat difficult to imagine what you are depicting. In any case, the answer to your question is a maybe. I'll explain why. The newest version of HEC-RAS has both 1-Dimensional and mxed 1-D / 2-Dimensional modeling capabilities.

I do not know if you are familiar with these terms. In case you are not I will briefly explain with examples. If you are, you can skip this part.

If you have nice straight rectangular channel reach (when I say channel you may also think about rivers), you may assume that the flow is one dimensional (in reality it never is unless you are dealing with very very small scales). That means, that the water moves preferentially parallel to the channel bed and normal to the cross section. Movement of water to the sides, or up and down may be neglected. For such conditions  you would then use a 1-Dimensional model.

Once you introduce some sort of disturbance to this nice straight flow, water can no longer be assumed to move in only one dymension. For example a curve on the channel induces movement of the water to the sides, but also a so called secondary current towards the bottom of the channel. Depending on your informations needs you may once again make some asumptions and simplify the problem as to stil be appropriate to use a 1-Dimensional model. For example if you are interested in the flow within the channel, you may use it. Of course the results will not match reality as well as in the first case. But if you are interested in modeling floods outside the curve, then you need a 2-Dimensional model. As you might imagine the mathematical and computational efforts required, increase significantly with an increase in dimensionality. It is no wonder that the first versions of Hec-RAS (and up to not such a long time ago) only provided 1-Dimensional modeling capabilities.

Now going back to your problem. A bridge pier also induces a distinctive three dimensional flow. That is for example the reaseon of pier scouring, as you may already know (look at the link I have attached from scientificamerican).The complex nature of this flows and the limited theory and computational power available up to a couple of decades ago, was the reason why such conditions were still being studied using physical models in flumes.

Some will say, but Hec-RAS has the option to modell bridges even in 1-Dimensional mode. Yes it does, but it makes use of a series of simplifications two describe what happens to the flow as a consequence of these disturbance. An example would be when the piers contract the channel so much that a supercritical flow (fast flow) occurs. After the pier or even along the pier, it will return to a subcritical flow (slow flow). In such a case a hydraulic jump ocurrs, where the water actually aquires a higher elevation (depth) and therefore slower. A hydraulic jump has a very complex flow structure and it is quite difficult to fully modell it. But the change in water elevation can be quite easily calculated (even by hand) after applying the momentum equation. So 1-Dimensional models use this solution to "modell" the hydraulic jump. The result within the modell will look like an upwards step. Yes, water may flow upwards. But the charcteristic swirling will not be by any means described.  If you require this, then you need a 3-Dimensional model. But guess what, even though software with 3-Dimensional modelling capabilities are readily available now, they are very much prone to uncertainties; there are simply to many parameters you can adjust. So even though the programm will give you some flow description, you still do not know if it's true. So there you have it, that is the reason for your flume experiment.

If I imagine your experimental setup properly, I would guess a flow develops which is likely of 3-Dimensional nature. Nonetheless, its effects on the flow might stil be modeled with a lower dimensionality, provided some mathematical model is available to describe its effects. Hec-RAS has some floating debry modelling capability (see chapter 5 of the reference manual that I have linked here). The way it it is done,  is basically through a change (reduction) in the cross section geometry and by assuming a rectangular cross section of the debris. This is quite crude, maybe enough for engineering purposes, not for research. But of course you can always use your flume experiment as a reference to see how good Hec-RAS performs. Your work in the flume could (I pressume) actually be used to improve such a model

I hope it helps.

https://blogs.scientificamerican.com/but-not-simpler/files/2013/05/LocalScour.gif

http://www.hec.usace.army.mil/software/hec-ras/documentation/HEC-RAS%205.0%20Reference%20Manual.pdf

Hello! Thank you so much for your detailed responses,

I have attached a drawing within the question of the experimental layout to clear up a bit what I was trying to describe. For my project HEC-RAS will be compared to CFD results, where the programs will be used to find velocities at certain points around and in front of the pier. Subsequently the results will be used to calculate scour and compare to the experimental scour maps we have obtained. I am new to HEC-RAS and have been using it for just under a month now.

I have been researching about splitting to pier up into 3 (two triangles and a rectangle) and representing the debris as an ineffective flow area. With regards to the debris, I don't believe hec-ras is capable as presenting it as a cyclindrical bar and so that it obviously a limitation...

i think I would be using a combination of the 1d/2d since I am interested in the 2d aspect of the velocities?

thanks again for the much needed help

^^^ also I am unsure on how the floating debris tool and ineffective flow area tool differ? Thanks

The debris tool is simply an easier way to define an ineffective flow area, nothing else. And you are right, it will not be modelled like a cylindrical bar but as a flat plate, which makes a big difference. To get a feeling look at the drag coefficients of both forms.

1D/2D in Hec-RAS means that 1-D will be used for flow within the channel and 2-D for flow outside the channel. It does not mean that it mixes and matches both approaches all together. So by using the pier/bridge and debris tool you will be at the end in the 1-D region. Little chance for 2-D. I have not worked with Hec-RAS for some months now, so please do not take my comment as the whole truth =).

i agree with Hansen