Routing through bridges/culverts
Routing through bridges/culverts
(OP)
When establishing a relationship between outflow and elevation, it is easy to see which equations to use for culverts or bridges flowing full or overtopping the roadway. What is the best way to calculate outflow from a bridge in low flow conditions? Would that be considered weir flow? or open channel flow?
Weir flow seems appropriate since it accounts for the constriction of flow at the opening, but H (height above "weir") wouldn't make sense - unless you use an average height above stream bottom? Would that make sense?
Open channel flow would be complicated because you'd have to take wetted perimeter and area at each elevation. Although, HEC RAS could do that - I already have a running model.
What has everyone else done?
Weir flow seems appropriate since it accounts for the constriction of flow at the opening, but H (height above "weir") wouldn't make sense - unless you use an average height above stream bottom? Would that make sense?
Open channel flow would be complicated because you'd have to take wetted perimeter and area at each elevation. Although, HEC RAS could do that - I already have a running model.
What has everyone else done?
RE: Routing through bridges/culverts
RE: Routing through bridges/culverts
If HW and TW have no effect, weir analysis might be logical only if you have a very wide, flat bottom, and very low head...like a box culvert during base flow conditions. Otherwise, it is certainly a channel, not a weir.
You state "I already have a running model". If I were reviewing, and your modelling software doesn't have culvert and step analyses built in, I would require you to use HEC-RAS, or another one that does.
Engineering is the practice of the art of science - Steve
RE: Routing through bridges/culverts
By the way, I think the procedure is to apply the outflow obtained from HMS at all the cross sections downstream of the bridge in HEC RAS to properly model the stream. Flow changes at a bridge/culvert/dam, etc.
For bridge flowing full, orifice equation is used; For overtopping roadway, orifice and weir equations are used. What equation is used for low flow through structure - no pressure flow? Has anyone done this?
RE: Routing through bridges/culverts
As far as modeling a bridge, it depends on what your bridge is. Is it a culvert, piers, or is the bridge not affecting flow and you really just have open channel? You have to balance energies. RAS for bridge design includes a lot of trial and error. You have to set your ineffective flow areas upstream and downstream, both widths and heights, if you want to model the whole spectrum of expected flows. This is why sometimes you make two models so you don't have to fine tune a single model as much.
If you want to model the bridge outside of RAS, a lateral constriction is similar to a flume. Instead of contracting the bottom you force critical flow by a side contraction.
As to your last question, I would again recommend you review the manual. You have to trial and error to determine which method is critical.
RE: Routing through bridges/culverts
RE: Routing through bridges/culverts
weir flow with avg flow depth might be appropriate if you have a drop / grade control structure at the bridge such that flow is going through critical depth / supercritical.
However, for most bridges you probably have subcritical conditions. Open channel flow analysis using hec-ras, HEC2 or other program is probably more accurate.
RE: Routing through bridges/culverts
As for determining your bridge model you may wish to review pages 5-9 and forward of the Hydraulic Reference Manual. This gives a pretty good overview of what goes on.
I am familiar with Modified Puls and HEC-HMS. Most attenuation is in the floodplain and using a detailed HEC-RAS model will more accurately model this. You normally apply modified puls in HMS to help account for this, it depends on where you want your accuracy. I know a lot of people shy away from running unsteady in RAS because you need a stable model and have to check your stage discharge curves to make sure. But if you are trying to look at floodplain attenuation it is the better tool.
Saying all of that, I did have a reservoir recently that I ended up going to a 2D model to make sure that the reservoir was adequately modeled along with attenuation.
RE: Routing through bridges/culverts
Original question: After determining storage versus elevation, how can you establish the relationship between elevation and outlow (not inflow - which we already know) for the low flow stages?
RE: Routing through bridges/culverts
Either treat the bridge as a outlet control structure and do an unsteady flow analysis in HEC-RAS using an inflow hydrograph at the upstream end of the system. This is theoretically "correct" but unsteady flow examples in HEC-RAS are notoriously difficult.
Or, treat the inflows as a series of steady flows and review each resulting profile to see if it seems reasonable. This is time consuming and tedious but easier understand and check for errors.
Or, Model the system in say, HydroCad, treating the bridge as a weir with a "lid" on it.
This may be the easiest to setup and do a couple of trial runs to see if results seem reasonable. Post your question in the HydroCad forum too. Peter Smart can direct you to and axample or two which can help a lot in setting up the model.
Good luck
RE: Routing through bridges/culverts
The other option is to do it by hand, which is the most fun, but you have to know what a weir is. It is actually worse than your original post lists, because you have to run an iterative step method on multiple cross sections at multiple flow rates. http:/
The weir equation is only an estimate of a very special case.
RE: Routing through bridges/culverts
RE: Routing through bridges/culverts