Culvert Capacity

[andyp]

Prior to using HEC-RAS I have calculated a culvert capacity to be 58 m3/sec for a rectangular concrete culvert using the Manning formula (width 3.5m and height 1.4m) using a Manning value of 0.010 and a channel gradient of 1:50.

However within HECRAS struggling to replicate the above – with the culvert showing flooding with a catchment flow of 34 m3/sec – well less than the culvert capacity of 58 m3/sec. To prevent flooding in HEC-RAS requires that the culvert be doubled in size which is unfeasible for the site.

I have modelled for a steady flow with critical depth reach boundary conditions and have tried altering Manning’s values, channel shape as well as culvert settings while troubleshooting.

Attempting to model the culvert as an open channel without the upstream and downstream cross sections also results in flooding despite the open channel having sufficient capacity to deal with the design flow and no obstructions or impediments to flow upstream or downstream.

Has anyone had experience in resolving capacity issues similar to this that can advise?

[Jarvus]

I created a simple data set that was 5000 meters long, 100 meter drop and cross sections every 100 meters, with a Manning N of 0.010. With an upstream boundary of critical depth. At the downstream end, I got a water depth of 1.389, fairly close to the 1.4 m you report.

1:50 gradient and a Manning N of 0.01 gives a normal depth that is very supercritical. I do not believe the RAS steady flow culvert routines will compute a supercritical flow going into the culvert. The flow inside of the culvert can be supercritical and the flow leaving the culvert can be supercritical.

But the water surface immediately upstream of the culvert is, I believe, going to be assumed to be subcritical.

I didn’t try modeling the culvert itself. However, if the water surface upstream of the culvert you describe is subcritical and the downstream boundary is low enough, then the culvert is probably going to be under inlet control. So the ability of the flow to get into the entrance of the culvert is going to be the limiting factor.

If you have a 3.5m by 1.4m channel flowing into a 3.5m by 1.4m box culvert and the flow doesn’t hit the lid, then there may not be any entrance losses. But if the flow touches the lid or there is any change in shape between the channel and culvert, there are going to be some type of entrance losses especially given the very high velocity.

Also, the capacity of a box culvert flowing full is not the same as the open channel capacity. The top of the culvert adds another 3.5 meters of wetted perimeter. So this culvert, or the equivalent cross section with lid would not have a capacity of 58 m3/sec at normal depth.

If you have a situation where there is a supercritical channel flowing into a culvert and the flow stays supercritical the entire way, in order to model that with RAS, I think using cross sections with lids is the only viable option. As this has the capability of staying supercritical. Depending on the transition from the channel to the culvert entrance, you would have to give a lot of thought to what the entrance loss should be. The entrance losses would probably be higher than the normal contraction/expansion loss that is computed. Since the normal contraction/expansion loss assumes a gradual change over the entire distance, not a sudden change like the entrance to a culvert. RAS has an option to enter an additional energy loss that is a coefficient multiplied by the velocity head. This could be used in order to model the entrance loss. Although I’m not sure how would be the best way to determine the value of the coefficient itself.

[Author]

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