max. gradient for open bottom corrugated metal arch

[foreng]

Looking for an pinion on what might be considered the max. design gradient for an open botton arch drainage structure.

I have a site that has a natural stream grade of 10%, a design flow of 3.5 m3/s, with a design HW/D of around 0.5 to minimize backwater effect and piping.

At what point should a tie back system be considered?

 

[cvg]

you need to calculate the scour depth and extend your footings down below that depth. Or line the bottom of the channel with concrete or corrugated steel. Or flatten the culvert grade and construct some sort of drop structure(s).

 

[gbam]

CVG hit an important point, local scour. You should also consider long term conditions. A sediment transport model or evaluation for the equilibrium slope may be necessary. The 10% slope seems steep to be in equilibrium unless the channel is armored/bedrock. What is the natural bed material?

 

[foreng]

The channel is located on a steep mountain stream with average stream gradients approx 10%. The D50 is around 250 mm or large cobble-small boulder. The Rosgen stream classification is an A2.

I have typically limited arches to under 6%, but the road alignment on this crossing is well suited for this type of structure. The arch was chosen as the stream is fish bearing, and in our jurisdiction all fish bearing stream are limited to open bottom structures.

 

[cvg]

one option that is recommended around here is to overexcavate, install the box culvert and replace some of the native material back in the box. A 6 foot high box can be installed with just 4 feet above grade and 2 feet buried. However, this is done primarily where agradation is the problem. I believe your problem is probably degradation, either local scour or long term bed scour. Have you done an equilibrium analysis to determine if the channel is sufficiently armored to be stable?

One option to protect against local scour if you intend to stick with the open bottom metal arch is to construct a cutoff wall across both the inlet and outlet ends of the culvert to control the invert grade inside the culvert. The wall would typically be part of the headwall structure.

 

[foreng]

The analysis did show the channel to be stable, I hadn't considered a grade control structure, I will look into it.
Thanks for the advice.

 

[gbam]

Is the 10" rock stable with the flow rate (Shear and Permissable Velocity)?

 

[foreng]

The analysis indicated that the 10" rock is stable.

 

[gbam]

If the 10" rock is stable through the culvert for the design event then the 10% grade should be fine. You may want to evaluate the flow rate that is erosive and determine the return frequency to show that protection is not warranted.