Long culvert on no slope

[SMIAH]

Reviewing a design:

2 BOX culvert with a length of 120' on 0 slope (identical upstream & downstream elevation).
To reach the rock bedding, they've put the BOX 3' under the stream elevation (embeddding).

Is it me or this will affect:

1) Capacity of te culvert (I have to see the calculations yet)
2) Frequency of maintenance (sedimentation issue)
3) ...

 

[Twinkie]

Capacity would be effected because you are losing 3 vertical feet of cross-sectional area, you're always going to have sedimentation issues with a culvert, probably less with 3' of head always in the culvert.

 

[OHbridgeguy]

Is this a 3-sided or 4-sided box?

If 3-sided I would recommend cast in place footings and knee walls to keep the box at the higher elevation, unless it is designed for the loss of height. Also, the stream elevation may lower due to contraction scour which may help with this. On the other hand if scour becomes too large due to contraction and local abutment/pier scour, the rock will be exposed and an evaluation of whether the rock is erodible should be performed. In this case rock embedment of the footings may need to be considered.

If a 4-sided box just check to make sure the culvert was designed with the sedimenation issues in mind. You could also grade the channel to lower the elevation to the box bottom.

 

[gbam]

Smiah - sounds like the design is keeping the natural bottom through the culvert; may for fish passage? The culvert may also be oversized from the start to accommodate the rougher bottom. I have seen and used this approach previously. As long as the designer/engineer accounts for scour/aggredation within the culvert and the analysis is correct, it should be fine.

 

[SMIAH]

Well... I have to see the calculations yet.

From my understanding, the 4-sided box isn't oversized and no analysis has been perform regarding scour/sedimentation issue.

Plus, from my own verifications, there's no rock 4' under the stream so...

 

[SMIAH]

I always try to design a culvert with at least a slope that can produce a velocity over 5 or 6 fps for a 2-year peak flow so it can be naturally washed out once in a while (I may be wrong here).

It's pretty hard to calculate a culvert with no slop (?!). You can do it but... you have to assume a minimum slope or a head differential from entrance to exit.

I'm trying to make my point about avoiding a 120' culvert on a flat slope. Option is to reprofilate the stream upstream (or downstream) to gain a little slope (say at least 0.1%).

 

[ACtrafficengr]

The Wiki culvert article has a paragraph on Australian culverts designed for high volumes and minimal slopes: http://en.wikipedia.org/wiki/Culvert#Minimum_energy_loss_culverts