HEC RAS culvert on flat terrain 1

[Jajosk]

Torn between HY-8 and HEC-RAS to model a culvert to carry about 1 m3/s under a road embankment with a allowable headwater of 1.5 m. The topo survey covers some 50 m radius from the middle of the proposed alignment of the culvert. The road embankment will be located in the middle of a pasture field with less than 1% slope. There is no identifiable channel located at the upstream or downstream end of culvert. Here lies the problem for choice of model - I do not know how wide should I be modeling the reach, and where the overbank will be (since however much wide I go, everything in cross section is at same elevation). How can I model this reach in HEC-RAS? HY8 will not fuss about cross section...but do I miss on something important if I am not using HEC-RAS? HY8 requires tailwater elevation though for which I'm thinking of adding normal flow depth for an assumed diameter of culvert, to culvert downstream invert. I will do an iterative process till I get the assumed diameter close to what I need for carry the flood flow. Does this sound reasonable?

If I do have to use HEC-RAS do I need to consider expansion and contraction reaches for this case, or is just one cross section on either end of the culvert just past the ends of the culvert sufficient (I might be able to find a wee bit of depression in topo just beyond the culvert, but the further I go from the culvert, the more the pasture becomes flat)?

 

[francesca]

If there's no identifiable channel, then how big is the drainage area? I would say HY-8. When I worked in Indiana, you modeled up to 20' span box culverts with HY-8 and only used HEC-RAS for bridges.

 

[Jajosk]

Thanks francesca for your response.

The drainage area is to the tune of 50,000 sq meter. Just out of curiosity, how is choice of software related to drainage area? Is there an upper limit for HY-8?

BTW, the HY-8 outlet velocity for this culvert is 3.15 m/s. Seems a bit on the higher side (approx 25 m long culvert on 5% slope, assumed TW @ normal depth of culvert). I know some jurisdictions put an upper limit on the outlet velocity at 15-20 fps (

http://www.lawrencevillega.org/user...WaterDesignManual/Chapter 4 section 4.3.pdf),

and I am below that, but still would like to know if there is good practice sort of upper limit for culvert design.

The jurisdiction I am working on will require a riprap apron of 2.5 x diameter of culvert for all culverts. Will that be sufficient for my culvert?

 

[francesca]

At that outlet velocity you will have a scour problem and should design accordingly. (Energy dissipating structure, etc.) A riprap apron won't cut it. I have seen spectacular scour where we had high outlet velocity on a steep slope and no geofabric under the riprap. It was pretty costly to the contractor to repair (they had to remove all the transported sediment and repair the slope) and I'm not sure how the designer got out of being held jointly liable.

Depending on the soil type, you will have erosive velocity around 3-5 fps, so I'm curious where the water from your 12 acre catchment is going. I have seen a drainage area that size not have a channel ONCE, but those were Type A soils in dense vegetation.