Outlet Protection for 60" Culvert 1

[debgallery]

Proposing a 60" culvert at an access road crossing of an intermittent creek on BLM land. Used HY-8 to analyze the culvert (file attached). Now I'm looking at outlet protection. The design flow is 54-cfs (10-yr storm) but want to protect against the velocity for 129-cfs (25-year storm). The outlet velocity is 9.5-fps and the channel velocity is 1.8-fps. Outlet protection is my weak point. Also we don't have much in the way of soils information, I'm assuming cohesionless soils and assumed a soil sigma of 2.10 (I backed into a D84=100mm and D16=22.68mm just to get the sigma. I calculated scour hole geometry (also in HY-8 file). What are your feelings on the appropriateness of a constructed scour hole? Would you line it with riprap and if that's the case would you recalculate the scour hole geometry? Would a riprap apron suffice? Is this all overkill?
Thanks for the advice?

 

[TerryScan]

The Energy Dissipation module of HY-8 allows you to design energy dissipators. Just change the pull down from "Estimate Scour Hole Geometry". I changed your design fow to 129cfs and used Streambed Level Structures->Riprap Basin which resulted in a 20ft. long rip rap basin.

Just curious, what area the stream conditions that have a 0.10 mannings value? (appears high)

 

[debgallery]

Thanks TerryScan - I hadn't found that you could change the energy dissipator. Also, you're right about the manning's n - thanks for pointing that out. I always feel that the recommended d50 is too small (4"!?). Anyone else have any thoughts on that?

 

[cvg]

Condition to be used to Compute Basin Outlet Velocity = Envelope Curve
this will give you 8 inch

 

[gbam]

debgallery,
The FHWA HEC14 document is what HY-8 incorporates. If you want to check out the emperical formula and approach go to:

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/