Maximum culvert HW/D =1.5 why?

[CEPEJack]

Does anyone have an idea of why we often limit our culvert headwater depth versus diameter (HW/D) to a maximum 1.5 to 2.0? I've seen these values mentioned in various stormwater design manuals.

Does it have something to do with uplift pressures on the conduit?

 

[Twinkie]

See Section 2.2.5 of Hydraulic Design of Highway Culverts for a very thorough discussion about allowable headwater.

 

[SMIAH]

I rather check WSE to be below the road elevation (with a freeboard), residence or infrastructure elevation near that could be flooded, entrance velocity vs obstruction/erosion concerns than 1.5HW/D.

 

[cvg]

any more than 1.5 or 2.0 HW/D and the velocity tends to get fairly high. Also, a small culvert which backs up to form a deep pool will trap sediment upstream which a larger culvert might allow to pass through. with proper construction such as headwalls, good backfill and cover, uplift should not be an issue