u/Emotional-Try-8380

Hi everyone,

I am new to HEC-RAS and working on a bridge in a very steep channel.

Site:

• Slope: 6.23%
• Flow: 489.33 m³/s
• Manning’s n: 0.055
• Bridge: 40 m clear span (no piers)
• Flood width: ~30 m (so bridge is wider than flow)
• No real contraction at bridge

Results:

Subcritical flow:

• Fr = 0.43 to 1.01
• Very unstable results
• High HFL = 1118.24 m
• Large afflux

Mixed flow:

• Fr goes up to ~2.3
• Flow becomes supercritical in reach
• Stable results
• HFL = 1116.31 m
• Afflux = 0

My confusion:

1. Is the high HFL from subcritical run just a numerical issue in steep flow?
2. Can I safely use mixed flow HFL for bridge design?
3. How do you justify mixed flow when reviewers ask for subcritical only?

Thanks for any advice.

https://preview.redd.it/8zyr08tmch3h1.png?width=954&format=png&auto=webp&s=c5f4496a092be6d6ec6f235decfc92e83c74d0ea

I’m new to HEC-RAS and I’m confused about which water surface is normally used for bridge freeboard/clearance checks.

In my bridge results:

• Upstream W.S. = 1299.77 m
• Inside Bridge Upstream W.S. = 1297.84 m

From what I understand, the upstream W.S. is the approach water level before contraction at the bridge, while the inside bridge upstream W.S. is the level within the bridge opening itself.

When measuring clearance/freeboard to the bridge low chord/soffit, which level is usually considered in practice?

Do engineers normally use:

• the approach/upstream W.S.,
• the inside bridge upstream W.S.,
• or something else like EGL in high velocity flow?

This is for a river with relatively low velocity and possible debris transport, so I wanted to understand standard practice.