General hydraulics question

[Tjaepp]

Hello all,

Say I want to model a stream with riverbed substrate dominated by large boulders, and want to make a simplified model with trapezoidal shaped cross sections. What is the most correct way to draw the cross sections; does one use the lowest bottom elevation and will a high Mannings number compensate for the parts where boulders block flow, or does one use the average bottom elevation. Further, there must be a “breaking point” where the substrate is so large in relation to the flow that you’d have to draw a cross sections taking into account every single rock, and the roughness is in fact lower since there is mostly a single smooth rock surface against the water, if that makes sense.

I realize this is more a question of the basics of calculating hydraulics. Can anyone point me towards a good resource for learning more about this?

Regards,
EB

[Chris Goodell]

Personally, I use the average bottom elevation. But a lot will really depend on how it’s surveyed. The detail that you pick up in your survey really depends on your study objectives and at what scale you are focusing on. Near-field, localized conditions may require you to pick up a lot more detail on the bed. A reach-wide flood study: you don’t really need that kind of detail. Go with an average bed elevation and adjust the Manning’s n value to represent the type of bed material you have.

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