Impact Structure Design using Momentum Eqn 1

[Ofy]

I have very little experience with designing energy dissipation devices at outlets to stormwater pipes and culverts. I have found some information, i.e formulae, for using the momentum equation to calculate size of impact structures required to reduce the outlet velocity to a non-erodible velocity. The non-erodible velocity is of course dependent on the downstream vegetation cover etc. So far so good.

There are two(2) formulae to use, one is (hopefully this makes sense as I don't know how to insert symbols here!!):

1) Sum(F)= discharge (m3/s) x fluid density (kg/m3) x change in fluid velocity (m/s).

Sum(F) is the total external force on a defined body of water required to render the desired change in velocity.

2) Sum(Fb) is approximately = sum(0.5 x Cd x S x p x Vo x Vo)

Fb = drag force component on a single impact block of column
Cd = dissipator block/column drag coefficient
S = impact surface area of the impact block
Vo = outlet jet velocity at headwall.

If I know my outlet pipe size, discharge and velocity from the pipe at full flow then the way I would use these formulae are as follows:

a) Calculate Sum(F)
b) Make Sum (F) = Sum (Fb)
c) Therefore can determine S if assume a coefficient value for Cd.

Where I get confused is that in the information it states that Sum(F) is the sum of the hydrostatic forces on the control volume plus the force exerted by the impact blocks of the water.....???! Do I need to calculate this...how do I do this?

Also is the fluid velocity the same as the outlet jet velocity? resaon I ask is that they have different symbols i.e. V and Vo.

Does anyone have practical experience and know how on designing impact structures using the momentum equation?

 

[letrab]

Ofy,
Your first equation calculates the force required to stop the water, i.e. all the momentum is lost.You would use this when calculating turbine forces. This force is consequently the maximum force you have to design for.
I do not think the second equation was derived to calculate the contact surface area, it was derived to allow for instances where some water spray past the block and therefore take cognisanse of the impact as well as drag forces.
Use the first equation, ensure your impact block is large enough to extend beyond the water jet and design the structure to withstand that force keeping in mind it is a dynamic load.

Hope this helps.

 

[cvg]

get a copy of FHA Hydraulic Engineering Circular No. 14 or BUREC Engineering Monograph No. 25 - these will have complete theory necessary for the understanding and design of energy dissipaters. I believe you can get both of these online.

 

[bltseattle]

The references named by cvg are essential reading. They contain design guidance based on extensive empirical performance assessments in addition to theory.