Added Pressure due to Flow 2

[dik]

Can anyone provide a 'quick formula' that will accommodate the increased pressure on a cofferdam due to the flow?

We have a cofferdam in a river that will have a static head of 18'. If we have a flow of 20 ft/sec, how is the static head affected by the increased velocity?

Dik

 

[cvg]

flow velocity does not increase pressure.

the forces are:

water pressure - which equals the depth of water
shear - caused by water flowing past the dam
thrust - caused by the change in momentum of the water when it changes flow direction (by hitting the dam)
impact - caused by debris striking the dam
bouyancy - equal to the displaced volume of water

 

[dik]

Thanks for the clarification...

Water pressure is the simple one...

By shear, do you mean the shear on the face of the cofferdam caused by the water being diverted laterally? Is there a means of estimating this based on the flow velocity?

Thrust is the one I was thinking about. Is it possible to calculate this based on flow velocity?

Impact has already been accommodated, including ice loading. It is small when considering the length of the cofferdam is in excess of 100'.

Buoyancy is not an issue since the cofferdam is anchored to a 50'+ thick concrete mass.

Can you point me in the direction to estimate the ones that are a direct concern?

thanks, Dik

 

[Drew08]

At least in pipes, the force on a bend is as follows. I would think it could also apply to an open channel situation. The caveat being that there may be other influences in the stream that contribute to velocity changes, not just the coffer dam, and portions of the coffer dam might contribute unequally.

Sum of Vector Force (F) = Density of Water (r) x Flow Rate (Q) x Vector Change in Velocity (V2 – V1)

Vector Arrows above F, V2 and V1, which become important if flow is impinging the coffer dam on an angle and not straigt on.

 

[dik]

Thanks...

Dik

 

[cvg]

shear / drag force is caused by water flowing longitudinally along a surface. without a sketch it is hard to elaborate much more.

you still need to calculate bouyancy, even the concrete base is subject to bouyant forces. And the reduced weight of both the concrete base and the coffer dam will not work in your favor when calculating overturning or sliding

 

[dik]

cvg...

The concrete base is secured to bedrock and it is not possible for water to enter from below and the cofferdam is secured to the concrete base. I don't think that bouyancy uplift is an issue. Water will not be flowing parallel to the surface, but will be impinging on the surface; the river flow is at right angles to the face of the cofferdam.

There are three large piers, one on each bank and a third one at mid stream. The concrete piers are supported on a massive concrete hydraulic structure (the riverbed at that location is mass concrete). The cofferdam frames between a pier at the bank and the one at mid stream. The cofferdam diverts the flow for 1/2 of the river width to the other half of the river. A sketch would not provide added info.

Dik

 

[cvg]

The flow will transition from full width to half width. With the transition, the velocity will increase. this transition will occur over some distance upstream of the cofferdam. in effect, the water directly in front of the cofferdam will stagnate and will likely have no velocity at all (except during installation of the cofferdam, where water will likely flow right over the top). You may experience some swirling action in this stagnant area, hard to predict but can be done with CFD analysis. I would expect that in a concrete lined channel at 20 fps, you will have supercritical flow. Check the Froude number and if it is close to 1.0 or over 2.0 then additional planning should be done for either hydrualic jump analysis or to check for standing waves.
Check to make sure that 20 fps and 18 feet head is the velocity and depth through the restricted channel width.
You might also want to install a temporary training wall to force the flow transition to half width. That might be useful prior to installing the dam.

 

[dik]

The hydraulics engineer has provided a formula:

Force = Cd * v^2 * Ad * GammaW / 2
Where Cd = coef = 1.4
V = velocity of flow = 2.5 m/sec
Ad = area of the cofferdam = 5.49m x 34.3m
GammaW = weight of 1m^3 water = 9.8 kN / m^3

This gives rise to a force = 8078 kN = 1816 Kips = which is equivalent to a pressure of approx 900 psf. Which seems a bit high… I work in Imperial and Metric... the project is Imperial.

Would anyone expect a force of this magnitude?

Dik

 

[chicopee]

The static head will decrease as the cofferdam fills up and obviously the flow rate will decrease also.