Added Pressure due to Flow
Added Pressure due to Flow
(OP)
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
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





RE: Added Pressure due to Flow
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
RE: Added Pressure due to Flow
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
RE: Added Pressure due to Flow
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.
RE: Added Pressure due to Flow
Dik
RE: Added Pressure due to Flow
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
RE: Added Pressure due to Flow
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
RE: Added Pressure due to Flow
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.
RE: Added Pressure due to Flow
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
RE: Added Pressure due to Flow