Weep Hole - Coefficient of Discharge

[emilywalters]

The problem:

A large, mass concrete sea wall with free-draining backfill is subjected to overtopping and is designed to allow for 1.0 m tidal lag (hydrostatic head).

The weep holes / drainage pipes therefore need to be capable of allowing a flow through the wall greater than the overtopping rate (with a 1.0 m hydrostatic head) otherwise the hydrostatic head will build up to a level greater than 1.0 m and the wall will fail.

I have used the following equation to calculate the capacity of the pipes under a 1.0 m hydrostatic head:

Q = Cd * A * √(2*g*H)

Where Cd = Coefficient of discharge, A = Cross-sectional area of pipe, g = Gravitational acceleration, H = Hydrostatic head (1.0 m)

The question:

The weep holes are 250 mm diameter pipes. The pipes are approx. 6 m long. The geometry of the pipes is such that they fall 0.5 m over the 6 m length (from inlet to outlet). What therefore is the coefficient of discharge? Is it even applicable because from my reading it seems to be relevant only to "orifices" - is a 6 m pipe too long to be considered an "orifice"?

More information:

In a previous calculation Cd had been taken as 0.7 - I do not know why and no justification was given.

 

[cvg]

I certainly hope you are not relying on just the weep holes for stability of a sea wall. the wall should be stable even if they are plugged

 

[burrphillips]

There are a lot of questions to that design that seem super hard to assess - what is the rate of overtopping with a storm surge, what will be the tailwater level when the weeps are draining, etc. It seems it would be most prudent to eliminate those questions and design the wall assuming the backside is full of water as the previous poster suggested.

However, to start to answer your question regarding the capacity of the weep drains/lines...
1) You need to check both the orifice capacity (inlet control) and the weep pipe flow capacity (outlet control). This would be very similar to highway culvert design - check both conditions and use the worse case. You Cd is in the ballpark of what I would use (I'd use 0.61) but the equation assumes the pipe inlet is wide open (not partially block with free-draining backfill. If you put the typically "screen and stone" at the weep inlet, the orifice capacity will be greatly diminished.
2) You need to know the tailwater elevation (ocean level when the weep is draining) in order to calculate the pipe flow capacity.
3) Obviously, seawater will flow both ways in the weep line. Consequently, it may make some sense to install duckbill valves (e.g., Tideflex valves) on the outlet of each weep line.

Again - so many unknowns. I'd just design the wall assuming the back side is filled to the brim.

 

[chicopee]

Barnacle build up in the holes for sure.

 

[beej67]

If I understand what you're describing, then the orifice equation is not applicable. The orifice equation is applied to culverts in inlet control on the presumption that there is a vena contracta at the entrance, and the geometry of the orifice plate creates differences in that vena contracta, which lead to different coefficients of discharge. None of that applies to what you're describing, if I understand your situation properly.

I don't routinely design seawalls, but if I did, I think I would make sure they didn't tip over even if the weep holes were all suddenly clogged by oysters and the like.

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