Flow Velocity in RCP 4

[sdynamic]

The minimum velocity of 2 fps - 3 fps shall be maintained as a self-cleaning velocity for storm drainage system.

In a County, the design criteria for flow velocity is 2 fts and 3 fps for full and half-full flow conditions, respectively.

Does this mean that we can assume full pipes with free outfall, e.g., Full Free Flow Velocity?

Most of the design cases, the actual flow velocity is much less than the above due to the HGLs of the exiting storm drainage system connected.

Is it OK to just assume full pipes with free outfall for the minimum velocity? Or the real velocity in the systems should be considered? I am confused.

Any information or advice would be highly appreciated!

 

[WILDWINGS27]

I typically assume a flowing full condition when setting the slopes of the sewer. In reality, it may be hard to ge a full 3 fps. This is usually the case in the upstream pipes that may have a small tributary area but are being installed to pick up a designed low spot. I never use less than a 12" RCP...so it is ussually difficult to produce enough water to get it up to 3 fps in a 10 year event.

 

[Hoagie]

short, open channels rarely flow at full flow capacity. so don't base anything on full flow conditions. review inlet/outlet control in culverts if you must know why.

for your situation, design for 1/2 full flow at a min 2% and you will be fine. yes, assume no tailwater conditions.

 

[civilperson]

Full flow and one half flow have identical velocities.

 

[cvg]

not necessarily, if flowing under pressure - full flow can have much higher velocities

 

[civilperson]

cvg
I would be amazed at pressure flow design in a storm sewer calculations, where do you live and who is the reviewer?

 

[cvg]

I'm amazed you have not seen a storm drain that is occasionally surcharged.

 

[civilperson]

And you include that in your calcs?

 

[bimr]

Typical stormwater culvert design specification:

Culverts - should have capacity to convey the following:
A. 10-year storm without the headwater depth exceeding the diameter of the culvert.
B. 50-year storm without the headwater depth exceeding one (1) foot over the top of the culvert.

If you have not seen a storm drain that is occasionally surcharged, then you have never seen a storm greater than the 10-year storm.


Here is a reference document:

http://rebar.ecn.purdue.edu/LTAP/Re...2008 STORMWATER DRAINAGE MANUAL - UPDATED.pdf

Assume flowing full in pipe at a design slope:

B. Design criteria and considerations

1. Design frequency--the return period of the design storm is 5 years for residential area.

2. Design flow depth--flowing full in pipe at a design slope, which generally is ground slope.

3. Design velocity

(a) The non-depositing (self-cleansing) velocity for silt and grit is vmin = 3 ft/s (0.9 m/s).

(b) The maximum velocity from scour consideration is vmax = 15 ft/s (5 m/s).

(c) The design velocity should satisfies vmin < vdesign < vmax.

4. Design slope

(a) Generally the ground surface slope.

(b) It may be adjusted to a larger slope than the surface grade from the non-depositing consideration.

(c) Minimize the design grade (slope) for minimum excavation, still meeting the criteria above.

http://te-webserver.cce.iastate.edu/courses/ce372/Course material/SEWER.DOC.

 

[CarlB]

I agree that design criteria which has different minimum velocities for half & full flow is nonsensical, or confusing at the least. If full flow is "gravity" conditions as should logically be assumed, then as civilperson noted it would have the same velocity as half full (unless you consider the well established n-value variation with depth, which shows that half-full velocity is about 80% of full flow). Why would a minimum criterion be set for full, pressurized flow, which could have a range of EGL's & velocities.

A criterion that would better achieve the desired(?) performance would be something like "minimum 2 ft/sec at the 2-yr design flow", though I can't say I've ever seen that.

I would think a more