As Built Drainage Pipeline Undersized?

Civil & Environmental Engineering gen discussion Storm & Flood
#21

Followup: At the meeting on Tuesday, I had a chance to discuss this situation with the long-time resident who took the recent photo of flooding (shown above). This resident in known to be a reliable observer. His input:

Area of concern floods, to varying extent, an average of 3 times a year.
More importantly, he emphatically stated that water flows up and out of the catch basin on these occasions.

Seems more indication that water is “backing up” in the 24" outfall pipe.

#22

Can you get a reliable maximum depth of the flooding above the catch basin/inlet of the 24" outfall pipe? This should allow you to calculate another, independent flow rate. Then, using the largest flow of the two you have now, estimate the diameter of outfall pipe needed such that it does not run full. Then, no more flooding.

I’ve never done this “not running full” calculation, so have no idea if you will get practical results, or not.

#23

@Latexman - Good idea. I asked the resident for his estimate of maximum water depth; his estimate is 1.5 feet. Also, got his permission to go on his property to the (July 2023) high water mark he described. I have a surveyor’s (antique) wye-level (my father’s pre-World War II) and know how to use it. When we get cooler weather (maybe in a week, or so) can get relative elevations for top or grating and high water mark. Should be able to measure down thru the grating to establish relative invert elevation of the outfall pipe, too. We’ll see how this develops.

1 Like
#24

Hydraulic jump…

#25

I know more about problem solving than about flow rates,but accepting the given information:
Will not the hydraulic jump have more impact on upstream flow rates than downstream flow?
Once the water level is much above the level of the inlet pipes, will not the outflow be more dependant on the head in the catchment?
When there is several feet of head in the catchment, will turbulence due to opposing flows affect the outflow?

  1. If the opposing flows does not much affect the outflow, then a larger outflow pipe should suffice.
  2. If the opposing flows do affect the outflow, consider 90 degree elbows on the inlet pipes so that their discharges are parallel.
    This could be done by moving the catchment so that the 90s are “in the dirt”.
    Or it may be possible to install 90s inside the catchment and possibly extend the catchment.
    An extension of the catchment may be completely below ground so that the original grate may remain in place.

A thought/question; In the case of a pipe discharging into a relatively small chamber at right angles to the discharge, with several head of water in the chamber, does the turbulence caused by the right angle entry affect the discharge rate or is the discharge rate mostly determined by the head?
The answer to this may indicate whether it is advisable to add 90s to the inlets.

#26

It’s something to noodle over, isn’t it?

I would think (idk) if the ongoing or outgoing flow was heavily affected by the hydraulic jump there would be significant turbulence seen in the flood water over the basin. IOWs, the hydraulic jump would be seen. If the flood water over the basin is not turbulent, then not so much.

Has the hydraulic jump been seen? The picture through the screen door doesn’t show it, but . . . it’s not perfectly clear.

#27

I’m still working toward getting elevations of the pipe invert and estimated flood water surface. Tropical storm Idalia passed over the site recently dropping about 6" of rain in 18 hours. However, rainfall was steady over the entire time… no high intensity “cloudbursts”. Result: no flooding reported.