STORM DRAIN IN A COMMERCIAL PARKING LOT

[chicopee]

I HAVE THREE QUESTIONS DEALING WITH STORM DRAINS:

1) I RECENTLY REVIEWED A SET OF OLD PRINTS OF A STORM DRAIN LAYOUT OF A PARKING LOT ABOUT TWENTY YEARS OLD. I NOTICED THAT THE INVERTS OF THE INFLUENT PIPES TO BE AT LOWER ELEVATIONS THAN THE EFFLUENT PIPES. THIS PRACTICE WAS CONSISTENT AT ALL CATCH BASINS OF THAT PARKING LOT THEREFORE I DID NOT CONSIDER THE SITUATION TO BE A MISTAKE; THEREFORE WHAT APPEARED TO BE THE INTENTION OF THE DESIGNER? I WAS ALWAYS UNDER THE IMPRESSION THAT THE INVERT ELEVATIONS OF THE INFFLUENT PIPE S/B HIGHER THAN THAT OF THE EFFLUENT PIPE.

2)DURING THE DESIGN STAGE OF STORM DRAIN SYSTEM, DOES THE DESIGNER DESIGN PIPE SIZE WITH FULL FLOW IN MIND OR DOES IT DESIGN IT W/ LETS SAY 90% FULL?

3)WHEN INFLUENT AND EFFLUENT PIPES AT A CATCH BASIN HAVE DIFFERENT GRADES AND BOTH PIPES HAVE DIFFERENT SIZES, IS THE HYDRAULIC GRADIENT CONSTANT FROM ONE PIPE TO THE OTHER PIPE AND WHAT HAPPEN TO THE HGL AT THE CATCH BASIN JUNCTION WHERE I WOULD ASSIME FLOW VELOCITY IS ALTERED?

 

[mpetner]

Inverts are probably different to keep sewer gases from surfacing. Kind of acts like a trap, especially when connecting to combined sewers.

 

[jheidt2543]

Is it possible that the step in grade was to allow debris to drop out of the flow? This would mean periodic maintenence at each catch basin, but could it mean a "cleaner" discharge?

 

[chicopee]

To jheidt2543: It is a possibility, however, would you agree the practice is unusual? Also do you think that the practice of two decades ago was not as crucial as in today's climate.

 

[jheidt2543]

I agree the design is unusual and creates a maintenence headache. But, if you think about it, inlet catch basins in most residential areas have a debris sump in the bottom that requires annual maintenence.

However, I think if I had designed a storm sewer with a grade change as you describe, my Sanitary Engineering Prof. would turn over in his grave - RIP.

Today, I would guess the storm line would discharge into a detention area or marsh rather than directly into a water course. Then the debris could settle out before entering and polluting the stream or lake.

 

[mpetner]

Was the main sewer at a higher elevation in relation to the bottom of the catch basins? This type of layout may be necessary because of the local land topography.

Take, for example, three catch basins linked in series that dump into a main sewer. Each catch basin at a higher elevation than the other. During a rain event, the lowest catch basin fills with water and eventually spills out into the effluent pipe. This effluent pipe drains into the next catch basin that is at a higher elevation than the previous. Then this catch basin then fills with water and spills out of the effluent to the third catch basin. When the third catch basin fills, it dumps into the main sewer.

It’s best to understand this if you draw it out. Of course, this only works if the difference in elevation between the lowest catch basin and the main sewer invert is only a couple of feet (depending on the size of the catch basin). This type of configuration allows debris to settle in the catch basin where it can be regularly cleaned, as was mentioned in the previous posts. Also, depending on the specific details of the catch basin, it allows the ‘trap’ action to prevent sewer gases from surfacing.