Flood Volume in Storm Sewer Network

[kolkata321]

Hi All,

I have created a spreadsheet model to calculate the hydraulic capacity of storm sewer using mannings equation. The spreadsheet can calculate HGL for a system. I am interested to calculate the flood volume from the system(volume excess of the system)to quanity the amount of spill. Please can anyone advice me in right direction.

 

[RWF7437]

If you know the size, shape, and length of all the conduits in the system it is easy to calculate the total volume, assuming they are all full. Whether or not is is possible for them all to be full simultaneously depends on other factors, such as slopes, entrance and exit losses and elevations at points of overflow.

But........ why would you want or need to know the volume of overflow (spill)? How would you use this number ?

 

[beej67]

Mannings equation is only valid for open channel flow. Any time your pipe is in a pressure flow situation (full pipe at capacity, possible flooding) Mannings equation is no longer valid.

Something to think about while you tackle this problem.

 

[RWF7437]

Manning's Equation is valid for pressure flow if you use the slope of the hydraulic grade line, rather than the slope of the invert or the slope of the free water surface.

 

[jgailla]

The Manning's slope is the slope of the HGL, not the slope of the pipe.
The invert slope is used in open channel flow because in a long channel with constant geometry the slope of the invert is the slope of the HGL.
In general the HGL slope should be used, not the pipe slope.

 

[puzzler77]

The volume of a flood is a different animal. Are you referring to the Ratioanal method (Q=CIA)? This method only gives a peak flow rate and is not valid for large basins (>150 Acres). I use the SCS method with a software such as the USACE Hydrologic Engineering Center's Hydrologic Modeling System (HEC-HMS) for obtaining a hydrograph and volume of runoff from a basin. TR-55 is another "free" public domain software developed by the Soil Conservation Service (now Natural Resources Conservation Service).