Hydraulic grade line

[wds1]

If you have the Req'd flow Q, diam. D, Length L, manning's "n", and entrance loss Ke, for each pipe length, than you can calc. the head losses thru the pipe. he=Ke*V^2/2g; hf=29*n^2*L/[(D/4)^1.333]; hv=V^2/2g where he, hf, hv are the entrance, friction, and veloc. heads. The req'd head to push the flow thru the pipe = the head losses. Any hv gained thru the pipe sys. is lost at the end of the pipe system. By starting at the downstream water elevations and by adding the losses as we go upsteam we can plot the hydraulic grade line(HGL). This can all be done on a spreadsheet. My question is: Do we need to include entrance losses at each Catch basin. The flow appears to be passing thru the CB with very little loss of speed. It's unlike the entrance to a culvert where the flow goes from zero velocity to pipe flow velocity.

 

[gbam]

FHWA has put out a document "Hydraulic Engineering Circular 22" that has a methodology to calculate the head loss through access holes, ie CBs. I would recommend obtaining that document from their website,

http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm.

This approach uses a series of coefficients to obtain the single coefficient for the structure. It is a bear to go through by hand but one can code a spreadsheet to ease the calculations. Storm Cad and other computer programs have this methodology built into them as well.

GB

 

[cvg]

yes, you should add a loss to account for flow contraction, suggest K = .05. Actually, under higher flows it is very similar to a culvert as you probably will have ponded water in your catch basin with velocity = zero.