## Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

## Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

(OP)

For a preliminary sizing I have looked at using the Chezy Manning's Equation for box culverts.

1. can you use Chezy Manning's on CBC that are multiple barrels? If yes do you sum up all the areas of each individual box culvert? Do you sum up the hydraulic radius of each individual barrel for the total hydraulic radius?

Q=(1.49/n)(A)(R)^.67(S)^.5

1. can you use Chezy Manning's on CBC that are multiple barrels? If yes do you sum up all the areas of each individual box culvert? Do you sum up the hydraulic radius of each individual barrel for the total hydraulic radius?

Q=(1.49/n)(A)(R)^.67(S)^.5

## RE: Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

Peter Smart

HydroCAD Software

www.hydrocad.net

## RE: Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

A = area of flow in one barrel

P = wetted perimeter in one barrel

n = number of barrels

r = n*A/(n*P) = A/P

## RE: Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

for example you have a CBC and assume flowing full

IE 5-8x4 CBC assuming flowing full

A=8*4=32 sf

∑A= 32+32+32+32+32= 160 sf

P = 4+4+8+8=24 feet

r=5*32/5*24=1.33

assuming n=.011, S=.0025

Q = (1.49/.011)(160)(1.33)^.67*(.0025)^.5= 1311.79 cfs

## RE: Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

for instance MDOT says 0.013

https://www.michigan.gov/documents/MDOT_MS4_App_91...

precast.org and INDOT says 0.012 to 0.015

http://precast.org/wp-content/uploads/2014/08/Box_...

http://www.in.gov/indot/design_manual/files/Ch31_2...

## RE: Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

## RE: Hydraulic Radius and Area of Multiple Box Culverts.(Chezy Mannings)

The hydraulic radius for a single barrel is the same as the hydraulic radius for the entire multi-barrel structure. Check the equation - you cannot add the radius for each barrel.

Again, the easy solution is to evaluate the flow for a single barrel and multiply by the number of barrels. Keep it simple.

Peter Smart

HydroCAD Software

www.hydrocad.net