Flow Measurement through Trapezoidal & Rectangular Channels

The basic formula for channel capacity is

Q = A x V

Manning's formula for velocity is   

By combining these two formulas , channel capacity or quantity of flow can be determine by 

where              

Q = Quantity of flow in cubic meters per second                     

V = Mean Velocity in meters per second                                  

A = Flow area in square meters                                                

P =  Wetted perimeter in meters                                               

R = Hydraulic radius in meters = A/P                                     

S = Slope or energy gradient in meters drop per meter length

n = Roughness coefficient                                                       

Trapezoidal Watercourse/canal or Channel

Flow measurement or hydraulic formulas are used to measure the discharge/volume of water through canals,irrigation channels,watercourses,drain lines etc. Following is the brief descripion used to calculate the flow measurement or discharge of the trapezoidal sectioned channel.

b = bottom width                                                                              d = flow depth              

B = width at water surface = b + ( 2 x z x d )                                  FB = Free board

D = Total depth of channel = d + FB                                               BW = Bank width

Z = Side slope, ( Such as 1: 1 - 1.5 : 1  & 2 : 1)                              FSL = Full supply level

TW = total width of the ditch = b + ( 2 x Z x D)

So formula for trapezoidal channel flow quantity is

 A = b x d + Z x d²

Rectangular Watercourse/canal or channel

A = b x d

where b is the width of the channel and d is the depth of the channels.