In this post, we determined the velocity of fluid passing through a rectangular open drain  by using Manning’s formula;                                                                                                         

V             =             RH ^2/3 *So^1/2 / n                

Here;                    

V            =             Velocity in m/sec                                                                              

So           =             Channel bed slope in fraction                                                    

RH          =             Hydraulic radius = Cross sectional area of channel / Wetted parameter                  

n             =             Roughness coefficient                                                                                                                                                  

We further calculate discharge by using  volumetric  flow rate

Discharge capacity of rectangular open drain in m³/sec                   QD          =             A ×  V  

you can also check the adequacy of the drain section then compare the calculated “Discharge capacity of rectangular open drain” to the rainwater runoff calculated in the previous post  “https://knowledgesharingplatform.com/calculation-of-storm-water-runoff”, if Discharge capacity of rectangular open drain is greater than the rain water runoff then drain section is adequate.