Hello,

As per my design, the storm water discharge from 35 acres of residential complex is disposed to a lake/wet land which is adjacent to the residential complex through 4 numbers of 1.2 m diameter (Cross-sectional area 4.5 sq.m) NP2 class Hume pipes. 

But the local authority is demanding to dispose the water to the same lake through 1 number 3.0m x 1.0m box culvert (cross-sectional area 3.0 sq.m) and 4 numbers of 1.0 m diameter (cross-sectional area 3.14 sq.m) NP2 class Hume pipe instead of 4 numbers of 1.2 m diameter NP2 class Hume pipes.

The total cross-sectional area as per the authorities is 3.0 sqm+3.14 sqm = 6.15 sqm. As per my design the final flow at the outlet chamber is around 3.93 cum/sec and for this flow of 4 numbers of 1.2 m diameters are sufficient.

The engineers of the authority claim that the cross-sectional areas of the pipes should be equal and not the discharge. 

My question is how could I explain that the discharge is dependent on velocity and cross-sectional area of pipe and not only on cross-sectional area?