Drainage Flow Calculator

Calculate stormwater runoff, open channel discharge, and full circular pipe flow using practical hydraulic engineering equations.

Rational Method Manning Equation Open Channel Flow Pipe Flow Analysis

Runoff Coefficient (C)

Dimensionless (Range 0.0 - 1.0)

Rainfall Intensity (I)

Catchment Area (A)

Rectangular

Trapezoidal

Triangular

Bottom Width (b)

Side Slope (z) [Horizontal : 1 Vertical]

Flow Depth (y)

Channel Bed Slope (S)

Manning Roughness (n)

Typical concrete = 0.013, earth = 0.022

Internal Pipe Diameter (D)

Pipe Slope (S)

Manning Roughness (n)

e.g., PVC = 0.010, Concrete = 0.013

* Assuming Full-Flow Condition

Peak Runoff Discharge

0.000 m³/s

Formula & Engineering Principles

1. Rational Method

The Rational Method is widely used to estimate peak surface runoff rate for small urban and rural watersheds. It assumes that rainfall intensity is uniform over the catchment area and lasts at least as long as the time of concentration.

Q = C × I × A

Q: Peak runoff rate
C: Runoff coefficient (ratio of runoff to rainfall)
I: Rainfall intensity
A: Catchment area

2. Manning Equation

The Manning formula is an empirical equation estimating the average velocity of a liquid flowing in a conduit that does not completely enclose the liquid (open channel flow) or in a pipe flowing full.

V = (1/n) × R^2/3 × S^1/2
Q = A × V

Q: Volumetric flow rate (Discharge)
V: Cross-sectional average velocity
n: Manning roughness coefficient
A: Cross-sectional flow area
R: Hydraulic radius (Area / Wetted Perimeter)
S: Slope of the hydraulic grade line or channel bed slope

Engineering Disclaimer: The results provided by this calculator are theoretical estimates based strictly on the input parameters and mathematical assumptions (such as uniform flow and full pipe conditions). Real-world drainage design must account for local hydrological data, return periods, freeboard requirements, backwater effects, and local regulatory standards. Do not use this tool as the sole basis for final structural or civil engineering design.

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