Stormwater Quantity Formulas Explained

Understanding how to calculate stormwater quantity is crucial in civil engineering and hydrology for designing drainage systems, bridges, and managing flood risks. Stormwater quantity refers to the volume or rate of water that runs off a surface during a rainfall event.

Common Stormwater Calculation Methods

Several formulas are used to estimate stormwater runoff. Let's look at the options provided:

Fuller's Formula

Fuller's formula is often used in hydrology to estimate the peak flow rate of runoff from a drainage basin. It relates the peak discharge to the basin area and empirical coefficients derived from studies, helping engineers predict the maximum flow expected.

Manning's Formula

Manning's formula is primarily used to calculate the average flow velocity (v) in open channels and pipes, based on the channel's slope, cross-sectional area, hydraulic radius, and roughness coefficient (n). The formula is typically written as:

$$ v = \frac{k}{n} R_h^{2/3} S^{1/2} $$

Where:

• v = average velocity
• n = Manning's roughness coefficient
• R_h = hydraulic radius (Area / Wetted Perimeter)
• S = slope of the channel or pipe
• k = conversion factor (1.0 for SI units, 1.486 for US customary units)

While it calculates velocity, this velocity is then multiplied by the cross-sectional area (A) to determine the flow rate (Q = vA), which is a measure of stormwater quantity in conduits like sewers.

Talbot's Formula

Talbot's formula is another empirical method used for estimating the peak rate of runoff, particularly useful for smaller drainage areas. It provides a way to calculate the peak discharge based on factors like catchment area and slope.

Fanning's Formula and Stormwater Quantity

Fanning's formula, also known as the Darcy-Weisbach equation (when using the Darcy friction factor f), is fundamentally used in fluid mechanics and hydraulics to calculate the head loss (or pressure drop) due to friction along a pipe or conduit.

The formula is generally expressed as:

$$ h_f = f \frac{L}{D} \frac{v^2}{2g} $$

Where:

• h_f = head loss due to friction
• f = Darcy friction factor
• L = length of the pipe
• D = diameter of the pipe
• v = average flow velocity
• g = acceleration due to gravity

This formula focuses on the energy loss during flow, not the direct calculation of the initial *quantity* of stormwater runoff generated from rainfall over a catchment area. While flow velocity (v) is a component, Fanning's formula itself isn't a primary method for determining the overall stormwater runoff quantity like the others.

Conclusion

Therefore, Fanning's formula is not typically classified as a formula for determining the quantity of storm water runoff from a catchment area, unlike Fuller's, Manning's (indirectly), and Talbot's formulas which are more directly applied to runoff estimation.