Manning’s Formula

Manning's formula is a valuable tool in designing drainage systems that are efficient, effective, and long-lasting. By accurately determining the friction loss and flow velocity of the water, it helps engineers and drainage experts to design systems that are tailored to the specific needs of each property.

At Yard Drain, we use Manning's formula as a key tool in our drainage system design process. This is particularly important when selecting the type of pipe to use in the system. For example, using the formula, we can accurately determine the necessary slope, flow velocity, and pipe size for a given property, allowing us to select the most appropriate type of pipe for the job.

In general, we recommend using perforated PVC or double wall corrugated pipe for drainage systems. Research has shown that the Manning's roughness coefficient for PVC pipe is approximately 0.009, while the coefficient for double wall corrugated polyethylene pipe is approximately 0.012 (source: "Hydraulic Performance Evaluation of Two Stormwater Infiltration Systems" by Kao-Tsung Tseng and Chyan-Deng Jan). This indicates that PVC pipe has a slightly smoother internal surface than double wall corrugated polyethylene pipe, which can result in lower friction loss and better hydraulic performance in drainage systems.

It's worth noting that higher water velocity through the drainage pipe can result in cleaner pipes, as it helps to wash away debris and sediment. On the other hand, lower water velocity may result in clogged pipes, as debris and sediment can accumulate and cause blockages. By using Manning's formula to accurately determine the necessary flow velocity and pipe size for a given property, we can help to ensure that the drainage system operates efficiently and effectively, without the risk of clogs and blockages.

By using Manning's formula in conjunction with PVC or double wall corrugated pipes, we can create drainage solutions that are effective and long-lasting, providing homeowners with the peace of mind that their properties are protected from the negative impact of standing water and muddy lawns.

Q = Flow Rate (cfs)

n = Manning’s Roughness Coefficient (friction coefficient)

A = Cross Sectional Flow Area (sqft)

R = Hydraulic Radius (ft)

S = Slope of Energy Gradient (ft/ft)