Coefficient of Runoff

The impermeability factor is also known as the coefficient of runoff. It is a measure of the ability of a surface to absorb water. In other words, it is the percentage of rainfall that runs off a surface instead of being absorbed into the ground. The coefficient of runoff is an important factor in hydrology and civil engineering as it helps in the design of drainage systems and stormwater management.

Calculation

The coefficient of runoff is calculated by dividing the amount of runoff by the amount of rainfall. It is expressed as a decimal or a percentage. For example, if 1 inch of rainfall produces 0.5 inches of runoff, the coefficient of runoff would be 0.5/1 = 0.5 or 50%.

Factors Affecting Runoff

The coefficient of runoff is affected by various factors such as:

- Surface type: Different surfaces have different abilities to absorb water. For example, concrete is impermeable, while grass is permeable.
- Slope: Steeper slopes produce more runoff as water flows downhill faster.
- Vegetation: The presence of vegetation can increase the absorption of water by the surface.
- Soil type: The soil's ability to absorb water affects the amount of runoff produced.

Importance in Civil Engineering

The coefficient of runoff is an important factor in civil engineering as it helps in the design of drainage systems and stormwater management. Civil engineers use the coefficient of runoff to determine the size of drainage pipes and the capacity of stormwater detention ponds. It also helps in the design of green infrastructure such as rain gardens and bioswales, which aim to increase the absorption of water by surfaces and reduce the amount of runoff.