How is the equivalent drainage area for site drainage calculated?

The equivalent drainage area for site drainage is calculated by multiplying the actual area in acres by the runoff coefficient. The runoff coefficient is a crucial factor in this calculation because it accounts for different surface types and their ability to absorb or repel water. Various surfaces, such as asphalt, concrete, grassy areas, or roofs, have different rates of runoff due to their distinct physical and hydrological properties. For example, impermeable surfaces tend to have higher runoff coefficients because they do not allow water to infiltrate, whereas permeable surfaces, like grass, have lower coefficients as they can absorb more water.

In practical terms, using the runoff coefficient in the calculation ensures that the equivalent drainage area accurately represents the potential runoff that can occur from a particular site under typical storm conditions. This is essential for designing effective drainage systems that can adequately handle stormwater and prevent flooding or erosion issues.