# How do you calculate the runoff coefficient?

## How do you calculate the runoff coefficient?

How to Calculate Runoff Coefficient

- The Rational Method is used for areas less than 50 acres.
- Find the appropriate values for each variable.
- Calculate the value for “i.” This is a measurement of rainfall intensity.
- Isolate and solve for “C.” Your altered equation should look like this:

**What is fraction impervious?**

In hydrological science, the most important surrogate measure of urbanisation is the fraction of impervious surfaces, which includes pavement, roof, paved parking, driveways and other sealed surfaces. Impervious surfaces are either hydraulically connected to stormwater pipe inlets or separated by pervious surfaces.

**What does a low runoff coefficient mean?**

The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land).

### What is the value of runoff coefficient?

Normally, runoff coefficient is applied to storm events, but it can also be used for monthly and annual rainfall and runoff values….Definition.

Type of area | Runoff coefficient |
---|---|

Commercial and industrial | 0.70–0.90 |

Parks and pastures | 0.05–0.30 |

Pavements and roads | 0.70–0.95 |

Flat agriculture | 0.10–0.50 |

**What is the runoff coefficient of an impervious surface?**

Impervious surfaces have runoff coefficients greater than 0.80 based on Table 1. These are hard surfaces which either prevent or significantly retard the entry of water into the soil mantle.

**What is the composite runoff coefficient in Table 1?**

The composite runoff coefficient is the weighted average of all of the land uses within the drainage area. Table 1 presents values of runoff coefficients for various surfaces. The Rational Method is most applicable to drainage areas that are 20 acres or less.

## How to calculate the volume of stormwater runoff?

The Simple Method was developed by measuring the runoff from many watersheds with known impervious areas and curve-fitting a relationship between percent imperviousness and the fraction of rainfall converted to runoff (the runoff coefficient). It uses a minimal amount of information to estimate the volume of runoff.

**What is the rational method for runoff factors?**

1. Review the runoff factors “C” values for the Rational procedure specified in the current Design and Procedure Manual. 2. Review other references. 3. Review the more detailed land use elements that are identified in the County general Plan and estimate the percent of area that is impervious associated with each. 4.