Roof Runoff Coefficient

Thus the runoff coefficient must have a value between zero and one.

Roof runoff coefficient.

A runoff coefficient is a number that relates the rainfall rate and runoff rate. There are two ways you can find the runoff coefficient. According to the rational method the widespread implementation of the green roofs facilities causes a reduction of the runoff coefficient of about 20 essentially originated by the increase of the permeable surfaces accordingly to the sustainable stormwater management principles. The average runoff coefficient is in this case equal to 0 45.

Runoff coefficient c this coefficient allows for the quantity of water which runoff a particular type of surfacing into a drainage system and discount the quantity of water which will soak into or through the surfacing. Green roofs can help to reduce the surface run off entering the drainage system. The runoff coefficient describes the percentage of rainfall which runs off a particular surface during a storm event. The runoff coefficient c is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received.

The higher the value the greater the quantity of water which will need to be dealt with by the drainage system. Alternatively the runoff coefficient is the factor by which depending on the type of surface the effective impermeable area is calculated. For example it is 0 9 for metal or tile roofs and up to 0 3 for green roofs. Surfaces with vegetation to intercept surface runoff and those that allow infiltration of rainfall have lower runoff coefficients.

This coefficient accounts for the fact that some roof surfaces are more efficient than others at collecting rainwater. F x p a 4 estimate the rainfall per event in your area r by dividing the annual precipitation rate by the number of rain days per year. Typical values are given below. Rational method runoff coefficient applications.

Using the runoff coefficient scientists and hydrologists can calculate how much water passes over a given area per second. Surfaces that are relatively impervious like streets and parking lots have runoff coefficients approaching one. In summer a green roof can typically retain between 70 80 of the runoff. Green roofs storm water run off.

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. The physical interpretation of the runoff coefficient for a watershed is the fraction of rainfall on that watershed that becomes storm water runoff.