Green Roof Runoff Coefficient

The amount will be dependent on the depth and type of substrate in the green roof system.

Green roof runoff coefficient. V indicate that for most green roof designs there is little to no runoff from small storm events e g 1 2 inches of rainfall. Stormwater runoff discussions of storm water runoff are typically concerned with the impervious surfaces within a certain development or a certain watershed. Why is it important. The mean runoff coefficient of green roofs was 0 58 and 0 75 for dry and wet substrate respectively.

A green roof will typically intercept the first 5mm and more of rainfall providing interception storage. In the summer a green roof can typically retain between 70 80 of the run off. 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. Averaged cn values ranged from 96 to 98 for wet substrate and from 93 to 97 for dry substrate.

Green roofs are one of the most used green infrastructure measures to alleviate the pressure on the urban drainage system through the detention and retention of runoff. Alternatively the runoff coefficient is the factor by which depending on the type of surface the effective impermeable area is calculated. For sample purposes a 55 gallon barrel will be used. However there may be considerable runoff for larger less frequent events.

I the design rainfall intensity for a storm of specified recurrence interval and duration equal to the watershed time of concentration. The average runoff coefficient is in this case equal to 0 45. The other parameters are a the area of a watershed. Also outlined are the steps to build your own rain barrel to collect water from your roof and downspout for all outdoor uses like watering and cleaning.

And q the peak storm water runoff rate due to a storm of intensity i on. 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. One of the parameters in the rational method equation q cia is the runoff coefficient c. For example it is 0 9 for metal or tile roofs and up to 0 3 for green roofs.

As president of green roof blocks i ve been involved in the industry since 1980 am a leed accredited professional and a green roof professional grp. The rational method with the runoff coefficient c is one of the most commonly used design tools for stormwater design in norway. The runoff coefficient c is a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. The system outlined below will walk you through the steps of determining your rain barrel sizes based on average roof runoff.