Quick Answer
When calculating runoff for cisterns in urban areas, it's essential to consider factors such as roof size, local rainfall intensity, and stormwater management systems. This requires accurate calculations to ensure the cistern is properly sized to handle the expected runoff. Proper sizing prevents overflow and minimizes water loss.
Determining Roof Area
To calculate runoff for a cistern, start by determining the roof area that will contribute to the runoff. This includes all impervious surfaces, such as roofs, sidewalks, and driveways. Use the following formula to calculate the roof area: Roof Area = Length (in feet) x Width (in feet). For example, a roof with a length of 40 feet and a width of 30 feet would have a roof area of 1200 square feet.
Calculating Rainfall Intensity
Next, calculate the rainfall intensity for your area. This can be done using a rainfall intensity map or by consulting local data. For urban areas, a rainfall intensity of 2-3 inches per hour is common. Use this value to determine the total rainfall amount over a given period, such as 24 hours. Multiply the rainfall intensity by the roof area to get the total runoff volume. For example, a rainfall intensity of 2.5 inches per hour and a roof area of 1200 square feet would result in a total runoff volume of 3000 cubic feet.
Accounting for Stormwater Management Systems
When designing a cistern system, it’s essential to account for stormwater management systems, such as gutters and downspouts. These systems can reduce runoff by directing it away from the cistern. Consider the efficiency of these systems when calculating runoff. For example, if a gutter system has a 75% efficiency rate, you would need to adjust the calculated runoff volume accordingly.
Find more answers
Browse the full Q&A library by topic, or jump back to the topic this question belongs to.