Before a rainwater management facility is designed, the storm runoff should be established, which refers to the volumes and rates of flow in individual rainfall events.The actual volume of runoff reaching a rainwater management facility depends on several factors. These include the design storm, the size of the catchment area, the degree of development in the basin (i.e., amount of impervious surface) as well as the the soil surface. It is often called "direct runoff", because it results from surface flow and other immediate responses to precipitation. A runoff gauging station would provide a direct, factual way to observe flows from a site in its existing condition. However, only few sites have gauging stations so that some sort of estimate is necessary. The estimate should be based on data about the site and general knowledge of runoff processes (Urbonas & Stahre, 1993). The factors affecting the runoff such as plant cover are combined in the runoff coefficient, which indicates the percentage of the total water volume actually becoming runoff. The runoff coefficient is also dependent on the slope of the area. For steep slopes as well as for impermeable soils or sealed areas the runoff coefficient has higher values. Table 1 shows runoff coefficients which can be used as approximations

Table 1: Runoff coefficients (Source: Ferguson & Debo, 1990)

For flat slopes or permeable soils use the lower values, for steep slopes or impermeable soils use the higher values.

If the catchment area consists of several areas with different land uses or surfaces, an average runoff coefficient is calculated according to the following formula:

with:

A₁ ... A_n:partial areas in square meters or hectares

C₁ ... C_n:runoff coefficients for the respecitve partial areas

Runoff calculation can be based on following formula:

Q_T = C * I_T * A

with:

Q_T:runoff rate for a T-year storm, in liters/second

C:runoff coefficient, nondimensional

I_T:rainfall intensity for a T-year storm at a storm duration t,

in liters/(second*hectare)

A: area of the catchment area, in hectares

The cumulative volume of rainwater over the storm duration can be calculated by multiplying the average runoff rate Q by the design storm duration t:

V_T = 3600 * Q_T * t

with:

V_T:total runoff volume at time t for a T-year storm, in liters

t:storm duration in hours

The total runoff volume for different design storms needs to be calculated by using different duration-intensity combinations for a given recurrence interval. Then, the largest runoff volume is used for the design of the rainwater management facility.