Runoff curve number

The runoff coefficient () is a term used in hydrology and refers to the ratio of the corresponding part of a precipitation event, the right to drain () entering (effective rainfall) and total precipitation ( ).

The difference between precipitation and runoff results from the following reasons:

• At the beginning of a rain event local wells are initially in the ground filled ( wetting and depression loss). This is consistent with the observation that at the beginning of a rain event takes place at all no outflow, the first part of a precipitation event goes down the drain so to speak, for lost
• Part of the precipitation seeps or fills the soil pores on ( seepage loss )
• Another part evaporates (evaporation loss )

The runoff coefficient is used for the design of stormwater and combined sewers, the calculation of flood runoff in rivers on the basis of observed ( historical ) rainfall events and flood forecasts.

The runoff coefficient is dependent on the catchment characteristics (percentage and permeability paved surfaces, soil properties, vegetation, terrain slope, antecedent soil moisture, ice cover ) as well as the intensity and duration of the precipitation event. The effect of area characteristics may vary with rainfall intensity and duration.

In addition to the above-defined ( middle ) runoff coefficient are in hydraulic va another for the planning and design of wastewater systems values ​​in use. The Spitzenabflussbeiwert: This is calculated as the quotient of the maximum rain runoff of surface considered to designed / sized donation.

Another important value is the Endabflussbeiwert. This is defined as the runoff coefficient, which after a prolonged rain event after filling up all temporary storage and losses ( here especially relevant: Dump loss, wetting loss ) results in a constant value - the runoff coefficient of a surface is at the beginning of a rain event equal to zero, it increases with then the storm duration to when, after and filled up all the latches, to finally reach a constant ( maximum ) value: the Endabflussbeiwert. The Endabflussbeiwert achieved for fixed and sealed surfaces such as concrete or asphalt in general the value of 1.0, that is, the total precipitation from flowing. In fortified with partially water-permeable materials surfaces, the value is typically between 0.3 and 0.6. To determine the stormwater fee such runoff coefficients are determined and contrived; it is for example for concrete and natural stone paving, Paving and composite stones 0.6; for grass pavers, gravel or crushed rock surfaces and green roofs 0.3.

In contrast to the runoff coefficient of the drainage coefficient is the ratio of successful precipitation to the measured runoff over a longer period. Therein including the portions of the precipitate are contained, which are initially stored in the field, but access to the drain later. Here, a difference is made ​​only from the evaporation.