Runoff concentration is calculated by means of a linear reservoir. For combined and separate sewer system, it contains catchment runoff and sewer system flow down to the overflow tank. For the natural catchment, it contains catchment runoff and channel routing down to the calculation point in the receiving water where the overflows of combined and separate sewer system are located. The following differential equation must be solved in each timestep Δt:
mit
Q Δt k A h |
runoff resp. discharge [m3/s] timestep [s] reservoir time constant [s] contributing area [m2] effective or net rainfall [m] |
The unknown parameter of this equation, the reservoir time constant k, corresponds to the retention time of the water in the reservoir. k is approximately equal to the mean flow time (=entry time + flow time in the sewer system) in the regarded system.
Spk: reservoir time constant