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Calculation results of the urban water balance model indicate that measures change the return periods of runoff volumes by a constant factor. Figure 1 shows the calculation results of several measures applied in an area in Laakhaven (The Hague, The Netherlands). Continuing the example above, where a runoff return period of 2 years is changed into a runoff period of 3 years after applying a measure, the same measure will change a runoff with a return period of 20 years into a runoff with a return period of 30 years and a return period of 0.2 years will change into a return period of 0.3 years. The latter is very interesting, because that means that the effectivity of measures can be tested (in practice) during short periods of time.
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Figure 1 Calculation Simulation results of the effect of applied measures on the return periods of urban runoff volumes for Laakhaven over a 30-year period with an hourly calculation time step
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