One of the most outstanding problems within urban drainage is to give a statistically correct description of the runoff effect from areal and temporal distributed rain storms. Due to the large number of parameters for catchments and storms involved in such a description it is important to derive dominating dimensionless quantities by combining the parameters.

The paper deals with dimensionless peak flow calculations using moving block rains. Two important dimensionless parameters p1 and p2 (see Eqs. (7), (8) and (14)) and their influence on peak flow is found in a deterministic way by applying the time area model (TAM) to a linear time-area curve. Dimensionless peak flows are found as functions of p1 and p2 and results are compared with computations from the Storm Water Management Model (SWMM) version 2. TAM overestimated the peak flows by appr. 5% of the full flow capacity relative to SWMM on an average of 26 comparisons. The standard deviation is appr. 10%.

Finally some perspectives are presented in relation to a general statistical runoff description of the influence of areal and temporal rain storms. The possibility is discussed of achieving such a description by combining existing knowledge within 1) synthetic point rainfall design storms, 2) areal rain depth reduction factors, 3) shape of moving storm cell isohyetographs and 4) distribution of storm direction.