The goal of the report is to evaluate the dimensioning methods of rain water storage volumes in Denmark. The analysis has been divided in two parts; Analysis of the input used for the dimensioning of rain water storage volumes (Part I), and Analysis of the maximum allowed discharge from the rain water basin (Part II).

The main analysis in Part I is based on the comparison of the two methods describing rain; intensity–duration–frequency (IDF) relationships and historical time series. It is recommended to use a historical time series as it accounts for the linkage of following rain events. When using the IDF-method a factor of linkage is needed. The factor of linkage accounts for the fact that the basin is not necessarily empty before the next rain event occurs. This report states that the factor of linkage is not valid as constant of 1,2, but is exponential dependent on the outlet of the basin. Basins are dimensioned with assumption of constant outlet. This assumption causes too small volumes since the outlet is pressure dependent. An analysis shows that the storage volume will increase with 11-16 % compared to a volume dimensioned with the assumption of a constant outlet. By defining a different return period for the overflow than the outlet it is possible to have a more effective use of the outlet permit. By defining the outlet for a 1-year return period, and the overflow for a 5-year return period, the storage volume will on average increase by 4 %.

Part II contains an analysis of the the hydraulic consequences of the outlet to the receiving streams. The analysis shows that the raise in water level caused by outlet from basins are exponential dependent on the flow in the stream. The water level also depends on the chosen manning-number. The manning-number can be difficult to assess due to natural variations of the vegetation in the streams. Table values for manning-number have been used in this report. An analysis of flooding in six specific streams shows, that the outlet can be larger than 1 l/s/hectare in five of the streams without causing unacceptable flooding. Erosion of the stream bed is a natural incident but discharges from rain water basins can aggravate the erosion.

By implementing the findings in this report the range of possible basin volume at a given location is narrowed and in general lowered. Some receiving streams has a high hydraulic capacity and a larger outlet than the normally used 1 l/s/hectare can possibly be allowed.