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2/90 events. This creates a conflict between local SCS hydrology guide and the WMOs recommendations. 4. The Type II distribution appears to be conservative for, design of urban facilities. It is based on the assumption that the 30- minute, 1 -hour, 2 -hour, 3 -hour, 6 -hour, 12 -hour, and 24- hour rainfall amounts for a given recurrence interval will occur in the same rainfall event. The probability of this occurring would be less than the recurrence interval it is modeling. For example, a 100 -year rainfall event designed with the Type II storm would have a probability of recurrence that is less than 1 percent. 5. In the Twin Cities, there are several types of detention facilities. They range from small ponds, to wetlands with large surface areas and shallow depths, to large open bodies of water. The use of an arbitrary freeboard is of concern. For a given rainfall event, a two foot freeboard will not provide the same level of protection for all facilities. Conveyance systems are typically designed for a five or ten - year recurrence interval. Therefore, inlets such as flared -end- sections or catch basins do not allow for full interception. This inadvertent detention is usually ignored in modeling large detention basins. The effect of each is to reduce the peak rate of runoff to the main detention facility and thus reduce the required storage. DS /FES CONCEPT All models require input of actual rainfall data or rainfall data derived from a synthetic distribution. The DS /FES concept is intended to be flexible so that it can be used for any model, storm distribution, or with changing methodologies. The DS and FES can be determined by actual rainfall events or sensitivity analysis of synthetic storms. The DS and FES should be reanalyzed to match new, available data and methodologies. DS /FES FOR ROSEVILLE SWMP The City of Roseville, Minnesota,. Storm Water Management Plan will be completed in 1988. As part of the study, the DS /FES concept was applied. Several recent large rainfall events have demonstrated that the Type I analysis used in the design of detention facilities has accurately predicted runoff. In small areas without detention, the Type II typically will indicate discharge rates approximately twice as large as those from a type I distribution. However, in large watersheds with available storage, the difference between the Type I and Type II storms is almost negligible. • •