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the benefits as a spectrum in the sense that Figure 11-3 showed a I spectrum of runoff events. Figures ~1-3 and 11-4 are therefore related through UDFC projects which would seek to control runoff events of different frequencies. This spectrum of benefits would at first glance, appear to be cumulative. That is, if a UDFC project is built to control a rare runoff event it automatically will control ~esser flows. This is not necessarily true, For example, a major runoff project which prevents damage does not automatically provide con- venience, although this feature can be built into the project, Figure 11-4 can be expanded somewhat into a classification of benefits and costs from UDFC projects as shown in Table 11-3. The Problem of Repetitive Occurrences Figure 11-4 lists certain project benefits that might be experienced each time a small flow occurred. Examples of these are increased con- venience and reduced traffic delays, Figure 11-3, a probability distri- bution for annual maxima, provided no way to consider repetitive occurrences of runoff events of less than one year return period. A basic dilemma for storm drainage criteria which specifies a "2-year design" or a "S-year design" is how to consider this problem. It is conceivable that two regions, one semi-arid and one humid, could have identical depth-frequency relations such as Figure 11-3, but that the humid region could have a hundred light rainstorms for each one on the semi-arid area and that the convenience benefit for the storm drainage system in the humid area would be correspondingly greater. The Problem of Intensity of Use and of Land Value Figure 11-4 shows benefits which will vary with intensity of use of streets and with adjacent land value. Using convenience again, this 30