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street (Figure II-Ia). If there is no storm sewer all flow will be carried in the street itself, which will act as a channel as shown on Figure II-lb. If depth of flow is measured from the gutter flow line, a depth-frequency curve for annual maxima can be developed for each street section (Figure 11-2). The methodology for development of this curve is a subject of controversy among urban hydrologists because of a number of unresolved problems, but for the purpose of this discussion it is convenient to assume that the relationship has been established. Figures II-lb and II-2 show two basic depths; depth "A" reflects a m,[no!t. flow which is contained within the gutter itself whereas depth "B" shows the entire street flooded, up to the building line. On Figure 11-2, depths A and B are shown as about one and 100 year flows respectively, but this is only a hypothetical example. For an actual case, the relationship shown on Figure 11-2 might turn out to fit some prescribed probability distribution such as, for example, log-normal. If the corresponding probability density function is drawn, the relationship shown in Figure 11-3 results. If a depth is selected on the abcissa of Figure 11-3 (such as depth B), the area under the curve to the right of the depth represents the probability that, in a given year, the depth will be exceeded (exceedance probability). Because of the typical skewed shape of this distribution, it is attrac- tive to delineate the curve into two portions, the frequent depths and the rare depths as shown, The exact point of division is, of course, subject to debate. As an attempt to show the spectrum of benefits of urban drainage and flood control projects, and to relate them to the objectives of the corresponding UDFCsystems, Figure 11-4 is presented, It shows 26