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):~~)t~ ENGINEERING CONSULTANTS, INC. Mr. L. Scott Tucker July 11, 1973 - DG 855/73 Page Two Rainfall and Runoff The rainfall and runoff studies are described in Chapter III. We used the Colorado Urban Hydrogroph procedure and the Urban Storm Drainage Criteria to estimate 10-, 25-, 50-, and lOO-year storms. The design rainfall for the lOO-year vollv,1 frequency is a two-hour storm, with a total rainfall of 2.25 inches. The Niver Creek Basin is composed of the main stream and two major tribu- taries; we further subdivided the basin into 15 small sub-basins and computed the hydrographic characteristics of each. Then, we routed the storms through the sub- basins, using the discharge characteristics of the existing drainage features and the conveyance characteristics of the intervening channels. These computations were made for both the existing conditions and for future conditions, assuming full devel- opment. ; At present, the basin is about 35 percent impervious, but that value will increase to more than 50 percent in five to ten years, according to development pro- ""c.,,~ \: jections by the officials of Federal Heights, Thornton and Adams County. This in- crease in impervious area will cause a corresponding increase in the peak discharge from the same stonn; for example, a IOO-year rainfall today will produce a peak discharge of about 1,800 cfs at 84th Avenue, while the same rainfall ten years from now will probably produce a peak discharge of about 2,700cfs at the same location. We emphasize this point to illustrate that a IOO-year flood is not a static event, but will be worse tomorrow than it is today. - Flooding and Damages The flooding and damage estimates are given in Chapter IV. We estimated the flooding and damage under a 'do-nothing' alternative, in which the drainage facilities of the basin would remain unchanged and would be restored to their present condition after each storm. The long-term average annual damages under such a pro- gram would be about $316,000 per year. - . At first glance it may seem absurd to compute the cost of keeping the basin drainage unchanged, because improvements will invariably take place, especially after major disasters. We have prepared this 'do-nothing' estimate nevertheless, because it provides a good reference point, or bench-mark, to measure the effec- tiveness of capital improvements, or 'do-something' alternatives. A PLANNING RESEARCH CORPORATION COMPANY