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<br />Afternoon clou?bursts have created flooding situations <br />in the past but three successive floods have created <br />severe erosion problems to occur in a short period <br />of time. <br /> <br />Debris from the open flume in Central City reached <br />the covered flume in Black Hawk and quickly became <br />a tangled mess. Gregory Street buckled and broke as <br />the flood waters pushed the cement cover from its founda- <br />tion as the flume filled with timbers ... Terry Aten <br />of the Black Hawk - Central City Sanitation District <br />reported that a section of sewer main and a manhole <br />had been exposed in Black Hawk. Aten said the plant <br />... received little damage .... <br /> <br />2.4 Flood Protection Measures <br /> <br />No flood-control structures exist or have been authorized in the <br />basin and none are under investigation. There are some water <br />diversions from the waterways for various purposes, but they are <br />small in quantity and are often curtailed during high water, the <br />result being that there is a negligible impact on flood peaks. <br /> <br />Black Hawk has adopted local ordinance Nos. 75-6 and 75-7 dealing <br />specifically with flood protection (Reference 4). <br /> <br />3.0 ENGINEERING METHODS <br /> <br />For the flooding sources studied in detail in the community, standard <br />hydrologic and hydraulic study methods were used to determine the flood <br />hazard data required for this study. Flood events of a magnitude which <br />are expected to be equaled or exceeded once on the average during any <br />10-, 50-, 100-, or SOO-year period (recurrence interval) have been selected <br />as having special significance for flood plain management and for flood <br />insurance rates. These events, commonly termed the 10-, 50-, 100-, <br />and SOD-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, <br />of being equaled or exceeded during any year. Although the recurrence <br />interval represents the long term average period between floods of a <br />specific magnitude, rare floods could occur at short intervals or even <br />within the same year. The risk of experiencing a rare flood increases <br />when periods greater than 1 year are considered. For example, the risk <br />of having a flood which equals or exceeds the 100-year flood (1 percent <br />chance of annual exceedence) in any 50-year period is approximately <br />40 percent (4 in 10), and, for any 90-year periOd, the risk increases <br />to approximately 60 percent (6 in 10). The analyses reported herein <br />reflect flooding potentials based on conditions existing in the community <br />at the time of completion of this study. Maps and flood elevations <br />will be amended periodically to reflect future changes. <br /> <br />10 <br />