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I I I I I I I I I I I I I I I I I I I - FLOOD HISTORY - Some localized flood damage along portions of Big Dry Creek upstream of the Study Reach is described in the Flood Hazard Area Delineation Report, Big Dry Creek and Tributaries, Westminster Area (Reference 3). Several sources were consulted for information concerning floods along the Study Reach of Big Dry Creek (Reference 12, 13). These sources included newspapers, historical societies, various libraries and residents. Unfortunately, little flood information was available regarding Big Dry Creek. Several factors contribute to this lack of information. A primary factor is the attenuation of flood peaks caused by several reservoirs that have been constructed along Bi g Dry Creek and its tri butari es. Standl ey Reservoir is the largest, collecting runoff from 16.8 square miles of watershed. Another is the Great Western Reservoir with 5 square miles of watershed. Numerous smaller reservoirs also exist along the tributaries to Big Dry Creek. Since the natural floodplain evolved from a drainage basin free of man-made reservoi rs, the current attenuated fl ood peaks are 1 ess damaging in the natural floodplain. - HYDROLOGY AND HYDRAULIC ANALYSES - Hydrologic Analysis The undeveloped character of the Big Dry Creek floodplain is another factor affecting the availability of flood history. There are no significant developed land uses adjacent to the channel. Consequently, moderate floods of the past may have damaged cultivated fields, but would not have affected valuable buildings. This flood nuisance has rarely reached the newspapers or been remembered by residents several years later. The objective of this portion of the study was the development of flood hydrographs and associated peak flows which could be expected to result from both la-year and lOa-year flood events in the Big Dry Creek watershed, under the assumption that the land surfaces would be fully developed to ultimate future conditions. A separate study, as reported in Reference 3, was in progress at the writing of the analysis reported herein. Reference 3 presents a complete analysis of flood hydrographs and associated peak flows on Big Dry Creek, upstream of Interstate 25 (the upstream limit of the Study Reach for this report). The hydrologic analysis reported herein has resulted from a combination of the analysis of Reference 3 and independent computat ions of storm runoff from the 26.2 square mi 1 es of watershed directly tributary to the Study Reach. The fact that floods have occurred along Big Dry Creek was verified by area residents. A past Water Commissioner for the area remembered some flash floods, but could not recall significant damage resulting from the floods other than bank erosion. A long-time farmer along Big Dry Creek recounted how his cultivated fields along the creek would flood two or three times during a la-year period. Flood depths over the fields would reach several feet. If a field proved to be particularly flood prone, it was returned to native pasture for livestock grazing. The farmer also could not recall significant damage caused by flooding. Figure 1 illustrates the portion of the Big Dry Creek watershed which is directly tributary to the Study Reach, between Hydrology Design Points A and F. The addit i ona 1 watershed area of 39. 2 square mil es upstream from Hydrology Design Point F is illustrated in Reference 3. Figure 1 also illustrates the boundaries of drainage sub-basins and land surface characteristics which are projected as future development. One-hour point rainfall depths were determined from the Rainfall Chapter of the Urban Storm Drainage Criteria Manual (Reference 4) for the la-year and lOO-year return periods. These values were used to develop the six-hour design storms based upon a time distribution of rainfall recommended by the Manual. Area reductions of rainfall for 30 to 50 square miles and for 50 3 ~