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A reference I ine Is shown on the Flooded Areas for Dry Creek. This I ine Is used for reference only and does not neces- sarily coincide with the existing stream channel even though it may be referred to as such in this report. The flood elevations shown on the profile and In table 3 apply laterally from the channel over most of the flood plain width. Road crossings and other topographic features can alter the lateral flood elevations. Depending on whether these features dlvert,pr block flows, the flood elevations at the edge of the flood plain may be higher or lower than at the channel. Flood plain widths vary from one area to another through- out the study reach. The lOa-year flood plain width of Dry Creek varies from a minimum of about 20 feet to a maximum of approximately 1,800 feet and has an average width of about 600 feet. In general, the flood plain continues to widen progressively downstream from Boulder Reservoir. The major portion of the lOa-year flood plain of Dry Creek is in agricultural use. Some areas, however, have been urbanized such as in.The Niwot area. . Obstructions to floodflows - Several roadways cross the flood plain of Dry Creek In the study reach. The physical character- istics of these roadways created conditions in the study reach which resulted in an Increase in water surface elevations immediately up- stream from these roadways ranging from 0 to II feet. The upstream water surface elevations would increase drastically If the bridge area is substantially reduced by debris - especially at those road- ways having high embankments. These higher water surface elevations would flood more area than shown In this ~eport and would increase velocities through the unobstructed portions of the bridges. 24 The profiles on plates 14 through 27 show the extent to which flood stages are increased at the bridges. An appreciable stage Increase occurs on Dry Creek upstream from the combination of Colorado Highway 119 and the Colorado and Southern Railway, Monarch Road, North 83rd Street, North 107th Street, and North I 19th Street. Quite large Increases In stage occur upstream from a county road at reference point 30, North 79th Street, and Nlwot Road. All roadways In the Dry Creek study reach wll I be overtopped by the 100-year flood at some point along the roadway except the northbound lane of Colorado Highway 119, the Colorado and Southern Railway, and Colorado Highway 52. Obstructions such as trees and buildings do exist on the flood plain In the study reach. The obstructive effect Is, however, In general relatively Insignificant because of the sparsity in number. Velocities of flow - Water velocities during floods depend upon such factors as size and shape of the cross sections, condi- tions of the stream, and bed slope; all of these factors vary on different streams and at different locations along the same stream. On Dry Creek In the study reach, the channel velocity for the 100- year flood, In general, ranges from 3 to 10 feet per second. The overbank velocity for the 100-year flood ranges, In general, from less than I to 2.5 feet per second. Rate of rise and duration of flooding - Rate of rise and duration of flooding, for floods comparable to a 100-year or 500- year flood, can vary over a wide range. A flood of rainfall origin on Dry Creek just upstream from the mouth would peak approximately 4.5 hours from the time It begins to rise. This flood would have an approximate duration of from 18 to 20 hours. This flOod would peak at Nlwot approximately 2 hours from the time of initial rise and would have a duration of about 10 hours. 25