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. I .. I I 1 I I . . I I I I I I I . I Two different conditions were evaluated to determine the flood way limits. The first condition was a maximum one-half foot rise in base flood energy grade line or water surface elevation as a result of encroachment. This is a standard for Urban Drainage and Flood Control District projects. In addition to the one-half foot rise flood way, a one-foot maximum flood way rise was also determined, which is consistent with the City of Arvada's municipal floodplain management ordinance. The floodways were determined using encroachment Method 5 and Method I of the HEC-RAS program. This method as applied herein sets left and right encroachment targets based upon equal conveyance reductions and adjusts them until the target level of a 0.5 or 1.0 foot rise in the 100-year energy grade line or water surface elevation was achieved. The f100dway statistics for both the half-foot and one-foot rise floodways are included in Tables 3 and 5. Table 2 and Table 4 present the results of the 10-, 50-, and 500- year flood elevations for Ralston Creek and Leyden Creek respectively. The tables include flooding source identification, cross section number, station number, design flood discharges, and resulting water surface elevations. Tables 3 and 5 present the 100-year floodplain data along with the 0.5 and 1.0-foot rise f100dway data for Ralston Creek and Leyden Creeks, respectively. The thalweg information for each cross section is included in these tables. The floodway data includes the distance to the left and to the right of the thalweg, along with the total width and the floodway water surface elevation. Tables 2 through 5 are intended to provide regulatory agencies with the information necessary to apply floodplain regulations. By providing this information, agencies are in a position to consistently regulate floodplain development. The tables can also be used by developers or property owners in evaluating the hazards and constraints associated with sites along Ralston and Leyden Creeks. 4.7 Flood Velocities Mean velocities in the floodway generally range between 3 and 11 feet per second (fps) with flow velocities in the channel exceeding 11 fps at certain locations. Velocities in the overbanks typically generally range between 0 and 6 fps; sometimes exceeding this range at certain locations. Velocities in excess of 6 to 8 fps can cause severe erosion of the stream banks, and are capable of transporting large rocks and debris. Velocities in this range can also erode fill around bridge abutments, potentially leading to bridge failure. Lower velocities of approximately 2 fps or less will deposit debris and silt. 12