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I I I I I I I I I I I I I I I I I I I 6 FLOOD HAZARDS FLOOD PLAIN AND FLOODWAY DATA The flood hazard area delineation maps show the outline of the 100-year flood event for fully developed basin conditions and present channel conditions. 1st Avenue Tributary: An undersized channel upstream of the tributary's con- fluence with Weir Gulch at Stuart Street is causing flooding even at high frequency storms. Lack of maintenance of the existing channel in all reaches is causing debris to reduce the available cross-section at culvert inlets sometimes by as much as 50 percent. Considerable hazards are associated with the flooding condition at Sheridan Boulevard just north of 1st Avenue. During the 100-year flood, the water may pond as deep as four: feet and cause prob- lems because of Sheridan Boulevard's function as a major north-south traffic arterial. West of Sheridan Boulevard, a 24-inch corrugated metal culvert is the only conduit conveying runoff toward Sheridan Boulevard, causing ponding and subsequent overflow into Sheridan Boulevard. Flood hazards are less severe west of Harlan Street. The maps are supplemented by Tables 4 and 5, "Flood Plain and F100dway Reference Data". The tables contain a listing of potential 100-year flood discharges, elevation of thalweg, elevation of flooding, width of flooding, and velocity of flow for all cross-sections utilized along each tributary. The floodway data presented shows the elevation of flooding, floodway width and velocity of flow based on the most restrictive of the following requirements: 1. An increase in water surface elevation of .5 feet; 2. A mean velocity in the floodway of 4 feet per second; 3. A total depth in the floodway of 2 feet. SUMMARY Dakota Avenue Tributary: At Sheridan Boulevard and immediately east, the main problems to be anticipated are high velocity flows overtopping Sheridan Boulevard at a depth exceeding one foot, and velocities of flow in Alaska Avenue reaching ten feet per second. Traffic congestion and hazard to life appear a real possibility. Between Sheridan Boulevard and Harlan Street, the hazards due to flooding are primarily limited to temporary loss of access to residences on Depew and Eaton Street. The area could be subject to considerable erosion. Between Harlan and Pierce Streets, major problems can be anticipated during a 100-year flood because of ponding, Pierce Street is used quite fre- quently as access to the residential area north of Villa Italia Shopping Center. The residences located south of Alameda Avenue and east of Pierce Street are valued fairly high. Flooding of the magnitude expected during a 100-year event could cause considerable damage because of the long time it may take the water to recede due to extremely flat slopes. The probable limits of flooding due to a 100-year storm occurring over the fully developed drainage basin but with presently existing channels have been identified in this report. The State of Colorado and the Urban Drainage and Flood Control District designate the 100-year floodflow under fully developed basin conditions as the basis for regulatory purposes. The lOO-year flood plain limits and water surface profiles are shown on the Flood Hazard Area Delineation Maps.