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I I I I I I I I I I I I I I I I I I I Commonly, regulations or ordinances are developed by local governing bodies restricting or limiting floodplain development within the floodway. The computed lOa-year flood elevations at each reference point are tabulated in Tables 6 and 7. A description of the extent of flooding is discussed in the following paragraphs. - FUTURE FLOODS - Sanderson Gulch. In the reach of Sanderson Gulch from the South Platte River to Sheridan Boulevard, channel and culvert improve- ments confine lOa-year flows to a grasslined channel with the following exceptions. Flood Frequency This Flood Hazard Area Delineation Report identifies the flood po- tential that exists today in the Sanderson Gulch Basin. High flows and flooding will continue to occur in the future at a more fre- quent rate reflecting the urbanization of the basin. With the ex- tensive channel improvements in recent years, however, severe flood damage will be minimized even during very large events. A short distance below Lipan Street, a box culvert with lOa-year capacity discharges into the channel immediately upstream of a railroad crossing. The railroad crossing creates a backwater condition, and approximately 800 cfs flows north along and around a warehouse building (Sheet 4). These flows then spread over an area approximately 800 feet wide and continue north to Kentucky Street before spilling into the South Platte River. West of Jason Street, flow depths reach one to three feet. East of Jason Street, depths are shallow, generally less than one foot. There are many residential structures located in this sheet flow area. The discharge information reported in Figures 6 and 7 is provided to guide planning, engineering and floodplain improvements. Future construction activity in the floodplain, such as new road crossings or channel improvements, should utilize these discharges for design, and follow criteria established in the Urban Storm Drainage Criteria Manual (Reference 3). Following the gulch downstream to Platte River Drive, the culvert under this crossing is not adequate to pass flows remaining in the channel. Most of the flow in excess of culvert capacity spills over Platte River Drive in the vicinity of the crossing. A small discharge spills over Jason Street near Platte River Drive and flows north. The lOa-year floodplain has been presented on the flood hazard maps for regulatory purposes and for the general information of local governing bodies and the public. The Urban Drainage and Flood Control District, the Colorado Water Conservation Board and the Federal Insurance Administration have adopted the lOa-year runoff event as the regulatory flood. Colorado recognizes the lOa-year floodplain as an area of state interest as defined in House Bill 1041. At the Florida Avenue crossing, backwater from this culvert spills east along Florida Avenue for 700 to 800 feet before flows are again confined in the gulch (Sheet 5). Approximately ten resi- dential structures are located in this shallow flow area. Flood magnitudes greater than the lOa-year event can and will occur. Land improvement adjacent to the lOa-year floodplain should consider the possibility of flood damage. At Xavier Street, much of the flow in excess of culvert capacity spills over the roadway and back into the gulch. However, Xavier Street slopes to the south carrying some flow to Colorado Avenue, and east along Colorado Avenue over 1,200 feet before the flow is confined by the channel (Sheet 9). No structures are affected by flood flows along this reach. (6) Flooded Areas The lOa-year floodplain limits and water surface profiles are shown on the attached Flood Hazard Area Delineation Maps, Sheets 4 to 19.