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I I I I I I I I I I I I I I I I I I I canals would be eliminated and storm flows contained within their respective basins. and is considered to be a matter of statewide interest as defined in H.B. 1041. Floods of greater magnitude than the 100-year event can and will occur and any land development adjacent to the flood plain should take this into account. A. LEE GULCH The 100-year flood plain limits and the 100-year and 10-year water surface profiles for Lee Gulch are shown on the attach- ed Flood Hazard Area Delineation Maps, Sheets 2 through 12. The 100-year water surface elevation, discharge, and flood- way for each cross-section are tabulated on Table II-A. From the South Platte River to the Highline Canal, Lee Gulch is a well defined channel that passes through several parks. Flooding is generally contained in the defined channel except for the backwater caused by the street crossings. There are three homes in the 100-year flood plain. Windermere Street and Elati Street will be overtopped by approximately one foot during the 100-year flood. From the Highline Canal to the Littleton-Arapahoe County border (S. Logan Street Extended), the channel is well de- fined and is generally in a natural state. Just east (up- stream) of the Highline Canal a tributary enters the main channel from the southeast. There are no commercial or residential structures within the flood plain along this reach, though shallow flooding will occur in the parking lot at a building in the industrial park just west of the Littleton city limits and south of Mineral Avenue. The wooden flume carrying the Highline Canal over Lee Gulch will pass the 100-year storm, but the timber truss construc- tion of this structure has a high debris catching potential and may be washed out. Broadway is overtopped by the 100- year flood, with the flow from the South Fork entering the ditch along the east side of Broadway and causing shallow flooding across the street. From the Littleton city limits to the Arapahoe-Douglas COunty border at County Line Road, Lee Gulch is well defined IV. HYDRAULICS The hydrological data was used to determine the 100-year and 10-year water surface elevations at various cross-sections taken along the streams. Cross-sections were taken and digitized by Bell Mapping Co. into the format required by the computer program developed by the Corps of Engineers Hydrological Engineering Center and known as HEC-II, which was used to determine the flood profile. All crossing structures were rated by hand in order to determine the reasonableness of the computer results. Manning's "n" values used in this study varied from 0.023 to 0.035 for the channels and 0.035 to 0.10 for the overbanks. The computed flood profile elevations are plotted on the Flood Hazard Delineation Map Drawings and the 100-year elevations are tabulated on Tables II.A. and II.B., with the 100-year flood- way information. The floodway is defined as the area required to pass the 100-year flow with a maximum of a 0.5 foot rise in water surface elevation, or less than a 2 foot depth of flow, or less than 4 F.P.S. velocity, whichever is more restrictive. All cross-section information is presented looking upstream with the thalweg being defined as the lowest point within the flood plain. V. FLOODING POTENTIAL Floods have occurred and will continue to occur along Lee Gulch and Little Creek. The potential for flooding will increase as the basins continue to develop and as storm flows are separated from irrigation flows. The lOO-year flood event, which can be expected to occur anytime, is that which has a one' percent chance of being equaled in any given year. The lOO-year flood plain is that which is considered by the Urban Drainage and Flood Control District to be designated for regulatory and improvement purposes -5-