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I I I I I I, I I I I I I I I I il I . I The upper portion of the basin is moderately steep, rolling terrain. The Croke Reservoir located immediotely west of Huron Street and the various small ponds along the south tributary, which were constructed as part of the irrigation ond stockwoter supply system prior to urbanization, provide some storm runoff peak flow reduction benefits. Inadvertent detention storage behind the Union Pacific Railroad embankment also provides some flood peak reduction during major storms. The soils of the basin are primarily derived by in-place weathering of the underlying shales and sandstones of the upper portion of the Dawson Arkose, although the soils of the lower reach below Riverdale Drive are largely derived from olluvial deposition from the South Platte River. The shales, sandstones, ond conglomerates of the Dawson Arkose Formation, including the Arapahoe Sands, range from approximately 600 feet to over 900 feet thick in the area of the Grange Hall Creek basin. Underlying the Dawson Formation ore the Laramie and Fox Hills Formations. The upper portion of the Dawson Formation dips gently east-southeast; and due to the relotively impervious nature of some of the shale- sandstone layer in a vertical direction, it is doubtful if 0 large omount of recharge to the deeper aquifer occurs in this basin. D. Present Lond Use The present land use in the Grange Hall Creek study area is divided into essentially two ports. The upper bosin (west of York Street) is essentiolly urbanized, except for that portion of Tributory S in Thornton between 104th Avenue and 1-25, whereas the lower basin (eost of York Street) is only sparsely developed. The net result on the overoll basin is thot 54% of the bosin is either undeveloped or in the open space land use category. A summary of the present land use for each land use category and each planning basin is presented in Table 11-2. Examining Table 11-2, we find that 48% of Tributary S, 60% of Tributary NE, and 68% of Tributary N are presently developed, while only 10% of Tributary SE and 37% of Grange Hall Creek main stem area are developed. E. Future Lond Use The probable future land use for the Grange Holl Creek study areo has been determined based upon the existing lond use, current zoning of the undeveloped areas, and from the future land use maps and concepts available for Thornton, Northglenn, and Adams County. Incorporating all the input, a future land use map, Figure 11-3, was prepared, reviewed, and approved by the participoting entities. The areas of existing zoning that were modified according to the future land use concepts available are shown cross hatched. A summary of the future land use for each category and each planning basin is also presented in Table 11-2. When full development of the basin is obtained approximately 20% will remain open space, with a majority of the open spoce along the main stem of Grange Hall Creek below the Union Pacific Railroad. About 60% of the basin will be developed into residentiol oreos, with a significant portion of the development in the upper reaches of Grange Hall Creek and its tributories. In addition to the development aspect of land use, other future land use aspects were considered. The City of Thornton has prepared plans and specifications for the construction of Carpenter Dom immediately west of Colorado Blvd. The effects of the dom on the alternative plans were investigated in Phase A, but after detailed review was deleted as a viable alternative for this study. The City of Northglenn is considering constructing a water supply reservoir at the eastern city limits (NW 1/4 Section 12, Township 2 South, Range 68 West). The Northglenn dam ond reservoir, however, was not evaluated for potential flood peak reductions at this time since details on the proposed configuration were not available. 6