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I I I I I I I I I I I I I I I I I I I additional flow could be directed to the west side of the valley from the main channel below Marshall Road. Upper and Middle Flatirons Levee Area (Drawing 5) The distinguishing features here include the upper portion of the levee which forces water back to South Boulder Creek, the middle levee section which sustains this deflection of flow, the adjacent Dry Creek Ditch No.2 which conveys the ditch flow and keeps some of the west bank floodplain flow from going back to South Boulder Creek. If the upper portion of the levee were to be breached or overtopped, most West Valley flow would continue on the west side to the approximate location of the Viele Channel west of the Foothills Parkway and Highway 36 Interchange. If the middle portion of the levee were to be breached, we would expect a smaller increase in spillage to the interchange than if the upper levee breached. The Middle or Hiahwav 36/South Boulder Main Channel Splits (Drawings 5 and 6) Most of the flood flow in the West Valley Overflow drops back into South Boulder Creek by the Middle Flatirons Levee (see element 1513). due to lack of West Valley Overflow carrying capacity and the lower elevation of South Boulder Creek. For the 1 OO-year event, the model shows the majority of the flow in South Boulder Creek (approximately 1800 feet upstream of Highway 36). Analysis has shown significant 1 OO-year spillage from South Boulder Creek back to the West Valley Overflow to begin about 500 feet upstream of Highway 36. (See element 261.) This spill is controlled by several components including: 1) the Highway 36 main stream bridge and the adjacent downstream McGinn Ditch Diversion, 2) the topography in the actual split area which is approximately 180 to 430 feet west of the creek centerline, and 3) the West Valley Overflow floodplain hydraulics and water surface profile which is controlled by the topography and roughness characteristics from the split area west for about 1200 feet. Aggradation of the main channel area could lead to degradation of the open space area and increased flows to the West Valley. This aggradation could potentially be caused by upstream sediment movement and limited transport capability of the bridge area channel hydraulics and the McGinn Ditch, or by erosion of the overbank spill zone. Permanent transfer of the main channel to the West Valley is possible, but not likely. However, some increase in spillage appears likely with time given present conditions. II - 5