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Jeffrey W. Schwarz Massey, $emcnofT, Schwarz, and Bailey Mobile Premix Concrete, Inc. October l0, 2001 Page 3 studies throughout the United States and overseas. Our clients have included the U.S. Army Corps of Engineers, States of Wyoming and Oregon, US Forest Service, numerous municipalities, National Audubon Society, San Luis Valley Water Conservancy District, in addition to private mining and development related interests. My firm includes both professional engineers and geologists licensed in numerous states, including Colorado. Basin Conditions and Hydrologic Regime The South Platte River above the MPC permit area includes approximately 4500 square miles of urban, high plains, submontane and mountainous drainage basin azeas. Land uses within the South Platte drainage basin include residential, commercial development, farming, ranching and wildlife. Immediately within and adjacent to the permit area land uses include extensive gravel mining, farming, grazing and wildlife usage. Dams along the South Platte River provide some flood water storage and flood detention. Gaging data at the Henderson Gage, immediately downstream of the MPC permit area provides approximately 70 years of continuous hydrologic records. As part of this study, Lidstone and Associates, Inc. (LA) completed a Log Pierson Type III transform of the annual peak flow data and determined the approximate flood frequency analysis: 2-year = 5,800 cfs 5-year = 9,200 cfs 10-year = 12,700 cfs 25-year = 18,800 cfs 50-year = 24,900 cfs 100-year = 32,700 cfs The May 5, 2001 flood event of 6000 cfs falls somewhere between a 2-year and 5-year flood frequency. At the location of the breach, the approximate stage height of this flood event is a bank full or potentially higher discharge event. Modeling data suggest that a certain amount of flow may have left the channel reach immediately upstream from this location, resulting in the bank full determination. The 2-year bank full even[ is typically the channel forming or dominant dischazge flood event. Within the South Platte River Basin typical spring flows are generated from snowmelt runoff. Peak flow events during the winter and spring are rainfall on snowmelt or rainfall on frozen soil conditions. Summer thunderstorms are flashy, intense, short duration storms and can result in catastrophic flood events. Prior to the intense development of the drainage basin, the South Platte River was a braided stream. During the 19'" century the river was a wide, shallow sediment dominated river system with multiple channels. As agricultural development and irrigation ensued, riparian vegetation developed along the channel banks resulting in a narrower and more confined floodplain condition. Late in the 19`h and 20'" Centuries, urban encroachment strongly influenced the morphology of the South Platte River. The nature of this influence includes flood water storage, irrigation diversions, channel confinement, channel straightening, establishment of artificial grade controls and bank revetment works.