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IV-1 SECTION I V FLOOD PROBLEMS REACH 1: CHATFIELD TO COLUMBINE VALLEY This reach, immediately below Chatfield Dam, is characterized by active and inactive gravel pits and a regional floodplain park. The channel in this reach is relatively undisturbed by man. Flood damages that occur are to utility crossings, bridges, gravel pit lakes, and extensive erosion to the river banks. The most significant utility damage would be to the Ken Caryl sewer crossing just below C-470. Damages could be significant to active and inactive gravel mines due to berm erosion and potential failure. topping. It does, however, cause a backup of about 2 feet as the result of inefficient piers and a low chord elevation 4 feet below the 100-year flood. This 2 foot rise, combined with the low banks upstream of the bridge on the left bank results in flooding on the left bank of approximately 50 acres. The area flooded consists of retail stores, warehouses, offices, a nursery, a lumber yard, and small manufacturing facilities, such as welding shops, sign painting shops, glass companies, etc. There are about 35 buildings in the floodplain which receive damage from the 100-year flood. In most cases, the flooding on these buildings is less than 3 feet. The major flood problem is erosion which would cause a breach in the berms protecting and separating the gravel mining lakes resulting in the release of water which could increase peak flows in the South Platte River. On the right bank water leaves the channel, crosses Evans Avenue and flows down South Huron Street and floods a portion of the Overl and Golf Course. No structures would be damaged by this flooding. REACH 3: OXFORD TO BATES In this reach the 100-year flood would be confined to the channel except for a small portion of floodplain at the mouth of Bear Creek. Because this area is currently in a natural state and is expected to remain so, the damages from flooding are expected to remain minimal. Vallejo to Colfax From the Evans Avenue bridge to the 3rd Avenue Dam the 100-year flood is contained within the channel. The 6th Avenue Bridge is inadequate to pass the 100-year flood without overtopping. This bridge causes almost a 2 foot rise in the 1DO-year flood elevation. When combined with the low left bank upstream of the bridge, extensive flooding occurs on the left bank from Vallejo Street to the 6th Avenue Bridge. The area flooded consists primarily of warehouses, retail stores, and small fabricating shops. The area con- tains about 55 acres and about 23 structures. Not all of the structures in this area are flooded. Depth of flooding is generally less than 2 feet. In addition to roads there are railroads which would be affected by flooding in thi s area. REACH 2: COLUMBINE VALLEY TO OXFORD The U.S. Army Corps of Engineers has designed a channel improvement project for this reach of the river. Construction is scheduled to begin in 1984 on the channel improvement project. This project is designed to prevent flood damages outside the channel for the 100-year event. Erosion is the most significant potential cause of damage in this reach. The USACE has two drop structures planned for this reach. There is a narrow channel from about 10th Avenue to. 13th Avenue. The flood- plain is 150 feet wide in some places and the channel is only 13 feet deep. In order to carry the required 100-year flow, a 150 foot wide channel would require a depth of over 20 feet. As a result flooding also occurs on the left bank between 6th Avenue and Colfax. This area covers about 50 acres, and affects approximately 30 structures. The structures located in this floodplain include a Public Service Power plant, public housing structures, a bakery, an ice cream plant, and warehouses. The floodplain ranges from REACH 4: BATES TO COLFAX Evans Avenue The 100-year flood is contained within the channel from Bates to Evans Avenue. The Evans Avenue Bridge will pass the 100-year flood without over