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I I I I I I I I I I I I I I I I I I I -, -9- FLOOD PROBLEMS source of flooding is the low bank of the river and the restriction of the 6th Avenue Bridge. FLOODED AREAS In the reach between Sand Creek and York, Street, the floodpl ain is generally confined by manmade facilities including levees, railroad embankments and roadway embankments. Fl oodp 1 ai n wi dths are generally about 200 feet. In the reach from York Street to Interstate 70, there is extensive flooding of the left (west) bank of the South Platte River. The right bank is protected by natural high banks and highway and railroad embankments. In some cases in this reach the floodplain may be as wide as 3,000 feet. Upstream of the 3rd Avenue Dam previous studies have shown flooding of the left bank, but analysis using the plans for the dam as it has recently been modifiedhows that this flooding would no longer occur, An area on the left bank of the South Pl atte River receives flood flows which leave the channel upstream of the Evans Avenue Bridge and return to the river about 1/3 mile downstream. The reach from Cherry Creek to Interstate 25 is subject to extensive flood- ing on the right bank. The floodplain in this area is as much as 3,200 feet wide. Depths, however, are low over most of the floodplain. In the remaining reaches, the 100-year flood would be confined to the chan- nel. From Interstate 25 to 13th Avenue flood water from the South Platte is split by the Interstate 25 embankment. Water enters the area east of Interstate 25 at 13th Avenue, the railroad north of 13th Avenue, and at the Colfax Avenue-Interstate 25 junction, These flows east of Interstate 25 are inde- pendent of flows in the South Platte River. They return to the South Platte River between Interstate 25 and Speer Boulevard. FLOOD PROBLEMS Dams, bridges, and low banks are the causes of flooding between York Street and Interstate 70. A low bank on the left side of the river, in combination with the Gardener's Diversion Dam upstream of York Street, allows water to enter low areas to the left of the river, The Burlington Diversion Dam downstream of Franklin Street causes a significant obstruction to 100-year flood flows. The Franklin Street Bridge is overtopped and causes a signifi- cant rise in the 100-year water surface elevation. In addition, the adja- cent railroad bridge consists of numerous piers which block flow. This rise in water surface elevation causes the left bank to be overtopped in numerous places upstream to Interstate 70. From 13th Avenue to 6th Avenue there is flooding on both sides of the South Platte River. On the ri ght bank water spill s onto Interstate 25 at about 10th Avenue. A low spot in the bank at about 8th Avenue allows water to leave the channel and flow under Interstate 25 where it flows north to join the flows from 13th Avenue. Because of the small flows, flood depths are shallow. On the left bank the floodplain is as much as 1,400 feet wide, The source of flooding is water coming under 6th Avenue west of the river and the low river banks from 6th Avenue to 8th Avenue. Flooding between Cherry Creek and Interstate 25 is the result of the Farmers and Gardeners Dam upstream of the confl uence of the South Pl atte River and Cherry Creek, combined with inadequate channel capacity. From 6th Avenue to the 3rd Avenue Dam major flooding occurs on the left bank. The floodplain in this area is as much as 2,400 feet wide. The Inadequate channel capacity also causes flooding from Interstate 25 to the 3rd Avenue Dam. The problem with inadequate capacity is compounded by the 6th Avenue, 13th Avenue, and Old Colfax bridges which will not pass the 100- year flood.