FLOOD03848 - Laserfiche WebLink

Home Browse Search

I if a I I I ; m I I I a m H I I I I I <J 44 I I I I I I I I I I I I I I I I I I I VII. EFFECTS OF CHANGES ~ significant rise due to backwater. 3. Inadequateopenfngs under some brldges also cause serious backwat er dur1ngfloods. Forexample.thecalculatedbackwaterforthe lOO-year urban flood peak passing over and under the Florida Avenuebrlc1ge(RiverStation1537+S0)1sapproximately5feet. 4. At the present time, vegetation does not appear to influence the water levels of the large floods. The vegetat10n 1s lIsually low and sparse and is anticipated to remain so. 5. Any stralghtenfng of the river channel should improve the flood- carrying capacity of the river. 6. The slight decrease in riverbed slope would reduce the flood-carrying capacfty of the river. However, without bed-control structures, the net effect of degradation and reduction in bed slope is anticipated to be a reduction 1n flood stages where there are no levees. In summary, the expected changes in the South Platte River in Metropolitan Denver due to past, current and anticipated future developments areas follows. 1. In the Urban reach, only minor degradation of the river bed is anticipated. It is presumed that any bank failures or damage to riverbed control structures will be repaired immediately. 2. In the Suburban reach where the Corps of Engineers are going to excavate a realigned river channel and stabilize its banks and bed, it is anticipated that a set of bars will develop in the reach. If the bars are at inappropriate locations it may be difficult to main- tain the bankline in its initial location. 3. In the Rural reaches it is anticipated that, in the long tenn, the river would widen, become more shallow, straighten slightly, decrease its slope, and degrade and armor its bed in subreaches where the river is not controlled. In other subreaches where the banks are leveed, and stabilized and the channel narrowed by filling and pro- tectingthefil1 from erosion, the river will be narrower, deeper, straighter, flatter and will have a coarser gravel or cobble bed. Man-Made Structures ,"d The undesired effects of the changes on the man-made structures in adjacent to the river include: Flood-Carrying Capacity The effects of these changes on the flood-carrying capacity of the riverincludethefollow1ng. 1. Where the river is leveed, confining all the flood water to the main channel causes increased degradation of the channel, higher flood levels. additional stresses to bridges, and larger flood peaks down- stream. Where bridges sit on pilings, driven into the Denver Formation. bridge pier scour and scour due to the contraction of the flow are not factors for consideration: however, repairs will be required to lower piling caps where the pilings become exposed to flow. Any bridges sitting on spread footings would be susceptible to failure from underscour. 1. The uncover1ng of buried pipelines, sewers and other conduits crossing the river due to degradation. 2. The undermining of bridge abutments and sewer and storTl'P<iaterdrain outlets along the river due to degradation or river bank erosion. 3. ThedamagingorlossoffacilitiesontheriverbankduetobiJ.nk erosion. One beneficial outcome of allowing the downstream Rura1 reach to degrade is that it would be much easier to drain the low-lying floodplain back from the channel. Some of this land is 2 to 3 feet lower than at the riverbank. 2. In general, degradation improves the flood-carrying capacity of the river over what it would be without degradation, If long reaches degrade unifomly. the water surface for floods will drop as the bed degrades. localized degradation may not affect the flood profile to any significant degree. Building check dams at frequent intervals along the river localizes degradation. Consequently, check dams negate the beneficial effect of degradation on the flood profile and can cause significant additional flooding upstream if the dams are high. The drop structures at York Street (River Station 1020+ 20l. at the Colorado and Southern Railroad bridge in downtown Denver (River Station 1390 + 80}. and between Alameda and Mississippi Avenues (River Station 1480 + 10) are three structures which cause Major Tributaries At its confluence with the South Platte River. Clear Creek is a gravel bed stream. In the future. Clear Creek will experience severe flooding at some time when the Platte discharge is low upstream from the confluence. Under such cond1tions degradation could extend up Clear Creek. InOctober 1981,Wright.Mclaughl1n Engineers showed a 6-footdrop in bed profile under Interstate 25 which is 2 miles upstream from the confluence. If that control is not penmanent, then degradation could extend up to the offtake for the Lower Clear Creek Canal another mile upstream. Additional grade control closer to the South Platte River may be needed if the river experiences continued degradation. -.