![]() Flows input to the models will be obtained from the most recent flood frequency estimates as published by the USGS (Wall and others, 2014) for the streamgages 04234000 on Fall Creek (about 1.2 miles upstream from the study reach) and 04233255 on Cayuga Inlet at the upstream end of the study reach. The hydraulic models will be used to compute water-surface profiles through the study reaches (as defined in the Scope section) for flows with recurrence intervals of 2-, 10-, 50-, 100-, and 500-years (annual exceedance probabilities of 50, 10, 2, 1, and 0.2 percent). These areas do not receive additional protection from flood waters, however, because the outfalls of the storm sewers that drain to these streams are below the top of banks and permit water to pass into these areas before the water level in the streams overtops the banks.Ħ. High flows in all four stream reaches are constrained by concrete flood walls or elevated earthen banks or levees, such that, in most of the area, the land surface adjacent to the streams is below the natural or man-made top of banks. In the Ithaca area, this floodplain is highly developed and is a main concern for flooding from Fall Creek, Cascadilla Creek, and Sixmile Creek all of which are westward-flowing tributaries that cut through the City of Ithaca to their confluences with Cayuga Lake or Cayuga Inlet. All lateral tributaries flow from hanging valleys, through narrow gorges, and over waterfalls before entering the flat Cayuga Lake floodplain. Cayuga Lake and its main tributary, Cayuga Inlet, follow a north-south glacial trough. The geomorphology of the creeks in Tompkins County are greatly influenced by the glacial history of the area. These flood-inundation maps associated with a flood stage could be used by emergency responders and the public to view the areal extent of potential flooding and to make timely decisions regarding road closures and evacuations. The flood maps would also be referenced to staff gages that will be installed at a selected point within each of the Fall, Cascadilla, and Sixmile Creeks study reaches and to a USGS streamgage on Cayuga Inlet. The proposed flood-inundation maps would depict flood extents associated with specific flood frequencies for each of the study reaches. They do not account for flooding further upstream on Fall, Cascadilla, and Sixmile Creeks. ![]() Army Corps of Engineers (USACE, 2011) however these maps reflect flooding only for the 100-year flood and one initial lake elevation, and only along Cayuga Inlet and the mouths of the tributaries. ![]() Flood maps are also available from a recent study conducted by the U.S. Flood maps for extreme floods are already available through the effective FEMA flood insurance studies, but these maps are dated (FEMA, 1981 1984) and changes in channel geometries due to sedimentation necessitate revising these maps. These models would be used to simulate high flows and to generate water-surface profiles from which flood-inundation maps could be created. Under extreme conditions, flooding can result from high lake levels alone by water moving through the storm-sewer system and filling low spots in parking lots and road ways.Īs part of the flood management plan, the City would like hydraulic models of the reaches of Fall, Cascadilla, and Sixmile Creeks and Cayuga Inlet that lie within the City limits. High lake levels can cause backwater in the downstream reaches of the streams that flow through the City, which, in turn, can slow flows and increase water levels above that which would otherwise occur. ![]() Also, all potential flooding can be aggravated when Cayuga Lake is at or above flood stage. Because the streamward ends of the storm sewers do not have backflow covers, this situation can permit stream water to flow through the sewers and out manholes on the landward sides of streambanks or floodwalls potentially causing flooding before the stream itself reaches the top of the banks. The storm sewers drain to the nearby streams at points below the tops of the streambanks. All of these flood types are compounded by two factors: the storm-sewer system in the City and the elevation of Cayuga Lake. Flash floods are produced by summer thunderstorms. Slow ice-melt and breakup can lead to ice jams and subsequent flooding. Flooding often is a result of snowmelt and rain during the winter and spring. Flooding in the City is an annual problem caused by a variety of distinct and sometimes interconnected reasons. The City of Ithaca, Tompkins County, N.Y., is in the process of developing a flood management plan for the streams that flow through the City.
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