Flood-Inundation Maps for Four Stream Reaches, Ithaca, Tompkins County, New York

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. Flooding in the City is an annual problem caused by a variety of distinct and sometimes interconnected reasons. Flooding often is a result of snowmelt and rain during the winter and spring. Slow ice-melt and breakup can lead to ice jams and subsequent flooding. Flash floods are produced by summer thunderstorms. All of these flood types are compounded by two factors: the storm-sewer system in the City and the elevation of Cayuga Lake. The storm sewers drain to the nearby streams at points below the tops of the streambanks. 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. Also, all potential flooding can be aggravated when Cayuga Lake is at or above flood stage. 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. 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.

 

As 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. These models would be used to simulate high flows and to generate water-surface profiles from which flood-inundation maps could be created. 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. Flood maps are also available from a recent study conducted by the U.S. 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. They do not account for flooding further upstream on Fall, Cascadilla, and Sixmile Creeks. The proposed flood-inundation maps would depict flood extents associated with specific flood frequencies for each of the study reaches. 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. 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.

 

Background 

The geomorphology of the creeks in Tompkins County are greatly influenced by the glacial history of the area. Cayuga Lake and its main tributary, Cayuga Inlet, follow a north-south glacial trough. All lateral tributaries flow from hanging valleys, through narrow gorges, and over waterfalls before entering the flat Cayuga Lake floodplain. 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. 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. 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.

 

The USACE conducted a hydraulic analysis of the decreased conveyance in Cayuga Inlet that has resulted from long-term sedimentation (shoaling) of the flood-control channel (USACE, 2011). The USACE also conducted a dam-break analysis of the reservoirs on Sixmile Creek. In both cases, HEC-RAS models were developed as part of these analyses.

 

Objective  

The objective of this project is to develop hydraulic models and generate a series of flood-inundation maps that will reflect flows with 2-, 10-, 50-, 100-, and 500-year recurrence intervals for selected reaches of Fall, Cascadilla, and Sixmile Creeks and Cayuga Inlet within the City limits of Ithaca.