Water-Surface Profiles and Discharges for Four Stream Reaches, Ithaca, Tompkins County N.Y.

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Background / Problem – 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 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 subseque...

Background / Problem – 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 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 before rising water in the streams 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 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 intends to have the USGS develop 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. In anticipation of the need to calibrate these hydraulic models, the USGS is proposing that flows and water-surface profiles for up to two storm events be measured on each of the four streams.

 Objective and Scope – The objective is to collect the streamflow and high-water-elevation data for calibration of hydraulic models for the reaches of Fall, Cascadilla, and Sixmile Creeks and Cayuga Inlet that lie within the City limits. Two high flows (between half- and full-bank depths) will be measured on each of the four streams listed above within their respective study reaches. Concurrently water-surface elevations will be flagged along the study reaches and subsequently surveyed to obtain a water-surface profile for each measured flow.

Approach

1.      The study reaches lie within the City limits of Ithaca along Fall, Cascadilla, and Sixmile Creeks and Cayuga Inlet (fig. 1). The three creeks enter the Cayuga Lake floodplain, where the City of Ithaca is situated, by way of gorges, which have been created by the downcutting streams since the end of the last glacial period. The study reaches extend from the point where each creek exits its respective gorge downstream to its respective confluence with Cauga Inlet. The study reach for Cayuga Inlet extends from its mouth at Cayuga Lake upstream to the flow-control structure at the upstream end of the flood-control channel on the Inlet.

2.      The reaches will be inspected to select an appropriate bridge from which to make discharge measurements and to identify accessible measurement sites for flagging and surveying water surfaces. 

3.      Streamflow will be measured during two storm events when the flow is at least half bankfull, but preferably close to bankfull, in depth. At the same time that the flow is being measured, the water surface (WS) along the study reach will be flagged at approximately 500- to 1000-ft spacing along the reaches. This should result in about 6 water-surface elevations on Fall Creek; 8 on Cascadilla Creek; 10 on Sixmile Creek; and 12 on Cayuga Inlet. Included with the data for Cayuga Inlet will be the recorded gage height from the USGS lake-level gage (station 04233500).

Realizing that (1) the optimal situation for obtaining a WS profile would be the peak stage of a storm event and (2) the slope of the WS profile during a rising- or falling-stage period might not reflect the peak-stage WS slope, but (3) being onsite at the exact time to measure a peak flow would be very difficult to achieve, the tentative plan for collecting the desired data would be to measure streamflow and flag WS elevations during the rising limb of one storm event and the falling limb of the same or second storm event. If discrepancies in WS slopes are noted between the pair of datasets from a given stream, this discrepancy will be accounted for when the data are used to calibrate the hydraulic models.

 4.      The elevations of any reference points that are used to tape-down to the water surface and (or) the flagged water-surface marks will be measured with a Trimble RTK-GPS (where possible) or a total station. All elevations will be recorded at North American Vertical Datum of 1988.

Project
Location by County

Tompkins County, NY