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

Science Center Objects

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 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 subse...

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 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.
 
Approach 
The guidelines for creation of flood-inundation map libraries that have been jointly developed by the NWS and the USGS (2008) will be followed.
1. Past flood-related reports will be obtained and reviewed. These documents will include the FEMA flood-insurance studies (FISs) for the four study reaches (FEMA 1981; 1984) and the USACE study of Cayuga Inlet (USACE, 2011).
2. A Digital Elevation Model (DEM) of the study area that was derived from lidar data collected during 2008 will be obtained from the City of Ithaca.
3. Two HEC-RAS models (USACE, 2010) will be developed for the study reaches; one will model Fall Creek and the other will model flows in the other three streams. Fall Creek-- A HEC-2 model for Fall Creek from the 1981 FIS is available. This model will be reviewed for its accuracy and applicability to present-day conditions. The City Department of Transportation will be consulted regarding the validity of bridge geometries. If bridges have been replaced since 1981, the new bridges will be surveyed. Channel geometry will most likely have to be updated. This will be accomplished by surveying the within-bank cross sections at selected points along the reaches and then merging these data with cross sections extracted from the current DEM.
Cayuga Inlet, Sixmile Creek, and Cascadilla Creek-- The second model will use the existing USACE HEC-RAS model for Cayuga Inlet (developed in 2010) as a starting point. The downstream segment of an existing USACE HEC-RAS model for Sixmile Creek (developed in 2010) will be joined to the Cayuga Inlet segment. A HEC-2 model for Cascadilla Creek from the 1981 FIS is available. As with the Fall Creek HEC-2 model, this model will be reviewed for its accuracy and applicability to present-day conditions. A new model segment for Cascadilla Creek will be added to the combined Cayuga Lake-Sixmile Creek segments to form the final second HEC-RAS model.
4. One deficiency in the existing USACE HEC-RAS models is the lack of current bathymetric data for Cayuga Inlet and the mouths of Fall, Cascadilla, and Sixmile Creeks, which are subject to shoaling in the reaches that are affected by backwater from Cauyga Inlet and Cayuga Lake. Bathymetric data will be collected in these reach segments from their mouths upstream to their respective intersections with NY Route 13. The surveys will be conducted with an ADCP and RTK GIS. A zig-zag pattern of data collection will be followed in each of the channel segments. An independent quality-assurance dataset (as described in Wilson and Richards, 2006) will be collected down the center of each channel. A statistical analysis will be performed on the coincident data points from the two datasets to assess the accuracy of the surveyed elevations. The bathymetric data will be incorporated into the cross sections used in the HEC-RAS models.
5. During the field surveys, a road crossing within each of the three creek reaches will be selected for installation of a staff gage on one of the downstream bridge wingwalls. Staff sections will be selected such that values read from the gage can easily be converted to elevations. The existing USGS streamgage 04233255 on Cayuga Inlet will serve the same function as these staff gages. The flood maps produced as part of this project will be referenced to these gages for use by emergency responders during times of high flows.
6. 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). 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. Flows for Sixmile Creek will be estimated by drainage-area adjustment of flood frequencies from streamgage 04233300, Sixmile Creek at Bethel Grove (about 4 miles upstream from the study reach). These estimates will be checked with those computed from StreamStats for reasonableness. The flood frequencies for Cascadilla Creek will be estimated from StreamStats. The flows determined as described will be compared with those from the City of Ithaca FIS and those used in the USACE HEC-RAS models for Cayuga Inlet and Sixmile Creek.
7. Flood-profile data that will have been collected by the USGS for two storm flows on each of the four study reaches (completed as part of a separate project) will be used for calibration of the models.
8. Three initial Cayuga Lake elevations will be simulated: a winter-time low of 381.0 ft, a summertime average of 384.0 ft, and 387.0 ft, which exceeds both the FEMA 100-year elevation of 386.3 ft and the period-of-record peak elevation of 386.46 (which occurred in 1993). The HEC-RAS models will simulate water-surface profiles for a given recurrence-interval flood for each of these lake elevations. A flood of the same magnitude (that is, same recurrence interval) will be assumed to occur on each stream coincidentally. This approach will result in 15 sets of water-surface profiles: 5 flows simulated at three different lake levels.
9. Water-surface profiles will be transferred to a GIS (HEC-GeoRAS; USACE, 2009) and, following the guidelines for interagency coordination regarding the production of flood-inundation map libraries documented by the NOAA (2011) and the IWRSS Consortium (2013), the DEM data will be used to -- create a water-surface Triangular Irregular Network (TIN) from the water-surface-elevation data for each simulated flood magnitude / lake elevation, and -- create a flood-inundation polygon and a flood-depth grid for each flood magnitude / lake elevation.
10.The polygon shapefiles and depth grids generated by each of the two models will be merged such that the final flood maps will display the entire study area.
11.The 15 flood maps will be available in electronic format as individual products of this project; that is, as shapefiles and depth grids. Three composite maps will be created in PDF format. These maps will display the flood-extent polygons of the 5 simulated flood magnitudes for each of the given lake elevations at 381.0, 384.0, and 387.0 ft. The polygons will be color-coded and overlain on the most recent low-altitude orthophotographic imagery available.
12.The final HEC-RAS hydraulic model, flood-inundation polygon shapefiles, depth grids, and associated metadata will be provided to the City of Ithaca.
13.A USGS scientific-investigations report that documents the process used to generate the flood-inundation and flood-depth maps will be written.
14.All models will be archived in amanner consistent with the requirements established in Office of Surface Water Technical Memorandum 2015.01
 
References
FEMA, 1981, Flood insurance study, City of Ithaca, New York, Tompkins County
FEMA, 1984, Flood insurance study, Town of Ithaca, New York, Tompkins County
IWRSS Consortium, 2013, Requirements for the National Flood Inundation Mapping Services: IWRSS Consortium, including NOAA, USACE, and USGS, 51 p.
National Oceanic Atmospheric Administration, 2011, NOAA partnered guidelines for the development of Advanced Hydrologic prediction Service flood inundation mapping: National Oceanic Atmospheric Administration, paginated by section.
U.S. Army Corps of Engineers, 2009, HEC-GeoRAS, GIS tools for support of HEC-RAS using ArcGIS, User’s Manual, version 4.2: U.S. Army Corps of Engineers, Hydrologic Engineering Center [variously paged].
U.S. Army Corps of Engineers, 2010, HEC-RAS River Analysis System, Hydraulic Reference Manual, version 4.1: U.S. Army Corps of Engineers, Hydrologic Engineering Center [variously paged].

Project
Location by County

Tompkins County, NY