Flood maps for extreme floods are available through the effective FEMA flood insurance study (FEMA, 1978), but these maps, which became effective in 1979, are considered no longer valid by the Village of Dryden owing to development in the basin, which presumably has increased runoff and, in at least one instance, encroached on the floodplain. In addition, similar data for lower, intermediate, and higher floods are non-existent. Emergency responders and the public would benefit from a series of flood-inundation maps for the portion of Egypt Creek that flows through the Village of Dryden.
(a) Available data on flows will be compiled from the FEMA Flood Insurance Study for the Village of Dryden, NYS Department of Transportation bridge-design data, and flood assessments conducted by consultants in regards to development projects in the basin.
(b) Flows for selected flood frequencies will be computed from the USGS program, StreamStats (USGS, 2014), which utilizes the regionalized flood-frequency equations for rural, unregulated streams in N.Y. that were developed by Lumia and others (2006).
(c) The probable effects of development on flood flows in the basin will be assessed by using the equations for estimating flood flows in urban watersheds that were developed by Sauer and others (1983).
These computed flows, along with any other available flow data, will be used as guidance for selecting realistic flow contributions to Egypt Creek from each of the north and south branches, as well as from the tributary near the Dryden High School (fig. 1), for input to the model.
2. The second phase of the project is to create flood-inundation maps for the study reaches. The guidelines for creation of flood-inundation map libraries that have been jointly developed by the NWS and the USGS (2008) will be followed.
- Lidar data will be compiled and a DEM created from which cross-section elevations will be extracted for the hydraulic model and flood-inundation maps will be generated.
- Five bridges or culverts will be surveyed along the main channel and north branch. One bridge and four culverts will be surveyed along the south branch. Surveys will be tied to the North American Vertical Datum of 1988. Personnel from the Tompkins County Soil and Water Conservation District will conduct the bridge and culvert surveys.
- A reference mark on or near the Route 13 bridge over the north branch of Egypt Creek will be established. A staff gage or stage recording device will be installed on the downstream side of the bridge. The flood maps will be referenced to the stages observed/recorded by this installation.
- A HEC-RAS hydraulic model will be created. The upstream extent of the model will be Livermore Road on both the north and south branches of Egypt Creek. The downstream extent will be the downstream side of Springhouse Road (about 1,200 ft upstream from the creek’s mouth).
- Flows derived from the hydrologic analysis in step 1 will be input to the model.
- The model will be used to generate water-surface profiles for flows that correspond to roughly 0.5 or 1-ft increases in stage at the stage-monitoring site. The stages will cover the range from top of bank to a stage greater than that associated with the estimated 500-year flood. The increment of stage that is used for the maps will depend on the overall range in flood stages and the change in inundation area that results from an incremental increase in stage. If the range in stage from top of bank to the approximate 500-year flood level is less than 10 ft and/or the area of inundation increases substantially with small increases in stage, then the maps will be created for 0.5-ft increments of stage; otherwise a 1-ft increment will be used.
- The model will be calibrated to any existing high-water elevations that the Village or NYS Department of Transportation has documented in the past (these data have not yet been identified nor compiled). Peak flows that occurred in Egypt Creek on a given date that these high-water elevations were recorded will be estimated by applying a drainage-area adjustment to concurrent flows recorded at the Fall Creek near Ithaca streamgage (04234000), the closest USGS gage, which is about 10 miles west of Dryden. This approach can be "tested" by comparing similar drainage-area-adjusted flows computed for Virgil Creek at Dryden (the stream that Egypt Creek flows into) and comparing these peaks flows with those recorded by a USGS gage that existed on Virgil Creek (0423368620) from 2002-04. Alternatively, a peak-flow versus drainage-area analysis can be performed for each past flood date (for which water-surface elevations are available) that would include data from the Fall Creek gage, as well as several other, more distant, streamgages in the general area. Peak flows for Egypt Creek could then be estimated based on this relation.
- Model calibration will also be attempted by measuring at least two medium or high flows on Egypt Creek during the spring of 2015. High-water marks will be flagged, surveyed, and used for calibrating the model.
- Simulated water-surface profiles will be compared with those included in the effective FEMA flood-insurance study (FEMA, 1978).
- Water-surface profiles generated by the HEC-RAS model for each flood flow will be imported to a GIS program, which, along with the previously developed DEM, will be used to create flood-inundation polygons and water-depth grids.
- Flood polygons will be shaded to depict varying flood depths and will be overlain on the most recent low-altitude orthophotographic imagery available.
3. USGS scientific-investigations maps (in PDF and KML formats), shapefile polygons and depth grids of flood-inundation areas, metadata, and a pamphlet that documents the process used to generate the flood-inundation maps for Egypt Creek will be posted on the USGS Publication Warehouse web site (http://pubs.er.usgs.gov/). KML files of the flood maps will enable viewing the flood-inundation areas using the Google Earth mapping application.
Reference
FEMA, 1978, Flood Insurance Study, Village of Dryden, New York: U.S. Department of Housing and Urban Development, Federal Insurance Administration, 19 p., 10 plates, 1 map.
Lumia, Richard, Freehafer, D.A., and Smith, M.J., 2006, Magnitude and frequency of floods in New York: U.S. Geological Survey Scientific Investigations Report 2006-5112, 152 p. plus DVD.
National Weather Service and U.S. Geological Survey, 2008, Flood inundation map library guidelines: accessed March 4, 2013, at http://water.usgs.gov/osw/flood_inundation/toolbox/
Sauer, V.B., Thomas, W.O., Jr., Stricker, V.A., and Wilson, K.V., 1983, Flood characteristics of urban watersheds in the United States: U.S. Geological Survey Water-Supply Paper 2207, 63 p.
U.S. Geological Survey, 2014, The StreamStats Program—New York, accessed November 19, 2014 at http://water.usgs.gov/osw/streamstats/new_york.html
Project
Location by County
Tompkins County, NY
