Flood-Inundation Mapping for Schoharie Creek at North Blenheim, NY

Background and Problem  

Flooding is a ubiquitous problem throughout New York State. The Schoharie Creek has experienced severe floods, including the devastating floods of August 2011 following Hurricane Irene, which severely damaged or destroyed many homes, businesses, roads, and other property within the Schoharie Valley.  In North Blenheim, homes, the Blenheim Town Hall, fire hall, and maintenance hall were inundated; road were washed out and the historic covered bridge was destroyed. In addition to the flood of 2011, major floods occurred in April 1987, January 1996, and April 2005. Before and during a flood, forewarning and emergency response are critical to minimizing loss of life and property. The rescue efforts of emergency responders are often hampered by lack of an understanding of where flooding is occurring at any given moment, but also where flooding is likely to occur during the projected flood peak. Emergency personnel would benefit from a series of flood-inundation maps that delineate the areal extent and depth of flooding.

 Flood-inundation maps are derived from high-accuracy elevation data, such as lidar (light detection and ranging) data, and hydraulic information, often a hydraulic model. The Lidar elevation data and hydraulic data are integrated with a geographic information system (GIS) to delineate flood-inundation areas for given water levels within the modeled reach. With this information, a series of accurate flood-inundation maps are produced for the reach. These maps are associated with real-time stage recorded at a USGS streamgage and the predicted stages from a National Weather Service (NWS) river forecast point, if available. There are two USGS streamgages on the Schoharie Creek in the vicinity of North Blenheim: 01350180, Schoharie Creek at north Blenheim, located on the left bank, upstream of North Blenheim, with a period of record from 1970 to current, and 01350212, Schoharie Creek near North Blenheim, located on the right bank just downstream of the State Route 30 bridge, with a period of record from 2017 to current. At high flows, the Schoharie Creek partially bypasses the main channel near streamgage 01350180, so use of the newer downstream streamgage 01350212, at the State Route 30 bridge is preferable for this study.

 By referring to inundation maps for a given stage, emergency responders and property owners can immediately obtain a picture of the extent and severity of a flood and make plans for near-future changes in flood levels. In addition to static maps that can be printed and, for example, mounted on the wall at the fire station, an online interactive mapper will also be available. More information on flood inundation mapping and the online mapper are available at: http://water.usgs.gov/osw/flood_inundation/; the USGS Flood Inundation Mapping Program works with the NWS, the U.S. Army Corps of Engineers, and the Federal Emergency Management Agency to connect communities with available federal resources, thereby ensuring the quality and consistency of flood inundation maps across the country.

 Objective and Scope

The objective of this project is to create a library of flood-inundation maps at incremental stream stages for Schoharie Creek at North Blenheim using streamgage 01350212, Schoharie Creek near North Blenheim. The maps will be derived from flood water-surface profiles computed using a one-dimensional step-backwater hydraulic model (HEC-RAS, U.S. Army Corps of Engineers, Hydrologic Engineering Center 2016a, 2016b) computed using the steady-flow option. Lidar data collected in 2014 and field surveys of channel bathymetry will be used to create cross sections for the hydraulic model. The results of the hydraulic model will be re-combined with the lidar data in a GIS to produce the inundation maps.

 The study reach will extend about 2.4 miles, from 0.3 miles downstream of the Blenheim-Gilboa Pumped Storage Lower Reservoir to 0.5 miles downstream of streamgage 01350212. The modeled reach will extend a further 0.7 miles downstream to ensure model convergence and stability in the mapped reach and at streamgage 01350212. Maps will be referenced to the stage recorded at the USGS streamgage 01350212 and generated at 1-ft intervals for stages ranging from 14 ft, gage datum (784 ft, North American Vertical Datum of 1988 [NAVD88]), which is approximately the 50 percent annual exceedance probability stage, to 24 ft, gage datum (794 ft, NAVD 88), which is just above the observed stage of the record flood during August 2011 at the location of streamgage 01350212 at the State Route 30 bridge.

 Approach 

1. Obtain high-accuracy elevation data (lidar) Digital Elevation Model (DEM) for the study reach.

2. Survey current bathymetric data for the channel and structures within the study reach.

3. Extend the rating for streamgage 01350212, Schoharie Creek near North Blenheim, to stage 24 feet, just above the high-water mark elevation and flow observed in August 2011.

4. Create a hydraulic model using lidar and observed bathymetry and structure data.

5. Calibrate model to streamgage ratings and to observed high water marks from 1987, 1996, and 2011 flood events.

6. Compute water-surface profiles through the study reach (as defined in the Scope section) at 1-ft increments stages ranging from 14 to 24 ft, gage datum.

7. Transfer water-surface profiles to GIS and, following the Flood-inundation map development and documentation standards (USGS, 2015), and using the DEM data,

–     create a water-surface Triangular Irregular Network (TIN) from the water-surface-elevation data for each flood stage, and

–     create a flood-inundation polygon and a flood-depth grid for each flood stage. The flood maps will be overlain on recent orthophotographs.

8. Link the final inundation and flood-depth maps to the USGS flood mapping site. The flood mapping site will show the flood-inundation polygons and the flood depth will be indicated when a point in the flooded area is “clicked.”

9. Produce a USGS scientific-investigations report that documents the process used to generate the flood-inundation and flood-depth maps and create a ScienceBase Data Release of the hydraulic model and GIS files

References