Use of GIS in Determination of the Probable Maximum Flood at Nuclear Plant Sites Presented by: Monica Anderson, GISP Tennessee Valley Authority Carrie Stokes, PE, CHMM Barge Waggoner Sumner & Cannon, Inc.

Presentation Overview Requirements for Probable Maximum Flood (PMF) Determination Definition of PMF TVA River System Overview Modeling Needs GIS data processing GIS-based inputs for model Lessons Learned Questions

Requirements for PMF Determination Nuclear Regulatory Commission (NRC) regulations require that nuclear plants be designed for the Probable Maximum Flood (PMF) as described in Regulatory Guide 1.59, “Design Basis Floods for Nuclear Power Plants”.

Probable Maximum Flood (PMF) The Probable Maximum Flood (PMF) is the flood that may be expected from the most severe combination of critical meteorological and hydrologic conditions that are reasonably possible in a particular drainage area.

Tennessee River System Overview  The Tennessee River is the fifth largest in the United States in terms of discharge and seventh largest in North America.  The drainage area covers 40,910 square miles and lies mostly in the state of Tennessee with parts in six other states- Kentucky, Virginia, North Carolina, Georgia, Alabama, and Mississippi.  There are over 49 dams on the Tennessee River watershed.  The river system is operated as a multi-purpose system.

Tennessee River System Overview

Summer Pool Elevations

Modeling Needs Inputs to the PMF model simulations included: calibrated reservoir models (geometry files and Manning’s n values), operating guides and initial reservoir levels, reservoir storage, initial dam rating curves, and storm-specific inflow hydrographs developed based on rainfall, rain/runoff relationship, and basin runoff response.

GIS Data Used USGS Digital Elevation Model (DEM) 10M & 30M & hard copy 1:24,000 Topographic Maps US Army Corps of Engineers – 500’ Underwater Elevation Points Rain/Stream Gage Data from TVA River Operations Hydro Engineers Watershed Boundaries were derived from the Hydrologic Units based on the 12-digit Hydrologic Unit Codes downloaded January Watershed boundaries for AL, GA, and VA were downloaded from NRCS ( for NC, from USDA (ftp://ftp-fc.sc.egov.usda.gov/NC); for TN, from Tennessee Spatial Data Server ( Additional boundaries were delineated at selected stream gages from 1:24,000 scale USGS topographic maps. State of Tennessee Digital Terrain Model (DTM) data

Data Processing Approach Convert DEM data to Contour Polygons Digitize data from hard copy 1:24,000 Topographic Maps from USGS Converted the USACE 500’ underwater elevation points to TIN – Perfected this process through a number of trial and error attempts. HUC delineation in areas of concern with existing sub-basin data Antecedent Precipitation Index (API) regions were geo-referenced and plotted with the watershed basins. Reservoir Storage – calculate total surface area of each contour polygon. Probable Maximum Precipitation Isohyets used to calculate the weighted average rainfall for each sub-basin.

Model Input - Watershed Delineation

Model Input – API Regions

Model Input – Channel Geometry

Model Input - Channel Geometry

Model Input – Reservoir Storage

Model Input – Critical Storm Selection

Lessons Learned DEM Conversion Inaccuracies – details matter! – 32-bit floating point option instead of 16-bit integer option – This change allows ArcGIS Spatial Analyst Contour Tool to create smoother (interpolated) contour linework. – US Survey Feet conversion factor (repeating) should have been used instead of international feet ( ).

Questions?