Engineering Services
PAR Government Systems Corporation (PGSC) has pioneered advanced methods and software tools that support an integrated, Geographic Information System (GIS) based approach to watershed modeling and mapping. The GIS database methodology integrates federal and state digital data sources, including high-resolution orthoimagery, LiDAR-based digital elevation models, and Global Positioning System (GPS) field survey technologies. Custom GIS database software interfaces with standard hydrologic and hydraulic modeling software.
This custom software provides accurate and effective data management, input and output data set creation, and map product generation. The GIS approach to floodplain modeling and mapping is compliant with Federal Emergency Management Agency's (FEMA's) Digital Flood Insurance Rate Map (DFIRM) standards and has been applied in support of FEMA's map modernization initiative under the Flood Hazard Mapping Program. PGSC is currently performing projects in support of the New York State Department of Environmental Conservation (NYSDEC) and FEMA. PGSC is an ESRI Business Partner. 
Software Solutions
PGSC's software methodology includes a suite of commercial GIS and custom Flood*Ware™ software tools which support database integration, generation of Hydrologic Engineering Center-River Analysis System (HEC-RAS) input files, nomination of peak discharges, modeling of water surface elevations, and encroachment analysis. A visual representation of flood extents and floodways is rendered using ArcView Spatial Analyst, based on HEC-RAS report files and a RAS-Plot profile database. This visualization may be displayed over digital base maps or orthorectified imagery to provide a detailed evaluation of flood extents, floodways, and potential property damage.
PGSC's Flood*Ware™ is PC Windows-compatible, and requires ArcView 3.x with the Spatial Analyst extension, HEC-RAS, and RAS-Plot. Optionally, Check-RAS and HEC-1 may be utilized. ArcView and Spatial Analyst are licensed ESRI software products. HEC-RAS and HEC-1 are licensed software products from the US Army Corps of Engineers.
Flood Model Generation
Flood*Ware™ is used to generate and format HEC-RAS geometric input files (detailing stream hydrogeometry) from a GIS database. The stream network is divided into segments along the stream centerline. Perpendiculars to the stream flow are constructed and elevations are sampled on both sides of the stream centerline. Flood*Ware™ interpolates between field surveyed cross sections and hydraulic obstructions, using Manning’s equation, to construct bed profiles.
Stream widths and slopes are computed for each segment, based on a high accuracy digital elevation model. A flow balance is applied to obtain stream depths between survey sites. The illustration to the right is an example of digital landcover and orthophotographs used to compute overbank Manning numbers.
Flood Modeling and Encroachment Analysis
The resulting geometric file is used as input to HEC-RAS and multiple flow profiles are processed. Flow profiles are generated for nominated return periods, such as 1.25, 2, 5, 10, 20, 25, 50, 100, 200, and 500 year events. In addition, discharges and high water marks, measured during recorded historical events, are used to calibrate and verify the model.
The results of running HEC-RAS are output as report files, loaded into RAS-Plot for profile and floodway data table creation. These report files, as well as the RAS-plot database, are loaded into ArcView for geospatial analysis and visualization. Sample HEC-RAS perspective view and cross-section displays are illustrated to the right.
Flood Map Development
HEC-RAS report files are processed using Flood*Ware™. Water surface elevations and encroachment stations, as predicted by HEC-RAS, are fit to contours generated from the elevation model. The result is a set of polygons that provide the extent of inundation, and define a floodway that may be used as overlays with digital maps, property parcel maps, or orthorectified imagery. In addition, the RAS-Plot database is used to generate labeled cross-sections, and BFE lines.
These digital products provide the basis for visualization, hard copy map production, property damage assessment, and emergency planning. The illustration to the right represents the results of applying the methodology for a 100-year discharge, including a floodway from encroachment analysis overlaid on a hill-shade of an elevation model.
