Source_Contribution:
The Bare Earth Digital Elevation Model (DEM) used to generate all mapping products was produced by the PAMAP Program and consists of a raster digital elevation model with a horizontal ground resolution of 3.2-feet. The model was constructed from PAMAP LiDAR (Light Detection and Ranging) elevation points. First, the Bare Earth DEM was used to cut cross-section data which was done in the HEC-GeoRAS environment. HEC-GeoRAS is a set of ArcGIS tools specifically designed to process geospatial data for use with HEC-RAS. HEC GeoRAS first serves as a pre-processor which feeds elevation data to HEC-RAS. The hydraulic modeling is completed using HEC-RAS and once the output is considered satisfactory to the engineer, the results are exported from HEC-RAS to HEC-GeoRAS, which is then used as a post-processor in order to visualize the results in the GIS environment. The HEC-RAS output file contained 28 modeled flood profiles from Stage 11-feet through Stage 37-feet as measured at the USGS Gage ID Number 01570500 (SUSQUEHANNA RIVER AT HARRISBURG, PA). The 28 modeled flood profiles were exported to the GIS environment using HEC-GeoRAS. All GIS data developed during HEC-GeoRAS post-processing is based on the content of the HEC-RAS GIS Export File and the Bare Earth DEM. The HEC-RAS GIS Export File contains GIS coordinate-based information that describe the modeling cross-section locations and the resulting water surface elevations at each modeling cross-section. The Export File is first read into the GIS. The next step is to create water surface TINs for each of the 28 modeled flood profiles. The TIN created is based on the water surface elevation at each cross-section. The water surface TIN is created without considering the Bare Earth DEM. The next step is to delineate a flood plain for each water surface TIN. A floodplain polygon is created based on the corresponding water surface TIN. Each floodplain polygon results from intersecting the water surface TIN with the Bare Earth DEM. The water surface TIN is converted to a grid and compared to the Bare Earth DEM. A depth grid is then created with values where the water surface grid is higher than the Bare Earth DEM. The depth grid is clipped by the bounding polygon to remove any areas outside the hydraulic model. The depth grid is then converted into a floodplain polygon feature class. This process resulted in the Main Stem floodplain polygon and Main Stem depth grids. For the study area, there were four major areas of backwater flooding that had to be mapped independently of the Main Stem. Backwater flooding occurs in the following four descriptive areas: 1) along the Paxton Creek from its confluence with the Susquehanna River to the Harrisburg Area Community College; 2) the area behind the Capital City Airport, which is partially along the Yellow Breeches; 3) the area behind the Harrisburg International Airport SR 230; and 4) the area along the Swatara Creek up until a point approximately 2,000-feet south of Interstate 76. These four areas were modeled separately by necessity. Water surface elevations consistent with the point on the Main Stem Susquehanna River which is generating the backwater elevation were mapped using a process similar to the HEC-GeoRAS post-processing routines. During this process, the backwater elevations were rendered as water surface TINs, which were intersected with the Bare Earth DEM to generate both depth grids and floodplain polygons. At this point in the post-processing, all depth grids and Bare Earth DEM data within the backwater areas were re-sampled from a 3.2-feet horizontal ground resolution to a 5-feet horizontal ground resolution in order to accommodate computing times, transfer of data, and legacy mapping software. Once the backwater areas were mapped, the four backwater areas were merged with the Main Stem Susquehanna River flood areas. This was done for the depth grids and the floodplain polygons. Prior to finalizing the data, the depth grids and floodplain polygons needed to be reviewed and edited. The review and editing process consisted of general smoothing and clean-up plus two major steps: 1) remove any disconnected water bodies and 2) bridge clips. Step 1 involved checking all hydraulically disconnected wet areas. If there was evidence that a wet disconnected pond was hydraulically connected (i.e. an underground pipe connects the Main Stem Susquehanna River flooding to the disconnected pond), no action was taken. If there was no evidence of a hydraulic connection, the disconnected pond was removed. Step 2 involved making the depth grids and floodplain polygons as accurate as possible by clipping bridges if they were still usable during a flood event. A clipped bridge means it is not shown as flooded and will remain usable. For Main Stem Susquehanna River bridges, a bridge was clipped (and shown as being usable) as long as the lowest portion of the bridge was not impacted by water. Once the lowest portion of the bridge was impacted, all subsequent and higher elevation flood profiles would not be clipped. For the non-main stem bridges, if the top elevation of the bridge was not flooded, the bridge was clipped and shown to be usable for that flood profile. Once the top elevation of the bridge was impacted by water, all subsequent and higher elevation flood profiles would not be clipped. Once the review and editing process was complete, the depth grids and floodplain polygons were renamed and projected to their final format.