GZA’s forensic engineering to determine the causal mechanisms associated with Superstorm Sandy flood impacts at a critical New York City structure was integral to the building owner’s insurance recovery efforts.
GZA was retained by the owner’s legal counsel to conduct forensic engineering that would determine the sequence of flooding caused by Superstorm Sandy’s storm surge, in hopes that such documentation could validate the owner’s disputed insurance eligibility.
GZA evaluated casual flood mechanisms, including flood elevation, inundation hydrographs, points of entry, etc. using the two-dimensional hydrodynamic numerical computer model FLO-2D. The model simulated the transient inundation during the storm in and around the three city-block site. FLO-2D simulates transient overland flow migration generated by tidal surges and rivers; using the dynamic wave momentum equation, FLO-2D effectively simulates waves as they move over the floodplain or down channels, streets, alleyways, flow over adverse slopes, wave attenuation, ponding and backwater effects. In urban areas such as this one, buildings and other hydraulic obstructions are simulated to account for the loss of storage and redirection of the flow path.
GZA’s ocean and hydraulic engineers conducted a detailed site reconnaissance to observe the hydraulic setting of the site and vicinity in support of model development. GZA also photo-documented the physical conditions and layout of building, entry points, streets and alleyways that were incorporated into the model. To construct the model, GZA used existing site/area-specific digital terrain models (DTM) to develop polygon shape files to create grid element elevations of the site and vicinity depicting surface contours and physical structures. The hydraulic model was calibrated to observed high water marks, time-stamped photographs and retrieved security videos. The resultant flow model of the site exterior was also used to simulate water migration through penetrations to interior building areas via doorways, windows, and other openings.
The comprehensive model produced by GZA and the documented forensic data were successful in conceptually representing the transient flood processes at the site during the storm, supporting the client’s insurance claim.