Technical Review of Flood Insurance Study ? RO Anderson

The provisional flood insurance study (FIS) and associated Flood Insurance Rate Maps (FIRMs) were received by Douglas County, Nevada in April 2008. Proposed changes to significant portions of the regulatory floodplain resulted in concern among many private citizens. As a result, R.O. Anderson was retained to perform a technical review of the methods used to revise the FIS and the FIRMs. As a part of the FEMA review, five watersheds along the eastern side of Carson Valley were restudied. The hydrology developed for those watersheds included a set of HEC-HMS models ?calibrated? using regional regression equations. The datasets that comprise the foundation for development of these regression equations included little data from watersheds in the Pine Nut Mountains, where the five eastern Carson Valley watersheds originate. In addition, the standard error of estimate was approximately 100 percent. The standard error of estimate for regional regression equations is usually between 30 and 40 percent. As a result, the conclusions of the provisional FIS hydrologic analysis were challenged because the models were calibrated with regression equations that had substantial uncertainty and were based on data not taken in the vicinity affected by the outcome of the hydrologic analysis.

A complex two-dimensional hydrodynamic model was used by the FEMA contractor to analyze shallow flows over the valley floor. The model was uncalibrated and no convergence test was performed. The conclusions of the hydraulic modeling were challenged on the basis of a lack of understanding of the application of the numerical method in the context of a complex hydraulic system. The lack of convergence testing and not including hydraulic structures (such as culverts) was an omission that would result in significant differences to the floodplain delineation. Conservation of mass in the model application was not achieved.

The floodplain boundary adjustments reflected in the provisional FIRMs were substantial. A review of the floodplain boundaries determined that the AE zone boundaries did not comply with FEMA 38 floodplain boundary accuracy standards for three panels of the provisional FIRMs that were available for detailed comparison.

A report was developed and submitted to Douglas County, Nevada. The purpose of the report was to document the findings of the R.O. Anderson review for submission to FEMA as part of their public-review process. The results of the submission of materials to FEMA are not known at this time.

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Source: http://www.roanderson.com/what-we-do/hydrologyhydraulics/technical-review-of-flood-insurance-study/

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