Hydrology/Hydraulics

The number of proposed projects along Martin Slough and post-FIRM changes to the Martin Slough floodplain were such that Douglas County, Nevada engineers determined that updated maps of the regulatory floodplain for Martin Slough were necessary. Martin Slough is a distributary of East Fork Carson River and receives flows from East Fork Carson River (through Cottonwood Slough) and from shallow Pine Nut Creek flows during extreme hydrologic events. The critical event is considered to be a cloudburst hydrologic event in the Pine Nut Mountains over the Pine Nut Creek watershed. Flows leave the mountains and are distributed over the valley floor as sheet flow or shallow overland flow and enter Martin Slough near the upstream end of the drainageway just north from Toler Avenue.

The original mapping was done for the Federal Emergency Management Agency (FEMA) by a contractor, probably the Sacramento District of the U.S. Army Corps of Engineers (USACE). The modeling was completed using the USACE HEC-2 steady-state, gradually-varied, water-surface profile model using cross sections collected by field surveying. Effective model cross sections were separated by distances of about 4,000–5,000 feet, too far apart for detailed hydraulic modeling. The topographic mapping used to delineate the floodplain was constructed using aerial topographic methods with a contour interval of five feet, greater than the current two-foot standard. A number of floodplain features were absent from the effective model. Subsequent to floodplain mapping adoption, a number of changes were effected to the floodplain, including addition of crossings (Gilman Avenue, Lucerne Street, Monte Vista Drive), modification of two crossings (U.S. 395 and Buckeye Road), and numerous developments adjacent to the drainageway.

Existing conditions were such that a significant fraction of the base flood discharge is diverted by U.S. 395 north along the east side of the highway. This represents a substantial difference from the formulation of the original modeling. R.O. Anderson was contracted to develop an existing conditions model and prepare a Letter of Map Revision for the study reach. The project was amended to include an approximate analysis of the reach downstream from U.S. 395 to East Fork Carson River.

LiDAR topography supplemented with field surveys was used to develop a suite of models of Martin Slough. The suite of models included an existing conditions model (as well as a duplicate effective model) using more than 80 cross sections for the analysis. The project culminated with reports and applications for a Letter of Map Revision.

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.