GEOLOGICAL MAPPING –AN ESSENTIAL (BUT FREQUENTLY IGNORED) COMPONENT OF FLOOD HAZARD ASSESSMENT ON DESERT PIEDMONTS

The distribution, frequency, and severity of flood hazards on desert piedmonts is intimately linked to the distribution of surficial geologic deposits and piedmont landforms. Therefore, detailed geological mapping of desert piedmonts provides essential scientific information for managing flood hazards. Southern Nevada is a prime area for illustrating this fact where burgeoning urban centers are rapidly encroaching on pristine desert piedmonts. A recent study of the surficial geology of the piedmont of the Newberry Mountains in Laughlin, Nevada demonstrates the value of geologic mapping by using GIS analysis to directly compare it with conventionally derived Flood Insurance Rate Maps (FIRMs). In the Laughlin area, geologic evidence indicates that large parts of the piedmont have been isolated from flooding for at least 10,000 years. This is an easily drawn geological conclusion that contrasts sharply with the implications of two generations of FIRMs for the area which include large tracts of non-flood-prone land in both the 100-year and 500-year flood hazard zones (Type 1 error). Paradoxically, these same regulatory maps exclude large areas of indisputably flood-prone land from floodplain zones (Type 2 error). Each type of error stems from critical deficiencies in the regulatory model typically applied in this setting. In the Laughlin area, approximately 50% of the piedmont is mischaracterized with respect to its flood-prone status on the FIRMs. Of this, 36% is not flood hazardous but is mapped as such, and 14% is hazardous but mapped as though it were not, or simply not mapped. These circumstances of disagreement pose an essentially irresistible argument for the inclusion of geologic information early in the regulatory process. Recognition of this fact by the Clark County Regional Flood Control District (CCRFCD) has led to their support of a geologic mapping program in the Ivanpah Valley area immediately south of Las Vegas along Interstate 15. Presently, this area is sparsely populated, but it is an inevitable target for significant commercial and residential development. The surficial geology in a set of 9 contiguous quadrangles is being mapped in detail to provide the CCRFCD with data necessary to focus engineering studies and related hazard mitigation efforts to areas that are actually hazard-prone.