in review: J.D. Pelletier, P.A. Pearthree, L. Mayer, K. House, K.A. Demsey, J.E. Klawon, and K.R. Vincent, Alluvial-fan flood-hazard assessment with hydrological modeling, field mapping, and remote sensing: Application to the southern Tortolita and Harquahala piedmonts, Arizona

Abstract: The complex and dynamic patterns of flood flow on alluvial fans presents a significant challenge for flood-hazard assessment because many standard flood-routing methods developed for confined-flow channels cannot be applied to the unconfined distributary flows common on alluvial fans. As a result, there is no widely-used analytical method for predicting flood-inundation extents on alluvial fans. We propose and test a new methodology for alluvial-fan flood-hazard assessment based on a two-dimensional hydrological model for arbitrary topography. The model simultaneously solves the continuity equation and Manning s equation using an implicit numerical method. A high-resolution DEM and peak flood stage are required inputs. Inundation extents and flow regimes of historical flood events can be modeled in conjunction with field- and satellite-based flood-inundation maps. By modeling the inundation extents for a range of input peak discharges, each with a corresponding probability of occurrence, we can construct a map with probability-of-annual flooding estimates. This map can be used in conjunction with a surficial geologic map to further refine flood-prone areas. To test the accuracy of the two-dimensional hydrological model for reconstructing historical floods, we compared model predictions against field- and satellite-based flood-inundation maps for two extreme historical floods on the southern Tortolita and Harquahala piedmonts in Arizona. Predicted inundation extents match field- and satellite-based maps for the Tortolita and Harquahala floods by 83% and 86%, respectively. The success of the model provides a concrete basis for using model-based probability-of-annual flooding maps in flood-hazard assessments. Probability-of-annual flooding maps were constructed for the study areas by modeling flood-inundation extents and flow depths for a range of input peak discharges. Stream-gage records and paleoflood deposits were used to constrain the annual probabilities of input peak discharges. The resulting maps predict a spatially-complex flood hazard that strongly reflects small-scale topography and is consistent with surficial geology. In contrast, FEMA Flood Insurance Rate Maps (FIRMs) predict uniformly-high flood risk across the study areas without regard for topography and surficial geology. The hydrological model is particularly useful for identifying the thresholds in input flood stage required to  activate  flood hazards on different segments of the fan.

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NSF project: Fluvial systems and climate in the Southwestern U.S.

Abstract: The proposed research will identify the key controlling processes of fluvial-system response to Quaternary climatic and tectonic changes in the southwestern U.S. with an integrated approach that combines numerical modeling, geochronology, and field observations. Alluvial-fan terraces record the effects of climate change and tectonics from time scales of thousands to millions of years. These terraces are a major component of the Southwestern landscape, controlling flood hazards, soil development, ecosystems, and even human history. Our ability to read this record is limited, however, by a poor understanding of fluvial-system response. For example, is climate change or tectonics the major trigger of episodes of alluvial-fan deposition in the Southwest? Can the internal dynamics of the fluvial system be responsible for many of the terraces we observe? Does deposition occur during humid periods, arid periods, or the transitions between them? What are the time lags between environmental changes and fluvial-system response? To answer these questions, the project will focus on two distinct times scales. We will reconstruct Quaternary cycles of erosion and deposition from time scales of 10 kyr to 1 Myr. This long-term focus will enable us to determine the effects of the magnitude and timing of different triggering events for the same basin.

In addition, we will reconstruct the spatial and temporal variability of latest Pleistocene and Holocene erosion and deposition from time scales of 1 kyr to 10 kyr in our study area in Cuyama Valley, CA. This work will enable specific fluvial-system
dynamics and hillslope-response mechanisms to be evaluated.

Preliminary cosmogenic and radiocarbon ages constrain the ages of entrenchment and deposition in the Black Mountains, AZ and Cuyama Valley, CA.These ages are consistent with climatic triggering of alluvial-fan deposition during humid-to-arid transitions. Geochronology, however, provides limited information on the spatial and temporal evolution of the fluvial system. Simulation modeling and spatial analysis of alluvial-fan terraces help to provide a more complete picture. Computer-simulation models are useful for linking climate changes with surficial processes because multiple working hypotheses can be generated and tested against field and laboratory observations. For this project, a numerical-simulation model has been developed for the coupled evolution of drainage basins and alluvial fans. This model uses existing digital datasets together with scenarios of climatic and tectonic changes to model the fluvial-system evolution for specific study areas.
The proposed research will determine the signatures of terrace age and morphology for each entrenchment mechanism
so that the dominant processes that shape alluvial fans of the Southwest can be determined. To do this, forward modeling of fluvial-system response to different climatic and tectonic histories will be performed and their results compared against the ages and geometries of alluvial-fan terraces in our study areas.

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